Forest Ergineering (Hydrology) presented on CARLTON STRATTON YEEforthedegreeof (Major Department)

AN ABSTRACT OF THE THESIS OF CARLTON STRATTON YEEforthedegreeof DOCTOR OF PHILOSOPHY (Degree) (Name) i Forest Ergineering (Hydrology) presented on (Major Department) (Date) Title: SOIL AND HYDROLOGIC FACTORS AFFECTING THE STABILITY OF NATURAL SLOPES IN THE OREGON COAST RANGE Abstract approved: R. Dennis Harr This study was conducted to examine certain soil and hydrologic properties of two major cohesionless soils Qccupying 55% of the central portion of the Oregon Coast Range. Knowledge of these properties was desired to determine the role each played in the stability of slopes in this region. Bohannon and Klickitat soils often occupy the steep mid- slopes where the greatest potential for stability problems exists. The Bohannon series is derived from Tyee sandstone and the Klickitat series is derived from intrusive, igneous parent material. Soil samples were obtained from four widely separated sites, two for each of the soil series and were examined for particle-size distribution, bulk density, porosity, pore-size distribution, aggregate stability, saturated and unsaturated hydraulic conductivity, and shear strength. A 1. 15 ha study site was instrumented with a recording aingage, 78 piezometers, and four tens iometers placed at varying depths in the soil profile. Field measurements were made t subsurface water movement in the Klickitat soil during the 1973-74 water year, one of the wettest on record for this area. An intensive subsurface geologic survey of this study site was also made. Both soils, although derived from very different parent materials, exhibited nearly identical ranges of values for soil and hydrologic properties. Both were found to be extremely porous, highly permeable, very well aggregated and graded, sandy to gravelly cohesionless soils. From engineering and hydrology standpoints, the two soil series can be considered as one. In spite of low bulk densities and high porosities, the dry effective angle of internal friction, 4), was found to be unusually large in both soils. For the Bohannon and Klickitat soils, respectively. Such large 4) was 400 and 410, values for such loosely packed soils were attributed to the high aggregation in both soils. Pseudomorphs were stable enough to function as primary particles and possessed increased surface roughness, angularity, and effective size over what they would have had as discrete particles. The effect of water on Reductions in 4) of 950 and was found to be atypical for both soils. 110 were noted when the two soils were were rested in a drained, saturated state. The severe reductions in attributed to aggregate disintegration under direct wetting conditions. A decrease in aggregate cotitetit of 29% in the Bohatinon soils was accompanied by a 28% decrease in 4). For the Klickitat soils, the 26% ti decrease in aggregate content was accompatiied by a 23% decrease Aggregate destruction by direct wetting is a possible mechanism for 4). some slope failures near roads. flow. Movement of subsurface water was predomitlantly by utisaturated While saturated flow was observed iti fractured bedrock near of saturated flow the sedimetitary-igneOus contact, otily otie instance minimum in the soil profile was noted. Tetisiometry indicated that capillary pressures of 5-10 cm of water existed duritig storm evetits. Atialysis of pore-size data and moisture_tetlsiOn relationships substantiated the effectiveness and adequacy of unsaturated flow as the prime mechanism of water transmissiOtl in these soils. Both soils were able to transmit water rapidly and at large fluxes even under unsaturated conditions. Large scale saturated subsurface flow is unncessary for dispersing the low intensity, long duration rainfall found in this region. Soil aid Hydrologic Factors Affectiig Stability of Natural Slopes it the Oregoti Coast Raige by Canton Stratton Yee A THESIS submitted to Oregon State University in partial fuLfillment of the requirements for the degree of Doctor of Philosophy Jute 1975 APPROVED: Assistant Professor of Forest Engineering in charge of major Professor of Civil Engineering Associate Professor of Forest Engineering Research Geologist (Assistant Professor) Graduate School Representative Head of Departmetit, Forest Engineering Dean of Graduate School Date thesis is presented Typed by Susie Kozl.ik for Canton Stratton Yee ACKNOWLEDGEMENTS The work upon which this thesis is based was supported by the Pacific Northwest Forest and Range Experiment Station under Cooperative Agreement Supplement No. 82, FS-PNW-1602 and the Water Resources Research Institute at Oregon State University. Personal words of thanks are extended to Drs. Harr, Bell, Froehlich, and Swanston for not only their advice and encouragement during the preparation of this thesis, but also for their helping me to learn in so many new areas. TABLE OF CONTENTS Page LNTRODUCTION 1 LITERATURE REVIEW 6 6 Slope Stability Strength and Failure The Principle of Effective Stress Analysis of Slope Stability Measurement of Strength Parameters Soil Structure arid the Shear Strength of Cohesionless Soils Geology General Geological Setting Topography arid Geologic Features Soil Morphology Energy State of Soil Water Total Energy Concept Darcy's Law Hydraulic Conductivity DESCRIPTION OF THE STUDY SITE Location Geology of the Area Physiography Climate Soil Vegetation METHODS AND MATERIALS Site Selection Criteria Site Mapping Precipitation Measurements Geologic Inivestigationi Soil Sampling Soil Pits Soil Sampling Laboratory Analyses of Soils Particle-size Distribution Triaxial Compress ion Tests Aggregate Stability Saturated Hydraulic Conductivity 6 12 14 20 28 34 34 37 39 43 43 45 47 53 53 53 53 56 56 57 60 60 61 63 64 66 66 67 70 70 72 74 78 Page Drainage Characteristics Bulk Density and Porosity Piezometry Tensiometry RESULTS General Geologic, Soil, and Site Characteristics Precipitation, Piezornetry, and Tensiometry Physical Properties of the Soils Strength Tests Drainage Characteristics Pore-size Distribution Moisture Characteristics DISCUSSION Physical Properties of Soils Soil Texture Saturated Hydraulic Conductivity Aggregate Stability Soil Aggregation, Shear Strength, atid Slope Stability Angle of Ititernal Frictioti of Dry Soils Angle of Intertial Friction of Wetted Soils Possible Effect of Man-Caused Direct Wetting on Slope Stability Movement of Subsurface Water Unsaturated Flow Hydraulic Gradient Analysis Flow Direction of Soil Water Soil Water Fluxes Threshold Storm Apparent Cohesion Influence of Bedrock 81 84 84 88 91 91 97 109 125 128 128 129 138 138 140 142 144 146 146 148 152 158 158 161 167 171 177 179 181 CONCLUSIONS 185 LITERATURE CITED 189 APPENDICES Apperdix A Common and Scientific Names of Species Found on Study Site 197 Appetdix B: Tables Table I. Saturation Values for Soils at 0-100 cm of Water Capillary Pressure 198 Page Table II. Soil Moisture, Percent of Total Volume (0), for Increasing 199 Capillary Pressure 200 Appendix C: Representative Soil Profile Descriptions Appendix D: Summary of Precipitation Statistics for Selected Stations in the Central Coast 204 Range of Oregon LIST OF FIGURES Page Figure 1 2 Failure envelope for a soil with cohesion and friction properties 8 Failure envelope and Mohr circles 12 Idealized section for an infinite slope with a piezometric surface present 16 4 Stress system for continuous medium 21 5 Stresses in triaxial shear 22 6 Schematic diagram of a triaxial compression device 23 7 Major geologic formations of the central portion of the Oregon Coast Range 36 3 8 9 10 11 12 Tyee sandstone formation overlain by the Bohannon soil series 38 Cuesta face and backslope of dipping arkosic sandstone bedrock 38 Soils and landforms of the Bohannon-Slickrock association 41 Soils and landforms of the Klickitat-Shotpouch association 42 Effective saturation as a function of capillary pressure 13 14 51 Relative hydraulic conductivity as a function of capillary pressure 51 Location of the study site in relation to the physiographic provinces of western Oregon 54 15 Location of the study site 16 Overview of the study area 58 Pag_e Figure 17 Sword-fern community on the study site 59 18 Contour map of study site with soil pit, piezometer, and tensiometer locations shown 62 19 Recording raingage, approximately 160 meters west of the study site 64 20 Acker rock drill used in the subsurface geologic investigation of the study site 65 21 Rock corings obtained by Acker drill 66 22 Soil core sampler used in this study 69 23 Soiltest triaxial compression chamber 73 24 Apparatus for testing aggregate stability 76 25 Constant head permeameter with soil core in place 80 26 Tension table apparatus for determining drainage characteristics of soil samples 82 27 Diagram of a piezometer installation 86 28 Tens iometers installed on the study site 90 29 Contour map showing geologic contact zone and surf icial indicators of apparent slope instability 92 30 Tyee sandstone as seen in micrographic thin section 93 31 Microscopic thin section of the Marys Peak intrusive rock 94 Graph of piezometric height in piezometers 39 and 40 and daily rainfall 99 33 Graph of otezometric height in piezometers 45 and 48 and daily rainfall 100 34 Graph of piezometric height in piezometers 56 and 59 and daily rainfall 101 32 Page Figure 35 Average piezometric head versus 48-hour rainfall 36 Soil capillary pressure at 30 and 60 cm depths and daily rainfall amounts during February and March, 1974 37 38 39 40 41 42 43 44 45 46 47 104 106 Soil capillary pressure at 90 and 120 cm depths and daily rainfall amounts during February and March, 1974 107 Particle-size distribution curve for Klickitat soil from the study site 115 Particle-size distribution curve for Klickitat soil from off site soil pit 116 Particle-size distribution curve for Bohannon soil from soil pit A 117 Particle-size distribution curve for Bohannon soil from soil pit B 118 Particles of Klickitat soil showing high degree of aggregation 122 Particles of Bohannon soil showing typical high degree of aggregation 122 An individual particle of Klickitat soil (less than 0.074 mm) which exhibits typical high degree of aggregation 124 A sand-sized particle of Klickitat soil (greater than 2. 0 mm) with smaller particles aggregated to a larger rock particle 124 Experimental relationships between effective saturation (Se) and capillary pressure for two cohesionless soils of the Oregon Coast Range 130 Change in pore-size distribution with depth for Klickitat soil from the study site (A) and from off site (B) 132 Pagç Figure 48 Change in pore-size distribution with depth for Bohannon soil from soil pits A and B 133 49 Moisture characteristic curves for the Klickitat soil pits 136 50 Moisture characteristic curves for the Bohannon soil pits 137 51 Diagram illustrating the suggested development of a progressive reduction in the factor of safety (F) due to direct wetting of soil by intermittent culvert discharges 155 52 Successive pressure potential profiles during February 4-20, 1974 165 53 Flow vectors on selected days during February 4-20, 1974 168 54 Rotation of a single isopotential (4') with time 170 LIST OF TABLES Page Table Correlation coefficients for piezometric height as a function of the 48-hour cumulative rainfall 102 Regression equations and related r2 coefficients for piezometers 45 and 48 105 Maximum depths of water in piezometers during January 11-16, 1974 storm 108 Mean values for bulk density, particle-size classes, porosity, and saturated hydraulic conductivity for two cohesionless Coast Range soils 110 Aggregate stability (AS) for Bohannon and Klickitat soils under direct and tension wetting 119 6 Results of vacuum and saturated triaxial shear tests 126 7 Mean porosity (n), bulk density (BD), bubbling and pore-size distribution index (X) pressure obtained from capillary pressure-desaturation data for two cohesionless soils of the Oregon Coast Range 134 Mean values of pore-size distribution as of total porosity 135 Estimated values of unsaturated hydraulic conductivity at maximum, mean, and minimum winter capillary pressures 172 Estimated unsaturated hydraulic conductivities for typical clay and sand soils 174 1 2 3 4 5 8 9 10 LIST OF SYMBOLS The following symbols are used in this thesis: Symbol Description Dimensions AS Degree of aggregate stability expressed as a % of total aggregates dimensionless C Cohesion gm/cm2 C Effective (apparent) cohesion gm/cm 2 Uniformity coefficient dimensionless D10 Effective grain size (10% is finer) cm D60 60% grain size (60% is finer) cm e Void ratio dimensionless F Factor of safety dimensionless f Fluidity 1/cm-sec Specific gravity of soil particles dimensionless g Acceleration of gravity cm/sec2 H Hydraulic head cm h Head of water cm I Hydraulic gradient dimensionless K Hydraulic conductivity coefficient cm/sec K(P) Unsaturated hydraulic conductivity coefficient of a soil at capillary cm/sec Kr Relative hydraulic conductivity dimensionless K Saturated hydraulic conductivity coefficient cm/sec C G U S pressure of Desc ription Symbol Dimensions 2 k Intrinsic permeability cm L Length cm m That portion of the depth, to the failure surface in an infinite slope, which is below the water table dimensionless N Normal force gm n Porosity dimensionless Bubbling pressure; approximately the minimum capillary pressure on the drainage cycle at which the nonwetting fluid is continuous cm P c Capillary pressure; the difference in cm pressure across the interface between the wetting and non-wetting fluid 15g Gravitational potential cm P Pressure potential cm Total potential cm p Applied stress gm/cm2 q Flux rate cm/sec R Radius of curveture of a point on the miniscus cm S Unit shear strength gm/cm 2 S Ratio of total shear strength aT,ailable along a failure plane to the factor of safety gm Degree of saturation dimensionless Degree of effective saturation dimensionless S S p a Description Symbol Dimensions Sr Degree of residual saturation dimensionless T Tangential shear force gm T Surface tension of liquid gm/cm u Porewater pressure gm/cm 2 v Volume cm3 Vr Volume of retaining ring cm3 W Total weight of soil and soil moisture gm W Weight of soil solids gm w Gravimetric water content, expressed as % of dry soil weight dimensionless Z Depth or height cm a Miniscus contact angle degrees Angle of slope inclination degrees Unit weight of soil, water, and air gm/cm2 Unit weight of water gm/cm2 Incremental head cm Incremental shear strength cm 1 Pore-size distribution index dimensionless o Volumetric moisture content, in dimensionless y a- % of total volume Pore-size distrtbution index dimensionless Density of water gm/cm3 Stress perpendicular to the surface which it is applied gm/cm2 Description Symbol 3 3 T Dimensions Major, intermediate, and minor principal stresses gm/cm2 Effective (intergranular) stress perpendicular to the surface to which it is appfled g rn/c rn Major and minor principal effective (intergranular) stresses gm/cm2 Shear stress gm/cm2 Angle of internal friction degrees Effective angle of internal friction degrees Isopotential line cm 2 SOIL AND HYDROLOGIC FACTORS AFFECTING STABILITY OF NATURAL SLOPES IN THE OREGON COAST RANGE INTRODUCTION The history of forest land use in the United States, and particularly for the Pacific Northwest, has been traditionally centered around timber growing and harvesting. In the past decade other uses, such as recreation and water, have increased in importance. Nevertheless, timber growing and harvesting is still the dominant use for forest land irt many areas. Historically, the flatter, more accessible areas were the first to be logged. These areas posed relatively few problems in terms of loggirtg, road cortstruction, or slope stability. Ir the past 20 years, an increasing demartd for forest products in the Urtited States, coupled with a rising export market, has resulted in the extension of harvestirtg operations to the steeper mountainous areas. Orte effect of this trend has been the disruption of the stability of natural slopes in many areas which has caused accelerated mass wasting. These slope failures have produced extensive damage to roads and other structures, degraded water quality by excessive sedimentation of streams, removed portiorts of watersheds from timber productior1. artd produced detrimerttal aesthetic effects. An additional result of these slope fail.ure has been to give erviroimeritaiist groups physical evidence upor which to base their charges of imprudent aid reckless forest management. 2 The problem of increased watershed degradation caused by slope failures, initiated by the cultural activities of man, is a recognized one, especially in the mountainous portions of the western United States. Some of the greatest stability problems are located in Oregon, particularly in its Coast Range. This region has high relief, characterized by very steep slopes and narrow ridges and valleys. Slopes are often above a stable angle for the soils upon them. Frequent winter storms of varying intensities and long durations may produce locally saturated conditions which greatly increase the inherent instability of the soils. Under such conditions, man's activities can easily upset the delicate equilibrium between the resistance of the soil to failure and the gravitational forces tending to move the soil downs lop . Of the many activities included under forest operations, road construction has been identified as one of the most damaging to slope stability. This fact has been documented by many researchers. Road construction activities can disrupt the equilibrium of steep slopes in basically three ways: 1) slope undercutting, 2) slope loading, and 3) alteration of slope drainage. However, before more conclusions can be drawn as to the mode and degree roads may influence Oregon's coastal slopes, one must know what is occurring on undisturbed slopes. Such essential data is lacking for the Oregon Coast Range. 3 The soils comprising over 60% of the central Coast Range of Oregon are classified as cohesionless (Corliss, 1973). These soils generally exhibit very high infiltration and percolation rates. Visual inspection of almost any undisturbed slope in these mountains, during a period of heavy rainfall, will substantiate that overland flow rarely occurs. The lack of overland flow leaves two courses for the precipitation once it is at the ground surface: retention and dispersion as unsaturated soil moisture and saturated subsurface flow. Either course can, under certain conditions, decrease slope stability. In the case of saturated flow, pore water pressure and seepage forces are known slope destabilizers. Rothacher, Dyrness, and Fredriksen (1967) have suggested a shallow and rapid lateral movement of water through thesoils and subsoils on the slopes of the western Cascades as being one of the dominant modes of dispersal. Unsaturated soil moisture can, in sufficient amounts, contribute to slope failure by reducing or eliminating the capillary tension of a soil. However, as mentioned above, the actual measurement of these subsurface processes is lacking for the coastal mountains of Oregon. Soil investigations are usually made so as to obtain arid organize knowledge about observed soil properties. This knowledge is often needed so that the soil response to a proposed use may be predicted. Such investigationis, therefore, must include all important basic soil characteristics. However, only a limited number of these 4 characteristics may be pertinent for a particular use. Characteristics that are not pertinent to the interpretation may vary widely within the soil groups which, according to the grouping criteria, are relatively similar. One such grouping will rarely suffice for other than its intended use. For engineering and hydrologic considerations, soils are often grouped by the characteristics that affect their response to stress and hydrologic events. It is seldom prudent and often impossible to carry out research on each different soil within a complex watershed. Thus, the most practical groupings are those based on readily discernible or previously known properties that most nearly reflect the soil's ability to withstand stress and to absorb, store, and transmit water. These may include the various physical properties, such as porosity, texture, depth, hydraulic conductivity, as well as direct tests to determine strength, plasticity, or other properties. The objective of this study is to characterize two major cohesionless soil series from the central Coast Range of Oregon. Special emphasis will be directed to their hydrologic properties and those physical properties known to be important to slope stability. Some of the questions for which answers are sought are: 1. What hydrologic and physical properties are identical, or nearly so, between the two soil series; and what are the ranges of values to be expected in these properties? What reason(s) can be given for the stability of many apparently over-steepened slopes in the area? And, Can large-scale subsurface flow be expected as a normal occurrence in these soils? 6 LITERATURE REVIEW Slope Stability Strength and Failure The forces acting upon any sloping soil mass can be segregated into two broad categories: 1) those forces that tend to resist failure, arid 2) those that tend to contribute to failure. The failure of a soil mass is commonly called a slide or mass movement. Other more definitive terms exist for slides of a particular shape, size, material, or process (Varnes, 1958). Regardless of its name, a slide involves a downward and outward movement of the entire mass of soil encom- passed in the failure zone. On natural steep slopes, slides frequently occur in the absence of man's activities (Ellison and Coaldroke, 1954; Dyrness, 1967; Fredrjksen, 1970 ). In addition, both natural and man-induced causes are often present simultaneously, and the occurrence of the slope failure cannot be accurately placed on any one cause. Because of the extraordinary variety of factors and processes, known and unknown, that may lead to mass movements, the conditions that determirte the stability of slopes usually defy strict theoretical analysis (Terzaghi artd Peck, 1967). However, there are certain theoretical concepts which are common to all slope stability analyses and deserve review here. 7 The resistance of a soil mass to sliding is termed its shear strength. Shear strength parameters are used to analyze the stability of natural and embankment slopes, excavations, and foundations. Compared to other building materials, the strength of soils can be classed as low, extremely variable, and subject to change with time and natural and operating conditions (Holtz, 1969). When a stress is applied, the soil particles tend to resist movement past one another due to friction and/or other forces present. For a failure to occur in a material, it is intuitively obvious that the strength of the material must be equaled or exceeded by some applied stress. (There are other definitions of failure, but for the purpose at hand we will confine ourselves to a stress-related definiS tion.) In soil mechanics, the most successful failure theories to date define failure in terms of three principal stresses (Wu, 1970). The most widely known and used theory of this type is the Mohr-Coulomb failure theory. This theory postulates that failure in a material will occur if the shear stress on any plane equals the shear strength (5) of the material and that the shear strength is a function of the normal stress (o-) on that plane (Mohr, 1871). Simply, in equation form, S = f(,-). (1) Coulomb's (1776) contribution to the general theory concerns defining f as a linear function of the normal stress. Thus, equation (1) becomes 8 S=C+ytan, (2) where C is a constant of cohesion, a- is the normal stress, arid is the angle between the normal stress and the shear stress; is also called the angle of internal friction. This equation is generally referred to c as Coulomb's equation and was a first attempt at an empirical analysis of the sliding of earth masses (Heyman, 1972). Coulomb's equation is shown graphically in Figure 1. The failure envelope represents the shear- normal stress combinations necessary for failure. In other words, any combination of shear and normal stresses that plot as a point above the failure envelope line cannot exist in this material because failure will have taken place before such stresses can be reached. The ir1tercept on the shear stress axis (T) is equal to C with the slope for the failure envelope equal to tan 4. -r Failure Envelope C a- Figure 1. Failure envelope for a soil with cohesion and friction properties. 9 which The quantity C is related to that portion of shear strength does not depend upon intergranular friction. It is therefore a function of water in clay, surface of the shearing strength of adsorbed layers forces, and cementing materials that bind individual soil particles cohesion, for true cohesion together (Wu, 1970). Not all soils possess Apparent cois usually a property of soils containing silt and clay. and is hesion, on the other hand, may be found in almost all soils which may be usually due to particular soil-moisture relationships One transitory arid disappear altogether under certain conditions. such condition leading to apparent cohesion is capillary tension. which the drivThe angle of internal friction ( ) is the angle at resisting ing f9rces in the soil mass are equal and opposite to the It is a measure of the frictional component of soil strength forces. been due to the interlocking of the soil grains. The juterlocking has (Wu, shown to be mainly a function of the relative density of the soil and 1957), particle shape, surface roughness, arid gradation (Sowers zero under Sowers, 1970). For some clay soils, may decrease to in certain conditions leaving C as the only soil strength component exclusively on this Coulomb's equation. Soils which rely almost cohesion intergraniular friction for their strength arid exhibit little or no are termed cohesionless soils. The effect of water on is often misunderstood. Because slope failures occur most commonly during periods of heavy rain-fall, a 10 decrease in shear strength is frequently ascribed to the lubricating action of water. For almost all soils, arid especially cohesionless soils, this explanation is unacceptable for several reasons. First of all, Terzaghi (1960) has shown that water in contact with most common minerals acts as an antilubricant and not as a lubricant. Secondly as Terzaghi further pointed out, soils of humid regions contain far more water than is needed for the lubrication of particles; yet almost all their mass movements also occur during rainstorms. A third reason is that very little water is needed to produce full lubrication (Hardy and Hardy, 1919). It would require only a few molecule thick layer to produce lubrication. Yet for cohesionless soils, the intense stresses at the intergranular contact points force water molecules aside so that the moisture does not appreciably effect the value of in a soil (Sowers and Sowers, 1970). Terzaghi and Peck (1967), in commenting on cohesionless soils, state that, "Since most of the shear ing strength is caused by interlocking of grains, the values of tb are not appreciably different whether the soil is wet or dry. This discussion illustrates that lubrication is not an acceptable causative factor in most mass movements and does not greatly reduce 4. It would be erroneous to conclude, however, that water does not play an important role n soil strength. The converse is true as will be discussed later. 11 The failure envelope is usually used in conjunction with another graphical procedure called a Mohr circle. This graphical aid is used for rapidly solving the equations for shear and normal stresses on any plane (Sowers and Sowers, 1970). Two Mohr circles with the major and minor principal stresses, 0-1. and a- are shown in Figure 2. The values for the two principal stresses are such that each circle is tangent to the failure envelope points at D and E. Therefore, on this particular plane, the shear stress (T) is equal to the shear strength (S) and failure occurs. Because the stress in a material can never exceed its strength, it is apparent that no part of the circle may extend above the failure envelope. Therefore, all combinations of stresses and their corresponding Mohr circles at failure are tangent to the failure envelope. One should note that according to the Mohr-Coulomb theory, the shear strength is not dependent on the third (intermediate) principal stress (a- and that all failures are, by definition, shear failures (Wu, 1970). The values of al, and a- are usually determined by triaxial shear tests on soil samples. The determination of meaningful shear strength parameters and their proper use constitutes the most difficult area in the practice of engineering (Holtz, 1969). Triaxial shear tests will be discussed in greater detail later. 12 T 7/7 C °3A °3B °1A ci B Figure 2. Failure envelope and Mohr circles. The Principle of Effective Stress Although Coulomb developed the fundamental equation by which soil strength is calculated today, he overlooked the important influence of porewater pressure. Previous to Terzaghi's effective stress concept, developed about 1923, (Terzaghi, 1936) the role of water in soil strength was poorly understood. Solutions to slope stability and other soil mechanics problems were largely empirical and often erroneous. Terzaghi recognized the three-phased nature of the soil medium, and the result was the birth of modern soil mechanics. Coulomb's equation, Equation (2), used the total normal stress in its original form. Terzaghi realized that in a saturated soil, 13 the soil voids were water-filled and must be subjected to a portion of the total normal stress applied. Concurrently, the soil particles would be carrying the remainder of the total normal stress transmitted through the points of intergranular contact. He termed the grain-to-grain stress as the effective stress (a-) and the water borne portion as the neutral stress (u). The term neutral stress is very descriptive because the water is incapable of supporting shear. There- fore, from Terzaghi's (1936) work, it is apparent that a- = a- - u. (3) This is frequently thought to be the most important equation of soil mechanics. Applying Terzaghi's effective stress principle to Coulomb's failure criterion, SC+a-tan4, or SC+(o--u)tan4i, where C and (4a) (4b) are the apparent values of C and for the soil in question. Although for a given soil, C and are known to vary due to many factors (rate of loading, drainage conditions, dilatancy, etc.), for most practical problems C and may be considered as unique parameters for that soil. The principle of effective stress need not be reserved solely for saturated soils. Obviously for a dry soil, u is equal to zero and = a- In partially saturated soils, the influence of the neutral stress 14 on shear strength is very complicated. For slope stability situations, the effect is usually favorable. The tension in porewater, produced by the minisci between particles, increases with decreasing water content, while the total normal stress on a given section through the soil remains basically unchanged. Because o = o -u, the pore-water causes an effective negative pressure. In other words, capillary pressure is equal to -u. Coulomb's equation under this condition becomes, S = C+(o- -(-u))tan S , or C-I-(o- -I-u)tan $. (5 a) (5 b) The increase in strength (as) due to this capillary pressure is, = c tan (6) assuming both C and 4 remain constant (Terzaghi and Peck, 1967). Analysis of Slope Stability The general procedure applied in analyzing the stability of a slope is to compute the shearing resistance available along a critical sliding surface and to compare this with the driving forces present along the surface (Sowers and Sowers, 1970). The critical surface may be planar or circular depending upon the type of soil, depth to bedrock, layering sequences, arid other factors. If the comparison, called a factor of safety (F) analysis, results in a value less than or equal to unity, the slope will or is about to fail. In general equation form, 15 F resisting forces driving forces (7) analysis, but There are usually several factors of safety in any slope the one of for this discussion the factor of safety against sliding is interest. The major driving force present in any slope is the combined shear resisweight of the soil mass and water. The fully mobilized tance'along the critical failure surface is the major resisting force. respect to Where the depth of the soil on a steep slope is shallow with several the length of slope and the bedrock or other firm layer is itfinite times more shear resistant than the soil material, use of the form of slope analysis is warranted. Rather than failing on some along circular surface, infinite slopes usually fail by sliding linearly than 50% in the Oregon the firm surface. For most slopes steeper A minimum Coast Range, the soils are usually less than six feet deep. soil slope length of 600 feet is not uncommon. With a slope length to 100:1, an infinite slope model for factor of safety depth ratio of over Therefore, computations appears logical for the majority of slopes. this section will concentrate on reviewing the development of factor of safety equations for infinite slope conditions. The derivation of a general equation for the factor of safety for such slopes can be done with the aid of Figure 3. This development modificais essentially that shown by Taylor (1948) with some slight tions in terminology. 16 dy Iticlitied Area = dx cos p \N a Figure 3. Idealized sectioti for ati in.fitiite slope with a piezometric surface presetit. For this discussion, assume that the soil has cohesion atid that a piezometric surface is present within the soil profile. The slope atigle (1) is equal to the slope of the bedrock, or firm layer, atid the soil depth is Z. A homogeneous soil will, also be assumed. The coefficient m represents the position of the water table iti terms of Z atid may vary from zero (no water present) to unity where the piezometric surface is coiticident with the ground surface. The width of the idealized section will be the increment dx with the dimetision itito the 17 hillside being equal to dy. Further, assume that the side forces of the section are colinear and equal in magnitude so that the net forces present in the section are the weight (W) of the section arid the other botton-i forces as shown. At this point, an introduction of the term S is necessary. S is defined as the ratio of the total shear strength available to the factor of safety. In ather words, . S(Area) F However, fron-i the prior discussion, it was shown in. (4a) that S = a- tan by Coulomb's equation. Therefore, by substituting (4a) in (8), SIC+a-tanlfC+tafl1dd LF F jcosP If a mass unit weight (y) and boundary neutral forces type of analysis is used, we can define, W = Z y dxdy T = Z ydxdy sin, and N=Zydxdy cos. At static equilibrium, the total downslope driving forces (T) will be equal to the necessary resisting force (S ). Setting T = S yields, Zdxdy ysin F tandxdy J cos3 To incorporate the Influence of the water table, an expression for the porewater pressure must be obtained. This can be accomplished 18 through the effective stress term (a- ) in (10). To obtain a dimensionally correct end product, it is necessary to derive both a- and u iri terms of forces rather than stresses. Recalling that a-N = a-N - u, for the normal plane, it can be deduced that the normal force (N) would be equal to (a-N)(Area) or + u) N (dxdY Further manipulacos ). tiori would yield Z y cos2 a- By direct substitution, I LF + (Z y cos2 y Z dxdy sinI3 (11) - u. - u)tan dxdy JcosI3 (12) Solving for F arid rearranging results in, F[ c cosl3 + (Z y cos - Z y sin 3 u cosJ' tan (13) Where a piezometric surface exists, it is apparent that the total porewater pressure (u) on the inclined surface is equal to the force, m Z cos2 3, where y is the unit weight of water. Substituting this expression for u into (13) arid rearranging terms gives the general factor of safety equation for a cohesive soil with a water table at some level mZ in the profile; 1 Zy cos 3 sinl3/ +1Ymta J tan I I, y (14) It is readily evtderit from equation (14) that the factor of safety is composed of a cohesive strength term, the first term, plus a frictional term, the second. However, the frictional term often may be 19 assumed to be zero for clay soils under certain conditions. The reader should also note that the stabiLity of a slope with a water table excLuis not dependent upon the velocity of flow, if any, but depends sively on the pressure in the porewater (Haefeli, 1948 arid many others). Using equation (14), but assuming rio free water surface exists in the profile of the cohesive soil, the porewater pressure and mZ S will both be zero and, F [()(cossia (15) J. Cohesionless soils can also be analyzed using variants of equacondition (14). For saturated non-cohesive soils in an infinite sLope tion, equation (14) reduces to, F ,y-mywjftan& / Jtaa y Berniatzik (1948) formulated a similar equation for this situation but accomplished his solution through testing arid slope circle analysis. Concurrently, for dry cohesionless soils, equation (14) further reduces to, tan F - tan 3 Equation (17) illustrates the well known fact that in the absence of other factors, a dry, cohesioniless soil can not exist on a slope of an angle greater than its angLe of internaL friction (4). This maximum 20 angle, where = 3, defines the critical slope of such soils and is often called "The angle of respose." Technically, the term TT angle of resposeU describes the maximum angle at which loose, clean, dry sand will stand unsupported arid is riot synionomous with the T!critical slope", although it is often misused in that context. Measurement of Strength Pa rameters The basic objective of strength measurement is to determine the failure envelope. As previously discussed, the failure envelope is the relationship between T and a- at failure. Because of the highly complex nature of the shearing resistance of soils, many methods of testing have been tried with varying degrees of success. The two most widely used laboratory methods for the measurement of soil shear strength parameters are the direct shear test and the triaxial test. The ultimate choice of apparatus is primarily determined by the conditions of drainage under which it is desired to carry out the test. However, most of the major authorities on strength testing of soils state that the triaxial test gives the most consistent and reliable resuits while allowing the most versatility in testing procedure (Skempton and Bishop, 1950; Bishop and Henkle, 1962; and Sowers, 1963). For these reasons, the triaxial test apparatus was used to measure soil strength for this study. The values obtained from the triaxial test can only be interpreted and appreciated from an adequate 21 understanding of the test apparatus, procedure, and theory. A brief review for this purpose is included at this point. A more complete background on triaxial testing may be obtained from the singularly authoritative text of Bishop and Henkle (1962). In an ideal test, the triaxial test should permit independent control of the three principal stresses, a- 1' 0-2 anda- (Figure 4). Figure 4. Stress system for continuous medium. However, as Bishop and Henkle (1962) point out, the relatively high compressibility of the soil skeleton and the magnitude of the shear strains required to induce failure leads to mechanical difficulties which make independent control too complicated for other than special research tests. To reduce the number of principal stresses involved 22 to two, the cylindrical compression test is most commonly used. In a cylindrical soil sample subject to triaxial test conditions, the stresses are as shown in Figure 5. 1 Figure 5. Stresses in triaxial shear test. In this type of test, the cylindrical sample is enclosed within a thin watertight membrane which is attached to porous end plates, and placed in a pressure chamber (Figure 6). The porous end plates are provided for saturating or draining the specimen. Pore pressure measurements can be made through these end plates or through porous inserts. The specimen is surrounded by a liquid, and an ambient pressure is applied through the liquid. Incremental axial loads are applied to the specimen until failure occurs. 23 CONSTANT PRESSURE SUPPLY PISTON METAL BEARING PLATE I OF 3 TIE RODS SAMPLE r' LUCITE ' .4-CYLINDER (1 1 1 RUBBER MEMBRANE 6 0 (-. I- t' I- PERVOUS STONE fJ r) DRAINAGE PORT Figure 6 -" Schematic diagram of a triaxial compression device. 24 The applied minor principal stress (cr 3) is considered to be that produced by the chamber pressure, and the applied major principal stress (cr1) is that produced by the axial load and the chamber pressure. The difference between 01 and 03 is often called the deviator stress. Usually the Mohr circles of failure stresses for a series of such tests, using different 03 values, are plotted and the failure envelope drawn tangent to them (Figure 2). For saturated cohesionless soils, Lambe (1951) describes a procedure by which c may be determined from only one set ofcr and cr3. Where dry cohesionless soils 1 are to be tested, a vacuum provides the lateral pressure (cr 3). Sample requirements for specimens range from undisturbed samples, as for clays, to reconstituted samples, as is often the case for cohesjonless soils. Where cohesionless soils are to be tested by reconstructing a test specimen inside the rubber membrane, the void ratio and density achieved should be within the range that exists in the field. Chen (1948) found that 4 was almost constant for a given soil if the same procedures and void ratio were used with different lateral pressures. However, it should be realized that 4 is, in actuality, a function of 0.3 and not independent of it. Triaxial shear tests can be performed with a variety of procedures. Depending upon the data desired and procedural differences, the tests will produce entirely different values for C and 4. Therefore, it is extremely importart that the procedures used be 25 carefully programmed to represent all past, present, and anticipated conditions. Test results which come from a test procedure not representative of field stress conditions are worse than no results. These values will be erroneous and probably highly dangerous if used to represent real field conditions. Currently, the classification of triaxial compression test is based upon the conditions of drainage obtained during shear. The variations available may be classified as follows (Skempton and Bishop, 1950): 1. No Drainage During Shear Undrained Test: This type of test is also known as a quick test. The samples are placed in the testing apparatus in any given state (undisturbed, compacted to a specific density, etc.) subjected to the applied lateral pressure, and then sheared with no drainage of porewater allowed. Be- cause no drainage is allowed, no dissipation of porewater pressure is possible during the application of a + or 3). Consolidated-Undrained Tests: This type of test is also termed a consolidated-quick test. Drainage is allowed during the application of until the sample is fully consolidated under this pressure. No drainage is allowed during the shearing phase. 26 2. Full Drainage During Shear (a) Drained Tests: In this test variant, drainage is permitted throughout the test. Full consolidation occurs due to and no excess pore pressure is built up during shear. The criterion of no excess pore pressure requires a very low strain rate. Thus, this type of test is often called a slow test. This type of test produces values of effective stress. The analysis of natural slopes for stability is often performed on an effective stress basis. This is quite logical because natural slopes represent the ultimate long term state of equilibrium for a profile formed by geological processes. The pore pressures are controlled by the prevailing ground water conditions which correspond to relatively steady seepage (Bishop and Bjerrum, 1969). Irt other words, the pore pressure is an irtdependent variable for natural slopes. An artalysis based on effective stress parameters of the soil would be expected to agree closely with observed slopes in limiting equilibrium. This preference usually also implies that strength parameters will be determirted by a drained test procedure. Up to this point, some of the advarttages available through the use of the triaxial test apparatus have beert discussed. To provide a balartced review, some of the limitations should be mentioned. First, triaxial tests require elaborate and complex equipmeiat. Secondly, the shear displacements in a slope occur under plane strain conditions while the triaxial test produces radially symmetric strain conditions. Bishop (1961) and others have shown that the angle of friction under plane strain conditions may be several degrees higher than that developed in triaxial strain conditions. This would lead to the value determined by a triaxial test being too low for predicting conditions of instability of the slope. Thirdly, at very low confining pressures, 200 psf or less, the envelope of failure for cohesionless soils often does not pass through the origin (Chen, 1948). This would correspond to a slope with shallow soil depth above the failure surface. The small 20 to 100 psf intercept at the origin is usually discounted as a test error. Consequently, the necessity of extrapolating the envelope back to the origin from a minimum confining force to 200 psf (1. 39 psi) or greater often precludes determining the true nature of the failure relationship at confining pressures of less than 200 psf. This would be quite important for steep slopes with soils 1-2 feet deep where small changes in shear strength may produce significant changes in the stability analyses. Other disadvantages and problems exist, but in the total, advantages of the triaxial test still make it the preferred method ror most soil strength analyses. 28 Soil Structure and Shear Strength of Cohesioriless Soils The importance of soil structure to crop productivity, infiltration, and surface erosion has long been recognized. However, relatively little literature is available concerning aggregate stability and its role in the stability of steep, natural slopes. Because much of future sections will concern aggregation, a review of the pertinent literature on this subject is in order. An aggregate is a group of two or more primary particles which cohere to each other more strongly than to surrounding particles (Kemper and Chepil, 1965). The size, distribution, and stability of aggregates are determined primarily by the stability forces binding the particles together and the strength of disrupting forces present. The stability forces causing and maintaining aggregation may be many, and more than one is usually present in any situation. The development of stable aggregates is a complex and not completely understood process which involves the binding together of soil particles into structural units which are not readily dispersed in water. The most accepted theory of aggregation formation hypothe- sizes that aggregates develop as a result of the alternate wetting and drying of soils. The alternate wetting and drying cycles produce unequal stresses arid strains which are set up by shrinkage and swelling. These strains and stresses along with the disruptive action of air 29 entrapped in the pores on wetting create groups of particles called aggregates (Lutz and Chandler, 1961). However, it should be noted that aggregates formed purely as a result of alternate wetting arid drying are usually riot very stable. Stability is normally imparted by a binding agent or combination of binding agents. One of the first agents to be recogmzed is organic matter. Bayer (1935) found a good correlation between organic matter content arid aggregates in 75 soils. A greater effect was noted in those soils containing less clay. Mixing fresh organic matter into soil has been shown to be important in encouraging microbial activity. Resulting polysaccharides (Macalla, 1942), microbial gums (Chesters, Attoe, and Allen, 1957), arid filamentous soil fungi (Martin, 1945) all contribute to the formation of stable aggregates. Kemper and Koch (1966), iri studies on 519 soils from the western portions of the United States arid Canada, also found that aggregate stability increased with organic matter content, but that above a content of 2%, aggregate stability inLcreased comparatively little. Organic matter content below 1% was highly correlated with large reductions in aggregate stability. Clay cortent has also beer long recognized by soil scientists as ar important factor in aggregate stability. The formatior arid stability of soil aggregates is deperdent largely upor the quantity arid type of clay (Hillel, 1971). Bayer (1935), Chester etal. (1957) and Kemper and Koch l966) quantified the close relationship between clay content 30 and aggregate stability. Emerson (1959) produced a model of soil crumbs based upon the various ways in which assemblage of clay particles associate with sand and silt to form aggregates. He found not only internal cementation by the clay, but also clay skins around the aggregates. Both factors aided aggregate stability. For a given set of circumstances, the more active clays, i. e. montmorillonite, bentonite, appeared to cause greater aggregation per unit volume of clay than the less active clays (Mazurak, 1950). Free iron and aluminum oxides (Fe203 and Al203) have also been found to be responsible for aggregate stability over a wide geo- graphic range of soils and types (Lutz, 1936; McIntyre, 1956; Deshpande, Greenland, and Quirk, 1968). Weldon and Hide (1942), in extracting both iron and aluminum oxides, concluded that most of the cementing effect was due to the iron oxides. However, Deshpande et al. (1964) concluded that aluminum oxides were more important to aggregate stability. Same, MacLean, and Doyle (1966) also published data tending to confirm the superiority of aluminum oxides over iron oxides, but concluded that more than one cementing agent was usually present in any aggregate. Soils with large amounts of free iron oxides have been found to be almost 100% stable in water. Soils of the western United States have relatively low concentrations of free iron oxide, yet the one to three percent available in western soils 31 appears sufficient to make it an important factor in aggregate stability (Kemper and Koch, 1966). Kemper and Koch found, however, that higher concentrations of free aluminum oxides were not always associated with significantly higher aggregate stability. Soils of the western portions of the United States and Canada, with over four percent exchangeable sodium, showed negative correlations between aggregate stability and concentrations of free aluminum oxide. These negative correlations, they supposed, suggested that there is a complex relationship between exchangeable sodium (a known deflocculator) and high levels of free aluminum oxide. Other known cementing agents imparting varying degrees of aggregate stability are calcium carbonate (Kroth and Page, 1947), soluble silicates (Laws and Page, 1947), and organic resins (Greenland, Lindstrom, and Quirk, 1962). Opposing these formative and cementing agents are the forces or agents tending to cause aggregate breakdown. One of the prime natural disrupters of aggregates is water. Water entry into soil aggregates is not the only disruptive force present in nature. Mechan- ical disturbance by roots and soil fauna, frost action, and rain drop impact are other recognized aggregate destroyers. Yet at the depths near the soil-rock interface and in absolute magnitude of effect and occurrence, the forces involved in the entry of water are probably 32 the most important of the disruptive forces. A major factor affecting the stability of wetted aggregates is the mode by which they are wetted. Direct immersion of a dry soil in water at atmospheric pressure causes the greatest disruption of aggregates (Kemper and Chepil, 1965). Water between closely adjacent mineral surfaces usually has less free energy than water in bulk. Cousequently, water teuds to move hi betweeu the mineral surfaces and forces the particles apart. Some or all of the bonds between particles are broken. Coucurreutly, one side of the aggregates may become wet and swollen while the other side is dry. This differeutial swelling often causes fractures just behind the wetting frout, and the aggregate is weakened (Kemper, 1965). Air is entrapped within the aggregate by such rapid wetting at atmospheric pressure, and the air is compressed by the advancing water film. This air bursts out of the aggregate when the aggregate structure is sufficiently weakened by hydration (Emerson and Grundy, 1954). These minature explosions disintegrate the aggregates. The unaggregated surface crusts of recently flooded lands are a result of this phenomenou (Kemper, 1965). The other mode of water entry is from one side of the aggregate under tension. This is the uormal mode of wetting for subsurface soils (Kemper aud Chepil, 1965). The slower wetting under tension results iu very little entrapmeut of air and hence less disruption of the aggregates in the maiu body of soil (Kemper, 1965). 33 As previously discussed, the shear strength of cohesionless soils is predominantly a function of the normal effective stress ( a-) and the angle of internal friction (4)). Because 4) represents the intergranular friction, any factor which affects the surface roughness and angularity of the primary particles in the soil framework affects the shear strength of that soil. Silt and clay particles, the more advanced products of weathering, tend to be smoother and generally more flake- shaped than sand particles. As individual particles, they have relatively little frictional resistance to shear and tend to rely more on surface forces for their strength (Terzaghi and Peck, 1967). However, if these small particles are bound to each other and to larger sand particles in a random fashion, the resulting aggregate will tend to have a greater surface roughness and angularity than the individual clay or silt particles (Paeth, 1970). Assuming a given set of conditions and assuming the stresses applied are not sufficient to crush the aggregates, the aggregates should exhibit greater shear strength through a higher angle. Soils with higher percentages of stable aggregates would, of course, benefit more. These aggregates would function as individual particles in the soil framework and tend to increase the solid-to-solid contact, forming a more continuous framework resistant to stress. Paeth (1970) concluded that aggregate stability was an important factor in explaining the greater resistance 34 to failure of two soils of the western Cascade mountains compared to two other slide-prone soils in the same area. Geology The geologic origin and history of an area has a strong influence topography, and runoff patterns of that area. Because on the s Oils, all three of these factors are also interrelated with the natural stability of slopes, the relationship between the geologic features of an area arid slope stability is an important one. For the Oregon Coast Range this is no less true. This review describes the geology of the region in order to aid the reader in understanding future discussion of the soil arid subsurface hydrology. General Geological Setting The Coast Range of Oregon extends from the Columbia River on the north to the Klamath Mountains on the south. The western' boundary Valley forms the is the Pacific Ocean' and to the east, the Willamette limit (Figure 14). Marys Peak is the highest mountain in the Coast Range of Oregon with an elevation of 4097 ft. As the other prominent peaks in the central portion of this range, it is capped by a layer of igneous rock. Granophyric gabbro, diorite, nephyline syenite, camptoniite, and basalt are the mcst common igneous rocks present. The Coast Range' s most 35 prominent intrusive bodies cap Saddleback Mountain, Laurel Mountain, Fanno Ridge, Table Mountain, Marys Peak, and Roman Nose Mountain. Almost aU of these peaks and ridges are remnants of thick sills or dikes that were intruded beneath relatively thick layers of sedimentary rock. As a result of this deep intrusion, the igneous magma cooled slowly to form mostly medium-g rained rocks (Baldwin, 1964). In the specific case of Marys Peak, the igneous rock present is predominantly granophyric gabbro with lesser amounts of pegmatic granophyric diorite, granophyric diorite, and aplitic rocks (Roberts, 1953). Some interstitial quartz is also present. The time of intrusion has not been definitely determined (Baldwin, 1964). Robert's (1953) study comprises the most detailed work to date on the Marys Peak intrusive body. Underlying the intrusive cap of Marys Peak is the Tyee formation, the most widespread geologic formation in the central Coast Range of Oregon (Figure 7). This formation is a bluish-gray to gray, rhythmically bedded, micaceous, and arkosic sandstone and sandy siltstone (Baldwin, 1964). The sandstone has appreciable mica and is generally firmly to very firmly compacted. It is well graded, and layering is sharply defined (Figure 8). The formation is over 7000 feet thick and has been lichologically assigned to the middle Eocene. The Tyee formation has been described in considerable detail by Snavely, Wagner, and Mac Leod (1964) and Lovell (1969). II . U) 0 C-) ITYEE S Baldwin, 1964). Figure 7. Major geologic formations of the central portion of the Oregon Coast Range (after ::.19GNEous . FORMATIONS I I 37 Topography and Geologic Features The varied landscapes of the central Coast Range of Oregon ref lect the differences in the underlying bedrock and other associated geologic features. Generally, the Tyee formation slopes to the west. However, local faulting may cause dips in any direction. The sandstone areas generally possess dendritic, high density drainage patterns. Valleys are quite narrow and have steep sides. Relief may vary from 1500 feet down to 500 feet (Corliss, 1973). There are usually several drainageways and ridges per square mile in areas of sandstone. Two distinct features characterize the sandstone areas. They are the cuesta face arid the backs Lope (Figure 9). Because the open arid exposed layers of sandstone are more easily eroded by running water, streams quickly undercut the cuesta face. The backslope is more resistant to such erosion, however, and streams cut more slowly through it. Consequently, the cuesta faces have steeper slopes, thinner soils, greater drainage densities, and more stability problems (Corliss, 1973). Areas of igneous bedrock exhibit two topographic patterns. First, they may exhibit landforms due mainly to the degree of fracturing and faulting previously experienced. A smooth slope will generally overlie a relatively unfractured bedrock with few or rio faults. 39 Conversely, a very fractured and faulted bedrock will exhibit an undulating topography often cut by several drainageways. In general, however, the number of drainageways is lower than in an area of sandstone (Corliss, 1973). The slopes are also usually longer and smoother. Zones of contact between sedimentary and igneous rocks frequently produce areas of special interest. Slopes are often unstable and visibly broken due to several factors. Differential weathering, metamorphism, fracturing, and water accumulation are but a few reasons for taking special interest in these areas (Burroughs, Chalfant, and Townsend, 1973). Soil Morphology The soils of any area are formed by various factors acting on the parent material. Some of these factors are climate, topography, biological organisms, and chemistry. None of these factors operates alone on the parent material, but all act as a total mechanism to produce soil from bedrock. The intensity of any one factor may, of course, vary from place to place. Most of the soils on the steep slopes of the central Coast Range area are formed in alluvial and coliuvial materials that have been transported. Because the rocks of the area have low silica content and a high content of easily weathered minerals, plant growth is 40 usually abundant (Rojanasoonthon, 1963). However, soil depths are not great nor are soil profiles exceptionally well developed because of soil movement on steep slopes and the relative geologic youth (less than 10,000 years old) of these soils (Corliss, 1973). The Alsea Area Soil Survey (Corliss, 1973) provides the best summary of the soils in the central Coast Range. The author estimates that 55% of the surveyed area is overlain by two cohesionless soil series, the Bohannon and Klickitat soils. The Bohannon soil series has Tyee sandstone as its parent material while the Klickitat series is derived from igneous material. Aside from the differences in parent material, the Klickitat and Bohannon soil series are quite similar. Both belong to the Inceptisol order of the National Cooperative Soil Survey (U. S. Soil Conservation Service, 1960). This order is characterized by relatively poorly defined horizon development. Inceptisols are believed to have been formed in a short period of time with little significant eluviation or illuviation. On a subgroup level, both soil series are classified as Typic Haplumbrepts, soils that are well-drained with depths greater than 20 inches. Base saturations are generally low because of leaching from high annual precipitation (Corliss, 1973). The two soil series also exhibit similar topographic positions on slopes. Both exist on the steepest portions of slopes on sandstone or igneous bedrock (Figures 10 and 11). 1964). Figure 10. Soils and laridforms of The Bohannori-Slickrock association (after Corliss, Figure 11. Soils arid landforms of the K1ickitatSh0utP0 association (after Corliss, 1964). Energy State of Soil Water Total Energy Coacept While the geology and soils, as just discussed, play great roles in influencing the stability of slopes, subsurface water must a].so be recognized as an essential component. Subsurface water is often listed as a prime cause of slope failures. However, a knowledge of the state and movemeat of such water is often more importaat than the mere knowledge of its presence ia a slope. Both the state and movement of soil water can be understood in terms of energy. The movement of soil water is directly determined by the energy state of the soil water. Because the flow through a porous media is usually quite slow, the kinetic energy component is generally very small. For this reason, the kinetic energy contribution to the tota]. energy of soil water is usually considered negligible and ignored (Sowers and Sowers, 1970). On the other hand, potential energy, which is due to position or iateraal condition, is of primary importance in determining the state and movement of soil water (Hillel, 1971). The potential energy of soil water, often called the total potential, usually varies from place to place as well as over a wide range of values. The differeaces ia magaitude and locatioa set up gradients of interaal energy which give rise to water flow ia the soil. Because soil water obeys the uaiversal tendency of all matter to seek a lower 44 energy state and to equilibrate with its surroundings, the gradient of potential energy is negative. Clearly as Hillel (1971) points out, it is not the absolute amount of potential energy, but the relative level of energy between soil regions that is the important factor in understanding soil water movement. The total potential of soil water is composed of many separate components. It may be mathematically expressed as Pt =Pg +Pp+ (18) where Pt is the total potential, Pg is the gravitational potential, and P is the pressure potential. The dots on the right side of (18) signify that additional terms, such as an osmotic potential, are possible. However, for most cases of soil water movement, only gravity and pressure potential are considered (Hillel, 1971). The gravitational potential of soil water at any point is a function of the elevation of that point above some arbitrary reference level. For convenience, the datum is often chosen at an elevation which allcws Pg to be positive or zero. The magnitude of the gravitational potential at any point is dependent only on the height above the reference datum (Z), the density of water w' (g), and the volume the water occupies (V). Pg =pg Z. the acceleration of gravity For a unit volume, (19) The pressure potential of any point can be positive or negative. 45 When the soil water is at a pressure greater than atmospheric, P is positive. If P is less than atmospheric, it is negative. Negative soil water pressures are also called "suction," tttension, U arid "capi1 lary pressure. ' For a positive P, which occurs below a free water surface, Pp =p w gh (20) where h is the submergence depth below the free water surface. In an unsaturated soil, where there is both air arid water, a surface tension exists at the interface between water and air. The presence of surface tension along the miniscus lowers the pressure in the water immediately below the miniscus. Hence, P is negative in relation to the atmospheric pressure. It has been shown through the phenomenon of capillarity that -2T cos P= p R (21) where T is the surface tension of water, a is the contact angle of the miniscus and R is the radius of curvature at a point on the miniscus. Darcys Law As mentioned previously, the existence of a gradient in potential energy between poicits within a soil mass causes soil water to move. In 1856, Darcy presented an equation which quantified this relationship (Hubbert, 1956). In studying seepage rates through saturated sand filters, Darcy fouud that q= (22) -K(th/L) where q is the specific discharge rate or flux, K is the proportiotlalitY constant called hydraulic conductivity. The ratio th/L is usually called the hydraulic gradient, i, and is the head loss per uuitdistarLCeitl the direction of flow. Darcy's Law is an empirical equation describwhere ing the gross aspect of flow in porous media. For the case deflow is unsteady or the soil nonuniform, Slichter (Hillel, 1971) veloped a more generalized, three_dimensional differential form of Darcy' s equation. Slichter' s version is q=-Kvh (23) where v h is the potential gradient in three dimensions. Therefore, from the two forms of Darcy's Law, it can be concluded that the velocity of flow and the quantity of discharge through the porous media are directly proportional to the hydraulic gradient. Conditions For this condition to be true, flow must remain laminar. for laminar flow in soils normally require a Reynolds number less five than unity (Hillel, 1971) or a hydraulic gradient of less than (Sowers and Sowers, 1970). 47 Hydraulic C onductivity The saturated hydraulic conductivity is essentially a constant for a soil. It possesses the dimensions of a velocity and expresses the ease with which water moves through saturated soil. In unsaturated soil, the hydraulic conductivity varies as a function of the soil's saturation or wetness. Any particular soil will show a drop in hydraulic conductivity as the soil becomes drier. This phenomenon occurs due to the decrease in transmission area as pores empty with increasing tension. Anyone who has worked with soils will soon find out the truth in the fact that no other soil property is as variable as the hydraulic conductivity. The range in hydraulic conductivity for the total spec- trum of soils, from gravels to clays, may exceed 10 orders of magnitude (Cedegren, 1968). The range is so great that its physical significance is difficult to comprehend. The hydraulic conductivity is affected by five major factors (Lambe, 1951): size of the soil grains, void ratio(e), or porosity(n) of the soil, shape and arrangement of the pores, properties of the pore fluid, and the degree of saturation. and are often' Factors I through 3 are functions of the soil matrix (k) compotietit of grouped under the intrinsic or specific permeability related solely to the fluid and hydraulic conductivity (K). Factor 4 is Combined, they relate is often termed the fluidity (f) component of K. the porous the fact that the hydraulic conductivity (K) is a product of can be expressed media arid the fluid. Mathematically the relationship as K = kf. fluid. (24) arid The degree of saturation is a function of both the matrix saturated The value of K for any soil is highest when the soil is and virtually all the pore space is contributing to flow (Wu, 1970). hydraulic conAs the degree of saturation decreases from 100%, the sharply reductivity of a soil decreases abruptly. The flow area is duced arid the water must flow through those pores still containing of the square of its water. Because the area of a pore is a function inverse radius arid large pores desaturate at low tensions due to the large pores lose flow area nature of the capillarity equation, soils with rapidly under only slight tension's. Conversely, a soil with many small pores will have a higher conductivitY at the same tension because the smaller pores will remain filled arid transmit water sizes nearly (Hillel, 1971). Soils exhibiting a wide range of pore both high evenly distributed over the total range, often car' exhibit Well saturated arid high unsaturated hydraulic coniductivities. 49 aggregated sandy-silt and sandy-clay soils often have this ability (Corey, 1969). The laboratory measurement of hydraulic conductivity of soils at different values of capillary tension is a routine, but tedious prosimplified cedure. Laliberte, Brooks, and Corey (1968) described a procedure for calculating the hydraulic conductivity of saturated and partially saturated soils that overcomes most of the tediousness of prior methods. Their method utilized parameters that can be obtained from capillary tension_desaturation data to calculate hydraulic conductivity at any tension K(P). Their procedure is currently applicable only to the drainage cycle and at moisture contents greater than field capacity. Briefly, Laliberte etal. (1968) tested the validity of the assumption that the hydraulic conductivity of a soil at any tension could be described by three parameters x, i, and Eb. Previous work by Brooks and Corey (1964) had defined >.. and 1 as pore-size distribution as the bubbling pressure. The bubbling pressure was found by Brooks and Corey (1964) to be closely related to the minimum indices and b capillary pressure on the drainage cycle at which gas permeability was probably related to the hyexists. They hypothesized that b draulic radius of the smallest pore in that pore sequence which, as capillary pressure increased, was the first to desatu rate. 50 The parameter X is a constant which is dependent on the nature of a particular porous medium. Essentially this index describes the absolute slope of a logarithmic relationship between the effective saturation (S) and the capillary pressure as shown in Figure 12. A soil of uniform pore sizes would have a high X value while a soil having a wide range of pore sizes would possess a low X value. Theoretically, if all subsections of the pore space had the same hydraulic radii, X would tend to infinity; that is, all portions of the pore space would desaturate at the same value of The second pore-size distribution index, 1, described the absolute slope of another logarithmic relationship, the relative hydraulic conductivity to capillary pressure. This is shown in Figure 13. Brooks and Corey (1964) showed that mathematically X andi were related by This relationship was shown to represent observed data very closely (Laliberte et al., 1968). Laliberte et al. (1968) further concluded that Brooks and Corey's (1964) original formula describing relative hydraulic conductivity as 1 was a valid one. 51 I.0 0.I .0I l.O I0 P (cm) 100 Figure 12. Effective saturation as a function of capillary pressure (after Corey, 1969). I.0 Kr 04 10 Pc(cm) b0 Figure 13. Relative hydraul.ic conductivity as a function capiflary pressure (after Corey, 1969). of 52 While the method described by Laliberte etal. (1968) is at present applicable to the desorption phase of soil wetting, the method does provide a valuable analytic tool with which to investigate the unsaturated hydraulic regime of a soil and to develop soil classification systems based on hydrologically relevant parameters for hydrologic problems. DESCRIPTION OF THE STUDY SITE Location A study area of 2.8 acres was selected as the most suitable in terms of slope, vegetation, soil, and future use. It is centrally located within the Oregon Coast Range province (Figure 14) in the NW 1/4, Sec. 19, T. 12S. , R. 7W., W. M.. Located in Benton County, approximately two miles west of the Marys Peak summit (Figure 15), the study site is on land administered by the Siuslaw National Forest. It is located at the headwaters of Shotpouch Creek, a tributary of Marys River. Geology of the Area The geologic features on Figure 15 are extracted from Baldwin's (1955) geologic map of the Marys Peak and Alsea quadrangles. As can be observed, both the Tyee sandstone formation and the Marys Peak intrusive body underlie the study area. Physiography The topography of the study site is characterized by very steep slopes, averaging 35 degrees (70%), which is typical of the general area. The slope has a northwest aspect and is slightly convex to l.inear with few dissections. The elevation of the study ranges from 54 ortland corvallis newport U 00 U) w 2 LI- coos bay U) 4 0 0 , 4 I C-) U) 4 0 I I z I w I Figure 14. Location of the study site in relation to the 55 ft 7W. Tyee(Te)& lgneous(lg) Contact Zone A I sea 1296 U.S.ES. Rd. No. Geologic Fault Figure 15. Location of the study site. 5.2 mi. 56 2180 to 2500 feet. A linear depression, possibly the result of a past slope failure, traverses the area in a generally south to north directioti. C 1 ima te The climate of the area, typical of Oregon's coastal area, is dominated by the marine influence. The summers are warm and dry, while the winters are cool and wet. Prolonged periods below 20°F or above 100°F are uncommon. The average annual rainfall for the immediate area varies from 60-80 inches (Corliss, 1964) with November through March containing 70% of the annual total. Inter- mittent periods of snow are common, but normally snow melts quite rapidly at elevations below 3500 feet. Soil The soil on the study site is predominantly a colluvial regosol, having developed primarily from the igneous Marys Peak intrusive. The soil type found on the study slope was originally identified by members of the Alsea Soil and Vegetation Survey (Corliss, 1964) as a Marty silt clay or loam. However, because the original soil typing was done from aerial photographs with very limited grourtd checking, the original classification is incorrect. The correct designation for 57 the soils on this slope is a Shotpouch gravelly loam1, a very loose, well aggregated, non-cohesive soil of shallow to medium depth. Vegetatioti large Vegetation2 on the study site is similar to that occupying a portion of the Coast Range and Douglas-fir region. The dominant tree species is Douglas-fir mixed with varying amounts of western hemlock. The overstory is 130-140 feet high arid 80-90 years old, young for a natural stand in this area. Several scattered large red alder trees can also be found on the study site. Crown density is 75-95% (Figure 16). The uniderstory vegetation is typical of that described for the sword-fern community. It is characterized by a lack of shrubs. Sword-fern covers 30-60% of the total area (Figure 17). According to Rothacher etal. (1967), the sword-fern community is found in areas where moisture is abundant. It is usually found along drainages, on steep north arid east facing slopes, and in seepage areas. Other the final published text of the Alsea Soil Survey (Corliss,and 1973), the Shotpouch series was grouped with the Klickitat series Klickitat the name Shotpouch eliminated. To avoid confusion, the name will be used in any future discussions, even though there are very 3Iight technical differences between the soils. in Appendix A. 2Scietitific names not shown in the text are listed from FrankIin Common names for tree and herbaceous species are arid Dyrness (1969). 11n 60 METHODS AND MATERIALS The intent of this study was to examine certain soil and hydrologic properties of a representative cohesionless slope in the central portion of the Oregon Coast Range. Knowledge of these soil properties arid their irtteractjort with water was desired in order to determine the role each property plays in the stability of these types of slopes in this region. Cohesjortless soils were selected because they comprise such a great portion of the area arid often occupy the steep midslope region where the greatest stability problems exist. Because rio previous studies had interisively investigated the soils of the area from an engineeririghydrology standpoint, it was necessary to desigri a study from the ugrourid up. Soil and hydrologic properties studied were those thought to be important in slope stability and in understariding how rainfall was disposed of by these soils and slopes. Site Selection C rite na Criteria used iri selecting a study area were accessibility, a relatively undisturbed nature, cohesion].ess soil type, and the possibility of future road construction across the study slope. The last criterion was required for the brig-term objective of everitually studyirig the hydrologic chariges caused by such a road. II Soil Pit LEGEND I locaUons bhown. Contour Interval= 50' Debris Fan 3. Piezometer * Ten siometer - Depression Scale I = 66 '-4 Fiuc I 8. C otour map of study site 'A'ift sod pit, piczocter, and tcnsiomeer MARYS PEAK STUDY SITE 22 Acres 63 Precipitation Measurements To understand the hydrologic changes which take place in the soil, a means of measuring the water input to the site was needed. Precipitatioti measuremetits were made during the 1973-1974 water year, begintiing October 1, 1973. Because rain, in the past 20 years, comprised over 95% of the annual precipitation in the area, rio provisions were made for stiowfall measurements. Incoming precipitatioti was measured contitivally with a Fischer-Porter, model 1559, recording raitigage (Figure 19). Observatiotis to the nearest 2.54 mm were recorded atid putiched oti a bitiary-coded paper tape every 15 minutes. Tapes were recovered each month. The raingage was mounted oti a stump iti a clear-cut approximately 160 m west of the study site. The gage had a 35° field-of-view oti the uphill side and an unrestricted field-of-view over the remainder of the horizoti. Periodic volumetric checks were made with a standard raingage. No differences greater than 5 mm were noted between the two raingage amoutits recorded for several storms. Hydrologic arid other properties of the Bohaririori soils were compared with those of the Klickitat soils to determine if sufficient similitude exists between them to make valid generalizations concerning cohe- siortless soils of the central Coast Range. Soil pits on the study site were positioned to sample visibly different topographic areas of the site (Figure 18). The depth of each soil pit was determined by the depth of soil at each location. Original plans called for digging down to unweathered bedrock, but in some cases this proved to be impractical because of the presence of a deep saprolitic layer. The exact depth of the unweathered bedrock at these locations was determined by core drilling. Large amounts of rock debris within the soil profile made soil sampling difficult in several cases. Two soil pits, 3 and 6, were either so rocky or shallow that urdisturbed soil cores could not be taken from these pits. The Soil Survey Manual (U. S. Soil Conservation Service, 1967) was used as a guide for sampling, and checks of field descriptions were made with previous descriptions for the same soil series (Corliss, 1964). Soil Sampling Two types of soil samples were taken., As the soil pits were being excavated, bulk representative samples were placed in double plastic bags, sealed, and labeled. Large stones over 7-8 cm in diameter were discarded after their percentage by volume was noted. 68 cold room These disturbed samples were stored in a high humidity arid later analyzed for aggregate stability, particle-size distribution, and soil strength. The second type of soil samples were relatively undisturbed These samples were used for tests of hydraulic con- core samples. This ductivity, porosity, bulk density, and drainage characteristics. sampling utilized an impact type bulk density sampler employing a fitted inside a stainbrass retainer ring, 6 cm ,c 5. 4 cm in diameter, not less steel cutting cylinder (Figure 22). Although this device does sampling devices, the meet Hvorlsevs (1948) criteria for undisturbed method was adequate for our purposes to date. Ranken (1974) found results, in his judgethis type of impact sampler to give satisfactory ment, in securing 452 samples for the types of tests listed above. The first samples were taken from 10 to 30 cm below the surface of the mineral soil. Excessive roots in the first few centimeters sampling above these of the mineral soil often precluded effective soil depths. Due to large rocks randomly scattered throughout the Samples were profile, rio exact sampling schedule was possible. taken wherever possible and at intervals such that each horizon was sampled at least twice. Samples were taken in both a vertical and horizontal direction at each selected depth to determine if saturated hydraulic conductivity was anis otropic. 70 total of 153 samples were takerL from the study site, 75 were taken from the two Bohanriort soil pits offsite, and 42 were taken from the Klickitat soil pit offsite. After the sampler was extracted from face of the soil pit, the retaining ring with the soil was removed from the sampler. Excess If soil was trimmed from the ends of the sampie with a sharp knife. large stones or roots were observed in the sample, the sample was discarded and another taken from near the same spot. A few samples contained these defects and escaped detection until later testing. A double-layer of cheesecloth was then placed over both ends of the soil containing retainer rings and secured with rubber bands. The sample was placed in a clean soil can whose lid was firmly held in place by another rubber band. Finally, the can was labeled for future use. Laboratory Analyses of Soils Particle-size Distributioti Particle-size distributions at several depths in each soil pit were determirLed from the large, bulk samples collected in the field. To fractionate the coarse particles greater than 0.074 mm a series Qf metal wire sieves were used. The sieve sizes used in this analysis Were 19. 1 mm, 4.76 mm (no. 4), 2.00 mm (no. 10), 0.420 mm (no. 40), 0.149 mm (no. 100), and 0.074 mm (no. 200). 71 The sieves were nested with the largest mesh size at the top and the smallest at the bottom. An enclosed pan formed the base for the nest and was used to collect particles which passed the 200 sieve. Completely air-dried specimens of 300 to 500 gm were then subjected to rolling for five minutes with an aluminum rolling pin on an aluminum tray. This served as a primary deaggregating treatment for the larger particles. The rolled samples were then passed through the sieve nest by shaking on a mechanized sieve shaker. After shaking for 10 minutes, each sieve, with the soil, retained on it, was weighted to the nearest 0. 1 gm. Subtracting the sieve weight gave the net weight of the soil retained on each sieve and in the pan. The material in the pan was saved for further differentiation into silt and clay sizes. To achieve this separation, a modified sedimentation procedure was used. Fifty grams of the material in the pan were put into a flask with 10 ml of 5% sodium hexametaphosPhate (Calgon) solution and placed on a reciprocating shaker table for 12 hours. The unusually long time was due to the recognized difficulty of dispersing soils of western forests (Youngberg, 1957). With one exception, the standard hydrometer procedure (Day, 1965) was followed from this point on. Because only total amounts of silt and clay were desired, observations were taken only at one and 120 mi nut e 5. 72 Triaxial Compression Tests To determine the angle of internal friction () for the two soil series being studied in this work, triaxial compression tests were performed. These tests used samples reconstructed to a soil density similar to field conditions. Both dry arid saturated samples were samples tested. Tested soil samples were constructed from composite arid had contribu of soil. The Klickitat soil was from the study site tionis from all sever' soil pits arid depths. The Bohaninioni samples were made up of soil from both offsite soil pits. For testing purposes, duplicating the very low bulk densities of the surface to near surface potential failure sursoils was very difficult. However, because the face would most likely be at the saprolite-sOil interface arid the bulk density at this depth was larger arid easier to achieve, the bul.k density of this depth served as the reconstruction density. Reconstruction was achieved by pouring the selected soil sample irito a rubber membrane held to a correct dimension by a vacuum-forming jacket arid tampirig the soil irito the membrane. Dry weight of the soil used arid the volume of the formed sample allowed the computation of the bulk density. The reconstructed sample was nominally 7.11 cm by 15.24 cm n height. It was then place in the triaxial chamber (Figure 23). 74 avoid sample consolidation. Lower values of the minor principal and vacuum gauges stress (a- 3) were desireable, but the pressure available on the testing equipment were inaccurate at pressures below conthat used. Five samples were tested in each of the two moisture at failure ditions. Knowing the major and minor principal stresses allowed for the calculation of the angle of internal friction according to -4sin -1 where rl'3 I - - 1 I (18) a-3) +1] L is the sum of the applied stress (p), at failure and the minor principal intergranular stress (3). Aggregate Stability Early analysis of the triaxial shear data pointed to a significant decrease in the angle of internal friction (4)) when the soils were tested in the two different moisture states (see Results). This behavior was unexpected arid contrary to the literature concerning the effect of water on cohesioniless soils. In order to determine whether water-stable aggregates could [ikely influence soil strength, two series of tests were performed. Each test was a modification of Kemper's (1965) procedure for anialyztrig aggregate stability during wetting. One test analyzed the stability of aggregates wetted under tension while the other tested the stability of aggregates during direct immersion at atmospheric pressure. 75 Representative, air-dried samples from soil pit 2 on the study site and the other three offsite soil pits were tested. Four to five hundred grams of soil were passed through a sieve nest made up of a 2.00 mm and 1.00 mm sieve. The material retained on the 1.00 mm sieve was used in these tests. The selection of one aggregate size class to be tested was based on the work of several authors (Pannabokke and Quirk, 1957; Bryant, Bendixen, and Slater, 1948) which showed that results from simple one- and two-sieve methods were closely correlated with results using several size ranges of aggregates. The 1-2 mm sieved portion was specified by Kemper (1965). The material retained on the 1. 00 mm sieve was then divided into seven subsamples, each weighing 25 gm or more. One subsample was oven-dried to determine moisture content. The other six subsamples provided material for three replications of each test method. Three replications were deemed adequate because 1emper (1965) cited a coefficient of variation of only 4% for coarse textured soils using his procedure. Tension wetting was accomplished in the apparatus shown in 'igure 24. The subsample was placed in a no. 60 (0.25 mm) sieve on a platform ir a desiccator. About 10 ml of distilled water was poured 1flder the platform, and the desiccator evaculated to between 0. 5 and atm. The soil subsample was subjected to this vapor saturated 76 From Deaired, Distilled Water Supply 0 e ssicoto r Figure 24. Apparatus for testing aggregate stability. 77 deaired, distilled vacuum for 10 minutes. After the 10 minutes, the aggrewater was allowed to enter through the entry tube arid cover maintained, the gates. After two minutes of soaking with the vacuum The wetted sample was then sieve was removed from the desiccator. rotary motion using a dissieved for five minutes in a vertical and all times. tilled water bath. The bath kept the aggregates cove red at The purpose of the sieving after wetting was to separate the fine, without disrupting the agslaked material from the stable aggregates gregates. then washed into a The remaining aggregates and sand were the amounts of weighing dish, oven-dried, and weighed to determine of sand was sand and stable aggregates. Determination of the amount the no. 60 accomplished by putting the oven-dried material back on minutes in a five percent sieve and resieving the material for five Any remaining solution of sodium hexametaPhosPhate dispersant. aggregates were broken with a waterjet or gentle fingertip abrasion against the sieve screen. The remaining material was again transBecause only ferred to the weighing dish, oven-dried, arid weighed. retained as individual those sand particles greater than 0. 25 mm were than particles, "sand in this procedure refers to particles greater O.Z5 mm. Percent aggregate stability (AS) was then calculated by 78 AS lOOx (wt. of sand + aggreg.) - (wt. of sand) (orig. wt. of sample) - (wt. of sand) (19) This formula dilferentiates the larger sand particles from the aggregates so that meaningful estimates of aggregate stability are obtained. Kemper (1965) found better correlations between aggregate stability and other factors (clay, organic matter, etc.) when the sand greater than 0. 25 mm was not considered as aggregates. He also reported lower standard deviations in his 1965 work. The test for aggregate stability under direct immersion at atmospheric pressure was done with the same equipment and procedures but without the vacuum application. Saturated Hydraulic Conductivity Saturated hydraulic conductivity measurements were made using undisturbed core samples. The undisturbed core samples selected for this test were placed in a large, deep, stainless steel pan and saturated with deaired, distilled water. All samples from one depth and soil pit were grouped together for testing. The water was added slowly by siphoning to avoid air bubbles and to avoid disturbing the samples by wave action. Samples taken during the initial sampling were often distrubed by such wave action and several samples col- lapsed in the rir.gs, requiring subsequent resampling. The siphoning 79 procedure eliminated this problem. The water level was brought up to one centimeter over the sample tops. Actual testing was performed with a constant-head permeameter of unique construction (Figure 25) which allowed testing of the soil directly in the retaining rings (Rankin, 1974). The permeameter consisted of a support frame, constant-head reservoir, and inlet and o.it1et chambers. A screen was attached to each chamber to provide full s.ipport for the soil samples. The permeameter support frame was mated with each soil sample under water. The inlet and outlet chambers were thus kept full of water when testing began, and no air bubbles were introduced into the sample. In all cases, the samples were mounted so that water flowed through the sample from top to bottom (relative to the soil itself). The screw clamp was then tightened forcing a seal between all parts. Next, the assembly was removed from the water pan and the constant head reservoir was adjusted to the desired height. Preliminary tests had shown a hydraulic head of 15 cm, operating over the 6 cm sample length, generally gave good results in a reasonable time with little or no soil piping. A few very loose samples, however, needed lower hydraulic heads to avoid piping. These samples were tested with a hydraulic head of 5 cm. With the 15 cm hydraulic head, a three minute measuring time was used. 80 SCREW SUP ORT FRAME IlL OUTLET CHAMBER TO DRAIN SOIL CORE CHEESECLOTH RING I INLET CHAMBER FFOM RESERVOIR Figure 25. Cotistatit head permeameter with soil core iti place (reservoir tiot showrt). 81 After the reservoir was set at the desired head, the inlet valve was opened. When the flow became steady, outflow was collected and measured in the selected time span. The saturated hydraulic conductivity for each sample was then computed using Darcy' s equation. Drainage Characteristics Des orption tests were performed to determine certain drainage characteristics of the soils, including pore-size distribution and unsaturated hydraulic conductivity at various tensions. Because statistical analysis of the soil data (porosity, density, etc.) did not indicate any significant differences between soil pits, one batch of 15 cores from soil pit 2 was selected to represent soil of the study site. This soil pit was also closest to the tensiometers and piezometers (see Figure 18). Samples from the three offsite soil pits were also tested. During the normal testing routine, samples were fully saturated and using a special C-clamp to retain the water (Ranken, 1974) saturated weight was determined. Each sample was then quickly transferred from the C-clamp to the prepared tension table. Once the tension table was filled with 24 soil rings, its top was sealed. Testing involved a tension range of 0-100 cm of water and Utilized tension tables (Figure 26). Each table was equipped with a SCREEN 6 OVERFLOW RESERVOIR TYGON TUBE DRAIN HOLE S4MPLES PLEXIGLAS -TENSION TABLE BLOTTER PAPER Figure 26. Tension table apparatus for determining drainage characteristics of soil samples. 30- 20- 10- 0 SOIL. Co 83 nylon screen measuring 26 cm by 40 cm. Deaired, distilled water was added to cover the screen completely. After a 40 cm by 50 cm sheet of blotter paper was carefully lowered into the water with special care being taken to avoid entrapping air bubbles, the drain tube was then unclamped, and excess water drained off. The blotter paper was smoothed out with a hard rubber roller ensuring a tight seal between the paper and tension table around the screen. A poor seal would have allowed air to enter the system and break the tensioninducing column of water. The tension applied to the surface of the blotter paper was controlled by an overflow reservoir of water connected to the tension table with tygon tubing. By lowering the reservoir a specified distance below the midpoint, of the samples, tensions up to 100 cm were induced. The outflow of the reservoir was first placed 5 cm below the samples and the tubing unclamped. Samples were allowed to equilibrate with the applied tension. Measuring the amount of water released showed that equilibrium had been attained within 24 hours in both soil series at all applied tensions. After the equilibrium period, the tygon tubing was reclamped. Each sample was removed and any condensation wiped off the retainer ring. The weight of the sample was then determined, and the sample rettirned to the tension table. After this was done for all the samples, the top was again sealed and the reservoir lowered to the 10 cm level. The tygon tubing was again unclamped. The cycle was then repeated 84 at 20, 30, 40, 50, and 100 cm. Oven-drying followed the 100 cm weighing. The moisture content and degree of saturation at each tension was then computed (Appendix B). The pore-size distribution and unsaturated conductivity estimates were calculated using the 0-100 cm tension data. Bulk Density and Porosity After each undisturbed sample had been utilized in other tests, it was oven-dried for 24 hours at 105°C and the oven-dried weight (W) recorded. Bulk density was calculated using the volume of the retaining rings. The porosity (n) was calculated by the formula 1- W nGV xlOO. Where G is the specific gravity of the solids, (20) ''w is the unit weight of water, and Vr is the volume of the retaining ring. Specific gravity Was determined by standard pycnometric means (Lambe, 1951). Piezometry Saturated flow in a soil profile can be a prime cause for slope faj1ure. The 3aturated zone causes an increase ic the neutral stress ard a corresponding decrease in the effective stress. (Piezometers 85 were installed on the site to detect and quantify the neutral stress effect of a saturated zone.) Seventy-eight piezometers were installed during the summer of 1973. Piezometers were constructed of 1.91 cm, inside diameter, polyvinylchloride pipe cut to appropriate lengths (Figure 27). The last 10 cm were perforated with 3 mm holes to allowwater entry. Ny1oi window screening was cemented over the holes to prevent the entry of the surrounding filter sand. Each piezometer was sealed at the bottom, and a removable, vented cap kept rain from entering through the top. The location for each piezometer was chosen by considering the topography, surficial indicators of possible subsurface water, and desired spacing. Approximate piezometer locations are shown in Figure 18. For the sake of clarity, the only piezometers numbered are those necessary in the future discussion of results. The first 30 piezometers were installed in holes drilled by a continuous full-flight auger. This auger did not have the ability to penetrate even soft rock. Approximately a month after piezometer installation began, an Acker Portable, coring drill became available and was used to install piezometers 31 through 78. These piezometers extended to varying depths into the bedrock. At this time the bedrock was thought to be a relatively impermeable layer and saturated water flowing through the bedrock was not seriously considered. 86 Vented Retrieving Cap Line I. 3/411 P.V.C.. Pipe Acrylic Tub.Float Assembly Bentonite Seals Is £ Filter Sand l/Drjlled Nylon Screening at ' I 5' £ S. I-, "7 27. Diagram of a piezometer installation. Holes 87 desired depth, enough clean, After each hole was drilled to the in to provide a 2-3 cm base. No. 4 (4.76 mm) quartz sand was poured into the hole aud enough of the same The piezoineter was thea inserted 30 cm of the piezometers. satid was poured ii to surround the bottom placed above the sand layers by A 7-10 cm bentonite seal was then Soil was thei repacked alternately pouring in dry clay and water. arouad the piezometer up to the ground surface. bottom was determi1ed to the The elevatio1 of each piezometer with a haid abiey arid PhiladelPh nearest 0. 1 foot (3 cm) by leveliug 0 iit to the top of the leveliig rod from the iearest gr Id control recorded piezometer leagth gave the piézoineter. SubtraCtio1 of the elevatio1 at the bottom of the pi ezometer. similar to that Piezometric head was measured by a method inside diameter, described by Swansto1 (1967). A clear 0.48 cm, inserted into each piezowas plastic foam float, acrylic tube, with a rise occurred, the float was meter (Figure 27). If a piezometric level fell before the carried upward by the water surface. If the water tens ion betweeu the float and next observation reading, capillary maximum crest level. The acrylic tube walls kept the float at the by retrievi1g Water level at the time of reading could be measured imum crest betweeti the last the acrylic tube and recorditkg the max thea repositioned at the reading atid the pres erkt time. The float was reinserted into the piezometer. bottom of the acrylic tube, and the tube 88 The float thus marked the current water level in a similar mariner as the maximum crest. Laboratory tests showed this method to have an accuracy of + 5 mm. In practice, a ring of soil particles deposited on the acrylic tube by the water allowed measurements to the nearest 1 mm. Piezometers were read at time intervals determined by weather conditions and unforeseen events. During storm periods, attempts were made to make daily readings. However, snow storms sometimes precluded reaching the site for up to six consecutive days. Vehicle problems, such as breakdowns and the acute fuel shortage of 197 3-74, also Umited observations. Consequently, readings could be taken, on an average, only every other day. Tens iometry Early piezometric data indicated that no substantial saturated flow was occurring in the soil profile even after 10 cm of daily rainfall. To determine the degree of saturation in the soil and to substantiate the piezometer data, a bank of four tensiometers was installed on the site in late January, 1974. Because most of the detected subsurface hydrologic activity was centered near the geologic contact Zone, the tens iometers were located close to this zone and to the Piezometers that were consistently the most active. Soil pit 2 was also located nearby (Figure 18). along the contour adjacent The four tensiometers were located 120 cm (Figure 28). This to one another at depths of 30, 60, 90 and made from near the ground arrangement permitted measurements to be The body of each tensiometel surface to the soil_bedrock interface. in diameter (O.D.). A was made of clear plastic tubing, 1.91 cm the body. The porous ceramic cup was permanentlY bonded to body, possessed a mercury manometer, clamped on to the plastic Each tensiometer scale graduated in millibars of water tension. The length of the plastic could measure from 0 to 850 cm of water. body varied according to the depth of installation. circular, steel insertion Installation was accomplished with a the ground. A clean tube designed for placing the tens iometers into by driving the tool hole, about 1. 9 to 2. 0 cm in diameter, was made inserted The tens iometer body could then be the porous cup and the soil and a good hydraulic contact made between against the bottom of the by lightly tamping the tensiometer snugly level completed the installation. around the ground Packing soil hole. water exPrior saturation of the porous cup allowed for immediate into the desired depth. change with the soil. whenever the piezometers were All four tensiometers were read centimeter of water read. Measurements were made to the nearest by 5ubtracting the attd later converted to actual capillary tensions the observed reading. elevation head constant of each tensiometer from 91 RESULTS General Geologic, Soil, and Site Characteristics sampling, and site mapping proThe geologic investigation, soil organize subsequent portions of the duced iaformation which helped to the study overall study plan and to reveal important facts concerning site. showed The 48 boreholes drilled with the Acker diamond-bit drill mapped by that the study site is crossed by the geologic contact shown in Figure 15. Figure 29 shows the approxi- Baldwin (1955) arid the Marys Peak igneous body and mate- location of the contact between formation. Inspection of the recovered rock cores the Tyee sandstone that the arid thin sections made from the selected cores corilirrned underlying rock types are Tyee sandstone arid igneous granophyric gabbro and diorite as described by Lovell (1969) arid Roberts (1953). made Figures 30 arid 31 are photomicrograPhs of the thin sections Figure 30 shows a moderate from the corings of both types of rocks. ti the upper third degree of metamorphism and mineraL crystallization of the Tyee sandstone section.. Drilling resistance varied with the Location on he slope. As the increased, eth.er iphiLL or distance from the geoLogic contact zone from very ow to extremely downhill, the drilling resistance increased quite eather'd so high. The sandstone near the contact zone is Fig;rc ). LEGEND Contour Interval= 50' -_te.. Slump Contact Debris Fan - Tension Crack Depression Scale I": 6 - Contour map of site showiug gcologic contact zone and surficial indicators of apparcFlt 5Iope instability. 2.8 Acres MARYS PEAK STUDY SITE / 95 drilling was relatively easy. At these points near the contact zone, the drill was able to penetrate the soft weathered strata at a rate often exceeding one foot per minute. Such rapid advancement through the rock usually precluded recovery of intact cores. However, as one moved a short distance (15-20 m) downhill, the sandstone improved in competence and intact cores of hard sandstone were recovered. In several boreholes uphill from the contact zone, the thin igneous rock layer was pierced, and highly weathered sandstone was found underlying the igneous rock. The thickness of the igneous sill increases so rapidly, however, that beyond 10 to 15 m uphill from the contact zone the igneous rock was too thick and hard to penetrate in a reasonable time. Therefore, it was not possible to determine if a weathered sandstone layer underlies the igneous sill at all points or whether the weathered sandstone is concentrated at the contact zone. Generally the igneous rock near the contact zone did not, by visual inspection, appear to be more weathered than the igneous rock from sampling points at higher elevations. The sole exception to this was the highly weathered igneous rock found at the head of the linear depression. The seven soil pits indicate that the entire study site is covered by the same soil series. The profile descriptions resemble the Klickitat seri.es as described by Corliss (1973). The seveni soil pits produced nearly identical soil descriptions, except for variations in depth. The soil is cohesionless, a sandy to gravelly loam, arid does 9t. not exhibit strong profile development. Because of the nearly idetiti- cal descri.ptions obtained from all seven soil pits, a sitigle composite description of the study site soils is given in Appendix C. Soil depth varied from 66 cm to over 215 cm, averaging 173 cm. Surficial features, such as small slumps and debris fans, contributed to the variation in depth. Numerous surficial and soil indicators thought to be important in analyzing slope stability were noted during the site mapping, geologic survey, and soil sampling. The most prominent surface indicators of apparent past slope instability are old slump scarps, a series of tension cracks, and a longitudinal depression with a debris fan at the lower end. All are shown on Figure 29. The slump scarps are relatively small and revegetated. The largest of the slumps has a headwall of 1-2 m. The tension cracks run parallel with the approximate location of the geologic contact zone. Probing of the major tension crack (point a on Figure 29) indicate a depth i.n excess of 180 cm, approximately the full soil depth at this point on the slope. Other shorter, less distinct tension cracks were also observed running parallel to the crack shown in Figure 29. Tension cracks were not noted at distances greater than about 20 m away from the apparent contact zone. The longitudinal depression extended uphill to approximately the 2400 foot elevation. No appre- Ciabl.e dilferences in rock hardness, nor any apparent fault in the rock, 97 were rioted from the drilling resistance experienced in the depression. The mean soil depth in the depression is one meter less, however, than the area as a whole. The debris fan located at the bottom end of the depression indicates the depression is a linear debris avalanche slide trace. The buried organic horizons uncovered in soil pit 5 (Figure 18) tend to support this. The presence of more than one buried horizon indicates a sequence of small slides down the track rather than one Large debris avalanche. The charcoal bits noted in every soil pit, for the full soil depth, tend to point towards a general colluvial nature for this area. This colluviation is most likely a slow, continuing process. Corliss (1973) also noted charcoal bits throughout his Klickitat arid Boharinon soil pits, as well as other soil series. The mean slope on the study site was 35 degrees (70%). The range in slope was from 4 1 degrees (27%), on the debris fan, to over degrees (100%) on the steepest portions. A sample standard deviation. of only 4. 6 degrees points to the relatively uniform steep nature of the study site. P recipitationi, Piezomet ry, arid T ens iomet ry The precipitation received on the study site during the 197 3-74 Water year is listed monthly in the Appendix. To provide some basis for later analyses and discussions, the 197 3-74 precipitation of the tw0 nearest cl.imatologically comparable stations, Alsea Fish Hatchery 98 arid Valsetz, are also listed as well as a summary of the previous 20 year monthly precipitation records for both stations. During November, 1973 through March, 1974, piezometer measurements were recorded. Only six piezometers (39, 40, 45, 48, 56, 59) showed consistent saturated flow. Figures 32 through 34 show the relationship between piezometric level and rainfall during December through March. In all cases, the flow recorded was occurring in the sandstone strata, well below the soil-rock interface. Because the depth of piezometer penetration was insufficient in three of the six cases (piezometers 40, 56, and 59), the figures shown are riot true piezometric heads, but are heights referenced to the bottom of the piezometer. For this reason, the term piezometric height or rise will be used in future discussions of the water level in each piezometer. The graphs, for the most part, show rapid fluctuations with rainfall variations and indicate an active layer of flow in the rock rather than water merely collecting in the boreholes. Piezometer 39 may be an exception to this, however, considering its relatively low response to differences in rainfall. In the cases where there was insufficient Piezorneter penetration, the water level actually dropped below the piezorneter bottom and was not detectable. Hence, a base level to reference a true piezometric head was not available for these piezomete rs. Figure 32. Graph of piezometric height in piezometers 39 and 40 and daiLy rainfall. 71 U) I' / DEC, 73 I J/\ f\ 'I \ \___ JAN.'74 I,' MAR Figure 33. Graph of piezornetric height in piezorneters 45 and 48 and daily rainfall. c o.ni ' 2- 0 k 0 5 I0 4- 5- 6- DEC,'73 [\ ii I Ld 1/ r Id JAN,' 74 7. FEB / I MAR -I 59] Figure 34. Graph of piezometric height in pieorneters 56 arid 59 arid daily rainfall. : .E3 o 4- '1) C, 0 x 71 I0 5 C D U 102 For the six piezometers which consistently exhibited water during the winter months, sample correlation coefficients (r) were computed to measure the degree of association between the piezometric height and a past period of rainfall. The cumulative rainfall periods tested were 12, 18, 24, 36, 48, 60, 72, 84, and 96 hours. It was found that in all cases, the 'tr'1 values increased to a peak at 48 hours and then decreased. Table 1 summarizes the peak htrH values obtained for each piezometer. The fact that the piezometric height was best described by the 48-hour cumulative rainfall is not too surprising in light of Figures 32, 33, and 34. The peaks in piezometric height appear, for the most part, to follow peaks in rainfall by 36 to 48 hours. Table 1. Correlation coefficients for piezometric height as a function of the 48-hour cumulative rainfall. Piez ometer r Numb e r 39 40 45 48 56 59 0.55. 0.68 0.84 0.87 0.80 0.70 Fluctuations in water level are associated with a quantity of rainfall (time x intensity) rather than a rate (inches/day) as was measured. 103 A further interesting note is the fact that piezometers 45 arid 48 give the highest correlation coefficients. Only these two piezometers had sufficient penetration into the bedrock to establish a base reference level for the piezometric heights measured. Base level was estimated to be approximately 35 cm arid 0. 2 cm above the bottoms of piezometers 45 and 48, respectively (see Figure 33). Regress ion analyses were performed on the two piezometers for which base water levels were available (piezometers 45 arid 48). Ad- justinig the recorded piezometric heights for each piezometer by the estimated base level yielded piezometric heads (Y) which were re- gressed against the amount of 48-hour rainfall (X). All units are in centimeters. The best fit obtained for each piezometer was a second degree polynomial, Y a + bX + cX2. Table 2 summarizes the regression equations arid corresponding coefficients of determination (r2). Because the two resulting equations were quite close when plotted, a combined equation is also included in the table. Figure 35 is a plot of the combined equation and the 95% confidence limits. As can be seen, the confidence limits are quite wide. The r2 values are high for this type of physical phenomenon arid indicate that 72% of the variation in piezometric head was associated with the 48-hour rainfall. Considering the influence such factors as / 6 / 5 / 4 0 / a3 -D a I C-) I- E 0 N a, =8.U2.I4X-.0I9X2 r2= 0.72 2 3 48-Hour Rainfall(cm) X 10 Figure 35. Average piezometric head versus 48-hour rainlall. 104 105 rock fracturing, varying degrees of weathering, and local channeling of flow can have on subsurface water flow, an r2 of 0. 72 is quite good. Table 2. Regression equations and related r2 coefficients for piezometers 45 and 48. 2 Equation r 48 Y = 9.00 + 2. 32X - 0. 032X2 Y = 6.78 + 2.04X - 0.009X2 0.75 0.72 45 + 48 Y = 8.11 +2.14X- 0.019X2 0.72 Piezometer Number 45 During January 11-16, the largest storm of the water year delivered over 40 cm of water on the study site. The greatest one day total was 15. 5 cm. As a result of this storm, 21 additional piezometers recorded brief rises in the saturated water zone. These piezometric rises did not l.ast more than 48 hours and receded to zero almost as soon as the storm abated. In some instances, the detected zone of saturated flow was in the soil profile. Table 3 lists the maximum piezometric height obtained during the January 11-16 period for all Z7 piezometers. Tensiometry was begun on February 2, 1974 after almost two complete months of piezometer measurements with no sustained 3aturated tiow detected in the soil, mantle. The tensiometer bank showed that unsaturated flow existed at all soil depths even after periods of substantial rainfall (Figures 36 and 37). A maximum two 8 FEB 1 severe freeze 3Ocm 1. -J U Figure 36. Soil capillary pressure at 30 and 60 cm depths arid daily rainfall amounts during February and March, 1974. a- -J -J 0:: >- Q4 w C/) (I) Lu 0:: C-, E6 c'J 0 freeze duriig February an March, 1974. Figure 37. Soil capillary pressure at 90 and 1Z0 cm depth5 and daily rainfall amouits I Isevere * 295.5 263.0 208.5 149.0 181.0 169.0 303.0 182.0 151.5 263.0 239.0 301.0 304. 5 333.0 355. 0 364.0 104.5 128.0 138.0 83.0 229.5 261.0 329.5 270.0 218.5 348.0 279.0 Piezometer Length (cm) 3. 3 5. 6 1.8 12. 6 13.4 182.8 169.4 88.0 12.7 12.7 1. 1 0.3 18.2 60. 1 125.8 86. 6 121.9 1.8 27. 6 34. 2 19.0 4.9 3.6 4.8 43.3 72. 1 5. 6 0. 2 0. 3 113.1 76. 2 88.5 25.6 45.6 146. 3 159. 3 134. 1 97. 0 70. 7 135. 1 1. 9 13.6 1.3 0.8 7.6 5. 6 93.3 2. 1 Max. Depth of Water Recorded During Period (cm) 0 0 0 0 0 0 Ht. of Soil-Bedrock Interface Above Piezometer Bottom (cm) Piezometer showing sustained piezometric activity during months of November, 1973 through March, 1974. 76 77 64 67 61 48* 51 52 53 56* 59* 22 32 33 34 37 39* 40* 41 43 44 45* 5 19 4 2 1 Number Piczmeter T.b1e 3. ILixiinum depths of water in piezometers during the January 11-16, 1974 storm. Middle 1/3 slope Middle 1/3 slope Upper 1/3 slope Above head of depression Above head of depression Neargeol. contact 25 ft. above geol. contact On geol. contact zone On geol. contact zone On geol. contact zone On tension crack On tension crack Above geol. contact zone Below geol. contact Lower 1/3 slope Lower 1/3 slope Near debris fan On geol. contact zone Head of depression In depression In depression Base of slump Head of debris fan East side of debris fan Base of slump Middle 1/3 of slope In depression Comments 109 day rainfall of 7. 0 cm produced the lowest capillary pressure readings, but unsaturated conditions remained. Another storm equaling or exceeding the January 11-16 storm did not occur during the period tensiometers were monitored. Figures 36 and 37 show the close relationship between the capillary pressures at the 30, 60, 90, and 120 cm depths and the daily rainfall during the period. The absolute minimum capillary pressure for the upper 60 cm of soil ranged from 4. 5 to 6. 5 cm of water and for the lower 60 cm of depth ranged from 10.6 to 20. 6 cm of water. During the two months of tensiometer measurements, capillary pressures failed to exceed 70 cm of water at any depth. When at least 0. 5 cm of rain fell per day, capillary pressures usually remained below 30-40 cm. The freezing weather during the last week of February to March 8 froze the tens iometer water columns and made readings impossible. Physical Properties of the Soils The physical properties for the sampled soils are summarized in Table 4. The appropriate Unified Soil Classification System symbol is also listed for each entry. No Atterberg Limits are shown because all samples proved to be nonpiastic. The data from the five on site soil pits point to the relatively homogeneous nature of the soils on the study site. The three off site soil pits also show listed properties quite simdar in value to the on site soils. The only trends noted from Depth (cm) Pit Average 4 Pit Average 2 Pit Average 1 0-30 4.2 86.0 30-45 45-60 60-75 75-90 90-105 0-30 9.8 --- --- 86. 3 1. 18 8.4 5.8 --84.4 7.9 --- 82.6 1.05 1.12 1.02 --- 11.0 --- 0.87 0.98 0.98 7.2 6.4 --- --- 89.8 6.3 5.2 --- 0.83 3.9 --- 5.8 89.0 0.81 0.85 0.91 --- --- 64.6 59. 1 66.1 66.1 63.6 61.2 69.9 71.4 71.2 71.3 73.6 70.4 68.4 72. 9 0.84 0.83 3.9 1. 7 94.4 0.78 0-30 30-45 45-60 60-75 75-90 90-105 69.2 68.6 71.1 68.4 68.9 62.0 7.5 7.0 9.9 8.0 82.1 85.5 1.09 --- --- 5.1 (%) Total Porosity % Clay (<.05mm) --- 6.0 % Silt (.074.-. 05mm) 0.90 30-45 45-60 60-75 75-90 88.9 (.074mm) (g/cm3) 0.90 0.83 0.91 0.88 % Sand + Gravel Bulk Density Ktickitat Soil Series (on site) Number Soil Pit SM SM -- SM -- SM SM -- SM -- SM SM SM -- SM Unified Soil Class 49 77 52 24 76 36 48 93 102 85 107 99 305 102 39 74 82 56 221 39 23 20 21 46 35 82 69 80 95 98 101 232 55 65 92 164 21 60 Saturated Hydraulic Conductivity (cm/hi,) Horiz. Vert. Table 4. Mean values for bulk density, particle-size classes, porosity, and saturated hydraniic conductivity for two cohesionless Coast Range soils. 4.3 --8.8 --86.9 87. 1 0.93 --10.6 --82.2 Overall Average --- 8. 6 84. 2 7. 3 9. 2 71.3 59.4 7. 2 --- --- 87. 3 66. 4 5.0 2.2 92.8 5.6 3.5 7.2 --- 67.7 68. 3 69.4 68.7 66.6 70.5 --- --- --- 6.8 7.3 85.9 0.82 0.88 0.90 0.96 0.97 0.83 1.17 0.92 45-60 60-75 75-90 90-105 105-120 30-45 8. 2 67.8 75.7 69.2 70.5 69.1 64.5 62.4 (%) 88.6 3. 2 2.1 --- Total Porosity 0.95 Pit Average 7 3-30 --- --- --- Pit Average --- --7.6 --90.3 (.074mm) (g/cm ) 0.88 0.85 0.88 1.03 1.08 % Clay (<.05mm) 0.70 % Silt (.074-..OSmrn) 0-30 Gravel 30-45 45-60 60-75 75-90 90-105 % Sand + Densit S Bulk Depth (cm) Soil Pit Number TabLe 4. Concinu.d. SM SM SM -- SM -- SM -- SM SM SM -- -- SM -- Unified Soil Class 92 72 110 38 45 52 51 117 72 38 36 --- 113 125 124 Honz. (cm/br) 89 67 79 62 83 85 55 85 22 174 20 98 134 Vert. Saturated Hydraulic Conductivity (gIcm) DensitX Bulk 1.8 1.10 --- --95.2 1.14 1.25 Overall and Pit Average 150+ 1.15 90.9 89.0 3.1 91.1 3.9 5.0 --- 2.2 93.0 --- --- --2.2 4.5 91.3 30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150 --- --- --- 1.08 1.15 1.06 1.08 1.12 1.23 1.32 1.28 1.31 92.8 5. 4 89.8 0.88 2.4 0-30 94.0 --- 2.3 --- 92.1 94. 1 2.4 3.0 5.2 6.0 --- 4.8 5.8 --- 5.0 --- 4.2 --- 4. 8 3.6 3.0 --- 3.6 --- 4.9 3.0 Porosity (<.05mm) (.074-.OSmm) 60.2 62.5 60.1 63.2 62.5 60.9 56.8 55.1 57.2 55.0 68. 9 61.9 70.6 63.1 61.2 58.9 60.5 56.9 (%) Total % Clay % Silt 1. 18 Bohannon Soil Series (soil pit A) Overall and Pit Average 30-45 45-60 60-75 75-90 90-105 94.6 Gravel .074mm) % Sand + 0.85 1.07 1.10 Kiickitat Soil Strie (off site soil pit) Number Depth (cm) 4. Continued. Soil Pit Tab1 SM SM -- SM SM -- SM -- SM -- SM SM SM -- SM -- SM SM Unified Soil Class 54 72 65 45 41 38 43 32 82 65 52 93 28 70 175 152 102 30 15 3 6 4 5 20 10 28 17 10 40 52 12 28 10 94 70 97 Saturated Hydraulic Conductivity (cni/hr) Horiz. Vert. Depth (cm) (g/cm3) Bulk Density Overall and Pit Average % Silt 3.2 1.16 92.5 1.8 94.4 1. 10 1.05 4. 1 91. 7 0.89 3. 8 (.074-.OSmm) 0-30 91. 4 .074mm) Gravel % Sand + 30-45 45-60 ohannon Soil Series (soil pit B) Soil Pit Number Table 4. Continued, 4.3 3.8 4. 2 4.8 63.9 59.8 64. 1 67.8 (%) Total Porosity % Clay (.05mm) SM SM SM SM Class Unified Soil 118 211 90 52 Honz. (cm/hr) 74 13 185 55 Vert. Saturated Hydraulic Conductivity 114 Table 4 were the expected increase in bulk density and decrease in hydraulic conductivity with depth. Considering the normal range of variability found in most soils, these changes with depth can be considered low. Inspection of Figures 38 through 41 illustrate the predominantly coarse nature of both soil series and the low proportions of fine materials. The soils are well graded in the coarse fraction having uniformity coefficients greater than 6.0, but possess enough fine particles to place them in the SM class of the Unified Soil Classi- fication System. The shapes of the particle-size distribution curves are very similar. Table 5 presents the results of the aggregate stability tests for the three off site soil pits and for soil pit 2 on the study site. A paired one-tailed t test was performed at each depth to test if there was a significant difference in the means found for tension and directed wetting. The test statistic used for t was derived from the standard equation for paired observations (Freeze, 1967) which reads x-Y with (n-l) degrees of freedom and where n is the number of pairs, X arid Yare the means for each set of observations, and S is the vana.1ce of the individual differences between the two sets of observations. In all cases, as the t statistics show, the aggregate stability is signifi¼aritly (a 0. 05) to very significantly (a = 0.001) higher for tension 100 60 10 U sieve 1 __ \ liii II .. SAND a \ I ' T 1 . D601.2 AVE.D10.O5 by IAVE. I ,, 0.05 dia. in mm \ ..._ FINES hydrometer rriri Figure 38. Particle-size distribution curve for Klickitat soil from the study site. 3) .c 80 100 bY GRAVEL Imean & range - C 4) I- >. I0 I 1 \\ \\ I ê 0.05 dia. in mm -. -4-1 AVE.C8.5 AVE. D10 .13 FINES hydrometer 1\ .J b AVE.D601-IO . SAND \I D 4 Ffl In sieve Figure 39. Particle-size distribution curve for Klickitat soil from off site soil pit. too 2C 60 60 I00 bY GRAVEL I mean 8i range JO' - c I- -c I0 !' eve 4m 1.0 SAND \ 0.05 dia, in mm 01 I I I FINES 00I 4VE..C=8.8 0JD01 IAVE.D6Q08 AVE. D1ç! .70 byJvdrpmeter Figure 40. Particle-size distribution curve for Bohannon soil from soil pit A. 100 20 60 80 100 bY GRAVEL . mean & range 0.00 C 0) - >, 0) -C I tO -A 54mm sieve 10 SAND 0.05 dia. in mm 0.1 .0744mm FINES hydrometer 120. 0.01 0.001 4. AVECu.. 12.0. AVE. D6 AVE. D10=. .10 by Figure 41. Particle-size distribution for Bohannon soil from soil pit B. 0 100 20 _60 80 100 GRAVEL by . mean & range 0.000I 119 Table 5. Aggregate stability (A. S.) for Bohannon and Klickitat soils under direct and tension wetting. Depth (cm) A.S. Direct Wetting A.S. Tension Wetting (%) t decrease (%) Klickitat soil, soil pit 2, onsite 0-20 60-75 150-175 83 83 22 63 60 87 31 82 86 82 85 20 17 26 29 87 33 18 65 24 2.93* 9.68** 16.33** Klickjtat soil, offsite 0-15 35-45 70-75 100-105 66 71 61 60 8. 30** 4.14* 4.82* 38.69*** Bohannon soil, soil pit A, offsite 20-30 40-50 75-85 95-105 125-130 150-160 180-205 58 75 68 75 61 54 92 89 90 90 89 51 83 24 17 32 39 39 6.45* 6.96* 6.84* 19.40** 16.42** 33.95*** 15. 49** Bohannon soil, soil pit B, offsite 5-15 0-30 45-50 62 60 46 86 84 28 29 89 26 significantly different at the 95% level significantly different at the 99% level ** significantly different at the 99. 9% level 20.08** 12.89** 8.67** wetting than for direct wetting. The differences in aggregate stability ranged from 17 to 39%. The range of values (4-9%) for tension wetting within any one soil pit was not great. For direct wetting, however, the range in values was wider, 5-24%. While trends in aggregate stability with depth may appear to be present for either wetting mode, statistical analysis indicated that generally there was no significant difference (a 05) between depths in any soil pit. The only two exceptions were in the two Bohannon soil pits where under direct wetting there is a significant difference between the maximum and minimum aggregate stability values. Figures 42 through 45 illustrate the high degree of aggregation to be found in various particle sizes for both soil series. The soils in these figures have been air-dried and sieved; yet the particles still possess great affinity for one another. 121 Figure 42. Particles of Klickitat soil showing high degree of aggregation. Particles shown are less than 0.074 mm. The mass shown is comprised of many of these minute particles. Figure 43. Particles of Bohannon soil showing typical high degree of aggregation. Particles are all less than 0.074 mm. Note the greenish color compared to the reddish color in Figure 42. Klickitat soils possess more iron from the igneous parent material and hence exhibit a more reddish color. 123 Figure 44. An individual particle of Klickitat soil (less than 0.074 mm) which exhibits typical high degree of aggregation. Figure 45. A sand-sized particle of Klickitat soil (greater than 2.0 mm) with smaller particles aggregated to a larger rock particle (dark areas). Sand-sized particles of Bohannon soil exhibited similar aggregation. 125 Strength Tests Table 6 summarizes the results of the five triaxial shear tests performed on the dry and saturated soils of both soil series. The initial porosities obtained during sample reconstruction were close enough betweeri the dry and saturated samples of each soil series that they may be considered as equal. The strairi at failure was riearly double for the saturated samples, compared to the dry samples, in both soil series. Iri the Klickitat soil, the average decrease iri the arigle of iriterrial frictiori () was almost 9.5 degrees arid approximately 11 degrees iri the Boharinon soils. These decreases in amourit to over a 29% decrease iri the Klickitat soil arid to over 27% ii the Bohaririori. The two differerices iri saturated arid dry arid strain at failure are riotable arid appear to vary sigriificantly from the traditional wet-dry behavior of cohesioriless soils, as described iri the literature. Paired t-tests were performed ori the two sets of Table 6 and the differerices iri the mean data iri values were fourid to be very highly significant (a = 0.001) for both soils. The reasons for these differences arid the implicatioris will be discussed iri greater detail later. 126 Table 6. Results of vacuum and saturated triaxial shear tests. Soil Tested: Klickitat soil, composite sample from soil pits 1-7. Comments Initial Strain at Test Failure (%) No. Porosity (%) VACUUM 61.5 62.0 63.0 41056! 6.0 5.5 6.0 5.0 6.0 Average: 410291 5.7 61.4 42°26' 1 41056 2 3 41055? 39°14' 4 5 59.8 60.6 All samples failedby bulging. No distinctfailure plane. SATURATED 33°34' 33°14' 32°14' 11.0 12.0 10.0 12.0 10.0 60.6 61.1 59.2 62.3 Average: 32°OO' 11.0 61.3 30013t 30046) 1 2 3 4 5 63.1 All samples failed by bulging. No distinct failure plane. For H: t t-table(d. f. 4, a0. 00l)=8. 610 8. 710 Soil Tested: Boharinon soil from soil pits A and B. Initial Strain at Test Failure (%) No. C ommetits Porosity (%) VACUUM 1 2 3 41052? 5.6 60.4 39°20' 39°23 4. 5 60. 0 4 420071 5 39°35 Average: 40026? = 4.5 7.1 4.5 61.2 61.0 60.2 5.2 60.6 All samples failed by bulging. No distinct failure plane. 127 Table 6. Continued. Test Strain at Failure (%) No. Initial Porosity (%) Comments SATURATED 1 30026? 2 3 4 29°09' 29°48' 29°18' 27°5U 9.5 10.2 11.2 10.9 12.3 60.0 60.3 59.7 60.2 60.0 Average: 29°18' 10.8 60.0 5 All samples failedby bulging. plane. For H: t 18. 710 No distitictfailure t-table(d. 1. 4, a0. 001)8. 610 128 Drainage Characteristics Pore-size Distribution The pore-size distribution for each soil was determined from desaturation tests using tensions equal to 0-100 cm of water. The and pore-size distribution index (X) were ob- bubbling pressure tamed from the logarithmic plots of the data in Appendix B. Figure 46 illustrates how b and )%. can be derived from such plots. For many soils, the saturation values obtained by desaturation methods must be adjusted by a residual saturation value (5) before a plot such as Figure 46 can be made. The residual saturation is the saturation value at which capillary pressure becomes very large and may be taken as the value of saturation where the hydraulic conductivity of water approaches zero. By removing that portion of the pore space not active in soil moisture flow, the saturation value is scaled to that portion effectively conducting water under capillary tensions, hence the term effective saturation (Se). The saturation (S a ) is related to the effective saturation (S) by Sa -s r r for Sr < Sa< 1.0. The value for Sr cat-i be obtained in several ways, but the method described by 3 rooks acid Corey (1964) is the simplest. It is not un- Ornmon, however, to encounter values of Sr equal to zero. This is 129 especially true of highly structured soils. For the soils sampled in this work, Sr was equal to zero and the plotted values of S in Figure 46 are equal to the saturation values in Appendix B. Table 7 summarizes the mean values for porosity, bulk density, bubbling pressure, and pore-size distribution index (X). The percentage of pores in each diameter class was computed through the relationship between pore diameter and capillary pressure (equation 21). Table 8 lists by soil and depth the percentage of pores by pore diameter class for Klickitat and Bohaninon soils. Pore-size distributions are compared in Figures 47 arid 48. In these figures, the pore-size classes from 0.030 to 0.592 mm have been combined so that only three classes are shown. Moisture Characteristics The moisture characteristics)expressed as water content in percent by volume (8)for each tension applied on the tension table, are given in Appendix B. All values are mean water contents for the soil series, soil pit, arid sampling interval indicated. A graphic representation of the change in water content with tenisjon is kniowni as a soil moisture characteristic or moisture release curve. Figures 49 and 50 are moisture release curves for the respective Klickitat and Bohaninion soil pits. The reclining J-shape of all the curves is typical for all soils, but the exact configuration for 1 30 1.0 2.0 cm Soil: 0.I- ?, =.012 Bohannorf (soil pit A) Depth: 0-60 cm I .01 10 1.0 Pc(Cm) i6o I.0.5 1.0 cm =.074 Soil: Klickifot (SoilIpit 2) 0.1 Depth:90-I2Ocm Io Pc() ibo Figure 46. Experimental relationships between effective saturatioti (Se) arid capillary pressure for two cohesionless soils of the Oregon Coast Ranc. 1 31 any soil is dependent on soil structure and texture. The use of 0 rather than w, the gravimetric water content, is often more convenient because it is more directly applicable to the computation of fluxes and water quantities added to the soil by irrigation or rain and to quantities subtracted from the soil by evapotranspiration or drainage. Examination of Figures 49 and 50 indicate that generally all four soil pits are quite homogeneous from the surface to the bottom, in terms of their moisture characteristic curves. The homogeneous results within each soil pit can be attributed to the well-mixed nature of the two soil series. The soil moisture-tension relationship varies little with depth in each soil pit, in a manner similar to the saturated hydraulic conducitivity (Table 4). .. .. . *** * 30-60 * ** * 60-9 ** -A- ** * ** ** * * ** ** * -12 (A) <,030 .30-592 0C 0 0 0 S. f\ 0-30 ** * ** * 25 ** * *** 50 ** 75- 00- ** * ** * ** * ** * 3* . S S DEPTH INTERVAL SAMPLED (CM) **- * ** * ** ** ** 0 00 S 00 0 00 . 60-90 ** * *** ** * *** * 0 .. ** .5 0 (B) Figure 47. Change in pore sizc distribution with depth for Klickitat soil from the study site (A) and from off site (B). 0 025 U- 0 0050 cr LIJ (I) 4075 w 100 >0.592 DIAMETER OF PORES (mm) DEPTH INTERVAL SAMPLED(CM.) - - 4 S 0-60 60-90 .S. S 55 .5 (B) Figure 48. Change in pore size distribution with depth for Bohannon soil from soil pits A and 13. 0 0-60 60-90 90-120 120-150 150-210 75 2 <.030 25 (A) S S. ,.592 5 .. .. 55 S. S. S. S S S S S. S. S. .__ S S S. S S. S ____ 50 75 100- DIAMETER OF PORES(mm) 134 Table 7. Mean porosity (n), bulk density (BD), bubbling pressure (rb)' and pore-size distribution index (X) obtained from capillary pressure_desaturation data for two cohesionless soils of the Oregon Coast Range. Depth Iiterva n (%) x BD (gm/cm) (cm) (cm) Klickitat soil, soil pit 2 30-60 60-90 90-120 76 70 69 0.78 0.98 0.97 1.8 0.7 1.0 0.071 0.072 0.074 0.92 1.14 1.20 2.0 2.0 0.022 0.020 0.021 1.07 1.13 1.14 1.24 1.28 2.0 2.0 2.0 1.7 0.012 0.009 0.006 0.004 0.005 0.99 1.19 1.8 1.8 0.019 0.010 Klickitat soil, off site soil pit 0-30 30-60 60-90 67 58 59 1.5 Bohannon soil, soil pit A 0-60 60-90 90-120 120-150 150-210 59 56 56 52 52 1.6 Boharinon soil, soil pit B 0-30 30-60 61 56 135 Table 8. Mean values of pore-size distribution as fractions of total porosity. Depth Interval (cm) Diameter Class of Pores (mm) .030 .030.049 >.592 .074.098 .099- .149- .073 .148 .294 .295.592 .020 .014 .016 .031 .018 .017 .029 .023 .024 .022 .024 .023 .059 .043 .049 .062 .132 .101 .007 .005 .003 .036 .022 .015 .065 .035 .025 .132 .080 .058 .089 .088 .089 .081 .071 .086 .017 .015 .012 .009 .005 .005 .002 .003 .002 .002 .019 .013 .016 .012 .007 .055 .046 .034 .026 .028 .044 .028 .027 .022 .030 .051 .041 .055 .024 .030 .016 .018 .024 .027 .041 .093 .087 .071 .052 .138 .039 .050- Klickitat Soil Series (soil pit 2) 30-60 60-90 90-120 .634 .645 .674 .144 .101 .095 Klickitat Soil Series (off site soil pit) 0-30 30-60 60-90 .539 .640 .672 .051 .059 .052 Bohannon Soil Series (soil pit A) 0-60 60-90 90-120 120-150 150-210 .711 .760 .751 .796 .783 .098 .095 .102 .109 .115 Bohannon Soil Series (soil pit B) 0-30 30-60 .551 .562 .066 .071 .044 5o 30 0 tO \ .1 20 40 "-. 50 - 60 TENSION (CM H20) 30 - .S 70 80 90 100 Figure 49. Moisture characteristic curves for the Klickitat soil pits. 0 140 0 0 .4- 0 >60 -J 0 -J ILl 0 80 30-60 60-90°---- 0-30 Offsite Soil Pit 90-120--- 60-90®® 30-60* Soil Pit 2(depth,cm) KLICKITAT 0 Cl) -J 0 0 10 . 20 30 50 - () 60 TENSION,(CM H20) 40 I 70 80 . 90 100 Figure 50. Moisture characteristic curves for the Bohannon soil pits. 20 30- 40 60 70 i 60-90 90-l20 I20-l50 30-60 0-30 --' Soil Pit B 150-210 XX c * 0-60* Soil Pit A(depth,cm) BOHAN NON 138 DISCUSSION The direct application of soil mechanics theory to the analysis of slope stability processes is difficult because of the normally heterogeaeous nature of most soils and the extreme variability of soil water iaconditions. [f, however, certain factors or processes kaown to fluerice slope stability can be identified and are found to exhibit significant similarity between soils of a group, valid generalizations may often be made to reduce this variability. One of the objectives of this study was to investigate whether such commoa, similar factors and processes existed among the cohesionless soils of the Oregon Coast Range and, if so, to identify them so that a better understanding of the stability of these soils could be achieved. Physical Properties of the Soils The two soil series selected for investigation in this study cover over 55% of the central Coast Range of Oregon and makeup over 90% of the cohesiociless soils in the same area (Corliss, 1973). They are derived from distinctly different parent materials. The basic igneous rocks from which the Klickitat soils are derived are rich in clayforming n'.ineral.s while the Tyee sandstore is typically poorer in clay. Yet, the two soils do not differ greatly in their physical properties. 139 Both are cohesionless soils exhibiting little or no plasticity. The most characteristic properties of these two soil series are the low bulk densities, high porosities, and high values of saturated hydraulic conductivity (Table 4). These three properties are, of course, highly interrelated. All three common characteristics occur in both the A and B horizons of both soils, with the Klickitat soils being a bit more porous. Because these properties do occur in both horizons, they are, therefore, not entirely due to a high organic matter content as one might first suppose. The high porosity may be related to such disruptive factors as the large number and rapid growth of roots and the activity of various soil fauna. Perhaps the best explanation of this high porosity can be derived from the extreme aggregation of both soils noted in Table 5 and seen in Figures 42-45. The exact causes of such high degrees of aggregation appearing in these soils are not completely understood. A great deal of the aggregation is undoubtedly due to the high free iron and organic matter contents of both soils. Corliss (1964) reported values for organic matter and iron content in both soils of sufficient magnitude to account for the excellent aggregation, using Kemper and Koch's (1966) work as a criterion. However, oxides of aluminum could also be another good binding agent and are often thought to surpass iron oxides in importance (Saint etal. , 1966). Unfortunately, no estimates of hydrous aluminum oxides in either soil are available to judge their effectiveness 140 in aggregation here. Silica could also be acting as an aggregate cementing agent, but it is not considered a very effective agent in soils of humid areas (Paeth, 1970). As can be seen from the above comments, it is obvious that our knowledge of aggregate binding agents is very incomplete and further research is needed. Soil Texture The texture of either soil changes little with depth (Table 4). Between soils there is also little difference in texture. The low amounts of clay detected appear to result from the youth of the soils, the loss of any generated fines through periodic removal by washing during colluvial movement, and possible piping loss through the soil macropores. (It was noted during piezometer measurements that fines were transported into over hail of the piezometers.,) Even the extreme weathering described by Rojanasoonthon (1963) as typical for soils of this area is not apparently sufficient to keep pace with the loss of fines from these steep slopes. In any event, the texture of both soils are typified by their apparent low silt and clay contents. Texture is one of the most commonly used criteria to differen- tiate between soils. It is used in almost all soil classification systems, including the Unified and Comprehensive Systems of soil classification. The first system is widely used in engineering work, while the second 1 primarily used by agronomists and soil scientists. The results of 141 texture analysis are often presented in the form of particle-size distribution curves such as those shown in Figures 38-41. In the early days of soil mechanics, many people thought the size of individual particles would prove to be the most important characteristic of a soil. The ensuing years of research have failed to substantiate this belief, especially for clays. Noncohesive, granular soils, however, are still classified in engineering work mainly by particlesize distribution because for these soils, many important pieces of engineering data can be gained from them. For the cohesionless soils studied here, however, the particle-size distribution curves should be viewed with suspicion. Soil scientists have known for many years that certain soils become finer textured as a result of prolonged rubbing. The particle- size distribution of these soils will depend on the intensity of the dispersion treatment in the laboratory. Although the mechanical analysis performed on these two soil series was carried out in such a manner that the soil samples were exposed to a standard dispersion treatment and consistent, reproducible results were obtained, the particle-size distribution curves are not unique for these two soils. The excellent aggregation produced many pseudomorphs up through the medium sand sizes which could be broken down into smaller sands, or into silt, and clay-sized particles, depending upon the dispersing treatment and the amount of energy used. In this respect, these two soil series resemble 142 residual soils rather than transported soils. Taylor (1948) and Deere and Patton (1971) point out the dubiousness of particle-size data for residual soils. Therefore, it appears Likely that while the steep slopes usually associated with the Bohannon and Klickitat soils imply substantial colluvial movement, these two soil series possess a development trait of residual soils. The transportation experienced is local rather than interregional and has not erased, to any degree, this residual soil trait. Residual or transported soil history notwithstanding, it is apparent that for a substantial portion of the cohesionLess soils of the central Coast Range the traditional particle-size anaLysis is not very definitive. Saturated Hydraulic Conductivity Besides affecting soil properties such as porosity, buLk density, and texture, the excellent aggregation also affects the saturated hydraulic conductivity of both soil series.. The saturated hydraulic conductivity (K) data from Table 4 indicates that both soiL series are extremely permeable and transmit water rapidly. As would be expected, the surface layers were found to have higher K values. These high rates of conductivity reflect the extremeLy porous and rtighly aggregated nature of the surface layers. Subsoil, layers were also highly permeable in the saturated state, and, in general, were only slightly less conductive than their respective surface layers. 143 Soil mixing through colluvial action and the high porosity arid aggregation of the subsurface soils helped to make high saturated hydraulic conductivities a common property of the entire soil mass of both soils. Both soil series have sufficiently high subsoil K rates that extensive saturation or near saturation would not be expected except for very short periods during prolonged rainstorms. One should keep in mind, however, that these K determinations were made using small soil samples contained in rings only 5. 4 cm in diameter. Consequently, the values presented probably have only an approximate relationship to values which might be expected under field conditions. However, the values do provide an index of the relative conductivity of the soils and their horizons and also make it possible to compare these soils with others in their ability to transmit water under saturated conditions. The saturated hydraulic conductivity data also indicate that only slight to moderate anisotropism exists between the vertical and horizontal K values. According to a common rule of thumb, the vertical and horizontal values of K should differ by at least four times for the strata being considered. Such differences were not noted for these soils. The lack of great differences in vertical and horizontal K is most likely due to the homogeneity induced by past colluvial action in both soil series. The lack of extensive profile development also aided in producing a relatively isotropic condition. 144 soils and can be The high values for K are typical for such granular volume in the larger associated with the high proportions of soil pore Table 8 and Figures pore sizes (larger than 0.030 mm), as shown in fluxes of water when 47 and 48. These large pores allowed for large the soils were saturated or nearly so. Aggregate Stability soil aggregates is The influence of the mode of wetting on the results obtained from the aggregate stability clearly illustrated by wetting has on tests. Besides showing the detrimental effect direct from the aggregates of any depth of both soil series, the results points concernthese tests, shown in Table S, present two interesting ing the aggregation in both soils. wetting First, the percentage of aggregate stability (AS) in both The Bohanrion soil modes is almost identical for both soil series. of values from soil pit A does, however, have a slightly higher series likely due to the lower posiThis is most mode. in the tension wetting While not aption on the slope from which this soil was sampled. content (Table 4) to account pearing to have substantially greater clay soil pit probably possess for increased aggregation, the soil from this This would be due to greater proportions of other cementing agents. agents to this lower elevaleaching and transportation of such soluble one must c onclude tion by movement of subsurface water. Nevertheless, 145 from Table 5 that the two soil series possess similar arid nearly equal aggregate stability percentages in either wetting mode. The near equality in aggregate stability values between the two soils is similar to information published by Wooldridge (1964) for basalt- derived and sandstone-derived soils in central Washington. The second point that can be observed from Table 5 is that in both wetting modes, the aggregate stability with depth is relatively constant in both series. Paired t tests between depth intervals in each soil pit, for both wetting modes, generally showed no significant (a = . 05) differences existed in aggregate stability with depth. These results are somewhat in opposition to those found by Kemper and Koch (1966) who reported that the subsurface soils usually possess higher aggregate stability than do the surface soils. Kemper and Koch attributed the increased aggregate stability with depth to the decreasing effect of biological and mechanical disruption mechanisms on the aggregates at the lower profile levels. However, the 519 soils they studied were predominantly from level to only slightly inclined agricultural and forested lands. Consequently, Kemper and Koch's soils probably did not possess the homogeneity induced by colluvial mixing as the two soil series studied here did. The extensive colluvial mixing apparent in the two cohesionless soils studied here would tend to produce the relatively equal percentage of stable aggregates at any depth seen in Table 5. 146 Soil Aggregation, Shear Strength, and Slope Stability Angle of Internal Friction of Dry Soils The triaxial shear tests (Table 6) of both soils produced unusually high values for the dry angles of internal friction (i). Terzaghi and Peck (1967) list representative values of for effective stresses less than 5 kg/cm2. For loose sands and silty sands, the values for range from 270 to 330 The values derived in this study were approximately 41° for the Klickitat soil and 400 for the B ohannon. Considering that both soil series are very loose (e > 1.0, n> 50%), one can see that these soils had appreciably higher than would be expected for such loose soils. values A review of the literature produced no comparable examples where such loose, highly aggregated soils were tested. An inspection of the soil grains (Figures 42-45) composing both soils revealed that the soil grains are actually aggregations of many particles. Such aggregations were found to persist in the fine silt ard clay, fine sand, medium sand, and into the very coarse sand. In their natural state, the aggregates were very stable, and produced high primary aad secondary porosity. The stability of these particles was sufficient to resist substantial dispersion techniques and these particles were assumed stable enough to function as individual primary 147 particles in the soil framework, at least for the range of effective stresses under which testing took place. Having large, composite particles such as these would tend to increase the solid-to-solid contact, forming a framework more resistant to stress. These large, composite primary particles would tend to have greater angularity, rougher surfaces, and larger effective sizes than they would as discrete individuals. This would be especially true for those smaller, more weathered particles which most likely would be more rounded and plate-like in shape. Therefore, it is logical to suggest that the high degree of aggregation, coupled with their stability, produced the unusually high values found in these two soils. Another factor which allows for such high values in spite of the very high porosities is the well-graded nature of both soils. Even with the previously noted limitations imposed by the aggregation on the particle-size analysis data, one can still conclude that the two soils are well-graded. The coefficients of uniformity, Cu (Figures 38-41), are all greater than 6, indicating a well-graded soil. Soils having high coefficients of uniformity usually have higher values of j than do uniform soils because there is more interparticle contact. The smaller particles can fill the voids between the larger particles (Sowers and Sowers, 1970). 148 Angle of Internal Friction of Wetted Soils The large differences in the dry stateand the drained, saturated values (950 to 110) for both soils are unusual. The reductions are very atypical when compared to the literature concerning the effect of water on the angle of internal friction of cohesionless soils. According to Terzaghi and Peck (1967) and others, the reduction in due to moisture in a cohesionless soil should not exceed 10 to In addition no significant change in the strain rate at failure should occur. However, most of the previous work was done on relatively clean, unaggregated soils, with very discernible, distinct cohesionless particles. At first glance, the 9.5°-l?reductions appear to be artifacts of excess porewater pressure buildups. Of course, if this was the case, the data would not yield reliable values for unless interpreted in terms of effective stress (cr). A review of the shear testing procedures (see Methods and Materials) will show that the strain rate was held to 1/4% per minute for the saturated tests. Because of the high saturated hydraulic conductivities found in both soils, the drainage occurred rapidly enough to avoid a buildup of porewater pressure. This drainage was verified by the monitoring of the water outflow in a graduated burette. Water outflow began as soon as the deviator stress was applied, and the soil began to compress immediately as would 149 does not appear also be expected. Hence, excess porewater pressures to be a plausible explanation for the large reductions. If excess porewater pressure is eliminated as a possible cause, all other test conditiors during the two test modes were the same, arid the results are valid differences, do these results indicate a departure from the accepted belief that water has little effect on the value of cohesionless soils? I believe they do not, but that the extreme aggre- gatiort of these soils offers a logical explanation. The nature of the effects of aggregation on the values of both soils can be deduced from the aggregate stability test results shown in Table 5. The influence of the wetting mode on aggregates of both soil series makes the shear test results quite understandable. During the saturated shear tests, the samples were saturated under only a slight vacuum of 0.07-0.14 kg/cm2. This saturation procedure was used to avoid sample consolidation, a potential problem due to the low sample detsities being used. Almost all the 0.703 kg/cm2 confiring stress and only a slight vacuum (0 3) was applied through the glycerin fluid, could be applied during the saturatior operation to help deair the samples. Consequently, the samples were saturated by direct wettirg, instead of tensior wetting. Marty soil aggregates were destroyed, and others weakened. Composite particles (aggregates) which had previously acted as single, primary units, with larger effective sizes, greater angularity, and increased surface roughness, were reduced 150 to smaller, more platelike arid rounded individual particles. This, of course, reduced the intergranular friction in both soils. The shearing resistance resulting from interlocking of grains, beyond that offered by friction alone, was also reduced. A substantial part of the resistance of loose, cohesionless soils is due to the interlocking of grains (Chen, 1948). It is not surprising that the effect of interlocking is large, because some grains must be lifted and rolled over others as sliding occurs along the failure planes. Because the motion of individual particles (in this case, composite aggregated particles) has a component normal to the plane of failure, a considerable amount of the work required to produce failure must be used in overcoming the resistance which the normal force offers to this motion. If a substantial amount of the large primary particles are reduced to their smaller individual components, the amount of interlocking resistance will probably be reduced because adjacent grains will have a smaller distance to be lifted. The same can be said for those aggregates not completely destroyed, but merely weakened. The weakening of the intra-aggregate bonds would allow failure to occur within the aggregate as well as between aggregates. Again the amount of work necessary to overcome interlocking would be reduced. Composite particl.es whose bonds were merely weakened also most likely experienced greater distortions before sliding 151 commenced and produced the greater strain rates at failure that were observed during the saturated tests. It is interesting to note, though perhaps only coincidentally, that the 23% decrease in the average angle of internal friction (Table 6) for the on site soils and the 28% decrease for the Bohannon soils were accompanied by 26% and 29% decreases, respectively, in the average aggregate stability (Table 5). Whether or not samples saturated under conditions of tension wetting and subjected to drained shear would yield values closer to those obtained under dry conditions is, as yet, unanswered and further research is needed on this subject. But considering the greater percentage of aggregates remaining intact under tension wetting for both soil series, it appears that the tension wetted sample should be closer to the dry ted . f. for a than to the satura- Thus, for these types of soils, one must consider the wetting mode used in soil saturation to be as important as the state of drainage during shear in determining a suitable value. 11 one accepts the above explanations, the implication of this type of aggregate behavior is that the values for highly aggregated cohesj.onless soils must be determined very carefully. The wetting mode, as well as the drainage mode, may have to be closely monitored in triaxially testing soils of the type under discussion. If one blindly accepts the belief that the angle of internal friction for coheSioriless soils was not greatly affected by water, then he may test 15Z such a soil in a dry state because such testing is simpler and faster. As a result, he would obtain a high value. Conversely, testing in a saturated state, as was done here, would result in a much lower value. Most likely, the smaller saturated value would be used for any slope stability analysis because the saturated condition is often considered the most critical in slope stability problems. For a natural slope, the saturated value would be too low to represent field conditions where wetting most likely occurs under tension. Therefore, the engineer making the analysis would get a low factor of safety (F) and may even find the slope angle (Is) exceeds over a great portion of the site he is considering. This situation, of course, leads to a factor of safety less than unity (see equation 17) for even a dry slope of cohesionless material. On a steep slope where no failure has occurred, the obvious erroneousness of the assumed evident. On the other hand, use of the dry value is value may be adequate for analyzing a dry or tension wetted slope, but would be hazardous for analyzing a slope subjected in the future to direct wetting, such as from a cross-draining culvert under a new road emptying runoff onto the slope below the road. Possible Effect of Man-Caused Direct Wetting on Slope Stability A system of cross-drairiinig culverts arid ditches is often the 153 most common means used by the engineer to divert and control runoff from forest roads in the Pacific Northwest. Most culverts empty into depressions, on to sidehill slopes, or into established watercourses. Where the depression is on a steep hillside and has never apparently carried surface flow, some shallow soil is usually present. The soil in the depression may have carried subsurface saturated flow, but tension wetting was the means of this saturation. Now, with the road present, the culvert periodically may deposit varying quantities of water into the depression. These periodic surges subject the soil profile below the culvert exit to direct wetting. The flow of water may riot be enough to wash away the soil nor to cause a sufficient increase in porewater pressure (u) to initiate sliding, but some aggregates may be destroyed. If the soil is shallow, less than 3-5 feet, many of the aggregates near the soil-rock interface may be destroyed. The data in Table 6 demonstrates that more aggregates are destroyed at deeper depths by direct wetting. Lowe ring the angle of internal friction in this portion of the soil creates a small weak point along the potential failure plane. However, the soil may riot fail at this time. Also, some of the deaggregated soil particles may even reaggregate over time, but usually neither the total number of aggregates nor the overall degree of aggregation is as great as that initially (Emerson and Grundy, 1954; Lutz and Chandler, 1761). However, as subsequent inundations occur, the 154 spread and inteasify zone of aggregate destruction may increase or perhaps after in areas previously deaggregated. At some future date, where failure many years, the factor of safety is lowered to the point described. The areas occurs. Figure 51 illustrates the process profil.e labeled 1, 2, 3 reflect the hypothesized portions of the soil have had their angl.e of affected by the inundation events arid, hence, ia the facinternal friction lowered. At some event, ni, the reduction arid the deprestor of safety to or below 1.0 causes the slope to fail relationship shown in sioni "sluices out." The factor of safety-time The storm Figure 51 graphically depicts this gradual lowering of F. riot even be an extreme during or after which the failure occurs may may merely event in terms of return period. The cumulative effects become critical at this time. of the 1.aboraOne important aspect concerning the comparison field should be tory test data arid the soil' s potential reaction in the kept in mind. Reductions ri aggregate stability and angles of iaternal friction resulting from direct wetting in this study are probably more The severe than what would be experienced under field conditions. soils will reason for this is that under normal winter conditions, the culvert be far from the air-dried state when subjected to the periodic main effects on discharges. rhe tension of the soil water has its The drier the soil aggregate stability through the rate of wetting. betweetl the wetted whea direct wetting occurs, the larger the gradient Fiurc N 1. 2/i/7 1.0 F fill 2 3 Inundation Events After Culvert is Installed n failure i/i. Diagram illustrating the suggested development of a progressive reduction in the factor o safety (F) due to direct wetting of soil by intermittent culvert discharges. 3 -- dl' '1 1 - .- - road 4; 156 area and the dry area and the more rapid the movement of the wetting front. As mentioned in the literature review, the destruction of aggregates is caused by the compression of air in the aggregates? voids and the rupturing of the weakened bonds along the planes of failure. Hence, the cLoser a soil is to saturation when inundated, the lower will be the percentage of aggregates destroyed because less air is entrapped at such higher moisture contents. The work of Quirk (1950) and pannabokke and Quirk (1957) showed that the rate of wetting was still a controlling factor ir aggregate destruction at low tensions (<100 cm of water). However, as would be expected, the percentage of stable aggregates was higher than if wetting took place from an air-dried state. For a loam soil, the decrease in aggregate stability ranged from 12-16% when wetting occurred at initial tensions of 18-70 cm of water. At an initial airdried state, they found that the loam experienced about a 42% decrease in aggregate stability when directly wetted. Using their work as a guideline suggests that the aggregate destruction experienced under the hypothetical field conditions previously described may be only about a third of that experienced in the laboratory tests. Assuming that the almost linear relationship of decrease in aggregate stability soils and q is a true approximation of the relationship for the two studied, it appears reasonable to assume that the decrease ir will also be a third of those found in the study. For the Klickitat soils 157 this would amount to approximatelY 30 and for Bohannon soils 40 Even with this lessened severity in aggregate destruction, the progressive reduction in the proportion of stable aggregates per unit of soil mass would still occur with subsequent direct wettings. The time span of the hypothetical process may merely be lengthened. Whether or not failure ultimately occurs, of course, still depends on the individual slope conditions, how close the factor of safety is to 1.0, and if the reduction in 4 due to aggregate destruction is sufficient to lower the factor of safety to the critical level. Assuming something of the nature of that just described does occur would, in part, explain why there often appears to be numerous slope failures below roads during winters which are not excessively probable wet. I'm not aware of any census of slope failures by year, cause, topographic association, soil, etc. in the Oregon Coast Range. However, through correspondence and conversations with several members of the Sius law National Forest, which covers a great portion of the central Coast Range, I have gained the impression that there is less than a reliable correlation between the number of road-associated slides and the rainfall in a year, winter, or even month. Future research into the association between aggregate stability and slope stability for other soils in the region and a census system for slides is needed to test this hypothesis. 158 Paeth (1970) hypothesized a similar aggregation-soil strength phenomenon to help explain the observed difference in slide proneness between two soils derived from green tuff and breccia and two other more slide-resistant soils derived from yellow-red tuffs and breccias in the Western Cascade Mountains of Oregon. These four soils were quite clayey, however, and Paeth' s conclusions concerning aggregation as an important soil stability factor were derived from clay-soil moisture retention relationships rather than by aggregate stability- shear strength tests. He also did not comment on the influence of wetting mode on the soil aggregation he observed. His work is apparently the only other that directly recognized the possible importance of aggregates to soil strength and slope stability in a specific situation. Movement of Subsurface Water Unsaturated Flow The lack of sustaitied saturated flow in the soil profile between November, 1973 and March, 1974 was riot expected because this rainy season was extremely wet. Comparisons between 197 3-74 and the previous 20 years were made for the two closest climatologically similar stations: Valsetz, Oregon and the Alsea Fish Hatchery on the north fork of the Alsea River. These precipitation data are 159 summarized in Appendix D. These data showed that the 1973-74 winter precipitation was the highest in 20 years for these two stations. The 197 3-74 winter month& precipitation at these two stations exceeded the 20 year average for those months by approximately 160%. On a per month basis, the 1973-74 winter exceeded the 20 year averages by 130% to over Z50%, depending upon the month and station analyzed. These comparisons show that it is logical to assume that the 1973-74 winter precipitation on the study site was undoubtedly also the highest in 20 years. However, in spite of excessive rainfall, only once during the winter months did saturated flow occur in the soil profile of the study site (Table 3). Even then, the saturation was spotty, shallow, and the period of saturation persisted, on the average, only Z4 hours. Overland flow was never observed; this, coupled with the singular occurrence of very limited saturated flow in the soil shows that unsaturated flow was by far the dominant mechanism of water movement on the study slope. The predominance of unsaturated flow in the soil profile on the study site is further supported by tens iometer data. These data (Figures 36 and 37) itidicate that capillary pressure varied inversely with daily rainfall received on the site. Maximum capillary pressures observed tiever exceeded 70 cm of water. A minimum capillary pressure of about 5 cm of water was observed during storms. 160 The tens iometry data in Figures 36 and 37 also point out the effects of the relatively homogeneous soil profile on the soil moisture distribution. The capillary pressures obtained on any particular day were very nearly equal for the 30, 60, and 90 cm depths. The 120 cm depth, on the other hand, tended to possess slightly lower capillary pressure values. The slightly lower capillary pressure values at the lowest depth were due to slightly greater moisture contents there. The higher moisture contents were caused by the downward moving moisture approaching the less conductive saprolite and by the wetting frotit advancement slowing. Also soil porositiès at this depth were slightly lower thati in the overlying soil. Iti addition, rapid adjustment of the soil to the daily rainfall amounts can also be observed. Usually the "valleys" in the plots of capillary pressure over time lagged only 18-24 hours behind the day of peak rainfall. This rapid redistribution arid equilibration of moisture was a direct consequence of the relatively homogeneous, highly porous nature of the entire soil profile and infers relatively high unsaturated hydraulic conductivities over the range of capillary pressure values measured during the winter. Considering the similarity of the soil properties in both soil series, one can reasonably conclude that unsaturated flow is a dominant process in both. Because soil-water interactionis play such an important role in the stability of steep slopes, the following discussion will focus oni the 161 observed on the study dynamics of the movement of subsurface water amounts of winter precipitransmit the large How these soils slope. should help in our derstanding tationi normally deposited upon them cohesi.oniless soils of this of the factors and processes that allow the Discussiori will concentrate on area to remain on such steep slopes. the changes that occur direction of flow, the magnitude of such flow, of the flow on the between' arid during storms, arid the possible effects stability of the slopes. Hydraulic Gradient Analysis of hydraulic gradients This discussion requires a knowledge Because the flow properties of the soil matrix. (1) and the hydrauLic predominantly in the unsaturated mode water movement appears to be of either the through these soils, it will be seen that determinati.on is more complihydraulic gradient or the hydraulic flow properties cated than if flow was in the saturated mode. it is well knoWn From considerations of just energy potentials areas of low that flow will take place from areas of high potential to hydraulic head (H) is usually defined potential. In saturated systems above a reference plane arid P is as H = Z + P where Z is the distance systems H must the head due to the depth of water. mi unsaturated pressure exerted by is a capillary H = ZPc where be redefined as c For practical purposes capillary and osmotic forces (Hillel, 1971). 162 and omitting osmotic forces, P may be interpreted directly as the reading on a tens iometer cell located at some point, Z, in the soil mass. With this adaptation, Darcyts law may be used for unsaturated flow if the assumptions of isotropism and uniform hydraulic conductivity are still met and if hydraulic conductivity is interpreted as the unsaturated hydraulic conductivity, K(P) (Richards, 1931). These assumptions were used in this discussion. In saturated flow, the potential flow net can be constructed fairly simply if elevation of the free water surface can be determined. For unsaturated flow, however, the potential field will be created in response to a number of interacting forces. In particular, gravitational forces tend to pull water down whereas capillary forces may tend to pull water up. Considering gravity alone and assuming a is a constant in a homogeneous soil if constant moisture content hysteresis is ignored), hydraulic head (H) will increase directly with height (Z) above the reference plane. This will lead to a downward flow of water. Such downward movement in an initially unsaturated soil, however, generally occurs under the combined influence of gradients in both capillary arid gravitational potentials. As the water penetrates deeper arid the wetted profile lengthens, the average gradient in capillary potential decreases because the overall difference in capillary pressure head between the surface arid the wetting front divides itself 163 along an ever increasing distance. This trend continues until eventually the gradient in capillary potential in the upper part of the profile becomes negligible, leaving the constant gradient in gravitational potential as the only force moving water downward in this upper zone. The gradient during such steady drainage therefore tends to approach unity, the gravitational potential decreasing at the rate of 1 cm with each centimeter of vertical depth below the surface of the soil. 11 rain intensity is always lower than the soil infiltrability (a common condition in many western forest soils), the soil will continue to absorb the water as fast as it is applied without ever reaching saturation. After a period of time, as the gradients in capillary potential become very small, the wetted profile will attain a wetness for which the conductivity is equal to the rainfall intensity. Then, according to Darcy's law, as the hydraulic gradient approaches unity the flux (q) approaches q = K(Pc) Under natural conditions, however, a constant steady-state drainage condition such as that just described usually exists only during periods of prolonged precipitation. Once precipitation ceases, the soil mass continues to drain, but under other than steady-state conditions. 11 no additional precipitation occurs, the continued water movement will result in a decrease in water content with height. The limit of the soil capillary pressure at any point should occur at the poiat where P = Z. 11 this state is attained, the gradient (i) becomes 164 zero. Water would cease to move under gravity and water flux would also cease. Theoretically, then, the hydraulic gradients in an unsaturated soil mass would be expected to vary from approximately zero to unity. For the situation under study here, the complicating factors of steep slopes, varying rainfall intensities, and departures from true homogeneity and isotropy leave some doubt as to the validity of the hydraulic gradient behavior just described. Analyses of actual gradients were made in order to more clearly define the gradients operating on the study slope and to see if they are approximately those described in theory. Gradient data were obtained from profiles of 'c for each day tensiometer measurements were made (Figures 36 and 37). Figure 52 is a representative example of such profiles for the period February 4-20, 1974. For the sake of clarity, only selected days are shown, and the time span is divided into two graphs. The profiles of c can again be seen to react to rainfall and drainage quite rapidly. One may also infer hydraulic gradients from these curves as shown. Re- lating the day number for each 'c profile with the rainfall histogram for the period, one can easily see the cyclic nature of the profiles as they wet up and drain. It can also be seen that as the wetting front moves down into the soil, hydraulic gradients tend to exceed unity briefly. However, eventually all of the profile comes to about the o lii a- o x Q 12 'S 5% 5% =0 6 adient I 3 0 I I0 I I I I P 4 15 20 U 0 2'-0 C 41 E 12 PRESSURE POTENTIAL(CM)X 10 'I hydrauliC gr'adient ' / 19 2Q VI6 Figure 5Z. Successive pressure potential profiles during February 4-ZO, 1974. Feb. 10 Days in Feb. PRESSURE POTENTIAL(CM)XIO hydrauliC ' tI2 166 with depth. During same moisture content, and P is nearly constant moisture content, the hydraulic gradimaximum soil these periods of arid rainfall ents are very close to 1.0. As drainage progresses do not achieve this towards zero, but gradients rotate ceases, the usually ceases well before i0, due to The drainage process value. As seen in Figure 52, the gradietits the occurretice of a new storm. the soil moisture again increases. back toward unity as then proceed Analyses of other time periods showed similar sequetices. it appears that the From the hydraulic gradient analysis, then, events and timing are consistent pressure profiles relative to rainfall gradient equal to nearly with theory. The assumption of a hydraulic S appears valid. Due to frequent 1.0 during steadystate drainage between storms does not storms, however, the drainage process equals zero. For hydraulic gradient where the develop to the point minimum gradient of approxithe longest periods of drainage, a the study slope. It is interesting mately 0. 57 frequently occurred on equal to the sine of the to note that this minimum gradient (0. 57) is 350, in this case. it appears that the sine of the slope angle, approximation of the lower limit of the slope angle may be a good period between storms. hydraulic gradient for the short drainage 167 Flow Direction of Soil Water The n.ext question is: In which direction is flow occurririg? To answer this question, flow vectors were analyzed. Figure 53 shows the is opoteritial arid flow vectors for the same time period as Figure 52. Hydraulic head (H) was determined from ZPc where Z is positive as measured from the hydraulic head reference and P is considered as a positive term. Figure 53 is drawn to scale arid represents a side view of a 75 cm section of the sloping soil profile. Bedrock is assumed to be parallel to the slope surface and at a depth below 120 cm. The 120-140 cm depth in the diagrams may be viewed as hydraulically active saprolite. The selection of 140 cm as the impermeable base was purely arbitrary. Due to the relatively homogeneous na.ture of the soil, in areal extent as well as with depth, pressure potentials at a given vertical distance above the bedrock were therefore assumed to be constant across the 75 cm section. S Figure 53 shows that flow vectors in this period (and for all other measurement periods) were predominantly vertical throughout the soil profile. During the days when the soil was wettest, such as February 19, the flow vectors were within 50 of the vertical. However, as drainage progressed, the flow rotated toward a more dowrislope direction, approximately 20°-. 22° from the vertical. The flow vectors thus followed a cyclic nature between vertical and some downslope angle. 0 c. 1 IL 4- -c Hydraulic 2 J__.___ Head Reference d>%Q?PS0 2-4-74 0 0 0 Q x 4 : ----- -5 2-11-74 I 1- I 0 2-19-74 T 4- 6- 8 T-9. v---- T 8-- -9----- -3 2-20-74 z I Figurc 53. Flow vectors of selected days during February 4-20, 1974. The hydraulic head was based on tens iometer measurements for the day indicated. > U z0 4- > 0 cO 4a30 U) ti2 0c', -o c'J 169 Harr (1974) usitig a similar, but more ititetisive terisiometer installation, found the same pattern of changes in flow direction with stages iti the soil drainage cycle. The soils he studied were less homogeneous with depth, however, arid exhibited more marked profile development. Consequently, his soils had greater changes in hydraulic conductivity with depth than do the soils studied in this work. The impeding effect of deeper, denser and less conductive layers tended to produce a more pronounced downslope flow at all soil moisture conditions in Harr!s study. Scholl and Hibbert (1973), using a relatively homogeneous and isotropic, large-scale soil model also observed a cyclic pattern in flow direction very similar to that observed in this study. Thus, the results of this study appear consistent with those of other similar studies. Inspection of the patterns of movement of the isopotenitial lines provides additional inis'ight to the drainage process during unsaturated flow. As an example, attention is directed to the 60 cm isopotential line in Figure 53. During the periods of sustained precipitation arid highest hydraulic gradients, such as occurred on February 4, the 60 cm isopotential is nearly horizontal. As drainage proceeds, the 60 cm isopotential rotates toward a position orthogonal to the slope. As might be imagined, the existence of consistently, purely horizontal or orthogonal isopotentials in a slope is impossible. 11 the sopotentials were all horizontal, no lateral movement downslope 170 could occu.r because because entirely vertical flow would be dictated. Conversely, purely orthogonal isopotenitials would preclude any vertical movement of rainfall into the soil. The answer, of course, is that each isopotential is neither purely horizontal or orthogonal over its entire possible length within the slope. Because capillary pressure gradients will be low at the start of drainage (and isopotentials are consequently nearly horizontal), it is possible to envision a continuum from near vertical to near lateral flow as drainage progresses (Weyman, 1973). Figure 54 demonstrates this situation. Isopotential lines will rotate until nearly orthogonal to the slope and simultaneously move upslope as moistu.re content decreases. The examples in Fig- ure 53 demonstrate this in only a limited scope because of the limitations imposed by the selected profile width and depth and the relatively short drainage periods experienced between storms. 1 Figure 54. Rotation of a single isopotential (4) with time (after: Weyman, 1973). 171 Soil Water Fluxes Having determitied the direction of flow arid the hydraulic gradients operating in the soil, attention will tiow be given to the soil water Estifluxes (q) possible under the unsaturated conditions observed. rnatioris of soil water flux under unsaturated conditions require knowledge of the unsaturated hydraulic conductivity, K(P), for the capillary pressures of interest. Table 9 presents the estimated unsaturated hydraulic conductivities for the Klickitat and Bohaninlo!1 the resoils. Due to the unequal depths of the four sampled sites, sults are presented in terms of the upper arid lower halves of the soil mean, and profile in order to facilitate comparisons. The maximum, minimum capillary pressure values are derived from the tensiometry data in Figures 36 arid 37. The average X and b values are derived from Table 7. The values for r are computed from the relationship = 2 + 3X (equation 25). The average saturated hydraulic conductivities (K) are computed from the original hydraulic conductivity data which are summarized in Table 4 and represent the combitied vertical arid horizontal values. The relative hydraulic cotiductivities (Kr) are, in turn, computed from equation 26 where the c used is either the maximum, mean, or minimum capillary pressure. The :.'alues of K(P), the hydraulic coniducti.vity at that capillary pressure, are merely the product of K and Kr Similar calculations could be (Oil pit B) Bohannon 2.030 1.8 60 17 1.8 2.015 .005 Bohannon .010 27 1.8 2.060 .020 Klickitat (off site) (Soil pit A) 90 0.9 2.219 106 .073 1.8 Klickitat (on site) Lower Half of Soil Profile (soil pit B) Bohannon 2.057 44) 1.9 2.027 .009 Bohannon .019 112 2.0 2.063 .021 Klickitat (off site) (Soil pit A) 156 1.8 2213 K 60 17 .014 .00083 .049 27 .018 .00068 .00081 90 .008 .00009 C Maximum P = 60 cm 00062 106 40 .031 .066 112 .085 .00076 .00078 1i6 .056 K 60 17 .042 .00245 .145 27 .056 .00208 .00242 90 .027 106 .00030 c Mean P =35cn .237 .109 00272 .00223 112 305 .00272 40 156 C 03078 .03052 .02751 .004Th C 5 12. 960 5.627 16. 915 16. 264 (cm/hr) K(Pc) cm 1.847 0.519 0.743 0.430 Minimum P = 10 cm .12227 .14068 .15102 .10426 Kr Minimum P = (cm/hr) Ave. K(Pc) (cm/hr) .219 .00141 K C Mean P = 35 cm (cm/hr) Ave. K(Pc) (cm/hr) .00036 Kr C Maximum p = 65 cm (cm/hr) K Ave. .071 (cm) Pb Klickitat (on site) 1 Average iL'tinated values of unsatw.:ed hydraulic conductivity at rnaxinurn, mean, and minimum winter capillary pressures. Upper Hail of Soil Profile Soil T.iL,le 9. 173 made for any specific depth or capillary pressure rather than for two generalized layers as was done here. Recalling that during periods of sustained rainfall the hydraulic gradient is nearly 1.0, when the moisture content of the soil is high, the soil water flux at the minimum capillary pressure may be assumed to be nearly equal to the unsaturated hydraulic conductivity, K(P). For the mean and maximum capillary pressure values, the soil water flux will be app roximately 0. 57 of the K(P) values shown in Table 9. At first glance, the K(P) values shown in Table 9 may appear to be low, even for the minimum capillary pressure values. However, one need only compare them with a Htypicalhf clay and sand, as shown in Table 10, to see that they are not low. The typical and K used in Table 10 were obtained from Corey values for b' the (1969). For the values of c analyzed, one can observe that sandy textured Klickitat and Bohannon soils maintain high unsaturated flow rates more like clays than sands. The high degrees of aggregaion common to both soil series results in high proportions of secondary porosity and a wide pore-size distribution in each. These attributes, represented by the very small X values (Table 7), allow both soils to keep large proportions of their pores filled with water, even at maximum winter capillary pressures. Analysis of pore size and tens iometry data illustrates this point in greater detail. At the 30 cm depth in the on site Klickitat soil, for 2. 15 8.00 .05 2.00 Clay Sand Pb) 'c 1 X Soil Average 1.0 100 200 .036 (cm/hr) K5 Ave. .036 200 63x10'3 3.1x10'5 c 4.8x1013 1.0* Kr Mean P = 35 ii (cm/hr) K Ave. .036 (cm/hr) K(Pc) 1.0* Kr c Maximum P = 65 cm Table 10. Estimated unsaturated hydraulic conductivities for typical clay and sand soils. 8.9x10'1 .036 (cm/hr) K(Pc) 200 .036 (cm/hr) K5 Ave. K(Pc) .036 5.1x104 26x106 (cm/hr) 1.0* Kr c Minimum P = 5 cm 1 75 example, a maximum capillary pressure of 66. 5 cm of water was measured (Figure 36). According to equation 21, this capillary pressure corresponds to a pore diameter of 0. 04 mm. All pores with diameters less than 0. 04 mm will remain filled under a capillary pressure of 66. 5 cm of water. According to data in Table 8, approximately 76% of the pores are filled when capillary pressure ecuals 66. 5 cm of water. Similar calculations for the 60, 90, and 120 cm depths at their maximum recorded capillary pressures yielded 76%, 76%, and 78% of the pores remaining filled when the soil was at the lowest moisture content. The homogeneity of the pore-size distribution with depth is apparent here. Similar near equality of the percentage of pores remaining filled at other capillary pressures was also found for this soil. The samples from the other three soil pits also exhibited the ability to keep large percentages of their pores filled. Hence, the ability to transmit water and to rapidly equilibrate moisture content throughout the soil mass is maintained. This occurs even under capillary pressures under which, in theory, coarse soils should experience near cessation of flow. Considering the similarity in physical properties, especially X, it is assumed that both soil series possess this ability to transmit water rapidly in the unsaturated state. Therefore, few soil saturation events would be expected to occur in soils with these properties. As was observed, only one 176 storm produced saturation of the soil profile on the study area. Looking more closely at this one storm, one can see that the calculated soil water fluxes (q) for the study site appear reasonable in light of the January 11-16 storm data. Prior to the limited saturation observed on January 15, several days of continuous rainfall occurred. In fact, over 19 cm of rainfall were recorded prior to the climactic 15. 5 cm on January 14-15. At this point, soil water drainage was near steady-state with soil capillary pressures in the 5-10 cm of water range. Utilizing the calculated data in Table 9 for the lower half of the on site soil profile one can also assume that during this period, just prior to saturation, the outgoing flux would have been approximately 0. 43 cm hr. However, with the onset of the heaviest period of rainfall, 15.5 cm in 24 hours on January 14-15, the upper profile would have had a soil water flux nearly equal to the rainfall rate being experienced, 0.65 cm hr1. This 0. 65 cm hr1 represents an incoming flux to the lower half of the profile. If the two soil water fluxes were initially in a relatively steady- state, an excess of 0.22 cm hr1 would accumulate at the base of the lower profile zone. In other words, each cubic centimeter of soil had 0. 22 cm3 of excess water incoming to occupy any unfilled pore space. From the moisture characteristic curve for the 90-120 cm depth interval of the study site soil (Figure 49), one can see that the available empty pore space would have been only approximately 177 0. 10 cm3 per cubic centimeter of soil volume when P equals 10 cm of water. Therefore, 0. 12 cm3 of water per hour would have been available for saturating the soil and raising the phreatic surface. Adding water to the soil would have another effect on soil water movement. There would be an increase in the hydraulic conductivity. The result of such an increase would serve to restore the balance between incoming and outgoing fluxes, but not immediately. Assuming an 18-24 hour delay in the adjustment of moisture contents and the balancing of fluxes, the excess 0. 12 cm3 of water per hour would produce a saturated zone 2-3 cm thick. Inspection of Table 3 shows thatfor the few piezometers (piezometers 1, 2., 4, 5, 19, and 22) recording saturated zones in the soil during the January 11-16 storm, the thickness of the saturated zone varied somewhat from this predic- tion, but were within the same order of magnitude. Considering variations in topography, soil, geology, and rainfall possible on this relatively homogeneous soil and site, plus the known rapid rise of K(P) with increasing moisture content, these few piezometric values appear reasonably close to that predicted. Threshold Storm Considering the demonstrated sufficiency of the January 11-16 storm to create a shallow saturated soil zone and the similarity in soil properties of both soil series, can one infer that this storm is a 178 threshold storm, the storm necessary to induce the onset of soil saturation and possible mass wasting resulting from increased pore water pressures, for the Klickitat and Bohannon soils? Unfortunately, it does not appear so. Inspection of precipitation-frequency maps for maximum recorded 24 hour rainfall in this area of the Coast Range (Miller, Fredricks, and Tracy, 1973) revealed that the January 11-16 storm had a return interval of about five years. Yet, even this piece of data tells us little about what size of storm actually constitutes a threshold storm, except for this specific site and the time period atialyzed. Using the information in Table 11 atid precipitation-fre- queticy maps for the other three soil sites, widely differing estimates of the threshold storm size would be obtained. The exact period of most ititetise rainfall producing soil saturation, iti this case 24 hours, would merely be a final factor in a chain of events. Preceding it there is almost always a rather complicated interactioti among site conditions which set the stage for saturatioti. Recognizing the existence of such stage setting factors, it appears hazardous to set some storm size or return period as the threshold storm for these two soil series or for any other soil series. Perhaps a more useful indicator of what constitutes a threshold storm should be described in terms of a sequence of meterological events, rather than gross amounts of rainfall over some time period as is done now. The determinatior of such a sequence or sequences necessary to cause 179 soil saturation in a particular soil group would be a large step forward in determining the probability of occurrence of possible landslideproducing storms. But for this study, no estimate of threshold storm appears applicable to both soils investigated. Apparent Cohesion The apparent ability of both soil series to minimize soil saturation events has obvious beneficial effects on maintaining slope stability. Not only is there no reduction in effective stress (a-) due to excess pore water pressure (u), but also the slight capillary pressure present, even during most storm periods, adds to the shear strength. This is because capillary pressure is, in effect, a negative neutral stress. Recalling that the effective stress is definable as a- a- -u (equation 3), a negative neutral stress (-u) results in a- = a- + u. If, as has been indicated, the soil is relatively homogeneous and at a more or less even capillary pressure throughout the profile, this favorable addition to o- can be assumed equal at all points in the soil. Therefore, it increases the shearing resistance of the soil along any section. Because of capillary pressure, even perfectly cohesionless material, such as these two soils, may temporarily acquire the characteristics of cohesive materials. Because the cohesion of such soils disappears completely after saturation, it is often referred to as apparent cohesion. 180 With the help of equatioa 15, F-- [(dy) /cos 1 s )+tan tan 1' it is possible to determine the influence of apparent cohesion of the factor of safety (F) oa a slope having soils like those studied here. As purely illustrative examples, let us look at two cases where the slope is precariously balanced (Fl.0), unsaturated, but still quite moist. The depth of the soil (d) in each case will be 0.60 m and 1.50 m. Other assigned parameters to be used in each case are: C = 10 cm of water = 100. 1 kg/m2, 400 y 1602 = 400, kg/m3, where the apparent cohesion is represented by C in the otherwise cohesionless soil. For the 1.50 m depth of soil, F increases from 1.0 to 1.08, an 8% increase. For the 0.60 m depth, F increases by 21% to 1.21. These two examples illustrate that the beneficial increase in slope stability due to apparent cohesion varies with the depth for a given soil and slope. If the soil profile can maintain unsaturated conditions, the apparent cohesion may materially add to the factor of safety of a shallow soil. But, in all likelihood, it would be the shallow soil which would saturate first during a given storm, negating the large beneficia effects described above. Considering that most cohesionless 181 soils in the central Coast Range are on very steep slopes and are deeper than 0.60 m (Corliss, 1973), any increase in F would becloser to the lower F value of 1.08 than the F value of 1.21. From these two examples, it appears quite hazardous to include capillary-pres sure induced apparent cohesion in a factor of safety analysis for such slopes. Even if the slopes never saturate, an improbable situation over the long term, the transient arid variable nature of the apparent cohesion makes its inclusion in a factor of safety analysis imprudent. mi additioni, saturationi would only have to occur at the potential failure zone to negate any apparent cohesion. Omitting apparent cohesioni appears more prudent because any error induced mi F is on the side of increased safety. For slopes where 4)> 3, the effect of capillary pressure-induced apparent cohesioni is only of academic importance, mi that F is already in excess of unity mi these slopes. Influence of Bedrock The unusual characteristics of the two cohesioniless soils studied that allow them to maintain unsaturated conditions for even high rainfall periods may not be the only reasons for the rare occurrence of soil saturation. The hydraulically active sandstone found oni the study site tends to complicate any explanation of nionisaturation due solely to soil properties. Figures 32-34 show rapid response of the piezometric surfaces to precipitation arid indicate that flow within the rock 18a is a direct consequence of rainfall. The composite curvilinear regression for piezometers 45 and 48 (Figure 35) indicates that the observed rises in piezometric level were best correlated with the 48hour rainfall. Therefore, some means of direct hydraulic linkage between the rainfall and the water flowing in the sandstone most likely exists. Other areas in the Coast Range may also have such hydraulically active bedrock zones. Undoubtedly, the intrusion of the igneous sill and its subsequent cooling was accomparded by the formation of many cracks and fis- sures in both rock types. Weathering and solution of the rock types over time has probably widened such cracks. The sandstone, being less resistant than the igneous rock, has been more affected by such weathering and ended up possessing the conducting zone (Burroughs etal., 1973). Such zones may aid in removing much excess water that would normally have accumulated in the soil mantle. However, for soils such as the Klickitat, where capillary pressures have been shown to exist more often than not, the passage of water into large macropores would appear improbable, at least on the continuous basis observed on the study site. Capillarity would preclude this happening. The remote possibility does exist, however, that there are other pathways for soil water flow not connected to the micropore system. Whipkey (1965, 1967, 1968) and Aubertin (1971) detected such rapid flow through macropores, such as root channels arid other 183 structural openings in the soil matrix, while the soil was still in an unsaturated state as a whole. But the soils they studied were very clayey and this type of flow was attributed to surface funneling of water into these interconnected channels rather than to normal gradient-induced infiltration. For the cohesionless, highly permeable soils studied here, however, surface funneling into macropores appears highly unlikely, even though such macropores do exist in the soil. Mac ropores in the rock strata may be operative, however under certain conditions, and provide a pathway for the water detected in the sandstone. Relatively shallow soil profiles, as those observed here, are often located over several feet of saprolite. Therefore, from a hydrologic standpoint, the soil mantle, including the saprolite, may be quite deep whereas the soils themselves are pedologically shallow. Such a situation was also described for the western Cascade Mountains of Oregon (Rothacher etal., 1967). A water table only a few centimeters deep could exist within the saprolite or in weathered iearns of the parent material and not be detected. Such a saturated zone, even if localized, could funnel water into the cracks and fis- sures while almost the entire soil mantle remains in an unsaturated state. This situation may be a common occurrence (Megahan, 197 3), but is highly conjectural at this point because of the relatively undetermined role bedrock plays in watershed hydraulics. It is obvious 184 from this study, though, that an. assumption. of impermeable bedrock cannot be made in analyzing soil water movement in this area. The potential importance of bedrock in affecting local soil-water relation ships may be greater thani previously supposed, and further research on the role and importance of bedrock in the watershed system is suggested. 189 LITERATURE CITED Aubertin, G. M. 1971. Nature and extent of macropores in forest soils and their influence on subsurface water movement. Upper Darby, Pennsylvania. U. S. Forest Service. Northeast Forest Experiment Station. Research Paper NE-192. 33 p. Baldwin, E. M. 1955. Geology of the Marys Peak and Alsea Quadrangles, Oregon. Oil and gas investigations map OM-162. U. S. Geological Survey. 1 p. 1964. Geology of Oregon. 2nd ed. Eugene, Oregon, University of Oregon Press, 165 p. Bayer, L. D. 1935. Factors contributing to the genesis of soil microstructure. American Soil Survey Association Bulletin 16:55-56. Bernatzik, W. 1948. The extreme inclination of slopes with groundwater passinlg through them. Proceeding of the 2nd International Conference on Soil Mechanics, Rotterdam, 5:19-20. Bishop, A. W. 1961. Soil properties and their measurement (Discussion). Proceedings of the 5th International Conference on Soil Mechanics, Paris 3:97-100. Bishop, A. W. and D. J. Henkel. 1962. The Measurement of Soil Properties in the Triaxial Test. London, England, Arnold. 227 p. Bishop, A. W. and L. Bjerrum. 1960. The relevance of the Triaxial Test to the solution of stability problems. In: Research Conference on Shear Strength of Cohesive Soils. ASCE, Soil Mechanics Division (Boulder, Colorado). p. 437-501. Brooks, R. H. and A. T. Corey. 1964. Hydraulic properties of porous media and their relationship to drainage design. Transaction, American Society of Agricultural Engineers, 7(l):26-28. Bryant, 3. C., T. W. Bendixen, and C. S. Slater. 1948. Measurement of the water-stability of soils. Soil Science 65:341-345. Burroughs, E. R., G. R. Chalfant, and M. A. Townsend. 1973. Guide to reduce road failures in western Oregon. Portland, Oregoti. Bureau of Land Management. 111 p. 190 Cedegreri, H. R. 1968. Seepage, Drairiage, arid Flow Nets. New York, Johri Wiley. 489 p. strength Cheri, L. 5. 1948. Ari irivestigatiori of stress -strairi arid compression characteristics of cohesioriless soils by triaxial Soil tests. Proceedirigs of the 2nd Initerriatiorial Corifererice ori MechariicS, Rotterdam 5:35-43. 1957. Soil aggregation Chesters, G., 0. J. Attoe, arid 0. N. Alleri. Soil Scierice Society of Amen- in relatiori to soil coristituerits. cari Proceedirigs 21:272 -277. Flow in Porous Media. Fort Colliris, Colorado, Colorado State Uriiversity. 259 p. Alsea Area, Corliss, J. F. 1964. Soil arid vegetation survey, 730 numb. leaves. Oregori. 2nddraftCorvailis, Oregon. Corey, A. T. 1969. Soil Survey of Alsea Area, Oregori. 82 p. (U. S. Dept. of Agriculture, Soil Conservatiotl Service arid Forest Service Soil Survey). des Rigles des Coulomb, C. A. 1776. Essai Sur urie Application Relatifs a ltArchi Maximis a quelques Problimes de Statique Sciences. 7:343-382 tecture. Memoris del' Acadamie des Geology arid Slope Failure (Cited in: Swaristoni, D. N. 1967. Alaska. Ph. D. iri the Maybeso Valley, Prince of Wales Islarid,206 riumb. leaves) Thesis. Larisirig, Michigan State University. particle-size arialysis. Day, P. R. 1965. Particle fractioriation arid arid MineralogiIn: Methods of Soil Arialysis, Part 1, Physical riumber iri the series cal Properties, ed. C. A. Black etal., Society9 of Agroriomy. Agroriomy, Madisori, Wiscorisiri, Americari 1973. p. 545-566. Slope Stability in residual Deere, D. U. arid F. D. Pattori. 1971. Pan_Americari Conlererice ori soils. Proceedirigs of the 4th Soil Mechariics arid Foundations, Caracas. p. 87-171. 1964. Role of Deshpande, T L., D. J. Greeriland, and J. P. Quirk. iron oxides in the boriding of soil particles. Nature 201:107108. with Changes iri soil properties associated -__- the remcr7al 1968. of iron and aluminum oxides. Soil Science 19:108122. 191 Dyrriess, C. T. 1967. Mass movements on the H. J. Aridrews Experimerital Forest. Portlarid, Oregon. U. S. Forest Service. Pacific Northwest Forest and Rarige Experiment Station Research Paper PNW-42. 12 p. Ellison, E. arid J. E. Coaldrake. 1954. Soil maritle movemerit iri relation to forest clearirig iti southeastern Queensland. Ecology 35:380- 388. Emerson, W. W. 1959. The structure of soil crumbs. Soil Scierice 10:235. Emersori W. W. arid G. M. F. Gruridy. 1954. The effect of rate of wettirig on water uptake and coehsion of soil crumbs. Journal of Agricultural Scierice 44:249-253. Frariklin, J. F. arid C. T. Dyrriess. 1969. Vegetatiori of Oregon and Washington. Portland, Oregon. U. S. Forest Service. Pacific Northwest Forest and Range Expe rimerit Statiori. Research Paper PNW - 80. 216 p. Fredriksen, R. L. 1970 Erosion arid sedimeritatiori following road constru,ction and timber harvestirig ari uristable soils iri three small westerri Oregori watersheds. Portlarid, Oregon. U. S. Forest Service Pacific Northwest Forest arid Range Experiment Statiori. Research Paper PNW-104. 15 p. Freeze, F. Elementary Statistical Methods for Foresters. Washington, D. C., Agricultural Handbook 317. U. S. Dept. of Agriculture. 87 p. 1967. Greenland, D. J., G. R. Liridstrom, arid J. P. Quirk. 1962. Or- gariic materials which stabilize natural soil aggregates. Soil Science Society of America Proceedirigs 26:336 -371. Haefeli, R. 1948. The stability of slopes acted upori by parallel seepage. Proceedirigs of the Zrid triterriational Conference ori Soil Mechatics, Rotterdam 6:57-62. Hardy, W. B. and J. K. Hardy. 1919. Note on static friction arid on the lubricating properties of c ertairi substarices. Philos ophical Magazine, Series 6, 38:32-48. 1 9Z Harr, R. D. 1974. Movement of subsurface water on a steep forested slope. Paper presented before the American Geophysical Union, Zlst Pacific Northwest Regioial Meeting. Richlarid, Washington. Oct. 17-18. Heymari, J. 197Z. Coulomb's Memoir on Statics: An Essay in the History of Civil Engineering. Cambridge, England. Cambridge University Press. 211 p. Hillel, D. 1971. Soil and Water Physical Principles and Processes. New York, Academic Press. 287 p. Holtz, W. G. 1969. Soil as an engineering material. Washington, D. C., U. S. Dept. of Interior, Bureau of Reclamation. Water Resources Technical Publication No. 17. 45 p. Hubert, M. K. 1956. Darcy's law and the field equations of the flow of underground fluids. American Institute of Mining Methods and Petroleum Engineering Transactions 207:222-239. Hvorslev, J. M. 1949. Subsurface exploration and sampling of soils for civil engineering purposes. Vicksburg, Mississippi, Waterways Experiment Station. 465 p. Kemper, W. D. 1965. Aggregate stability. In: Methods of Soil Analysis, Part 1, Physical and Minerological Properties, ed. C. A. Black etal., number 9 in the series Agronomy, Madison, Wisconsin, American Society of Agronomy. p. 512-519. Kemper, W. D. and W. S. Chepil. 1965. Size distribution of aggregates. In: Methods of Soil Analysis, Part 1, Physical and Minerological Properties, ed. by C. A. Black etal., number 9 in the series Agronomy, Madison, Wisconsin, American Society of Agronomy. p. 449-5 10. Kemper, W. D. and E. J. Koch.. 1966. Aggregate stability of soils from the western portion of the United States and Canada. Washington, D. C. 52 p. (U. S. Dept. of Agriculture. Technical bulletin 1355). Kroth, F. M. and J. B. Page. 1947. Aggregate formation in soils with specia reference to cementing substances. Soil Science Society of America Proceedings 11:27-34. 193 Permeability calculated from desaturation data. ASCE, Journal of Irrigation and Drainage, IR 1: 55843, p. 5 7-71. Laliberte, G. E., R. H. Brooks, and A. T. Corey. 1968. Lambe, T. W. 1951. Soil Testing for Engineers. New York, John Wiley, 165 p. Silicate of soda as a soil stabilizing agent. American Society of Agronomy Journal ZO: Laws, W. D. arid J. P. Page. 1947. 921-941. Lovell, J. P. B. 1969. Tyee formation: Underformed turbidites and their lateral equivalents: Minerology and paleogeography. Geological Society of America Bulletin 80(l):9 -22. The relation of free iron in the soil to aggregation. Soil Science of America Proceedings 1:43-45. Lutz, J. F. 1936. Lutz, H. and R. F. Chandler. 1961. Forest Soils, New York, John Wiley. 514 p. Macalla, T. M. 1942. lafluence of biological products on soil structure and infiltration. Soil Science Society of America Proceedings 7:209-214. Martin, J. P. 1945. Micro-organisms and soil aggregation, part I. Soil Science 59:163-174. Mazurak, A. P. 1950. Aggregation of clay separates from bentonite, kaolinite, and a hydrous mica soil. Soil Science Society of America Proceedings 15:18-24. McIntyre, D. 5. 1956. The effect of free ferric oxide on the structure of some terra rosa and rendzina soils. Soil Science 7: 302-306. Megahan, W. F. 1973. Role of bedrock in watershed management. ASCE, Proceedings of the Irrigation and Drainage Division Speciality Conference at Fort Collins, Colorado, April 22-24: 449-470. Precipitation-freuency atlas for the western United States, V. 10, Oregon. Washington, D. C., U. S. Dept. of Commerce, Miller, J. F. , R. H. Fredricks, and R. J. Tracey. NOAA. 43 p. 1973. 194 Mohr, 0. 1871. Beitr.ge zur Theorie des Eddruckes: Z. arch. U. Ing. Ver. Hannover, V. 17, l.344andv. 18, pp. 67, 245. (Cited in: Swanston, D. N. 1967. Geology and Slope Failure in the Maybeso Valley, Prince of Wales Island, Alaska. Ph. D. Thesis, Lansing, Michigan State University. 206 numb, leaves.) Paeth, R. C. 1970. Genetic and Stability Relationships of Four Western Cascade Soils. Ph.D. thesis. Corvallis, Oregon State University. 126 numb, leaves. Panabokke, C. R. and J. P. Quirk. 1957. Effect of initial water content on soil aggregates in water. Soil Science 83:185-195. Quirk, J. P. The measurement of stability of soil microaggregates in water. Australian Journal of Agricultural 1950. Research 1:276-284. Rankin, D. W. 1974. Hydrologic Properties of Soil and Subsoil on a Steep, Forested Slope. Master's thesis. Corvallis, Oregon State University. 117 numb, leaves. Richards, L. A. 1931. Capillary conduction of liquids in porous mediums. Physics 1:318-333. Roberts, A. E. 1953. A petrographic study of the intrusive at Marys Peak, Benton County, Oregon. Northwest Science, v. 27(2):43- 60. Rojanasoonthon, 5. 1963. State of Weathering on Some Upland Soils of the Alsea Basin, Oregon. Master's thesis. Corvallis, Oregon State University. 98 numb, leaves. Rothacher, J., C. T. Dyrness, and R. L. Fredriksen, 1967. Hydro- logic and related characteristics of three small watersheds in the Oregon Cascades. Portland, Oregon. U. S. Forest Service. Pacific Northwest Forest and Range Experiment Station. 54 p. Saini, G. R., A. A. MacLean, and J. J. Doyle. 1966. The influence of some physical and chemical properties on soil aggregation and respcnses to VAMA. Canadian Journal of Soil Science 14: 267- Z 81. Scholl, D. G. and A, R. Hibbert, 1973, Unsaturated flow properties used to predict outflow and evapotranspiration from a sloping ysirneter. Water Resources Research 9:1645-1655. 195 Skempton, A. W. and A. W. Bishop. 1950. The measurement of the shear strength of soils. Geotechnique 2:90-108. Snavley, P. D., H. C. Wagner, and N. S. Macleod. 1964. Rhythmicbedded eugeosyncline deposits of the Tyee formation, Oregon Coast Range. In: Symposium of Cyclic Sedimentation, ed. D. F. Merriam, Kansas Geological Survey Bulletin 169. 636 p. Sowers, G. B. and G. F. Sowers. 1970. Introductory Soil Mechanics and Foundations. 3rd ed. New York, MacMillan. 556 p. Sowers, G. F. 1963. Strength test of soils. In: Laboratory Shear Testing of Soils, ASTM STP 361. p. 3-31. Swanston, D. N. 1967. Soil-water piezometry in a southeast Alaska landslide area. Portland, Oregon. U. S. Forest Service. Pacific Northwest Forest and Range Experiment Station. Research Paper PNW-68. 17 p. Taylor, D. W. 1948. Fundamentals of Soil Mechanics. New York, John Wiley. 700 p. Terzaghi, K. 1936. The shearing resistance of saturated soils and the angle between planes of shear. Proceedings of the lst International Conference on Soil Mechanics, 1:54. 1960. Mechanisms of landslides. In: From Theory to Practice in Soil Mechanics, eds. L. Bjerrum, A. Gas sag rande, R. B. Peck, and A. W. Skempton. New York, John Wiley, p. 202-245. Terzaghi, K. and R. B. Peck. 1967. Soil Mechanics in Engineering Practice. 2nd ed. New York, John Wiley, 729 p. U. S. Soil Conservation Service. 1960. Soil classification, a comprehensive system, 7th approximation. Washington, D. C. U. S. Dept. of Agriculture. 265 p. 1967. Supplement to soil classification system (7th approximation). U. S. Dept. of Agriculture. 207 p. Varnes, D. J. 1958. Landslide types and processes. In: Landslides and Engineering Practice. Washington, D. C. Highway Research Board Special Report 29. p. 20-47. 196 Weldon, T. A. and J. C. Hide. 1942. Some physical properties of soil organic matter and of sesquioxides associated with aggregation in soils. Soil Science 54:343-351. Weyman, D. R. 1973. Measurements of the downslope flow of water in a soil. Journal of Hydrology 20:267-268. Whipkey, R. Z. 1965. Subsurface stormllow from forested slopes. Bulletin of the International Association of Scientific Hydrology l0(2):74-85. flow. 1967. Theory and mechanics of subsurface stormIn: International Symposium on Forest Hydrology, ed. W. E. Sopper and H. W. Lull. New York, Pergamon Press. p. 255-258. 1968. Storm runoff from forested catchments by subsurface routes. In: Floods and Their Computation: Proceedings, of the Leningrad Symposium. Leningrad, U. S. S. R., August 1967. Volume 2 International Association of Scientific Hydrology. p. 773-779. Wooldridge, D. M. 1964. Effects of parent material and vegetation on properties related to soil erosion in central Washington. Soil Science Society of America Proceedings 28:430-432. Wu, T. H. Relative density and shear strength of sands. ASCE Journal of Soil Mechanics 83:no. SM1, Paper No. 1161, 23 p. 1970. Soil Mechanics. Boston, Allyn and Bacon. 431 p. Youngberg, C. T. 1957. A modification of the hydrometer method of mechanical analysis for certain western forest soils. Soil Science Society of America Proceedings 21:655-656. APPENDICES 197 APPENDIX A COMMON AND SCIENTIFIC NAMES OF SPECIES FOUND ON STUDY SITE Common Name (Franklin arid Dyrniess, 1969) Scientific Name Overstory Trees Tsuga heterophylla (Raf.) Sarg. western hemlock Douglas-fir Pseudotsuga meniziesii (Mirb. ) Franco. red alder Alnius rubra Bong. salal Shrubs arid Small Trees Gaui.theriá shalloni, Pursh. bag-leaved Oregon-grape Berbis niervosa, Pursh. vine maple Acer circinatum, Pursh. red huckleberry Vacciniium parvifolium, Smith western red cedar Thu.ja plicata, Donin. little wild rose Rosa gymnocarpa, Nutt. thimbleberry Rubus parviflorous, Nu.tt. sword -ferrL Herbaceous Species Polystichum muniitum (Kaulf.) Presi. Oregon oxalis Oxalis oregaria, Nutt. western gold-thread Coptis lacinata, Gray western trillium Trillium ovatum, Pursh. fragrant bedstraw Galium trilorum, Micbx. grasses Graminae family 198 APPENDIX B Table I. Saturation values for soils at 0-100 cm of water capiUary pressure. Capillary pressure (cm of waters Depth Interval (cm) 0 5 100 20 30 40 50 0.88 0.85 0.83 0.86 0.83 0.80 0.83 0.81 0.78 0.80 0.79 0.76 0.78 0.77 0.75 0.73 0.72 0.74 0.83 0.70 0.76 0.77 0.64 0.73 0.74 0.60 0.70 0.73 0.59 0.70 0.73 0.54 0.94 0.85 0.88 0.88 0.93 0.91 0.83 0.87 0.87 0.92. 0.90 0.83 0.87 0.86 0.91.0.90 0.81 0.85 0.85 0.90 0.90 0.77 0.83 0.83 0.88 0.88 0.75 0.92 0.65 0.83 0.61 0.79 0.58 0.76 0.57 0.75 0.51 0.70 10 Klickitat Soil Series (soil pit 2, on site) 30-60 60-90 90-120 1.00 1.00 1.00 0.94 0.90 0.87 Klickitat Soil Series (off Site) 0-30 30-60 60-90 1.00 1.00 1.00 0.92 0.93 0.89 0.84 0.83 0.65 0.66 Bohannon Soil Series (soil pit A) 0-60 60-90 90-120 120-150 150-210 1.00 1.00 1.00 1.00 1.00 0.95 0.96 0.95 0.98 0.97 0.91 0.93 0.92 0.95 Bohannon Soil Series (soil pit B) 0-30 30-60 1.00 1.00 0.83 0.96 199 Table II. Soil moisture, percent of total volume (&), for increasing capillary pressure. Depth Interval (cm) Capillary pressure (cm of water) 0 10 20 30 40 50 100 63.2 60.6 62.4 57.9 57.4 59.0 55.4 56.0 57.6 53.8 54.2 55.6 52. 9 50.8 52.0 53.4 46. 2 61.0 54.8 52.7 55.0 49.2 48.5 46.5 44.8 45.0 42.5 42.5 43.7 4.0.0 41.2 42.7 39.0 40.8 42.6 36.0 37.5 40.3 53.0 52.8 51.5 49.8 48.8 50.0 50.5 49.8 48.8 47.5 49.0 49.2 48.8 48.2 47.0 48.3 48.8 48.5 48.2 47.0 47.3 48.0 47.8 47.6 46.5 44.5 45.5 45.1 46.2 46.0 48.2 51.0 42.4 46.2 40.0 43.6 38.4 42.0 37.4 41.2 35.0 38.2 5 Klickitat soil, soil pit 2, oa site 30-60 60-90 90-120 72. 1 69.6 69.4 53.0 54.5 45.0 47.0 Klickitat soil, off site 0-30 30-60 60-90 66.5 58.5 58.8 Bohannon soil, soil pit A, off site 0-60 60-90 90-120 120-150 150-210 58.7 56.5 56.5 51.8 51.5 55.3 54.0 53.5 51.0 50.2 Bohannon soil, soil pit B,off site 0-30 30-60 61.4 56.4 52.6 54.0 200 APPENDIX C REPRESENTATIVE SOIL PROFILE DESCRIPTIONS Unit: Klickitat Soil Series (on site) Parent Material: Granophyric gabbro Landform: Colluvial slope Slope: 70%, north-northwest aspect Erosion: past slumping, slight raveling Drainage: Well-drained Vegetation: Swordfern community Elevation: 2200-2500 ft. Horizon Depth Desc riptioni (cm) 0.I 10-0 A1 0-23 Litter, leaves, twigs from Douglas-fir, western hemlock, swordfern, etc. Very dark brown (1OYR 3/3 moist), heavy silty sand/gravel loam, very granular, very friable, slightly plastic; numerous fine roots; approx. 10% 1-2 inch rock cobbles; 10-15% charcoal bits; clear wavy boundary 2-3 inches 2 3-46 thick. Dark brown (1OYR 3/4 moist), very sandy/ gravelly loam, granular to subangular blocky structure, weak, slightly to non-plastic; abundant roots, 1-2 inch gravel and small cobbles; B2 46-122 C IZZ-ZlOf weak color change at boundary. Dark brown (1OYR4/3 moist), sandy loam, granular to subangular blocky, slightly to nonplastic; marty roots; 20-30% small cobbles, many angular and relatively unweathered, both igneous arid sandstone included; clear wavy boundary. Strong brown (5YR 4/4 moist), very rocky loam, silty to very coarse sand, massive friable, slightly more plastic than above horizons; well weathered igneous and sandstone saprolite. 201 REPRESENTATIVE SOIL PROFILE DESCRIPTIONS (corit.) Unit: Klickitat Soil Series (off site) S.27, T.145.,R.7W., W.M. Parent Material: Granophyric gabbro Landlorm: Colluvial sideslope Slope: 45%, south_southeast aspect Erosion: None apparent, other than slight raveling Drainage: Well-drained Vegetation: Clear cut area Elevation: 1800 ft. Horizon Depth (cm) 4-0 A1 0-20 B1 20-45 Des c r ipt ion Litter from salal, ferns, Oregon-grape, and past logging. Dark brown and very dark brown (7. 5YR 3/3 moist); very gravelly loam, strong very fine granular structure; friable, slightly sticky; abundant roots; many fine and very fine interstitial pores; 10-15% charcoal bits; many large rock fragments; gradual wavy boundary. Dark reddish brown (5YR 3/3 moist), very gravelly loam; moderate to weak very fine subangular blocky to granular structure; friable, slightly to non-plastic; many roots; B2 45-94 wavy boundary. Dark reddish brown (5YR 3/4 moist), very gravelly loam; moderate to very fine subangular blocky to granular structure; friable; non-plastic; many roots; many interstitial pores fine to tubular; many cobbles and flags, wavy C 94-1CC boundary. Strong brown (7. 5YR 4/6 moist), very flaggy loam; massive, friable, slightly plastic; fewer roots; common, fine pores; grades into saprolite. 202 REPRESENTATIVE SOIL PROFILE DESCRIPTIONS (corit.) Unit: Bohannon Soil Series, Soil Pit A SE 1/4,5.26, T.135.,R.9W.,W.M. Landlorm Cutbank at base Parent Material: Tyee sandstone of 65 orig slope, Slope: 65%, west_northwest aspect Erosion: none apparent Drainage: Well-drained Vegetation: Alder, hemlock, Douglas-fir Elevation: 400 ft. Horizon A1 A3 Depth (cm) Description 10-0 Litter from alder, Douglas-fir, arid other 0-8 vegetation. 1OYR (3/3) graveUy loam; fine granular struc- 8-30 small pebbles and coricretiot1S; abrupt wavy boundary. 1OYR (3/3) gravelly loam; subanigularbocky 30-50 33 50-165 C 165-200'- ture; friab'e, non-plastic many roots; many structure; friable, noti-pastic; 20% rotten sandstone cobbles and flags; many roots; gradual boundary into B horizon. Dark brown (7. 5YR 4/4 moist) gravelly loam; subaniguar to blocky structure; friable, slightly plastic; trending to yellowish color; 20% rotten sandstonie gradual boundary. 1OYR (5/4 moist) gravelly loam; yellowish brown co'or; subangular blocky structure; slightly plastic; hard to friable; roots throughout; 20-25% rotten sandstone. Fractured_rotten arkosic sandstone; 40% 33 in fractures of this horizon. 203 REPRESENTATIVE SOIL PROFILE DESCRIPTIONS (cont.) Unit: Bohannon Soil Series, Soil Pit B Cl/4, S.4, T. 15S., R. 8W. , W. M. Parent Material: Tyee sandstone Landlorm: Colluvial slope Slope: 55%, east-southeast aspect Erosion: none apparent Drainage: Well-drained Vegetation: Sec ond-growth Douglas-fir and hemlock Elevation: 1200 ft. Horizon 01 8-0 A1 0-8 A3 Description Depth (cm) 8-25 Litter layer from Douglas-fir and hemlock, plus ground cover. Dark brown (1OYR 3/4) gravelly loam; granular structure; friable, non-plastic; many fine roots; 10-20% small pebbles and concretions; very soft sandstone rocks, well weathered; abrupt, smooth boundary. 1OYR (3/3 moist) gravelly loam; granular, friable structure; subangular to blocky structure; 10-20% rotten sandstone stones; clear wavy boundary. 3 25-56 C 56+ 7. 5YR (3/4 moist) gravelly loam; trending to yellowish-brown color; weak subangular blocky structure; friable; many roots; many wellweathered sandstone flags. Well-weathered saprolite, very yellowish color; slightly plastic; fewer roots. 59. 3 43. 6 86.8 31.5 Feb. 35. 1 March 20.6 25. 2 96. 3 20.5 18.6- 23.6102. 2 10.683. 2 17. 6 11.086.8 16.3 66.4 6.3- 47.9 1953-1973. 167.5-322.6 142 47.0 257.2-416.9 155 130 154 133 157 202 +136.6 +132. 1 +11. S +20. 3 +18.8 463.0 34.1 +32. 3 +49.0 34.0 326. 4 57.0 11.4 371.5 239.4 Valsetz. Oregon 12.7 4.951.4 50.8 39.0 48. 3 9.663.8 58. 1 37. 9 89. 3 97.3 18.9 14.468.0 76.0 57.2 17.3 16.982.0 13.2 10.558.8 147 183.9-307.4 163 143 159 138 136 251 137.2-251.9 +116.2 +115.4 +13.3 +17.8 31. 3 30. 2 43. 2 +16.5 246.0 182.9 +15.6 362. 1 298. 3 44.6 280.2 48.0 235.5 59.7 Alsea Fish Hatchery, Oregon 58.2 Jan. For the Water Year +52.2 34. 7 45.2 65.5 Dec. For the Period Source U.S. Dept. of Commerce (N.O.A.A. Environmental Data Service). Climatological Data -Oregon. 1953-73 standard deviatiOn (cm) 1973-74 rainfall (cm) 1953-73 mean rainfall (cm) 1973-74 difference from mean rainfall (cm) 1973-74 % over mean rainfall 1953-73 range (cm) 1953-73 standard deviatiOn(cm) from mean rainfall (cm) 1973-74 % over mean rainfall 1953-73 range (cm) 1973-74 differenCe 1973-74 rainfall (cm) 1953-73 mean rainfall (cm) 1973-74 raiiaU (cm) for study sit. Nov. Month of SUMMARY OF PRECIPiTATION STATISTICS FOR SELECTED STATIONS IN THE CENTRAL COAST RANGE OF OREGON APPENDIX D

Forest Ergineering (Hydrology) presented on CARLTON STRATTON YEEforthedegreeof (Major Department)

Related documents

Products

Support

Forest Ergineering (Hydrology) presented on CARLTON STRATTON YEEforthedegreeof (Major Department)

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib