Technical Note Engineering Soils Maps PAUL M. SANTI Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401 JAMIE L. MARTENS URS Corporation, 10975 El Monte, Suite 100, Overland Park, KS 66211 Key Terms: Engineering Properties, Soils, Hazards, Mapping INTRODUCTION For many applications, ‘engineering soils maps’ may be preferable to comprehensive engineering geology maps. Engineering soils maps, which focus on soil thickness, soil type, and soil engineering properties, may be more expedient to develop than maps that include details on rock properties, geomorphology, and geologic hazards. These details are a critical component of the input offered by engineering geologists, but for many smaller sites, they may be summarized as caveats in the text while the map concentrates on issues of more immediate application to engineering design. A technique to prepare engineering soils maps has been developed that is visually simple and uncluttered and that uses an extensive, standardized database. In this technique, rose-pie charts provide an overall assessment of soils hazards and capabilities. Data and base maps come directly from National Resources Conservation Service (NRCS, formerly U.S. Department of Agriculture [USDA]) soil surveys. As many as three levels of six different hazards or engineering characteristics may be shown simultaneously, in addition to shorthand notation that indicates soil thickness and land capability class. The suggested map scales may be appropriate for developments; linear structures such as pipelines, power lines, and roads; and regional evaluations of sources of aggregate, clay, or sand. The authors have successfully used this technique for several years as a teaching tool for undergraduates (Santi and Gregg, 1999). It should be recognized that the NRCS maps were developed as a tool for agronomists, rather than geotechnical engineers or engineering geologists. Their use is therefore most beneficial as a tool to plan geotechnical investigations, not to replace them. Engineering geologists should keep in mind several other precautions when translating NRCS soil survey data into geotechnical data: The maps were compiled on the basis of air photo interpretation, followed by field checking. As a result, individual map units may have a percentage of their area consisting of soils other than the named deposit. Units are identified on the basis of composition rather than genesis. As a result, an individual soil unit may consist of a durable, stable parent formation as well as its eroded and re-deposited residuum, which can have vastly different engineering properties. Many of the enumerated capability limitations and hazards can be mitigated by appropriate geotechnical designs. EXAMPLES OF PREVIOUS WORK Engineering soils maps in various configurations are not uncommon, although they are not usually referred to by that name. They are often developed by subdividing and adding detail to the typical ‘Quaternary Alluvium’ (Qal) category from geologic maps. Some methods provide a substantial level of useful engineering information and are quite successful at delineating various alluvial, colluvial, residual, and other soil deposits. A few of these include Genesis-Lithology-Qualifier mapping (Keaton, 1984), a recent system developed by Moore and others (1999) to delineate and provide thorough descriptions of various surficial units, and two Canadian systems: the British Columbia Environmental Land Use Committee (ELUC) Terrain Classification System and the Northern Ontario Engineering Geology Terrain Study (Gartner, 1984). The U.S. Geological Survey has produced numerous engineering geologic maps that sub-divide soil units and discuss the expected engineering behavior of each unit (examples of such maps are Miller and Bryant, 1976; Lemke and Maughan, 1977; Berg et al., 2000; and SCAMP, 2000). In addition to sub-dividing soils deposits, some maps also display engineering properties Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183 179 Santi and Martens Table 1. Summary and method of indicating engineering information available in NRCS (USDA) soil surverys. Hazard or Engineering Property Slope Flooding Low strength Shrink-swell Stony soil Depth to rock Permeability Roadfill quality Topsoil quality Construction sand quality Construction gravel quality Soil thickness Land capability class Liquid limit, plasticity index Method of Data Summary in Engineering Soils Maps Location of information in Soil Survey Table 11: Building Site Development Table 11: Building Site Development Table 17: Soil Water Features Table 11: Building Site Development Table 11: Building Site Development Table 16: Physical and Chemical Properties of the Soils Table 11: Building Site Development Table 11: Building Site Development Table 16: Physical and Chemical Properties of the Soils Table 13: Construction Materials (see soil survey text for specific properties) Table 13: Construction Materials (see soil survey text for specific properties) Table 13: Construction Materials Open Pie Slice Entirely Filled Slice Low slope Moderate Severe Hazard is absent Moderate Severe Hazard is absent — Severe Hazard is absent Moderate Severe Not stony Moderate Severe Hazard is absent — Severe (,60 in. of soil) ,0.06 in./h Good 0.06–0.2 in./h or 0.2–0.6 in./h Fair 0.6–2.0 in./h or 2.0–6.0 in./h Poor Good Fair Poor Acceptable — Excess fines Table 13: Construction Materials Acceptable — Excess fines Table 15: Engineering Index Properties Table 17: Soil and Water Features Table 6: Categories are summarized in soil survey text section entitled ‘‘Use and Management of the Soils’’ Table 15: Engineering Index Properties When ‘‘depth to unweathered bedrock’’ is noted, use this value as a minimum value of soil thickness (e.g., 35 in.). If not given, express soil thickness as greater than the deepest unit described (e.g., .65 in.). e ¼ erosion; w ¼ water; s ¼ shallow, droughty, or stony soil; I ¼ few limitations; II ¼ moderate limitations; III–VIII ¼ severe limitations (e.g., IIe or VIw) Give ranges for either: thickest layer or layer with worst properties. on the map (Mulder and Hillen, 1990; Dearman, 1991). Rockaway and Lutzen (1970) and Lutzen and Rockaway (1971) use three-dimensional block diagrams, similar to those included in soil surveys, to show the geomorphic relationships between the engineering geologic units delineated on their maps. Thomas (1991) uses pie-chart graphs to illustrate relative gravel, sand, and silt/clay content across a site, similar to the use of pie charts in this study. He suggests that other engineering parameters could be similarly graphed (and displayed in cross-section format), including strength, plasticity, permeability, or Standard Penetration Test (SPT) friction ratio. None of these methods makes extensive use of USDA Soil Surveys, which contain a wealth of engineering data, especially for the upper few feet of soil and sediment units. Soil surveys also contain 1:24,000 air photo overlay maps on which various soil types are delineated. 180 Half-Filled Slice An overview of the type of engineering information available in soil surveys and potential applications to geotechnical studies is provided in Hasan (1994). Prokopovich (1984) illustrates the grouping of soil survey units to delineate geologic and geomorphic units such as ‘‘Holocene Deltaic Alluvium,’’ ‘‘Intermediate Flood-Plain Terrace,’’ and ‘‘Recent or Historic Lake Beds.’’ Bergstrom and others (1976) include summary tables of soil survey information and use this information to develop geology maps for construction and waste disposal. ENGINEERING SOILS MAPS The purpose of an engineering soils map is to extract generalized engineering data from an NRCS soil survey and display it such that a large area can be judged for hazards or engineering properties. Such maps can be used to: Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183 Engineering Soils Maps Plan a geotechnical exploration program for a proposed project Select zones within a larger area that minimize one or several hazards Identify zones with particular desirable characteristics, such as thick soils or low-permeability soils Group materials by land capability class or hazard type. Many natural hazards, such as sinkholes, faults, or landslides, are not shown on soil survey maps and would not be adequately recognized in an engineering soils map. Table 1 summarizes the various sources of engineering information in typical soil surveys, as well as how this information may be consolidated into simple ratings. Recent soil surveys share a common list of tables and common text layout, so the location of the information noted in Table 1 may be consistently applied. For example, information on slope steepness is expressed in several tables within the soil survey, namely tables 9, 11, 12, and 14. In the interest of consistency and referring to the fewest number of tables possible, the authors suggest obtaining slope information exclusively from soil survey table 11. The soil survey suggests that slope hazards are either ‘moderate’ or ‘severe.’ A third category, that of ‘absent,’ is implicit when slope hazards are not noted in the soil survey. Figure 1. Proposed method of displaying engineering soils information from NRCS soil surveys. Two or three hazard levels may be simultaneously displayed for up to six hazards. In each case, the hazard or engineering property is displayed so that higher hazard levels correspond to filled pie-chart slices. The example chart describes a 35in.-thick soil with severe erosion limitations, severe depth to rock (shallow rock) and low strength conditions, and moderate slope and shrink-swell conditions. Figure 2. Example engineering soils map of a portion of Iron County, Missouri. Base map is from USDA (1991). Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183 181 Santi and Martens Figure 3. Example use of a block diagram from a soil survey (USDA, 1991) to show both geomorphic relationships and engineering characteristics of soil units. Figure 1 details proposed methods of displaying engineering soils information, using rose-pie charts. These charts display up to six simultaneous hazards or engineering properties at two (present/not present) or three (high/moderate/low or absent) levels. For example, the slope hazard may be shown as absent (pie slice is open), moderate (slice is half filled) or severe (slice is entirely filled). Each hazard or engineering property is displayed such that higher hazard levels correspond to filled pie-chart slices. Any combination of categories may be displayed within the charts, depending on the intended use of the map, but all are developed from the same rose-pie format, so only an abbreviated key is necessary to interpret the maps. Because engineering soils maps created directly on the 1:24,000-scale orthophoto maps included in soil surveys may be cluttered, the authors recommend mapping at 1:12,000 scale or larger. The inherent inaccuracy of these maps is exaggerated when expanding the base map, so 182 contacts between units should be considered approximate, at best. Alternatively, the 1:24,000-scale base map could be retained, and similar units could be combined or inset maps used to reduce clutter. Figure 2 is an example engineering soils map using data exclusively from a published soil survey (USDA, 1991), representing a site in eastern Missouri. Using Figure 2, one may quickly identify areas susceptible to flooding, areas with particularly thin or thick soils, or areas underlain by expansive soils. At the scale shown (originally created at 1:12,000), the area covered is roughly 3 km by 1.5 km, which may be appropriate for planning investigations for housing developments; linear structures such as pipelines, power lines, and roads; and regional evaluations of sources of aggregate, clay, or sand. Figure 3 is an example use of a block diagram contained in a soil survey (USDA, 1991) to show both geomorphic relationships and engineering characteristics of soil units. In this example, the same rose-pie charts Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183 Engineering Soils Maps used in Figures 1 and 2 are overlain on the block diagram. CONCLUSIONS The purpose of an engineering soils map is to extract preliminary engineering data from an NRCS (USDA) soil survey and display it such that a large area can be judged for hazards or engineering properties. The NRCS soil surveys comprise an extensive, standardized database and include 1:24,000 air photo overlay maps on which various soil types are delineated. Rose-pie charts may be developed from this database to display up to three levels of six hazards simultaneously. With some loss of accuracy, the map scale may be doubled to 1:12,000, to cover an area that may be appropriate for developments; linear structures such as pipelines, power lines, and roads; and regional evaluations of sources of aggregate, clay, or sand. REFERENCES BERG, R. C.; BLEUER, N. K.; JONES, B. E.; KINCARE, K. A.; PAVEY, R. R.; AND STONE, B. 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