PRELIMINARY GEOTECHNICAL INVESTIGATION
UCCS ACADEMIC OFFICE BUILDING
COLORADO SPRINGS, COLORADO
Prepared for:
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
Facilities Services
1420 Austin Bluffs Parkway
Colorado Springs, Colorado 80918
Attention: Mr. Gary Reynolds
CTL|T Project No. CS17986-115
October 16, 2012
5240 Mark Dabling Blvd | Colorado Springs, Colorado 80918 | Telephone: 719-528-8300 Fax: 719-528-5362
TABLE OF CONTENTS
SCOPE..................................................................................................................................... 1
SUMMARY............................................................................................................................... 1
SITE CONDITIONS.................................................................................................................. 2
PROPOSED DEVELOPMENT ................................................................................................ 3
SITE GEOLOGY ...................................................................................................................... 3
FIELD INVESTIGATION.......................................................................................................... 3
SUBSURFACE CONDITIONS ................................................................................................ 4
Existing Fill ....................................................................................................................... 4
Natural Clay and Sand..................................................................................................... 5
Bedrock ............................................................................................................................. 5
Groundwater..................................................................................................................... 5
Seismicity.......................................................................................................................... 6
SITE GRADING AND UTILITIES ............................................................................................ 6
CONSTRUCTION CONSIDERATIONS .................................................................................. 7
Foundations...................................................................................................................... 8
Floor Systems and Slabs-on-Grade............................................................................... 8
Subsurface Drainage ....................................................................................................... 9
Pavements ........................................................................................................................ 9
CONCRETE ............................................................................................................................. 9
SURFACE DRAINAGE.......................................................................................................... 10
CONSTRUCTION OBSERVATIONS .................................................................................... 10
GEOTECHNICAL RISK......................................................................................................... 10
LIMITATIONS ........................................................................................................................ 11
FIG. 1 – LOCATION OF EXPLORATORY BORINGS
FIG. 2 – SUMMARY LOGS OF EXPLORATORY BORINGS
APPENDIX A – SWELL CONSOLIDATION TEST RESULTS
TABLE A-1 – SUMMARY OF LABORATORY TESTING
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SCOPE
This report presents the results of our Preliminary Geotechnical Investigation for
the proposed Academic Office Building to be constructed on the campus of the
University of Colorado at Colorado Springs. This report includes a description of
subsurface and groundwater conditions found in our borings and our opinions regarding
the potential influence of these conditions on site development and building
construction. The report also includes preliminary geotechnical design and construction
criteria for installation of buried utilities and site grading, concepts for building
foundations, floor slabs and pavement sections, and our opinions with respect to the
influence of subsurface conditions on site development. We believe this study was
completed in general conformance with our proposal (CS-12-0133) dated September 12,
2012.
No preliminary documents or construction plans were available for the proposed
building, at the time of this report. Our understanding of the project is based on the
owner’s verbal description of the proposed construction. The report was prepared based
on conditions interpreted from conditions found in our exploratory borings, results of
laboratory tests, engineering analysis, and our experience. As the project documents
become more definitive, we should review the plans to formulate design and construction
recommendations. Evaluation of the site for the possible presence of potentially
hazardous materials (Environmental Site Assessment) is beyond the scope of this
investigation.
The following section summarizes our evaluation. A more complete description of
the conditions found, our interpretations, and our recommendations are included in the
report.
SUMMARY
1.
Subsurface conditions encountered in our exploratory borings drilled
within the vicinity of the planned building consisted of a layer of existing
fill underlain by natural sand and clay soils. Claystone bedrock with
occasional layers of sandstone was encountered in both borings
underlying the natural soils. Samples of the natural clay and claystone
exhibited low to moderate measured swell values when wetted.
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2.
At the time of drilling, groundwater was not encountered in either of the
exploratory borings. When water levels were checked again two days after
the completion of drilling operations, water was measured in one of the
borings at a depth of 21.5 feet below the existing ground surface.
Groundwater conditions may affect the installation of drilled pier
foundations.
3.
We believe grading and utility installation can be accomplished using
conventional heavy-duty equipment.
4.
We expect the soils encountered at the anticipated lowest floor elevation of
the proposed construction will consist of grading fill, natural sands and
clays, and possibly sandstone and claystone bedrock if a basement level is
included. To reduce the risk of excessive total and differential movements
of potentially heavily-loaded foundation walls underlain by a widely varying
combination of non-expansive soil, and expansive soil and bedrock
materials, we anticipate drilled piers bottomed in the underlying bedrock
will be an appropriate foundation alternative.
5.
New moisture conditioned and densely compacted grading fill and the onsite, natural soils likely provide good support characteristics for lightly
loaded slab-on-grade floors. Slabs bearing on or near expansive claystone
may experience movement and associated damage. Where claystone
bedrock is present near the finished floor level, which may occur in a
basement area, subexcavation of the claystone and replacement with
moisture conditioned fill will likely be an appropriate method to mitigate
the effects of the expansive materials and should allow for the installation
of a slab-on-grade floor. If some floor movement cannot be tolerated, a
structurally supported floor should be provided. The risk of poor slab
performance cannot be fully evaluated until plans and floor elevations are
defined.
6.
For planning purposes, pavement thicknesses for full-depth asphalt
sections for parking lots and access driveways in the range of 4 to 6
inches are anticipated. Concrete pavement, 6 inches thick, is expected at
the trash dumpster site.
7.
Surface drainage should be designed for rapid runoff of water away from
the proposed building. Conservative irrigation practices should be
followed to avoid excessive wetting. Water should not be allowed to pond
adjacent to the structure or over exterior slabs or pavements.
SITE CONDITIONS
The site of the proposed Academic Office Building is situated between Regent
Circle and Austin Bluffs Parkway at the location of the existing CITI Building, on the
camps of the University of Colorado at Colorado Springs. The CITI Building is a oneUNIVERSITY OF COLORADO AT COLORADO SPRINGS
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ACADEMIC OFFICE BUILDING
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story, wood-frame structure with brick veneer and wood siding exterior wall treatments. A
basement level that “walls out” at the southwest corner of the structure is present
beneath the building. The ground surface at the site slopes downward to the south and
southwest near Regent Circle and then flattens out to the north of Austin Bluffs Parkway,
in the area of existing building. Vegetation on the site consists of irrigated sod,
deciduous trees, and pine trees. An existing paved automobile parking lot is situated to
the west of the proposed building location.
PROPOSED DEVELOPMENT
We understand the proposed Academic Office Building is in the preliminary
conceptual planning stage. Preliminary concepts indicate the building will be a three or
four-story structure, and may include a basement level. The building is to provide space
for academic offices. Foundation loads are expected to be moderate to high. Paved
access driveways and additional automobile parking area are anticipated as part of the
overall development.
SITE GEOLOGY
Published geologic mapping (“Geologic Map of the Pikeview Quadrangle, El Paso
County, Colorado,” Jon P. Thorson, Christopher J. Carroll and Mathew L. Morgan,
Colorado Geological Survey, 2001) indicates the site is underlain locally by alluvial
deposits (Qfo). The upper member of the Laramie Formation (Klu) comprises the
underlying bedrock found beneath the near-surface soils. Conditions encountered in our
borings generally confirm the mapping, although our borings suggest some man-made
fill has been placed to adjust site grades.
FIELD INVESTIGATION
Our field investigation included drilling two exploratory borings at the general
location of the proposed building footprint. The borings were advanced to depths of 25
and 35 feet using 4-inch diameter, continuous-flight auger and a truck-mounted drill rig.
Drilling was observed by our field representative who logged the conditions found in the
borings and obtained samples. Graphical logs of the conditions encountered in the
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borings, the results of field penetration resistance tests, and laboratory test data are
presented in Fig. 2. Swell-consolidation test results are presented in Appendix A.
Laboratory test data are summarized in Table A-1.
Soil and bedrock samples obtained during this study were returned to our
laboratory and visually classified. Laboratory testing was then assigned to representative
samples. Testing included moisture content and dry density, swell-consolidation, sieve
analysis, and water-soluble sulfate content tests. To evaluate potential heave, the swell
test samples were wetted under applied pressures that approximated the overburden
pressure (the weight of overlying soil).
SUBSURFACE CONDITIONS
Subsurface conditions encountered in our exploratory borings consisted of a layer
of existing fill underlain by natural sand and clay soils. Claystone bedrock with
occasional layers of sandstone was encountered in both borings underlying the natural
soils. The pertinent engineering characteristics of the soils and bedrock encountered are
discussed in the following paragraphs.
Existing Fill
A layer of existing fill, about 3 to 7 feet thick, was encountered at the ground
surface in both borings. The fill consisted of clayey sand and sandy clay. The fill was
dense (sand) or very stiff (clay), based on the results of field penetration resistance
testing and our observations during drilling operations. The fill was apparently placed
when the CITI Building was constructed, or possibly when Regent Circle was extended.
We are not aware of specific documentation regarding the placement and compaction of
the fill material, such as the results of field density testing, and so the fill is of suspect
quality. If free from deleterious substances, we anticipate the fill can probably be
excavated, moisture conditioned, and densely compacted as new fill within the planned
development. If documentation of fill placement and testing is available, we should
review the information to determine if the fill is suitable for the proposed construction.
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Natural Clay and Sand
About 10 feet of natural, sandy to very sandy clay was encountered beneath the
existing fill in boring TH-1. The clay was stiff to very stiff based on the results of field
penetration resistance tests. Two samples of the clay tested in our laboratory exhibited
low measured swell values of 0.1 and 0.2 percent when wetted under estimated
overburden pressure.
About 16 feet of natural, clayey or silty sand was found underlying the fill layer in
boring TH-2. The sand was medium dense to dense based on the results of field
penetration resistance tests. Two samples of the sand tested in our laboratory contained
14 and 15 percent clay and silt-size particles (passing the No. 200 sieve). Our experience
indicates the sands are typically non-expansive when wetted.
Bedrock
Sandy claystone bedrock and slightly silty sandstone were found in the borings
drilled at the site beneath the natural soils, at depths of 13 and 23 feet below the existing
ground surface. Claystone appeared to be the predominant bedrock material. Field
penetration resistance test results indicated the bedrock was medium hard to very hard.
Two samples of the sandy claystone tested in our laboratory exhibited measured swell
values of 0.1 and 1.2 percent when wetted, which is indicative of slightly to moderately
expansive material. A sample of the sandstone contained 8 percent silt and clay-sized
particles (passing the No. 200 sieve). The sandstone is typically non-expansive or
exhibits low measured swell values when wetted.
Groundwater
At the time of drilling, groundwater was not encountered in either of the
exploratory borings. When water levels were checked again two days after the
completion of drilling operations, water was measured in one of the borings at a depth of
21.5 feet below the existing ground surface. Our experience indicates groundwater can
occur in different forms below this site. Water can be found in the fissures within the
bedrock. “Perched” groundwater tables can also form at the interface between the
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overlying granular materials and underlying bedrock. The occurrence of groundwater, the
volume, and elevation will fluctuate in response to seasonal precipitation variations,
surface drainage, and landscaping irrigation. Groundwater conditions may affect the
installation of drilled pier foundations.
Seismicity
This area, like most of central Colorado, is subject to a degree of seismic activity.
We believe the soils and bedrock on the site classify as Site Class C (dense soil and soft
rock) according to the 2009 International Building Code (2009 IBC). A geophysical study
is required to evaluate the shear wave velocity (V100) profile at the site to potentially
allow for an upgrade to Site Class B. Our firm can provide a site-specific geophysical
seismic study using the ReMi micro-tremor, surface method, if desired.
SITE GRADING AND UTILITIES
No grading plans were available for our review during the preparation of this
study. Based on the existing site topography and our understanding of the planned
construction, we estimate maximum cuts and fills of 5 to 10 feet will be necessary to
achieve the desired building pad elevation and grades within the associated parking
areas and access driveways. Permanent cut and fill slopes should be no steeper than 3:1
(horizontal to vertical). Our office should be contacted to review the site grading plans
once they are prepared.
All remnants of the existing CITI Building, including foundation elements, floor
slabs, below-grade walls, exterior concrete flatwork, the superstructure, and buried
utilities will need to be removed from the site before grading of the property can begin.
Prior to grading fill placement, existing fill and vegetation should also be removed from
the site. The existing fill should be excavated to expose the underlying natural soils.
Organic topsoil can be stockpiled and placed in landscaped areas. Fill materials within
the building footprint should consist of the on-site sand and clay soils and silty to clayey
sandstone (if encountered). If free from deleterious substances, we anticipate the existing
fill can probably be incorporated into the new site grading fills. The grading fill should be
moisture conditioned to within 2 percent of optimum moisture content and compacted in
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thin lifts to at least 95 percent of maximum standard Proctor dry density (ASTM D 698).
Sandstone, if placed as grading fill, should be mechanically broken down into particles of
less than 2 inches in diameter. Expansive claystone bedrock (if encountered) should be
placed as fill outside of the planned building footprint as much as possible or be
removed from the site. The placement and compaction of the grading fill should be
observed and tested by a representative of our office during construction.
Our borings suggest the on-site soils and bedrock (if encountered) can be
excavated using conventional, heavy-duty equipment. The grading fill and natural soils
will likely cave into unsupported, near-vertical utility trench excavations. Based on the
Occupational Safety and Health Administration (OSHA) criteria governing excavations,
the grading fills and natural soils will probably classify as Type C soils. The bedrock will
probably classify as Type B soil. Temporary excavations in Type B and Type C materials
require a maximum slope inclination of 1:1 and 1.5:1 (horizontal to vertical), respectively,
unless the excavation is shored or braced. Should groundwater seepage occur, flatter
slopes may be necessary. The contractor’s competent person should evaluate the soils at
the time of excavation and determine appropriate safety measures.
We recommend utility trench backfill be placed in thin, loose lifts, moisture
conditioned to within 2 percent of optimum moisture content, and compacted to at least
95 percent of maximum standard Proctor dry density (ASTM D 698). Personnel from our
firm should periodically observe utility trench backfill placement and test the density of
the backfill materials during construction.
CONSTRUCTION CONSIDERATIONS
Our preliminary opinions regarding foundations, floor systems, subsurface
drainage, and pavements are presented below for the anticipated construction. Once
plans for the office building become more defined, our office should be contacted to
provide specific design criteria and construction recommendations.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
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Foundations
Based on data from our exploratory borings, subsurface conditions across the site
will likely consist of grading fills and natural sand and clay soils underlain by claystone
and sandstone bedrock. To reduce the risk of excessive total and differential movements
of potentially heavily-loaded foundation walls underlain by a widely varying combination
of non-expansive and expansive soil and bedrock materials, we anticipate drilled piers
bottomed in the underlying bedrock will be an appropriate foundation alternative for the
proposed office building. For planning purposes, we anticipate a maximum allowable end
pressure of 25,000 to 35,000 psf and an allowable skin friction of 2,500 to 3,500 psf for the
portion of pier in comparatively unweathered bedrock will be appropriate for preliminary
pier sizing. We anticipate a minimum deadload pressure of 10,000 to 15,000 psf will be
appropriate for initial design. The presence of potentially caving sands and groundwater
at the site may require the use of temporary casing to install some of the drilled piers. Our
office should provide site-specific foundation recommendations and design criteria for
the planned structure after site grading plans have been prepared and the building
location and floor elevations have been established.
Floor Systems and Slabs-on-Grade
We expect a conventional slab-on-grade floor is considered to be an attractive
floor system alternative for at-grade portions of the proposed building. We judge the risk
of poor slab performance will likely be low where new, sandy to clayey, moisture
conditioned and densely compacted grading fill, natural sand, and/or sandstone are
present at or near floor slab elevations. Should claystone bedrock be encountered at or
near floor slab elevations, such as in a basement level, subexcavation of the claystone
and replacement with moisture conditioned granular fill will likely be an appropriate
method to mitigate the effects of the expansive materials and should allow for the
installation of a slab-on-grade floor. If some floor movement cannot be tolerated, a
structurally supported floor should be provided. The risks associated with poor slab-ongrade performance cannot be evaluated fully until the plans become better defined and
floor slab elevations are established.
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Subsurface Drainage
At the time of our investigation, we were not aware whether or not the proposed
construction will include habitable, below-grade areas such as a basement level. For
planning purposes, foundation drain installation should be anticipated around all belowgrade construction within the project site with the potential exception of non-habitable
areas that can be constructed as water-tight structures, such as elevator pits.
Pavements
Pavement subgrade soils across the site will likely consist of new, sandy to clayey
grading fills and natural, silty to clayey sands and sandy to very sandy clay. We anticipate
the grading fills and natural sands will generally provide good subgrade support
characteristics for pavement systems. The natural clay will provide comparatively poor
pavement support qualities. Where granular subgrade soils are encountered, pavement
thicknesses for full-depth asphalt concrete sections for parking lots and access
driveways in the range of 4 to 6 inches are likely. Pavement thicknesses for full-depth
asphalt concrete sections in areas where clay is the predominant subgrade material will
likely be on the order of 1 to 2 inches thicker. We recommend a concrete pavement be
provided at the trash dumpster site. The concrete pavement should be at least 6 inches
thick and large enough to support the entire length of the trash truck and dumpster
during the emptying process. When possible during grading, sands should be placed in
the upper 2 feet of the subgrade in pavement areas to reduce the required asphalt
concrete thickness. Final pavement section recommendations can be provided once site
development and grading plans are prepared.
CONCRETE
Concrete in contact with soils can be subject to sulfate attack. We measured the
soluble sulfate concentration in one sample from this site at less than 0.1 percent. Sulfate
concentrations less than 0.1 percent indicate Class 0 exposure to sulfate attack for
concrete in contact with the subsoils, according to ACI 201.2R-01, as published in the
2008 American Concrete Institute (ACI) Manual of Concrete Practice. For this level of
sulfate concentration, the ACI indicates Type I/II cement can be used for concrete in
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
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contact with the subsoils. In our experience, superficial damage may occur to the
exposed surfaces of highly permeable concrete, even though sulfate levels are relatively
low. To control this risk and to resist freeze-thaw deterioration, the water-to-cementitious
material ratio should not exceed 0.50 for concrete in contact with soils that are likely to
stay moist due to surface drainage or high water tables. Concrete subjected to freezethaw cycles should be air entrained.
SURFACE DRAINAGE
The performance of this project will be influenced by surface drainage. When
developing an overall drainage plan, consideration should be given to drainage around
the proposed building and away from paved areas. Drainage should be planned such that
surface runoff is directed away from foundations and is not allowed to pond adjacent to
the building or over pavements. We recommend slopes of at least 6 inches in the first 10
feet for the area surrounding the building, where possible. Roof downspouts and other
water collection systems should discharge well beyond the limits of all backfill around
the structure. Proper control of surface runoff is also important to prevent the erosion of
surface soils. Sheet flow should not be directed over unprotected slopes. Water should
not be allowed to pond at the crest of slopes. Permanent slopes should be seeded or
mulched to reduce erosion. Special attention should be paid to compact soils behind the
curb and gutter sections adjacent to streets and in utility trenches.
CONSTRUCTION OBSERVATIONS
We recommend that CTL | Thompson, Inc. provide observation and testing
services during construction to allow us the opportunity to verify whether soil conditions
are consistent with those found during our investigation. If others perform these
observations, they must accept responsibility to judge whether the recommendations in
this report remain appropriate.
GEOTECHNICAL RISK
The concept of risk is an important aspect with any geotechnical evaluation
primarily because the methods used to develop geotechnical recommendations do not
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comprise an exact science. We never have complete knowledge of subsurface
conditions. Our analysis must be tempered with engineering judgment and experience.
Therefore, the recommendations presented in any geotechnical evaluation should not be
considered risk-free. Our preliminary recommendations represent our judgment of those
measures that are necessary to increase the chances that the structure will perform
satisfactorily. It is critical that all recommendations in this report are followed during
design and construction.
LIMITATIONS
Plans for the proposed building were in the preliminary schematic phase at the
time of this report. The recommendations presented should be considered to be
preliminary. Once the plans become better defined, our firm should be contacted to
formulate geotechnical design criteria and construction recommendations.
Our borings were located to obtain a reasonably accurate indication of subsurface
foundation conditions. The borings are representative of conditions encountered at the
exact boring location only. Variations in subsurface conditions not indicated by the
borings are possible. We recommend a representative of our office observe the
completed foundation excavation. Representatives of our firm should be present during
construction to perform construction observation and materials testing services.
We believe this investigation was conducted with that level of skill and care
normally used by geotechnical engineers practicing in this area at this time. No warranty,
express or implied, is made.
If we can be of further service in discussing the contents of this report or in the
analysis of the influence of the subsoil conditions on design of the structure from a
geotechnical engineering point-of-view, please call.
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APPENDIX A
SWELL CONSOLIDATION TEST RESULTS
TABLE A-1 – SUMMARY OF LABORATORY TESTING
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EXPANSION UNDER CONSTANT
PRESSURE DUE TO WETTING
2
COMPRESSION % EXPANSION
1
0
-1
-2
-3
-4
0.1
APPLIED PRESSURE - KSF
1.0
CLAY, VERY SANDY (CL)
TH-1 AT 4 FEET
100
10
DRY UNIT WEIGHT=
MOISTURE CONTENT=
111
14.6
PCF
%
3
EXPANSION UNDER CONSTANT
PRESSURE DUE TO WETTING
2
COMPRESSION % EXPANSION
1
0
-1
-2
-3
-4
0.1
APPLIED PRESSURE - KSF
1.0
CLAY, VERY SANDY (CL)
TH-1 AT 9 FEET
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DRY UNIT WEIGHT=
MOISTURE CONTENT=
100
103
22.9
PCF
%
Swell Consolidation
Test Results
FIG. A-1
3
EXPANSION UNDER CONSTANT
PRESSURE DUE TO WETTING
2
COMPRESSION % EXPANSION
1
0
-1
-2
-3
-4
0.1
APPLIED PRESSURE - KSF
1.0
CLAYSTONE, SANDY
TH-1 AT 19 FEET
100
10
DRY UNIT WEIGHT=
MOISTURE CONTENT=
109
19.2
PCF
%
3
EXPANSION UNDER CONSTANT
PRESSURE DUE TO WETTING
2
COMPRESSION % EXPANSION
1
0
-1
-2
-3
-4
0.1
APPLIED PRESSURE - KSF
1.0
CLAYSTONE, SANDY
TH-2 AT 24 FEET
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DRY UNIT WEIGHT=
MOISTURE CONTENT=
100
108
18.9
PCF
%
Swell Consolidation
Test Results
FIG. A-2
TABLE A-1
SUMMARY OF LABORATORY TESTING
CTL|T PROJECT NO. CS17986-115
BORING
TH-1
TH-1
TH-1
TH-1
TH-2
TH-2
TH-2
TH-2
DEPTH
(FEET)
4
9
14
19
4
9
19
24
MOISTURE
DRY
CONTENT DENSITY
(%)
(PCF)
14.6
111
22.9
103
8.8
107
19.2
109
11.2
119
2.8
107
7.4
114
18.9
108
ATTERBERG LIMITS
LIQUID PLASTICITY
LIMIT
INDEX
(%)
(%)
* SWELL MEASURED WITH ESTIMATED IN-SITU OVERBURDEN PRESSURE.
NEGATIVE VALUE INDICATES COMPRESSION.
SWELL TEST RESULTS*
APPLIED
SWELL
SWELL PRESSURE PRESSURE
(%)
(PSF)
(PSF)
0.1
500
0.2
1100
PASSING
WATER
NO. 200
SOLUBLE
SIEVE
SULFATES
(%)
(%)
8
1.2
2400
14
15
0.1
3000
<0.1
DESCRIPTION
CLAY, VERY SANDY (CL)
CLAY, VERY SANDY (CL)
SANDSTONE, SLIGHTLY SILTY
CLAYSTONE, SANDY
FILL, SAND, CLAYEY
SAND, CLAYEY (SC)
SAND, SILTY (SM)
CLAYSTONE, SANDY
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