HomeMy WebLinkAboutSoils Report 07.27.2020Kumar & Associates, Inc.®
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Company
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumaizusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT E-6, ASPEN GLEN
16 PUMA AND DIAMOND A RANCH ROAD
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-371
JULY 27, 2020
PREPARED FOR:
SCOTT SAMBORSKI
995 HOLLY STREET
DENVER, COLORADO 80220
scottsarnborski r(�i,icioud.conn
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 3 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 5 — GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 20-7-371
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot E-6, Aspen Glen, 16 Puma and Diamond A Ranch Road, Garfield County, Colorado. The
project site is shown on Figure 1. The purpose of the study was to develop recommendations for
the foundation design. The study was conducted in accordance with our agreement for
geotechnical engineering services to Scott Samborski dated July 1, 2020. Chen -Northern, Inc.
previously conducted a preliminary geotechnical study for preliminary plat design of the Aspen
Glen development under their Job No. 4 112 92, dated December 20, 1991 and May 28, 1993.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification, compressibility or
swell and other engineering characteristics. The results of the field exploration and laboratory
testing were analyzed to develop recommendations for foundation types, depths and allowable
pressures for the proposed building foundation. This report summarizes the data obtained during
this study and presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a 1 and 2 -story structure about 2,000 square feet in size plus the
garage and located approximately as shown on Figure 1. Ground floors could be structural
above crawlspace or slab -on -grade. Grading for the structure is assumed to be relatively minor
with cut depths between about 3 to 6 feet. We assume relatively light foundation loadings,
typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot is located on the southwest corner of Puma and Diamond A Ranch Road as shown on
Figure 1. The ground surface is relatively flat with about 1 foot of elevation difference down to
the west across the building footprint. The lot appears to have been disturbed possibly during the
Kumar & Associates, Inc. ® Project No. 20-7-371
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subdivision development. Vegetation consisted of grass and weeds. There was also scattered
gravel and cobbles on the ground surface.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. There is a possibility that massive
gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. Several sinkholes were observed by Chen -Northern
(1992 and 1993) scattered throughout the Aspen Glen property during the subdivision
development. These sinkholes appear similar to others associated with the Eagle Valley
Evaporite in areas of the Roaring Fork River valley.
The lot is located along the southeast perimeter of a broad subsidence area but sinkholes were
not observed in the immediate area of the subject lot. The closest mapped sinkhole within the
broad subsidence area is located about 600 feet northwest of the subject lot. No evidence of
cavities was encountered in the subsurface materials; however, although the exploratory borings
were relatively shallow, for foundation design only, a variable soil depth down to the dense river
terrace gravel was encountered in the exploratory borings. The potential impacts on the building
due to the variable subsurface profile conditions are discussed in the Design Recommendations
sections of this report. Based on our present knowledge of the subsurface conditions at the site,
it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence
on Lot E-6 throughout the service life of the proposed residence from ground subsidence due to
subsurface voids, in our opinion, is low but the owner should be made aware of the potential for
sinkhole development. If further investigation of possible cavities in the bedrock below the site
is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on July 2, 2020. Three exploratory borings
were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. Two
borings were initially drilled but due to the variable subsurface profile, a third boring was drilled.
The borings were advanced with 4 -inch diameter continuous flight augers powered by a truck-
mounted CME -45B drill rig. The borings were logged by a representative of Kumar &
Associates.
Kumar & Associates, Inc. ® Project No. 20-7-371
f low bearing capacity
-3 -
Samples of the subsoils were taken with 1% -inch and 2 -inch I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our
laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below a thin topsoil root zone, consist of about 3 to 251/2 feet of very stiff,
silty sandy clay overlying dense, slightly silty sandy gravel and cobbles with probable boulders.
Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and
boulders and drilling refusal was encountered in the deposit at Borings 2 and 3.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell -consolidation testing performed on
relatively undisturbed drive samples of the clay soils, presented on Figure 4, indicate low
compressibility under light loading and natural low moisture condition and low to moderate
expansion potential when wetted. The samples showed moderate compressibility under
additional loading after wetting. The results of gradation analyses performed on small diameter
drive samples of the gravel soils (minus 11/2 -inch size) are shown on Figure 5. The laboratory
testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the soils were typically
slightly moist.
FOUNDATION BEARING CONDITIONS
The soils encountered at proposed excavation depths typically consist o
clay with variable compressibility/heave potential mainly when wetted. The underlying dense
gravel has relatively high bearing capacity and low settlement potential. Since the clay soil
depth is much greater in the proposed garage area (Boring 3); there will be a risk of differential
movement between the deep clay soil area and the shallow gravel depth area: (Borings 2 and 3) of
the proposed residence. Lightly loaded spread footings can be used for building support and
accepting a risk of differential settlement mainly if the clay soils become wetted. Extending the
Kumar & Associates, Inc. ® Project No. 20-7-371
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foundation down to bear on the dense gravel soils and moving the building outside of the deep
clay soil area of the garage are ways to mitigate the differential foundation movement potential
If the risk of settlement and distress is not acceptable, a deep foundation extending down to the
dense gravel and cobble soils should be used.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils with a risk of settlement as described below.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf
Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. Additional settlement on the order of to 11/2 inches is
possible if the underlying clay soils are wetted and would likely be differential
between shallow and deeper gravel soil areas. Footings extended down to bear
entirely on the gravel soils in Borings 2 and 3 area can be designed for an
allowable bearing pressure of 3,000 psf with settlement potential of about 1 inch
or less.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet
especially in the area from the garage to the residence where additional
reinforcement should be used. Foundation walls acting as retaining structures
should also be designed to resist lateral earth pressures as discussed in the
"Foundation and Retaining Walls" section of this report.
Kumar & Associates, Inc. ® Project No. 20-7-371
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5) The topsoil and loose or disturbed soils should be removed down to the
undisturbed natural soils. The exposed soils in footing area should then be
moistened and compacted. The exposed clay soils should be further evaluated for
expansion/compression potential and the need for sub -excavation and replacement
with compacted structural fill at the time of excavation. Structural fill placed
below footing areas (if any) should extend horizontally out from the edge of the
footing to a distance equal to at least 1/2 the depth of fill below the footing and be
compacted to at least 98% of standard Proctor density at near optimum moisture
content.
6) A representative of the geotechnical engineer should evaluate structural fill for
compaction and observe all footing excavations for bearing conditions prior to
concrete placement.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site soils. Cantilevered retaining structures (if any) which are separate from the
residence and can be expected to deflect sufficiently to mobilize the full active earth pressure
condition should be designed for a lateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95% of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
Kumar & Associates, Inc. ® Project No. 20-7-371
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The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 300 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade
construction with a risk of settlement mainly if the bearing soils are wetted. To reduce the
effects of some differential movement, floor slabs should be separated from all bearing walls and
columns with expansion joints which allow unrestrained vertical movement. Floor slab control
joints should be used to reduce damage due to shrinkage cracking. The requirements for joint
spacing and slab reinforcement should be established by the designer based on experience and
the intended slab use. A minimum 4 -inch layer of relatively well graded sand and gravel such as
road base should be placed beneath interior slabs for support. This material should consist of
minus 2 -inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing
the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area where clay soils are present that local perched groundwater can develop during times of
heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
condition. We recommend below -grade construction, such as retaining walls, crawlspace and
basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
Kumar & Associates, Inc. ® Project No. 20-7-371
-7 -
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free -draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to
a suitable gravity outlet or drywell based in the underlying gravel and cobble deposit. Free -
draining granular material used in the underdrain system should contain less than 2% passing the
No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The
drain gravel backfill should be at least 11/2 feet deep. Where footings bear on clay soils, an
impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.
SURFACE DRAINAGE
Proper grading and drainage will be very important to keeping the bearing soils dry and limiting
the building settlement and potential distress. The following drainage precautions should be
observed during construction and maintained at all times after the residence has been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Free -draining wall backfill should be
covered with filter fabric and capped with at least 2 feet of the on-site soils to
reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
10 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
Kumar & Associates, Inc. ® Project No. 20-7-371
-8 -
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, Inc.
Steven L. Pawfak,
Reviewed by:
K --
Daniel E. Hardin, P.E.
SLP/kac
Kumar & Associates, Inc.
Project No. 20-7-371
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20-7-371
Kumar & Associates
LOCATION OF EXPLORATORY BORINGS
Fig. 1
- 0
5
BORING 1
EL. 6022'
18/12
// 13/12
/ WC=19.4
f DD=102
//
//
//
10
/ /11 28/12
f WC=15.8
/ DD=101
-200=99
f
30/12
/ WC=17.6
/ DD=103
f
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/ /] 24/12
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//1 20/12
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— 15
— 30
BORING 2
EL. 6022.5'
54/12
WC=1.3
+4=56
- 200=12
77/12
WC=1.6
+4=39
- 200=16
BORING 3
EL. 6022'
50/1.75
0
5 —
10-
0-
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15-
20
20 —
25
30-
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35 35
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w
20-7-371
Kumar & Associates
LOGS OF EXPLORATORY BORINGS
Fig. 2
LEGEND
18/12
TOPSOIL, ORGANIC SILT AND CLAY, SLIGHTLY MOIST, RED.
CLAY (CL); SILTY, SLIGHTLY SANDY TO SANDY, VERY STIFF, SLIGHTLY MOIST TO MOIST, RED,
LOW PLASTICITY, RED, CALCAREOUS.
GRAVEL AND COBBLES (GM—GP); SLIGHTLY SILTY TO SILTY, SANDY, PROBABLE BOULDERS,
DENSE, SLIGHTLY MOIST, GRAY—BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
DRIVE SAMPLE BLOW COUNT. INDICATES THAT 18 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JULY 2. 2020 WITH A 4—INCH—DIAMETER
CONTINUOUS—FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
0
20-7-371
Kumar & Associates
LEGEND AND NOTES
Fig. 3
CONSOLIDATION - SWELL
CONSOLIDATION - SWELL
0
—1
— 2
— 3
—4
3
2
1
0
— 3
SAMPLE OF: Sandy Silty Clay
FROM: Boring 1 0 5'
WC = 19.4 %, DD = 102 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
1.0 APPUED PRESSURE — KSP 10
Too
SAMPLE OF: Sandy Silty Clay
FROM: Boring 1 0 15'
WC = 17.6 %, DD = 103 pcf
Thew Leer r..tra oppy uny •n ene
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
1.0 APPUED PRESSURE — KSF 10 100
20-7-371
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
HYDROMETER ANALYSIS
SIEVE ANALYSIS
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COBBLES
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LIQUID LIMIT
SAMPLE OF: Slightly Silty Sandy
SAND 32 X SILT
PLASTICITY INDEX
Gravel FROM:
AND CLAY 12 X
Boring 2 0 5'
HYDROMETER ANALYSIS
SIEVE ANALYSIS
too
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7 NRS
10 $111$111904119
TIME READINGS
191494
414(14
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IN MILLIMETERS
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LIQUID LIMIT PLASTICITY INDEX
Thum fool ro.ult9 apply only le the
SAMPLE OF: Silty Sand and Gravel FROM: Boring 2 0 10' } Irrpr1q.4 np.rth.hallene ohrd. r. Tho
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A2111 0135 and/ar ASTM 01140.
20-7-371
Kumar &Associates
GRADATION TEST RESULTS
Fig. 5
Komar & Associates, Inc. ®
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20.7.371
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITYPASSING
(ucf)
GRADATION
PERCENT
NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(Psfl
SOIL TYPE
BORING
DEPTH
(ft)
GRAVEL
(%)
SAND
(74
LIQUID LIMIT
lal
PLASTIC
INDEX
(%)
1
5
19.4
102
Sandy Silty Clay
10
15.8
101
99
Silty Clay
15
17.6
103
Sandy Silty Clay
2
5
1.3
56
32
12
Slightly Silty Sandy Gravel
10
1.6
39
45
16
Silty Sand and Gravel