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HEPWORTH· PAWLAK GEOTECHNICAL
SUBSOIL STUDY
l-lepw0rch-f~a~·lak (ieo1eduucal, 111..:..
j(i](1 C:ounrr Road 1 S.:J
Glenwood Springs. Coiornck. 81001
Ph(•ne: 970-9-15-7988
Fax: 970-94j·84S4
email: hpgco®hrgeorech.con1
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT M-8, ROARING FORK MESA AT ASPEN GLEN
BROOKIE
GARFIBLD COUNTY, COLORADO
JOB NO. 104 891
JANUARY 31, 2005
PREPARED FOR:
JANICE AND SPENCER YOUNGBLOOD
c/o MUIR ARCHITECTS, INC;
ATTN: KRISTEN MULE
201 MAIN STREET, SUITE 304
CARBONDALE, COLORADO 81623
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ -I -
PROPOSED CONSTRUCTION ..................................................................................... -I -
SITE CONDITIONS ....................................................................................................... - 2 -
SUBSIDENCE POTENTIAL ......................................................................................... - 2 -
FIELD EXPLORATION ................................................................................................. - 3 -
SUBSURFACE CONDITIONS ...................................................................................... -3 -
DESIGN RECOMMENDATIONS ................................................................................. - 4 -
FOUNDATIONS ......................................................................................................... - 4 -
FOUNDATION AND RETAINING WALLS ............................................................ - 5 -
FLOOR SLABS .......................................................................................................... - 6 -
UNDERDRAIN SYSTEM .......................................................................................... - 7 -
SITE GRADING ......................................................................................................... - 8 -
SURFACE DRAINAGE ............................................................................................. - 8 -
LIMITATIONS ............................................................................................................... - 9 -
REFERENCES .............................................................................................................. -10 -
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
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located
on Lot M-8, Roaring Fork Mesa at Aspen Glen, Garfield County, Colorado. The project
site is shown on Figure I. The purpose of the study was to develop recommendations for
the foundation design. The study was conducted in accordance with our proposal for
geoteclmical engineering services to Janice and Spencer Youngblood dated December 10,
2004. Chen-Northern, Inc. previously conducted a preliminmy geotechnical engineering
study for the development (Chen-Northern, 1991) and another geotechnical engineering
study for preliminary plat design (Chen-Northern, 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 mid allowable pressures for the proposed building foundation.
This report smmnarizes the data obtained during this study 811d presents our conclusions,
design recommendations and other geotechnical engineering considerations based on the
proposed construction and the subsmfacc conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a single story wood frame slrucl11l'e over a walkout
basement level. The attached garage and basement floors will be slab-on-grade. Grading
for the structure is assumed to be relatively minor with cut depths between aboul 3 lo 12
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.
JobNo.104 891
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SITE CONDITIONS
The lot was vacant and covered with up to 6 inches of snow at the time of our field
exploration. The ground surface in the building envelope slopes moderately to
moderately steep down to the east at grades between about 5% and 17%. There is about
14 feet of elevation difference in the building envelope. The terrain becomes steeper on
the west and east sides of the lot with slopes of about 40% to 50% down to the east.
There is some fill on the lot from overlot grading as part of the subdivision. Vegetation
consists of grass and weeds. Several cobbles and boulders are exposed on the ground
surface on the south and southeast sides of the lot.
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/siltstone and limestone with some massive beds ofgypsum. 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. During previous
work in the area, several sinkholes were observed scattered throughout the Aspen Glen
development (Chen-Northern, Inc., 1991). These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of
cavities was encountered in the subsurface materials; however, the exploratory borings
were relatively shallow, for foundation design only. 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 M-8 tlu·oughout the service life of
the proposed residence, in our opinion, is low; however, the owner should be made aware
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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 December 22, 2004. Two
exploratory borings were drilled at the locations shown on Figure I to evaluate the
subsurface conditions. 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 Hepworth-Pawlak Geoteclmical, Inc.
Samples of the subsoils were taken with l '/s inch and 2 inch I.D. spoon samplers. The
samplers were driven into the subsoils at various depths with blows from a I 40 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 laboratmy 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 in Boring 2 consist of up to 4 feet of sandy clayey gravel fill
overlying medium stiff to stiff, sandy silt and clay. Relatively dense, silty sandy gravel
with cobbles and possible boulders was encountered beneath the silt and clay at a depth of
about I 5 feet. Drilling refusal was encountered in the fill soils in Boring I at a depth of
2Y, feet due to the cobbles and boulders.
Laboratory testing performed on samples obtained from the borings included natural
moisture content, density and gradation analyses. Results of swell-consolidation testing
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performed on relatively undisturbed drive samples of the sandy silty clay soils, presented
on Figure 4, generally indicate low to moderate compressibility under conditions of
loading and wetting. The 5 foot sample from Boring 2 showed a low expansion potential
when wetted under a constant light surcharge. The 10 foot sample from Boring 2 showed
a minor collapse potential (settlement under constant load) when wetted. Results of
gradation analyses performed on small diaineter drive samples (minus 1 Y, inch fraction)
of the coarse granular subsoils are shown on Figure 5. The laboratory testing is
summarized in Table 1.
Groundwater was encountered in Boring 2 at a depth of 17 feet at the time of drilling.
The boring had caved at 17 feet when measured 13 days later. The subsoils were slightly
moist to wet below the groundwater
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, we reconunend the building be founded with spread
footings bearing on the natural subsoils.
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
settlement of footings designed and constructed as discussed in this section
will be about I inch or less. There could be some additional settlement if
the bearing soils become wetted.
2) The footings should have a minimum width of 18 inches for continuous
walls and 2 feet for isolated pads .
lob No. 104 89 I
3)
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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 reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 12
feet. Foundation walls acting as retaining structures should also be
designed to resist lateral earth pressmes as discussed in the "Foundation
and Retaining Walls" section of this repmt.
5) All existing fill, topsoil and any loose or disturbed soils should be removed
and the footing hearing level extended down to the firm natural soils. The
exposed soils in footing area should then be moistened and compacted. If
water seepage is encountered, the footing areas should be dewatered
before concrete placement.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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
eati11 pressme computed on the basis of an equivalent fluid unit weight of at least 50 pcf
for backfill consisting of the on-site soils. Cantilevered retaining structures which are
separate from the residence and can be expected to deflect sufficiently to mobilize the full
active emth pressure condition should be designed for a lateral earth pressure computed
on the basis of an equivalent fluid unit weight of at least 40 pcf for backfill consisting of
the on-site soils. Backfill should not contain vegetation, topsoil or oversized rock.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and
equipment. The pressmes recommended above assume drained conditions behind the
Job No. 104 891 ~tech
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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 a moisture content near optimum. Backfill 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.
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 of0.35. 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, are suitable to support lightly loaded slab-
on-grade construction. The clay soils are typically compressible when wetted which
could result in some slab settlement and distress it the subgrade soils become wet. To
reduce the effects of some differential movement, floor slabs should be separated from all
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bearing walls and columns with expansion joints which allow muestrained vertical
movement. Floor slab control joints should be used to reduce damage due to sluinkage
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 free-draining gravel should be placed beneath basement level slabs to
facilitate drainage. This material should consist of minus 2 inch aggregate with at least
50% retained on the No. 4 sieve and less than 2% 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 encountered below probable excavation depth during our
exploration, it has been our experience in the area and 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 and basement areas, be
protected from wetting and hydrostatic pressure buildup by an underdrain system.
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 l foot below lowest adjacent finish
grade and sloped at a minimum l % to a suitable gravity outlet. Free-draining granular
material used in the underdrain system should contain less than 2% passing the No. 200
sieve, Jess than 50% passing the No. 4 sieve and have a maximum size of 2 ii1ches. The
drain gravel backfill should be at least l Y, feet deep. 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.
Job No. 104 891 G8ffitech
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SITE GRADING
The risk of construction-induced slope instability at the site appears low provided the
building is located above the steep slope as planned and cut and fill depths are limited.
We assume the cut depths for the basement level will not exceed one level, about I 0 to 12
feet. Fills should be limited to about 8 to I 0 feet deep, especially at !be downhill side of
the residence where the slope steepens. Embankment fills should be compacted to at least
95% of the maximum standard Proctor density near optimum moish1re content. Prior to
fill placement, the subgrade should be carefully prepared by removing all vegetation and
topsoil and compacting to at least 95% of the maximum standard Proctor density.
Permanent umetained cut and fill slopes should be graded at 2 horizontal to I vertical or
flatter and protected against erosion by revegetation or other means. The risk of slope
instability will be increased if seepage is encountered in cuts and flatter slopes may be
necessary. If seepage is encountered in pennanent cuts, an investigation should be
conducted to detennine if the seepage will adversely affect the cut stability. This office
should review site grading plans for the project prior to construction.
SURFACE DRAINAGE
The following drainage precautions should be observed during constrnction and
maintained at all times after the residence has been completed:
1) Immdation 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)
Job No. 104 891
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 I 0 feet in unpaved
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areas and a minimum slope of 3 inches in the first 10 feet in paved areas.
Free-draining wall backfill should be capped with about 2 feet of the on-
site finer graded soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Irrigation sprinkler heads and landscaping which requires regular heavy
irrigation, such as sod, should be located at least 5 feet from foundation
walls.
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. 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 I, 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 subsmface
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 infomiation. As the
project evolves, we should provide continued consultation and field services during
construction to review and monitor the implementation of our reconu.nendations, and to
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications to the recommendations
Job No. 104 891 ~ech
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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,
HEPWORTH -PAWLAK GEOTECHNICAL, INC.
Jordy Z. Adamson, .Tr., P.E.
Reviewed by:
Daniel E. Hardin, P .E.
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REFERENCES
Chen-No11hem, Inc., 1991, Preli111in01y Geotechnical Engineering Study, Proposed
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated December20, 1991, Job No. 4 112 92.
Chen-No1thern, Inc., 1993, Geotechnical Engineering Study for Prelimi1101y Plat Design,
Aspen Glen Development, G01jie/d County, Colorado, prepared for Aspen Glen
Company, dated May 28, 1993, Job No. 4 112 92.
Job No. 104 891 ~tech
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APPROXIMATE SCALE
1 • =30'
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1010-----( --
LOT
M-48
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990 -
-
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104 891
-
/
----..
,,,, BORING 2
/
/
/ LOT M-8
/ I i,,,,' I I
I LOT M-7
--e BORING 1 ~LDING SETBACK LINE 990
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/
/ ---/
/
/ ------980
980---
--
HEPWORTH-PAWLAK
GEOTECHNICAL, INC. LOCATION OF EXPLORATORY BORINGS
BROOKIE
Figure 1
1000
995
-990
( " " '~ "-
c:
0 :;;
0 > " 985 w
980
975
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104 891
BORING 1 BORING 2
ELEV.~997' ELEV.~1 ooo'
36/12
TTT~ 14/12
WC=14.7
00=96
7/12
WC=13.6
00=95
-200-65
24/12
} wc-10.s ++=30
-200=16
25/3,10/0
Note: Explanation of symbols is shown on Figure 3.
HEPWORTH-PAWLAK
GEOTECHNICAL. INC.
LOGS OF EXPLORATORY BORINGS
1000
995
990 -" " "-
c:
0 :;; c >
985 " w
980
975
Figure 2
I LEGEND:
~) lXJ · ~ FILL; sandy clayey gravel with cobbles and boulders, firm, slightly moist, brown.
Q
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SILT AND CLAY (ML-CL); sandy, medium stiff to stiff, slightly moist, browns, low plasticity.
GRAVEL (GP-GM); sondy, slightly silty to silty, with cobbles and probable small boulders, medium
dense to dense, moist to wet below groundwater, brown.
Relatively undisturbed drive sample; 2-inch l.D. California liner sample.
~ Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-1586.
Drive sample blow count; indicates \hot 36 blows of o 140 pound hammer foiling 30 inches were
36 /12 required to drive the California or SPT sampler 12 inches.
0 Free water level in boring and number of days following drilling measurement wos token.
Depth at which boring had coved when checked on January 4, 2004.
T Practical drilling refusal. Where shown above bottom of log, indicates that multiple attempts were
mode to advance the boring.
'-....-NOTES:
1. Exploratory borings were drilled on December 22, 2004 with 4-inch diameter continuous flight power
auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site
pion provided.
3. Elevations of exploratory borings were obtained by interpolation between contour lines on the site pion
provided and checked by instnument level.
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 transitions may be gradual.
6. Water level readings shown on the logs were made at the time and under the conditions indicated.
Fluctuations in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content ( % )
DD = Dry Density ( pcf )
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
104 891 HEPWORTH-PAWLAK
GEOTECHNICAL, INC. LEGEND AND NOTES Figure 3
.~ Moisture Content = 14.7 percent
Dry Density = 96 pcf
Sample of: Sandy Silty Clay
~ From:Boring 2 at 5 Feet
c 1 0 ·u;
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0
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I ~""' c -._._~
""' 0 ·u; 1
"' ~ '\ Q)
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0 2 ' () Expansion ~
upon
3 wetting
' 0.1 1.0 10 100
APPLIED PRESSURE -ksf
Moisture Content -= 13.6 percent
Dry Density = 95 pcf
Sample of: Sandy Silty Clay
From:Boring 2 at 10 Feet
0
~ H1
1 c ~"' .Q ~~ Compression
"' "'
1-. upon
Q) ' L 2
wetting
Q.
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3
4
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0.1 1.0 10 100
APPLIED PRESSURE -ksf
104 891
HEPWORTH-PAWLAK SWELL-CONSOLIDATION TEST RESULTS Figure
GEOTECHNICAL, INC.
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24 HR. 7 HR
O ~ MIN. 15 MIN.
10
20
0
w "" z
~ w
"' I-.. z w
()
"' w a. 60
70
80
100
.001 .002
HYDROMETEft AHAL VSIS I
TIME REAOINGS I
601.4W. 19MIN. .f-MIN. I MIN. ""
,005 .OOSI ,019 .037 .074 ,150
I
U.S. STANDAl!O S£RIE$ I a.EAR SOUAR£ OPENINGS I
f4 J/8" J/1," 1 112· J" s• s•
90
80
70
60
50
20
10
0
.300 .600 1.18 2.36 4.7~ 9,512.5 19.0 37,5 76.2 152 203
127
DIAMETER OF PARTICLES IN MILLIMETERS
104 891
CLAY 10 SILT ANE
SAND
MED"IM 100-I FINE I COARSE I 00881£5
GRAVEL 30 % SAND 54 % SILT AND CLAY 16
LIQUID LIMIT %
SAMPLE OF: Gravelly Silty Sand
HEPWORTH-PAWLAK
GEOTECHNICAL, INC.
PLASTICITY INDEX
FROM: Boring 2 at 15 and 20 Feet Combined
GRADATION TEST RESULTS Figure 5
"' z m
<( a.
1-z w
()
"' w
0..
u 0 ,,,.-·., u
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 104 891
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT ATTERBERG LIMITS UNCONFINED
MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOlL OR BORING DEPTH CONTENT DENSI1i' NO. 200 LlMIT INDEX STRENGTH BEDROCK TYPE (%) (%) SIEVE
((1:) (%) (pcf) (O/o) (O/o) (PSFl
2 5 14.7 96 Sandy silty clay
10 13.6 95 64 Sandy silty clay
15 & 20 10.6 30 54 16 Gravelly silty sand
combined