HomeMy WebLinkAboutSubsoil StudylGrf åiffilflffitHniïiå**
An Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email ; kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
.ilji I !: rLü7
GARFIELD COUNTY
TOMMUNITY DEVELOPMENT SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 2S4,IRONBRIDGE
BLUE HERON VISTA
GARFTELD COUNTY, COLORADO
PROJECT NO. 21-7-s95
AUGUST tt,202l
PREPARED FOR:
scrB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUrTE 52018
ASPEN, COLORADO 81611
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY.
BACKGROUND INFORMATION
PROPOSED CONSTRUCTION
SITE CONDITIONS.....
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOI.INDATIONS
FOUNDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS
UNDERDRAIN SYSTEM
LIMITATIONS...
FIGURE 1 . LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 - SV/ELL.CONSOLIDATION TEST RESULTS
FIGURE 4 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Aseociatos, lnc, @ Project No.21-7-595
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot2S4,Ironbridge, Blue Heron Vista, Garfield County, Colorado. The project site is shown on
Figure 1. The pu{pose 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 SCIB, LLC dated July 9, 2021.
An exploratory boring was drilled 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, expansion-compression potential and other engineering characteristics. The
results of the field exploration and laboratory testing were analyzedto 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.
BACKGROUND INFORMATION
The proposed residence is located in the existing Ironbridge development. Hepworth-Pawlak
Geotechnical, lnc. (now Kumar & Associates) previously conducted subsurface exploration and
geotechnical evaluation for the development of Villas North and Villas South parcels, Job No.
105 115-6, report dated September 14,2005, and performed observation and testing services
during the infrastructure construction, Job No. 106 0367, between April 2006 and April 2007.
The information provided in these previous reports has been considered in the current study of
Lot284.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The residence
will likely be a one or two-story, wood-frame structure with structural slab foundation and no
basement or crawlspace. Grading for the structure is assumed to be relatively minor with cut and
fill depths up to about 2 to 3 feet. We assume relatively light foundation loadings, typical of the
proposed type of construction.
If builcling loaclings, location or gracling plans change significantly from those clescribed above,
we should be notified to re-evaluate the rscomrlìerìdations contained in this reporl.
Kumar & Associates, lnc. 6 Project No. 21.7-595
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SITE CONDITIONS
The subject site was vacant at the time of our field exploration, The lot is located in the north
part of the Villas North Parcel. The natural terrain prior to development in 2006 sloped down to
the east at about 5o/o grade. Thc subdivision arca was clcvatcd by filling on thc ordcr of 15 fcct
above the original ground surface to create a relatively flat and gently sloping building site off
Blue Heron Vista. A 2-tier MSE retaining wall up to around 10 to 12 feet high supports the
north perimeter of the fill section as shown on Figure 1. Vegetation consists of sparse grass ancl
weeds with scattered sage brush.
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with sinkholes
and localized ground subsidence in the lower Roaring Fork River valley. A sinkhole opened in
the cart storage parking lot located east of the Pro Shop and west of the Villas North parcel in
January 2005. Irregular surface features were not observed in the Villas North parcel that could
indicate an unusual risk of future ground subsidence. Variable depths of the debris fan soils
were locally encountered by the previous September 14,2005 geotechnical study which indicates
there could have been localized subsidence of the river gravel deposits. The current subsurface
exploration performed in the area of the proposed residence on Lot 284 did not encounter voids.
ln our opinion, the risk of future ground subsidence on Lot 284 thróughout the service life of the
proposed residence is low and similar to other areas of the lower Roaring Fork River valley
where there have not been indications of ground subsidence.
FIELD EXPLORATION
The field exploration for the project was conducted on August 2,202I. One exploratory boring
was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring
was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
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. Dcpths at which the samplcs wcrc takcn and thc pcnctration resistance values are
shown on the Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
Kumar & Aoeoclatee, lnc. o Project No.21'7-595
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SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about a 6-inch thick root zone overlying mixed sand, silt and gravel fill to a
depth of 15 feet where stiff sandy silt and clay soils (alluvial fan deposit) were encountered to a
depth of 2I feetwhere dense, silty sandy gravel and cobble soils (river gravel deposit) were
encountered down to the maximum explored depth of 26 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and gradation analysis. Results of swell-consolidation testing performed on
a relatively undisturbed drive sample of the sandy silt and clay, presented on Figure 3, indicate
low compressibility under existing low moisture conditions and a minor expansion potential
when wetted under constant light surcharge. Results of a gradation analysis performed on a
small diameter drive sample (minus Ilz-inch fraction) of the gravelly sand and silt fill soils are
shown on Figure 4. The laboratory testing is summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were
typically slightly moist.
FOUNDATION BEARING CONDITIONS
The upper 15 feet of soils encountered in the boring consist of fill placed mainly in 2006 as part
of the subdivision development. The field penetration tests and laboratory tests performed for
the study, and review of the field density tests performed during the fill construction indicate the
structural fill was placed and compacted to the project specified minimum 95Yo of standard
Proctor density. Alluvial fan soils which tend to collapse (settle under constant load) when
wetted were encountered below the fill. The tested sample of the alluvial soils exhibited a minor
expansion potential. Our experience in the area indicates that the expansion is an anomaly and
can be ignored in foundation design. The amount of settlement will depend on the thickness of
the compressible soils due to potential collapse when wetted, and the future compression of the
wetted soils following construction. Relatively deep structural fill as encountered will also have
some potential for long-term settlement but should be significantly less than the alluvial fan
deposits. Proper grading, drainage and compaction as presented in the Surface Drainage section
will help to keep the subsoils dry and reduce the settlement risks. A heavily reinforced structural
slab or post-tensioned slab foundation designed for significant differential settlements is
recommended for the building support. As an alternative, a deep foundation that extends down
into the underlying dense, river gravel could be used to reduce the building settlement risk.
Kumar & Associates, lnc. o Project No.21-7-595
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DESIGN RECOMMENDATIONS
FOUNDA'I'IONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with a heavily reinforced
structural slab or post-tensioned slab foundation bearing on about 15 feet of the existing
compacted structural hll. The structural engineer should consider the close proximity of the
MSE wall to the north side of the residence in the foundation design to not adversely impact wall
stability and for potential differential settlement. If a deep foundation system is considered for
building support, we should be contacted for additional recommendations.
The design and construction criteria presented below should be observed for a heavily reinforced
structural slab or post-tensioned slab foundation system.
1) A heavily reinforced structural slab or post-tensioned slab placed on compacted
structural fill should be designed for an allowable bearing pressure of 1,500 psf.
The post-tensioned slab placed on structural fill should be designe¿ fohtá
distance of 1 0 feet or at least half of the slab width, whichever is greater.
Settlement of foundation is estimated to be about 1 to 2 inches based on the long-
term compressibility of the fill. Additional settlement of about 1 inch is estimated
if the rurderlying debris fan soils were to becolne wet. Settlement fi'om the deep
wetting would tend to be uniform across the building area and the settlement
potential of the fill section should control the design.
2) The thickened sections of the slab for support of concentrated loads should have a
minimum width of 20 inches.
3) The perimeter turn-down section of the slab should be provided with adequate soil
cover above their bearing elevation for frost protection. Placement of foundations
at least 36 inches bclow cxtcrior gradc is typically used in this area. If a frost-
protected foundation is used, the perimeter tum-down section should have at least
18 inches ofsoil cover.
4) The foundation should be constructed in a "box-like" configuration rather than
with irregular extensions which can settle differentially to the main building area.
The f'oundation walls, where provided, should be heavily reinforced top and
bottom to span local anomalies such as by assuming an unsupported length of at
least 14 feet. Foundation walls acting as retaining structures, if any, shoulcl also
be designed to resist lateral earth pressures as discussed in the "Foundation and
Retaining Vy'alls" section of this report.
Kumar & Associates, lnc. @ Project No.21-7-595
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The root zone and any loose or disturbed soils should be removed. Additional
structural fill placed below the slab should be compacted to at least 98%o of the
maximum standard Proctor density within 2 percentage points of the optimum
moisture content.
A representative of the geotechnical engineer should evaluate the compaction of
the fill materials and observe all footing excavations prior to concrete placement
to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures (if any) 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 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 earth 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.
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 90o/o of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95o/o 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 sefflement of deep foundation wall
backfrll 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 of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 325 pcf. The
s)
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Kumar & Associates, lnc. @ Project No.21-7-595
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coeftìcierrt of t.r'iction arrcl passive pl'essure vahres recorrunenrlerl ahove assunle ultinrate soil
strength, Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, pafticularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95o/o of the
maximum standard Proctor density at a moisture content near optimum.
NONSTRUCTURAL FLOOR SLAB S
Compacted structural fill can be used to support lightly loaded slab-on-grade construction
separate from the building foundation. The fill soils can be compressible when wetted and can
result in some post-construction settlement. To reduce the effects of some differential
movement, nonstn¡ctrrra.l floor slabs should he separated from huildings to allow unrestrained
vertical movement. Floor slab control joints should be used to reduce damagc duc to shrinkagc
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 slabs as subgrade
support. This material should consist of minus 2-inch aggregate with at least 50% retained on
the No. 4 sieve and less than l2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
stanclard Proctor dcnsity at a moisture contcnt ncar optimum. Rcquircd fill can consist of thc on-
site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
It is our understanding the finished floor elevation at the lowest level is at or above the
surrounding grade. Therefore, a foundation drain system is not required. It has been our
experience in the area that local perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
condition. Wc rccommcnd bclow-gradc construction, such as rctaining walls, be protected from
wetting and hydrostatic pressure buildup by an underdrain and wall drain system.
If the finishecl floor elevation of the proposecl strucflrre has a floor level below the surrouncling
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
SITE GRADING
Extensive grading was performed as part of the existing Villas North development. Additional
placement and compaction of structural fill soils could be needed to elevate the'site to design
Kumar & Associates, lnc. o Projcct No.2l-7.595
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grades and reduce the risk of excessive differential settlements and building distress. In addition,
the water and sewer pipe joints should be mechanically restrained to reduce the risk ofjoint
separation in the event of excessive differential settlement. Additional structural fill placed
below foundation bearing level should be compacted to at least 98% of the maximum standard
Proctor density within 2%o of optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing any vegetation and organic soils and compacting to at
least 95o/o of the maximum standard Proctor density at near optimum moisture content. The fill
should be benched into slopes that exceed 20o/o grade.
Permanent unretained cut and fill slopes should be graded at2horizontal to 1 vertical or flatter
and protected against erosion by revegetation or other means. This office should review site
grading plans for the project prior to construction.
SURFACE DRAINAGE
Precautions to prevent wetting of the bearing soils, such as proper backfill construction, positive
backfill slopes, restricting landscape irrigation and use of roof gutters, need to be taken to help
limit settlement and building 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 building structural slab foundation excavations should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and
nonstructural slab areas and to at least 90%o 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 6 inches in the first 5 feet in unpaved areas and a minimum slope of
3 inches in the f,rrst 10 feet in paved areas. Graded swales should have a
minimum slope of 3%.
4) Roof downspouts and drains should discharge at least 5 feet beyond the
foundation and preferably into a subsurface solid drainpipe.
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.
Kumar & Associates, lnc. @ Project No.2l-7-595
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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 boring drillcd at the location 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 concemed about MOBC, then a professional in this special field of
practice should be consqlted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory boring and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear diflerent 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. W'e 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 recoütmendations, and to verifu 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.
Respectfu lly Submitted,
Kumar & Associatcs. Inc.
}rtn trË. Pq*<:>u.¿
James H. Parsons, P.E.
Reviewed by:
Steven L.
JHPlkac
3t,15222
Kumar & Associates, lnc. o Project No.21,7-595
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MSE WALL
LOT 283
o
LOT 285
o
Y.
1
APPROXIMATE SCALE-FEET
LOl 284
5958
OIi
5959
o
BORING I
21 -7 -595 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
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BORING 1
EL. 595s'
LEGEND
R00T ZONE; SAND AND SILT, CLAYEY, GRAVELLY, FIRM,
MOIST, BROWN,
0
24/12
WC=6.4
DD= 1 04
-200=7 1
FILL: SAND, SILTY, GRAVELLY, SCATTERED COBBLES, VERY
STIFF TO HARD OR MEDIUM DENSE TO DENSE, SLIGHTLY
MOIST, MIXED BROWN.
srLT AND CLAY (ML-CL);
VERY STIFF, MOIST, LIGHT
SANDY TO VERY SANDY, STIFF TO
BROWN.
5
50/ 12
GRAVET (CU); SlHOv, SILTY, COBBLES, PROBABLE BOULDERS,
DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
46/12
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
'10
35/ 12
WC=6.5
+4=29
-200=37
DRTVE SAMPLE, 1 5/8-|NCH t.D. SPLTT SP00N STANDARD
PENETRATION TEST.
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15
,7¡¡1¡DRIYE SAMPLE BLOW COUNT. INDICATES THAT 24 BL0WS 0F-'I '' A 14o-POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
11 /12
NOTES
1. THE EXPLORATORY BORING WAS DRILLED ON AUGUST 2, 2021
WITH A 4-INCH DIAMETER CONÏINUOUS FLIGHT POWER AUGER.
20
20/12
WC=9.8
lJlJ= 1 15
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLÀN PROVIDTD.
3. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.25
37 /6, 55/6 4, THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD BE
CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE
METHOD USED.
50
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE
GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE
TIMF OF DRII I ING.
7, LABORATORY TEST RESULTS:
WC = WATER coNTtNT (%) (ASTM D 2216);
DD = DRY DENSTTY (pcf) (lSrU O ZZr0);
14 = PERCENTAGT RETAINED ON NO.4 SIEVE (ISTU O CSI¡);
-2OO = PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1140).
21 -7 -595 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
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SAMPLE OF: Sondy Silt ond Cloy
FROM:Boringl @20'
WC = 9.8 %, DD = 115 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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1.0 APPLIED PRESSURE - KSF 10 100
21 -7 -595 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 3
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90
80
70
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50
40
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HYDROMETER ANALYSIS
lIME REAOINCS
7 HRS
SIEVE ANALYSIS
U.S.gIANDARD SERIES CLEAR SOUARE OPENINGS
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=
.425 2.O 152
DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 29 % SAND 34 %
LIQUID LIMIT - PLASTICITY INDEX
SAMPLE OF: Grovelly Sill ond Sond (Fill)
SILT AND CLAY 37 %
FROM:Borlng1O10'
Thosc l6sl rcaulls opply only io lh.
sqmpl6s wh¡ch w€r6 l€sl.d. Th€
l€sllng roporl sholl nol b. rcproducrd,
€xcept ln full, wllhoul lhe wrlllon
opprovol of Kumo¡ & Assocloles, lnc,
Sl6yo onolysls losllng ls porformod ln
qccordonc6 wlth ASTM D6913, ASTM D792E,
ASTM C156 qnd,/or ASTM Dl1,+0.
SAND GRAVEL
FINE MEDTUM lCOanse FINE COARSE
21 -7 -595 Kumar & Associates GRADATION TIST RESULTS Fig. 4
I(+rf iiffilf;#itrÉ:if 'IÊ;å'**TABLE 1SUMMARY OF LABORATORY TEST RESULTSGravelly Silt and Sand(Fill)Sandy Silt and ClaySOIL TYPEGravelly Sandy Silt (Fill)losf)UNCONFINEDCOMPRESSIVESTRENGTH(%lPLASÏICINDEXI7AÏTERBERG LIMITS("/rlLIQUID LIMITPERCENTPASSING NO.200 srEVE)tSAND(%)34GRADATIONGRAVEL(%)29SAMPLE LOCATIONDEPTHBORINGNATURALDRYDENSITYNATURALMOISTURECONTENTt0411s6.46.59.82%0120INo. 2f'7-595