HomeMy WebLinkAboutSubsoil Study for Foundation Design 09.29.2021l(ln Kumar&Assoc¡ates, lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO g1601
and Environmentalsc¡ent¡sts phonà: (g7O) g45_7ggS
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc Owncd Compony www'kumarusa'com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT ?0,IRONBRIDGE, PHASE 3
1820 RIVER BEND WAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-684
SEPTEMBER29,202l
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
lu ke.gosda@sunriseco.com
TABLE OF CONTENTS
PURPUSH AND STJUPE U¡'S'I'UDY
PROPOSED CONSTRUCTION
SITE CONDITIONS..
SUBSIDENCE POTENTIAL....
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS ...
DESIGN RECOMMENDATIONS .
FOUNDATIONS
FLOOR SLABS
UNDERDRAIN SYSTEM.
SURFACE DRAINAGE...............
LIMITATIONS.
FIGURE 1 . LOCATION OF EXPLORATORY BORINGS
FIGTIRE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc.Project No. 21-7-684
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot21,Ironbridge, Phase 3,1820 River Bend V/ay in 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 SCIB, LLC dated August 2,2021'
A fîeld exploration program consisting of two 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 potential, 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
At the time of our study, design plans for the residence had not been developed. The building is
proposed within the building envelope shown on Figure 1. For the purposes of our analysis, we
assume the proposed residence will be a one- or two- story wood-frame structure over a
crawlspace with an attached slab-on-grade garage. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 5 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 was vacant and appeared to have had minor cut and fill grading, likely during the
subdivision development. According to historical Google Eartho aerial images dating back to
lgg3,minimal site disturbance occurred on Lot20 between successive photos dated from 2007
and20l1. The surface of the lot slopes gently down to the northeast with about 3 feet of
elevation difference across the assumed building area. A moderately steep slope then descends
Kumar & Associates, lnc.Project No, 21-7-684
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in the middlo part and becomes gentle in the north part indicated by the contour lines on
Figurc 1. Elevation diffr:r'ence aùl'oss the lot is about 20 feet. Vegetation consists of sparse
grasses and weeds on the upper lot portion, oak brush on the steeper transition slope, and grasses,
weeds atld scattered brush on the lower level. The Roaring Fork River is located immediately to
the northeast.
SUBSIDENCE POTENTIAI,
The geologic conditions were described in a previous report conducted for planning and
preliminary design of the overall subdivision development by Hepworth-Pawlak Geotechnical
(now Kumar & Associates) dated October 29, TggT,Job No. Ig7 32j. The natural soils on the
lot mainly consist of sandy silty clay alluvial fan deposits overlying gravel terrace alluvium of
the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of rounded
gravel, cobbles and boulders typically up to about2 fo 3 feet in size in a silty sand matrix and
overlies siltstone/claystone bedrock.
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge subdivision.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum and limestone. Dissolution of the gypsum under certain conditions can
cause sinkholes to develop and can produce areas of locahzed,subsidence. A sinkhole occurred
in the parking lot acljoining the golf cart storage tent in January 2005 was backf,rllcd and
compaction grouted. To our knowledge, that sinkhole has not shown signs of reactivation such
as ground subsidence since the remediation. Sinkholes possibly related to the Evaporite were not
observed in the immediate area of the subject lot. Based on our present knowledge of the
subsurface conditions at the site, it cannot be said for certain that sinkholes related to the
underlying Evaporite will not develop. The risk of future ground subsidence on Lot 20
tlrmughout the service life of the proposed building, in our opinion, is low; however, 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 f'or the project was conducted on August 25, 2021. Two explo ratory
borings were drilled at the approximate locations showr on Figure 1 tg evaluate the subsurface
conditions. The borings were advanced with 4-inch diameter continuous flight augers powered
by a truck-mountcd CME-458 drill rig. The borirrgs were logged by a representative of Kumar
& Associates, Inc.
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Samples of the subsoils were taken with l%-inch and 2-inch LD. California or split-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. Below
a thin topsoil or f,rll layer about T to l% feet thick and 4% feet of stiff to very stiff, slightly sandy
to sandy silty clay in Boring 2,very dense, silty sandy gravel and cobbles with probable boulders
was encountered down to the maximum explored depth of 11 feet. This fill was probably placed
as part of initial subdivision development. Drilling in the coarse granular materials with auger
equipment was diffîcult due to the cobbles and probable boulders and practical drilling refusal
was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, swell-consolidation and gradation analyses. Results of swell-consolidation
testing performed on a relatively undisturbed drive sample of clay soils, presented on Figure 4,
indicate low compressibility under existing low moisture conditions and light loading and a
moderate expansion potential when wetted under a constant light surcharge. Results of gradation
analyses performed on small diameter drive samples (minus l%-inch fraction) of the granular
subsoils 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 subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper clay soils encountered in the borings possess low bearing capacity and typically have
a low to moderate settlement potential if wetted. Testing indicates the clay soil has a moderate
expansion potential when wetted. Our experience in the area indicates the swell potential is
minor (if any) and can be discounted in foundation design. Shallow spread footings placed on
the upper natural soils can be used for support of the proposed residence with a risk of
foundation movement mainly if the clay bearing soils become wetted. Alternatively, potential
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movcment can be reduced hy removing ancl replacing the clay soils below foundatiolts with
compacted structural fill. Proper surface drainage as described in this report will bc critical to
the long-term performance of the structure.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encouutered in the exploratory borings antl the nature of
the proposed construction, the building can be founded with 1) spread footings bearing on the
upper natural soils, or 2) spread footings bearing on compacted structural fill or densc granular
subsoils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) F'ootings placed on the upper natural soils can be designed for an allowable
bearing pressure of 1,500 psf. Footings placed on undisturbed natural granular
soils or structural fill can be designed for an allowable bearing
pressure of 3000 psf. Based on experience, we expect initial settlement of
footings aesrgi;trand constructed as discussed in this section will be about I inch
or less. Additional differential movement up to about 1 inch could occur mainly
if the clay bearing soils are wetted.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isqlated 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 I
area.
below exterior grade is typically used in this
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 12 feet.
Founclation walls acting as retaining structures should also be designed to resist a
lateral earth pressure of 55 pcf for the on-site fine-grained materials or 45 pcf for
the on-site granular materials.
Topsoil, fill and any loose disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils or compacted structural fiIl.
The exposed soils in footing area should then be moistened and compacted.
5)
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Structural fill should be compacted to at least 98% of standard Proctor density at
near optimum moisture content and extent beyond the footing edges a distance at
least one-half the depth of fill below the footing.
6) A representative ofthe geotechnical all
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural clay soils possess an expansion potential and slab heave could occur if the subgrade
soils were to become wet. Vy'e should observe the soil conditions exposed at the time of
excavation and evaluate them for swell-compression potential and possible mitigation such as
sub-excavation and replacement with structural fill. Slab-on-grade construction can be used
provided precautions are taken to limit potential movement and the risk of distress to the
building is accepted by the owner. A positive way to reduce the risk of slab movement, which is
commonly used in the area, is to construct structurally supported floors over crawlspace.
To reduce the effects of some differential movement, nonstructural 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. Slab reinforcement and control joints should be established by the designer based on
experience and the intended slab use.
A minimum 4-inch layer of sand and gravel should be placed immediately beneath garage level
slabs-on-grade for support. This material should consist of minus 2-inch aggregate with less
fhan 50Yo passing the No. 4 sieve and less than I2o/o passing the No. 200 sieve.
Required fill beneath slabs should consist of suitable onsite sandy gravel or imported granular
material, excluding topsoil and oversized rocks. The fill should be spread in thin horizontal lifts,
adjusted to near optimum moisture content, and compacted to at least 95o/o of the maximum
standard Proctor density. All vegetation, topsoil and loose or disturbed soil should be removed
prior to fill placement.
UNDERDRAIN SYSTEM
It is our understanding the ground level, finished floor elevation of the residence is at or above
the surrounding grade. Therefore, a foundation drain system is not recommended. It has been
our experience in the area and where clay soils are present that local perched groundwater can
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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, if provided, be protected from wetting and hydrostatic
pressure buildup by an underdrain and wall drain system. An underdrain is not recommended
around the slab-at-grade garage and crawlspase area to help limit the potential lor wetting below
the shallow fbotings.
If the finished floor elevation of the proposed structure has a floor level below the surrounding
grade or a taller crawlspace is constructed, we should be contacted to provide recommendations
for an underdrain system. All earth retaining structures should be properly drained.
SURFACE DRAINAGE
It willbe critical to the building performance to keep the bearing soils dry. The following
drainage precautions should be observed during construction and maintained at all times after the
residence has been completed:
1) Inundation ofthe 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 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90Yo of the maximum standard Proctor tlensity 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. Graded swales should have a
minimum slope of 3%.
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
l0 feet from foundation walls. Consideration shoulcl 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 conductcd in accorclance with generally accepted geotechnical engineering
principles and practices in this area atthis time. We make no warranty either express or implied.
Kumar & Associates, lnc.Project No.21-7-684
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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 theìecommendations may be made.
This report has been prepared for the exclusive use by our client for design pulposes. 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 veriff 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,
Kurnar & Associateso Inc.
-1\r^nV--
Mark Gayeski, E.I.T.
Reviewed by:
Steven L.
SLPlkac
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P u,15222
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Kumar & Associates, lnc Project No, 21-7-684
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21 -7 -684 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig.
WC= 1 .0
+4=54
-2QO=12
BORING 1
EL. 5936'
BORING 2
EL. 5959.5'
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DD=99
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Fig. 2LOGS OF EXPLORATORY BORINGSKumar & Associates21 -7 -684
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LEGEND
N roesorr-; SANDy ro vERy sANDy srLT wrïH GRAVEL AND oRGANrcs,
'LTGHTL'
Morsr, TANNro BRowN.r-¡I
FILL: SILTY CLAY AND SAND WITH GRAVEL AND SCATTERED CoBBLES, FIRM oR MEDIUMDENSE, SLIGHTLY MOIST, TAN.
71gyy..lçL); ^sltGHrly sANDy ro sANDy, srlry, TRA.E cALcAREous, srFF To vERy stFF,
t,/isLrcHTLv Morsr, TAN, Low puslCrry.
F:Vt
|.."f.J9.RåY+ ¡fD- g9BP.lql_(9!)¡._s_l-LTY, SANDY wtrH PRoBABLE SMALL B0ULDERS, vERy DENSE,
[é.lsucHrly Morsr, LTGHT TO'MED|UM TAN AND ORAV. ROUñOEO ROCX.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
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DRIVE SAMPLE, 1 J/B-tNcH r.D. spLrr spooN sTANDARD pENETRATToN TEST.
66/12 PilT'^.'Tl'-.i',1*uu','#-Iid[?JEå'i: 'JHJ.'f,F'31u,',3å ii iR;fiP8ND
HAMMER
f enncrrcAL AUGER REFUSAL.
NOTES
THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 25, 2021 WITH A 4-INCH DIAMETERCONTINUOUS-FLIGHT POWER AUGER.
THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACINGFROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEENCONTOURS ON THE SITE PLAN PROVIDED.
THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATEONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THEAPPROXIMATE BOUNDARIES BETWEEN MATERTAL TypES AND lu¡ rRnrsnoNs MAy BE cRADUAL.
GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
2.
3.
4.
5.
6
7 LABORATORY TEST RESULTS:wc = wATER CONTENT (%) (ASTM D2216);DD = DRY DENSTTY (pct) (lsrv D2216);+4 = pERCENTAGE RETATNED ON NO. 4 STEVE (ASTM D69r5);-200= PERCENTAGE PASS|NG NO. 2OO SteVe ladrv D114O).'
21 -7 -684 Kumar & Associates LEGEND AND NOTES Fig. 3
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SAMPLE OF: Silty Cloy
FROM:Boring2@2.5'
WC = 9.5 %, DD = 99 pcf
-ZQQ = 86 %
D-4546.
tn
lnc. S{€ll
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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1.0 APPLIED PRESSURE - KSF 10 r00
21 -7 -684 Kumar & Associates SWELL-CONSOLIDATION TEST RTSULTS Fig. 4
100
90
80
70
60
50
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20
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60
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DIAMETER OF
132
MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 54 % SAND
LIQUID LIMIT
SÁMPLE OF: Silly Sondy Grovet
31 %
PLASTICITY INDEX
SILT AND CLAY 12 %
FROM: Boring 1 O 2.5' & 5' (Comblned)
Thsso l€sl raeulls opply only lo lhc
sompl€s whlch wôrå loslêd. Thslosllng roport shqll nol bo rcÞroducsd,
exc€pl lñ full, wllhout lhe wrillen
opprovql of Kumor & AsloclqlÇs, lno.
Slovo qnolyll! lcsllng l! pcrformcd ln
occordonco wlth ASIM D6915, ASTM D7928,
ASTM C156 qndlor ASTM Dll¡lo.
I.{YDROMSTER AIIALY6IE SIEVE AI.IALTSIS
CLEAR SQUARE OPENINGS
t/A' a/^ô I t/r'
READINCS
HRS
MIN
7 HRSI5 MIN
U.S. SÎANÞARD SERIES
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SAND GRAVEL
FINE MEDTUM lCOanSE FINE COARSE
21 -7 -684 Kumar & Associates GRADATION TEST RESULTS Fig. 5
I (+rt *r#*['fflî:Ë:inlrÍå *' "TABLE 1SUMMARY OF LABORATORY TEST RESULTSSilty Sandy GravelSilty ClaySOIL TYPEUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEXLIMITSLIQUID LIMIT86PERCENTPASSING NO.200 stEvE2134SAND(%)54GRADATION(%)GRAVEL99NATURALDRYDENSITY019.5NATURALMOISTURECONTENT2/22Y2 &.5combinedDEPTH2BORING1SAMPLENo.2l-7-684