HomeMy WebLinkAboutSubsoil StudytGrtåffiffiffiir#'i*"5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
An Erçloyae Owmd Compony wwwkumarusa.com
Ofifice Locations; Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Corurty, Colorado
RECEIVED
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GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 15
THE FAIRWAYS AT ASPEN GLEN
GOLDEN BEAR DRTVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 19-7-738
JANUARY t7,2020
PREPARED FOR:
DANN COF'FEY
P.O. BOX 4308
EDWARDS, COLORADO 81632
idcregroup@gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY.
PROPOSED CONSTRUCTION
SITE CONDITIONS
SIIBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ...............
FOI.INDATIONS
FOTINDATION AND RETAINING V/ALLS ....
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE,..........
LIMITATIONS
FIGURE 1 . LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 . LEGEND AND NOTES
FIGURES 4 8t 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No. 19.7.738
PURPOSE Ai\D SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 15, The Fairways at Aspen Glen, Golden Bear Drive, 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 general accordance with our agreement for
geotechnical engineering services to Dann Coffey dated December 23,2019.
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 iharacteristics. 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 single-story structure with an attached garage. The ground
floor for the residence and garage will be strucfural over crawlspace and slab-on-grade,
respectively. Grading for the structure is assumed to be relatively minor with cut depths between
about 3 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 re,port.
SITE CONDITIONS
The site was vacant and ground surface was covered with about 6 to 10 inches of snow at the
time of our field exploration. The site is vegetated with grass. The terrain is relatively flat with
Kumar & Associates, lnc. o Project No. 19.7-738
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about a l-foot elevation difference across the building footprint. Golden Bear Drive is to the
north and east, Aspen Glen golf course is to the west, and single-family residences are to the
south, east, and west.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site. These rocks
are a sequence of g¡rpsiferous 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 sinlc*roles to develop and can produce areas of localized subsidence.
During previous work in the area, sinkholes have bee¡ observed scattered throughout the lower
Roaring Fork Valley. These sinl*roles appear similar to others associated with the Eagle Valley
Evaporite in this area. The closest mapped sinkhole is a few hundred feet north of the project
site.
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 sink*roles will not develop. The risk of
future ground subsidence on Lot 15 throughout the service life of the proposed residence, 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.
F'IELD EXPLORATION
The field exploration for the project was conducted on January 6,2020: 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 auge{s powered by a kuck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates.
Kumar & Associates, lnc. o Projec-t No. 19.7-738
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Samples of the subsoils were taken with 1% inch and 2 inc.}l' 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 retumed 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. T\e
subsoils consist of about I foot of topsoil overlying about 3Yz to 4Yz feú of very stiff sandy silt
and clay, underlain by dense, silty sandy to very sandy gravel with cobbles and possible
boulders. Drilling in the dense granular soils with auger equipment was difficult due to the
cobbles and possible boulders and drilling refusal was encountered in both borings at depth of
approximately 8 to \Yz feet.
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 silt and clay soils, presented on Figures 4 and 5,
indicate low to high compressibility under loading and a low to moderate collapse potential when
wetted. Results of gradation analyses performed on a small diameter drive sample (minus l%-
inch fraction) of the coarse granular subsoils are shown on Figure 6. 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 natural sandy silt and clay soils within about the upper 4Yzto 5Yzfeet are low density and
highly compressible especially when wetted. The underlying silty sandy to very sandy gravel
soils possess a moderate bearing capacity and a relatively low settlement potential. At assumed
excavation depths we expect the subgrade will expose sandy silt and clay soils. Excavations of
Kumar & Associates, lnc. @ Projecl No. 19.7-738
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less than 4y, feetin depth should be deepened to expose less compressible granular soils and the
sub-excavated depth can be backfiiled with compacted structural fill. spread footings should be
feasible for foundation support ofthe residence'
DESIGN RECOMMENDATIONS
FOIINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural granular soils or properly compacted structural fiI1.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural gtanular soils or compacted structural
fill should be designed for an allowable bearing pressure of 2,500 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be about 1 inch or less. Structural fill should be
compacted to a minimum oî98% of the standard Proctor density at a moisture
content near oPtimum.
Z) 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 soii cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
atea.
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 pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
5) The topsoil, silt and clay soils and any loose or disturbed soils should be removed
and the footing bearing level extended down to the relatively dense natural
granular soils. The exposed soils in footing area shouid then be moistened and
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compacted to a minimum of 95o/o of the standard Proctor density. Structural fiIl
used to reestablish design footingbearing level should extend at least l%feet
beyond footing edges and be compacted to at least 98% of standard Proctor
density at near optimum moisture content.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOLTNDATION AND RETAINTNG 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. 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 45 pcf for backfill consisting of the on-site soils,
All foundation and retaining strucfures 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 a moisfure content near optimum. 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. Backfill should not contain organics, debris or rock larger
than about 6 inches.
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
6)
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the side of the footiàg. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.45. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 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 with the accepted risk of movement. The risk of rnovernent can be reduced by
placing slabs-on-grade on a minimum of 2 feet of compacted structural fill or by using structural
floors over crawlspace, which is commonly done in the area. The structural fill should consist of
CDOT Class 5 or 6 base course material.
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 sand and gravel base
course should be placed beneath floor "slabs at grade" for support. This 4 inch thickness can be
included in the recommended 2 feet of base course below the slabs. 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 lTYo 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, below the
recommended depth of base course, can consist of the on-site soils devoid of debris, topsoil and
oversized rocks (plus 4-inch).
Kumar & Associates, lnc. @ Project No. 19.7.738
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LINDERDRAIN SYSTEM
Although free water was not encountered during our exploration, 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. We
recommend below-grade construction, such as retaining walls and basement areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain should
not be placed around shallow crawlspace areas to help limit the potential for wetting the bearing
soils.
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 lo/o to
a suitable gravity outlet. 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 1% feet deep.
SURFACE DRAINAGE
The following drainage precautions should be observed duriag 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 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 inpaved areas.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfil1.
5) Landscaping which requires regular heavy irrigation should be located at least 5
feet from foundation walls.
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LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area atthis 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 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 {indings 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 verifu that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recomrnend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Ässociates, fnc.
Shane J. Robat, P.E.
Reviewed by:
Steven L. Paw
SJR/kac
Cc: RM
Kumar & Associates, lnc. @ Project No. 19.7.738
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19-7 -738 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1
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19-7 -738 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2
LEGEND
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TOPSOIL; SANDY SILT WITH SCATTERED GRAVEL, ORGANICS, FIRM, SLIGHTLY MOIST, BROWN
stLT AND CLAY (ML-CL); SANDY, VERY STtrF, SL'GHTLY MOTST, SL|GHTLY PoROUS,
RED-BROWN.
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GRAVEL
DENSE,
(GM)¡ SILTY, SANDY TO VIRY SANDY W|TH COBBLES AND POSS|BLE BOULDERS,
SLIGHTIY MOISI, BROWN, GRAY. ROUNÐED ROCK,
DRIVE SAMPLE, z_INCH I,D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 3/8-rNCH r.D. SPL|T SPOON STANDARD PENETRATTON TEST.
2\/1t DRIVE SAMPLE BLOW COUNT. INDICATES THAT 23 SLOWS OF A 14O-POUND HAMMER
FALLING 30 INCHES WTRE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
f rnlcrrcal AUcER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 6,2020 WITH A 4_INCH_DIAMETER
CONTINUOUS-FLIGHÏ 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 MEASURED BY HAND LEVEL AND REFER
TO BORING 1 AS 1OO'.
4, THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5, THE LINES BEÏWEEN MA|ERIALS SHOWN ON THE TXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRÀDUÄ1.
6, GROUNDWATER WAS NOT ENCOUNTERED IN ÏHE BORINGS AT THE TIME OF DRILLING.
7, LABORATORY TEST RESULTS:
Wc = WATER coNTrNT (%) (ASTM 02216);
DD = DRY DENSTTY (pcr) (lSrU D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIIVE (NSTU OOSIS);
-200= PERCENTAGE PASSINC NO. 200 SIEVE (ASTM D1140).
19-7 -738 Kumar & Associates LTGTND AND NOTES Fí9. 3
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SAMPLE OF: Sondy SilÌ cnd Cloy
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WC = 3.9 %, DD = 93 pcf
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UNDER CONSTANT PRESSURE
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19-7 -738 Kumar & Associates SWELL_CONSOLIDATION TTST RISULTS Fig. 4
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SAMPLE OF: Sondy Silt cnd Cloy
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19-7 -738 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig,5
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HYDROMEÍER ANALYSIS SIEVE ANÀLYSIS
NME RruINCS
2¿ HRS 7 HRSr< vrF ß6vrN roulN
U.S. SÏAXDÀRO SERIES CLüR SQUÆE OPENINOS
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LIAUID LIMIT
SAMPLE 0F: Silly Very Sondy crov€l
36%
PLASTÍCITY INDEX
SILT AND CLAY 13 %
FROM:Boring2oS'
lh.s. t.st rn3ulb opply only lo lh€
sompl€s whlch w.re losl€d. Ìhot.¡llng rcporl sholl nol bo râproduc6d,
oxc€pl ln lull, wffhoul tho wrlll.n
opprovol of Kumqr & Assoclofos,:nc.
Slovs onolyrls losfl¡g l! porlom.d Inqccordo¡e. slth ASTM 06913, ASÍM D7928,
ASTM C136 qnd/ôr ÄSTY 0lt4O,
19-7 -738 Kumar & Associates GRADATION TEST RTSULTS Fig. 6
lGrtm#mmi*ï'-"TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOILTYPESandy Silt and ClaySandy Silt and ClaySilty Very Sandy GravelfosflUNCONFINEDCOMPRESSIVESTRENGTHlo/olPLASTICINDEXATÏERBERG LIMTTSLISUID LIMIT(ol"lPERCENTPASSING NO.200 slEvEJ136I5$tSAND(%)GRAVELNATURALDRYDENSTY93100f/"1NATURALMOISTURECONTENT3.97.0t.6tf0DEPTH521/,5SAIIPLÊ LOCATIONBORING12