HomeMy WebLinkAboutSubsoil Studyrcrf iiffiifl'triffË:in''IË;'n""
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.cotn
Office Locations: Denver (HQ), Parker, Colorado Springs, Foú Collins, Glenwood Springs, and Sumrnit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT H-10, THE HOMESTEAD
ASPEN GLEN SUBDIVISION
GARFIELD COUNTY, COLORADO
PROJECT NO.20-7-521
ocroBERz,2o2o
PREPARED FOR:
BELLA VILLA BUILDERS LLC
ATTN: RUSSELL BURTON
P.O. BOX 875
CONIFER, COLORADO 80433
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL
FIELD EXPLORATION ........
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS ........
DESIGN RECOMMENDATIONS ......................
FOUNDATIONS
FOUNDATION AND RETAINING WALLS.
FLOOR SLABS
UNDERDRAIN SYSTEM .......
SURFACE DRAINAGE...........
LIMITATIONS.......
FIGURE 1 - LOCATIONS 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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Kumar & Associates, lnc. o Project No.20-7-521
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot H-10, The Homestead, Aspen Glen, 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 proposal for geotechnical engineering services
to Bella Villa Builders LLC dated September 2,2020.
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 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.
PROPOSED CONSTRUCTION
The proposed residence will be a single story wood-framed structure over a full basement with
attached garage. Ground floors will be slab-on-grade. Grading for the structure is assumed to be
relatively minor with cut depths between about 4 to 10 feet. V/e 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 subject site was vacant at the time of our field exploration. The ground surface is relatively
flat. The elevation difference across the proposed building area is estimated at about llz feet.
Vegetation consists of grass and weeds.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. There is a possibility that massive
gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. During previous work in the area, several sinkholes were
observed scattered throughout the development, mostly east of the Roaring Fork River. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the
Roaring Fork River Valley.
Sinkholes were not observed in the immediate area of the subject lot. The nearest mapped
sinkhole is located approximately 1100 feet southeast of the site. 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 H10 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.
FIELD EXPLORATION
The fîeld exploration for the project was conducted on September 21,2020. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with l% inch and 2 inch LD. 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-l586
The penetration resistance values are an indication of the relative density or consistency of the
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subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our
laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about Yz foot of topsoil overlying very stiff, sandy clay to between 5Yz and
6 feet deep underlain by dense, silty sand and gravel to the maximum explored depth of 16 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content, density and gradation analyses. Results of swell-consolidation testing performed on a
relatively undisturbed drive sample, presented on Figure 4, indicate low to moderate
compressibility under existing moisture conditions and light loading and a low expansion
potential when wetted under constant light surcharge. Results of gradation analyses performed
on small diameter drive samples (minus lYz-inch fraction) of the coarse granular subsoils are
shown on Figure 5. The laboratory testing is summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist.
FOUNDATION BEARING CONDITIONS
The upper clay soils encountered at the site possess an expansion potential when wetted.
Foundations placed on these soils may have a risk of movement especially if the bearing soils
become wetted. The expansion potential of the clay soils should be further evaluated at the time
of construction in spread footings or slab-on-grade construction is planned to bear on these soils.
To reduce the movement potential a depth of the clay soils could be sub-excavated and replaced
as compacted structural fîll. The underlying sand and gravel soils possess a moderate bearing
capacity and typically a low settlement potential. Spread footing bearing on the granular soils
should be suitable for support of the proposed residence with a low risk of settlement.
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DESIGN RECOMMENDATIONS
FOUNDATIONS
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 soils.
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 3.0Q0 psf. Footings placed on the undisturbed
-
natural clay soils should be designed for an allowable bearing pressure of
2)
1,500 psf. The expansion potential of any exposed clay soils should be fuither
evaluated at the time of construction. Based on experience, we expect settlement
of footings designed and constructed as discussed in this section will be about
I inch or less.
The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations atfg4g1|]s-i@g¡ below exterior grade is typically used in this
atea.
Continuous foundation walls should be reinforced top and bottom to span local
anomalies and across transitions between clay and gravel soils (if any) such as by
assuming an unsupported length of at least 10 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.
Any topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the 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.
3)
s)
4)
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6)A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOLTNDATION AND RETAINING V/ALLS
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 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 fuIl 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 backfîll consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traff,rc, 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 90Yo of the maxtmum
standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway
areas should be compacted to at least 95o/o of lhe 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 of 0.50 for the gravel soils and 0.40 for the clay soils. Passive
pressure of compacted backfill against the sides of the footings can be calculated using an
equivalent fluid unit weight of 400 pcf. The coefficient of friction and passive pressure values
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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
a granular material compacted to at least 95Yo of the maximum standard Proctor density at a
moisture content near optimum.
FLOOR SLABS
Expansive clay soils can present a problem where slab-on-grade construction is used. The
expansion potential of the clay soils should be further evaluated at the time of construction prior
to concrete forming or placement.
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction with a possible risk of movement due to the upper expansive clay soils. 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 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%o passing the No. 200
steve.
All lrll materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
I-INDERDRAIN SYSTEM
Although free water was not encountered 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,
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
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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 I foot below lowest adjacent finish grade and sloped at a minimum Io/o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yo passingthe 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 IYz feet deep.
SURFACE DRAINAGE
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 density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. 'We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Free-draining wall backflrll 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) Landscaping which requires regular heavy inigation should be located at least
10 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 thiÁ 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
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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 consulted. Our findings include interpolation and extrapolation of the
subsrlrface conditions identifred 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 recoÍlmendations, 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 frll by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kum¿tr & Associates,lnc.
James H. Parsons, E.I.
Reviewed by:
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l_._. .¿'' .,, _r.-'-
Daniel E. Hardin, P
JHP/kac
Kumar & Associates, lnc. 'Project No, 20"7-521
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APPROXIMATE SCALE-FEET
20-7 -521 Kumar & Associates LOCATIONS OF EXPLORATORY BORINGS 1Fig'
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BORING 1
EL. 99.5'
BORING 2
EL. 98.5'
0 0
16/ 12
22/12
28/ 12
WC=9.4
DD= 1 05
5 5
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20-7 -521 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
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TOPSOIL; CLAY, SANDY, ORGANICS, FIRM, BROWN, SLIGHTLY MOIST
CLAY (CI-); SLIGHTLY SANDY TO SANDY, VERY STIFF, SLIGHTLY MOIST, BROWN
GRAVEL (GM); SANDY TO VERY SANDY, SILTY, DENSE, SLIGHTLY MOIST, LIGHT BROWN.
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DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE
DRTVE SAMpLE, 1 5/8-|NCH LD. SPLrr SPOON STANDARD PENETRATIoN TEST
16/ 12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 1 6 BLOWS OF A 1 4O-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON SEPTEMBER 21,2O2O WITH A 4_INCH-DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO THE BENCHMARK ON FIG. 1.
4, THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCt) (NSTV D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (NSTU OOSIS);
-2oO= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
20-7 -521 Kumar & Associates LEGEND AND NOTES Fig. 3
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SAMPLE OF: Sondy Cloy
FROM:Boring1@2.5'
WC = 9.4 %, DD = 105 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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1.0 APPLIED PRESSURE - KSF 10
20-7 -521 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULTS Fig. 4
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SIEVE ANALYSISHYDROMETER ANALYSIS
CLilR SOUARE OPENINCSU.S. STANDARD SERIES
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150 .300 .600 t.la 2.ı6 1.75.125 2.O
PARTICLES IN MILLIMETERS
9.5 l9 5a.t
CLAY TO SILT COBBLES
GRAVEL 33 % SAND
LIQUID LIMIT
SAMPLE OF: Sllly Sond ond Grovol
47%
PLASTICITY INDEX
SILT AND CLAY 20 %
FROM: Borlng 1 @ 10 & 15 (Combined)
Thoso loll rcsulls opply only lo tho
somplss wh¡ch w€ro losl6d. The
losllng r€porl shqll nol b! raproducod,
cxcopl ln full, wllhoul lh6 wrlllcn
opprovol of Kumor & Aesoclol6s, lnc,
Sl€vo qnqlysis lesllng ls prrform.d ln
occordonc. wlth ASTM 06913, ASTM 07928,
ASTM ct36 ond/or ASTM D'|1,10,
SAND GRAVEL
FINE MEDIUM COARSE FIN E COARSE
20-7 -521 Kumar & Associates GRADATION ÏEST RTSULTS Fis. 5
lcrtKumar & Associates, lnc.'Geotechnical and Materials Engineersand Environmental ScientistsTABLE 1SUMMARY OF LABORATORY TEST RESULTSSilty Sand and Gravel2041JJr.610& lsSOIL TYPESandy Clay1059.42t/t(%)SAND$tGRAVELDEPTH1BORINGATTERBERG LIMITSLIQUID LIMITUNCONFINEDCOMPRESSIVESTRENGTHPERCENTPASSING NO,200 srEvENATURALDRYDENSITYNATURALMOISTURECONTENTPLASTICINDEXNo.20-7-521