HomeMy WebLinkAboutSubsoil Study - Foundationl(l Xumar & Associates, Inc.'
Geotechnical and Materials Engineers
and Environmental Scientists
RECEIVED
GARFIELD COUNTY
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5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.com
Offrce Locatiorrs: Denver (HQ), Palker, Colorado Springs, Forl Collins, Glenwood Springs, and Surnmit County, Colorado
January 13,202I
Terry Thompson
233 Yaquero Road
Carbondale, Colorado 81623
terry@thompsonframer'com
project No. 21-7-90r
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 14, Callicotte
Ranch, Sopris Lane, Garfield County, Colorado
Dear Terry:
As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at
the subject site. The study was conducted in accordance with our agreement for geotechnical
engineering services to you dated December I,202L The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report.
Proposed Construction: The proposed residence will be a single-story structure with attached
garage located on the site as shown on Figure 1. Ground floors will likely be a combination of
structural over crawlspace and slab-on-grade. Cut depths are expected to range between about2
to 5 feet. Foundation loadings forthis type of construction are assumedto be relatively light and
typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The subject site was vacant at the time of our f,reld exploration. The ground
surface is sloping down to the south at a grade between about 5 and 10 percent. Vegetation
consists ofjuniper trees with an understory of grass and sagebrush near the front (south) side of
the lot and sagebrush, grass and weeds in the rear area of the lot.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered, below about I to l% feet of topsoil, consist of
dense, silty gravel and sand with cobbles and boulders down to the maximum explored depth of
7 feet. Results of a gradation analysis performed on a sample of silty gravel and sand (minus
3-inch fraction) obtained from the site are presented on Figure 3. No free water was observed in
the pits at the time of excavation and the soils were slightly moist.
a
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural soil designed for an allowable soil bearing pressure of
2,000 psf for support of the proposed residence. The matrix soils tend to compress after wetting
and there could be some post-construction foundation settlement. Footings should be a
minimum width of 18 inches for continuous walls and2 feet for columns. Utility trenches and
cut areas deeper than about 5 feet may require rock excavation techniques such as chipping or
blasting. Loose disturbed soils encountered at the foundation bearing level within the excavation
should be removed and the footing bearing level extended down to the undisturbed natural soils.
Voids created from boulder removal at footing grade should be filled with an imported structural
material such as road base compacted to 98 percent standard Proctor density at a moisture
content near optimum or concrete. Exterior footings should be provided with adequate cover
above their bearing elevations for frost protection. Placement of footings at least 36 inches
below the exterior grade is typically used in this area. 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 be designed to resist a
lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site
soil as backfill.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded
slab-on-grade construction. 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 less Íhan 50o/o passing the No. 4
sieve and less than 2o/o passing the No. 200 sieve.
All fill 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 soils or a suitable imported granular material devoid of vegetation, topsoil and oversized
rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the areathat local perched groundwater can develop during times of
Kumar & Associates, lnc. @ Project No. 21-7-901
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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 crawlspace
areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system.
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent hnish 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 2o/o 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 lYz 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 of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 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.
Free-draining wall backfill should be capped with about 2 feet of the on-site, finer
graded soils to reduce surface water infiltration.
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 pavement and walkway areas. A swale may be
needed uphill to direct surface runoff around the residence.
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
5 feet from the building. Consideration should be given to the use of xeriscape to
limit potential wetting of soils below the foundation caused by inigation.
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 wananty either
express or implied. The conclusions and recommendations submitted in this report are based
upon the data obtained from the exploratory pits excavated at the locations indicated on Figure 1
and to the depths shown on Figure 2,the proposed type of construction, and our experience in
Kumar & Associates, lnc. @ Project No. 21-7-901
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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 pits 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 at once so re-evaluation of the
recoilrmendations 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 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.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Kumar & Associates, Inc.
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James H. Parsons, P.E
Reviewed by:
Daniel E. Hardin, P
JHP/kac
attachments Pits
Figure 2 - Logs Exploratory Pits
Figure 3 -Gradation Test Results
Table 1 - Summary of Laboratory Test Results
Figure 1
Kumar & Associates, lnc. I Project No. 21-7-901
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LEGEND
TOPSOIL: SANDY, ORGANICS, FIRM, MOIST, BROWN.
GRAVEL AND COBBLES (GM): SANDY TO VERY SANDY, SILTY, BOULDERS, DENSE, SLIGHTLY
MOIST, TAN.
DISTURBED BULK SAMPLE
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON DECEMBER 14,2021.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY PIT 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 PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION
7. LABORATORY TEST RESULTS:
wc = WATER CONTENT (%) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D A22);
-200= PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1 1 a0);
LL = LIQUID LIMIT (ASTM D a318);
PI = PLASTICITY INDEX (ASTM D 4518).
21 -7 -901 Kumar & Associates LOGS OF EXPLORATORY PITS lig. 2
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DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 36 % SAND
LIQUID LIMIT 50
SAMPLE OF: Silty Sond ond Grovel
33%
PLASTICITY INDEX
SILT AND CLAY 31 %
9
FROM:Boringl@4'-4.5'
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Siovg onolysis l€sling ls psrformod ln
occordonco wlth ASIM 06913, ASTM D7928,
ASTM c136 ond,/or ASTM Dll,fo.
HYDROMETER ANALYSIS SIEVE ANALYSIS
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7 HRS
U.S. SIANDARO SERIES
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CLEAR SQUARE OPENINGS
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21 -7 -901 Kumar & Associates GRADATION TTST RTSULTS Fis. 3
lGrtiiffifiåinftniiÍå**TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-901Silty Sand and GravelSOIL TYPEATTERBERG LIMITSGRADATIONLIQUID LIMITUNCONFINEDCOMPRESSIVESTRENGTHPERCENTPASSING NO,2(¡(l SIEVENATURALDRYDENSITYNAÏURALMOISTURECONTENTSAND(%)GRAVEL(%)PLASTICINDEX5091aJ1-JJ3613.94to 4%(ft)DEPTHSAMPLE LOCATIONPtïI