HomeMy WebLinkAboutSubsoil Studyl(Î'tiiffi'*ffinffirTåú**
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.kumatusa.com
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit C"ËËfÈ"lVË
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November 13,2020
Cathey Massey
l80B Seçberg Circle
Carbondale, Colorado 81 623
(Cathey.massey@, gmail. com)
i,:lAN ü 4 2üil
GARFIELD üOLífdI''J
COMIIiUNITY DËVELOPMh.¡I I
Project No.20-7-688
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 3, Wooden Deer
Subdivision, Wooden Deer Road, Garfield County, Colorado
Dear Cathey:
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 November 10,2020. 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 one story wood frame structure over
a crawlspace or slab-on-grade floor, located on the site in the area of the pits shown on Figure l.
Cut depths are expected to range befween about 3 to 4 feet. Foundation loadings for this type of
construction are assumed to 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 building area is located on the south side of the lot in a relativeiy flat area
with steeper slopes down to the south along the property line. The area is near the top of a broad
ridge. Vegetation consists of pinon and juniper forest with an understory of sage brush and
weeds. Basalt rocks were observed on the ground surface in the building area.
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 Yzfoot of topsoil, consist of
medium dense, silty sand and gravel with basalt rocks up to about 4 feet in size. Refusal to
digging with a small backhoe was encountered at 5% to 6% feet in the pits. No free water was
observed in the pits at the time of excavation and the soils were slightly moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
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placed on the undisturbed natural soil designed for an allowable soil bearing pressure of
1,500 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 and 2 feet for columns. Loose and 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. 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 (if any) should be designed to resist a lateral earth pressure based on an equivalent
fluid unit weight of at least 45 pcf for the on-site soil as backfill excluding organics and rock
larger than 6 inches.
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 road base gravel should be placed beneath slabs to provide support. This material
should consist of minus 2-inch aggregate with less than 50Yo passing the No. 4 sieve and less
than l2'Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fillcan consist of the on-
site soils or a suitable imported gravel 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 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, deep crawlspace
and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. The proposed slab-on-grade construction should not require an underdrain.
If installed, 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 2Yopassing the No. 200 sieve, less than 50% passing
Kumar & Associates, lnc. o Project No. 20-7-688
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the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at
Least Ilz feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
l) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjustecl to near optimum moistr¡re and compacterl 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 6 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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 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
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 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
recommendations may be made.
This report has been prepared for the exclusive use by our client for design pu{poses. 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
Kumar & Associales, lnc. @ Project No. 20-7-688
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of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
If you have any questions ır if we may be of further assistance,þlèase let us know.
Respectfully Submitted,
Kum¡r & Associ¡tcs, Inc.
Daniel E. Hardin, P.E.
Rev. by: SLP
DEH/kac
attachments Figure 1 - Location of Exploratory Pits
Figure 2 * Logs of Exploratory Pits
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LEGEND
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TOPSOIL. ORGANIC SILTY SAND WITH GRAVEL, ROOTS, SOFT, MOIST, DARK BROWN.
BASALT GRAVEL AND COSBLES (GM-SM); SANDY, SILTY, BASALT ROCKS UP TO 1l FEET lN
PIT 2 AND UP TO 4 FEET IN PIT 1. GRAY BASALT ROCKS WITH WHITISH GRAY SAND AND
SILT MATRIX.
REFUSAL TO BACKHOE DIGGING.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON NOVEMBER 10, 2020.
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 LOT PLAN.
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 AÏ THE TIME OF EXCAVATION. PIÏS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
20-7 -688 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2