HomeMy WebLinkAboutSubsoil Study
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
April 20, 2021
Mountain West Construction
Attn: Darren Gallegos
58 Surrey Street
Carbondale, Colorado 81623
darrengallegos@hotmail.com
Project No.21-7-238
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot E-30, Aspen Glen
Subdivision, 89 Fox Prowl, Garfield County, Colorado
Dear Darren:
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 Mountain West Construction dated March 3, 2021. 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- or two-story wood frame
structure with attached garage located on the site as shown on Figure 1. Ground floor will be a
combination of structural over crawlspace and slab-on-grade. Cut depths are expected to range
between about 2 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 subject site was vacant at the time of our field exploration. A small berm
is on the north side of the lot. The ground surface is relatively level with a gentle slope down to
the south. Vegetation consists of grass and weeds.
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 ½ foot of topsoil, consist of 1 to
2 feet of sandy clay soils underlain by dense, sandy gravel to the maximum excavated depth of
6½ feet. Results of a gradation analysis performed on a sample of clayey gravel (minus 5 inch
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Kumar & Associates, Inc. ® Project No. 21-7-238
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 to moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend the topsoil and clay
soils be removed and spread footings placed on the undisturbed natural granular soil designed for
an allowable soil bearing pressure of 2,500 psf for support of the proposed residence. Footings
should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Clay soils,
topsoil and 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. 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 10 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 slabs-on-grade to facilitate drainage. This
material should consist of minus 2 inch aggregate with less than 50% passing the No. 4 sieve and
less than 2% 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 can consist of the on-
site soils 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 and crawlspace
areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system.
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Kumar & Associates, Inc. ® Project No. 21-7-238
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 1% 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 1½ 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 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.
Free-draining wall backfill should be covered with filter fabric and 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.
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 irrigation.
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 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
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-238
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE PIT DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf)
2 4 to 4½ 67 29 4 Sandy Gravel