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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 Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
November 22, 2021
Dr. Ben Alter
2563 Dolores Way
Carbondale, Colorado 81623
drbenjaminalter@gmail.com
Project No. 21-7-811
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 61, Spring Ridge
Reserve, Hidden Valley Drive, Garfield County, Colorado
Dr. Alter:
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 October 11, 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 located off the end of the driveway
within the northern part of the building envelope as shown on Figure 1 with a partial walkout
basement level. Ground floors could be structural above crawlspace and slab-on-grade. Cut
depths are expected to range between about 2 to 8 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 residence area was undisturbed natural hillside vegetated with pinon and
juniper trees and understory of grass, weeds and sage brush. The ground surface slope is
moderate down to the west with around 8 feet of elevation difference across the residence
footprint. The driveway was cut into the hillside from Hidden Valley Drive to the residence site
and exposed sandstone/siltstone bedrock in the uphill cut face.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
4 exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered, below a thin topsoil/root zone depth and about
one foot of silt in Pit 1, consist of weathered to very hard sandstone/siltstone bedrock to the pit
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Kumar & Associates, Inc. ® Project No. 21-7-811
depths of 2½ to 5 feet where practical digging refusal was encountered. Results of finer than
sand size gradation analysis performed on a sample of the silt soil obtained from Pit 1 are
summarized in Table 1. No free water was observed in the pits at the time of excavation and the
soils and bedrock were slightly moist.
Foundation Recommendations: Considering the subsurface conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural bedrock materials designed for an allowable bearing pressure
of 4,000 psf for support of the proposed residence. Foundation settlements should be minor, less
than 1 inch. Footings should be a minimum width of 16 inches for continuous walls and 2 feet
for columns. The topsoil, silt soil and loose disturbed rock encountered at the foundation bearing
level within the excavation should be removed and the footing bearing level extended down to
the undisturbed natural bedrock. 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. It may be acceptable to reduce frost depth
where very hard cemented rock is encountered. 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 45 pcf for the on-site
soil and well broken rock or imported granular material such as base course as backfill.
Floor Slabs: The natural on-site soils and rock, 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 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
onsite soils and well broken rock devoid of vegetation, topsoil and oversized rock or imported
granular material such as road base.
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Kumar & Associates, Inc. ® Project No. 21-7-811
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the area and where bedrock is shallow 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.
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 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 will be
needed uphill to direct surface runoff around the residence.
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
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-811
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)
1 1 4.1 88 92 Slightly Sandy Silt