HomeMy WebLinkAboutSubsoils Report for Foundation Design
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
December 29, 2025
Paul and Wendy Gabica
429 Meadow Court
Basalt, Colorado 81621
wsg429@comcast.net
Project No. 25-7-576
Subject: Subsoil Study for Foundation Design, Proposed Steel Building, 6060 County Road
313, Garfield County, Colorado
Dear Paul and Wendy:
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 September 9, 2025. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered
are presented in this report.
Proposed Construction: The proposed structure will be a steel-framed building located on the
site in the area of the pits, as shown on Figure 1. Ground floor will be slab-on-grade construction.
Cut depths are expected to range between about 4 to 6 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 proposed building location is not occupied and is vegetated with grasses
and oak brush. Several small outbuildings are located nearby. The site is nearly flat, with Divide
Creek located about 150 feet to the south.
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 one foot of topsoil, consist of very
sandy silt down to a depth of about 5½ feet, underlain by dense silty, sandy gravel down to the
explored depth of about 6 feet. Results of swell-consolidation testing performed on relatively
undisturbed samples of the upper sandy silt soil, presented on Figures 3 and 4, indicate low
compressibility under existing moisture conditions and light loading and low to moderate
compressibility when wetted under light loading. 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
placed on the undisturbed natural upper silt soil designed for an allowable soil bearing pressure
of 1,500 psf. The silt soils tend to compress after wetting and there could be some post-
construction foundation settlement. Footings placed on the natural gravel soils or structural fill
Paul and Wendy Gabica Job No. 25-7-576
December 29, 2025 Page 2
Kumar & Associates, Inc.®
bearing on the gravel soils can be designed for an allowable soil bearing pressure of 2,500 psf
for support of the proposed building. 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 or structural fill 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 should be designed to
resist a lateral earth pressure based on an equivalent fluid unit weight of at least 55 pcf for the
on-site silt 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 ground level slabs-on-grade for support
and to facilitate drainage. This material should consist of minus 2-inch aggregate with less than
50% passing the No. 4 sieve and less than 12% 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: It is our understanding the proposed finished floor elevation at the lowest
level is at or above the surrounding grade. Therefore, a foundation drain system is not required.
It has been our experience in mountainous areas 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 and wall drain system. If the finished floor elevation of the proposed structure is
revised to have a floor level below the surrounding grade, we should be contacted to provide
recommendations for an underdrain system.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the building 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.
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.
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 25-7-576
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 2 7.6 89 59 Very Sandy Silt
2 3 7.7 80 66 Very Sandy Silt