HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.13.2020
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
October 13, 2020
Lavern Schultz
1122 Florence Avenue
Eau Claire, Wisconsin 54703
vns.s@icloud.com
Project No.20-7-505
Subject: Subsoil Study for Foundation Design, Proposed Residence, 125 Primrose Point,
Lot 59, Filing 6, Elk Springs, Garfield County, Colorado
Dear Mr. Schultz:
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 2, 2020. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report.
Proposed Construction: Design plans for the residence have not been developed. We assume
the residence will be one to two stories of wood frame construction over a shallow crawlspace
located in the area of Pits 3 and 4 shown on Figure 1. Ground floor will be structural over
crawlspace or slab-on-grade. Cut depths are expected to range between about 3 to 5 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 relatively flat with a gentle slope down to the west. The
slope steepens to the west of the building envelope. The building area is vegetated with pinon
and juniper trees with an understory of weeds and scattered sagebrush. The front of the lot, in
the area of Pits 1 and 2 is open meadow, vegetated with grass, weeds and sagebrush. Basalt
rocks were observed on the ground surface in the northwest part of the lot. Some basalt rocks
were also observed on the ground surface in the northern part of the building envelope.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
four exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered in the building area, below about ½ foot of
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Kumar & Associates, Inc. ® Project No. 20-7-505
topsoil, consist of up to 1 foot of very stiff, sandy silty clay overlying 1 to 1½ feet of hard,
calcareous, sandy silt. Hard basalt rocks were encountered at 2½ feet down to refusal to backhoe
digging at 3½ to 4 feet. Results of swell-consolidation testing performed on a relatively
undisturbed sample of the clay soils, presented on Figure 3, indicate low compressibility under
existing moisture conditions and light loading and a minor expansion potential when wetted. 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 soil below the topsoil designed for an allowable soil bearing
pressure of 2,500 psf for support of the proposed residence. Our experience in this area is that a
conventional heavy duty excavator typically used for building excavation should be able to
excavate about two feet deeper than our pits in the open, larger excavation anticipated for the
proposed residence. Deeper foundation excavations or confined trench excavations for utilities
may require rock excavation techniques such as chipping or blasting. We should observe the
completed excavation for bearing conditions. Footings should be a minimum width of 16 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 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 excluding organics and rock larger than about 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 sandy gravel, such as road base, should be placed beneath 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 12% passing the No. 200 sieve.
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Kumar & Associates, Inc. ® Project No. 20-7-505
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 finer-graded soils or a suitable imported 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 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 deep
crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. Shallow crawlspaces or 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 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 (if any) 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