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
March 2, 2021
Armando Rivas
P.O. Box 964
Glenwood Springs, Colorado 81602
armandorivas_19@hotmail.com
Job No. 21-7-142
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 2, Gobblers Knob,
Donegan Road, Garfield County, Colorado
Dear Mr. Rivas:
As requested, Kumar and 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 January 19, 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 design was conceptual at the time of our
study. In general, it will be a one story, wood frame structure with slab-on-grade floor with an
attached slab-on-grade garage. Cut depths are expected to range between about 3 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 site is situated in a broad, undefined hillside drainage. The lot is vacant
and vegetated with grass and weeds, with juniper and oakbrush on the periphery of the building
area. At the northern, uphill corner, the lot slopes gently to moderately down to the southwest.
In the building area, the lot slopes moderately down to the south with a gentle slope from east
and west toward the center of the lot. The ground surface is relatively smooth and firm with
around 5 to 6 feet of elevation change across the building area.
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. Below about ½ foot of topsoil consisting of organic sandy silty clay, a
very stiff sandy silty clay that became calcareous with depth was encountered. In Pits 2 and 3,
the sandy silty clay continued to the total excavated depth of 6½ feet. In Pits 1 and 4, beneath
from 1 to 4 feet of clay, gravel with cobbles and boulders in a silty, sandy, light brown to white
calcareous clay matrix was encountered down to the maximum excavated depths of 8½ and
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Kumar & Associates, Inc. ® Project No. 21-7-142
6 feet. Digging was difficult due to the very stiff clay and the cobbles and boulders and practical
digging refusal was encountered in both the clay and the gravel deposits. Results of swell-
consolidation testing performed on a relatively undisturbed sample of the sandy silty clay soils,
presented on Figure 3, indicate low compressibility under existing moisture conditions and light
loading, and low to moderate compressibility potential when wetted and additionally loaded.
Results of USDA gradation analyses performed on a sample of the natural coarse granular soils
(minus 5-inch fraction) obtained from the site are presented on Figure 4. The laboratory test
results are summarized in Table 1. 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 either the undisturbed natural clay soils or the undisturbed natural gravel soils, and
designed for an allowable soil bearing pressure of 1,500 psf for support of the proposed
residence. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for
columns. The 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. We should observe the completed foundation excavation for bearing
conditions. 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 heavily 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 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 or free-draining gravel 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 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
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Kumar & Associates, Inc. ® Project No. 21-7-142
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.
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 content 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. Swales should be
constructed upslope of the building area to divert runoff. Water should not be
allowed to pond upslope of the building area which could impact slabs-on-grades
and foundations. Graded swales should have a minimum slope of 2%.
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.
Septic Disposal Area: The subsoil conditions in the planned septic disposal area were evaluated
by digging Pit 4 at the location shown on Figure 1. The subsurface profile mainly consists of
gravelly sandy loam. Results of a USDA gradation test performed on a sample of the coarse
grained soils are shown on Figure 4. Based on these findings, the tested area appears suitable for
an infiltration septic disposal system. A civil engineer should be engaged to design the septic
disposal system.
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
Kumar & Associates
Kumar & Associates
Kumar & Associates
1 MIN.
4 MIN.19MIN.15 MIN.60MIN.#325 #140 3/4"3/8"1 1/2"3"5"6"8"
DIAMETER OF PARTICLES IN MILLIMETERS
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
SIEVE ANALYSIS
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HYDROMETER ANALYSIS
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20315276.237.519.09.54.752.001.00.500.025.106.045.019.009.005.002.001
SILT COBBLESLARGE
GRAVEL
MEDIUMCOARSEMEDIUMV. FINE
SANDCLAY FINE V. COARSE SMALL
USDA SOIL TYPE:
GRAVEL %SILT %CLAY %
FROM:Pit 4 @ 4.5'-5'
20 31 13SAND %36
Gravelly Sandy Loam
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-142
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION USDA SOIL TEXTURE SOIL TYPE PIT DEPTH GRAVEL SAND SILT&CLAY GRAVEL SAND SILT CLAY
(ft) (%) (pcf) (%) (%) (%) (%) (%) (%) (%)
2 3½ 8.4 98 Sandy Silty Clay
4 4½-5 20 31 36 13 Gravelly Sandy Loam