HomeMy WebLinkAboutSubsoil Report
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
July 7, 2021
Red House Architecture
Attn: Bruce Barth
815 Blake Avenue
Glenwood Springs, Colorado 81601
bruce@redhousearchitecture.com
Project No. 21-7-454
Subject: Subsoil Study for Foundation Design and Septic Disposal Soils, Proposed
Residence, Lot 20, Teller Springs, 280 Lariat Lane, Garfield County, Colorado
Gentlemen:
As requested, Kumar & Associates, Inc. performed a subsoil study for foundation design and
septic disposal soils at the subject site. The study was conducted in general accordance with our
agreement for geotechnical engineering services to Red House Architecture dated May 13, 2021.
The data obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report. Evaluation of potential geologic hazard
impacts on the site are beyond the scope of this study.
Proposed Construction: The proposed residence will be a one- and two-story wood-frame
structure and garage located on the site as shown on Figure 1. Ground floors are assumed to be
structural over crawlspace in the residence and slab-on-grade in the garage. Cut depths are
expected to range between about 2 to 5 feet. Foundation loadings for this type of construction
are assumed to be relatively light and typical of the proposed type of construction. The septic
disposal area is proposed to be located south of the proposed residence.
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 and previously
used as horse pasture and riding areas. The ground surface was relatively flat with a gentle slope
down to the southeast. Vegetation consists of sparse grass and weeds in the proposed building
area.
Subsidence Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the
Teller Springs subdivision. These rocks are a sequence of gypsiferous shale, fine-grained
sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility
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Kumar & Associates, Inc. ® Project No. 21-7-454
that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the
lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. Teller Springs is known to contain several sinkholes
mainly scattered throughout the lower, eastern part of the subdivision. These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork
River valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities
was encountered in the subsurface materials; however, the exploratory pits were relatively
shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot 20 throughout the service life of the proposed residence, in our
opinion, is low; however, the owner should be made aware of the potential for sinkhole
development. If further investigation of possible cavities in the bedrock below the site is desired,
we should be contacted.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits in the proposed building area and one profile pit in the designated septic
disposal area at the approximate locations shown on Figure 1. The logs of the pits are presented
on Figure 2. The subsoils encountered, below shallow depth fill soils, consist of stiff, sandy silty
clay underlain by dense, silty sandy gravel with cobbles at depths of about 2 to 4 feet. Results of
swell-consolidation testing performed on a relatively undisturbed sample of sandy silty clay,
presented on Figure 3, indicate low compressibility under existing moisture conditions and light
loading, minor collapse potential when wetted under constant light loading and moderate
compressibility under additional loading after wetting. The results of gradation analysis
performed on a sample of gravel (minus 5-inch fraction) are shown 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 to moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, spread footings placed on the
undisturbed natural gravel soil below existing fill and clay soils and designed for an allowable
bearing pressure of 3,000 psf are recommended for foundation support. The gravel soils are
relatively dense and post-construction foundation settlement should be relatively minor.
Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns.
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Kumar & Associates, Inc. ® Project No. 21-7-454
The existing fill, clay soils 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 gravel soils. We should observe the completed foundation excavation prior
to forming footings to confirm suitable bearing conditions. Structural fill used to re-establish
design bearing level should consist of granular material such as road base compacted to at least
98% of standard Proctor density at near optimum moisture content. 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 50 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 relatively well graded sand and gravel, such as road base, should be placed beneath slabs
for subgrade support. 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: Although free water was not encountered during our exploration, it has
been our experience in the area and where clay soils are present 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
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Kumar & Associates, Inc. ® Project No. 21-7-454
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to
a suitable gravity outlet or drywell. 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: Providing proper surface drainage will be critical to the long-term
performance of the residence. 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 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.
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 System Suitability: The USDA gradation testing results presented on Figure 5, indicate a
soil type of R-0 (Very Gravelly Loamy Sand) for the sample taken from Profile Pit 1 at 6½ to
7½ feet. A civil engineer should design the infiltration septic disposal system. If the system is
designed to be based in the upper clay soils, additional soil classification testing should be
performed for design parameters.
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,
the proposed type of construction and our experience in the area. Our services do not include
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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:PP-1 @ 6.5'-7.5'
58 29 5SAND %8
Very Gravelly Loamy Sand
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-454
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) (%) (%) (%) (%) (%) (%) (%)
1 1 12.0 107 Sandy Silty Clay
2 4-4½ 44 39 17 Silty Sandy Gravel
Profile
Pit
1 6½-7½ 58 29 8 5 Very Gravelly Loamy
Sand