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
June 30, 2021
Louise Marron
P.O. Box 157
Carbondale, CO 81623
lamarron421@hotmail.com
Project No. 21-7-331
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 7, Mountain
Springs Ranch, Mountain Springs Road, Garfield County, Colorado
Ms. Marron:
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 April 4, 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 construction consists of a new residence located about
100 feet north of the existing cabin on the lot as shown on Figure 1. Ground floors could be
slab-on-grade or structural over crawlspace. Cut depths are assumed 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 subject site was mostly native hillside terrain at the time of our site visit.
The ground surface is moderately sloping down to the west at a grade of about 15% with about
5 feet of elevation difference across the proposed building footprint. Vegetation consists of
scrub oak stands with grass and weed meadows.
Subsurface Conditions: The subsurface conditions were evaluated by excavating 2 exploratory
pits in the designated building site and 2 profile pits in the designated septic disposal site at the
approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The
subsoils encountered, below about 2 to 3 feet of topsoil, mainly consist of sandy clay with
scattered gravel and cobbles to the typical explored depth of 8 feet. Pit 3 encountered sandy clay
with basalt cobbles and scattered boulders at about 7 feet to the pit depth of 9½ feet. Results of
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Kumar & Associates, Inc. ® Project No. 21-7-331
swell-consolidation testing performed on relatively undisturbed samples from the exploratory
pits, presented on Figures 3 and 4, indicate low compressibility under existing moisture
conditions and light loading and a low to moderate expansion potential when wetted. No free
water was observed in the pits (except for slight seepage at 2 feet in Pit 1) and the soils were
moist to very moist.
Foundation Bearing Conditions: Testing of the clay soils taken from the pits at the site
generally show low to moderate expansion potential. With the current information and test
results, we recommend the foundation areas be sub-excavated at least 2 feet below design
bearing level and backfilled with imported, relatively well graded granular material (such as
CDOT Class 6 road base). When the foundation excavation for the building has been cut to
design bearing level, a representative of the geotechnical engineer should evaluate the exposed
soils for the need to sub-excavate and place structural fill to help mitigate the expansion
potential. A low movement risk option would be to extend the foundation bearing down to a
depth of relatively stable moisture content such as with micro-piles possibly around 20 feet deep.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on at least 2 feet of imported granular structural fill designed for an allowable soil bearing
pressure of 2,000 psf for support of the proposed structure. The clay soils tend to expand after
wetting and there could be post-construction foundation movement of around 1 to 2 inches
depending on the soil conditions and depth of wetting. Footings should be a minimum width of
16 inches for continuous walls and 2 feet for columns. The topsoil, recommended depth of clay
sub-excavation and loose disturbed soils within the footing areas should be removed to expose
the undisturbed natural soils. Structural fill should be placed in thin lifts and 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 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 also be designed to resist a lateral earth pressure based on an equivalent fluid
unit weight of at least 60 pcf for the on-site soil as backfill excluding topsoil and rock larger than
6 inches.
Floor Slabs: The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded
slab-on-grade construction with a risk of heave and building distress. Sub-excavation of the clay
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Kumar & Associates, Inc. ® Project No. 21-7-331
soils at least 2 feet and replacement with structural fill should be provided to help mitigate the
heave potential or a structural floor above crawlspace should be used. 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 should consist of
imported granular soils devoid of vegetation, topsoil and oversized rock.
We recommend vapor retarders conform to at least the minimum requirements of ASTM E1745
Class C material. Certain floor types are more sensitive to water vapor transmission than others.
For floor slabs bearing on angular gravel or where flooring system sensitive to water vapor
transmission are utilized, we recommend a vapor barrier be utilized conforming to the minimum
requirements of ASTM E1745 Class A material. The vapor retarder should be installed in
accordance with the manufacturers’ recommendations and ASTM E1643.
Underdrain System: Although free water was generally not encountered in the exploratory pits,
it has been our experience in mountainous areas and where there are clay soils that local perched
groundwater can develop during times of heavy precipitation or seasonal runoff (such as the
slight seepage at bottom of the topsoil at Pit 1). 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:
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Kumar & Associates, Inc. ® Project No. 21-7-331
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction. Drying could increase the expansion potential of the clay
soils.
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. A swale may be
needed uphill to direct surface runoff around the structures.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation, such as sod, should be
located at least 5 feet from the building.
Septic System: Profile Pits 1 and 2 located within the proposed septic disposal area encountered
moderately blocky clay with scattered gravel to the pit depths of 8 feet. The USDA gradation
testing results of the sample taken from Profile Pit 2, presented on Figure 6, indicate a soil type
of 2A (Silt Loam). A civil engineer should design the infiltration 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
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
the area. Our services do not include determining the presence, prevention or possibility of mold
or other biological contaminants (MOBC) developing in the future. If the client is concerned
about MOBC, then a professional in this special field of practice should be consulted. Our
findings include interpolation and extrapolation of the subsurface conditions identified at the
exploratory pits and variations in the subsurface conditions may not become evident until
excavation is performed. If conditions encountered during construction appear different from
those described in this report, we should be notified at once so re-evaluation of the
recommendations may be made.
Kumar & Associates
LOT 7
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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
TIME READINGS
HYDROMETER ANALYSIS
#4#10#18#35#60
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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-2 @ 4'-5'
11 14 24SAND %51
Silt Loam
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-331
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 5½ 18.3 105 Sandy Clay
8-9 20 7 73 Clay with Gravel
2 4 32.8 80 Clay
7 37.6 67 Clay
Profile
Pit
2 4-5 11 14 51 24 Silt Loam