HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.20.2021
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
January 20, 2021
Mountain West Construction
Attn: Darren Gallegos
36 Surry Street
Carbondale, Colorado 81623
darrengallegos@hotmail.com
Project No. 21-7-108
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot D44, Aspen Glen,
175 Diamond A Ranch Road, Garfield County, Colorado
Dear Mr. Gallegos:
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 Mountain West Construction, dated January 7, 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 will be a two story wood frame structure over
a crawlspace or full basement level located on the site in the area of the exploratory pits shown
on Figure 1. Ground floor will be structural over a crawlspace or slab-on-grade. Cut depths are
expected to range between about 3 to 8 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 was vacant at the time of our site visit. The lot is relatively flat with a
slight slope down to the south. There is a landscape berm adjacent to the golf course on the west
side of the lot. Vegetation on the site consists of grass and weeds. There was about 4 inches of
snow on the site.
Subsidence Potential: The Aspen Glen Subdivision is underlain by Pennsylvania Age Eagle
Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum
under certain conditions can cause sinkholes to develop and can produce areas of localized
subsidence. During previous work in the area, several broad subsidence areas and smaller size
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sinkhole areas were observed scattered throughout the Aspen Glen development, predominantly
on the east side of the Roaring Fork River (Chen-Northern, Inc. 1993). Lot D44 is mapped as
being within the broad subsidence area. Sinkholes were mapped about 500 feet south and 400
feet northwest of the subject lot. Based on our present knowledge of the site, it cannot be said
for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at
Lot D44 is low and similar to other lots in the area but the owner should be aware of the potential
for sinkhole development. We have in the attached appendix, the Chen-Northern
recommendations for building in the broad surface depression area. We believe these
recommendations are conservative but will reduce structural distress in the event of future
ground movement and should be considered in the residence design.
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 1½ to 3 feet of loose to medium
dense clay fill mixed with scattered cobbles and small boulders, consist of stiff sandy silty clay
down to a depth of 8 feet. Relatively dense, slightly silty sand and gravel with cobbles and
boulders was encountered below the clay from 8 to 10 feet. 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 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 to 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 clay soil designed for an allowable soil bearing pressure of
2,000 psf for support of the proposed residence. Footings placed entirely on the underlying dense
gravel soils can be designed for an allowable soil bearing pressure of 4,000 psf. The clay soils
tend to compress after wetting and there could be some post-construction foundation settlement.
Extending the bearing down to the dense gravel soil should achieve a low settlement risk.
Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns.
Loose and disturbed soils and existing fill 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
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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.
Floor Slabs: The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded
slab-on-grade construction with a movement risk for bearing on the clay soils. 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 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
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 or drywell based in the gravel soils. 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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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 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
2½ 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. Consideration should be given to the use of xeriscape to
limit potential wetting of soils below the foundation caused by irrigation.
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.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
Kumar & Associates
Kumar & Associates
Kumar & Associates