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
May 17, 2022
Becky Ciani
P.O. Box 2913
Glenwood Springs, Colorado 81602
becky@masonmorse.com
Project No. 22-7-294
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 80, Filing 9,
Elk Springs, 0146 Kingbird Drive, Garfield County, Colorado
Dear Becky:
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 15, 2022. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report.
Proposed Construction: Plans for proposed residence were conceptual at the time of our study.
The proposed residence will likely be a one or two-story wood-frame structure with an attached
garage located on the site within the area of the pits shown on Figure 1. Ground floors could be
slab-on-grade or structural over crawlspace. 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.
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. The ground
surface is gently sloping down to the south at a grade of about 5 percent in the general building
area. Vegetation consists of sagebrush, grass and weeds.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three 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 2 feet of topsoil, consist of
dense, basalt gravel, cobbles and boulders in a highly calcareous sandy silt matrix down to the
maximum explored depth of 2½ feet. Results of a gradation analysis performed on a sample of
silty sandy gravel (minus 3-inch fraction) obtained from the site are presented on Figure 3. No
free water was observed in the pits at the time of excavation and the soils were slightly moist.
- 2 -
Kumar & Associates, Inc. ® Project No. 22-7-294
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 granular soil designed for an allowable soil bearing pressure of
2,500 psf for support of the proposed residence. The matrix soils could tend to compress after
wetting and result in relatively minor post-construction foundation settlement. Footings should
be a minimum width of 16 inches for continuous walls and 2 feet for columns. 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 granular soils.
Difficult excavation conditions could be encountered at the site due to the cobbles and boulders
in the cemented matrix. Foundation excavations will probably require a large excavator and/or
chipping equipment. 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
interior slabs for 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 that local perched groundwater can develop during times of
heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
condition. For slab-on-grade or shallow crawlspace construction where the finish floor grade is
at or above surrounding grade an underdrain system is not required. We recommend below-
- 3 -
Kumar & Associates, Inc. ® Project No. 22-7-294
grade construction, such as retaining walls and deeper crawlspace areas, be protected from
wetting and hydrostatic pressure buildup by an underdrain system.
Where used, 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 sump and pump. 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 and covered with filter fabric such as Mirafi 140N or
160N.
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 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. A swale may be
needed uphill to direct surface runoff around the residence.
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.
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
Kumar & Associates
Kumar & Associates
Kumar & Associates