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 10, 2021
Matt and Julie Fadden
2815 W. 115th Drive
Westminster, Colorado 80234
matt.fadden@comcast.net
julie.fadden@comcast.net
Project No. 21-7-305
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 9, Roaring Fork
Preserve, TBD Willow Lane, Garfield County, Colorado
Dear Matt and Julie:
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 Matt and Julie Fadden dated March 25, 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 single-story wood-frame residence
with attached garage. The proposed barn will be a tall single-story structure with a partial loft
level. The proposed residence will be located on the site in the area of the pits shown on
Figure 1. Ground floor will be structural over crawlspace for the residence and slab-on-grade for
the garage and barn. Cut depths are expected to range between about 2 to 4 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 a vacant pasture at the time of our field exploration. The
ground surface is relatively flat with varied gentle slopes. Irrigation ditches are north and
southwest of the building area. Vegetation consists grass and weeds with scattered aspen trees
along the ditches to the north and east.
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Kumar & Associates, Inc. ® Project No. 21-7-305
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits and a profile pit at the approximate locations shown on Figure 1. The logs of the
pits are presented on Figure 2. The subsoils encountered, below about 1 foot of topsoil, consist
of sandy gravel with cobbles down to the maximum explored depth of 8 feet. Results of a
gradation analysis performed on a sample of sandy gravel (minus 5-inch fraction) obtained from
Pit 1 at 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 to moist. Perforated PVC pipe was installed in the
two pits for long term monitoring of the groundwater level. The nearby profile pit for septic
design was logged by Carlo Ostberg of CBO Septic Consulting.
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 soil designed for an allowable soil bearing pressure of
2,500 psf for support of the proposed residence and barn. Footings should be a minimum width
of 16 inches for continuous walls and 2 feet for columns. Loose disturbed soils, topsoil 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 to span local anomalies such
as by assuming an unsupported length of at least 10 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 45 pcf for the on-site gravel soil as backfill.
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 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.
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Kumar & Associates, Inc. ® Project No. 21-7-305
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: It is our understanding the proposed finished floor elevation at the lowest
level is at or above the surrounding grade. Therefore, a foundation drain system is not required.
If the finished floor elevation of the proposed structure is revised to have a floor level below the
surrounding grade, we should be contacted to provide recommendations for an underdrain
system. All earth retaining structures should be properly drained.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence and barn have 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 (if any) 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-305
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE PIT DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf)
1 6-7 62 33 5 Slightly Silty Sandy Gravel