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
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 14, SPRING RIDGE RESERVE
144 SPRING VIEW DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-647
DECEMBER 2, 2021
PREPARED FOR:
JAKE MCKITTRICK
997 BRUSH CREEK LANE
GLENWOOD SPRINGS, COLORADO 81601
jake@flywateroutdoors.com
Kumar & Associates, Inc. ® Project No. 21-7-647
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 1 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS .................................................................................................................... - 3 -
FLOOR SLABS ...................................................................................................................... - 4 -
UNDERDRAIN SYSTEM ..................................................................................................... - 4 -
SURFACE DRAINAGE ......................................................................................................... - 5 -
LIMITATIONS ........................................................................................................................... - 5 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 21-7-647
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located in the
western part of Lot 14, Spring Ridge Reserve, 144 Spring View Drive in Garfield County,
Colorado. The project site is shown on Figure 1. The purpose of the study was to develop
recommendations for the foundation design. The study was conducted in accordance with our
agreement for geotechnical engineering services to Jake McKittrick dated July 27, 2021.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classification,
compressibility or swell potential and other engineering characteristics. The results of the field
exploration and laboratory testing were analyzed to develop recommendations for foundation
types, depths and allowable pressures for the proposed building foundation. This report
summarizes the data obtained during this study and presents our conclusions, design
recommendations and other geotechnical engineering considerations based on the proposed
construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. In general, we
assume the residence will be a one to two-story wood-framed structure over crawlspace or slab-
on-grade. Grading for the structure is assumed to be relatively minor with cut depths between
about 2 to 7 feet. We assume relatively light foundation loadings, typical of the assumed
proposed type of construction.
If building location, grading or loading information is significantly different than described, 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 slopes about 10 percent down to the
northeast. Vegetation consists of grasses and weeds. Nearby lots are developed with one to two-
story wood-framed houses.
FIELD EXPLORATION
The field exploration for the project was conducted on August 27, 2021. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
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Kumar & Associates, Inc. ® Project No. 21-7-647
The borings were advanced with 4-inch diameter continuous flight augers powered by a truck-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils and bedrock were taken with 1⅜-inch and 2-inch I.D. spoon samplers.
The samplers were driven into the subsurface materials at various depths with blows from a 140
pound hammer falling 30 inches. This test is similar to the standard penetration test described by
ASTM Method D-1586. The penetration resistance values are an indication of the relative
density or consistency of the subsoils and hardness of bedrock. Depths at which the samples
were taken and the penetration resistance values are shown on the Logs of Exploratory Borings,
Figure 2. The samples were returned to our laboratory for review by the project engineer and
testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils, below about 1 to 1½ feet of topsoil, consist of medium dense or stiff to very stiff,
occasionally clayey interbedded sand and silt down to the maximum explored depth of 26 feet in
Boring 2. In Boring 1, the sand and silt soil was underlain by hard sandstone/siltstone bedrock at
a depth of about 47 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content, density, swell-consolidation and percent silt and clay-sized particles. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the sandy silt and clay
soils, presented on Figure 4, indicate minor compressibility under existing moisture conditions
and light loading. When the samples were wetted under a constant light load, the results were
variable and one sample was non-expansive and one exhibited low expansion potential. The
laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were moist.
FOUNDATION BEARING CONDITIONS
The upper clayey sand and silt soils encountered in the borings possess low bearing capacity and
typically variable compressibility potential when wetted. Our experience in the area indicates
the swell potential is minor (if any) and can be discounted in foundation design. We should
observe the soil conditions exposed at the time of excavation and evaluate them for swell-
compression potential and possible mitigation such as sub-excavation to a certain depth and
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Kumar & Associates, Inc. ® Project No. 21-7-647
replacement with compacted structural fill. Shallow spread footings placed on the clayey sand
and silt soils can be used for support of the proposed residence with a risk of foundation
movement mainly if the fine-grained bearing soils become wetted. Proper surface drainage as
described in this report will be critical to the long-term performance of the structure.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the residence be founded with spread footings bearing
on the natural soils below the topsoil.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
4) 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 also be designed to resist a
lateral earth pressure corresponding to an equivalent fluid unit weight of at least
50 pcf for the on-site soils as backfill.
5) The topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the firm natural subsoils or replaced with
structural fill. The exposed soils in footing area should then be moistened and
compacted. Structural fill can consist of the onsite soils, compacted to at least
98 percent of standard Proctor density at near optimum moisture content. The fill
should extend beyond the footing edges a distance at least equal to one-half the
fill depth below the footings.
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Kumar & Associates, Inc. ® Project No. 21-7-647
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. The exposed subgrade should be evaluated for expansion potential and the need
for sub-excavation and placement of structural fill. 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 the garage slab to
facilitate drainage. This material should consist of minus 2-inch aggregate with at least 50%
retained on 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 and topsoil or imported structural material, such as road base.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
mountainous areas 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. 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. (An
impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils).
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Kumar & Associates, Inc. ® Project No. 21-7-647
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.
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 paved areas. Free-draining wall backfill (if any)
should be covered with filter fabric and capped with about 2 feet of the on-site
soils to reduce surface water infiltration.
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 foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building 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 borings drilled at the locations indicated on Figure 1, 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 borings 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 so
that 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
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-647
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf)
1 5 12.5 118 Sandy Clayey Silt
10 7.2 113 45 Very Silty Sand
15 11.0 126 55 Clayey Very Sandy Silt
2 5 10.5 112 Sandy Silty Clay
15 8.1 110 60 Clayey Very Sandy Silt
25 3.2 133 15 Silty Sand with Gravel