HomeMy WebLinkAboutSubsoils Report for Foundation Design
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
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 DETACHED GARAGE
LOT 8 LIONS RIDGE ESTATES
318 LION’S RIDGE ROAD
GARFIELD COUNTY, COLORADO
PROJECT NO. 25-7-247
JUNE 9, 2025
PREPARED FOR:
JACKIE DALY AND LARRY SWIFT
318 LIONS RIDGE ROAD
GARFIELD COUNTY, COLORADO 81623
jackiedalyrealy@gmail.com
Kumar & Associates, Inc. ® Project No. 25-7-247
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ................................................................................... - 1 -
PROPOSED CONSTRUCTION............................................................................................. - 1 -
SITE CONDITIONS ................................................................................................................. - 1 -
SUBSIDENCE POTENTIAL ................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ............................................................................................... - 2 -
FOUNDATION BEARING CONDITIONS ............................................................................. - 2 -
DESIGN RECOMMENDATIONS .......................................................................................... - 3 -
FOUNDATIONS ................................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 3 -
FLOOR SLABS ..................................................................................................................... - 4 -
UNDERDRAIN SYSTEM .................................................................................................... - 4 -
LIMITATIONS ........................................................................................................................... - 5 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURES 3 and 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 25-7-247
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed detached garage to be located
on Lot 8, Lions Ridge Estates, 318 Lions Ridge Road, 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 Jackie Daly and Larry Swift dated April 2, 2025.
An exploratory boring was drilled to obtain information on the subsurface conditions. Samples of
the subsoils obtained during the field exploration were tested in the laboratory to determine their
classification, compressibility or swell 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
The proposed detached garage will be a single-story wood-frame structure located on the site as
shown on Figure 1. Ground floor will be slab-on-grade. Grading for the structure is assumed to
be relatively minor with cut depths between about 1 to 4 feet. We assume relatively light
foundation loadings, typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject property was developed with a two-story residence at the time of our field exploration.
The ground surface was sloping down to the west at grades estimated at between 5 and 10
percent. Vegetation consisted of sagebrush, juniper and pinon trees, grass and weeds. There
was evidence of minor cut and fill grading for the existing developement.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site. These rocks
are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive
beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated
with the Eagle Valley Evaporite underlie portions of the lot. 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, sinkholes have been observed scattered throughout the lower
Roaring Fork River Valley which appear similar to others associated with the Eagle Valley
Evaporite.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was
encountered in the subsurface materials; however, the exploratory boring was relatively shallow,
for foundation design only. Based on our present knowledge of the subsurface conditions at the
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Kumar & Associates, Inc. ® Project No. 25-7-247
site, it cannot be said for certain that sinkholes will not develop. The risk of future ground
subsidence at 318 Lions Ridge Road throughout the service life of the proposed building, in our
opinion, is low; however, the owner should be made aware of the potential for sinkhole
development. If further investigation of possible cavities in the bedrock below the site is desired,
we should be contacted
FIELD EXPLORATION
The field exploration for the project was conducted on May 2, 2025. One exploratory boring was
drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring was
advanced with 4-inch diameter continuous flight augers powered by a truck-mounted CME-45B
drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with a 2-inch I.D. spoon sampler. The sampler was driven
into the subsoils 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 the bedrock. Depths at which the samples were taken and the
penetration resistance values are shown on the Log of Exploratory Boring, Figure 2. The samples
were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about ½ foot of topsoil overlying medium stiff to stiff, sandy silt to 19 feet deep
where medium hard to hard sandstone/siltstone bedrock was encountered down to the maximum
explored depth of 25 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples, presented on Figures 3
and 4, indicate low to moderate compressibility under light loading and low collapse potential
when wetted. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist.
FOUNDATION BEARING CONDITIONS
The upper sandy silt soils possess low bearing capacity and low to moderate settlement potential
primarily when wetted. The underlying sandstone/siltstone bedrock possesses moderate bearing
capacity and typically low settlement potential. The proposed detached garage can be designed
with a spread footing foundation bearing on the natural sandy silt soils with a risk of foundation
settlement mainly if the bearing soils become wet. A lower risk option would be to extend the
bearing level down to the underlying sandstone/siltstone bedrock with a deep foundation system
such as helical piers or driller piers. Provided below are recommendations for a spread footing
foundation system. If a deep foundation system is desired, we should be contacted for additional
analysis and design recommendations.
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Kumar & Associates, Inc. ® Project No. 25-7-247
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils with a risk of settlement.
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 initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. Additional, post-construction differential settlement could
occur if the bearing soils become wetted. The magnitude of additional settlement
would depend on the depth and extent of wetting but could be on the order of 1 to
2 inches.
2) The footings should have a minimum width of 20 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 heavily reinforced top and bottom to span
local anomalies and better resist differential settlement such as by assuming an
unsupported length of at least 14 feet. Foundation walls acting as retaining
structures should also be designed to resist lateral earth pressures as discussed
in the "Foundation and Retaining Walls" section of this report.
5) Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the firm natural soils. The exposed soils in footing area
should then be moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 60 pcf for backfill consisting of
the on-site soils. Cantilevered retaining structures which are separate from the garage and can
be expected to deflect sufficiently to mobilize the full active earth pressure condition should be
designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of
at least 50 pcf for backfill consisting of the on-site soils. Backfill should not contain topsoil,
organics, or rocks larger than about 5 inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment.
The pressures recommended above assume drained conditions behind the walls and a horizontal
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Kumar & Associates, Inc. ® Project No. 25-7-247
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard
Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway
areas should be compacted to at least 95% of the maximum standard Proctor density. Care
should be taken not to overcompact the backfill or use large equipment near the wall, since this
could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the sliding
resistance of the footing on the foundation materials and passive earth pressure against the side
of the footing. Resistance to sliding at the bottoms of the footings can be calculated based on a
coefficient of friction of 0.35. Passive pressure of compacted backfill against the sides of the
footings can be calculated using an equivalent fluid unit weight of 325 pcf. The coefficient of
friction and passive pressure values recommended above assume ultimate soil strength. Suitable
factors of safety should be included in the design to limit the strain which will occur at the ultimate
strength, particularly in the case of passive resistance. Fill placed against the sides of the footings
to resist lateral loads should be compacted to at least 95% of the maximum standard Proctor
density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction with a settlement risk. 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 CDOT Class 6 base course should be
placed beneath slabs-on-grade for support. This material should consist of minus 2-inch
aggregate with at least 50% retained on 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. We
recommend below-grade construction, such as retaining walls and basement areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain system
should not be required for the assumed slab-on-grade construction.
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Kumar & Associates, Inc. ® Project No. 25-7-247
If installed, the drains should consist of rigid perforated PVC 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 ½% 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 and covered with filter fabric such as Mirafi 140N or 160N. 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.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the garage 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 12 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 should be
covered with filter fabric and capped with about 2 feet of the on-site finer-graded
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 boring drilled at the location 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 boring 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.
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1 SUMMARY OF LABORATORY TEST RESULTS
Project No. 25-7-247
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 2 3.3 97 Sandy Silt
4 2.9 91 71 Sandy Silt
9 3.6 92 Sandy Silt