HomeMy WebLinkAboutSubsoils Study for Foundation DesignKumar & Associates, Inc.
K Geotechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
email: kaglenwood@kumarusa.com
www.kumarusa.co
An Employee Owned Company
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
PRELIMINARY SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED EMPLOYEE HOUSING DEVELOPMENT
INTERSECTION OF COUNTY ROADS 103 and 104
GARFIELD COUNTY, COLORADO
PROJECT NO. 25-7-601
OCTOBER 6, 2026
PREPARED FOR:
RYOBI FOUNDATION AND POWERS ART CENTER
ATTN: BOBBI HAPGOOD
13114 COLORADO HIGHWAY 82
CARBONDALE, COLORADO 81623
bobbi obifoundation.or
TABLE OF CONTENTS
PURPOSEAND SCOPE OF STUDY.....................................................................................-
1 -
PROPOSEDCONSTRUCTION.............................................................................................-
1 -
SITECONDITIONS...............................................................................................................-
1 -
SUBSIDENCEPOTENTIAL....................................................................................................2-
FIELD EXPLORATION ..........................................................................................................
- -
SUBSURFACECONDITIONS................................................................................................
3-
FOUNDATION BEARING CONDITIONS...............................................................................-
3 -
DESIGNRECOMMENDATIONS...........................................................................................
- 4-
FOUNDATIONS.................................................................................................................
- 4 -
FOUNDATION AND RETAINING WALLS..........................................................................
- 5 -
FLOOR SLABS ................................................................................................................ .
- 6 -
UNDERDRAINSYSTEM................................................................................................
- 7 -
SURFACEDRAINAGE......................................................................................................
- 7-
LIMITATIONS.........................................................................................................................
8-
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 through 6 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 7 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. 0 Project No. 25-7-501
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed employee housing development
to be located at the intersection of County Roads 103 and 104, Garfield County, Colorado. The
project site is shown on Figure 1. The purpose of the study was to develop preliminary
recommendations for the foundation design. The study was conducted in accordance with our
agreement for geotechnical engineering services to Ryobi Foundation and Powers Art Center
dated August 1, 2025.
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
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
Development plans were conceptual at the time of this study. The proposed employee housing
development may consist of two residences and two detached garages. Ground floors could be
slab -on -grade or structural over crawlspace. Grading for the structures is assumed to be relatively
minor with cut depths between about 5 to 8 feet. We assume relatively light foundation loadings,
typical of the proposed type of construction.
When building locations, grading and loading information have been developed, we should be
notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The subject site is currently vacant. Topography at the site consists of valley side with gently to
moderately sloping terrain down to the south-southwest. Vegetation at the site consists of native
grass and weeds and scrub oak. Crystal Spring Creek borders the site to the west.
Kumar & Associates, Inc. 0 Project No. 25-7-501
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the project 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 property. Dissolution of the gypsum under
certain conditions can cause sinkholes to develop and can produce areas of localized subsidence.
Sinkholes have been observed scattered throughout the project area and appear similar to others
associated with the Eagle Valley Evaporite in the lower Roaring Fork Valley.
Sinkholes were not observed in the immediate area of the subject property. No evidence of
cavities was encountered in the subsurface materials; however, the exploratory borings were
relatively shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future
ground subsidence on the subject property throughout the service life of the proposed residences,
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 August 7, 2025. Two exploratory borings
were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. 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 were taken with 1'/ inch and 2 inch I.D. spoon samplers. The samplers
were 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 Logs of Exploratory Borings, Figure 2. The
samples were returned to our laboratory for review by the project engineer and testing.
Kumar & Associates, Inc. 0 Project No. 25-7-501
-3-
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about'/z to 1 foot of topsoil overlying very silty sandy clay underlain by firm to
hard claystone/siltstone bedrock at a depth of 15 feet down to the maximum depth drilled of 30
feet in Boring 1 and by medium dense sand and silt at a depth of 18'/z feet down to 36 feet where
dense silty sandy gravel and cobbles was encountered down to the maximum depth drilled of 40
feet in Boring 2. The upper 6 feet of claystone/siltstone encountered in Boring 1 was weathered.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell -consolidation testing performed on
relatively undisturbed drive samples of the sandy clay, presented on Figures 4 through 6, indicate
high compressibility under conditions of loading and wetting and a moderate to high
hydrocompression potential when wetted under a constant 1,000 psf surcharge load. Results of
gradation analyses performed on a small diameter drive sample (minus 1Y2 inch fraction) of the
coarse granular subsoils are shown on Figure 7. 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 slightly
moist.
FOUNDATION BEARING CONDITIONS
The upper (above about 15 and 36 feet in Borings 1 and 2, respectively) soils at the site are
alluvial fan deposits which generally tend to settle when wetted. Shallow foundations placed on
the upper soils will have a risk of movement and building distress. The risk of movement is due
to the assumed variable bearing conditions and especially if the bearing soils were to become
wetted. To reduce the risk of foundation movement and building distress, and provide more
uniform bearing conditions, shallow spread footings can be placed on a minimum 5 feet of onsite
soils removed and replaced as QgmpaLted structural
The buildings could also be supported by a relatively deep foundation system such as helical
piers or micro -piles that extends the foundation bearing down into bedrock or dense coarse
Kumar & Associates, Inc. 0 Project No. 25-7-501
-4-
granular soils for a relatively low risk of movement and building distress. Provided below are
recommendations for spread footings bearing on structural fill. Helical piers and micro -piles are
typically design/build by contractors with experience in the area. We can review the helical pier
or micro -pile design if this type foundation system is selected. If recommendations for other
foundation types such as a heavily reinforced mat system are desired, we should be contacted.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Provided below are recommendations for spread footings bearing on a minimum of 5 feet of
compacted structural fill with some risk of movement. The risk of movement is if the natural soils
below the structural fill were to become wetted and precautions should be taken to prevent
wetting. The on -site soils, excluding topsoil and oversized (plus 6 inch) rocks, are suitable as the
structural fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on a minimum of 5 feet of removed and replaced onsite soils as
structural fill can 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 settlement could occur if
the bearing soils below the structural fill were to become wetted. The magnitude of the
additional movement would depend on the depth and extent of the wetting but may be on
the order 1 to 2 inch.
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 withstand the effects of some differential movement such as
by assuming an unsupported length of at least 14 feet. Foundation walls acting as
Kumar & Associates, Inc. 0 Project No. 25-7-501
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retaining structures should also be designed to resist lateral earth pressures as discussed
in the "Foundation and Retaining Walls" section of this report.
5) The topsoil, loose or disturbed soils and the required depth of natural soils to
provide for 5 feet of structural fill depth should be removed down to firm natural soils. The
exposed soils should then be scarified, moistened to near optimum and compacted to at
least 95% standard Proctor density. The structural fill used below footing areas can
consist of the onsite soils and should be compacted to at least 98% of standard Proctor
density at a moisture content within about 2% of optimum. The structural fill should extend
laterally beyond the footing edges a distance equal to at least one-half the depth of fill
below the footing (2'h feet).
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions and test structural
fill on a regular basis for compaction.
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 55 pcf for backfill consisting of
the on -site soils. Cantilevered retaining structures which are separate from the buildings 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 45 pcf for backfill consisting of the on -site soils.
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
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
Kumar & Associates, Inc. 0 Project No. 25-7-501
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 350 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 tend to settle when wetted. Floor slabs -on -grade will have a risk of
settlement and distress, mainly if the subgrade were to become wetted. Structural floors over
crawlspace, which are commonly used in area, would have a relatively low risk of movement and
distress. Floor slabs -on -grade can be used provided a depth of structural fill (typically 2 or 3 feet)
is provided below the slabs and the risk of some slab movement and distress is acceptable. The
structural fill can consist of the on -site soils, excluding topsoil and plus 6-inch size rocks, or a well
graded imported material such as CDOT Class 5 or 6 base course. We should review the
exposed soil conditions for depth of structural fill below floor slab areas at the time of 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 immediately beneath basement floor slabs -on -grade to facilitate drainage. This
material should consist of minus 2-inch material with at least 50% retained on the No. 4 sieve and
Kumar & Associates, Inc. 0 Project No. 25-7-501
SO
less than 2% passing the No. 200 sieve. A well graded sand and gravel material such as CDOT
Class 6 road base is more desirable below floor slabs -at -grade, to help reduce the potential for
water infiltration below the slabs.
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 topsoil and oversized (plus 6 inch) rocks or imported road base.
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.
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 1'%
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
Proper surface grading and drainage will be critical to keeping the bearing soils dry below the
buildings.
The following drainage precautions should be observed during construction and maintained at all
times after the residences have been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
Kumar & Associates, Inc. 0 Project No. 25-7-501
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 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 such as sod and sprinkler
heads should be located at least 10 feet from foundation walls.
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 planning and preliminary
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 monitor the implementation of our recommendations, and to verify that
the recommendations have been appropriately interpreted. Significant design changes may
require additional analysis or modifications to the recommendations presented herein. We
Kumar & Associates, Inc. 0 Project No. 25.7-501
WE
recommend on -site observation of excavations and foundation bearing strata and testing of
structural fill by a representative of the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, Inc.
57 22
Robert L. Duran P.E.
�SSlO�
Reviewed by: rVAL
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Steven L. Pawlak, P.E.
RLD/I f
cc: Sopris Engineering — Stephanie Helfenbein (stephanie@a_.__sor)riseng.com}
Kumar & Associates, Inc. 0 Project No. 25-7-501
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APPROXIMATE SCALE —FEET
LOCATION OF EXPLORATORY BORINGS I Fig. 1
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DD=75
-200=90
LL=28
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26/12
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DD=127
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WC=5.2
DD=96
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DD=92
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25-7-501 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
LEGEND
® TOPSOIL; SILT AND CLAY, FIRM, MOIST, DARK BROWN, ORGANIC.
CLAY (CL); VERY SILTY, SANDY, MEDIUM STIFF TO VERY STIFF, SLIGHTLY MOIST TO MOIST,
BROWN.
SAND AND SILT (SM—ML); SCATTERED GRAVEL, MEDIUM DENSE, MOIST, GRAY —BROWN.
GRAVEL AND COBBLES (GM); SANDY, SILTY, DENSE, SLIGHTLY MOIST, RED —BROWN.
WEATHERED CLAYSTONE/WEATHERED SILTSTONE; FIRM, MOIST, GRAY —BROWN.
CLAYSTONE/SILTSTONE; VERY HARD, SLIGHTLY MOIST, GRAY, EAGLE VALLEY EVAPORITE.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
18/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 18 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
Q DEPTH TO WATER LEVEL ENCOUNTERED AT THE TIME OF DRILLING.
DEPTH AT WHICH BORING CAVED.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 7, 2025 WITH A 4—INCH—DIAMETER
CONTINUOUS —FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF
THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE
DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER LEVELS SHOWN ON THE LOGS WERE MEASURED AT THE TIME AND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140);
LL = LIQUID LIMIT (ASTM D4318);
PI = PLASTICITY INDEX (ASTM D4318).
25-7-501 1 Kumar & Associates I LEGEND AND NOTES Fig. 3
SAMPLE OF: Slightly Sandy Clay
FROM: Boring 1 ®4'
WC = 10.1 %, DD = 77 pcf
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HYDROMETER ANALYSIS
SIEVE ANALYSIS
THE READINGS
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
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CLAY TO SILT COBBLES
FINE MEDIUM ICOARSEI FINE I COARSE
GRAVEL 56 % SAND 33 % SILT AND CLAY 11 %
LIQUID LIMIT — PLASTICITY INDEX -
SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 2 ® 39'
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25-7-501
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