HomeMy WebLinkAboutSubsoil Study for Foundation Design 09-12-2025K+A
Kumar ni fates, Inca Geotechnical a! and and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
1 email: kaglenwood@kumarusa.com
�— www.kumarusa.com
An Employee Owned Company
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 7, WHITECLOUD RIDGE
542 WHITECLOUD ROAD
GARFIELD COUNTY, COLORADO
PROJECT NO. 25-7-162.07
SEPTEMBER 12, 2025
PREPARED FOR:
TOM LESTER
P.O. BOX 2329
BASALT, COLORADO 81621
tomi(cDlesterdevelopment.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY...................................................................................- 1 -
PROPOSEDCONSTRUCTION.............................................................................................- 1 -
SITECONDITIONS.................................................................................................................. 1 -
SUBSIDENCE POTENTIAL
FIELDEXPLORATION............................................................................................................- 2-
SUBSURFACECONDITIONS...............................................................................................- 2-
FOUNDATION BEARING CONDITIONS.............................................................................- 2-
DESIGN RECOMMENDATIONS -
FOUNDATIONS.................................................................................................................... 3-
FOUNDATION AND RETAINING WALLS.......................................................................- 3-
FLOORSLABS.....................................................................................................................- 4 -
UNDERDRAIN SYSTEM -
SURFACE DRAINAGE -
LIMITATIONS 5-
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. 0 Project No. 25.7-162.07
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
7, Whitecloud Ridge, 542 Whitecloud Road, Garfield County, Colorado. The project site is shown
on Figure 1. The purpose of the study was to develop recommendations for foundation design.
The study was conducted in accordance with our agreement for geotechnical engineering
services to Tom Lester, dated February 14, 2025.
A field exploration program consisting of exploratory borings was conducted 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, recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Plans for the proposed residence were conceptual at the time of our study. The proposed
residence is assumed to be a one- or two-story structure with attached garage possibly over a
basement level located on the site in the area of the exploratory borings shown on Figure 1.
Ground floors could be structural over crawispace or slab -on -grade. Cut depths are expected to
range between about 3 to 10 feet. Foundation loadings for this type of construction are assumed
to be relatively light and typical of the proposed type of construction.
If building loadings, location or grading plans are significantly different from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The ground surface was gently
sloping down to the south/southeast at grades estimated at between 3 and 5 percent. Vegetation
consists of grass, weeds, sage brush and juniper trees.
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, several sinkholes were observed scattered throughout the lower
Roaring Fork River Valley. These sinkholes appear similar to others associated with the Eagle
Valley Evaporite in areas of the lower Roaring Fork River valley.
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 borings were relatively
Kumar & Associates, Inc. 0 Project No. 25-7-162.07
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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 Lot 7 throughout the service life of the proposed residence, 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 April 23, 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 auger 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. 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 consist of about Y2 foot of topsoil overlying stiff to hard, sandy clay to between 18Y2 and
20 feet deep. Below the clay in Boring 2, dense, sand clay and gravel with cobbles and possible
boulders was encountered to the maximum explored depth of 20 feet.
Laboratory testing performed on samples obtained from the borings 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 4
and 5, indicate low compressibility under existing low moisture conditions and light loading and
variable minor collapse (settlement under constant load) to minor expansion when wetted under
constant light surcharge. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at time of drilling. The subsoils were slightly moist.
FOUNDATION BEARING CONDITIONS
The clay soils encountered in the borings possess low bearing capacity and typically minor
collapse or minor expansion potential when wetted. The underlying granular soils encountered
in Boring 2 possess moderate bearing capacity and typically low settlement potential. At assumed
excavation depths we expect the foundation excavation to expose sandy clay. The proposed
residence can be supported on a spread footing foundation system placed on the natural soils
with a risk of movement if the bearing soils become wetted. A lower risk option would be to extend
the bearing level down to the underlying granular soils with a deep foundation system such as
helical piers or micro -piles. Surface runoff, landscape irrigation, and utility leakage are possible
sources of water which could cause wetting. We should be contacted at the time of excavation
Kumar & Associates, Inc. 0 Project No. 25-7-162.07
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to evaluate the exposed soil for collapse/expansion potential and the need for mitigation such as
compacted structural fill for footing support.
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 placed
on the natural soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils can be designed for an allowable
bearing pressure of 2,000 psf. Based on experience, we expect initial settlement
of footings design and constructed as discussed in this section will be about 1 inch
or less Additional post -construction movement could occur if the bearing soils
become wet. The magnitude of additional movement would depend on the depth
and extent of wetting but could be on the order of 1 inch.
2) The footings should have a minimum width of 16 inches for continuous footings
and 24 inches for isolated pads.
3) Continuous foundation walls should be well reinforced top and bottom to span local
anomalies and limit the risk of differential movement. One method of analysis is
to design the foundation wall to span an unsupported length of at least 12 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure as discussed in the "Foundation and Retaining Walls"
section of this report.
5) 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 the exterior grade is typically used in this
area.
6) Topsoil, expansive clay soil and any loose disturbed soils should be removed from
footing areas and the exposed soils moistened and compacted.
7) A representative of the geotechnical engineer should observe all footing
excavations and test structural fill for compaction 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 fine-grained soils. Cantilevered retaining structures which are separate from the
residence 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 fine-grained soils.
Kumar & Associates, Inc. ® Project No. 25-7-162.07
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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 slightly above optimum. Backfill placed in pavement 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 0.30. 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 and expansive clay soil, 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 -on -grade 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, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
the area and where clay soils are present that local perched groundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a
Kumar & Associates, Inc. 0 Project No. 25-7-162.07
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perched condition. We recommend below -grade construction, such as crawlspace and basement
areas, be protected from wetting by an underdrain system. The drain should also act to prevent
buildup of hydrostatic pressures behind foundation walls.
The underdrain system should consist of rigid perforated PVC drainpipe surrounded by free -
draining granular material placed at the bottom of the wall backfill. The drain lines should be
placed at each level of excavation and at least 1 foot below lowest adjacent finish grade, and
sloped at a minimum Y2% grade to a suitable gravity outlet. Free -draining granular material used
in the drain system 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. The drain gravel should be at least 1'h feet
deep.
SURFACE DRAINAGE
Providing and maintaining proper surface drainage will be critical to the long term performance of
the proposed residence. The following drainage precautions should be observed during
construction and maintained at all times after the residence has been completed:
1) Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction. Drying could increase the expansion
potential of the soils.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement areas and to
at least 90% of the maximum standard Proctor density in landscape 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.
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.
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
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 to be different from those described in this report, we should be notified
at once so re-evaluation of the recommendations may be made.
Kumar & Associates, Inc. 0 Project No. 25-7-162.07
li.
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 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 of the recommendations presented herein. 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.
James H. Parsons, P.E.
Reviewed by:
Steven L. Pawlak, P.E.
JHP/kac
Kumar & Associates, Inc. 0 Project No. 25-7462.07
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25-7-162.07 1 Kumar & Associates I LOCATION OF EXPLORATORY BORINGS I Fig. 1
_LEGEND
TOPSOIL; SILTY CLAY WITH ROOTS AND ORGANICS FIRM, MOIST, MEDIUM BROWN.
CLAY (CL) SANDY, STIFF TO VERY STIFF, SLIGHTLY MOIST, MEDIUM BROWN, TRACE TO
MODERATELY CALCAREOUS, SLIGHTLY POROUS.
GRAVEL AND CLAY (GC—CL); SANDY, SILTY, WITH COBBLES AND POSSIBLE BOULDERS, DENSE,
SLIGHTLY MOIST, LIGHT BROWN, CALCAREOUS, BASALT ROCKS.
DRIVE SAMPLE; 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE; 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
9/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 9 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON APRIL 23, 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 WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
1 25-7-162.07 1 Kumar & Associates I LEGEND AND NOTES I Fig. 3
SAMPLE OF: Calcareous Sandy Silty Clay
FROM: Boring 2 ® 9'
WC=8.2%,DD=86pcf
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