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 RESIDENCE
LOT E51, ASPEN GLEN
88 KINGFISHER LANE
GARFIELD COUNTY, COLORADO
PROJECT NO. 25-7-691
DECEMBER 18, 2025
PREPARED FOR:
KATIE AND JAY WILLOUGHBY
85 SEAPORT BOULEVARD #1418
BOSTON, MASSACHUSETTS 02210
katie.b.willoughby@gmail.com
Kumar & Associates, Inc. ® Project No. 25-7-691
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 ............................................................................. - 3 -
DESIGN RECOMMENDATIONS .......................................................................................... - 3 -
FOUNDATIONS ................................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 4 -
FLOOR SLABS ..................................................................................................................... - 4 -
UNDERDRAIN SYSTEM .................................................................................................... - 5 -
SURFACE DRAINAGE ....................................................................................................... - 5 -
LIMITATIONS ........................................................................................................................... - 6 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 25-7-691
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
E51, Aspen Glen, 88 Kingfisher Lane, 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 Katie and Jay Willoughby dated November 18, 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, design recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a one- and two-story wood-frame structure with attached garage
located as shown on Figure 1. Ground floors will be structural over crawlspace and slab-on-
grade. Grading for the structure is assumed to be relatively minor with cut and fill depths less
than 10 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 site was vacant at the time of our field exploration. The ground surface was gently
sloping down to the west/northwest. Cobbles were visible on the surface of the lot. Vegetation
consists of grass and weeds with evergreen trees along the lot perimeter. During previous
geologic studies performed for Aspen Glen development a broad subsidence area was identified.
The boundary of the subsidence area passes through the building area of Lot E51 approximately
as shown on Figure 1.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
Development. 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 broad or localized subsidence. During previous geologic studies performed in
the area, several sinkholes were observed scattered throughout the Aspen Glen Development.
These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of
the lower Roaring Fork River Valley.
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Kumar & Associates, Inc. ® Project No. 25-7-691
A sinkhole was observed approximately 500 feet southwest of the subject lot and a broad
subsidence area was mapped with the boundary of the subsidence area passing through the
building envelope of Lot E51 as shown on Figure 1. Relatively loose backfill material was
encountered in Boring 1 to a depth of approximately 10 feet, apparently placed to level the lot.
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 Lot E51 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 November 21, 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.
Samples of the subsoils were taken with 1⅜-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 encountered in Boring 1 below about ½ foot of topsoil consist of about 1 foot of sandy
silt with gravel overlying dense, silty sandy gravel and cobbles with possible boulders. The
subsoils encountered in Boring 2 below about ½ foot of topsoil, 2 feet of sandy silt and silty sand
and gravel fill to 10 feet deep consist of dense silty sandy gravel and cobbles with possible
boulders to the maximum drilled depth of 20 feet. Drilling in the coarse granular soils with auger
equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in
the deposit at Boring 1.
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
a relatively undisturbed drive sample of the gravelly silt, presented on Figure 4, indicate moderate
compressibility under conditions of loading and wetting. The sample may have been disturbed
during the sampling process due to the rock content. Results of a gradation analysis performed
on small diameter drive samples (minus 1½-inch fraction) of the coarse granular subsoils are
shown on Figure 5. 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 to moist.
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Kumar & Associates, Inc. ® Project No. 25-7-691
FOUNDATION BEARING CONDITIONS
The upper silt soils and fill soils in Boring 2 possess low bearing capacity and moderate to high
settlement potential, mainly when wetted. The underlying natural gravel and cobble soils possess
moderate bearing capacity and typically low settlement potential. The subsidence area backfill
material encountered to a depth of 10 feet in Boring 2 is variable in density and not suitable for
support of the proposed residence. The proposed residence can be founded with spread footings
bearing on the natural gravel and cobble soils below the silt and backfill soils in the subsidence
area. At assumed excavation depths we expect the excavation to expose natural granular soils
in the eastern part of the excavation and relatively loose backfill soils on the western part of the
excavation. Backfill encountered in the excavation should be removed to expose the underlying
natural dense granular soils. The spread footings can be extended down to the natural dense
granular soils or the sub-excavated depth can be backfilled with structural fill consisting of the
onsite granular soils or a suitable imported granular soil such as CDOT Class 6 base course.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural dense gravel and cobble soil or compacted structural fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural dense gravel and cobble soils or
compacted structural fill should be designed for an allowable bearing pressure of
2,500 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 16 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 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) The existing fill, relatively loose backfill, topsoil, silt and any loose disturbed soils
should be removed and the footing bearing level extended down to the dense
natural gravel and cobble soils. Sub-excavated depths can be backfilled with
compacted structural fill consisting of the on-site granular soils processed to
remove rock larger than about 6 inches or a suitable imported granular material
such as CDOT Class 6 base course. Structural fill should be spread in thin
horizontal lifts, moisture conditioned to near optimum moisture content and
compacted to at least 98 percent of maximum standard Proctor density. The
structural fill should extend laterally out from the footings a distance equal to at
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Kumar & Associates, Inc. ® Project No. 25-7-691
least ½ the depth of fill below the footings. The exposed soils in footing area should
be moistened and compacted.
6) 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 50 pcf for backfill consisting of
the on-site granular 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 40 pcf for backfill consisting of the on-site granular soils. Backfill should not
contain organics or rock larger than 6 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
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 0.45. Passive pressure of compacted backfill against the sides of the
footings can be calculated using an equivalent fluid unit weight of 400 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 a granular material 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 loose backfill soils are suitable to support lightly
loaded slab-on-grade construction. The loose backfill soils in slab-on-grade areas should be sub-
excavated to expose the natural granular soils. The sub-excavated depth should be backfilled
with compacted structural fill as recommended in the FOUNDATIONS section above. To reduce
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Kumar & Associates, Inc. ® Project No. 25-7-691
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
basement level 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 granular 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, crawlspace areas and basement
areas be protected from wetting and hydrostatic pressure buildup by an underdrain system.
Where 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, sump and pump system or drywell.
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.
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 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.
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 25-7-691
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 0.7 11 Silty Sandy Gravel
2 1 8.5 80 Slightly Gravelly Sandy
Silt (Fill)
3 7.3 103 30 Silty Sand and Gravel (Fill)
14 & 19
combined 0.4 54 33 13 Silty Sandy Gravel