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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 E ASPEN GLEN
FOX PROWL
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-491
JULY , 2021
PREPARED FOR:
ASPEN GLEN E LLC
c/o SHERMAN & HOWARD
ATTN: JOSEPH KRABACHER, MANAGER
730 EAST DURANT AVENUE, SUITE 200
ASPEN, COLORADO 81611
jkrabacher@shermanhoward.com
Kumar & Associates, Inc. ® Project No. 21-7-491
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
SUBSIDENCE POTENTIAL ..................................................................................................... - 2 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 3 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 3 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS ............................................................................................................ - 3 -
FOUNDATION AND RETAINING WALLS ............................................................... - 5 -
FLOOR SLABS .............................................................................................................. - 6 -
UNDERDRAIN SYSTEM ............................................................................................. - 6 -
SURFACE DRAINAGE ................................................................................................. - 6 -
LIMITATIONS ........................................................................................................................... - 7 -
REFERENCES: .......................................................................................................................... - 8 -
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
APPENDIX – DEVELOPMENT IN SURFACE DEPRESSION AREAS
Kumar & Associates, Inc. ® Project No. 21-7-491
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot E26, Aspen Glen, Fox Prowl, 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 Aspen Glen E26 LLC dated May 26, 2021. Chen-Northern previously conducted a
geotechnical study for the subdivision development and presented their findings in a report dated
December 20, 1991, Job No. 4 112 92.
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-story wood-frame structure with attached garage. Ground
floors are assumed to be a combination of structural over crawlspace and slab-on-grade. Grading
for the structure is assumed to be relatively minor with cut depths between about 2 to 5 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 is relatively
flat and gently sloping down to the north. Vegetation consists of grass and weeds. A small ditch
was flowing near the rear, southeast side of the lot.
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Kumar & Associates, Inc. ® Project No. 21-7-491
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision.
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 Aspen Glen development (Chen-Northern, Inc., 1991). These sinkholes
appear similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring
Fork River valley.
The site is mapped as lying within a broad depression area and a sinkhole was mapped about
50 feet southeast of the building envelope. The surface depression area is thought to be
associated with long-term ground subsidence. 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
E26 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 June 16, 2021. 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 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.
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Kumar & Associates, Inc. ® Project No. 21-7-491
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about 3 to 5 feet of very stiff, sandy silty clay overlying medium dense to
dense, slightly silty sandy gravel and cobbles with possible boulders. Drilling in the dense
granular soils with auger equipment was difficult due to the cobbles and boulders and practical
drilling refusal was encountered in Boring 1 at a depth of 12½ feet and in Boring 2 at a depth of
13 feet.
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 sample of the upper clay, presented on Figure 4, indicate low
compressibility under light loading and moderate collapse potential when wetted. Results of
gradation analyses 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.
FOUNDATION BEARING CONDITIONS
The upper sandy clay soils encountered in the borings possess low bearing capacity and low to
moderate settlement potential, particularly when wetted. The underlying sandy gravel soils
possess moderate bearing capacity and typically low settlement potential. At assumed
excavation depth we expect the subgrade will expose either sandy clay or gravel subsoils. We
recommend the upper clay soils be removed and the residence be supported on spread footings
bearing on the underlying gravel soils or compacted structural fill.
We have attached the Chen-Northern (1991) recommendations for building in a broad surface
depression area. We believe these recommendations are conservative but will reduce structural
distress in the event of future ground movement and should be considered in the building design.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
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Kumar & Associates, Inc. ® Project No. 21-7-491
natural granular soils below the upper clay soils or on compacted structural fill placed on the
natural granular soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils or compacted structural
fill should be designed for an allowable bearing pressure of 2,000 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 heavily 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
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
5) The topsoil, clay soils and any loose or disturbed soils should be removed and the
footing bearing level extended down to the relatively dense natural granular soils.
The exposed soils in footing area should then be moistened and compacted. If
structural fill is used to re-establish design bearing grade it should consist of a
relatively well graded granular material such as CDOT Class 6 (¾-inch) road
base. 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 fill should extend laterally beyond the
footing edges a distance at least equal to one-half the depth of fill below the
footing.
6) A representative of the geotechnical engineer should test any structural fill during
placement for compaction and observe footing excavations prior to concrete
placement to evaluate bearing conditions.
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Kumar & Associates, Inc. ® Project No. 21-7-491
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 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 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 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 375 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 compacted to at least 95% of the maximum
standard Proctor density at a moisture content near optimum.
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Kumar & Associates, Inc. ® Project No. 21-7-491
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction with a potential for settlement where underlain by clay soils. 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
road base should be placed beneath slab-on-grade construction 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 (plus 6-inch) rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where clay soils ae present 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 or drywell based in the underlying gravel soils. 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.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
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Kumar & Associates, Inc. ® Project No. 21-7-491
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 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.
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
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-491
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 2½ 9.1 93 Sandy Silty Clay
5 and 7½
combined 0.6 56 33 11 Slightly Silty Sandy Gravel
2 2½ 11.9 109 92 Sandy Silty Clay