HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.06.2021
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 32, SPRING RIDGE RESERVE
HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 21-7-818
DECEMBER 6, 2021
PREPARED FOR:
BRAD JENNINGS
517 WEST 13th STREET
GLENWOOD SPRINGS, COLORADO 81601
(brad.jennings@icloud.com)
Kumar & Associates, Inc. ® Project No. 21-7-818
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ........................................................................... - 1 -
PROPOSED CONSTRUCTION .................................................................................... - 1 -
SITE CONDITIONS ....................................................................................................... - 1 -
FIELD EXPLORATION ................................................................................................ - 2 -
SUBSURFACE CONDITIONS ..................................................................................... - 2 -
DESIGN RECOMMENDATIONS ................................................................................ - 3 -
FOUNDATIONS ........................................................................................................ - 3 -
FOUNDATION AND RETAINING WALLS ........................................................... - 4 -
FLOOR SLABS .......................................................................................................... - 5 -
UNDERDRAIN SYSTEM ......................................................................................... - 5 -
SURFACE DRAINAGE ............................................................................................. - 6 -
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
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 21-7-818
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 32, Spring Ridge Reserve, Hidden Valley Drive, 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 general accordance with our agreement for
geotechnical engineering services to Brad Jennings dated October 13, 2021. Hepworth-Pawlak
Geotechnical previously performed a preliminary geotechnical study for the subdivision
development and reported the findings in a report dated February 26, 2001, Job No. 101 126
and updated the study in a report dated June 22, 2004.
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
Building plans were not available at the time of our study. In general, the proposed residence
will be a one-story wood frame structure over crawlspace with an attached garage and slab-on-
grade floor and located within the building envelope as shown on Figure 1. Grading for the
structure is assumed to be relatively minor with cut depths between about 3 to 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 property was vacant at the time of our field exploration. The site is vegetated with grass,
weeds and sage brush with pinyon and juniper trees above the general building area. The ground
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Kumar & Associates, Inc. ® Project No. 21-7-818
surface in the general building area slopes moderately down to the north-northeast at about 8 to
10%. The grade steepens slightly in the upper lot area. Maroon Formation sandstone is exposed
on the hillside to the west of the lot.
FIELD EXPLORATION
The field exploration for the project was conducted on October 21, 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.
Samples of the subsoils were taken with a 2-inch I.D. spoon sampler. The sampler was driven
into the subsurface materials 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.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils, below about 1 foot of topsoil consist of 1½ to 6 feet of sandy clay and silt overlying
sandstone bedrock at depths of about 2½ to 7 feet. The bedrock became very hard with depth
and practical drilling refusal was encountered in the formation rock in Boring 1 at 11 feet and
Boring 2 at 16 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on a relatively undisturbed drive sample of the clay soils, presented on Figure
4, generally indicate low to moderate compressibility under light loading and a low collapse
potential (settlement under constant load) when wetted. 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. 21-7-818
FOUNDATION BEARING CONDITIONS
The top of bedrock slopes down to the east and may be encountered in the upper part of the
building excavation and transition to sandy clay and silt in the remaining areas of the excavation.
The sandy clay and silt soils have variable compressibility potential and could tend to settle
especially when they become wetted. A shallow foundation placed on the sandy silt and clay
soils will have a risk of settlement if the soils become wetted and care should be taken in the
surface and subsurface drainage around the house to keep the bearing soils dry. It will be critical
to the long term performance of the structure that the recommendations for surface grading and
subsurface drainage contained in this report be followed. Presented below are recommendations
for shallow spread footings with a risk of settlement. A low settlement risk foundation support
can be achieved by extending the bearing down into the underlying bedrock.
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
natural soils below topsoil provided the owner accepts the risk of settlement and potential
building distress.
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 1,200 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 differential settlement could be on the order of
½ inch for a limited wetted depth of around 5 feet below the footings. Footings
placed entirely on the underlying sandstone bedrock can be designed for an
allowable bearing pressure of 5,000 psf.
2) The footings on the clay and silt soils 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.
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Kumar & Associates, Inc. ® Project No. 21-7-818
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 topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils or bedrock. 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 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 near optimum moisture content. 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
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Kumar & Associates, Inc. ® Project No. 21-7-818
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 on the clay soils and 0.50 for the sandstone bedrock.
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. There could be differential settlement potential from wetting of the bearing soils
similar to that described above for footings. 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 slabs 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 there are clay soils and shallow bedrock 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, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
Where installed, 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
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Kumar & Associates, Inc. ® Project No. 21-7-818
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 1½ feet deep. In clay soil areas 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 limiting subsurface wetting below the
building. 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 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. 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 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
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-818
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)
2 2½ 8.2 106 76 Sandy Clay and Silt
5 8.4 95 55 Sandy Clay and Silt