HomeMy WebLinkAboutSubsoil Study
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 AND OUTBUILDING
LOT 51, SPRING RIDGE RESERVE
HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-650
DECEMBER 21, 2020
PREPARED FOR:
RED HOUSE ARCHITECTURE
ATTN: BRUCE BARTH
815 BLAKE STREET
GLENWOOD SPRINGS, COLORADO 81601
bruce@redhousearchitecture.com
Kumar & Associates, Inc. Project No 20-7-650
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
GEOLOGY ................................................................................................................................. - 2 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 3 -
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
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. Project No 20-7-650
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence and outbuilding to be
located on Lot 51, 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
foundation design. The study was conducted in accordance with our proposal for geotechnical
engineering services to Red House Architecture, dated October 23, 2020.
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
The proposed residence, garage and detached outbuilding will be 1 and 2-story structures,
located as shown on Figure 1. The ground floors will be structural above crawlspace or
slab-on-grade. We assume excavation for the buildings will be cut about 2 to 6 feet below the
existing ground surface. Foundation loadings for the structures were 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 property was vacant with patchy snow cover at the time of our field exploration. The site is
vegetated with grass, weeds and sage brush. The ground surface slopes gently to moderately
down to the northeast with around 4 to 7 feet of elevation difference in the general area of each
building site. Maroon Formation sandstone is exposed on the hillside to the west of the lot.
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GEOLOGY
According to the Geologic Map of the Cattle Creek Quadrangle, Garfield County, Colorado, by
Krikham, Steufert, Hemborg, and Stelling, dated 2014, the site is underlain by alluvium and
colluvium deposits of the Holocene age overlying Maroon Formation.
FIELD EXPLORATION
The field exploration for the project was conducted on November 2, 2020. Three 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.
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 profiles encountered at the site are shown on Figure 2. Below
about 1 foot of organic topsoil, the subsoils consist of very stiff, sandy silty clay down to about
16 to 20 feet underlain by relatively dense, silty sand and gravel underlain by hard to very hard
sandstone bedrock at depths of about 23 to 32 feet.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density and finer than sand size gradation analyses. Swell-consolidation
testing performed on relatively undisturbed drive samples of the clay soils, presented on
Figures 4 and 5, indicate low compressibility under relatively light surcharge loading and minor
to low expansion potential when wetted under a constant light surcharge. The laboratory testing
is summarized in Table 1.
No free water was encountered in the borings at time of drilling and the subsoils were slightly
moist.
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FOUNDATION BEARING CONDITIONS
The subsoils encountered at the site possess variable low to moderate movement potential mainly
when wetted. The expansion potential measured in the samples from depths down to 10 feet
generally appears to be anomalous and the expansion potential should be further evaluated at the
time of excavation. Sub-excavation to 3 feet below footing bearing level and placement of
structural fill could be used to help mitigate movement potential. Surface runoff, landscape
irrigation, and utility leakage are possible sources of water which could cause wetting. Footings
placed on the natural soils can be used for foundation support with the accepted risk of
movement. Deep foundations, such as drilled piers or micro-piles extending down into the
gravel soils, can be used if the risk of movement cannot be tolerated. We should be contacted if
deep foundation recommendations are desired.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the residence and outbuilding can be founded with spread footings
placed on undisturbed natural soils with a risk of movement mainly if the bearing soils are
wetted.
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 1,500 psf. Based on experience, we expect initial settlement
of footings designed and constructed as discussed in this section will be up to
about 1 inch. Additional movement could be around 1 to 1½ inches depending on
the depth and extent of wetting.
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 heavily 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.
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4) 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.
5) Prior to the footing construction, the topsoil and loose or disturbed soils should be
removed and the footing bearing level extended down to competent bearing soils.
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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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, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction. There could be differential settlement/heave 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. Basement
slabs should be underlain by a 4-inch layer of relatively free-draining gravel with 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 (plus 6-inch) 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
perched condition. Therefore, 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.
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The underdrain system should consist of a 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
1% 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 1½ feet deep. An
impervious liner such as 20 mil PVC should be placed below the drain gravel in a trough shape
and attached to the foundation wall with mastic to keep drain water from flowing beneath the
wall and to other areas of the building.
SURFACE DRAINAGE
Providing proper surface grading and drainage will be critical to prevent wetting of the bearing
soils and limiting building settlement and distress. 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.
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.
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. Consideration should be given to use of xeriscape
to prevent wetting of bearing soils from landscape 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
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-650
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 6.3 107 Sandy Silty Clay
5 9.4 120 79 Sandy Silty Clay
10 8.4 106 Sandy Silty Clay
20 3.8 132 Silty Sand and Gravel
2 5 6.3 109 Sandy Silty Clay
10 4.7 117 54 Very Sandy Silty Clay
20 1.7 131 Silty Sand and Gravel
25 2.4 131 35 Silty Sand and Gravel
3 5 5.4 116 Sandy Silty Clay
10 5.8 110 52 Very Sandy Silty Clay
15 6.9 126 66 Sandy Silty Clay
25 3.9 116 Silty Sand and Gravel