HomeMy WebLinkAboutSubsoil Study for Foundation
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 54, SPRING RIDGE RESERVE
950 HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 22-7-671
NOVEMBER 16, 2022
PREPARED FOR:
TERRY AND HEIDI RUONAVAARA
1015 HIDDEN VALLEY DRIVE
GLENWOOD SPRINGS, COLORADO 81601
thruonavaara@msn.com
Kumar & Associates, Inc. Project No 22-7-671
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 22-7-671
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 54, Spring Ridge Reserve, 950 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 agreement for
geotechnical engineering services to Terry and Heidi Ruonavaara, dated October 5, 2022.
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 single-family residence will be a single-story wood-frame structure located within
the central part of the lot shown on Figure 1. Ground floors will be structural above crawlspace
in living areas and slab-on-grade in the garage. We assume excavation for the building will be
cut about 2 to 5 feet below the existing ground surface. Foundation loadings for the structure 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 property was vacant and vegetated with grass, weeds and brush at the time of our field
exploration. The ground surface slopes gently to moderately down to the north-northeast with
around 6 to 8 feet of elevation difference in the general building area. The small valley bottom
drainage channel flows down to the north immediately east of the building area. 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 October 12, 2022. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions
across the lot. 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 1⅜ inch and 2-inch I.D. spoon samplers. The samplers
were 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 1½ feet of organic topsoil, the subsoils consist of stiff to very stiff, sandy silt to sandy silty
clay down to about 9 to 13 feet and underlain by silty to very silty sand. In Boring 1, the silty
sand was underlain at about 18½ feet by silty sandy gravel with scattered cobbles to the boring
depth of 31 feet. In Boring 2, the silty sand was underlain at about 17 feet by medium hard
siltstone/sandstone bedrock to the boring depth of 21 feet.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density and percent finer than sand size (passing the No. 200 sieve).
Swell-consolidation testing was performed on relatively undisturbed drive samples of the silt and
clay soils. The swell-consolidation test results, presented on Figures 4 and 5, indicate low
compressibility under relatively light surcharge loading and minor collapse or expansion
potential when wetted under a constant light surcharge. The laboratory testing is summarized in
Table 1.
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No free water was encountered in the borings at time of drilling and the subsoils and bedrock
were slightly moist.
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 sample from Boring 1 at 10 feet appears
to be an anomaly and the expansion potential should be further evaluated at the time of
excavation. Our experience is that the upper soils are compressible when wetted under load.
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, 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 can be founded with spread footings placed on
undisturbed natural soils with a risk of settlement 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 ½ to 1 inch depending on
the depth of wetting.
3) The footings should have a minimum width of 16 inches for continuous footings
and 24 inches for isolated pads.
4) 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. Below grade levels are not currently planned. If a basement level is
planned, the 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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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) 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.
7) 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
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
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based on a coefficient of friction of 0.40. 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 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 groundwater was not encountered during our exploration, it has been our experience in
the area and where clayey 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 (if provided), 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 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
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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
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
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 22-7-671
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 5 9.4 100 74 Sandy Silt and Clay
10 9.8 102 Sandy Silty Clay
15 7.2 113 56 Very Sandy Silt
2 2½ 8.1 95 Sandy Clayey Silt
10 2.6 114 28 Silty Sand