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 GARAGE
LOT 6, MINEOTA ESTATES
MINEOTA DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 22-7-127
FEBRUARY 24, 2022
PREPARED FOR:
CODY AND KARLA FERGUSON
P.O. BOX 792
SILT, COLORADO 81652
cf.masonry@yahoo.com
Kumar & Associates, Inc. ® Project No. 22-7-127
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ........................................................................... - 1 -
PROPOSED CONSTRUCTION .................................................................................... - 1 -
SITE CONDITIONS ....................................................................................................... - 1 -
FIELD EXPLORATION ................................................................................................ - 2 -
SUBSURFACE CONDITIONS ..................................................................................... - 2 -
FOUNDATION BEARING CONDITIONS .................................................................. - 2 -
DESIGN RECOMMENDATIONS ............................................................................ - 3 -
FOUNDATIONS ........................................................................................................ - 3 -
FOUNDATION AND RETAINING WALLS ........................................................... - 4 -
FLOOR SLABS .......................................................................................................... - 5 -
UNDERDRAIN SYSTEM ......................................................................................... - 6 -
SITE GRADING ......................................................................................................... - 6 -
SURFACE DRAINAGE ............................................................................................. - 7 -
LIMITATIONS ............................................................................................................... - 7 -
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. 22-7-127
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence and garage to be
located on Lot 6, Mineota Estates, Mineota 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 Cody and Karla Ferguson, dated January 18, 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
Design plans for the lot were preliminary at the time of our study. The proposed buildings will
be located as shown on Figure 1 and be cut into the north sloping terrain. Excavation for the
buildings is expected to be between about 3 to 12 feet below the existing ground surface. For the
purpose of our analysis, 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 building area was vacant and appeared mostly natural except for a small disturbed area in
the general proposed garage site which had probably been flattened by fill grading. The
proposed residence site had been staked. There were scattered items including trailers on the
property. The ground surface slopes gently to moderately down to the north at grades between
about 10 to 15% through the building area and steepens to about 20% uphill of the building area.
About 6 to 12 inches of snow covered the lot and vegetation appeared to consist of native grass
and weeds.
Kumar & Associates, Inc. ® Project No. 22-7-127
- 2 -
FIELD EXPLORATION
The field exploration for the project was conducted on January 26, 2022. 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, Inc.
Samples of the subsoils were taken with a 2-inch I.D. spoon sampler. The sampler was 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 profiles encountered at the site are shown on Figure 2. Below
about 1 foot topsoil or 3 feet of loose organic clay fill in Boring 3, the subsoils consist of stiff to
very stiff, silty sandy clay transitioning to medium dense/hard, clayey sand and sandy clay with
gravel at depth. Hard to very hard sandstone bedrock was encountered below the sand and clay
soil at depths of about 26½ and 17 feet in Borings 1 and 2, respectively. Clay soils like those
encountered in the borings can possess an expansion potential when wetted.
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 and bedrock
were slightly moist.
FOUNDATION BEARING CONDITIONS
The clay and clayey sand soils encountered at the site possess an expansion potential when
wetted under light loading. The expansion potential can probably be partly mitigated by load
Kumar & Associates, Inc. ® Project No. 22-7-127
- 3 -
concentration on spread footings to reduce or prevent swelling in the event of wetting below the
foundation bearing level. Surface runoff, landscape irrigation, and utility leakage are possible
sources of water which could cause wetting. A lower movement risk foundation would be to
place the footings on 3 feet of compacted road base or support the foundation with piers or piles
that extend down into the sandstone bedrock.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the buildings can be founded with spread footings placed on
undisturbed natural soils with a risk of movement mainly if the bearing soils are wetted. If a
deep foundation of piles or piers is desired, we should be contacted for additional analysis and
recommendations.
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 2,500 psf. The footings should also be designed for a
minimum dead load pressure of 800 psf. In order to satisfy the minimum dead
load pressure under lightly loaded areas, it may be necessary to concentrate loads
by using a grade beam and pad system. Wall-on-grade construction is not
recommended at this site to achieve the minimum dead load. The minimum dead
load pressure requirement is not needed if at least 3 feet of structural fill is placed
below footing grade. The fill should extend laterally out from the edge of the
footing at least 1½ feet on both sides. Structural fill used below the foundation
should be a relatively well graded granular material such as CDOT Class 6 road
base compacted to at least 98% of standard Proctor density.
2) Based on experience, we expect initial settlement of footings designed and
constructed as discussed in this section will be up to about 1 inch. There could be
additional movement of around 1 inch if the bearing soils were to become wet.
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 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.
Kumar & Associates, Inc. ® Project No. 22-7-127
- 4 -
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.
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, any existing fill, 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 fine-grained soils and at least 45 pcf for backfill consisting of imported granular
materials. Cantilevered retaining structures which are separate from the buildings 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 50 pcf for backfill consisting of the on-site fine-grained soils and at least 40 pcf for
backfill consisting of imported granular materials.
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 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
Kumar & Associates, Inc. ® Project No. 22-7-127
- 5 -
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 0.35. 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 on-site soils possess an expansion potential and slab heave could occur if the subgrade soils
were to become wet. Slab-on-grade construction may be used provided precautions are taken to
limit potential movement and the risk of distress to the building is accepted by the owner. A
positive way to reduce the risk of slab movement, which is commonly used in the area, is to
construct structurally supported floors over crawlspace. Where slab-on-grade is used, we
recommend at least 2 feet of relatively well graded granular material such as CDOT Class 6 road
base be placed below floor slabs to help mitigate the expansion potential.
To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with
a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be
transmitted to the upper structure. This detail is also important for wallboards, stairways and
door frames. Slip joints which will allow at least 1½-inches of vertical movement are
recommended. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. Slab reinforcement and control joints 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 immediately beneath basement
level slabs-on-grade. This material 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 free-draining gravel
will aid in drainage below the slabs and should be connected to the perimeter underdrain system.
Kumar & Associates, Inc. ® Project No. 22-7-127
- 6 -
Required fill beneath slabs can consist of the on-site gravelly soils or a suitable imported
granular material, excluding topsoil and oversized rocks. The fill should be spread in thin
horizontal lifts, adjusted to at or above optimum moisture content, and compacted to at least 95%
of the maximum standard Proctor density. All vegetation, topsoil and loose or disturbed soil
should be removed prior to fill placement.
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. All plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
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.
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. Void form
below the foundation can act as a conduit for water flow. 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 above the void form with mastic to keep drain water from flowing beneath the wall and to
other areas of the building.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided cut and fill
depths are limited to about 8 to 10 feet. Embankment fills should be compacted to at least 95%
of the maximum standard Proctor density near optimum moisture content. Prior to fill
placement, the subgrade should be carefully prepared by removing existing fill, vegetation and
topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should
Kumar & Associates, Inc. ® Project No. 22-7-127
- 7 -
be benched into slopes that exceed 20% grade. Permanent unretained cut and fill slopes should
be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or
other means.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after each building has been completed:
1) Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction. Drying could increase the expansion
potential of the clay soils.
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. 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.
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 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
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
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 22-7-127
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 4.3 114 Silty Sandy Clay
15 7.3 120 67 Sandy Silty Clay
2 2½ 4.5 110 Sandy Clay
5 4.6 111 50 Sand and Clay with Gravel
15 5.4 123 54 Sand and Clay with Gravel
3 2½ 5.1 108 50 Sand and Clay
10 8.5 112 Sandy Clay