HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.20.2022
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 13, BLOCK 7, WILLOW CREEK VILLAGE SECTION 2
BATTLEMENT MESA
48 RIDGE VIEW PLACE
GARFIELD COUNTY, COLORADO
PROJECT NO. 22-7-176
APRIL 7, 2022
PREPARED FOR:
RUSSELL CARTWRIGHT
35 WILLOWVIEW WAY
PARACHUTE, COLORADO 81635
russecart@gmail.com
Kumar & Associates, Inc. ® Project No. 22-7-176
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 1 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS .................................................................................................................... - 3 -
FLOOR SLABS ...................................................................................................................... - 4 -
SURFACE DRAINAGE ......................................................................................................... - 4 -
LIMITATIONS ........................................................................................................................... - 5 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 22-7-176
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 13, Block 7, Willow Creek Village Section 1, Battlement Mesa, 48 Ridge View Place,
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 Russell Cartwright dated
February 10, 2022.
An exploratory boring was drilled to obtain information on the general 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 residence will be a single-story wood framed structure with an attached garage located in the
middle portion of the lot and in the area of our exploratory boring shown on Figure 1. Ground
floors will be structural over crawlspace in the living area and slab-on-grade in the attached
garage. Grading for the structure is expected 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 lot was vacant and had scattered snow cover on the ground surface at the time of our field
exploration. The terrain is relatively flat and gently sloping down to the north. Vegetation
consists of grasses and weeds. There are existing residences on most of the adjacent lots.
FIELD EXPLORATION
The field exploration for the project was conducted on February 16, 2022. One exploratory
boring was drilled at the approximate location shown on Figure 1 to evaluate the subsurface
conditions. The boring was advanced with 4-inch diameter continuous flight augers powered by
a truck-mounted CME-45B drill rig. The boring was logged by a representative of Kumar &
Associates.
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Kumar & Associates, Inc. ® Project No. 22-7-176
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 Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below about 1 foot of topsoil, consisted of very stiff to stiff, slightly sandy
clayey silt underlain at a depth of about 17 feet by relatively dense, silty sandy gravel and
cobbles which consisted primarily of basalt fragments to boulder size. Drilling in the dense
coarse granular materials with auger equipment was occasionally difficult due to the cobbles and
boulders.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, and percent finer than sand size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the fine-grained
subsoils, presented on Figure 3, indicate low compressibility potential under conditions of
loading and wetting. The samples showed a low expansion potential when wetted under a
constant light surcharge. The laboratory testing is summarized in Table 1.
No groundwater was encountered in the boring at the time of drilling and the subsoils were
slightly moist below the moist topsoil layer.
FOUNDATION BEARING CONDITIONS
The clayey silt soils encountered in the borings possess low bearing capacity and typically a
moderate settlement potential when wetted. The laboratory testing indicated these soils to have a
low expansion potential, however, our experience in the area is that the fine grained soils tend to
settle some when wetted under load.
Lightly loaded spread footings bearing on the clayey silt soils can be used for foundation support
of the residence with some risk of settlement, primarily if the bearing soils were to become
wetted. A lower risk option would be to sub-excavate 3 feet of the fine-grained soils and replace
the soils as moisture conditioned compacted structural fill, or extend the foundation bearing
down to the underlying dense coarse granular soils such as with a pile or pier foundation.
Provided below are recommendations for spread footings bearing on the natural clayey silt soils.
If recommendations for a pile or pier foundation system are desired, we should be contacted.
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Kumar & Associates, Inc. ® Project No. 22-7-176
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we believe the building can be founded with spread footings bearing
on the natural soils with some risk of movement. The risk of settlement is primarily if the
bearing soils were to become wetted and precautions should be taken to prevent wetting.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Spread footings placed on the undisturbed natural soils or properly placed
structural fill can be designed for an allowable bearing pressure of 1,200 psf.
Based on experience, we expect settlement of footings designed and constructed
as discussed in this section will be about 1 inch or less. Additional settlement
could occur if the bearing soils were to become wetted. The magnitude of the
additional settlement would depend on the depth and extent of the wetting but
may be on the order of 1 to 1½ inches. Structural fill below the footings would
reduce the estimated settlements.
2) The footings 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.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies and better withstand the effects of some differential settlement
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 corresponding to an equivalent fluid unit weight of at least 55 pcf.
5) All topsoil and any loose or disturbed soils should be removed and the foundation
bearing level extended down to the firm natural soils. The exposed soils in
footing areas should then be moistened and compacted. Structural fill below
footings should be compacted to at least 98% of standard Proctor density at near
optimum moisture content.
6) A representative of the geotechnical engineer should observe all footing
excavations and test structural fill compaction as need on a regular basis prior to
concrete placement to evaluate bearing conditions.
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Kumar & Associates, Inc. ® Project No. 22-7-176
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. There could be some slab movement if the subgrade were to become wetted.
To reduce the effects of some differential movement, non-structural 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 base course should be placed immediately slabs at grade for support
and to facilitate drainage. 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 topsoil.
SURFACE DRAINAGE
A perimeter foundation drain around shallow crawlspace areas (less than 4 feet deep) should not
be needed with adequate compaction of foundation backfill and positive surface drainage away
from foundation walls as discussed below.
Positive surface drainage is a very important aspect of the project to prevent wetting of the
bearing soils. 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation, such as sod, and lawn
sprinkler heads should be located at least 5 feet from foundation walls.
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 22-7-176
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 1 6.5 90 92 Slightly Sandy Clayey Silt
4 5.1 105 Slightly Sandy Clayey Silt
10 4.6 100 89 Slightly Sandy Clayey Silt
15 4.6 106 Slightly Sandy Clayey Silt