HomeMy WebLinkAboutSoils Report 10.19.2020Kumar & Associates, Inc.®
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Company
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 27, SPRING RIDGE RESERVE
SPRING VIEW DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-191
OCTOBER 19, 2020
PREPARED FOR:
TODD YOUNG
2920 HAGGAR LANE
GLENWOOD SPRINGS, COLORADO 81601
(toddtoddyoungarchitect.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
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 - 7 -
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. 20-7-191
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 27, Springs Ridge Reserve, Spring View 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 accordance with our agreement for geotechnical
engineering services to Todd Young dated March 17, 2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock 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 proposed residence will be a one and two story structure with detached garage located on the
lot as shown on Figure 1. Ground floors will be slab -on -grade. Grading for the structure is
assumed to be relatively minor with cut depths between about 2 to 6 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 subject site was vacant at the time of our field exploration. The ground surface is sloping
moderately down to the north at a grade of around 15 percent. Vegetation Consists of grass,
weeds, and rabbit brush. Outcrops of the Maroon Formation are visible throughout the
subdivision.
Kumar & Associates, Inc. ® Project No. 20-7-191
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FIELD EXPLORATION
The field exploration for the project was conducted on October 9, 2020. 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, 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 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 consist of about %2 to 1 foot of topsoil overlying stiff silt and sand in Boring 1 to a depth
of 3 feet. Below the silt and sand in Boring 1 and topsoil in Boring 2 weathered siltstone/
sandstone was encountered to a depth of between 3 and 7 feet where very hard siltstone/
sandstone bedrock was encountered to the maximum explored depth of 11 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell -consolidation testing performed on
a relatively undisturbed drive sample of the silt and sand soil, presented on Figure 4, indicate low
compressibility under conditions of loading and wetting. 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.
Kumar & Associates, Inc. ® Project No. 20-7-191
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FOUNDATION BEARING CONDITIONS
The sand and silt soils and weathered sandstone/siltstone bedrock encountered in the borings
possess a moderate bearing capacity and low settlement potential. The underlying very hard
sandstone/siltstone bedrock possesses high bearing capacity and low settlement potential.
Excavations into the harder bedrock could be difficult and require rock excavation techniques.
We recommend spread footings bearing on the sand and silt soils or bedrock materials for
support of the proposed residence. There could be a risk of differential foundation settlement
where the spread footings transition from sand and silt soils to weathered bedrock. To reduce
this risk, the spread footings could be deepened to bedrock material or placed on structural fill
extending down to the bedrock. Structural fill can consist of the onsite soils and well broken
bedrock.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural soils or weathered bedrock.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils or weathered bedrock should be
designed for an allowable bearing pressure of 2,000 psf. Based on experience, we
expect settlement of footings designed and constructed as discussed in this section
will be about 1 inch or less. Footings deepened to bedrock can be designed for an
allowable bearing pressure of 4,000 psf.
2) The footings should have a minimum width of 16 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.
Kumar & Associates, Inc. ® Project No. 20-7-191
-4
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies and the soil/bedrock transition areas, such as by assuming an
unsupported length of at least 12 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 and rock should be removed and the
footing bearing level extended down to the relatively dense natural granular soils
and rock. The exposed soils in footing area should then be moistened and
compacted. _,,, a
6) A representative of the geotechnical engineer should observe all footing G"�G
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 50 pcf for backfill consisting
of the on -site soils and well broken bedrock. 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 40 pcf for backfill consisting of the on -site soils and
well broken bedrock.
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 a moisture content near optimum. 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
Kumar & Associates, Inc. ® Project No. 20-7-191
5
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
based on a coefficient of friction of 0.45. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 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 a granular material 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, are suitable to support lightly loaded slab -on -grade
construction. 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 free -
draining gravel should be placed beneath basement level slabs 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 2% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% Q of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on -
site granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered durinLpur exploratipn, it has been our experience in
the area and where bedrock is shallow that local perched groundwater can develop during times
Kumar & Associates, Inc. ® Project No. 20-7-191
6
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,
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
The drains should consist of rigid 4-inch diameter drainpipe placed in the bottom of the wall
backfill surrounded above the invert level with free -draining granular material. The drain should
be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and
sloped at a minimum '/2% 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 11/2 feet deep.
SURFACE DRAINAGE
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 thie.building .shouldhe. sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first.10feet 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.
Kumar & Associates, Inc. ® Project No. 20-7-191
7
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
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on -site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, Inc.
James H. Parsons, E.I.
Reviewed by:
Steven L. Pawlak, S.E.
JHP/kac
Kumar & Associates, Inc.
Project No. 20-7.191
1
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20-7-191
Kumar & Associates
LOCATION OF EXPLORATORY BORINGS
Fig. 1
BORING 1
EL. 6497.5'
BORING 2
EL. 6491'
0
5
10
15
ti.
f " 8/6, 22/6
WC=5.2
DD=124
43/6, 50/1
WC=6.7
DD=134
50/1
0
0
50/0
35/6, 50/5
WC=2.9
DD=128
50/2
50/1
50/1
0
5
10
15
20 20
20-7-191
Kumar & Associates
LOGS OF EXPLORATORY BORINGS
Fig. 2
0
7
LEGEND
f
1
TOPSOIL; SILT AND SAND, CLAYEY, ORGANICS, FIRM, SLIGHTLY MOIST, RED BROWN.
SAND AND SILT (SM—ML); SLIGHTLY CLAYEY, STIFF, SLIGHTLY MOIST, RED.
WEATHERED SILTSTONE/SANDSTONE; SLIGHTLY CALCAREOUS, MEDIUM HARD TO HARD,
SLIGHTLY MOIST, RED.
SILTSTONE/SANDSTONE BEDROCK; VERY HARD, SLIGHTLY MOIST, RED. MAROON FORMATION.
SANDSTONE BEDROCK; VERY HARD, SLIGHTLY MOIST, RED. MAROON FORMATION.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
30/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 30 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON OCTOBER 9, 2020 WITH A 4—INCH—DIAMETER
CONTINUOUS —FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216).
20-7-191
Kumar & Associates
LEGEND AND NOTES
Fig. 3
CONSOLIDATION - SWELL
1
-2
-3
-4
SAMPLE OF: Slightly Clayey Sand and Silt
FROM: Boring 1 c 2.5'
WC = 5.2 %, DD = 124 pcf
NO MOVEMENT UPON
WETTING
These test mute oppy only to the
' tenpin toted. The terinp report
e heA not be reproduced, vessel In
full. without the written approval of
i[wnw and Neoclata, Inc. Swell
Conealldalfon feeling performed In
ocelwdance with /Sit% 0-4S4S.
1.0 APPLIED PRESSURE - KSF 10 100
20-7-191
Kumar & Associates
SWELL —CONSOLIDATION TEST RESULTS
Fig. 4
ICA
Kumar & Associates, Inc.
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-191
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITY
Oct)
GRADATION
PERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(psi)
SOIL TYPE
BORING
DEPTH
(ft)
GRAVEL
(o �°)
SAND
(o�°)
LIQUID LIMB
CM
PLASTIC
INDEX
(%)
1
2'/2
5.2
124
Slightly Clayey Sand and
Silt
5
6.7
134
Weathered Sandstone/
Siltstone
2
1
2.9
128
Weathered Sandstone