HomeMy WebLinkAboutSubsoil Study for Foundation Design 11.28.16H-P~l<UMAR
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 49, SPRINGRIDGE RESERVE PUD, PHASE 3
HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 16-7-568
NOVEMBER 28, 2016
PREPARED FOR:
JORDAN ARCHITECTURE
ATTN: BRADJORD,AN
P.O. BOX 1031
GLENWOOD SPRINGS, COLORADO 81602
(bradjordam rchitec t @gmail.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ -1 -
SITE CONDITIONS .................................................................................................................. -2 -
FIELD EXPLORATION ............................................................................................................ -2 -
SUBSURFACE CONDITIONS ................................................................................................. -2-
DESIGN RECOMMENDATIONS ............................................................................................ -3 -
FOUNDATIONS .................................................................................................................... -3 -
FOUNDATION AND RETAINING WALLS ....................................................................... -4 -
FLOOR SLABS ...................................................................................................................... -5 -
UNDERDRAIN SYSTEM ..................................................................................................... -6 -
SITE GRADING .................................................................................................................... -6 -
SURFACE DRAINAGE ........................................................................................................ -7 -
LIMIT A TIO NS ........................................................................ ~ ................................................... -7 -
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2 -LOGS OF EXPLORATORY BORINGS
FIGURE 3 -LEGEND AND NOTES
TABLE 1-SUMMARY OF LABORATORY TEST RESULTS
H-P~ KUMAR Project No. 16-7-568
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at Lot
49, Springridge Reserve PUD, Phase 3, Hidden Valley 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 Jordan Architecture dated November 2, 2016. Hepworth-
Pawlak Geotechnical, Inc. previously performed a preliminary geotechnical study for the
subdivision and reported the findings June 22, 2004, Job No. 101 126.
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 one story wood frame construction above a partial basement and
partial crawlspace with an attached garage. Basement and garage floors will be slab-on-grade.
Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 12
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.
H-P~ KUMAR Project No. 16-7-568
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SITE CONDITIONS
The vacant lot is vegetated with scattered sage brush, grass and weeds. The ground surface in
the south part of the site slopes moderately down to the northwest at grades of 18 to 24 percent
and the grade flattens in the northern part of the lot to 7 percent or less. Cobbles and boulders up
to 3 feet in size were observed scattered on the ground surface.
FIELD EXPLORATION
The field exploration for the project was conducted on November 4, 2016. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
Access was limited in the southern part of the building envelope due to the relatively steep
slopes. 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 H-P/Kumar.
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 below about six inches of topsoil consist of l l/2 to 71/2 feet of silty sand with sandstone
fragments overlying sandstone bedrock down to the maximum depth explored, 11 feet. Drilling
in the hard bedrock with auger equipment was difficult due to the hardness of the bedrock and
drilling refusal was encountered in both borings .
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Laboratory testing performed on samples obtained from the borings included natural moisture
content, density and percent finer than sand size gradation analyses. The laboratory testing is
summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils and bedrock
were slightly moist to moist.
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 sandstone 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 bedrock should be designed
for an allowable bearing ressure oif 2,000 psf. Footings placed entirely on
undisturbed hard bedrock can be designed for an allowable bearing pressure of
4,000 psf. Based on experience, we expect settlement of footings designed and
constructed as discussed in this section will be about 1 inch or less.
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.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 10 feet.
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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) Any fill, topsoil and loose or disturbed soils should be removed and the footing
bearing level extended down to the relatively undisturbed soils or bedrock. The
exposed soils in footing areas should then be moistened and compacted. Deeper
parts of the excavation into bedrock (including utility trenches) may require rock
excavation techniques such as chipping or blasting.
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 50 pcf for backfill consisting
of the on-site soils or 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 45 pcf for backfill consisting of the on-site soils or 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 in pavement and walkway
H-P~ KUMAR Project No. 16-7-568
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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
based on a coefficient of friction of 0.45. Passive pressure of compf!Cted 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 can consist of the on-site soils compacted to at least
95% of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils and bedrock, 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 expan sion 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.
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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 rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
mountainous areas that local perched groundwater can develop during times 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 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 1 % 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 1 Y2 feet deep.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided cut and fill
depths are limited. We assume the cut depths for the basement level will not exceed one level,
about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep. 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 all
vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density.
The fill should be benched into the portions of the hillside exceeding 20% grade.
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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. The risk of slope instability will
be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is
encountered in permanent cuts, an investigation should be conducted to determine if the seepage
will adversely affect the cut stability.
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 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. Free-draining wall backfill should be
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.
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
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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,
Louis E. Eller
Reviewed by:
H-P~ KUMAR Project No. 16-7-568
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NOTE : CONTOURS SHOWN ARE
FROM SUBDIVISION PLANS.
APPROXIMATE SCALE-FEET
BORING 1 BORING 2
EL. 6462' EL. 6450'
0 0
44/12
WC:::3.4
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WC:::3.8 50/0
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LEGEND I TOPSOIL; ORGANIC SANDY SILT AND CLAY, FIRM, SLIGHTLY MOIST, BROWN.
~SAND (SM); SILTY, GRAVELLY, SANDSTONE FRAGMENTS TO COBBLE SIZE, DENSE,
SLIGHTLY MOIST, RED. DEEPER PORTION IN BORING 1 MAY BE WEATHERED
BEDROCK.
I SANDSTONE BEDROCK; WEATHERED TO VERY HARD, SLIGHTLY MOIST, RED.
MAROON fORMATION • .
p RELATIVELY UNDISTURBED DRIVE SAMPLE; 2-INCH l.D. CALIFORNIA LINER SAMPLE.
/ DRIVE SAMPLE BLOW COUNT. INDICATES THAT 44 BLOWS OF A 140-POUND HAMMER 44 12 FALLING 30 INC HES WERE REQUIRED TO DR IV E THE CALIFORNIA OR SPT SAMPLER 12 INCHES.
t PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 4, 2016 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 D 2216);
DD = DRY DENSITY (pct) (ASTM D 221 6);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
H-P ~l<UMAR
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No.16-7-568
SAMPLE L OCATION NATURAL NATURAL GRADATION A TTERBERG LIMITS UNCONFINED PERCENT
MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%)
{ft\ /%) {ccfl
SIEVE
(%) (%) (PSF}
26 Weathered Sandstone
1 2V2 3.4 131 Bedrock
5 3.8 118 Sandstone Bedrock
'
.