HomeMy WebLinkAboutSubsoil StudyI Crt im*,ffiilllriinli,sü "' "
Án 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
Ofïìce Locations: Denver (HQ), Palker, Colorado SprÌngs, Fofi Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
JUL ? O ?T??
SUBSOIL STUDY GáRFIELD COUNTY
FOR FOUNDATION DESIGN UOMMUNITY IJEVELOPMENT
PROPOSED RESIDENCE
LOT 2, GRAND HOGBACK SUBDIVISION EXEMPTION
HARVEY GAP ROAD
GARFTELD COUNTY, COLORADO
PROJECT NO.22-7-348
JUNE 29,2022
PREPARED FOR:
JAMES MATLOCK
P.O. BOX 1453
FORT COLLINS, COLORADO 80522
iames ¡3jlck@,vahoo.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION....
LIMITATIONS
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
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SUBSURFACE CONDITIONS a
FOUNDATIONS .',......,..-2 -
............ - 3 -
"...........- 4 -
FOUNDATION AND RETAINING WALLS ....
FLOOR SLABS
UNDERDRAIN SYSTEM....4-
SITE GRADING 5-
6
Kumar & Associates, lnc. @ Project No. 22-7-348
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on Lot 2,
Grand Hogback Subdivision Exemption, Harvey Gap Road, 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 proposal for
geotechnical engineering services to James Matlock dated May 3,2022.
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 analyzedto 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 story, wood frame structure, 30 feet by 50 feet in plan size,
over a walkout basement. The basement floor will be slab-on-grade. Grading for the structure is
assumed to be relatively minor with cut depths between about 3 to 10 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 except for a mobile home. The building arca is located on a southeast down-
trending ridge. The building area slopes moderately steep to steep down to the northeast and had
been cleared of vegetation. The surrounding areas were vegetated with pinyon, juniper and sage
brush with an understory of grass, weeds and cactus.
F'IELD EXPLORATION
The field exploration for the project was conducted on }/.ay 27,2022. 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 truck-mounted
CME-458 drill rig. The borings were logged by a representative of Kumar & Associates, Inc.
Kumar & Associates, lnc. @ Project No. 22-7-348
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Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was 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-l586
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 I to 2 feet of stift silty clay and sand overlying weathered to very hard,
interlayered sandstone/siltstone/claystone bedrock of the Iles Formation. The formation material
was moderately hard to hard based on drilling with auger equipment.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and Atterberg limits testing. Results of swell-consolidation testing
performed on relatively undisturbed drive samples of the shallow weathered bedrock, presented
on Figure 4, indicate a minor expansion potential when wetted under a constant low surcharge
load and low compressibility under additional loading after wetting. Atterberg limits testing
indicated that the bedrock had medium plasticity. 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.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the residence be founded with spread footings bearing
on the hard bedrock which was encountered within about 2 feet of the ground surface.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils should 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 less than 1 inch.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
Kumar & Associates, lnc. @ Project No.22-7-348
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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.
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.
Foundation walls acting as retaining structures should also be designed to resist
lateral eafih pressures as discussed in the "Foundation and Retaining 'Walls"
section of this report.
The upper soils and highly weathered/broken bedrock should be removed and the
footing bearing level extended down to the relatively hard bedrock materials.
Loosened bedrock fragments should be removed prior to forming footings.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
4)
s)
FOLINDATION 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 well-broken bedrock materials mixed with limited amounts of overburden soils
devoid of organics. 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 well-broken bedrock materials mixed with
limited amounts of overburden soils devoid of organics.
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 90Yo of the maximum
standard Proctor density at optimum moisture content to slightly above optimum. Backfill in
slab and walkway areas should be compacted to at least 95Yo 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
6)
Kumar & Associates, lnc. @ Project No. 22-7-348
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foundation wall backfill should be expected, even if the material is placed correctly, and could
result in distress to facilities constructed on the backfill. Backfill should not contain organics,
debris or rock fragments larger than about 6 inches.
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%o of the
maxirnum standard Proctor density at a moisture slightly above optimum.
FLOOR SLABS
The natural on-site well-broken bedrock materials, exclusive of organics, 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 2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
on-site well-broken bedrock devoid of vegetation and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
mountainous areas and where bedrock is shallow 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.
Kumar & Associates, lnc. @ Project No. 22-7-348
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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 1o/o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yo 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 lVzfeet deep.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided the building is
located as planned and cut and fill depths are limited. We assume the cut depths for the
basement level will not exceed one level, about l0 to 12 feet. Fills should be limited to about 8
to 10 feet deep. Embankment fills should be compacted to at least95o/o 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
95o/o of the maximum standard Proctor density. The fill should be benched into the portions of
the hillside exceeding 20o/o grade (if any).
Permanent unretained cut and fill slopes should be graded at2horizontal to I 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.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfrll should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/o of fhe 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 l0 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
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.
Kumar & Associates, lnc. @ Project No. 22-7-348
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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 concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions id'entifred 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 verifu 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,
â€anwsæx & åss**åaå*e,
Daniel E. Hardin, P.E.
Reviewed by:
Steven L. Pawlak, P.E.
DEH/kac
KuE"nar & åecreiåtea, lne.'r;Fr*ject No.22-T-34fi
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22-7 -348 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
BORING 1
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BORING 2
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DD=132
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Fig. 222-7 -348 Kumar & Associates LOGS OF EXPLORATORY BORINGS
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LEGEND
CLAY AND SAND (CL-SC); SILTY, WITH ROCK FRAGMENTS, VERY STIFF/MEDIUM DENSE,
SLIGHTLY MOIST, LIGHT BROWN.
WEATHERED SANDSTONE/SILTSTONE/CLAYSTONE; MEDIUM HARD, MOIST, GRAYISH BROWN
m
SANDSTONE/STLTSTONE/CLAYSTONE (|LES FORMATION); VERY HARD, SLIGHTLY MO|ST, LtcHT
BROWN TO GRAY.
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE"
^^/'t2 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 46 BLOWS OF A 14O-POUND HAMMER'-,'- FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1 THE EXPLORATORY BORINGS WERE DRILLED ON MAY 27,2022 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.
5. 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);
LL = LIQUID LIMIT (ASTM DA518);
Pl = PLASTICITY INDEX (ASTM 04318).
22-7 -348 Kumar & Associates LEGEND AND NOTES Fig. 3
SAMPLE OF: Weothered Sillslone/Cloyslone
FROM: Boring 1 @ 1'
WC = 5.5 %, DD = 132 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
1.0 APPLIED PRESSURE - KSF t0 100
1.0 APPLIED PRESSURE - KSF 100
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SAMPLE OF: Weolhered Sillstone/Cloyslone
FROM:Boring2@1'
WC = 8.2 %, DD = 127 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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22-7 -348 Kumar & Associates SWELL-CONSOLIDATION TTST RESULTS Fig. 4
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l(+rti,+i1,ffitffff$jn''.nå;*.*TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPEWeathered Siltstone/ClaystoneClaystone/SiltstoneV/eathered Siltstone/Claystone(psf)UNCONFINEDCOMPRESSIVESTRENGTHlo/"1PLASTICINDEX22ATTERBERG LIMITS(%lLIQUID LIMITt3238PERCENTPASSING NO,200 SIEVE%tSANDGRADATION(%)GRAVEL(pc0NATURALDRYDENSITYr278.2(%)NATURALMOISTURECONTENT5.56.8tftlDEPTT{I011SAMPLE LOCATIONBORING12No. 22-7-348