HomeMy WebLinkAboutSubsoil StudyI(lrtiiffilfl','i::ï:tif '"'Ëd*'*
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5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
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Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 280, TRONBRTDGE
BLUE HERON VISTA
GARFTELD COUNTY, COLORADO
PROJECT NO.2t-7-144
FEBRUARY 23,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81601
lu ke.gosda@,sunriseco.com
1
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .
BACKGROUND INFORMATION
PROPOSED CONSTRUCTION
SITE CONDITIONS.....
SUBSIDENCE POTENTIAL
FOUNDATIONS
FOUNDATION AND RETAINING WALLS ....
NONSTRUCTURAL FLOOR SLABS
UNDERDRAIN SYSTEM...
SURFACE DRAINAGE
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 - S'üIIELL-CONSÒLIDATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
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a
FIELD EXPLORATION J-
SUBSURFACE CONDITIONS J
FOUNDATION BEARING CONDITIONS .....-4-
DESIGN RECOMMENDATIONS .............-4-
4
5
6
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1
Kumar & Associates, Inc. o Project No.2'l-7-1¿Ã
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 280, Ironbridge, Blue Heron Vista, Garfield County, Colorado. The project site is shown on
Figure 1. The pu{pose 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 SCIB, LLC dated January 20,202I.
A field exploration program consisting of an exploratory boring 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 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.
BACKGROUND INFORMATION
The proposed residence is located in the existing lronbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted subsurface exploration and geotechnical
evaluation for development of Villas North and Villas South parcels, Job No. 105 115-6, repoft
dated September 14,2005, and performed observation and testing services during the
infrastructure construction, Job No. 106 0367 between April 2006 and April2007. The
information provided in these previous reports has been considered in the current study of
Lot 280.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. For the pulpose
of our analysis, we assume the proposed residence will be a one or two story, wood frame
structure with an attached garage about 1,800 square feet in size and located within the property
boundary shown on Figure 1. Ground floor is assumed to consist of a structural slab-on-grade
with no basement or crawlspace.
Kumar & Associates, lnc. @ Project No.21-7-144
I
Grading tbr the structure is assumed to be relatively minor with cut or fill depths between about
3 t¿ 4feet. l[e assume relatively light foundation loadings, typical of the proposed type of
construction.
If building loadings, location or gracling plans change signif,rcantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The site was vacant at the time of our field exploration, and covered with about 6 inches of snow.
Vegetation consists of sagebrush and weeds, and boulders are visible on the surface. There was
a soil stockpile approximately 8 feet high in the center of the lot as shown on Figure 1. The
terrain was relatively flat (excluding the soil stockpile) with about I to Ilz feet of elevation
difference across the lot with a gentle slope down to the north. Fill had been placed to elevate
the lot and surrounding area by the previous subdivision grading.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanianage Eagle Valley Evaporite underlies the Ironbridge development.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits
associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the
gypsum under certain conditions can cause sinkholes to develop and can produce areas of
localized subsidence. A sinkhole opened in the cart storage parking lot located east of the Pro
Shop and west of the Villas North parcel in January 2005. Other irregular bedrock conditions
haie been identified in the affordable housing site located to the northwest of the Villas Ñorth
parcel. Irregular surface features that could indicate an unusual risk of future ground subsidence
were not observed in the Villas North parcel, but localized variable depths of debris fan soils and
bedrock quality encountered by the previous September 14,2005 geotechnical study in the Villas
North development area could be the result of past subsidence. The subsurface exploration
performed in the area of the proposed residence on Lot 280 did not encounter voicls. In our
opinion, the risk of future ground subsidence on Lot 280 in the Villas North paroel throughout
the seryice life of the proposed residence is low antl similar to other areas of the Roaring Fork
River valley where there have not been indications of ground subsidence, but the owner should
be made aware of the potential for sinkhole development. If further investigation of possible
cavities in the bedrock below the site is desired, we should be contacted.
Kumar & Associates, lnc. @ Project No.21-7.144
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FIELD EXPLORATION
The field exploration for the project was conducted on January 27 ,202I. One exploratory
boring was drilled at the 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-458 drill rig. The boring was logged by a representative of Kumar & Associates.
Samples of the subsoils were taken with l% 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 is shown on Figure 2. Beneath
about 6 inches of topsoil (root zone), the subsoils consist of about 6Yz feet of medium dense,
mixed sand, silt, clay and gravel fill overlying about 8 feet of medium stiff to stiff sandy clay.
Relatively dense sandy silty gravel with cobbles and possible boulders was encountered below
the clay soils at a depth of about 15 feet down to the bottom of the boring at2l feet. Drilling in
the dense granular soils with augsr equipment was difficult due to the cobbles and possible
'boulders. I
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on a relatively undisturbed drive sample of the clay soils, presented on
Figure 3, indicate low to moderate compressibility under light loading. The laboratory test
results are summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist.
Kumar & Associates, lnc. o Project No.2'l-7-144
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FOUNDA'I'ION BSARTNG CONDI'I'IONS
The upper 7 feet of soils consist of fill placed mainly in 2006 as paft of the subdivision
clevelopment. The held penetration tests (blow counts) and laboratory tests performed for the
current study, and review of the field density (compaction) tests performed during the f,rll
construction indicate the structural fill was placed and compacted to the project specified 95o/o of
standard Proctor density. Debris fan soils which tend to collapse (setf.le under constant loacl)
when wetted were encountered below the fiIl. The amount of settlement will depend on the
thickness of the compressible soils due to potential collapse when wetted, and compression of
the underlying soils after wetting. Relatively deep structural fill will also have some potential for
long term settlement but should be considerably less than the alluvial fan deposit. Sources of
wetting include irrigation, surface water runoff and utility line leaks. A heavily reinforced
structural slab or post-tensioned slab tbundation designed f'or significant diflèrential settlements
is recommended for thc building support.
DESIGN RECOMMENDATIONS
FOI.INDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with a heavily reinforced
structural slab foundation bearing on at least 6 feet of (existing) compacted structural fill. A
post-tensioned slab foundation could also be used.
The design and construction criteria presented below should be observed for a structural slab
foundation system. ,
1) A heavily reinforced structural slab placed on around 6 feet of (existing) structural
fill should be designed for an allowable bearing pressure of 1,500 psf or subgrade
modulus of 125 tcf. A post-tensioned slab if used should be designed for a wetted
distance of 10 feet but at least half of the slab width, whichever is more. Based
on experience, we expect initial settlement of the slab foundation designed and
constructcd as discusscd 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 wetting but
may be on the order of I to I% inches.
2) The thickenecl sections of the slab for support of concentratecl loads should have a
minimum width of 20 inches.
Kumar & Assoclates, lnc. @ Project No.2'l-7-144
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3)The perimeter turn-down section of the slab should be provided with adequate soil
cover above the bearing elevation for frost protection. Placement of foundations
at least 36 inches below exterior grade is typically used in this area. If a frost
protected foundation is used, the perimeter tum-down section should have at least
18 inches of soil cover.
The foundation should be constructed in a "box-like" configuration rather than
with irregular extensions which can settle differentially to the main building area.
The foundation walls, where provided, should be heavily reinforced top and
bottom to span local anomalies such as by assuming an unsupported length of at
least 14 feet. Foundation walls acting as retaining structures (if any) should also
be designed to resist laterul earth pressures as discussed in the "Foundation and
Retaining Walls" section of this report.
The topsoil or organic root zone and any loose or disturbed soils should be
removed. Additional structural fill placed below the slab bearing level should be
compacted to at least 98%:o of the maximum standard Proctor density at a moisture
content near optimum.
A representative of the geotechnical engineer should evaluate the compaction of
fill materials and observe all footing excavations prior to concrete placement to
evaluate bearing conditions.
4)
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 soils. Cantilevered retaining structures (if any) which are separate from the
building and can be expected to deflect suff,rciently 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.
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.
s)
6)
Kumar & Associates, lnc, o Project No.21-7-144
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tsacktill should be placed in unifonn lifts and compacted to at least 90% of the rnaximurn
standard Proctor density at a moisture content ncar optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95o/o 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 cxccssivc latcral pressure on the wall. Sonle settlement of deep foundation wall
backfill shoultl 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 backhll against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefltcient of friction and passive pressure values recommended above assume ultimate soil
strcngth. 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 95Yo of the
maximum standard Proctor density at a moisture content near optimum.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support lightly loaded slab-on-grade construction
separate from the building foundation. The fill soils can be compressible when wetted antl result
in some post-constnrction settlement. To reduce the effects of some differential movement,
slabs-on-grade should be separated'from the building to 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 well-graded sand and
gravel, such as road base, should be placed beneath slabs for support. This material should
consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sievc and lcss than
I2o/o passing the No. 200 sieve.
All f,rll 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.
Kumar & Associates, lnc. @ Project No.2'l-7-'144
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LTNDERDRAIN SYSTEM
It is our understanding that the finished floor elevation at the lowest level of the proposed
residence will be at or above the surrounding grade. Therefore, a foundation drain system is not
required. Although free water was not encountered during our exploration, it has been our
experience in the areathat 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, be protected from
wetting and hydrostatic pressure buildup by an underdrain system.
If finished floor elevation of the proposed residence has a floor level below the surrounding
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
SURFACE DRAINAGE
Precautions to prevent wetting of the bearing soils, such as proper backfill construction, positive
backfill slopes, restricting landscape irrigation and use of roof gutters need to be taken to limit
settlement and building distress. 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 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90Yo 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 6 inches in the first 5 feet in unpaved areas and a minimum slope of
2Y"inches in the first 10 feet in paved areas. Graded swales should have a
minimum slope of 3%.
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.
Kumar & Associates, lnc, o Project No.21-7-144
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LIMITATIONS
This study has been conducted in ¿ûcordance with generally accepted geotechnical engineering
principles and practices in this area at the time of this study. We muke no wurrunty either
express or implied. The conclusions and recornmendations submitted in this report are based
upon the data obtained from the exploratory boring drilled at the location 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. Ifthe 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 boring 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 recornmendations rnay be ¡nade.
This report has been prepared for the exclusive use by our client for design pu{poses. 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 veriff 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.
David A. Noteboom, Staff Engineer
Reviewed by:
Steven L. Pawlak,
SLPlkac
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Kumar & Associates, ln6. o'ProJect No. 21.7.144
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APPROXIMATE SCALE-FEET
21 -7 -1 44 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
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BORING 1
Fl . 5960'LEGEND
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TOPSO|L: SANDY GRAVELLY CLAY, FIRM, SLIGHTLY MO|ST, BROWN
ROOT ZONE.
42/ 12
FILL: MIXED SAND, SILT CLAY AND GRAVEL, MEDIUM DENSE,
SLIGHTTY MOIST, BROWN,25/6,41/6
WC=11.2
DD=1 17
-200=62
CLAY (CL) SANDY, MEDTUM STTFF T0 ST|FF, MOIST, BRoWN, LOW
PLASTICITY.
13/ 12
WC= 13.0
-200=72
GRAVEL (CM), SANDY, SrLTy, W|TH RoUNDED RoCKS, DENSE,
SLIGHTLY MOIST, BROWN. ROUNDED ROCK,
10
5/ 12
WC= 14.7
DD=1 1 1
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DRIVE SAMPLE, 2-INCH I.D, CALIFORNIA LINER SAMPLE.
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DR|VE SAMPLE, 1 3/8-|NCH r.D. SpLtT Sp00N STANDARD
PENETRATION TEST.
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3e /6, 50/5.5
¡7¡11DR|YE SAMPLE BLOW COUNT. INDICATES THAT 42 BLOWS 0F,-I '- A 14o-POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 1 2 INCHES.
NOTES
20 THE EXPLORATORY BORING WAS DRILLED ON JANUARY 27,
2021 WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER
AUGER.
50/3
2, THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON IHE
SITE PLAN PROVIDED.
tq,3. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE
GRADUAL,
6, GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE
TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CoNTENT (%) (ASTM D 2216);
DD = DRY DENSTTY (pcf) (nSrU O ZZ1O);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1 1 4o),
21 -7 -1 44 Kumar & Associates LOG OF EXPLORATORY BORING îig. 2
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SAMPLE OF: Sondy Cloy
FROM: Boring 1 @ 10'
WC = 14.7 %, DD = 1'l I pcf
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21 -7 -1 44 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULTS Fig. 3
rcrf Hif;lfiffi:îËtrn¡',iÊ;n''*"TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-144SOIL TYPESandy Silty Clay withGravel (Fill)Sandy ClaySandy Clay(psf)UNCONFINEDCOMPRESSIVESTRENGTH(%lPLASTICINDEXATTERBERG LIMITS(%\LIQUID LIM]TPERCENTPASSING NO.200 stEVE6272SAND(f/"1GRADATION(%)GRAVELSAMPLE LOCATIONDEPTHBORINGNATURALDRYDENSfTYNATURALMOISTUREOONTENÏt17111,21113.014.74701I