HomeMy WebLinkAboutSubsoil Studyl (+rt irf;;l['tr#r:ffiini':i,Êü "' "
An Employcc Owncd Gompony
5020 Counfy Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.col"r'r
Office Locations: Denver (HQ), Patker, Colorado Splings, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 256, rRONBRrDGE
RIVER VISTA
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-480
JUNE 29,2021
PREPARED FOR:
SCIBO LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN' COLORADO 81611
TABLE OF CONTENTS
PURPOSE ANIJ SCOPH OF' S'I'UDY
BACKGROUND INFORMATION
PROPOSED CONSTRUCTION
SITE CONDITIONS.......
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ...
FOUNDATIONS..
FOI-INDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS .
LINDERDRAIN SYSTEM
SITE GRADING.............
SURFACE DRAINAGE.
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No.21-7.480
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot256,Ironbridge, River Vista, 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 SCIB, LLC dated }l4ay 25,202I.
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 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 Ironbridge development. Hepworth-Pawlak
Geotechnical, Inc. (now Kumar & Associates) previously conducted subsurface exploration and
geotechnical evaluation for the development of Villas North and Villas South parcels, Job No.
105 115-6, report dated September 14,2005, and performed observation and testing services
during the infrastructure construction, Job No. 106 0367, between April 2006 and April 2007.
The infonnation provided in these previous reports has been considered in the current study of
Lot256.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The residence
will likely be a one or two-story, wood-frame structure with structural slab foundation and no
basement or crawlspace. Grading for the structure is assumed to be relatively minor with cut
depths between about 2 to 3 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.
Kumar & Associates, lnc. o Project No. 21-7-480
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SITE CONDITIONS
Lot256 is bordered on the east by River Vista, and on the west hy the Tronhridge Golf Course.
The lot was vacant at the time of our fielcl exploral.ion. Vegetation consists of grass and weeds
with some sage brush. The lot slopes gently down to the west through most of the building
envelope then steeply dowr in the western part about 5 feet to the adjacent golf course. River
Vista is on a fill bench made for residence construction that was placed during the subdivision
development in 2006 to 2001 .
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with sinkholes
andlocalized ground subsidence in the Roaring Fork Valley. A sinkhole opened in the cart
storage parking lot located east of the Pro Shop and north of the Villas South parcel in January
2005. Irregular surface features were not observed in the Villas South parcel that could indicate
an unusual risk of future ground subsidence. Localizedvariable depths of debris fan soils which
could indicate ground subsidence were generally not encountered by the previous September 14,
2005 geotechnical study in the Villas South parcel. The subsurface exploration pert-ormed in the
area of the proposed residence on Lot 256 did not encounter voids. In our opinion, the risk of
future ground subsidence on Lot 256 throughout the service life of the proposed residence is low
and similar to other areas of the Roaring Fork Valley where there have not been indications of
ground subsidence, however, 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.
FIELD EXPLORATION
The field exploration for the project was conducted on June I I,202L 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-
45B drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with l3A 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 all indication of the relaLive 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.
Kumar & Associates, lnc. @ Project No, 21.7.480
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SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. Below
about a 6-inch root zone, the subsoils consist of very stiff to dense, gravelly sandy silt f,rll down
to a depth of about 15 feet. This was underlain by medium dense, slightly gravelly sand and silt
alluvial fan deposits down to about 20 feet where medium dense, silty sand and gravel alluvial
fan deposits were encountered down to about 37% feet. This was underlain by dense, slightly
silty sandy river gravel and cobble alluvium to the drilled depth of 41 feet. Drilling in the coarse
granular soils with auger equipment was diff,rcult due to the cobbles and possible boulders.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density, and gradation analyses. Results of gradation analyses performed on a
sample of the more granular soils (minus l%-inch fraction) are presented on Figure 3. The
laboratory testing is summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist.
FOUNDATION BEARING CONDITIONS
The upper 15 feet of soils encountered in the boring consist of fill placed mainly in2006 as part
of the subdivision development. The field penetration tests and laboratory tests performed for
the current study, and review of the field density tests performed during the fill construction
indicate the structural fill was placed and compacted to the project specified minimum 95% of
standard Proctor density. Alluvial fan soils which tend to collapse (settle under constant load)
when wetted were encountered below the fi1l. The amount of settlement will depend on the
thickness of the compressible soils due to potential collapse when wetted, and compression of
the wetted soils following construction. Relatively deep structural fill as encountered will also
have some potential for long-term settlement but should be significantly less than the alluvial fan
deposits. Proper grading, drainage and compaction as presented below in the Surface Drainage
section will help to keep the subsoils dry and reduce the settlement risks. A heavily reinforced
structural slab or post-tensioned slab foundation designed for significant differential settlements
is recommended for the building support. As an alternative, a deep foundation that extends
down into the underlying dense, river gravel alluvium could be used to reduce the building
settlement risk.
Kumar & Associates, lnc. @ Project No. 21-7-480
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DESIGN RECOMMENDA'T ON S
FOI-INDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the naturc of
the proposed construction, we recommend the building be founded with a heavily reinforced
structural slab or post-tensioned slab foundation bearing on at least 15 feet of the existing
compacted structural fill. If a deep foundation system is considered for building support, we
should be contacted for additional recommendations.
The tlesign and construction criteria presented below should be observed for a heavily reintbrced
structural slab or post-tensioned slab foundation system.
1) A heavily reinforced structural slab or post-tensioned slab placed on compacted
structural fill should be designed for an allowable bearing pressure of 1,500 psf.
The post-tensioned slab placed on structural fill shoulcl be clesigned for a wetted
distance of 10 feet or at least half of the slab width, whichever is greater.
Foundation settlement is estimated to be about I to I% inches basetl on the long-
term compressibility of thc fill. Additional settlement of about I to 2 inches is
estimated if the underlying clebris fan soils were to become wet. Settlement from
the deep wetting would tend to be uniform across the buildin g area and the
settlement potential of the fill section should control the design.
2) The thickened sections of lhe slab for support of concentrated loads should have a
minimum width of 20 inches.
3) The perimeter turn-down section of the slab 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. If a frost-
protected foundation is used, the perimeter turn-down section should have at least
1a ;-^1"^. ^f .^;1r u rrrvtlwù vI JvlI vuvul.
4) 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 f'oundatiou walls, where provided, should be heavily reinforced top and
bottom to span local anomalies such as by assuming an unsupportcd lcngth of at
least 14 feet. Foundation walls acting as retaining structures, if any, should also
be designed to resist lateral earth pressures as discussed in the "Foundation and
Retaining Walls" section of this report.
5) The root zone and any loose or disturbed soils should be removed. Additional
structural fill placed below the slab should be compacted to at least 98% of the
Kumar & Associates, lnc. @ Project No. 21.7.480
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maximum standard Proctor density within 2 percentage points of the optirnum
moisture content.
A representative of the geotechnical engineer should evaluate the compaction of
the fill materials and observe all footing excavations prior to concrete placement
to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures (if any) 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 backhll
consisting of the on-site soils. 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 laferal 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.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffrc, 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. Backhll 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 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 backfìll.
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 eafth 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 0.3 5. Passive pressure of compacted backf,rll 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
6)
Kumar & Associates, lnc. @ Project No,21-7-480
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strerrgth. Suitable factors of safety should bc inclucled in the design to limit the strain which will
occur at the ultitttate slrertglh, particularly tn the case ot'prrssivc rcsistancc. Fill placocl against
the sides of the footings to resist lateral loads should bc compactcd to at lcast 959lo of the
maximum standard Proctor density at a moisture contenl 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 and can
lesult i¡t sorne posl-construction settlement. To reduce the effects of some differential
movement, nonstructural floor slabs should be separated from buildings 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 relatively
well-graded sand and gravel, such as road base, should be placed beneath slabs as subgrade
support. This material should consist of minus Z-inch aggregate with at least 50% retained on
the No. 4 sieve and less than I2o/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. Rcquircd fill can consist of the on-
site predominantly granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
It is our understanding the finished floor elevation at the lowest level is at or above f.he
surrounding grade. Therefore, a foundation drain system is not required. 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 and wall drain system.
If the finished floor elevation of the proposed structure 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.
SITE GRADING
Extensive grailing was performed as part of the existing Villas South dcvelopment. Additional
placement and compacfion of the debris fan soils could be needed to elevate the site to design
Kumar & Associates, ln6. @ Project No.21.7.480
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grades and reduce the risk of excessive differential settlements and building distress. In addition,
the water and sewer pipe joints should be mechanically restrained to reduce the risk ofjoint
separation in the event of excessive differential spttlement. Additional structural fill placed
below foundation bearing level should be compacted to at least 98% of the maximum standard
Proctor density within 2o/o of optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing any vegetation and organic soils and compacting to at
leastg5o/o of the maximum standard Proctor density atnear optimum moisture content. The fill
should be benched into slopes that exceed 20Yo grade.
Permanent unretained cut and f,rll slopes should be graded at2horizontal to I vertical or flatter
and protected against erosion by revegetation or other means. This office should review site
grading plans for the project prior to construction.
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 help
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 building structural slab foundation excavations 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
nonstructural slab areas and to at least 90Yo of Lhe 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
3 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 at least 5 feet beyond the
foundation and preferably into a subsurface solid drainpipe.
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 inigation.
Kumar & Associates, lnc. @ Project No. 21-7-480
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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 recomrnendations subrnitted in this reporl are based upon thc data obtaincd
from the exploratory boring drilled at the location indicated on Figure 1, the proposed type of
construction zurd 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 ooncerned 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 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 pro"iect 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, lnc.
\ . lll)ffi
David A. Noteboom, Staff Engineer
Reviewed by:
Steven L. Pawlak, P.E
DNlkac
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Kumar & Associates, lnc, r'Project No. 21"7-480
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APPROXIMATE SCALE-FEET
Fig. 1LOCATION OF EXPLORATORY BORING21 -7 -480 Kumar & Associates
BORING 1
EL. 5980'
LE
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R00T 70NF; SANDY T0 VFRY SANIìY, GRAVELLY SILT, FILL,
FIRM, SLIGHTLY MOIST, BROWN.
32/ 12
WC=6.1
DD=l l9
-200=57
FILL: SANDY TO VERY SANDY SILT WITH GRAVFI, VFRY STIFF
TO DENSE, SLIGHTLY MOIST, GRAYISH TAN TO BROWN.
5 41/6,50/4
SAND AND SILT (SM-ML); SLIGHTLY GRAVELLY, MEDIUM
DENSE/STIFF, SL|GHTLY MO|ST, BROWN. ALLUVTAL FAN DEpoStT
30/ 12
WC=6.5
DD=1 12
-200=41
GRAVEL AND SAND (GM-SM); SILTY, WITH SILTY LAYERS,
MEDIUM DENSE, SLIGHTLY MOIST, BROWN, ALLUVIAL FAN
DEPOSITS.
GRAVEL (Cp-CU); SANDY T0 VERY SANDY, SLtcHTLy StLTy,
COBBLES AND POSSIBLE BOULDERS, DTNSE, MOIST, LIGHT
BROWN AND GRAY MIX. ROUNDED ROCKS, RIVER ALLUVIUM.
10
35/ 12
!
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DRIVE SAMPLE, z-INCH I.D. CALIFORNIA LINER SAMPLE.
15
13/12
WC=5.8
DD=1 10
-200=42
DR|VE SAMPLE, 1 3/8-tNCH t.D. SpLtT Sp00N STANDARD
PENETRATION TEST.
117leDR|VE SAMPLE BLOW COUNT. INDICATES THAT 31 BL0WS 0F-,1 '. A 14o-POUND HAMMER FALLING 30 INcHES wERE REQUIRED
TO DRIVI THE SAMPLER 12 INCHES,
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20 38/6, 50/6 NOTES
THE EXPLORATORY BORING WAS DRILLED ON JUNE 11,2021
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER
25
2, THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIDED.
44/12
WC=4.6
+4=26
-20Q=26
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5. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.
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4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDTRED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
34 / 1'2
5 THI LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BT
GRADUAL.
<Ã 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE
TIME OF DRILLING.
40
7, LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (PCf) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (MrV O OSIS);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1i40).
50/3
21 -7 -480 Kumar & Associates LOG OF EXPLORATORY BORING Fî9. 2
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150 .300 r .600 f.18 2.35 1.75.425 2.O
PARTICLES IN MILLIMETERS
3a.f 76.2
DIAM
CLAY TO SILT COBBLES
GRAVEL 26 % SAND
LIQUID LIMIT
SAMPLE 0F: Silty Grovelly Sond
4A%
PLASTICITY INDEX
SILT AND CLAY 26 %
FROM:Boringl@-25'
Thes6 lssl resulls opply only lo ihe
somples which were l6sl€d, Th€
losllng r€porl sholl nol be r€produc€d,
oxcapl ln full, wllhoul lh€ wrlll€n
opprovol of Kumor & Associolos, lnc.
Slev€ qnolysls l€sllng ls performed lnqccordoncs wlth ASTM D6913, ASÎM D7928,
ASTM C136 ond/or ASTM Dl140.
SIEVE ÂNALYSISHYDROMETER ANALYSIS
CLEAR SQUÀRE OPENINGSTIME READINOS
24 HRS 7 HRS ¡loô
U.S. STANDARD SERIES
45ô 4¡O 450 4t6 ¡10 4A
/':
i:l:.1
GRAVELSAND
FINE MEDIUM COARSE FINE COARSE
21 -7 -480 Kumar & Associates GRADATION TTST RESULTS Fis.3
rcrf iji,.rih:'ffill:Ëtrn':'È'ü*'*TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-480SOIL T'PEGravelly Sandl- Sih (Fill)Very Silty Gra.'ell:¿ Sand(^Fill)Sand and Silt u'ith Grave-(psflUNCONFINEDCOMPRESSIVESTRENGTH571442Silty Gravelly Sand264826119t121106.16.55.84.6at,'L /27v;{1251LIQUID LIMITGRADATIONSAMPLE LOCABORINGDEPTHPLASTICINDEXPERCENTPASSING I,IO.200 stEvENATURALDRYDENSITYNATURALMOISÏURECONTENTSAND(f/"1GRAVEL(%)