HomeMy WebLinkAboutSubsoil Studyrc iiffih:'tr:il:tin':'Êü'*^
An Employcc Owncd Compony
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 282, TRONBRTDGE
BLUE HERON VISTA
GARFTELD COUNTY, COLORADO
PROJECT NO. 21-7-594
AUGUST 9,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY....
BACKGROLIND INFORMATION
PROPOSED CONSTRUCTION
SITE CONDITIONS....
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOLINDATION BEARING CONDITIONS
DESIGN RECOMM EN DATIONS
FOUNDATIONS
FOUNDATION AND RETAINING V/ALLS
NONSTRUCTURAL FLOOR SLABS
LINDERDRAIN SYSTEM
srTE GRADING.................
SURIACtr DRAINACE.....
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No. 21-7-594
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot2S2,Ironbridge, Blue Heron 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 July 9, 2021.
An exploratory boring was drilled 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, expansion-compression potential 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 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 1 1 5-6, report dated September 14,2005, and performed observation and testing services
during the infrastructure construction, Job No. 106 0361, between April 2006 and April 2007.
The information provided in these previous reports has been considered in the current study of
Lot282.
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 and
fill depths up to 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 recornmendations contained in this report.
Kumar & Associates, lnc. @ Project No. 2l-7-594
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The lot is located in the north
paft of the Villas North Parccl. Ths natural tcrrain prior to dcvcloprncnt in 2006 slopcd clown l.o
the east at about 5Yo grade. The subdivision area was elevated by filling on the order of 14 feet
above the original ground surface to create a relatively flat and gently sloping building site off
Blue Heron Vista. Vegetation consists of sparse grass and weeds with scattered sage brush.
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with sinkholes
and localized ground subsidence in the lower Roaring Fork River valley. 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. Irregular surface features were not observed in the Villas North parcel that could
indicate an unusual risk of future ground subsidence. Variable depths of the debris fan soils
were locally encountered by the previous September 14,2005 geotechnical study which indicates
there could have been localized subsidcncc ofthc rivcr gravcl dcposits. Thc currcnt subsurfacc
exploration performed in the area of the proposed residence on Lot 282 did not encounter voids.
In our opinion, the risk of future ground subsidence on Lot 282 throughout the service life of the
proposerl resiclence is low ancl similar to other areas of the lower Roaring Fork River valley
where there have not been indications of ground subsidence.
FIELD EXPLORATION
The field exploration for the project was conducted on July 21,202I. One exploratory boring
was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring
was advanced wittr 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, Inc.
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 desøibed by ASTM Method D-1586.
The penetration resistance values aÍe 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.
Kunrar & Associates, lnc, o Project No.21-7-594
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SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils encountered, below about a 6-inch thick root zone, consist of mixed, sand, silt and
gravel fill to a depth of 14 feet underlain by stiff, sandy silt and clay soils (alluvial fan deposit) to
a depth of l8 feet where dense, silty sandy gravel and cobble soils (river gravel deposit) were
encountered down to the maximum explored depth of 22 feet. Drilling in the dense granular
soils with auger equipment was difficult due to the cobbles and possible boulders and drilling
refusal was encountered at a depth of 22 feet in the deposit.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on a relatively undisturbed drive sample of the sandy silt and
clay, presented on Figure 3, indicate low compressibility under existing low moisture conditions
and light loading and a minor expansion potential when wetted under constant light surcharge.
The laboratory testing is summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were
typically slightly moist.
FOUNDATION BEARING CONDITIONS
The upper 14 feef of soils encountered in the boring consist of fill placed mainly in 2006 as part
of the subdivision development. The field penetration tests and laboratory tests performed for
the study, and review of the field density tests performed during the fill construction indicate the
structural fiIl 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 fill. It is our experience that the expansion potential of the
tested sample can be ignored in the foundation design. The amount of settlement will depend on
the thickness of the compressible soils due to potential collapse when wetted, and the future
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 typically less
than the alluvial fan deposit. Proper grading, drainage and compaction as presented 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 deposit could be used to
reduce the building settlement risk.
Kumar & Associates, lnc. @ Project No. 21-7-594
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DRSIGN RIT,COMMN,NDATIONS
FOUNDATIONS
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 or post-tensioned slab foundation bearing on about t4 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 design and construction criteria presented below should be observed for a heavily reinforced
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 should be designed for a wetted
distance of 10 feet or at least half of the slab width, whichever is greater.
Settlement of foundation is estimated to be about I to 2 inches based on the long-
term compressibility of the fill. Additional settlement of about 1 inch is estimated
if the underlying debris fan soils were to become wet. Settlement from the deep
wetting would tend to be uniform across the building area and the settlement
potential of the fill section should control the design.
2) The thickened sections of the slab for support of concentrated loads should have a
minimum width of 20 inches.
3) The perimeter tum-down section of the slab should be provided with adequate soil
cover above their bearing elevation for frost protection. Placement of foundations
aL leasl.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.
4) The foundation should be constructed in a "box-like" configuration rather than
wilh irrcgular exlensions which oan settle diflèrentially 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 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 & Assoclates, lnc, o Projêct N0.21-7-594
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maximum standard Proctor density within 2 percentage points of the optimum
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.
FOLINDATION AND RETAINING V/ALLS
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 backfill
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 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.
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 90o/o of the maxlmum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95%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 comectly, 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 backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 325 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
Kumar & Associates, lnc. o Project No. 21-7-594
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strength. Suitable factors of safety should he incluclecl in the clesign to limit the strain which will
occur at thc ultimatc strcngth, 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% of the
maximum standard Proctor density at a moisfure 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 and can
result in some post-construction settlement. To reduce the effects of some differential
movement, nonstructural floor slabs should be separated from buildings to allow unrestrained
vertical tttovettent. 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 2-inch aggregate with at least 50% retained on
the No. 4 sieve and less than T2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be cornpacted to at least 95Yo 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.
LTNDERDRAIN SYSTEM
It is our understanding the finished floor elevation at the lowest level is at or above the
surrouncling gracle. Therefore, a foundation drain system is not required. It has been our
experience in the areathatlocal perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can crcatc a pcrched
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 grading was performed as part of the existing Villas North development. Additional
placement and compaction of structural fill soils could be needed to elevate the site to design
Kumar & Assoclates, lnc. o Project No, 21.7.594
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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 settlement. Additional structural fill placed
below foundation bearing level should be compacted to at least 98o/o 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 aîy vegetation and organic soils and compacting to at
least 95Yo of the maximum standard Proctor density at near optimum moisture content. The fill
should be benched into slopes that exceed 20o/o grade.
Permanent unretained cut and fill slopes should be graded at2horizontal to 1 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 95o/o of the maximum standard Proctor density in pavement and
nonstructural 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
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 frorn 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. @ Project No.21-7-594
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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. V/e make no warranty either express or implied.
The conclusions and recontmendations 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. 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 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 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 verift 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,
Kumnr & Associ¡tcs, fnc.
yt"rlt2-T. ?¿*caa.-
James H. Parsons, P.E.
Reviewed by:
Steven L. Paw
JHPikac
Kumor & Associotos, lnc.'Project No. 21-7-594
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BENCHMARK:
MANHOLE COVER
+'L. 100,, ASSUMED
COMMON
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LWI ¿OZo
BORING 1
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oLOT 281
10 0 0
APPROXIMATE SCALE-FEET
21 -7 -594 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
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BORING 1
E1.99.96'LEGEND
0 N
TOPSOIL; SAND AND SILT, CLAYEY, SCATTERED GRAVEL,
ORGANICS, FIRM, SLIGHTLY llOIST TO lt,iÔIST, BRÔWN. ROOT
ZONE.
22/6,50/4.5
FILL: SAND AND SILT, GRAVELLY, CLAYEY, HARD/DENSE,
SLIGHTLY MOIST, GRAY AND BROWN.
-5
31/6,50/5
WC=9.9
DD= 1 30.0
-200=77
CLAY AND SILT (CL-ML); SANDY, STIFF, SLIGHTLY MOIST,
LIGHT BROWN.
P. 1l
l. ?¿s.l(:::.)
IA
GRAVET
DENSE,
(çU); SITTY, SANDY, COBBLES, POSSIBLE BOULDERS,
SLIGHTLY MOIST, BROWN.
_ 10
5ols.s !
i
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
FtJ
L¡JtL
I-t--
o_LIâ
DRTVE SAMPLE, 1 3/8-|NCH t.D. SpLtT Sp00N STANDARD
PENETRAÏION TEST.
1 5 22/6 DRIVE SAMPLE BLOW COUNT. INDICATES IHAT 22 BLOWS OF
A 14o-POUND HAMMER FALLING 50 INCHES WERE REQUIRTD
TO DRIVE THE SAMPLER 6 INCHES.13/ 12
WC=8.9
DD=112 I PRACTICAL AUGER REFUSAL.
_20 NOTES52/6
1 THE EXPLORATORY BORING WAS DRILLED ON JULY 21, 2021
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER
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2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURIS SHOWN ON THE
SITE PLAN PROVIDED.
I 3. THE ELEVATION OF THE EXPLORATORY BORING WAS MEASURED
BY INSTRUMENT LEVEL AND REFERS TO THE BENCHMARK ON
Ftc. 1.
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
5. THI LINES BETWEEN MATERIALS SHOWN ON THE IXPLORATORY
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) (ASrU O ZZr0);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D
68/ 12
WC=1I.0
DD= 1 26
-200=62
1140)
21 -7 -594 Kumar & Associates LOG OF EXPLORATORY BORING Fis. 2
I
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SAMPLE OF: Sondy Sill ond Cloy
FROM: Boring 2 @ 15'
WC = 8.9 %, DD = 112 pcf
to the
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21 -7 -594 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 3
I Crt iiçlffiifr:ffiri'riå*'"TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-594SOIL TYPESandy Silt with GravelVery Sandy Silt withGravelSandy Silt and Clay(psf)UNCONFINEDCOMPRESSIVESTRENGTH62ALIQUID LIMITGRADATIONPLASTICINDEXPERCENÏPASSING NO.200 stEvENATURALDRYDENSITYSANDl:/"1GRAVELti/"|1309.977126I12('/,1NATURALMOISTURECONTENT11.08.957k51ISAMPLE LOCATIONDEPTHBORING