HomeMy WebLinkAboutSubsoil Studyl$rtiäffii'[,ff'}"fËtr¡,'.'Ê;'*.*
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
RECEIVED
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 2550 TRONBRIDGE
RIVER VISTA
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-787
JANUARY 29,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY....
tsACKGROUND INFORMATION
PROPOSED CONSTRUCTION
SITE CONDITIONS
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION.
SUBSURFACE CONDITIONS
FOTINDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ..
FOUNDATIONS...
FOUNDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS ..
TINDERDRAIN SYSTEM .....................
SITE GRADING..........
SURFACE DRAINAGE...............
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1
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1
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. @ Project No. 20.7.787
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lof 255,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 December 3I,2020.
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, 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.
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 information provided in these previous reports has been considered in the current study of
Lot255.
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 I to 3 feet. We assume relatively light foundation loadings, typical of the
proposed type of construction.
Kumar & Associates, lnc. o Project No. 20-7-787
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If building loaditrgs, looation or gracling plans change significantly from those described above,
we shoulcl be notitied to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The lot is located in the
southwest part of the Villas South Parcel. The natural terrain prior to development in 2006
sloped down to the east at about 5%o grade. The subdivision area was elevated by filling on the
order of 15 feet above the original ground surface to create a relatively Ilal builcling site off River
Vista. Vegetation consists of 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 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 and localized variable depths of the debris fan soils
were generally not encountered by the previous September 14,2005 geotechnical study. The
current subsurface exploration performed in the area of the proposed residence on Lot 255 did
not encounter voids. In our opinion, the risk of future ground subsidence on Lot 255 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.
FIELD EXPLORATION
The field exploration for the project was conducted on January 12,2021. One exploratory
boring was drilled at the location shown on Figure I 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, Inc.
Sanrples uf the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows lrom a 140 pound hammer falling 30
inchcs. This tcst is similar to the standard penetration test described by ASTM Method D-l586.
Kumar & Associates, lnc. o Project No. 20.7-787
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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. The
subsoils consist of about lzfoot of topsoil overlying compacted fill soils to 18 feet deep
overlying medium dense/stift sand and silt soils (alluvial fan deposits) underlain by dense, sandy
gravel with cobbles (river gravel alluvium) at a depth of about 32 feef to the maximum drilled
depth of 36 feet. The fill materials were mainly placed in2006 and consist of relatively dense,
mixed silt, sand and gravel.
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 sand and silt soil,
presented on Figure 3, indicate low to moderate compressibility under conditions of loading and
wetting. 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 with depth.
FOUNDATION BEARING CONDITIONS
The upper 18 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 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 fill. 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 hll as encountered will also
have some potential for long-term settlement but should be significantly less than the alluvial fan
Kumar & Associates, lnc. @ Project No. 20-7-787
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deposits. Ploper grading, drainage artd uortrpautiort as presentetl in the Surþce Druinuge seu[iorr
will help to keep the subsoils clry and reduce the settlement risks. A heavily reinforced structural
slab or post-tensioned slab foundation designed for significant differential settlements is
recommenclecl for the builcling sr.rpport. 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.
DESIGN RECOMMENDATIONS
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 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 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 the foundation is cstimatcd to bc about I to lYz inches based on the
long-term compressibility of the f,rll. Additiona! settlerrrent of about I to 2 inches
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 f,rll 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 perirneter tun¡tlown section of the slab should be provitletl with atlequate soil
cover above their bearing elevation for frost protection. Placement of foundations
at least 36 inchcs bclow cxtcrior grade is typically used in this area. If a frost-
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4)
protected foundation is used, the perimeter turn-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 buildingarea.
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.
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
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 frll materials and observe all footing excavations prior to concrete placement
to evaluate bearing conditions.
FOLINDATION 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 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 backf,rll 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.
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Kumar & Associates, lnc. @ Project No. 20-7-787
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Backfìll slrould lre plauetl in unil'onn lills ancl compacl.ed t<l at least 90% ol thc maximum
stanclard Proctor clensity at a moisture content near optimum. Backfill placed in pavement and
wallovay areas should be compacted to at least 95o/o of tl'rc rnaxirnurn 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 pressule on the wall. Sone settlenent of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. 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
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 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 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 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 baseo should be placed beneath slabs as subgrade
support. This material should çonsist of minus Z-inch aggregate with at least 50% retained on
the No. 4 sieve and less than l2Yo passing the No. 200 sieve.
Kumar & Associates, lnc. @ Project No.20.7.787
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All fill materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site 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 the
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 grading was performed as part of the existing Villas South development. Additional
placement and compaction of the debris fan soils could be needed to elevate the site to design
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 hll placed
below foundation bearing level should be compacted to at least 98% of the maximum standard
Proctor density within 2%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
leastg5%o of the maximum standard Proctor density atnear optimum moisture content. The fill
should be benched into slopes that exceed 20%o grade.
Permanent unretained cut and f,rll 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.
Kumar & Associates, lnc. o Project No.20-7-787
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SURFACE DRAINAGE
Precautions to ptevent wetting of thc bcaring soils, such as proper backfill construction, positive
backtill slopes, restricting landscape irrigation and use of roof gutters, need to be taken to help
limit settlement and building distress. 'l'he tbllowing drainage precautions should be observed
during construction and maintained at all times after the residence has been completed:
1) Itrundation of the builcling structural slab foundation excavations should be
avoided during construction.
2) Exterior backflrll 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 90% of the maximum standard Proctor
density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 6 inches in the first 5 feet in nnpavecl 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 healy irrigation should be located at least 10
feet from founclation walls. Consicleration shoulcl be given to use of xeriscape to
rcducc thc potcntial for wetting of soils below the building caused by inigation.
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 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 shnuld he consultecl. Our findings inclnclç interpolation and extrapolation of the
Kumar & Associates, lnc. @ Project No. 20-7-787
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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 project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recolnmendations, 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 &, fnc.
J H.E.I.
Reviewed by:
Steven L.
JHPlkac
CI,l52n
Kumar & Associates, lnc. o Project No, 20-7-787
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APPROXIMATE SCALE_FEET
255+o,o'?
BORING 1
20-7 -787 Kumar & Associates LOCATION OF EXPLORATORY BORING 1Fig.
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BORING 1
ELEV. 5981
LEGEND
5985
TOPSO|L: CLAYEY SILTY SAND, GRAVELLY, R00TS AND ORGANICS,
FIRM, MOIST, BROWN.
FILL: MIXED SAND, GRAVEL AND SILT, PROBABLE COBBIES,
DENSE, SLIGHTLY MOIST, MIXED BROWN.
5980
50/5
SAND AND S|LT (SM-ML), MEDTUM DENSE/Srrrr, SI|GHTLV
MOIST TO MOIST WITH DEPTH, BROWN.
50/5
WC=4.5
-200=66
GRAVEL (GM-GP): SANDY, SILTY, COBBLES, VERY DENSE,
SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
5975 !
I
DRIVI SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
50/5
DRTVE SAMPLE, 1 3/8-|NCH t.D. SPTLT SP00N STANDARD
PENETRATION TEST.
5970
51/12
WC=7.9
DD=1 1 1
-200=57 50/5 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 50 BLOWS OF
A 14o-POUND HAMMER FALLING 50 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 5 INCHES.
t-
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LL
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F
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---> DEPTH AT WHICH BORING CAVED FOLLOWING
DRILLING.
5965
28/12 NOTES
THE EXPLORATORY BORING WAS DRILLED ON JANUARY 12,2021
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
11/12
WC=6.1
DD=83
-200=30
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIDED.
5960 3. THE ELEVATION OF THI EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.
5955
11/12
Y,IC=12.7
DD= 1 08
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.
5950
18/ 12 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) (lSrV O ZZ1O);
-200 = PERCENTAGE PASslNc N0. 200 SIEVE (ASTM D 1140).
5945
50/3
WC=2.0
-200=26
20-7 -787 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
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SAMPLE OF: Silty Sond
FROM:Boringl@25'
WC = 1 2.7 %, DD = 108 pcf
-2OO = 26 %
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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20-7 -787 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fîg. 3
I (+rt iiffil,*åi'ffË::;i iTiå' *' "TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESandy Gravelly Silt (Fill)Sandy Gravelly Silt (Fill)Silty SandSilty SandSilty Sandy Gravel(psflUNCONFINEDCOMPRESSIVESTRENGTH66(%lPLASTICINDEXATTERBERG LIMITS(o/"1LIQUID LIMITPERCENTPASSING NO,200 stEvE573026(f/"1SANDGRADATION(%)GRAVELlocf)NATURALDRYDENSITY11183108(o/olNATURALMOISTURECONTENT4.57.96.t12.72.0410202535tft)DEPTHSAMPLE LOCATIONBORING1No.20-7-787