HomeMy WebLinkAboutSubsoil Studyl$rtiäffi1fi'ff*"fÉ:i¡''lsü'**
An Employcc Owncd Compony
5020 Counfy Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
emai I : 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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GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 2S5, rRONBRrDGE
BLUE HERON VISTA
GARFTELD COUNTY, COLORADO
PROJECT NO.2t-7-219
APRrL 13,202t
PREPARED FOR:
' scrB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD' SUrTE 52018
ASPEN, COLORADO 81611
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
BACKGROIIND INFORMATION ..
PROPOSTD CONSTRUCTION
SITE CONDITIONS...
SUBSIDENCE POTENTIAL...
FIELD EXPLORATION...
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ...............
FOLINDATIONS ....
FOUNDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS
I-INDERDRAIN SYSTEM .............
SITE GRADING......
SURFACE DRAINAGE
LIMITATIONS
FIGURE 1 . LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 _ SWELL.CONSOLIDATION TEST RESULTS
FIGURE 4 _ GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Assoclates, lnc. o Project No.21-7-219
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot2S5,Ironbridge, Blue Heron Vista, Garfield County, Colorado. The project site is shown on
Figure 1. The pulpose 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 February 18,2021.
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 f,reld 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 15-6, 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 cunent study of
Lot 285.
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 3 to 4 feet. V/e assume relatively light foundation loadings, typical of the
proposed type of construction.
Kumar & Associates, lnc. @ Project No. 21-7-219
a
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The lot is located in the north
part of the Villas North Parcel. The natural terrain prior to development in 2006 sloped down to
the east at about 5o/o grade. The subdivision area was elevated by filling on the order of 18 feet
above the original ground surface to create a relatively flat and gently sloping building site off
Blue Heron Vista. A2-lier MSE retaining wall up to around 10 to 12 feethigh supports the
northeast perimeter of the f,rll section as shown on Figure 1. 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
andlocalized ground subsidcncc in thc Roaring Fork 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 subsidence ofthe river gravel deposits. The current subsurface exploration
performed in the area of the proposed residence on Lot 285 did not encounter voids. In our
opinion, the risk of future ground subsidence on Lot 285 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 fielcl exploration for the project was conducted on March 22,202I. One exploratory boring
was drilled at the location shown on Figure I to evaluate the subsurfaoe conditions. The boring
was advanced with 4-inch diameter continuous flight augers powered by a truck-mountcd CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
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Samples of the subsoils were taken with I% 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. The
subsoils encountered, consist of compacted fîll soils to 18 feet deep overlying medium
dense/stiff, silt and sand with gravel soils (alluvial fan deposits) underlain by dense, silty sandy
gravel with cobbles at a depth of about 2I/zfeetto the maximum drilled depth of 26 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 gradation analyses. Results of swell-consolidation testing performed on
a relatively undisturbed sample of the sandy silt fill soil, shown on Figure 3, indicate low to
moderate compressibility under conditions of loading and wetting. Results of gradation analyses
performed on a small diameter drive sample (minus \%-inch fraction) of the natural sand and silt
soils are shown on Figure 4. 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.
FOUNDATION BEARING CONDITIONS
The upper 18 feet of soils encountered in the boring consist of fill placed mainly in 2006 as part
of the subdivision development. The flreld 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 minimurn 95% of standard
Proctor density. The penetration test and sample taken at a depth of 4 feet indicate a lower
Kumar & Associates, lnc, @ Project No.21-7-219
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compaction level but appears to be a localized condition. Alluvial fan soils which tend to
collapse (settle under constant load) when wetted were encor¡nterecl below the fill. The amount
of settlement will depend on the thickness of the compressible soils chle 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 signif,rcantly less thau the alluvial fan deposits. 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 altemative, a deep foundation that extends down into the underlying dense, river
gravel could be used to reduce the building settlement risk.
DESIGN RECOMMENDATIONS
FOTINDATIONS
Considering the subsurface conditions encountered in the exploiatory 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 18 feet of the existing
compacted structural f,rll. The structural engineer should consider the close proximity of the
MSE wall to the north side of the residence in the foundation design to not adversely impact wall
stability and for potential differential settlement. 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-tetxiortetl slab plaoed 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 1 to 2 inches based on thc long-
term compressibility of the fill. Additional settlement of about I inch is estimated
if the underlying debris fän soils were to become wet. Settlement from the deep
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2)
wetting would tend to be uniform across the buildingarea and the settlement
potential of the fill section should control the design.
The thickened sections of the slab for support of concentrated loads should have a
minimum width of 20 inches.
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
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 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 fill materials and observe all footing excavations prior to concrete placement
to evaluate bearing conditions.
3)
4)
s)
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures (if any) which are lateralTy 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 backf,rll
consisting of the on-site soils. Cantilevered retaining structures which are separate from the
residence 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 40 pcf for backf,rll consisting of the on-site soils.
6)
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All foundation and retaining structures shuuld be designetl fur appropriale hytlrostalic antl
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressuies 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 backf,rll 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 bc cxpcctcd, even if the material is placed conectly, 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, particulally in the case ulpassive resistanuc. Fill placud 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
Compactcd structural fill can be used to support lightly loadcd slab-on-grade constructiorr
separate from the building foundation. The fill soils can be compressible when wetted and can
result in some post-construction sctllement. 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. Thc rcquircmcnts for joint spacing and slab reinforcement should be established by the
Kumar & Associates, lnc. o Project No.21.7.219
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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 lTYo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo 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.
I-INDERDRAIN 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 groundwaier 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 anunderdrain 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
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 98% of the maximum standard
Proctor density within 2Yo 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
least 95Yo of the maximum standard Proctor density at near optimum moisture content. The fill
should be benched into slopes that exceed 20Yo grade.
Kumar & Associates, lnc. o Project No.21-7-219
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Permanent unretained cut and filI slopes should be graded af.2horizontal 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 cornpleted:
1) lnundation 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 f,rrst 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 irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
prirrciples ancl 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 drillcd at the location indicated on Figure 1, the proposed type of
Kumar & Associates, lnc. @ Project No.21-7-219
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construction and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions 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 reporl, we should be notified so
that re-evaluation of the recoÍrmendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to veriff that the reconlmendations
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,
Steven L.
Reviewed by:
r I
Daniel E. Hardin, P.E
SLP/kac
Kumar & Associates, lnc. ù Project No. 21-7-219
EXISTING 2 TIER
MSE WALL
LOT 284
LOT 286
10 0 20
APPROXIMATE SCALE-FEET
LOT 285
PROPERTY
!INE
o
O
BORING 1
21 -7 -219 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
e
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LEGEND
BORING 1
EL. 5558'
0
FILL; MIXED SANDY SILT AND SILTY SAND WITH VARIABLE
GRAVEL CONTENT, MEDIUM DENSE/STIFF TO VERY STIFF,
SLIGHTLY MOIST, MIXED BROWN.
22/12 SAND AND SILT (SM-ML), GRAVELLY, MEDIUM
DENSE/ST|FF, SLTGHTLY Mo|ST, BRoWN.
q
1o/ 12
WC=6.2
DD=97
-200=78
GRAVEL (GM);
MOIST, BROWN,
SILTY, SANDY, COBBLES, DENSE, SLIGHTLY
ROUND ROCK.
DRIVI SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
1e/12
10
31 /12
WC=6.8
-200=44
¡
DRIVE SAMPLE, 1 3/8-|NCH t.D. SPLTT SP00N STANDARD
PENETRATION TEST.
"" r." DRIVE SAMPLE BLOW COUNT. INDICATES IHI'I 22 BLOWS 0FLLl tL A 14o-pouND HAMMER FALuNc J0 rNcHEs wtRE REQUTRED
TO DRIVE THE SAMPLER 12 INCHES.
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16/ 12
NOTES
THE EXPLORATORY BORING WAS DRILLED ON MARCH 22, 2021
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIDED.
20
12/12
V,lC=4.7
+4=15
-200=49
3. ÏHT ELEVAÏION OF THE EXPLORAÏORY BORING WAS OBÏAINED
BY INTERPOTATION BETWEEN CONTOURS ON THE SIÏE PLAN
PROVIDED.
25
4, THE EXPLORAÏORY BORING LOCATION AND ELEVATION SHOULD BE
CONSIDERED ACCURATE ONLY TO THE DEGRET IMPLITD BY THE
METHOD USED,
67 /12 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE
GRADUAL.
30
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 DENSITY (PCf) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 6913);
-2OO = PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1140).
LOG OF EXPLORATORY BORING fig. 221 -7 -219 Kumar & Associates
SAMPLE 0F: Flll: Sondy Silt
FROM:Boringl@4'
WC = 6.2 %, DD = 97 pcf
-2OO = 78 %
of
I
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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1.0 APPLIED PRESSURE - KSF 10 r00
21 -7 -219 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 3
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DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 15 % SAND
LIQUID LIMIT
SAMPLE 0F: Sondy Sill with Grqvel
36%
PLASTICITY INDEX
SILT AND CLAY 49 %
FROM:Boringlre^20'
Th6sê l€sl rosulls opply only lo lh€
sompl€s wh¡ch were lesled. The
l€sling r6porl sholl nol b6 reproduced,
oxcepl ln lul¡, wllhoul lhe wrlllen
opprovol of Kumor & Associol€s, lnc,
Sl€v6 onolysls lesllng ls porformed ln
qccordqncs wlth ASTM 06913, ASTM 07928,
ASTM C'156 ond/or ASTM Dl140,
HYDROMETER ANALYSIS SIEVE ANALYSIS
lIME REÄDINGS
2,1 HRS 7 HRs
U.S. STANDARÞ SERIES CLEAR SQUARE OPENINGS
./tn t/t" I t/tn
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SAND GRAVEL
FIN E MEDIUM COARSE FINE COARSE
21 -7 -219 Kumar & Associates GRADATION TEST RISULTS Fig. 4
lcrtåiçi[#ffi,vÊ;;*.*TABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No.21-7-2'19Lot 285SOIL TYPESandy Silt (Fill)Sand and Silt with Gravel(Fill)Sandy Silt with GravelUNCONFINEDCOMPRESSIVESTRENGTHlosflATÏERBERG LIMITSPLASTICINDEX(o/"1LISUID LIMIT(o/"1PERCENTPASSING NO.200 stEvÊ784449GRADATIONSAND(f/"136GRAVEL("/")15NATURALDRYDENS]TYlocfl97NATURALMOISTURECONTEMT("/"\6.26.84.7SAMPLE LOCATIONDEPTH(ftt41020BORING1