HomeMy WebLinkAboutSubsoil StudyI Crt ii:l''ihi'.ii*:tif ':'Ê;
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An Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.coln
www.kumarusa.corn
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 281,IRONBRIDGE
BLUE HERON VISTA
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-s93
AUGUST 9,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
TABLE OF CONTI,NTS
PURPOSE AND SCOPE OF STIJDY
BACKGROLIND INFORMATION ...
PROPOSED CONSTRUCTION
SITE CONDITIONS...
SUBSIDENCE POTENTIAL...
FIELD EXPLORATION...
SUBSURFACE CONDITIONS .........
FOTINDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ............
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS ..
UNDERDRAIN SYSTEM .....................
SITE GRADING.......
SURFACE DRAINAGE...............
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. 21-7-593
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lof 2Sl,Ironbridge, Blue Heron Vista, Garfield County, Colorado. The project site is shown on
Figure 1. The putpose 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 9,202I.
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 classif,rcation, 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.
105 115-6, 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
Lot 281.
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 about2 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.
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SIT[, CONDITIONS
Thc subjcct site was vacant at the time of our field exploration. The lot is located in the nofth
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 hlling on the order of 10 feet
above the original ground surface to create a relatively flat and gently sloping building site off
Blue Heron Vista. Vcgctation consists of sparse grass ancl weecls with scattered sage bnrsh.
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 grouncl subsiclence. Variable clepths of the debris fan soils
were locally encountered by the previous September 14,2005 geotechnical study which indicates
there could have been localized subsidence of the river gravel deposits. The current subsurface
exploration performed in the area of the proposed residence on Lot 281 did not encounter voids.
In our opinion, the risk of future ground subsidence on Lot 281 throughout the service life of the
proposed residence is low and 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 20,2021 One exploratory boring
was drilled atthe location shown on Figure 1 to evaluate the subsurface conditions. The boring
was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, 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 14O-pound hammer falling 30
inches. This test is sirnilar to the standard penetration test described by ASTM Method D-1586.
T'he 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 T,og of Exploratory Boring, Figure 2. The samples were returned to our laboratory
lbr review by the project engineer and testing.
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SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figule 2. The
subsoils encountered, consist of compacted fill soils to about 8/zfeet deep overlying stiffto very
stiff, sandy silt and clay soils (alluvial fan deposit) underlain by dense, silty sandy gravel with
cobbles (river gravel deposit) at a depth of about 16 feet to the maximum drilled depth of 2l 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 sample of the sandy silt and clay
soils, 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
typically slightly moist.
FOUNDATION BEARING CONDITIONS
The upper SYzfeet 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 fiIl 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 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.
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountercd in thc cxploratory boring and thc nature of
lhe proposecl construction, we recommencl the builcling be founclecl with a heavily reinforced
stntctural slab or post-tensioned slab foundation bearing on the existing compacted structural fill.
If a deep foundation system is considered t'or building support, we should be contacted f'or
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 I% inches based on the
long-terur contpressibility of the fill. Additional setllerncnt of about 1 inoh 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 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 bel,rw exterior grade is typically used in this alca. If a h'ust-
protected foundation is used, the perimeter turn-down section should have at least
18 inches ofsoil cover.
4) The foundation should be constructed in a "box-like" conf,rguration rather than
with irregular extensions which can settle differentially to the rnain 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 laferal earth pressures as discussed in the "Foundation and
Retaining Walls" section of this report.
5) Thc root zone and any loose or disturbed soils should be removed. Additional
structural fill placed below the slab shoulcl he compacted to at least 98% of the
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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.
FOTINDATION AND RETAINING WALLS
Foundation walls and retaining structures (if any) which arelaterally 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 unifonn lifts and compacted to at least 90Yo of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas.should be compacted to at least 95o/o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the rnaterial 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 recomlnended above assume ultirnate soil
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strength. Suitablc täctors of satbty should be included in the design to limit the strain which will
occur at the ultimate streugth, particularly in the case of passive resistance. Fill plaoecl against
tlre sides of the footings to resist lateral loads should be oompacted to af. leas[ 95Yo ol Lhe
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 base, should be placed beneath slabs as subgrade
support. This material should cotrsist of rttirtus 2-inchaggregate with at least 50% retained on
the No. 4 sieve and less than |2o/o passing the No. 200 sieve.
All f,rll 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 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
surrounding grade. 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 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 ftrr 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
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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 98% of the maximum standard
Proctor density within 2o/o of optimum moisture content. Prior to hll placement, the subgrade
should be carefully prepared by removing any vegetation and organic soils and compacting to at
least95%o of the maximum standard Proctor density atnear optimum moisture content. The fill
should be benched into slopes that exceed 20Yo grade (ifany).
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:
l) 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 909io 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 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.
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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 makc no warranty either express or implicd.
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 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 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. Vy'e 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 verifii 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,
Kunral' & .,{ssocintes" hlc.
/r'rÌrtfÈ.PW
James H. Parsons, P.E.
Reviewed by:
Steven L. Pawlak, P
JHPlkac
Kumar & Associates, lnc.6'Project No. 21.7-593
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LOT 281
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BORING 1
LOT 282
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BENCHMARK:
SANITARY MANHOLE
EL. 100,, ASSUMED
LOT 280
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APPROXIMATE SCALE_FEET
21 -7 -593 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
b
BORING 1
EL. 102.3'
LEGEND
0
FILL: SILT, SANDY TO VERY SANDY, CLAYEY, GRAVELLY, VERY
STIFF TO HARD, SLIGHTLY MOIST TO MOIST, MIXED BROWN,
50/12
WC=7.1
DD= 1 20
-200=7 4
SILT AND CLAY (l'lL-CL); SANDY TO VERY SANDY, SLIGHTLY
CALCAREOUS, STIFF TO VERY STIFF, MOIST, LIGHT BROWN.
5 m
GRAVEL
DENSE,
(cu); surv, SANDY, C0BBLES, PosstBLE BoULDERS,
SLIGHTLY MOIST, GRAY AND ÏAN.
6?/ 1?
34/ 12
WC= 13.4
DD=115
-200=58
DRIVI SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
10
14/12
WC=8.6
DD=1 1 1
-200=77
i DRTVE SAMPLE, 1 5/8-|NCH t.D. SPLTT SP00N STANDARD
PENETRATION TEST.
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q¡712DR|VE SAMPLE BLOW COUNT. INDICATES THAT 50 BLOWS 0F
14o-POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
15
16/12
NOTES
THE EXPLORATORY BORING WAS DRILLED ON JULY 20, 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 41/6,50/4 5. THE ELEVAÏION OF THE EXPLORATORY BORING WAS MEASURED
BY INSTRUMENT LEVEL AND REFERS TO THE BENCHMARK ON
FtG. 1.
25
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
q THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESTNT THE APPROXIMATE BOUNDARIES
BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE
GRADUAL.
6. GROUNDWATER WAS NOT INCOUNTERED 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);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1 r 40).
21 -7 -593 Kumar & Associates LOG OF EXPLORATORY BORING Fis. 2
IF
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SAMPLE OF: Sondy Sill ond Cloy
FROM: Boring 't ó 1O'
WC = 8.6 %, DD = 111 pcf
-2OO = 77 %
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -593 Kumar & Associates SWELL-CONSOLIDATION TESÏ RESULTS Fig. 3
I (+rI Hffii'ffiy:ffin*Ê; ä' *' *TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESandy Silt \4'ith Gravelßill)Very Sandy Silt withGravel CFill)Sandy Silt and Clay(psfìUNCONFINEDCOMPRESSIVESTRENGTH(%lPLASTICINDEXAÏTERBERG LIMITSLIQUID LIMIT(o/0,PERCENTPASSING NO.200 stEvE745877GRADATIONELl:/")GRAV(%)SANDfocfìNATURALDRYDENSITYr20115111(ololNATURALMOISTURECONTENTI713.48.6lftìDEPTH.tt /7%01SA¡ìIPLE LOCATIONBORING1No.21-7-593