HomeMy WebLinkAboutSubsoil StudyH-PryKUMAR
Geotechnlcal Englnearing I Engineering Geology
Maledals Testlng I Environmentat
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Loætions: Pa*er, Glenwood Springs, and Silverlhome, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 264, IRONBRIDGE
RIVER VISTA
GARFIELD COUNTY, COLORADO
PROJBCT NO. U-7.4r0
JUNE t3,2ot7
PRDPARED FOR:
BH HOLDINGS
ATTN: JOHN YOUNG
43O IRONBRIDGE DRIVE
GLENWOOD SPRINGS, CO 8160I
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
BACKGROUND INFORMATION
PROPOSED CONSTRUCTION .
SITE CONDITIONS
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ...................
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
NONSTRUCTURAL FLOOR SLABS ...
UNDERDRAIN SYSTEM ............
SURFACE DRAINAGE
LIMITATIONS
FIGURE I - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SV/ELL-CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
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H-PtKUÎVIAR
Projecl No. 17.7-41O
PURPOSB AND SCOPE OF STUDY
This report Presents the results of a subsoil study for a proposed residence to be located on Lot
264, Ironbridge, River Vista, Garfield County, Colorado. The project site i.s shown on Figure l.
The purpose of the study was to develop recommendation.s for the foundation design. The study
was conducted in accordance with our agreement for geotechnical engineering services to BH
Holdings dated May 19,2017.
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 analyzed to 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 TNFORMATION
The proposed residence is located in the existing lronbridge development. Hepworth-pawlak
Geotechnical, Inc. (now H-P/Kumar) previously conducted subsurface exploration nnd
geotechnical evaluation for the development of Villas North and Villas South parcels, Job No.
105 I l5-6, 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.
Additional subsurface exploration, laboratory testing and geotechnical evaluation was conducted
for proposed residential construction throughout the Villa parcel.s, Job No. I I 3 471 A, report
dated February 28,2014. The information provided in these previous reports has been
considered in the current study of Lot264.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The residence
will likely be a two-story, wood-frame structure with structural slab foundation and no basemenl
H-PIKUMAR
Project No. 17-7-410
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or crawlspace. Grading for the structure is assumed to be relatively minor with cut depths
between about 3 to 4 feet. V/e assume relatively light foundation loadings, typical of the
proposed type of construction.
If building loadings, location or grading plans change signiFrcantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot is vacant and located in the northeast part of the Villas South parcel. The natural terrain
prior to development in 2006 sloped down to the east at about SVo grade. The subdivision area
was elevated by filling on the older of l5 feet above the original ground surface to create ¿t
relatively flat building site off River Vista. The lot is currently mostly gently sloping down to the
west with a steep fill slope on the eastern side of the lot. Vegetation consists of sparse grass and
weeds with scattered sagebrush. The Robertson ditch buried pipeline and asphalt paved
pedestrian path roughly follow the toe of the steep slope.
SUBSIDENCE POTBNTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with sinkhole
and localized ground subsidence in the Roaring Fork Valley. A sinkhole opened in the carr
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 Villa.s South parcel that could indicate
an unusual risk of future ground subsidence and localized variable depths of the debris fan soils
were generally not encounlered by the previous September 14,20A5 nor Februa ry 28,2014
geotechnical studies in the Villus South parcel. The subsurface exploration performed in the area
of the proposed residence on Lot 264 did not encounter voids. In our opinion, the risk of future
ground subsidence on Lot 264 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.
H.P*KUIVIAR
Projecl No.17-7-4f 0
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FIELD EXPLORATION
The field exploration for the project was conducted on May 22,2017. 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 represenrarive of H-P/Kunrar.
Samples of the subsoils were taken with l% inch and 2 inch I.D. spoon samplers. The samplers
wete 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 re.sistance value.ç 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 grapltic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about I5 feet of compacted fill soils overlying stiff ro very .stiff/medium
dense, silty .sand with gravel and cobbles and sandy silt with scattered gravel (altuvial fan
deposits) underlain by dense, sandy gravel and cobble (river gravel alluvium) ar a depth of about
47 feet. The fTll materials were mainly placed in 2006 and consist of medium dense, mixed silt,
sand and gravel. Drilling in the underlying, coarse river gravel alluvium with auger equipment
was difficult due to the cobbles and boulder.s.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density, and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on a relatively undisturbed drive sample of sílty sand with gravel soil,
presented on Figure 3, indicate low compressibility under conditions of light loading and a low
collapse potential (settlement under constant load) when wetted. The laboratory testing is
summarized in Table l.
H-PI KUÍVIAR
ProjeclNo. 17-7-410
4
No free water was encountered in the boring at the time of drilling and the sub.soils were slightly
moist.
FOUNDATION BEARING CONDITIONS
The upper 15 feet of soils encountered in the boring consist of fill placed mainly in 2006 as pârt
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 construcrion indicate the
structural fill was placed and compacted to the project specified minimum 95Vo of standard
Proctor density. Alluvial fan soils which tend to collapse (settle under constant load) when
wetled were encountered below the fîll. The amount of settlement will depend on the thickness
of the conrpressible soils due to potential collapse rvhen wetted, and the future compression of
the wetted soils following construction. Relatively deep structural fìll as encountered will also
have some potential for long-term settlement but should be significantly less than the alluvial fan
deposits. Proper grading, drainage and compaction as presented in the Sutfuce 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 exl.ends down
to the underlying dense, river gravel alluvium could be used to reduce the building settlement
risk.
DDSIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
tlte proposed construction, we recommend the building be founded with a heavily reinforced
structural slab or post-tensioned slab foundation bearing on at least l5 feet of the existing
compacted structural fill. If a deep loundation system is considered for building support, we
should be cont¿¡cted for additional recomrnendations.
The design and construction criteria presented below should be observed for a heavily reinforced
structural slab or post-tensioned slab foundation system.
H.P\KUMAR
Projecl No. 17-7.410
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A heavily reinforced structural slab or post-tens¡oned 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 fîll should be designed for a wetted
distance of l0 feet or at least half of the slab width, whichever is greater.
Settlement of the foundation is estimated 1o be about I to lVz inches based on the
long-term compressibility of the fill. Additional settlement of about I to 2 inches
is estimated if the wetting of the debris fan soils were to occur. Settlement from
the deep wetting would tend to be uniform across the building area and the
settlement potential of the fill section slrould control the design.
The thickened sections of the slab for support of concentrated loads should have a
minimum rvidth 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
l8 inches ofsoil cover.
The foundation should be constructed in a "box-like" configuration rather than
with irregular extensions which can settle differentialty to the main building are¿I.
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 l4 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 Vy'alls" section of this report.
The root zone and any loose or disturbcd soils should be removed. Additional
structural fÏll placed below the slab should be compacted to at least 987o of the
maximum 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.
H.PtKUMAR
Projecl No. 17.7.410
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FOUNDATTON AND RETAINING WALLS
Foundation walls and retaining structures 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 backfìll consisting
of the on-site soils. Cantilevered retaining structures which âre separate from the residence and
can be expected to deflect sufficiently to mobilize the full ¡¡ctive 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, traffic, construction mäterials and equipment. The
pressures recommended above assurne drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an up\À,¿trd sloping backlill surface rvill
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 uniforrn lifts and compacted to at least 90Vo oÍ the maximum
standard Proctor density al. a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95Vo of the maximunt standard Proctor density.
Care should be taken not to overcompact the backfill or u.se 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 nraterial 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 calculared
based on a coefficient of friction of 0.40. 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 assurne uttimate soil
strength. Suitable factors of safety should be included in the design to limit rhe strain which will
H.PtKUMAR
Projecl No. 't7-7-410
7
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 a compacted to at least 95Vo of ¡he
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
sepffate from the building foundation. The fTll soils can be compressible when wetted and can
result in some post-consLruction 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 est¡blished by the
designer based on experience and the intended slab use. A mininrum 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 507o retained on the
No.4 sieve and less than l27o passing the No. 200 sieve.
A¡l fill mûterials for support of floor slabs should be compacted to at least g57o of maximum
standard Proctor density at a moisture content neâr optimum. Required lìll can consist of the on-
site granular soils devoid of vegetation, top.soil 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 area that local perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
condition. rlVe recommend below-grade construction, such as retaining walls, be protected from
wetting and hydrostatic pres.sure 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. Allearth
retaining structures should be properly drained.
H.PìKUMAR
Project No. 17-7-410
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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 precaurions should be observed
during construction and maintained at all times after the residence has been completed:
I ) 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 95c/a of the maximum standard Proctor density in pavement and
nonstructural slab areas and to at least 90Vo of the maximum standard Proctor
density in landscape areas.
3) The ground sulrface 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 fìrst l0 feet in paved areas. Graded swales should have a minimum
slope of 37o.
4) Roof downspouts and drains should discharge at least 5 feet beyond the
foundation and pre ferably into a subsurface solid drainpipe.
5) Landscaping which requires regular heavy inigation should be located ar least l0
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 irigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. lile 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 l, 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
H-P+KUTWAR
Project No. 17-7-410
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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 reporr, we should be notifÏed 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 nor
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 verify that the recommendations
have been appropriately inl.erpreted. 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 fîll by ¿r representative of
the geotechnical engineer.
Respectful ly Submi tted,
H-Pt KUMAR
Steven L. Pawlak, P
Reviewed by:
Daniel E. Hardin, P.E.
SLP/kac
H.PtKUMAR
Project No. 17.7.410
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17 -7 -410 H-PryKUMAR SWELL-CONSOLIDATION TEST RESULT Fig. 3
H.PtKUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No. 17-7-410SOILTYPESilty Sand and Gravel (Fill)Silty Sand with GravelSilty Sand and GravelSilty SandUNCONFINEDCOMPRESSIVESTRENGTH(PSRATTÊRBÊRG LIÍIIIITSPIáSTICINDEX(o/olLtoutDUilIITlo/"1PERCENTPASSINGNO.200SIEVE301438GRADATIONSANDt%tGRAVELl"/"1NATURALMOISTURECONTENTNATURALDRYDENSITYt20t22t285.75.42.O5.8LOCATIONDEPTH{ft}5l52530BORINGI