HomeMy WebLinkAboutSubsoil Study 07.27.2015SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE, LOT 267
0038 VISTA RIVER
JRONBRIDGE DEVELOPMENT
GARFIELD COUNTY, COLORADO
JOB NO. 113 471N
JULY 27, 2015
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 81612
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......................................................................... -I -
BACKGROUND INFORMATION .............................................................................. - I -
PROPOSED CONSTRUCTION .................................................................................. ,. 2 -
SITE CONDITIONS .................................................................................................... -2 -
SUBSIDENCE POTENTIAL ...................................................................................... :-2 -
FIELD EXPLORATION .............................................................................................. - 3 -
SUBSURFACE CONDITIONS ................................................................................... ,. 3 -
ENGINEERING ANALYSIS ...................................................................................... :-4 -
DESIGN RECOMMENDATIONS ............................................................................... -4 -
FOUNDATIONS ...................................................................................................... - 4 -
FOUNDATION AND RET AJNJNG WALLS .......................................................... :-6 -
NONSTRUCTURAL FLOOR SLABS ..................................................................... -7 -
UNDERDRAIN SYSTEM ....................................................................................... :-7 -
SURFACE DRAINAGE .......................................................................................... :-8 -
LIMITATIONS ............................................................................................................ - 8 -
FIGURE l -LOCATION OF EXPLORATORY BORING
FIGURE 2 -LOG OF EXPLORATORY BORING
FIGURE 3 -LEGEND AND NOTES
FIGURE 4 -SWELL-CONSOLIDATION TEST RES UL TS
TABLE 1-SUMMARY OF LABORATORY TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a residence to be located on Lot 267,
Ironbridge Development, 0038 Vista River, Garfield County, Colorado. The project site
is shown on Figure l. The purpose of the study was to develop recommendations for the
foundation design . The study was conducted in accordance with our proposal for
geotechnical engineering services to Aspen Signature Homes of lronbridge, LLC dated
June l, 2015. The current engineering services includes a lot specific subsoil exploration
and laboratory testing study and has considered subsurface infonnation collected for
previous geotechnical studies at the lronbridge development.
A field exploration program consisting of an exploratory boring was conducted 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 current proposed building. 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 Jronbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted subsurface exploration and
geotechnical evaluation for development of Villas North and Villas South parcels, Job
No. l 05 115-6, report dated September J 4, 2005, and performed observation and testing
services during the infrastructure construction, Job No. J 06 0367, between April 2006
and April 2007. Additional subsurface exploration, laboratory testing and geotechnical
evaluation was conducted for the current proposed residential construction throughout the
Villas parcels, Job No . 113 471A, report dated February 28, 2014. The infonnation
provided in these previous reports has been considered in the current study of Lot 267.
Joh No. 113 471N ~ech
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PROPOSED CONSTRUCTION
The proposed residence wil1 be a two-story, wood frame structure with structural slab
foundation and no basement or crawlspace, and located as shown on Figure 1. A post-
tensioned (PT) slab foundation is expected at this time. Grading for the structure is
assumed to be relatively minor with cut and fill depths on the order of a few feet or less.
We assume relatively light foundation loadings, typical of the proposed type of
construction.
If building loadings, location or grading plans change signifiGantly from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The proposed residence is located in the north-central part of the Villas South parcel. The
natural terrain prior to development in 2006 sloped down to the east at about 5% grade.
The subdivision in this area was elevated by filling on the order of 15 feet above the
original ground surface to create a relatively level building site off of Vista River.
Vegetation consists of grass and weeds with scattered 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 River 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. Other irregular bedrock conditions have been identified in
the affordable housing site located to the northwest of the Villas North parcel. Irregular
surface features were not observed in the Villas South development area 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 in the Villas South development area. The subsurface exploration
performed in the area of the proposed residence on Lot 267 did not encounter voids. In
our opinion, the risk of future ground subsidence in the Villas South project area is low
and similar to other areas of the Roaring Fork River valley where there have not been
indications of ground subsidence.
Joh No. I I 3 4 71 N ~h
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FIELD EXPLORATION
The field exploration for current Lot 267 development was conducted on June l, 2015 at
the location shown on Figure l 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 Hepworth-Pawlak
Geotechnical , Inc .
Samples of the subsoils were taken with Bi's inch and 2 inch l.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 in the proposed residence area is
shown on Figure 2 . The subsoils encountered in the boring consist of about 17 feet of fill
above stiff, sandy silt (alluvial fan deposits) overlying dense, sandy gravel and cobble
soils (river gravel alluvium) at a depth of about 26Yl feet. The fill materials were mainly
placed in 2006 and consist of medium dense and sl ightly moist, mixed silt, sand and
gravel. Drilling in the underlying, coarse river gravel alluvium with augers was difficult
due the cobbles and boulders and drilling refusal was encountered in the deposit. Free
water was not encountered in the boring and the natural soils were slightly moist. The
subsurface conditions are generally consistent with the profiles encountered by the
February 28 , 2014 preliminary geotechnical study.
Laboratory testing perfonned 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 the
Joh No. 113471N ~ech
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natural sandy silt soils, presented on Figure 4, generally indicate low to moderate
compressibility under conditions of loading and when wetted. The laboratory testing is
summarized in Table l.
ENGINEERING ANALYSIS
The upper 17 feet of soils encountered in Boring I located at the north side Lot 267
consist of fill place mainly in 2006 as part of the subdivision development. The field
penetration tests and laboratory tests performed for the study, and review of the field
density tests performed during the fill construction indicate the structural fill was placed
and compacted to the project specified 95 % of standard Proctor density. Debris 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
folJowing construction. Relatively deep structural fill as encountered in Boring 1 will
also have some potential for long tenn settlement but should be significantly less than the
alluvial fan deposits. Proper grading, drainage and compaction as presented below in the
Swface 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
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 about 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 spread
footing foundation system.
Joh No . 113 471N ~tech
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1) A conventionally reinforced structural slab or post-tensioned slab placed
on about 15 feet or more of 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
but at least half of the slab width whichever is more. Settlement of the
foundation is estimated to be about 1 to 1 Yi inches based on the long term
compressibility of the fill. Additional settlement of about 1 inch is
estimated if 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 should control the design.
2) The thickened sections of the slab for support of concentrated loads should
have a minimum width of20 inches.
3) The perimeter tum-down section of the slab should be provided with
adequate soil cover above the 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 tum-down section should have at least 18 inches of soil cover.
4) The foundation should be constructed in a "box-like" configuration rather
than 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.
5) The root zone and any loose or disturbed soils should be removed.
Additional structural fill placed below the slab bearing level should be
compacted to at least 98% of the maximum standard Proctor density
within 2 percentage points of optimum moisture content.
6) A representative of the geotechnical engineer should evaluate the
compaction of the fill materials and observe all footing excavations prior
to concrete place ment for be aring conditions.
Joh No . 113 471 N ~tech
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FOUNDATION 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 buildings 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, 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 .
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density near optimum moisture content. Backfill placed in pavement
and walkway areas should be compacted to at least 95% 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 retaining 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 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 300 pct: The coefficient of friction and passive pressure values recommended
above assum e ultimate soil stre ngth. Suitable factors of safety should be included in the
Joh No. I 13 471N ~tech
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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 95% of the maximum standard Proctor density at near
optimum moisture content.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support Jightly loaded slabs-on-grade separate
from the building foundation. The fill soils can be compressible when wetted and result
in some post-construction settlement. To reduce the effects of some differential
movement, nonstructural floor slabs should be separated from buildings to allow for
unrestrained vertical movement. Slab control joints should be used to reduce damage due
to shrinkage cracking. The requirements for joint spacing and slab reinforcement should
be established by the designer based on experience and the intended slab use. A
minimum 4-inch layer of relatively well graded sand and gravel, such as road base,
should be placed beneath slabs as subgrade support. This material should consist of
minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 12 %
passing the No. 200 sieve.
All fill materials for support of slabs should be compacted to at least 95% of maximum
standard Proctor density at near optimum moisture content. Required fill can consist of
the on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
[t is our understanding the finished floor elevation at the lowest level of the proposed
residence will be 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. We recommend below-grade construction,
such as retaining walls, be protected from wetting and hydrostatic pressure buildup by an
underdrain and wall drain system.
Jnh No. 113 47 1 N ~tech
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If the finished floor elevation of the proposed residence 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.
SURF ACE 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:
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 95% 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. The slope
should be at least 6 inches in the first 5 feet in unpaved areas and at least
2 lh inches in the first 10 feet in paved areas. Graded swales should have a
minimum slope of 3%.
4) Roof gutters should be provided with downspouts that discharge at least 5
feet beyond the foundation and preferably into subsurface solid drain pipe.
5) Landscaping which requires regular heavy irrigation, such as sod, should
be minimized and located at least I 0 feet from the building foundation.
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 principles and practices in this area at this time. We make no warranty either
Joh No. 11 3 471 N
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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 in the future. If the client is
concerned about MOBC, then a professional in this special field of practice should be
consulted. Our findings include interpolation and extrapolation of the subsurface
conditions identified at the exploratory boring and variations in the subsurface conditions
may not become evident until excavation is performed. If conditions encountered during
construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design 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
verify 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 ,
HEPWORTH -PA WLA l,.,_"-'.....,
SLP/ksw
cc: Silich Homes -John Silich (fil.!rn 1a 1si I ichcon~truction.<.:0111)
Silich Homes -Jodi Thimsen (jodi«Vsilichhor11es.c01 1l)
Job No. 11 3 471N ~ech
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IRONBRIDGE DRIVE
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LOT266
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--~~------_JI -BORING 1 e
LOT267
38 RIVER VISTA
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PROPOSED RESIDENCE
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APPROXIMATE SCALE
1· 30'
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113 471 N ~ LOCATION OF EXPLORATORY BORING
Hll:PWORTH·PAWL.AK GEOTl!:CHNICAL
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Figure 1
.. .
..
5975
5970
5965
5660
Q)
Q) u..
I
c
.Q
(ij a;
w 5955
5950
5945
5940
113 471 N
BORING 1
ELEV .... 5972'
LOT267
FLOOR LEVEL 5972 86'
46/6,5013
WC e 12 7
DD == 112
·200 w51
38/12
28/12
WC •3.0
OO ac 119
·200 =17
26/12
20/12
WC 47
DD -113
·200 -71
15/12
90/11
NOTE : Explanation or symbols is shown on Figure 3.
LOG OF EXPLORATORY BORING
5975
5970
5965
5660
Q)
Q) u..
t
c::
0
·~
Q)
5955 UJ
5950
5945
5940
Figure 2
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LEGEND :··
~ FILL, mixed gravel , sand and silt , some cobbles , medium dense, slightly moist, brown. Man-placed.
~ SILT (ML); sandy to very sandy , slightly clayey , stiff to very stiff , slightly moist, light brown to brown.
~ GRAVEL AND COBBLES {GM-GP): silty, sandy, boulders . dense. slightly moist, rounded granitic rock .
p Relatively undisturbed drive sample ; 2-inch LD California liner sample .
• Drive sample, standard penetration test (SPT). 1 3/8 inch l.D . split spoon sample ,ASTM-1586 .
Drive sample blow count; indicates that 38 blows of a 140 pound hammer falling 30 inches were
38/12 required to drive the California or SPT sampler 12 inches .
T Practical drilling refusal.
NOTES:
1. The exploratory boring was drilled on June 12 , 2015 with a 4-mch diameter continuous Hight power auger.
2 . The exploratory boring location was measured approximately by pacing from features shown on the site plan
provided.
3 . The exploratory boring elevation was interpolated from contours shown on the site plan provided .
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 transitions may be gradual
6 . No free water was encountered in the boring at th e time of drilling. Fluctuation in water level may occur with time
7 Laboratory Testing Results:
WC == Water Content (%)
DD = Dry Density (pcQ
-200 "" Percent passi ng No . 200 sieve
113 471 N ~
HEPWORTl+PAWt.AK G£OTECHNICAI.
LE GEND AND NOTES Figure 3
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. Moisture Content ::: 4 .7 percent
Dry Density ; 113 pcf
• Sample of : Sandy Silt
From : Boring 1 at 20 Feet
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:,....-....... _,,...
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c ~ i-i-_ ,.. No movement 0
'(ii ~ upon C/l a.> wetting 0. 2
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3
0 1 1.0 10 100 . APPLIED PRE SSURE -ksf
113471N ~ SWELL -CONSOLIDATION TEST RESULT S Figure 4
HEPWORTl+PAWLA I< GEOTECHNI CAL
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HEPWORTH-PAWLAK GEOTECHNICAL, INC. . .
TABLE 1 Job No. 113 471N
SUMMARY OF LABORATORY TEST RESULTS
Lot267
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT
MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%)
SIEVE
(ft) (%) (pcf) (%) (%) (PSF)
I 2 12.7 112 51 Silt and Sand with Gravel (Fill)
10 3.0 119 17 Silty Sand and Gravel (Fill)
20 4.7 113 71 Sandy Silt
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