HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.26.15G4;&ec:h
HEPWORTH-PAWLAK GEOTECHNICAL
H-.puorth-Paw ak Georcchnical. Inc.
5020 Giunry Road 154
GI%.m twx1 Springs, Gflomdo 81601
Phone. 970.945.7988
fax 970.945-8454
cinail hpkeor17,hp eotech.corn
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE, LOT 291
MODEL 1947
RIVER BEND WAY
IRONBRIDGE DEVELOPMENT
GARFIELD COUNTY, COLORADO
JOB NO. 113 471T
OCTOBER 26, 2015
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 81612
lecclestone@pblhfl.net
Parker 303.841 -1119 9 Colorado Springs 719-633-5567 * Silverrhorne 970-468-19S9
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY............................................................................- l
BACKGROUND INFORMATION................................................................................- 1 -
PROPOSED CONSTRUCTION..................................................................................... 2 -
-SITESITE
CONDITIONS....................................................................................................... 2 -
SUBSIDENCE POTENTIAL......................................................................................... 2 -
-FIELFIELD
D EXPLORATION.................................................................................................- 3 -
SUBSURFACE
-SUBSURFACE CONDITIONS...................................................................................... 3 -
ENGINEERING
-ENGINEERING ANALYSIS.........................................................................................- 4 -
DESIGN
-DESIGN RECOMMENDATIONS................................................................................. 5 -
FOUNDATIONS......................................................................................................... 5 -
FOUNDATION AND RETAINING WALLS............................................................ 6 -
NONSTRUCTURAL FLOOR SLABS....................................................................... 7 -
UNDERDRAIN SYSTEM..........................................................................................- 8 -
SURFACE DRAINAGE.............................................................................................- 8
LEN41TATIONS................................................................................................................ 9 -
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a residence to be located on Lot 291,
Ironbridge Development, River Send Way, Garfield County, Colorado. The project site
is shown on Figure 1. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our proposal for
geotechnical engineering services to Aspen Signature Homes of Ironbridge, LLC dated
October 19, 2015. The current engineering services consist of a lot specific study using
subsurface information collected for previous geotechnical studies at the Ironbridge
development.
A field exploration program consisting of exploratory borings was previously 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 previous
field exploration and laboratory testing were analyzed to develop recommendations for
foundation types, depths and allowable pressures of 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 Ironbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted subsurface exploration and
geotechnical evaluation for 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. Additional subsurface exploration, laboratory testing and geotechnical
evaluation was conducted for the current proposed residence construction throughout the
Villas parcels, Job No. 113 471 A, report dated February 28, 2014. The information
provided in these previous reports has been considered in the current study of Lot 291.
JobNo 113471T Gggbech
-2 -
PROPOSED CONSTRUCTION
The proposed residence will 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 slab foundation is expected at this time. Grading for the structuress 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 significantly 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 east -central part of the Villas North parcel. The
natural terrain prior to development in 2006 sloped down to the east at about 5% or less.
The subdivision in this area was elevated by filling on the order of a few feet above the
original ground surface to create a relatively level building site off of River Bend Way. A
two tiered retaining wall about 12 feet high was built to support Slue Heron Way
immediately west of Lot 291. Vegetation consists of grass and weeds. A fairly large
stockpile of soil from Lot 292 basement excavation covers about half of the proposed
building area.
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 west of the Villas
North 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 North development area that could
indicate an unusual risk of future ground subsidence, but localized variable depths of the
debris fan soils encountered by the previous September 14, 2005 geotechnical study in the
Villas North development area could be the result of past subsidence. The subsurface
Job No. 113 47 Cr
-3 -
exploration performed in the area of the proposed residence on Lot 291 did not encounter
voids or subsurface irregularities indicative of sinkhole development. In our opinion, the
risk of future ground subsidence in the Villas North and 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.
hllMA1l0A9to] 7AIIllli]►I
The field exploration for current planned development of Villas North and Villas South
parcels was conducted between December 24, 2013 and January 2, 2014. Boring 6
(2014) was drilled adjacent to Lot 291 at the location shown on Figure l to evaluate the
subsurface conditions. Boring 10 (2005) from our September 14, 2005 subsurface study
report was drilled on the south side of Lot 291 as shown on Figure 1. The borings were
advanced with 4 -inch diameter continuous flight augers powered by a truck -mounted
CME -45B drill rig. The borings were logged by a representative of Hepworth-Pawlak
Geotechnical, Inc.
Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The
samplers were driven into the subsoils at various depths with blows from a I40 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 Logs of Exploratory Borings,
Figure 2. The samples were returned to our laboratory for review by the project engineer
and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered in the proposed residence area are
shown on Figure 2. The subsoils encountered in Boring 10 (2005) consist of a shallow
topsoil depth above sandy clayey silt and silty sand and gravel (alluvial fan deposits)
Job No. 11347 IT G
-4 -
overlying dense, sandy gravel and cobble soils (river gravel alluvium) at a depth of about
14 feet. The natural soils encountered at Boring 6 (2014) consist of about 12 feet of
similar alluvial fan soils below about 4 feet of fill material mainly placed in 2006. The
fill soils are medium dense and slightly moist to moist, and the underlying natural alluvial
fan soils are loose to medium dense/stiff and slightly moist. 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.
Laboratory testing performed on samples obtained from the borings 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
natural sandy silt soils obtained from Boring 6 (2005) and Boring 10 (2014), presented on
Figure 4, indicate low compressibility under light loading and a low to moderate collapse
potential (settlement under constant load) when wetted and moderate compressibility
under additional loading. The laboratory testing is summarized in Table 1.
ENGINEERING ANALYSIS
The upper 4 feet of soils encountered in Boring 6 (2014) located adjacent to Lot 291
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 and in Boring 10 (2005). The amount of settlement will depend on the thickness
of the compressible soils and their wetted depth. The structural fill as encountered in
Boring 6 (2014) will also have some potential for long term settlement but usually
significantly less than the alluvial fan deposits. Proper grading, drainage and compaction
as presented below in the Surface Drainage sections 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
Job Nov 113 471T G h
-5 -
underlying dense, river gravel alluvium could be used to reduce the building settlement
risk.
DESIGN RECONIN ENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings 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 4
feet of 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.
I) A conventionally reinforced structural slab or post -tensioned slab placed
on about 4 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 2 inches based on the
compressibility of the natural soils and depending on the depth and extent
of wetting.
2) The thickened sections of the slab for support of concentrated loads should
have a minimum width of 20 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
Job No. 113 471T
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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 placement for bearing conditions.
FOUNDATION 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 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 [cast 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.
Job No 113471T G&tBch
-7 -
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 pcf. The coefficient of friction and passive pressure values recommended
above assume ultimate soil strength. Suitable factors of safety should be included in the
design to limit the strain which will occur at the ultimate strength, particularly in the case
of passive resistance. Fill placed against the sides of the footings to resist lateral loads
should be compacted to at least 95% of the maximum standard Proctor density at near
optimum moisture content.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support lightly loaded slabs -on -grade separate
from the building foundation. The fill soils and underlying natural 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. 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
Job No. 113 471T G99beCI
ICE
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 floor 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
It 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 limes 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, and basement areas (if provided), be protected from wetting and
hydrostatic pressure buildup by an underdrain and wall drain system.
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.
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:
1) 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
]ob Na 113471T GgReCh
-9 -
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%2 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 10 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.
MUTATIONS
This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area at this time. We make no warranty either
express or implied. The conclusions and recommendations submitted in this report are
based upon the data obtained from the exploratory borings drilled at the locations
indicated on Figure 1, the proposed type of construction and our experience in the area.
Our services do not include determining the presence, prevention or possibility of mold or
other biological contaminants (MOBC) developing in the future. If the client is
concerned about MOBC, then a professional in this special field of practice should be
consulted. Our findings include interpolation and extrapolation of the subsurface
conditions identified at the exploratory borings 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
Job No. t 13 471T G99ted7
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 - PAWLAK GEOTECHNICAL, INC.
Ste
Ret
Daniel E. Hardin, P.E.
SLP/ksw
cc. Silich Homes — John Silich 'cion @0 ichconstruction.corn
Silich Homes — Eric Lintjer(clintler@silichconstruction.comi
Job No 113471T
APPROXIMATE SCALE
V=20'
RIVER BEND WAY
113 471 T f I I LOCATION OF EXPLORATORY BORINGS I Figure 1
5gA3
1
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PATIO !
EXISTING
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RESIDENCE
EXISTING
1
I
1
LOT 292
RESIDENCE
(
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1 PROPOSED
I
(BASEMENT)
LOT 290
i
I RESIDENCE
I
(PIT SLAB)
+
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1 LOT 291
I
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1
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BORING 6
BORING 10,
(2014)
«
/ / I
(2005) _
l
/ I /
GARAGE
/
'L 6°�aA
5940 a ,
5
RIVER BEND WAY
113 471 T f I I LOCATION OF EXPLORATORY BORINGS I Figure 1
BORING 10
BORING 6
(2005)
(2014)
ELEV,= 5938'
ELEV. = 5943'
5945
5945
MAIN FLOOR LEVEL
5942 86
19112
5940
5940
9112
WC 7.7
DID 104
-200=79
5935
5935
m
18112
L-
WC -40
8112
U;
o
DD -96
WC=78
DD- 105
0
Q
5930
-200=44
5930
w
M
10112
6/12
�1
5925
5925
60112
.4' 7519
5920
5920
Note: Explanation of symbols is shown on Figure 3,
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113 471T
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LOGS OF EXPLORATORY BORINGS
Figure
2
k warlh—Pawlak Geolschnkd
LEGEND:
®FILL; mixed clayey silt, sand and gravel with cobbles, medium dense, slightly moist to moist, constructed mainly
in 2006.
® TOPSOIL; organic sandy silty clay, firm, dark brown.
®SILT (ML); sandy to very sandy, slightly clayey, gravel layers, stiff to very stiff, slightly moist , light brown to brown,
slightly calcareous.
�q SAND AND GRAVEL (SM -GM); silty, with cobbles, medium dense, slighty moist, mixed brown, subangular rock
fragments.
FUR
GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable boulders, dense, moist, brown, rounded Over
• rock.
Relatively undisturbed drive sample; 2 -inch LD Cal fomia liner sample.
Drive sample; standard penetration test (SPT), 13/8 inch I.D. split spoon sample, ASTM D-1586.
18112 Drive sample blow count; indicates that 16 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
TPractical drilling refusal.
NOTES:
1. Exploratory Boring 10 was drilled in July 2005 and Boring 6 was drilled between December 24, 2013 and January 2,
2014. The borings were drilled with 4 -inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
provided.
3. Elevation of exploratory Boring 10 was provided by High Country Engineering in 2005. Elevation of exploratory
Boring 6 was obtained by interpolation between contours shown on the site plan provided
4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the
method used.
5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time
7. Laboratory Testing Results:
WC = Water Content (°/6)
DD = Dry Density (pc f)
-200 = Percent passing No. 200 sieve
113 471T
LEGEND AND NOTES I Figure 3
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Moisture Content = 7.7
Dry Density = 104
Sample of: Sandy Clayey Silt
From: Baring 6 (2014) at 5 Feet
Compression
upon
1.0 10
APPLIED PRESSURE - ksf
Moisture Content = 4.0
Dry Density = 96
Sample of: Sandy Silt
From: Boring 10 (2005) at 4 Feet
Comression
upon
wetting
percent
PC(
percent
pcf
100
8 L 1 I I I 11 I 1 I I „1 I I I I I I I I
0.1 1.0 10 100
APPLIED PRESSURE - ksf
113471T ~'"] SWELL -CONSOLIDATION TEST RESULTS Figure 4
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