HomeMy WebLinkAboutSoils Report 09.14.2015HEPWORTH-PAWLAK GEOTECHNICAL
Hcrworth•P.m l A. Getit«hnical, Inc
5020 County Road 1541
Glenwuxod Springs, Colorado 81601
Phone 970-9.15- 7958
Fax: 970,945-8454
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE, LOT 269
EAGLE CLAW CIRCLE
IRONBRIDGE DEVELOPMENT
GARFIELD COUNTY, COLORADO
JOB NO. 113 471P
SEPTEMBER 14, 2015
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 81612
eccleston e @ pb1h 11.n et
ParL•r 303-84I-7119 • Colorado Springs 7I9-633-7562 • Sib/erthome 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
BACKGROUND INFORMATION - 1 -
PROPOSED CONSTRUCTION - 2 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
ENGINEERING ANALYSIS - 4 -
DESIGN RECOMMENDATIONS - 5 -
FOUNDATIONS - 5 -
FOUNDATION AND RETAINING WALLS - 6
NONSTRUCTURAL FLOOR SLABS - 7 -
UNDERDRAIN SYSTEM - 8 -
SURFACE DRAINAGE - 8 -
LIMITATIONS - 9 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 AND 5 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1- 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 269,
Ironbridge Development, Eagle Claw Circle, 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
September 1, 2015. The current engineering services consist of a lot specific subsurface
study based on subsurface information collected for previous geotechnical studies at the
Ironbridge 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 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. I06 0367, between April 2006
and April 2007. Additional subsurface exploration, laboratory testing and geotechnical
evaluation was conducted throughout the Villas parcels, Job No. 113 47IA, report dated
February 28, 2014. The information provided in these previous reports has been
considered in the current study of Lot 269.
Joh No. I l 3 47I P Gegtech
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 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 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 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 25 feet above the
original ground surface to create a relatively level building site off of Eagle Claw Circle
and just off of an MSE retaining wall located immediately to the east of the lot.
Vegetation consists of grass and weeds.
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with
sinkholes and Iocalized 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, 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
exploration performed in the area of the proposed residence on Lot 269 did not encounter
voids or subsurface irregularities indicative of sinkhole development. In our opinion, the
Job No. 1 11 471 P GLticrytech
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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 caused by underlying voids.
FIELD EXPLORATION
The field exploration for the planned residence on Lot 269 was conducted on September
4, 2015. Boring 1 was drilled adjacent to Lot 269 at the location shown on Figure 1 to
evaluate the subsurface conditions. Boring 21 (2005) from our September 14, 2005
subsurface study report was drilled near the east side of Lot 269 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 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 arc 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 21 (2005) consist of a shallow
topsoil depth above about 9 feet of silty sand with gravel underlain by sandy silt and clay
(alluvial fan deposits) to the boring depth of 20 feet. The natural soils encountered at
Boring 1 were similar alluvial fan sandy silt and clay deposits below about 27 feet of
compacted fill material mainly placed in 2006. The fill soils are medium dense and
lob No 111 471P ate,
-4 -
slightly moist, and the underlying natural alluvial fan soils are loose to medium
dense/stiff and slightly moist. Drilling became difficult at a depth of about 45 feet in
Boring 1 apparently due to cobbles and possible boulders in the underlying river gravel
alluvium 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 and clay soils, presented on Figures 4 and 5, indicate low
compressibility under light loading and a low 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 27 feet of soils encountered in Boring 1 consist of fill placed mainly in 2006 as
part of the subdivision development. The field penetration tests and Iaboratory tests
performed for this 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. We expect that the eastern part of the fill material
includes geogrid reinforcement of the adjacent MSE retaining wall which the proposed
building will overlie. This condition was identified in the February 28, 2014 geotechnical
study report conducted for the current Villas parcels development. 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 and their
wetted depth. Relatively deep structural fill as encountered in Boring 1 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 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
Job No. 113 471P G ch
-5 -
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 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 27
feet of compacted structural fill. The structural engineer should consider the close
proximity of the MSE wall to the east side of the residence in the foundation design. 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.
1) A conventionally reinforced structural slab or post -tensioned slab placed
on about 20 feet or more of compacted structural fill should be designed
for an allowable bearing pressure of I,500psf. 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 11, 2 to 2 inches based on the long term
compressibility of the fill. Additional settlement of about 2 inches is
estimated if deep wetting of the debris fan soils were to occur. Settlement
from the deep wetting would tend to be uniform across the
building/development 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 the bearing elevation for frost protection.
1�.,hNo I1147IP Gtech
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Placement of foundations at least 36 inches below exterior grade is
typically used in this area. If a frost protected foundation is used, the
perimeter turn -down section should have at least 18 inches of soil cover.
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 compaction
of the new materials and observe all footinj 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 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
Job No. I I { -t7 I P Gtech
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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 9070 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 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
Job No. I ! 47 I P Gated.,
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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 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 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 (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
Joh No 113 411P Gaech
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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
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' 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 Ieast 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.
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
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
Job No 113 471P
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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 - PAWLAK GEOTECHNICAL, INC.
Steven L. Pawlak, P.
Reviewed by:
ry
i 1522
Daniel E. Hardin, P.E.
SLP/ksw
cc: Silich Homes - John Silich (jyohnC'silichconstructionxom)
Job No. 111471P eegtech
APPROXIMATE SCALE
1" = 20
113 471P
LOT 270
5995 —
5996 —
BORING 1
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BORING 21
1 (2005)i • 1
1 PROPOSED
RESIDENCE
} F F. - 5995.4
1 LOT 269
1 I
W�
GARAGE FLOOR
= 5994.92'
EAGLE CLAW CIRCLE
Hepworth—Powlak Geotechnical
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LOCATION OF EXPLORATORY BORINGS
LOT 266
EXISTING
RETAINING
WALLS
LOT 267
Figure 1
5
- 10
15
- 20
25
30
35
40
45
113471P
F F 5995 4
BORING 1, LOT 269
ELEV.= 5996
N
81/10
49/12
WC 67
DD 119
-200 65
25/12
36/12
38/12
WC 7.6
DD 122
-200 45
20/12
WC 6.6
DD- 120
-200- 61
16/12
WC 85
DD 108
52/12
WC -38
DD= 125
-200=66 BOTTOM OF BORING
AT 45 FEET
BORING 1
CONT
50/6
BORING 21 (2005)
ELEV.- 5976'
52/12
ice
�•� 15/12
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- WC 33
DD 108
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15;12
Note: Explanation of symbols is shown on Figure 3
Hepworth—Pawlak Geotechnical
LOGS OF EXPLORATORY BORINGS
0
5
10
15
20
25
30
35
40
45
Depth - Feet
Figure 2
LEGEND:
FILL; si ty sand to sandy silt, gravelly, sl ghtly clayey, medium dense to dense, slightly moist, brown.
® TOPSOIL; root zone. sandy silt, moist, brown
SAND AND GRAVEL (SM -GM), s-Ity, some sandy si.t zones, medium dense slightly moist, brown, subangular to
rounded rock.
SILT AND CLAY (ML -CL), sandy, scattered grave=, stiff to very stiff, slightly moist, light brown, slightly calcareous
and porous
Relatively undisturbed drive sample, 2 -inch I.D California finer samp'e.
Drive sample, standard penetration test (SPT), 1 318 inch I.D. split spoon sample, ASTM D-1586.
Drive sample blow count: indicates that 49 blows of a 140 pound hammer falling 30 inches were
49,12 required to drive the California or SPT sampler 12 inches
fi
NOTES.
Practical drilling refusal in gravel.
1. Exploratory Boring 21 was drilled on July 6, 2005. Exploratory Boring 1 was drilled on September 4, 2015. 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. Elevations of exploratory borings were 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 an 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 (%)
DO = Dry Density (pcf)
-200 = Percent passing No. 200 sieve
113 471P
Hepworth—Pawlak Geotechnical
LEGEND AND NOTES
Figure 3
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Moisture Content 8.5 percent
Dry Density - 108 pct
Sample of: Sandy Silt and Clay
From: Boring 1 at 29 1/2 Feet
_____No movement
upon
wetting
0.1
113 471P
1.0
Hepworth—Pawlak Geotechnical
10
APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 4
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Moisture Content = 3.3 percent
Dry Density = 108 pcf
Sample of: Sandy Silt and Clay
From. Boring 21 at 14 Feet
Compression
upon
��wetting
0.1
113 471P
1.0
Hepworth—Powlok Geotechnical
10
APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 5
Job No. 113 471P
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Very Silty Clayey Gravelly Sand
(Fill)
Very Sandy Gravelly Silt (Fill)
Sandy Silt and Clay
Sandy Silt and Clay with Gravel
Sandy Silt and Clay II
UNCONFINED
COMPRESSIVE
STRENGTH
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ATTERBERG LIMITS
PLASTIC
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