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HEPWORTH—PAWLAK GEOTECHNICAL
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 71, RIVER BEND WAY
GARFIELD COUNTY, COLORADO
JOB NO. 113 471G
OCTOBER 27, 2014
REVISED OCTOBER 31, 2014
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 81612
leccicstonerrr jhlbft nef
Parker 303-841-7119 • CttlulSpr•ir7,uti 119-633-'5562 • Silvcrtlitlrne 97CL468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
GEOLOGY -2-
FIELD EXPLORATION 3 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS 4 -
DESIGN RECOMMENDATIONS - 4 -
STRUCTURAL SLAB FOUNDATION - 5 -
DEEP FOUNDATION ALTERNATIVE - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS (NON-STRUCTURAL) - 7 -
UNDERDRAIN SYSTEM - 8 -
SURFACE DRAINAGE _ g -
LIMITATIONS - 9 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - 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 proposed residence to be located
on Lot 71, River Bend 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 August 27,
2014. The current study is an update of our previous subsoil study report conducted for
the Lot 71 building foundation design, dated September 28, 2007, Job No. 107 0486.
An exploratory boring was drilled on the lot 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.
PROPOSED CONSTRUCTION
The proposed residence will be a 2 -story, wood frame structure supported on a structural
slab foundation in both the living area and the garage. The garage and front porch slab
grades will be close to the main building floor level. Grading for the structure is assumed
to be relatively minor with cut depths between about 3 to 5 feet. We assume relatively
light foundation Ioadings, 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.
Job No. 113 471G
-2 -
SITE CONDITIONS
The lot is located on a strongly sloping alluvial fan along the uphill, western side of River
Bend Way. The Robertson Ditch (now buried) and easement borders the uphill side of
the lot. A detention basin for surface runoffborders the north side of the lot and
contained cattail and other marsh type vegetation at the time of this report preparation.
The ground surface of the lot has been graded relatively flat with Iikely shallow fill
placed in the central to eastern part during the subdivision development and is nearly
level across the proposed building area. Vegetation consists of mainly weeds on the lot.
The underground utilities to the lot are complete and the lot is essentially unchanged since
its original grading in 2006-2007. Lot 72 located to the south is vacant.
GEOLOGY
The geologic conditions were described in our previous report conducted for planning and
preliminary design of the overall subdivision development dated October 29, 1997, Job
No. 197 327. The surficial soils on the lot mainly consist of sandy silt alluvial fan
deposits with inter -bedded sandy and gravelly layers overlying gravel terrace alluvium of
the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of
rounded gravel, cobbles and boulders up to about 3 feet in size in a silty sand matrix
which extends down to depths on the order of 35 to 40 feet below ground surface and
overlies siltstone/claystone bedrock in the area of Lot 71.
The underlying bedrock consists of the Eagle Valley Evaporite which contains gypsum
and is generally associated with scattered sinkhole development in the Roaring Fork
River valley. An apparent sinkhole was observed along the south side of River Bend
Way and River Bank Way intersection about 900 feet southeast of Lot 71. The sinkhole
was excavated and backfilled during construction of the roadway. A sinkhole occurred in
the parking lot adjoining the golf cart storage tent in 2005, located about 700 feet to the
northwest of Lot 71, which was backfilled and compacted. Both sinkholes have not
shown signs of reactivation such as ground subsidence since their remediation.
Subsurface voids have not been encountered in borings drilled into the bedrock near Lot
71 and the potential for subsidence due to dissolution of the evaporite throughout the
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service Life of the residence, in our opinion, is low, but the owner of the Iot should be
aware of the sinkhole potential and the risk of future subsidence.
FIELD LB EXPLORATION
The field exploration for the project was conducted on July 20, 2007. An exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface
conditions. The boring was advanced with 4 -inch diameter continuous flight augers
powered by a truck -mounted CME -45B drill rig. The boring was logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with 1% 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 -I 586. 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 subsoil profile encountered in the boring is shown on Figure 2. The
subsoils consist of about 6 feet of silty sand and gravel overlying 7 feet of loose to
medium dense, sandy silt and silty sand above 10 feet of medium stiff sandy silt and clay
(debris fan deposits) overlying dense, slightly silty sandy gravel, cobbles and boulders
(river alluvium) at a depth of 23 feet down to the drilled depth of 34 feet. Drilling in the
dense river alluvium with auger equipment was difficult due to the cobbles and boulders
and drilling refusal was encountered in the deposit. The upper few feet of the boring may
be fill material placed during the subdivision development.
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 the
Job No. 113 471G
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sandy silt soil, presented on Figure 4, indicate low compressibility under existing low
moisture condition and light loading and a low collapse potential (settlement under
constant load) when wetted. The sample showed moderate compressibility under
additional loading after wetting.
No free water was encountered in the boring at the time of drilling in 2007 and the
subsoils were slightly moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The upper sand, silt and clay (debris fan) soils typically have low bearing capacity and
low to moderate settlement potential when wetted under loading and extend down about
20 feet below a shallow foundation such as spread footings. Considering the
compressible nature of the debris fan soils and the potential for (or continued) wetting
from the adjacent detention pond, spread footings could have a high risk of excessive
settlement and are not recommended for the building foundation support. With a risk of
differential settlement and minor distress, the building could be founded with a heavily
reinforced structural (mat) slab or post -tensioned slab foundation bearing on at least 5 feet
of compacted structural fill and is recommended for the building support. As an
alternative, foundations that extend down to the dense, river gravel alluvium (such as
piers or piles) could be used and would have moderate bearing capacity with low
sett:ement and building distress risk.
DESIGN RECOMMENDATIONS
STRUCTURAL SLAB FOUNDATION
The design and construction criteria presented below should be observed for a slab
foundation system.
1) A structural slab or post -tensioned slab placed on at least 5 feet of
compacted structural fill should be designed for an allowable bearing
pressure of 1,000 psf. Post -tensioned slabs 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. Initial settlement of the foundation is estimated
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to be about 1 inch or less. Additional differential settlement of about 1 to
2 inches is estimated if deep wetting of the debris fan soils were to occur.
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 (if used) 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 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.
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 and can consist of the onsite soils.
6) A representative of the geotechnical engineer should evaluate the
compaction of the fill materials during its placement and observe all
footing excavations prior to concrete placement for bearing conditions.
DEEP FOUNDATION ALTERNATIVE
Considering the compressibility potential of the debris fan soils encountered al. the site
and the nature of the proposed construction, piles or piers that extend down into the
underlying river gravel alluvium could be used for building support. Deep foundations
can typically consist of drilled or pushed micro -piles or helical piers. We expect the piles
or piers will be at least 20 feet long and penetrate the river gravel to achieve downward
allowable pile load capacity on the order of 20 to 40 kips. Uplift capacity will depend on
Job No. 113 4710
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All fill materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor density at a moisture content near optimum. Required fill can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, 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 and
basement areas (if provided), be protected from wetting and hydrostatic pressure buildup
by an underdrain system. An underdrain should not be provided around shallow
foundations (such as structural slabs and crawlspace areas if used).
Where installed around basement areas (if constructed), the drains should consist of
drainpipe placed in the bottom of the wall backfill surrounded above the invert level with
free -draining granular material. The drain should be placed at each level of excavation
and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a
suitable gravity outlet, sump and pump or drywelI based in the underlying river gravel
deposit. Free draining granular material used in the underdrain system should contain
less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least 1 feet deep. An
impervious membrane, such as a 30 mil PVC liner, should be placed in a trough shape
below the drain gravel and attached to the foundation wall with mastic to prevent wetting
of the bearing soils.
SURFACE DRAINAGE
Providing proper perimeter surface grading and drainage will be critical in the satisfactory
performance of the building. The following drainage precautions should be observed
during construction and maintained at all times after the building has been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
JobNo. tI347IG
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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.E.
Reviewed by:
Daniel E. Hardin, P.E.
S LP/ksw
cc: Silich Construction , David Guthrie (dguthrie(i silichconstruction.crnn)
Silich Construction - Jodi Thimsen (Jodi ri silichhomes.corn)
Job No, 11347IG
APPROXIMATE SCALE
DETENTION POND
•
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LOCATION OF EXPLORATORY BORING 1 FIGURE 1
Elevation - Feet
- 5950
- 5945
- 5940
5935
- 5930
5925
- 5920
5915
LOT71
BORING
ELEV. 5951'
MAIN FLOOIR ELEV. 5951'
INVERT LEVEL OF
ADJ. DETENTION BASIN
354112
' :
.tl°:: 18112
Xti� WC==3.6
DD=99
{•• -200-18
g
18/12
WC -11.1
DD=107
4
"/
5/12
r J 5935
5950
5945
5940
4
.p
Q1
.b.•
4112
60/12
NOTE: Explanation of symbols is shown on Figure 3.
5930
5925
5920
5915
Elevation - Feet
113 471G
G VStech
HF.PWORm.PAW W K GEOTECHNICAL
LOG OF EXPLORATORY BORING
FIGURE 2
1
LEGEND:
oPO'
-71
2
54/12
T
NOTES:
SAND AND GRAVEL (SM -GM); silty, medium dense, slightly moist, brown, subangular rock. Poss b e compacted
fill in upper part.
SAND AND SILT (SM -ML); loose to medium dense, slightly moist, Tight brown, stratified.
SILT AND CLAY (ML -CL); slightly sandy, medium stiff, moist, light brown.
GRAVEL AND COBBLES (GM -GP); slightly silty, sandy, probable boulders, dense, brown, rounded rock.
Relatively undisturbed drive sample; 2 -inch I.O. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch 1.0. split spoon sample, ASTM -1586.
Drive sample blow count; indicates that 54 blows of a 140 pound hammer falling 30 Inches were
required to drive the California or SPT sampler 12 inches.
Practical drilling refusal,
Caved depth when checked on July 23, 2007.
1. The exploratory boring was drilled on July 20, 2007 with a 4 -inch diameter continuous flight 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 interpreted from the contours shown on the site plan provided by H.C.E.
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 the time of drilling or when checked 3 days later. Fluctuation in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pci)
-200 = Percent passing No. 200 sieve
113 471G
HEIMORTH.pgyyLAK GEOTECHNICAL
LEGEND AND NOTES
FIGURE 3
Compression
0
1
2
3
4
Moisture Content = 11.1 percent
Dry Density = 107 pct
Sample oft Very Sandy Silt with Gravel
From: Boring 1 at 10 Feet, Lot 71
upon npression
up
wetting
0.1 1.0 10 100
APPLIED PRESSURE - ksf
113 471G
G&ec.1 1
H EA W ORTI+PAW LAK GEOTECHNICAL
SWELL -CONSOLIDATION TEST RESULTS
FIGURE 4
Job No. 113 471G
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