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HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 257, IRONBRIDGE DEVELOPMENT
203 RIVER VISTA
GARFIELD COUNTY, COLORADO
JOB NO. 113 471K
MAY 13, 2015
PREPARED FOR:
ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC
ATTN: LLWYD ECCLESTONE
P.O. BOX 7628
ASPEN, COLORADO 8161 2
lccclcstonc@ phlhfl.ncr
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......................................................................... - 1 -
BACKGROUND INFORMATION .............................................................................. -I -
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 -
SITE GRADING ...................................................................................................... -8 -
SURFACE DRAINAGE .......................................................................................... :-9-
LIMITATIONS .......................................................................................................... -10-
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 Jocated
on Lot 257, Ironhridge Development, 203 River Vista, Garfield County, Colorado. The
project site is shown on Figure I . 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 April 28, 2015. We previously conducted a preliminary subsoil
study for residences in the Villas North and South Parcels and presented our findings in a
report dated February 28, 2014, Job No. 113 471A.
A field exploration program consisting of an exploratory boring was conducted during the
preliminary subsoil study to obtain infonnation on the subsurface conditions. Samples of
the subsoils obtained during the field exploration were tested in the laboratory to
detennine their classification, compressibility or swell and other engineering
characteristics. The results of the field exploration and laboratory testing were analyzed
to develop recommendations for foundation types, depths and allowable pressures for the
proposed building foundation. This report summarizes the data obtained during this study
and presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface
conditions encountered.
BACKGROUND INFORMATION
The proposed residence is located in the existing lronbridge subdivision development.
Hepworth-Pawlak Geotechnical previously conducted s~bsurfacc 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. The info1mation provided in the previous reports has been considered in
the current study of Lot 257 .
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PROPOSED CONSTRUCTION
The proposed residence will be a two-story, wood frame structure with structural slab
foundation and no basement or crawlspace. 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 south end of the Villas South parcel. The natural
terrain prior to development in 2006 sloped down to the east at about 5 to 7% grade. The
subdivision in this area was elevated by filling on the order of 15 to 20 feet above the
original ground surface to create a relatively level building site at the end of River Vista
with a relatively steep slope down to the Robertson Ditch at the southeast side and down
to the 18 111 fairway at the southwest side 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 localized ground subsidence in the Roaring Fork River valley. A sinkhole
opened in the cart storage parking lot east of the Pro Shop located to the north of the
Villas South parcel in January 2005. Other irregular bedrock conditions have been
identified in the affordable housing site located to the west of the Vil1as North ,parcel.
Indications of ground subsidence were not observed in the Villas 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
Joh No. 113 47JK ~tech
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study in the Villas development area could be the result of past subsidence. In our
opinion, the risk of future ground subsidence in the Villas North and 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.
FIELD EXPLORATION
The field exploration for the Villas project was conducted between December 24, 2013
and January 2, 2014 and consisted of drilling ten ex.pl oratory borings. The previous
exploratory Boring 2 was drilled at the front part of the adjacent Lot 258 as 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% 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
relalivc 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.
suesuro~ACE CONDITIONS
A graphic log of the subsurface conditions encountered at Boring 2 is shown on Figure 2.
The subsoiJs encountered, consist of about 19 feet of fill O"{erlying about 22 feet of
alluvial fan deposits consisting of roughly stratified, sandy silt and silty sand and gravel
with cobbles to the drilled depth of 41 feet where practical drilling refusal was
encountered in apparent river gravel alluyiwn. The fill soils arc medium dense and
slightly moist to moist, and the underlying natural alluvial fan soils arc loose to medium
Job No. l 13 471K ~h
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dense and relatively dry to slightly moist. The underlying river gravel alluviwn was
encountered below the alluvial fun deposits in the otl1er borings drilled in the Villas South
parcel.
Laboratory testing perfonncd on samples obtained from the boring included natural
moisture content and density and finer than sand size gradation analyses. Results of
swelJ-consolidation testing performed on a relatively undisturbed drive sample of the
natural sandy silt soils obtained from Boring 1 (Lot 252), presented on Figure 4, indicate
low compressibility under light loading and a low collapse potential (settlement under
constant load) when wetted. The laboratory testing is summarized in Table 1.
ENGINEERING ANALYSIS
The upper 19 feet of soils encountered in the boring located on the adjacent Lot 258,
consist of fill place mainly in 2006 as part of the subdivision development. The field
penetration tests and laboratory tests performed during the study, and review of the field
density tests performed during the fill construction indicate that 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 and their wetted depth. The settlement potential and risk of excessive
building distress can be reduced by compaction of the soils to a certain depth below the
foundation bearing level (as has already been done) and by heavily reinforcing the
foundation to resist differential settlements. The compaction should also extend to below
driveway and utility areas. The compacted soils can consist of the existing structural fill
used to elevate the project site. Foundation levels deeper than 5 feet below the existing
ground surface on this site are not recommended. Relatively deep structural fills will also
have some potential for long tem1 settlement. Proper grading, drainage and compaction
as presented below in the Site Grading and Surface Drainage sections will help 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
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underlying dense, river gravel alluvium and structural floor slabs could also be used to
reduce the 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 at least 15
feet of compacted structural fill in the Villas South parcel. If n deep foundation system is
considered f01: 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 strµctural slab or post-tensioned slab placed on at least 15 feet of
compacted structural fill should be designed for an allowable bearing
pressure of 1,500 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. Settlement of the foundation is estimated to be
about I to 1 Yz inches based on the long term compressibility of the fill.
Additional settlement between about 2 to 3 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 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 turn-down section should have at least 18 inches of soil cover.
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4) The foundation should be constructed in a ''box-like" configuration rather
than with iITegular 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.
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 lateralJy 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 SO 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. TI1e 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
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retaining structure. An undcrdrain should be provided to prevent hydrostatic pressure
buildup behind walls. Site walls with a maximum back slope of 2 horizontal to I vertical
should be designed for an active earth pressure of at least 60 pcf equivalent fluid unit
weight.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density near optimum moisture content. Backfill 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 of300 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
unrestrained vertical movement. Floor slab control joints should be used to reduce
damage due to shrinkage cracking . The requirements for joint spacing and slab
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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 sub grade 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 optimwn moisture content. 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 nmoff. Frozen ground during spring runoff can create a perched
condition. We recommend below-grade construction, such as grade change site retaining
walls, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. An underdrain should not be provided around structural building foundation
slabs and separate slabs-on-grade.
Where installed, 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 I foot below lowest adjacent
finish grade and sloped at a minimum 1 % to a suitable gravity outlet. 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 of2
inches. The drain gravel backfill should be at least 1 Y2 feet deep. An impervious
membrane, such as a 20 mil PVC liner, should be placed beneath the drain gravel in a
trough ·shape and attached to the wa11 with mastic to prevent wetting of the bearing soils.
SITE GRADING
Extensive grading was performed as part of the existing Villas South development.
Additional placement and compaction of the debris fan soils could be needed to elevate
Job No. 113 47 I K
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the site to design grades and reduce the risk of excessive differential settlements and
building distress. In addition, the water and sewer pipe joints should be mechanically
restrained to reduce the risk of joint separation in the event of excessive differential
settlement. Additional structural fill placed below foundation. bearing level should be
compacted to at least 98% of the maximum standard Proctor density within 2 percentage
points of optimum moisture content. Prior to fill placement, the subgrade should be
carefully prepared by removing any vegetation and organic soils and compacting to at
least 95% of the maximum standard Proctor density at near optimum moisture content.
The fill should be benched into slopes that exceed 20% grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or
flatter and protected against erosion by revegetation or other means. This office should
review site grading plans for the project prior to construction.
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 maintain~ 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 content 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 3
incbes in the first 10 feet in paved areas. Drain gravel of retaining walls
should be covered with filter fabric and capped with at least 2 feet of the
Job No. 113 471K ~tech
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on-site soils to reduce surface water infiltration. Surface swa}es in
landscape areas should hav<? a minimum grade of 4%.
4) Roof gutters should be provided with downspouts that discharge at least 5
fee~ 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 foundation walls.
Consideration should be given to use ofxeriscape 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 boring drilled at the location indicated
on Figure I, the proposed type of construction and our experience in the area. Our
services do not include detennining 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-evalunt~on 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
Job No. 113 471K
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presented herein. We recommend on-site observation of excavations and foundation
beating strata and testing of structural fill by a representative of the geotechnical
engineer.
Respectfully Submitted,
Reviewed by:
Daniel E. Hardin, P .E.
SLP/ksw
cc: Silich Construction -John Silich (jill!nra 1siliclH..on-,truction.com)
Silich Construction -Jodi Thimsen (j ndi fri·sil ic hcon strui.:tion.wm)
Silich Construction -Hayden Horsford (hhorsford fCt,silichcunstruction.com)
JobNo.113471K
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APPROXIMATE
tllQH COUNtl\Y IHlilNllRlNO,INC. ~ ........................
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BORING2 ·1
SCALE '--~~~~~~~~~~~~~~~~~~~~~~~~~~~~--J
1· = 20'
113 471K LOCATION OF EXPLORATORY BORINGS Figure 1
0
5
10
15
.....,
Q) ce 20 • s::. a
Q) a
25
30
35
40
113 471K ~
HEPWORTH·PAWLAK G!O°"'CHNICAL
80RING2
ELEV.= 5980'
LOT25B
33/12
53/12
61/12
WC=9.7
DD=122
-200=-57
31/12
11/12
WC =8.7
-200 ... 59
29/12
25/5,20/0
Note: Explanation of symbols is shown on Figure 3.
LOG OF EXPLORATORY BORING
. 0
5
10
15
Qi
20 if
' .r::
Q.
Q)
Q
25
30
35
40
Figure 2
LEGEND:
IXJ FILL; mixed clayey slit, sand and gravel with cobbles, medium dense, slightly moist to moist, constructed main ~y ~ in 2006.
SILT (ML); sandy to very sandy, slightly clayey, gravel layers, stiff to very sti!f, slightly moist, light brown to brown,
slighUy calcareous.
SAND AND GRAVEL (SM-GM): silty, with cobbles, medium dense , slightly moist, mixed brown, subangular rock
fragments.
38/12 -T
NOTES .
Relatively undisturbed drive sample; 2-inch l.D. California liner sample
Drive sample; standard penetration test (SPl), 1 3/8inch1.0. split spoon sample, ASTM D-1586.
Drive sample blow count; indicates that 38 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Cave depth when checked on January 2, 2014.
Practical drilling refusal.
1. The exploratory boring was drilled between December 24, 2013 and January 2, 2014 with 4-inch diameter continuous
flight power auger.
2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan
provided.
3. Elevation of the exploratory boring was obtained by interpolation between 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 the time of drilling or when checked 3 or more days later.
Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD =-Dry Density (pc0
-200 = Percent passing No. 200 sieve
113 471K ~
Hl!'.PWORTH-PAWLAK GE:OTECHNICAL
LEGEND AND NOTES Figure 3
Moisture Content .. 4.6 percent
Dry Density = 105 pct
Sample of: Very Sandy Clayey Sill
From: Boring 1 at 25 Feet (Lot 252)
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APPLIED PRESSURE · ksf
113471K ~ech SWELL-CONSOLIDATION TEST RESULTS Figure 4
HEPWORTI+PAWl.J\K Gl'OT£CHNICAI.
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HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No.113 471K
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY (%) (%) N0.200 LIMIT INDEX STRENGTH BEDROCK lYPE
SIEVE lft) (%l (ncfl !%\ (%l (PSA
1 25 4.6 105 Very Sandy Clayey silt (Lot 252)
2 13 9.7 122 57 Very Sandy Silt and Clay
(Lot 258) with Gravel -Fill
23 8.7 59 Very Sandy Silt and Clay
with Gravel
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