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HEPWORTH-PAWLAK GEOTECHNICAL
I lcpxinrilh-i'a++•l.rl. Gem rLhnILO], Inc.
5020 County Road 154
Glenwood Spring:, C01or, 81601
Phone: 970-945-7985
F.tx. 970.945.8454
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT S-7, ASPEN GLEN
GARFIELD COUNTY, COLORADO
JOB NO. 115 090A
MARCH 27, 2015
PREPARED FOR:
CRAWFORD DESIGN BUILD, LLC
ATTN: SIMON BENTLEY
P.O. BOX 1236
CARBONDALE, COLORADO 81623
(ct1Walton(_coins i'l.nel)
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silvcrthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - I -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - g _
REFERENCES _ 9 _
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4, 5, 6 AND 7 - 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 at
Lot S-7, Aspen Glen, 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 agreement for geotechnical engineering
services to Crawford Design Build LLC dated March 13, 2015. Chen -Northern, Inc.
previously conducted a preliminary geotechnical engineering study for development of
Aspen Glen and geotechnical engineering study for preliminary plat design (Chen -
Northern, 1991 and 1993).
A field exploration program consisting of exploratory pits and 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 Iaboratory 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 one and two story wood frame construction above a
crawlspace with an attached garage. Garage floor will be slab -on -grade. Grading for the
structure is assumed to be relatively minor with cut depths between about 3 to 5 feet. 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.
Joh No, 115 090A
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SITE CONDITIONS
The vacant lot was free of snow cover at the time of our exploration. Vegetation consists
of sparse grass and weeds. The ground surface appears to have been graded during
subdivision development and gently slopes down to the east.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
development. These rocks are a sequence of gypsiferous shale, fine-grained
sandstone/siltstone and limestone with some massive beds of gypsum. There is a
possibility that massive gypsum deposits associated with the Eagle Valley Evaporite
underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause
sinkholes to develop and can produce areas of localized subsidence. During previous
studies in the area by Chen -Northern (1991 and 1993), several broad subsidence areas and
smaller size sinkholes were mapped scattered throughout the Aspen Glen development.
These sinkholes were primarily located east of the Roaring Fork River and appear similar
to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River
valley. The nearest mapped sinkhole is located about '/ mile southeast of Lot S-7.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of
cavities was encountered in the subsurface materials; however, the exploratory borings
were relatively shallow, for foundation design only. Based on our present knowledge of
the subsurface conditions at the site, it cannot be said for certain that sinkholes will not
develop. The risk of future ground subsidence on Lot S-7 throughout the service life of
the proposed residence, in our opinion, is low; however, the owner should be made aware
of the potential for sinkhole development. If further investigation of possible cavities in
the bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project consisted of exploratory pits observed on March 13,
2015 and an exploratory boring drilled on March 18, 2015. The exploratory pit and
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boring locations are shown on Figure 1. The pits were dug with a mini -excavator prior to
our site visit and the boring was advanced with 4 inch diameter continuous flight augers
powered by a truck -mounted CME -45B drill rig. The pits and boring were logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with a hand driven liner in the pits and I3/n inch and 2
inch I.D. spoon samplers in the boring. The samplers were driven into the subsoils at
various depths with blows from a 140 pound hammer falling 30 inches. This test is
similar to the standard penetration test described by ASTM Method D-1586. The
penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values
are shown on the Logs of Exploratory Pits and Boring, 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 at the site are shown on Figure 2.
The subsoils below about 1 foot of topsoil consist of about 20 feet of sandy silt and clay
overlying relatively dense silty sandy gravel with cobbles and boulders. Drilling in the
dense granular soils with auger equipment was difficult due to the cobbles and boulders
and drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the pits and boring included
natural moisture content and density and percent finer than sand size gradation analyses.
Results of swell -consolidation testing performed on relatively undisturbed samples,
presented on Figures 4, 5, 6 and 7, indicate Iow to moderate compressibility under
existing moisture conditions and light loading with a moderate collapse potential when
wetted (settlement under constant load) and high compressibility under additional loading
after welting. The sample tested from Boring 1 at I5 feet showed low expansion potential
when wetted. The laboratory testing is summarized in Table I.
Job No. 115 09(}A
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No free water was encountered in the pits or the boring at the time of drilling and the
subsoils were slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The silt and clay soils appear to possess a collapse or expansion potential when wetted
which could result in movement of footings bearing on the soils if they become wetted.
Surface runoff, landscape irrigation, and utility leakage are possible sources of water
which could cause wetting. An alternative with a Iower risk of settlement would be to
remove and replace a certain depth of silt and clay soils with compacted structural fill.
The lowest risk of settlement would be to place the foundation entirely on the underlying
relatively dense granular soil with a deep foundation system such as helical piers or
micropiles. If a deep foundation down to dense gravel is proposed, we should be
contacted for additional recommendations. The subgrade should be observed for bearing
conditions and further evaluated for settlement potential at the time of construction.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory pits and boring and
the nature of the proposed construction, the building can be founded with spread footings
bearing on compacted structural fill on the natural subsoils with a risk .
The design and construction criteria presented below should be observed for a spread
footing foundation system.
I) Footings placed on at least 3 feet of structural fill which will bear on the
undisturbed natural soils should be designed for an allowable bearing
pressure of I,500 psf. Based on experience, we expect initial settlement of
footings designed and constructed as discussed in this section will be about
I inch or less. There could be additional settlement if the silt and clay
soils become wetted. The movement would be differential and could be
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about'/ to 11/2 inches for a wetted depth on the order of I0 feet below
structural fill bearing Ievel.
2) The footings should have a minimum width of 20 inches for continuous
walls and 2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided
with adequate soil cover above their bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typically used in this area.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14
feet.
5) Compacted structural fill should be a granular material such as 3/., inch road
base (CDOT Class 6) and compacted to at least 98% of standard Proctor
density at a moisture content near optimum. The fill should extend
laterally beyond the footing a distance at least equal to half the depth of fill
below the footing. Structural fill placed to reduce the settlement risk
should be at least 3 feet deep below the footings.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls acting as retaining structures should be designed to resist a lateral earth
pressure corresponding to an equivalent fluid unit weight of at least 50 pcf. All
foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and
equipment. The pressure recommended above assumes 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 system should be provided to prevent hydrostatic
pressure buildup behind walls.
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Backfill should be placed in uniform lifts and compacted to at Ieast 90% of the maximum
standard Proctor density at a moisture content near optimum. 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 foundation wall backfill should be expected, even if the material is
placed correctly, and could result in distress to facilities constructed on the backfill.
Fill placed against the sides of the footings to resist lateral loads should be a compacted to
at least 95% of the maximum standard Proctor density at a moisture content near
optimum.
FLOOR SLABS
The natural soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade
construction. The silt and clay soils possess a settlement potential when wetted which
could result in slab movement and distress if the bearing soils become wetted. The risk of
slab movement can be reduced by removing the silt and clay soils and placing at least 3
feet of compacted structural fill, such as road base, below the slab. To reduce the effects
of some differential movement, floor slabs should be separated from all bearing walls and
columns with expansion joints which allow 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 sand
and gravel, such as road base, should be placed beneath interior slabs -on -grade for
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 a moisture content near optimum. Required fill can
consist of the on-site soils or imported granular material devoid of vegetation, topsoil and
oversized rock.
Jul) No. 1 15 (190A
-7
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
crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an
underdrain system. Shallow crawlspaces (less than 4 feet) should not require an
underdrain.
If 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 1 foot below lowest adjacent finish
grade and sloped at a minimum 1% to a suitable gravity outlet or sump and pump. 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 20 mil PVC should be placed beneath the drain gravel in a
trough shape and attached to the foundation wall with mastic to prevent wetting of the
bearing soils.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and
maintained at all times after the residence has been completed:
1) Inundation of the foundation excavations and underslab areas 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 slab areas and to at least 90% of the maximum standard
Proctor density in landscape areas.
Joh No 115 090A GgEftech
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3) The ground surface surrounding the exterior of the building should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in paved areas.
Free -draining wall backfill should be capped with about 2 feet of the on-
site soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at
least I0 feet from foundation walls. Consideration should be given to use
of xeriscape to reduce the potential for welting 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
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. 115 090A
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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.
Louis E. Eller
Reviewed by:
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REFERENCES
Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated December 20, 1991, Job No. 4 112 92.
Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design,
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated May 28, 1993, Job No. 4 112 92.
Job No. 115 090A
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115 090A
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LOCATION OF EXPLORATORY
PITS AND BORING
Figure 1
u-
0
5
10
15
20
PIT 1
ELEV.= 6068'
N
PIT 2
ELEV 6067'
WC 50
DD 95
51 WC=39
/ DD- 98
Q WC 59 -200= 59
DD 91
-200 79
BORING 1
ELEV.= 6068'
15/12
22/12
r
r
23/12
WC 81
DD 109
2
84/6
0
5
10
15
20
25 25
115 090A
Note: Explanation of symbols is shown on Figure 3.
H
Hepworth—Powlak Geotechnical
LOGS OF EXPLORATORY
PITS ANDBORING
Depth - Feet
Figure 2
LEGEND:
® TOPSOIL; organic sandy silt and clay, soft, slightly moist, dark brown, possible fill.
L SILT AND CLAY (ML -CL); sandy, medium stiff to very stiff, slightly moist, brown.
GRAVEL (GM); with cobbles and boulders, sandy, silty, dense, slightly moist, light brown, rounded rock.
—79
15/12
T
NOTES:
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586.
Drive sample blow count; indicates that 15 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
2" Diameter hand driven liner sample.
Practical drilling refusal.
1. The exploratory pits were observed on March 13, 2015. The exploratory boring was drilled on March 18, 2015 with
4 -inch diameter continuous flight power auger.
2. Locations of exploratory pits and boring were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits and boring were obtained by interpolation between contours shown on the site plan
provided and their relative elevation checked by instrument level.
4. The exploratory pit and 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 pit and boring logs represent the approximate boundaries
between material types and transitions may be gradual.
6. No free water was encountered in the pits or boring at the time of excavating and drilling. Fluctuation in water level
may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
-200 = Percent passing No. 200 sieve
115 090A
Hepworth—Pawkik Geotechnical
LEGEND AND NOTES
Figure 3
Compression °/O
0
1
2
3
4
5
6
7
Moisture Content 5.0 percent
Dry Density - 95 pcf
Sample of: Sandy Silt and Clay
From: Pit 1 at 3 Feet
Compression
_ upon
wetting
0.1
115 090A
1.0
Hepworth—Pawlak Geotechnical
10
APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 4
Compression
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Moisture Content -- 5.9 percent
Dry Density = 91 pct
Sample of: Sandy Silt and Clay
From: Pit 1 at 6 Feet
Compression
upon
��wetting
0.1
115 090A
1.0
Hepworth—Pawlak Geotechnicol
10
APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 5
Compression
0
1
2
3
4
5
6
7
8
9
10
11
Moisture Content 3.9 percent
Dry Density � 98 pcf
Sample of: Sandy Silt and Clay
From: Pit 2 at 4 Feet
Compression
upon
wetting
0.1
1.0
APPLIED PRESSURE - ksf
10
100
115 090A
Hepworth—Pawlak Geotechnical
SWELL -CONSOLIDATION TEST RESULTS
Figure 6
Compression - Expansion %
2
1
0
1
2
Moisture Content 8.1 percent
Dry Density - 109 pct
Sample of: Sandy Silt and Clay
From: Boring 1 at 15 Feet
Expansion
upon
wetting
0.1
115 090A
H
1.0
Hepworth—Pawlak Geotechnlcal
10
APPLIED PRESSURE - ksf
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 7
Job No. 115 090A
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