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HEPWORTH-PAWLAK GEOTECHNICAL
Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970.945-8454
email: hpgeoghpgeotech.com
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 22, SPRINGRIDGE RESERVE
HIDDEN VALLEY DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 113 126A
MAY 10, 2013
PREPARED FOR:
TIM PRENGER
8315 EAST JAMISON CIRCLE NORTH
CENTENNIAL, COLORADO 80112
Parker 303-841-7119 • Colorado Springs 719-633-5562 0 Silverthome 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY........................................................................ - 1 -
PROPOSED CONSTRUCTION................................................................................. - 1 -
SITECONDITIONS................................................................................................... - 2 -
FIELD-
FIELDEXPLORATION............................................................................................ - 2 -
SUBSURFACE
-
SUBSURFACE CONDITIONS.................................................................................. - 2 -
FOUNDATION
-
FOUNDATION BEARING CONDITIONS............................................................... - 3 -
DESIGN
-
DESIGN RECOMMENDATIONS............................................................................. - 3 -
FOUNDATIONS.................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS......................................................... - 4 -
FLOORSLABS...................................................................................................... - 6 -
UNDERDRAINSYSTEM...................................................................................... - 6 -
SITEGRADING.................................................................................................... - 7 -
SURFACEDRAINAGE......................................................................................... - 7 -
LIMITATIONS
-
LIMITATIONS.......................................................................................................... - 8 -
FIGURE
-
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 proposed residence to be located
on Lot 22, Springridge Reserve, Hidden Valley Drive, 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 Tim Prenger dated May 1,
2013. We previously conducted a preliminary geotechnical study for the subdivision
development and presented our findings in a report dated June 22, 2004, Job No. 101 126.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock 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 single story above a basement will be an attached garage
located as shown on Figure 1. Ground floor will be slab -on -grade. Grading for the
structure is assumed to be relatively minor with cut depths between about 2 to 8 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.
Job No. 113 126A Go &.L -Ch
-2 -
SITE CONDITIONS
The site is located on a colluvial slope below a bedrock knoll of Maroon Sandstone. The
ground surface is strongly sloping down to the east in the building area with about 6 feet
of elevation difference. The slope becomes moderate to steep in the uphill, west part of
the lot. Vegetation consists of grass.and weeds with scattered sandstone fragments on the
ground surface in the building area.
FIELD EXPLORATION
The field exploration for the project was conducted on May 2, 2013. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface
conditions. 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 ofHepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was
driven into the subsurface materials 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 and hardness of the bedrock. Depths at
which the samples were taken and the penetration resistance values are shown on the
Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for
review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils consist of about %2 to 1 foot of topsoil overlying 7%2 to 11'/z feet of stiff to
hard silty sandy clay above hard to very hard siltstone bedrock at depths of 8 and 12'/2
feet.
Job No. 113 126A HPtech
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Laboratory testing performed on samples obtained from the borings included natural
moisture content and density, finer than sand size gradation analyses and unconfined
compressive strength. Results of swell -consolidation testing performed on relatively
undisturbed drive samples of the upper clay soils, presented on Figures 4 and 5, indicate
low to moderate compressibility under conditions of loading and wetting with a low
collapse potential (settlement under constant load) when wetted. The unconfined
compressive strength of the moist clay sample showed a stiff to very stiff consistency.
The laboratory test results are summarized in Table 1.
No free water was encountered in the borings at the time of drilling or when Boring 2 was
checked the day following drilling and the subsoils were slightly moist at Boring 1 and
moist at Boring 2.
FOUNDATION BEARING CONDITIONS
The subsurface conditions are variable with respect to soil types, depths, consistency and
compressibility. The clay soils were relatively dry in the uphill boring (Boring 1) and
about 8 feet deep, and moist in the downhill boring (Boring 2) and about 12'/� feet deep.
The soils can support lightly loaded spread footings with a low bearing pressure and
moderate settlement potential. The underlying bedrock is hard and could support heavily
loaded footings with low settlement potential.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the, subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, spread footings bearing on the natural clay soils can
be used for the building support with low to moderate settlement risk. Placing footings
on the underlying bedrock would have low settlement, risk.
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The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural clay soils should be designed
for an allowable bearing pressure of 1,500 psf. Based on experience, we
expect initial settlement of footings designed and constructed as discussed
in this section will be about 1 inch or less. Additional settlement of about
1 inch could occur if the clay bearing soils are wetted. Footings placed on
the underlying siltstone bedrock can be designed for an allowable soil
bearing pressure of 3,000 psf with relatively low risk of settlement.
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. Foundation walls acting as retaining structures should also be
designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) The topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the relatively firm natural soils.
The exposed soils in footing area should then be moistened to near
optimum and compacted.
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 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
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earth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf
for backfill consisting of the on-site soils. Cantilevered retaining structures which are
separate from the residence 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 45 pcf for backfill consisting of
the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and
equipment. The pressures recommended above assume drained conditions behind the
walls and a horizontal backfill surface. The buildup of water behind a wall or an upward
sloping backfill surface will increase the lateral pressure imposed on a foundation wall or
retaining structure. An underdrain should be provided to prevent hydrostatic pressure
buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at 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 foundation 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 foundation or 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 0.30. 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
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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 a
moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -
on -grade construction. There could be some settlement below slabs if the bearing soils
are wetted. 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 free -draining gravel should be placed beneath
basement level slabs to facilitate drainage. This material should consist of minus 2 inch
aggregate with at least 50% retained on the No. 4 sieve and less than 2% 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 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 and where there are clay soils that local perched groundwater can
develop during times of heavy precipitation or seasonal runoff. Frozen ground during
spring runoff can also create a perched condition. We recommend below -grade
construction, such as retaining walls, crawlspace and basement areas, be protected from
wetting and hydrostatic pressure buildup by an underdrain system.
Job No. 113 126A GE Fhech
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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. 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 %2 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 foundation wall with mastic to prevent wetting of the bearing soils.
SITE GRADING
The risk of construction -induced slope instability at the site appears low provided the
building is located below the steep hillside as planned and cut and fill depths are limited.
We assume the excavation and backfill depths for the basement level will not exceed one
level, about 10 feet. Embankment fills should be compacted to at least 95% of the
maximum standard Proctor density near optimum moisture content. Prior to fill
placement, the subgrade should be carefully prepared by removing all vegetation and
topsoil and compacting to at least 90% of the maximum standard Proctor density. The fill
should be benched into the portions of the hillside exceeding 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.
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
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pavement and 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. 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 at least 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 10 feet from foundation walls. 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
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.
Job No. 113 126A G8'65teC
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 GEOTXJr.�I ICAL. INC.
LAI
Steven L. Pawlak, P.E. UL- 15222
Rev. by: :P�F
OP
Daniel E. Hardin, P.E.
SLP/ljg
cc: Jim Mason Omason�n rofnet)
Job No. 113 126A G89tech
---- KIMATE SC
1"=40'
113 126A
LOT 21
LOT 22
LOT 20
BORING 1
•
PROPOSED
RESIDENCE
-BeMNG 2
BENCH MARK: GROUND AT ENVELOPE
CORNER; ELEV. = 100.0', ASSUMED.
HIDDENVA�LEY DRIVE
LOCATION OF EXPLORATORY BORINGS I Figure 1
BORING 1
BORING 2
ELEV.= 105.5'
ELEV.= 100.5'
110
110
105
105
12/12
WC=7.4
DD=94
ASSUMED LOWER FLOOR LEVEL
100
65/12
100
WC=4.8
a)
DD=98
G-
-200=75
tl
C
O
0
8/12
g5
80/6
WC=14.6
95
W
DD=111
W
-200=66
UC=3,700
21/12
90
50/1
WC=16.6
ZI
DD=110
90
60/4
85
85
Note: Explanation of symbols is shown on Figure 3.
H
113
126A
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LOGS OF EXPLORATORY BORINGS
Figure
2
He worth—Pawlak Geotechnical
LEGEND:
® TOPSOIL; organic silt and clay, firm, moist, dark red.
CLAY (CL); silty, sandy, scattered sandstone/siltstone fragments, slightly moist and very stiff to hard at Boring 1,
moist and stiff at Boring 2, red, low plasticity.
SILTSTONE BEDROCK; hard to very hard with depth, slightly moist, red. Maroon Formation.
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
65/12 Drive sample blow count; indicates that 65 blows of a 140 pound hammer falling 30 inches were
required to drive the California sampler 12 inches.
NOTES:
1. Exploratory borings were drilled on May 2, 2013 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 measured by hand level and refer to the Bench Mark shown on Figure 1.
4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the
method used.
5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
-200 = Percent passing No. 200 sieve
UC = Unconfined Compressive Strength (psf)
113 126A
LEGEND AND NOTES I Figure 3
Moisture Content = 7.4 percent
Dry Density = 94 pcf
Sample of: Silty Sandy Clay
From: Boring 1 at 1 .1 Feet
0
1
Compression
upon
2
wetting
c
0
.N
2
3
a
E
0
U
4
5
6
7
8
9
0.1 1.0 10 100
APPLIED PRESSURE - ksf
N
113 126A
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SWELL -CONSOLIDATION TEST RESULTS
Figure 4
He worth—Pawlak Geotechnical
Moisture Content = 16.6 percent
Dry Density = 110 pcf
Sample of: Silty Sandy Clay
From: Boring 2 at 9 Feet
0
0
0
1
0
N
a)
C
E
2
O
Q
No movement
upon
g
wetting
0.1 1.0 10 100
APPLIED PRESSURE - ksf
113 126A
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SWELL -CONSOLIDATION TEST RESULTS
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Figure 5
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