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HEPWORTH - PAWLAK GEOTECHNICAL
62:3:3FiF1d244 p - 4
Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email: hpgeo@hpgeotech.com
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 45, 4 -MILE RANCH
MAROON DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 106 0745
SEPTEMBER 29, 2006
PREPARED FOR:
BARBARA HERNANDEZ
17342 WEST KONG CANYON DRIVE
SURPRISE, ARIZONA 85387
Parker 303-841-7119 • Colorado Springs 719-633-5367 • Silverthome 970-468-1989
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION -1 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS., - 5 -
FLOORSLABS -6-
UNDERDRAIN SYSTEM - 7 -
SITE GRADING - 7 -
SURFACE DRAINAGE - g -
LIMITATIONS - g -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
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This report presents the results of a subsoil study for a proposed residence to be located
on Lot 45, 4 -Mile Ranch, Maroon 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 Barbara Hernandez dated August 18, 2006.
A field exploration program consisting of exploratory borings 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 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
Design plans for the proposed residence had not been developed at the time of our study.
We assume the residence will be a 2 story wood frame structure with a walkout lower
level and located in the building envelope shown on Figure 1. The garage would probably
be located at the main level closest to the road. Ground floors could be slab -on -grade or
structural above crawlspace. Grading for the srructure is assumed to be relatively minor
. with cut depths between about 4 to 10 feet. We assume relatively light foundation
loadings, typical of the proposed type of construction.
When building location, grading and loading information have been developed, we
should be notified to re-evaluate the recommendations presented in this report.
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-2-
SITE CONDITIONS
The lot was vacant and covered with grass and weeds with sage brush in the middle to
westem part. The front part of the lot appears to have been disturbed with possible
shallow fill from the subdivision development. The ground surface slope is moderate
down to the west at about 15% with an elevation difference of about 10 feet across the
building envelope. Cobbles and small boulders are scattered on the ground surface.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the 4 -Mile Ranch
subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. 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. Sinkholes are common to the
nearby Roaring River valley generally further south of Glenwood Springs.
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 45 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 was conducted on August 24, 2006. 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
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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 X.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 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, below about 2 feet of topsoil, typically consist of stiff to hard, sandy silty
clay overlying dense, silty sandy gravel and cobbles with boulders below a depth of about
13 feet. At Boring 1, about 5 feet of the gravel and cobbles was encountered above the
clay soils. 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 at Boring 2.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and gradation analyses. Results of swell -consolidation testing
performed on relatively undisturbed drive samples, presented on Figure 4, indicate low to
moderate compressibility under conditions of loading and wetting. The sample from
Boring 2 at 5 feet showed a low swell potential when wetted. Results of gradation
analyses performed on small diameter drive samples (minus 1%2 inch fraction) of the
coarse granular soils from Boring 1 are shown on Figure 5. The Iaboratory test results are
summarized in Table 1.
No free water was encountered in the borings at the time of drilling or when checked
several days later. The subsoils were slightly moist.
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FOUNDATION BEARING CONDITIONS
The site is suitable for residential construction with precautions to reduce potential
movements mainly if the bearing soils become wetted. The subsurface conditions are
somewhat variable but should support lightly loaded spread footings with low to
moderate bearing capacity and relatively low settlement potentia]. The clay soils could
possess an expansion potential that should be further evaluated at the time of
construction.
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 spread
footings bearing on the natural soils below the topsoil.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 2,000 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section
will be about 1 inch or less. There could be some additional differential
movement if the bearing soils become wetted.
2) The footings should have a minimum width of 16 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 12
feet. Foundation walls acting as retaining structures should also be
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designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) Any existing fill, the topsoil and loose or disturbed soils should be
removed and the footing bearing level extended down to the relatively
dense natural granular soils. The exposed soils in footing area should then
be moistened 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
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. The backfill materials should contain as much of the on-site granular soil
as practical excluding rocks larger than about 6 inches.
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 to slightly above 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
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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 0.35. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 325 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 a
moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, generally appear suitable to support lightly
loaded slab -on -grade construction. Some of the clay soils are expansive when wetted and
the potential impacts of expansive soils on slabs should be further evaluated at the time of
construction. 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.
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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
should consist of predominantly granular 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 at bearing level 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, crawlspace and basement areas, be protected from
wetting and hydrostatic pressure buildup by an underdrain system.
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 1Y2 feet deep.
SITE GRADING
The risk of construction -induced slope instability at the site appears low provided cut and
fill depths are limited. We assume the cut depths for the basement level will not exceed
one level, about 10 feet. Fills should be limited to about 8 to 10 feet deep. 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 slopes that exceed
20% grade. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1
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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.
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.
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 5 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.
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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
concemed 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
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.
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APPROXIMATE SCALE
1" =80'
BENCH MARK: BACK OF CURB;
0 ELEV. = 100.0', ASSUMED.
LOT 44
1
4,0M-17116-11
1 BUILDING 1
1 ENVELOPE 1
•1
L _BORING 2J
LOT 45
OPEN SPACE
EASEMENT
i
1
1
1
LOT 46
HEPWORnN'AW AK GEDTECHNICA
LOCATION OF EXPLORATORY BORINGS l Figure 1
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Elevation - Feet
90
85
80
75
70
65
BORING 1
ELEV.= 88.3'
24/6,55/5
WC=3.5
46/12.+4=52
200=10
13/12
WC=13.4
DD- 114
-200=86
40/6,40/3
BORING 2
ELEV.= 81.2'
r 29/12
7 39/12
WC=10.1
DD=127
25/12
WC=10.4
DD=124
-200=82
Note: Explanation of symbols is shown on Figure 3.
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90
85
80
75
70
65
Elevation - Feet
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LOGS OF EXPLORATORY BORINGS
Figure 2
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LEGEND:
® TOPSOIL; organic sandy silt and clay, gravelly at Boring 1, brown.
2
29/12
T
NOTES:
CLAY (CL); silty, sandy, stiff to hard, slightly moist, mixed brown and Tight brown, calcareous at Boring 2, low
plasticity.
GRAVEL AND COBBLES (GM); slightly silty to silty, sandy, scattered boulders, dense, slightly moist, mixed brown
and gray, some basalt rocks.
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 29 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Practical drilling refusal.
1. Exploratory borings were drilled on August 24, 2006 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 instrument 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 or when checked on September 29, 2006.
Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
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LEGEND AND NOTES
Figure 3
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Compression %
Compression - Expansion °,6
1
0
1
2
0
1
2
3
4
Moisture Content = 13.4 percent
Dry Density = 114 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 10 Feet
No movement
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
0.1
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1.0 10
APPLIED PRESSURE - ksf
100
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FIEPwOR H•PAWLAK GEOTECHNICAL
1 SWELL -CONSOLIDATION TEST RESULTS
Figure 4
Moisture Content = 10.1 percent
Dry Density = 127 pcf
Sample of: Sandy Clay
From: Boring 2 at 5 Feet
:r..
Expansion
upon
wetting
0.1
106 0745 I
1.0 10
APPLIED PRESSURE - ksf
100
G
FIEPwOR H•PAWLAK GEOTECHNICAL
1 SWELL -CONSOLIDATION TEST RESULTS
Figure 4
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IHYDROMETER ANALYSIS SIEVE ANALYSIS
24 R. 7 HR TIME READINGS U.S. STANDARD SERIES
1 CLEAR SQUARE OPENINGS 1
0 45 MIN. 15 MIN. 60MIN19MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8" 3/4" 1 1/2" 3' 5"6" 8' 100
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40
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.001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 2.36
DIAMETER OF PARTICLES IN MIWMETERS
4.75 9.5 19.0 37.5 76.2 152 203
12.5 127
CLAY TO .T
GRAVEL 52 %
LIQUID UMfT %
SAND WAYS.
flrE I MEDIUM ICOARSE I FINE 1 COARSE
1 COBBLES
SAND 38 % SILT AND CLAY 10 %
PLASTICITY INDEX' %
SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 1 at 3 and 5 Feet Combined
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GRADATION TEST RESULTS
Figure 5
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