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HEPWORTH-PAWLAK GEOTECHNICAL
Hep Orth -Pa lak Geotechnical, Inc.
5010 County Road 154
Glcnnr l ~brings, Color:+du $1601
Phone: 970-945-79S$
Fax: 970-945-8454
email: Iirget %Iij'geutech.ccan
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
44157 US HIGHWAY 6
GARFIELD COUNTY, COLORADO
JOB NO. 114 160A
MAY 23, 2014
PREPARED FOR:
ERIC WILLIAMS
981 COUNTY ROAD 245
NEW CASTLE, COLORADO 81647
ecw5226(a)comcast.net
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - I -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS - 3 -
DESIGN RECOMMENDATIONS - 3 -
FOUNDATIONS -3-
FOUNDATION AND RETAINING WALLS - 4 -
FLOORSLABS 5_
UNDERDRAIN SYSTEM 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
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 at
44157 US Highway 6, 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 Eric Williams dated May 5, 2014.
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 a one and two story wood frame structure with a walkout
lower level in most of the building. Ground floors will be both structural above
crawlspace and slab -on -grade. The slab -on -grade portion will incorporate CMU block for
air circulation below the slab. Grading for the structure will be relatively minor with cut
depths up to about 10 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. 114 160A
GecGtech
-2 -
SITE CONDITIONS
The proposed building site was vacant and had been graded relatively flat with a gentle
slope down to the south. The topography in the area is hilly with an open pasture to the
south. Vegetation consisted of grass outside of the stripped area and burnt trees from a
past wildfire. The building corners had been stacked by others at the time of our site
visit.
FIELD EXPLORATION
The field exploration for the project was conducted on May 8, 2014. 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 of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was
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 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 4 to 8 feet of very stiff to hard, sandy clay overlying
relatively dense, silty clayey sand overlying hard to very hard siltstone/sandstone
bedrock. The dense sand appears to be weathered bedrock.
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Laboratory testing performed on samples obtained from the borings included natural
moisture content and density and finer than sand size gradation analyses. Results of
swell -consolidation testing performed on relatively undisturbed drive samples, presented
on Figures 4 and 5, indicate low to moderate compressibility under conditions of loading
and wetting. The clay sample from Boring 1 showed a low expansion potential when
wetted. The sand samples did not show expansion potential when wetted. The laboratory
testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils and
bedrock were slightly moist.
FOUNDATION BEARING CONDITIONS
The subsurface profiles encountered in the borings indicate variable bearing conditions
across the proposed building excavation. The upper clay soils are potentially expansive
and could heave lightly loaded spread footings. The underlying sand soils and bedrock
should support moderately loaded footings with low settlement potential. Spread footings
placed on the natural soils can be used with a risk of movement mainly if the clay soils
are wetted. The expansion potential of the bearing soils should be further evaluated at the
time of construction and clay soils removed as needed to limit the potential heave of the
structure. The sub -excavation of clay soils could include slab -on -grade areas.
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 granular soils.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural granular soils should be
designed for an allowable bearing pressure of 2,500 psf. Based on
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experience, we expect settlement of footings designed and constructed as
discussed in this section will be about 1 inch or less. Expansive clay soils
below footings could cause heave on the order of 1/2 inch and should be
further evaluated at the time of construction.
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
designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) Any topsoil and loose disturbed soils should be removed and the footing
bearing level extended down to the relatively dense natural granular soils.
Expansive clay soils may also need to be removed. 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. 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.
Job No. 114 160A
Ge`'cPitech
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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. 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 0.40. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 350 pd. 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 soils can be used to support lightly loaded slab -on -grade construction with a
risk of movement from wetting of clay soils. The expansion potential of the soils exposed
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at subgrade level should be evaluated at the time of excavation for possible removal of
the expansive clay soils. 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 a non -expansive granular soil
compacted to at least 95% of maximum standard Proctor density at a moisture content
near optimum. Required fill can consist of the on-site 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 bedrock is shallow 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 I% 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 11/2 feet deep.
Job No. 114 160A
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SURFACE DRAINAGE
Providing proper grading and drainage around the building will be critical to the
satisfactory performance of the foundation and floor slabs. 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 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.
Job No. 114 I60A
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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
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
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
SLP/ksw
Job No. 114 I60A
GaGtech
APPROXIMATE SCALE
1" = 10'
DECK
BORING 2
•
PROPOSED
RESIDENCE
44157 HIGHWAY 6
DECK
BORING 1
•
114 153A
Ge~Beech
Hepworth—Pawlak Geotechnical
LOCATION OF EXPLORATORY BORINGS
Figure 1
0
5
10
15
BORING 1 BORING 2
-7
r— 29/12
WC=9.7
DD=103
43/12
C• •:
,; ,r— 24/12
WC=4.1
D00=43
-
-200=43
•
108/12
4
44
24/12
40/12
WC=10.9
DD=107
75/12
31/12
o
5
10
15
20 20
Note: Explanation of symbols is shown on Figure 3.
Depth - Feet
114 153A
Ge&tech
Hepworth–Pawlok Geotechnical
LOGS OF EXPLORATORY BORINGS
Figure 2
LEGEND:
-7
L
CLAY (CL); silty, sandy, very stiff to hard, slightly moist, brown to light brown, calcareous, low plasticity.
SAND (SM -SC); silty, clayey, medium dense to dense, slightly moist, brown.
SILTSTONE-SANDSTONE BEDROCK; hard to very hard, slightly moist, mixed brown.
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
29/12 Drive sample blow count; indicates that 29 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 8, 2014 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 not measured and the logs of exploratory borings are drawn to depth.
4. The exploratory boring locations 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
114 153A
Ge�itech
HEPWORTH.PAWLAK GEOTECHNICAL
LEGEND AND NOTES
Figure 3
Compression - Expansion %
Compression %
1
0
1
2
0
1
2
Moisture Content = 9.7 percent
Dry Density = 103 pcf
Sample of: Sandy Clay
From: Boring 1 at 2 Feet
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 4.1 percent
Dry Density = 117 pcf
Sample of: Very Silty Clayey Sand
From: Boring 1 at 10 Feet
No movement
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
114 153A
Gmech
Hepworth—Pawlak Geotechnical
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
Compression %
N
Moisture Content = 10.9 percent
Dry Density = 107 pcf
Sample of: Silty Clayey Sand
From: Boring 2 at 5 Feet
No movement
upon
wetting
0.1 to 10 100
APPLIED PRESSURE - ksf
114 153A
GH Ch
Hepworth—Pawlak Geotechnical
SWELL-CONSOLIDATION TEST RESULTS
Figure 5
Job No. 114 153A
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Very Silty Clayey Sand II
Silty Clayey Sand II
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COMPRESSIVE
STRENGTH
(PSF)
ATTERB ERG LIMITS
u x
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g z A
LIQUID LIMIT
(%)
PERCENT
PASSING NO.
200 SIEVE
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GRAVEL
(%)
NATURAL
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SAMPLE LOCATION
DEPTH
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