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Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
hpgeo@hpgeotech.com
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PRO OSED RESIDENCE
LOT 2, BLOCK WILLOW CREEK VILLAGE
88 GREEN MESA PLACE
BATTLEMENT MESA, COLORADO
JOB NO. 100 465
JUNE 27, 2000
PREPARED FOR:
TRENT STEVENS
64 LITTLE ECHO DRIVE
PARACHUTE, COLORADO 81615
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
June 27, 2000
Trent Stevens
64 Little Echo Drive
Parachute, Colorado 81615 Job No. 100 465
1
Subject: Report Transmittal, Subso;Study for Foundation Design, Proposed
Residence, Lot 2, Blockk, Willow Creek Village, 88 Green Mesa Place,
Battlement Mesa, Colorado
Dear Mr. Stevens:
As requested, we have conducted a subsoil study for the proposed residence at the
subject site.
Subsurface conditions encountered in the exploratory boring drilled in the proposed
building area, below about 1 foot of sandy silt and clay fill, consist of relatively stiff
sandy silt.and clay soils. Relatively dense clayey basalt gravel with cobbles was
encountered in the boring at a depth of 101/2 feet. Groundwater was not encountered in
the boring at the time of drilling.
The proposed residence can be founded on spread footings placed on the natural
subsoils and designed for an allowable bearing pressure of 1,200 psf.
The report which follows describes our exploration, summarizes our findings, and
presents our recommendations. It is important that we provide consultation during
design, and field services during construction to review and monitor the implementation
of the geotechnical recommendations.
If you have any questions regarding this report, please contact us.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Trevor L. Knell
Rev. by: DEH
TLK/ksm
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................... 1
PROPOSED CONSTRUCTION ..................... ............ I
SITE CONDITIONS .......................................... 2
FIELD EXPLORATION ....................................... 2
SUBSURFACE CONDITIONS ................................... 2
FOUNDATION BEARING CONDITIONS ................... ... 3
DESIGN RECOMMENDATIONS .............................. .. 3
FOUNDATIONS..................................... .. 3
FOUNDATION AND RETAINING WALLS ..................... 4
FLOOR SLABS ........................................ 5
UNDERDRAIN SYSTEM .................................. 6
SURFACE DRAINAGE ................................ .. 7
LIMITATIONS ................................. ............ 7
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE I.- SUMMARY OF LABORATORY TEST RESULTS
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be
located on Lot 2, Willow Creek Village, 88 Green Mesa Place, Battlement Mesa,
Colorado. The project site is shown on Fig. 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 Trent Stevens
dated May 31, 2000.
A field exploration program consisting of an exploratory boring was conducted
to obtain information on 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 subsoil conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a one story wood frame structure over a walkout
basement with an attached garage. Basement and garage floors will be slab -on -grade.
Grading for the structure is assumed to be relatively minor with cut depths between
about 3 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.
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SITE CONDITIONS
The site was vacant at the time of our field work and is located near the south
end of Green Mesa Place. The ground surface in the building area is flat and
moderately sloping down to the northwest. There is up to 1 foot of fill across the lot
due to past overlot grading during subdivision development. Vegetation consists of
grass and weeds.
FIELD EXPLORATION
The field exploration for the project was conducted on June 13, 2000. One
exploratory boring was drilled at the location shown on Fig. 1 to evaluate the
subsurface conditions. The boring was advanced with 4 inch diameter continuous flight
augers powered by a truck -mounted Longyear BK-51HD drill rig. The boring was
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. Depths at which the samples were taken
and the penetration resistance values are shown on the Log of Exploratory Boring,
Fig. 2. The samples were returned to our laboratory for review by the project engineer
and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on
Fig. 2. The subsoils consist of about 1 foot of sandy silt and clay fill overlying
relatively stiff, sandy silt and clay soils. Relatively dense clayey basalt gravel with
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cobbles was encountered in the boring at a depth 101/2 feet. Drilling in the dense basalt
gravel with auger equipment was difficult due to the cobbles and boulders and drilling
refusal was encountered in the deposit at a depth of 111/2 feet. .
Laboratory testing performed on samples obtained from the boring included
natural moisture content and density and percent finer than No. 200 sieve gradation
analyses. Results of consolidation testing performed on a relatively undisturbed drive
sample, presented on Fig. 4, indicate low compressibility under conditions of a constant
light surcharge and low collapse potential under wetting. The laboratory testing is
summarized in Table I.
No free water was encountered in the boring at the time of drilling and the
subsoils were slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The natural -soils should be suitable for support of lightly loaded spread footings.
The upper silt soils can possess a collapse potential when wetted. Care should be taken
to limit the possibility of wetting of these soils both during and after construction of the
residence.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsoil conditions encountered in the exploratory boring and the
nature of the proposed construction, we recommend the building be founded with
spread footings bearing on the natural soils.
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 soil bearing pressure of 1,200 psf. Based on experience,
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we expect settlement of footings designed and constructed as discussed in
this section will be about 1 inch or less.
2) The footings should have a minimum width of 18 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) All existing fill, topsoil and any loose or disturbed soils should be
removed and the footing bearing level extended down to firm natural
soils. If water seepage is encountered, the footing areas should be
dewatered before concrete placement.
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 50 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 40 pcf for backfill consisting
of the on-site soils.
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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 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 betaken 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 0.35.
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 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. To reduce the effects of some differential
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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 gravel base should be
placed beneath the basement level and garage slab 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 devoid of vegetation or topsoil.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our
experience in mountainous areas that local perched groundwater may 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 basement areas, be protected from wetting and hydrostatic pressure
buildup by an underdrain system.
The drains should consist of drainpipe placed m 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 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.
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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 6 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 10 feet from foundation walls. Consideration should be given to
use of xeriscape to reduce the potential for wetting of soils below the
foundation caused by irrigation.
LD41TATIONS
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 expressed 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 Fig. 1, the proposed type of construction and our experience in the
area. Our findings include interpolation and extrapolation of the subsurface conditions
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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-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.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Trevor L. Knell
Reviewed by:
Daniel E. Hardin, P.E.
TLK/ksm
cc: Kurtz & Associates - Attn: Brian Kurtz
H -P GEOTECH
100 465 EPWORTH - PAWLAK LOCATION OF EXPLORATORY BORING Fig. 1
k GEOTECHNICAL, INC_
I
W
105
100
95
9c
BORING 1
ELEV. = 102.6'
22/12
WD --4.5
DD=109
—200=88
16/6.27/5
We=7.7
DD -105
—200=84
NOTE: Explanation of symbols is shown on Fig. 3.
105
100
95
100 465 HEPWORTH — PAWLAK LOG OF EXPLORATORY BORING Fig. 2
GEOTECHNICAL, INC.
W
LEGEND:ful
I
FILL; sandy silt and clay, firm, slightly moist, reddish brown.
- 1.
SILT (ML); slightly sandy to sandy, very stiff, slightly moist, light brown.
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CLAY (CL); sandy, slightly gravelly, very stiff to hard, moist, reddish brown.
GRAVEL (GC); sandy, clayey, with cobbles, possible boulders, dense, moist. reddish brown, basalt rock.
Relatively undisturbed drive sample; 2—inch I.D. California liner sample.
22/12 Drive sample blow count; indicates that 22 blows of a 140—pound hammer falling 30 inches were
required to drive the California sampler 12 inches.
TPractical drilling refusal in basalt boulders.
NOTES:
1. The exploratory boring was drilled on June 13, 2000 with a 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 measured by instrument level and refers to the Bench Mark
shown on Fig. 1.
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.
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.
HEPWORTH — PAWLAK
GEOTECHNICAL, INC.
LEGEND AND NOTES
Moisture Content = 4.5 percent
Dry Density = 109 pcf
Sample of: Slightly Sandy Silt
From: Boring 1 at 3 Feet
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Compression
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APPLIED PRESSURE — ksf
100 465 HEPWORTH — PAWLAK SWELL—CONSOLIDATION TEST RESULTS Fig_ 4
GEOTECHNICAL, INC.
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