HomeMy WebLinkAboutSoils Report 02.12.2015ech
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 6, PINYON MESA
SAGE MEADOW ROAD AND PINYON MESA DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 115 003A
FEBRUARY 12, 2015
PREPARED FOR:
JORDAN ARNOLD
292 RED BLUFF VISTA
GLENWOOD SPRINGS, COLORADO 81601
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Paikct' 303.1841-7119 ® ('rril,nid4.1Spting' 719-633-5)62 0 Silvealtiltnr 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............... ....... ...... ......... ......a........,........ ..... ,.....-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....................................................,: 6 —
FLOOR SLABS ..........................,,,,,....: 7 —
UNDERDRAIN SYSTEM . — 8 —
SURFACE DRAINAGE. — 8 —
LIMITATIONS — 9 —
FIGURE 1 — LOCATION OF EXPLORATORY BORING
FIGURE 2 — LOG OF EXPLORATORY BORING
FIGURE 3 - LEGEND AND NOTES
FIGURES 4, 5 AND 6 — 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 6, Pinyon Mesa, Sage Meadow Road and Pinyon Mesa 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 Jordan Arnold,
dated January 8, 2015.
An exploratory boring was drilled 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
The proposed residence design was preliminary at the time of our study and will generally
be a two story wood frame structure over a basement or crawlspace with an attached
garage at the main level. The basement and garage floors will be slab -on -grade. Grading
for the structure is assumed to be relatively minor with cut depths between about 5 to I2
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 exploration and covered with about 6 foot of
snow. The site slopes strongly down to the southwest with on the order of a 5 to 6 feet of
elevation difference across the assumed building area. Vegetation consists of, grass and
weeds with scattered sage brush.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa
development. 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. During previous
work in the area, sinkholes have been observed scattered throughout the lower Roaring
Fork River valley.
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 boring
was 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 6 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 January 14, 2015. One
exploratory boring was drilled at the location shown on Figure 1 to evaluate the
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subsurface conditions. The boring was advanced with 4 inch diameter continuous flight
augers powered by a truck -mounted CME -45B 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 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 or hardness of the bedrock. Depths at
which the samples were taken and the penetration resistance values are shown on the Log
of Exploratory Boring, Figure 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 Figure 2.
The subsoils encountered, below a thin root zone and topsoil, consist of about 40 feet of
stiff, slightly calcareous sandy silt and clay with scattered gravel interlayered with very
stiff, sandy clay with gravel down to the drilled depth of 41 feet.
Laboratory testing performed on samples obtained from the boring 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, 5 and 6, indicate low compressibility under light loading and existing low
moisture content, low to moderate collapse potential (settlement under constant load) in
the upper silt and clay soils and low expansion potential in the interlayered very stiff clay
soils when wetted, and moderate to high compressibility under increased loading after
wetting. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils and
bedrock were slightly moist.
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FOUNDATION BEARING CONDITIONS
The sandy silt and clay soils encountered at typical shallow foundation depth tend to
settle when they become wetted. The lower compressibility potential of the interlayered
very stiff, sandy clay soils may also impact a shallow foundation if deep wetting were to
occur but the risk appears low. A shallow foundation placed on the silt and clay soils will
have a high risk of settlement if the soils become wetted and care should be taken in the
surface and subsurface drainage around the house to prevent the soils from becoming wet.
It will be critical to the long term performance of the structure that the recommendations
for surface drainage and subsurface drainage contained in this report be followed. The
amount of settlement, if the bearing soils become wet, will mainly be related to the depth
and extent of subsurface wetting. We expect that initial settlements will be less than 1
inch. If wetting of the shallow soils occurs, additional settlements of 2 to 3 inches could
occur. Settlement in the event of subsurface wetting will likely cause building distress
and mitigation methods such as deep compaction, a deep foundation (such as piles or
piers extending down roughly 40 feet below existing ground surface) or a heavily
reinforced mat foundation, on the order of 2 feet thick, and designed by the structural
engineer should be used to support the proposed house. If a deep foundation or mat
foundation is desired, we should be contacted to provide further design recommendations.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction, the building can be founded with spread footings
bearing on compacted structural fill with a risk of settlement, mainly if the bearing soils
become wetted, and provided the risk is acceptable to the owner. Control of surface and
subsurface runoff will be critical to the long-term performance of a shallow spread
footing foundation system. The garage footing areas should be sub -excavated down
about 8 to 10 feet below existing ground surface and the excavated soil replaced
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compacted back to design bearing level but to a depth of at least 6 feet below footing
bearing level. Basement level footings should be placed on at least 3 feet of compacted
fill.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on a minimum 6 feet of compacted structural fill for the
garage and at least 3 feet of compacted structural fill for the basement
level of the residence should be designed for an allowable bearing pressure
of 1,200 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 settlements of about 1 to 1.4 inches could occur if the
silt and clay soils below the bearing level become wetted. A V3 increase in
the allowable bearing pressure can be taken for toe pressure of
eccentrically loaded footings.
2) The footings should have a minimum width of 24 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 heavily reinforced top and bottom
to span Iocal anomalies such as by assuming an unsupported length of at
Least 14 feet. The foundation should be configured in a "box like" shape to
help resist differential movements. 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 below the
building area. The exposed soils in footing areas after sub -excavation to
design grades should then be moistened and compacted. Structural fill
should consist of low permeable soil (such as the on-site sandy silt and
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clay soils) compacted to at least 98% standard Proctor density within 2%
of optimum moisture content. The structural fill should extend laterally
beyond the footing edges equal to about h the fill depth below the footing.
6) A representative of the geotechnical engineer should evaluate the
structural fill as it is placed for compaction and 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 fine-grained 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 fine-grained 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 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
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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, can be used to support lightly loaded slab -
on -grade construction with settlement risk similar to that described above for foundations
in the event of wetting of the subgrade 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 relatively well graded sand and
gravel such as road base should be placed beneath interior slabs to limit capillary
moisture rise. 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.
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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 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 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
basement areas, be protected from wetting and hydrostatic pressure buildup by an
underdrain system. An underdrain should not be placed around shallow footing depth
structures such as the garage area and crawlspace, if provided.
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 an interior sump of solid casing. 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 l % 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.
SURFACE DRAINAGE
It will be critical to the building performance to keep the bearing soils dry. 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.
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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 covered with filter fabric and 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. Natural vegetation lined drainage swales should have a minimum
slope of 3%.
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.
LIMI'T'ATIONS
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 boring drilled at the location 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 boring and variations in the subsurface conditions
may not become evident until excavation is performed. If conditions encountered during
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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 GEOTE HNICAL, INC.
Steven L. Pawlak, P.E.
Reviewed by;
Daniel E, Hardin, P.E.
SLPlksv
Job No. 115 003A
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LOT 6
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APPROXIMATE SCALE
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LOT 7
SAGE MEADOW ROAD
115 003A
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Hepworth—Pawlak eohbchnlcct
LOCATION OF EXPLORATORY BORING
Figure 1
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15
20
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BORING 1
ELEV.= 105*
NOTE: Explanation of symbols is shown on Figure 3.
16/12
14/12
WC=8.6
DD=103
24/12
we=7.7
00=97
-200=90
21/12
WC. -7.3
DD=98
33/12
WC=6.8
D0=1 13
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-200=71
34112
WC=7.2
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-200=53
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15
20
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115 003A
Hepworth—Porrlak Ceotaehnleal
LOG OF EXPLORATORY BORING
Figure 2
LEGEND:
® TOPSOIL; organic sandy sill and clay, firm, moist, ciark brown.
L
SILT AND CLAY (ML -CL); slightly sandy to sandy, scattered gravel with depth, interlayered with gravelly sandy
clay, stiff to very stiff, slightly moist, light red -brown.
Relatively undisturbed drive sample; 2 -Inch I.D. California liner sample.
16/12 Drive sample blow count; indicates that 16 blows of a 140 pound hammer falling 30 inches were
required to drive the California sampler 12 Inches.
NOTES:
1. The exploratory boring was drilled on January 14, 2015 with a 4 -Inch diameter continuous flight power auger.
2. The exploratory boring location was measured approximately by pacing from features at the site.
3. The exploratory boring elevation was measured by hand level and refers to pavement edge of Sage Meadow Road
as 100', assumed.
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 lime of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pc()
-200 = Percent passing No. 200 sieve
115 003A
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LEGEND AND NOTES
Figure 3
0
1
4
5
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Moislure Content = 8.6 percent
pry Density = 103 pcf
Sample of: Sandy Sift and Clay
From: Boring 1 at 5 Feet
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APPUED PRESSURE - ksf
Compression
upon
wetting
10
100
115 003A
Hepworth—Patrick Gaoteehnleal
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
Compression %
0
1
2
3
4
5
6
7
8
9
10
11
Moisture Content = 7.3 percent
Dry Density = 98 pcf
Sample of: Sandy Silt and Clay
From: Boring 1 at 15 Feet
Compression
upon
,,/wetting
0.1
1.0
APPLIED PRESSURE - ksr
10
1ao
115 003A
Hepworth—Pawlok eotechnical
SWELL -CONSOLIDATION TEST RESULTS
Figure 5
Compression - Expansion %
1
0
1
2
Moisture Content = 6.8 percent
Dry Density = 113 pcf
Sample of: Sandy Clay with Gravel
From: Boring 1 at 20 Feet
Expansion
upon
wetting
0.1
1.0
APPLIED PRESSURE ksf
10
100
H
115 003A
Hepworth—Pawlak Geotechnical
SWELL -CONSOLIDATION TEST RESULTS
Figure 6
Job No. 115 003A
Sandy Silt and Clay
Slightly Sandy Silt and Clay
Sandy Silt and Clay
Sandy Silt and Clay
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