HomeMy WebLinkAboutSoils Report 06.16.2016ech
HEPWORTH - PAWLAK GEOTECHNICAL
Elclmtxth•Pawhiz Geotechnical, Inc
5020 County hold 15.4
Glenwood Springs, Colum o 81601
Phone: 970.945.7988
Fax 970-945-8454
c ul: hpgcor@h08corcch.com
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 1, PINYON MESA
PINYON MESA DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 116 214A
JUNE 16, 2016
PREPARED FOR:
JAY BILLINGTON
179 RIVER VISTA
GLENWOOD SPRINGS, COLORADO 81601
(jdbron @yahoo.cont)
Parker 303-841-71 19 • Colorado Springs 719-633-5562 • Silverrhume 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - I -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS . - 3 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4 -
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 8 -
LIMITATIONS .. - 8 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 AND 5 - SWELL -CONSOLIDATION 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 1, Pinyon Mesa, Pinyon Mesa Drive, Garfield County, Colorado. The project site
is shown on Figure I. 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 Jay Billington, dated May 25, 2016.
Two exploratory borings were drilled 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 design was conceptual at the time of our study and will generally
be a two story wood frame structure over a crawlspace with an attached garage. The
garage floor will be slab -on -grade. Grading for the structure is assumed to be relatively
minor with cut depths between about 3 to 7 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.
SITE CONDITIONS
The site was vacant at the time of our field exploration. The site slopes steeply down to
the east with approximately 20 feet of elevation difference across the building envelope.
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Vegetation consists of grass and weeds with scattered sage brush, juniper and pinyon
pine. Scattered basalt boulders and gypsum fragments were observed on the surface.
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 1 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 May 27, 2016. Two exploratory
borings were drilled at the location shown on Figure I to evaluate the 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 and bedrock were taken with 1% inch and 2 inch I.D. spoon
sampler. The sampler was driven into the subsurface materials at various depths with
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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 consist of about 2 to 4 feet of very stiff, slightly moist, sandy
silt and clay with gravel overlying about 2 to 3 feet of firm to hard weathered siltstone.
Below 4 to 7 feet, very hard siltstone/gypsum bedrock (Eagle Valley Evaporite) was
encountered down to the drilled depths of 21 feet. Drilling through the bedrock was
relatively slow due to the rock hardness.
Laboratory testing performed on samples obtained from the boring included natural
moisture content and density analyses. Results of swell -consolidation testing performed
on relatively undisturbed drive samples of the upper silt and clay soils and weathered
bedrock, presented on Figures 4 and 5, indicate low compressibility under light loading
and existing low moisture content with a low to moderate collapse potential (settlement
under constant load) when wetted. The samples showed moderate to high compressibility
under increased loading after wetting. The deeper bedrock was too hard to obtain
undisturbed samples for swell -consolidation testing. 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.
FOUNDATION BEARING CONDITIONS
The sandy silt and clay soils and weathered bedrock encountered at typical shallow
depths tend to settle when they become wetted. A shallow foundation placed on the silt
and clay soils will have a risk of settlement if the soils become wetted and care should be
Job No. 116 214A
bearing on the bedrock
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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 1
to 11/2 inches could occur. Settlement in the event of subsurface wetting will likely cause
building distress and mitigation methods such as extending to bearing level down onto
bedrock or removing the upper layer of sandy silt and clay and replacing with properly
compacted structural fill should be used to reduce the settlement potential.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature of the proposed construction,
the building should be founded with spread footings
with a low risk of settlement. Control of surface and subsurface
runoff will be critical to the long-term performance of a shallow spread footing
foundation system. The garage and shallow crawlspace footing areas should be sub -
excavated (if needed) down to bedrock and the excavated soil replaced compacted back to
design bearing level.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1)
Footings placed on the bedrock should be designed for an allowable
bearing pressure of 2,000 psf.
Based on experience, we expect settlement
of footings bearing on the bedrock to be about 1 inch or less and
essentially occur during construction.
2) The footings should have a minimum width of I6 inches for continuous
walls and 2 feet for isolated pads.
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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 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) The topsoil and any loose or disturbed soils should be removed below the
building area.
7) 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
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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 placed 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 0.35. 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 onsite 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. The upper soils and weathered bedrock tend
to be compressible and there is a risk of slab settlement and distress mainly if the
subgrade soils are wetted. To reduce the effects of some differential movement, floor
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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 free draining gravel should be placed
beneath basement slabs for drainage and 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 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, oversized rock and gypsum.
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 also 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
needed 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
finishgrade and sloped at a minimum 1% to gravity outlet or 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 11/2 feet deep. An
impervious membrane such as a 20 to 30 mil PVC liner should be placed beneath the
drain gravel in a trough shape and attached to the foundation wall with mastic to prevent
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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.
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.
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
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indicated on Figure 1, the general 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.
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.
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Shane Mello
Reviewed by:
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Daniel E. Hardin, P.E.
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Job No. 116 214A G�tECh
APPROXIMATE SCALE
1"=30'
•
BORING 2
BORING 1
•
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116 241A
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Hopwoskh-Pawlois Goot chnicd
LOCATION OF EXPLORATORY BORINGS
Figure 1
Elevation - Feet
BORING 1
ELEV. = 6307
BORING 2
ELEV. — 6312'
6315 6315
6310
6305
6300
6295
6290
6285
25/12
WC -7.7
D0-109
22/12
5C/'2
50/2
A 50/1
f 65112
f� WC=41
DD -106
50/2
50/1
50/1
Note: Explanation of symbols is shown on Figure 3.
6310
6305
6300
6295
6290
6285
Elevation - Feet
LEGEND:
—7
7
z
7
SILT AND CLAY (ML -CL); sandy, gravelly, very stiff, slightly moist, brown.
WEATHERED SILTSTONE; very stiff to hard, slightly moist, brown
SILTSTONE BEDROCK; very hard, slightly moist, mixed brown and gray, gypsum layers. Eagle Valley Evaporite
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch 1.D. split spoon sample, ASTM D-1586.
25/12 Drive sample blow count; indicates that 25 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
NOTES:
1. Exploratory borings were drilled on May 27, 2016 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 obtained by interpolation between contours shown on the site plan provided,
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)
116 214A
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Hepworth—Powlok Gnotechnlool
LEGEND AND NOTES
Figure 3
Compression %
0
1
2
3
4
5
6
7
8
9
10
11
Moisture Content = 7.7 percent
Dry Density = 109 pcf
Sample of: Sandy Silt and Clay
From: Boring lat 2 y Feet
Compression
upon
wetting
0i
10
APPLIED PRESSURE - ksf
10
100
Compression %
1
0
1
2
3
4
5
6
7
Moisture Content = 4.1 percent
Dry Density = 106 pcf
Sample of: Sandy Silt and Clay
From: Boring 2 at 2 y2 Feet
Compression
upon
� — wetting
•
01
1.0
APPLIED PRESSURE - kst
10
100
116 214A
SWELL -CONSOLIDATION TEST RESULTS
Figure 5