HomeMy WebLinkAboutSoils Report 05.25.2016HEPWORTH-PAWLAK GEOTECHNICAL
Hepu arch-Pawlak Georcchnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email. hpgeorgthrgeotech.cora
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 5, PINYON MESA
107 PINYON MESA DRIVE
GARFIELD COUNTY, COLORADO
JOB NO. 116 157A
MAY 25, 2016
PREPARED FOR:
JULIAN ULRYCH
226 SOUTH 2ND STREET
CARBONDALE, COLORADO 81623
(1 ul rych @ nepine. com)
Parker 303-841-7119 ® Colorado Springs 719-633-5562 ® Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
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 _ 7 -
UNDERDRAIN SYSTEM - 7 -
SITE GRADING - 8 -
SURFACE DRAINAGE - 8 -
LIMITATIONS - 9 -
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 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1- 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 at
Lot 5, Pinyon Mesa, 107 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
professional services to you dated May 2, 2016.
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 story structure over a walkout basement. Ground
floor will be slab -on -grade. Grading for the structure is assumed to be relatively minor
with cut depths between about 4 to 12 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 lot was vacant at the time of our field investigation and the ground surface appeared
mostly natural. The lot is situated on a west facing hillside. The slope is moderately steep
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in the rear, western, portion of the lot and transitions to gently sloping near the street.
Several basalt and sandstone cobbles were observed on the ground surface. Vegetation
consisted of grass and widely spaced sagebrush with pinyon pine on the upper portion of
the lot.
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 5 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 10, 2016. 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.
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Samples of the subsoils were taken with a 1% inch I.D. spoon sampler and 2 inch
California sampler. 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 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 1/2 a foot of topsoil overlying 7 to 20 feet of stiff sandy silty
clay. Gypsiferous claystone/siltstone was encountered below the clay at depths of 71 to
20 feet down to the bottom of the boring at 26 to 31 feet.
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 Figures 4 and 5, indicate
limited expansion or compressibility when wetted and low to moderate compressibility
under additional loading. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The sandy silt and clay soils encountered at typical shallow foundation depth tend to
settle when they become wetted. 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
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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 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 2 inches
could occur. Settlement in the event of subsurface wetting will likely cause building
distress and mitigation methods such as a deep foundation (such as piles or piers
extending down into the bedrock could be used to support the proposed house. If a deep
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 bedrock or 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 footing areas should be sub -excavated down
about 4 feet below design footing grade and the excavated soil replaced compacted back
to design bearing level but to a depth of at least 4 feet below footing bearing level.
bedrock is encountered before excavation 4 feet below design footing grade, then the
excavation can be stopped and design footing grade be re-established with compacted
structural fill.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on bedrock a minimum 4 feet of compacted structural fill
should be designed for an allowable bearing pressure of 1,200 psf. Based
on experience, we expect initial settlement of footings designed and
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constructed as discussed in this section will be about 1 inch or less.
Additional settlements of about 1/2 to 11/2 inches could occur if the silt and
clay soils below the bearing level become wetted. A'/3 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 local 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 silty 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 1/ the fill depth below the footing.
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
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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
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
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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 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 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 local area that 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,
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
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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 11 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.
SITE GRADING
The risk of construction -induced slope instability at the site appears low provided the
building is located as planned and cut and fill depths are limited. We assume the cut
depths for the basement level will not exceed one level, about 10 to 12 feet. 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 95% of the
maximum standard Proctor density. The fill should be benched into the portions of the
hillside exceeding 20% grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or
flatter and protected against erosion by revegetation or other means. The risk of slope
instability will be increased if seepage is encountered in cuts and flatter slopes may be
necessary. If seepage is encountered in permanent cuts, an investigation should be
conducted to determine if the seepage will adversely affect the cut stability. This office
should review site grading plans for the project prior to construction.
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:
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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 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.
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
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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.
Tom C Brunner — Staff Engineer
Reviewed by:
Daniel E. Hardin, P. E.
TCB/ksw
cc: Patrick W. Stuckey Architects — Patrick Stuckey (stucarch@comcast.net)
Job No. 116 157A
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APPROXIMATE SCALE
1"=20'
1
1
i
i
BORING 1
LOT 5
BUILDING SE7B
ACK LINE
PINYON MESA DRIVE
00
i
LOT 4
BORING 2
-
a)
w
Q
a>
0
0
5
10
15
20
r— 25
30
35
BORING 1 BORING 2
' 7/12
WC=5.1
✓ DD=90
-200=82
✓ 10/12
WC=10.4
1DD=84
38/12
WC=4.4
DD=109
50/2
28/12
22/6,50/4
10/12
']WC=13.7
DD=111
, -200=85
17/12
f WC=11.4
DD=108
r
r
r
r
r
r
•
r
r
r� 16/12
WC=7.8
00=102
' -200=86
r
r
' r
16/12
WC=9.4
DD=103
12/12
29/6,50/5
50/1
Note: Explanation of symbols is shown on Figure 3.
0
5
10
15
20
25
30
35
LEGEND:
7
h
7/12
NOTES:
TOPSOIL; organic sandy silt and clay, firm, slightly moist, brown.
CLAY (CL); sandy, silty, scattered gravel, medium stiff to very stiff, slightly moist to moist, light brown.
CLAYSTONE/SILTSTONE; weathered to hard with depth, slightly moist, gypsiferous. Eagle Valley Evaporite.
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 7 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
1. Exploratory borings were drilled on May 10, 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 not measured and the logs of exploratory borings are drawn to depth.
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. 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
Compression
0
c
co
a
x
Lu
Compression
0
1
2
3
4
5
6
7
1
0
1
2
Moisture Content = 10.4 percent
Dry Density = 84 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 5 Feet
Compression
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 4.4
Dry Density = 109
Sample of: Sandy Silty Clay
From: Boring 1 at 10 Feet
percent
pcf
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
116157A
I-1
He worth—Pawlak Geotechnical
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 4
Compression - Expansion %
Compression %
1
0
1
2
0
1
2
3
4
5
6
Moisture Content = 11.4 percent
Dry Density = 108 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 5 Feet
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 9.4 percent
Dry Density = 103 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 15 Feet
Compression
��,upon
wetting
C)
0.1
116 157A
1.0 10
APPLIED PRESSURE - ksf
HEPWORTH-PAWLAK GEOTECHNICAL
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 5
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 116157A
SAMPLE LOCATION
GRADATION 1ATTERBERG
LIMITS
SOIL OR
BEDROCK TYPE
BORING
NATURAL
MOISTURE
DEPTH CONTENT
(ft) (%)
NATURAL
DRY
DENSITY
1 (Pcf}
PERCENT
GRAVEL 1 SAND PASSING
(%) (%) NO. 200
1 SIEVE
LIQUID
LIMIT
PLASTIC
INDEX
UNCONFINED
COMPRESSIVE '
STRENGTH
1
21/2
5.1
90
1 82
Sandy Silty Clay
5
10.4
84
Sandy Silty Clay
10
4.4
109
Sandy Silty Clay
2
21/2
13.7
111
85
Sandy Silty Clay
5
11.4
108
Sandy Silty Clay
10
7.8
102
86
Sandy Silty Clay
15
9.4
103
Sandy Silty Clay