HomeMy WebLinkAboutSoils Report 12.15.2014HEPWORTH-FPAWLAK GEOTECHNICAL
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 68, IRONBRIDGE DEVELOPMENT
RIVER BEND WAY
GARFIELD COUNTY, COLORADO
JOB NO. 114 522A
DECEMBER 15, 2014
PREPARED FOR:
DR. MATTHEW BURT
1195 RIVER BEND WAY
GLENWOOD SPRINGS, COLORADO 81601
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING CONDITIONS - 4 -
DESIGN RECOMMENDATIONS - 5 -
FOUNDATIONS - 5 -
FOUNDATION AND RETAINING WALLS - 6 -
FLOOR SLABS -7-
UNDERDRAIN SYSTEM - 8 -
SURFACE DRAINAGE . - 8 -
LIMITATIONS - 9 -
FIGURE 1 - LOCATIONS 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
on Lot 68 of Ironbridge Development, River Bend Way, 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 proposal for geotechnical study to Dr. Matthew Burt dated November 21, 2014.
Hepworth-Pawlak Geotechnical previously conducted geotechnical engineering studies
for the subdivision development and presented their findings in reports dated October 29,
1997 and February 12, 1998, Job No. 197 327.
A field exploration program consisting of exploratory borings was conducted 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
Development plans were not available at the time of our study and we understand the
findings will be considered in the purchase of the lot. In general, we expect the building
will be one and two stories with a walkout lower level and garage at the main level, and
located within the building envelope shown on Figure 1. Basement and garage floors will
likely be slab -on -grade. Grading for the structure is assumed to be relatively minor with
cut depths between about 4 to 9 feet. We assume relatively light foundation loadings,
typical of the proposed type of construction.
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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 is an undeveloped residential lot located on the downhill, east side of River Bend
Way. Vegetation consists of grass and weeds with scattered sage brush. The ground
surface in the building area is somewhat irregular and gently to moderately sloping down
to the east with a broad shallow drainage swale along the south side of the lot. The
ground surface beyond the building envelope to the east slopes steeply down to the
Roaring Fork River and is vegetated with brush, willows and cottonwood trees.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge
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
studies for the subdivision development, several sinkholes were observed scattered
throughout the Ironbridge Development. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork River
valley. A fairly large sinkhole was mapped about 800 feet north of Lot 68. Numerous
small sinkholes were also observed in undisturbed areas located roughly within 200 feet
to the north of Lot 68. These small depressions appear to be from subsurface erosion
(piping) of the fine-grained alluvial fan soils into coarser subsoils.
No small surface depressions, open voids or indications of sinkholes were encountered by
the subsurface exploration on Lot 68 but the borings were 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 in the remaining parts
of the lot. The risk of future ground subsidence on Lot 68 throughout the service life of
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the proposed residence, in our opinion, is low, but 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 December 1 and 4, 2014. Four
exploratory borings were drilled to evaluate subsurface conditions as shown on Figure 1.
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 1% inch and 2 inch I.D. spoon samplers. 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. 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, below about %i to 1 foot of topsoil or root zone, consist of about 3 to 8 feet
of stiff to very stiff, sandy silt and clay with scattered gravel overlying relatively dense,
silty sandy gravel with cobbles and small boulders. Drilling in the dense granular soils
with auger equipment was difficult due to the cobbles and boulders and drilling refusal
was encountered in the deposit.
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 of the
sandy silt and clay soils, presented on Figures 4 and 5, indicate low compressibility under
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existing low moisture content and light loading and a low to moderate collapse potential
(settlement under constant loading) when wetted. Moderate to high compressibility was
observed in the samples under further loading after wetted. The laboratory test results are
summarized in Table 1.
Free water was not encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The relatively dense, silty sandy gravel soils encountered in the exploratory borings at
depths of about 4 to 81/4 feet below the existing site grade are suitable for support of the
proposed structure with low compressibility potential and appear to be at or near expected
cut depths for the basement level. The depth to granular soils may vary across the
building footprint and sub -excavation below proposed basement footing level may be
required. If sub -excavation below basement level footing grade is required to encounter
the granular soil layer, foundation grade could be reestablished with compacted structural
fill or lean mix concrete slurry.
The silt and clay soils were encountered in our exploration to depths of about 71/4 to 81/2
feet below existing site grade in the uphill, western part of the building envelope. Sub -
excavation of the fine-grained soils below shallow footing bearing level (such as for the
garage) will serve to limit potential settlement of the foundation. It should be feasible to
found the shallow footings on a minimum 3 feet of properly compacted structural fill
placed on the undisturbed natural, fine-grained site soils with some risk of movement.
Dense granular soils, if encountered at shallow foundation bearing elevation, are suitable
for support of the structure as noted in the previous paragraph. Other shallow foundation
elements, such as porch or patio footings should be constructed in the manner noted
above for shallow foundations.
As an alternative to compaction of the site soils, a deep foundation system such as
micropiles or helical piers may be used to extend the shallow foundation level (and porch
and patio supports) down to the gravel site soils with low risk of potential settlement. If a
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deep foundation system is desired we should be contacted to provide appropriate
recommendations.
Structural fill in foundation areas can consist of imported granular material such as
CDOT Class 6 aggregate base course or the onsite soils free of vegetation and topsoil.
The fill should be placed in maximum 8 inch loose lifts and compacted to a minimum of
98 percent of the standard Proctor value for the material at near optimum moisture
content. Prior to placement of the structural fill the excavation subgrade should be
moisture conditioned and compacted. The structural fill should extend beyond the edge
of the foundations a distance equal to at least one half the fill depth but at least 2 feet.
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 as noted in the sections below.
1) Shallow spread footings placed on a minimum 3 feet of properly
compacted structural fill founded on the undisturbed sandy silt and clay
soils should be designed for an allowable bearing pressure of 1,500 psf.
Footings for the basement level bearing entirely on the undisturbed silty
sandy gravel soils or properly compacted structural fill bearing on the
gravel soils can be designed to impose a maximum soil bearing pressure of
2,500 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. There could be additional differential settlement of about %z to 1
inch if the relatively shallow silt and clay bearing soils are wetted.
2) 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.
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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 14
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) Topsoil and any loose or disturbed soils should be removed and the
foundation excavations extended down to the undisturbed natural soils.
The exposed soils in footing areas should then be moistened and
compacted. If water seepage is encountered, we should be contacted for
further evaluation. Structural fill placed below the building footings
should be compacted to at least 98% of standard Proctor density at near
optimum moisture content.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions and
evaluate compaction of structural fill during its placement.
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 onsite soils 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
50 pcf for backfill consisting of the onsite 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
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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 0.45 for foundation elements founded on
the silty sandy gravel soils or properly compacted structural fill. Passive pressure of
compacted backfill against the sides of the footings can be calculated using an equivalent
fluid unit weight of 375 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 with a risk of movement if the clay bearing soils are wetted.
Subgrade in floor slab areas should be scarified, moisture conditioned and compacted
prior to placement of concrete, drain gravel or leveling course. To reduce the effects of
some differential movement, non-structural 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
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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 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.
Shallow crawlspace areas (less than 4 feet) should not need
underdrain protection with properly placed and compacted foundation wall backfill and
positive surface slope away from the foundation.
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 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.
SURFACE DRAINAGE
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 covered with filter fabric and capped
with at least 2 feet of the on-site finer graded soils to reduce surface water
infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill and foundation areas.
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
conditions may not become evident until excavation is performed. If conditions
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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.C.
Reviewed by:
(Aar ,
Daniel E. Hardin, P.E.
SLP/ksw
Job No. 114522A
GecPtech
6
5930
1
r
E r
59135
/
1? 5940 ,l
_5945_
_./
/ BORING 1
/ •
114 522A
•
BORING 2
5935
68 •
BORING 3
BORING 4
•
RIVER BEND WAY
h
HEPWORTH-PAWLAK GEOTECHNICAL
APPROXIMATE SCALE:
5930
69
LOCATIONS OF EXPLORATORY BORINGS
IGURE 1
Elevation - Feet
5945
5940
5935
5930
5925
BORING 1
ELEV.= 5938'
18/12
WC=4.7
DD= 103
15/12
WC=4.9
DD=92
-200=76
62/12
BORING 2
ELEV.= 5932'
18/12
WC=3.9
DD=93
-200=50
39/10
BORING 3
ELEV.= 5936'
14/12
BORING 4
ELEV.= 5941'
12/12
15/12
WC=7,5
DD=87
5945
5940
5935
40/9,15/0
WC=1.4 5930
-200=17
5925
5920 5920
114 522A
]H
Note: Explanation of symbols is shown on Figure 3.
Hepworth--Powl ok Geotechnteal
LOGS OF EXPLORATORY BORINGS
Figure 2
Elevation - Feet
LEGEND:
nen
18/12
T
TOPSOIL; organic sandy silt and clay, soft, moist, dark brown.
SILT AND CLAY (ML -CL); sandy, scattered gravel, stiff to very stiff, slightly moist, light brown.
GRAVEL, COBBLES AND BOULDERS (GP -GM); sandy, silty, medium dense to dense, slightly moist, light brown,
subrounded rock.
Relatively undisturbed drive sample; 2 -inch I.Q. 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 18 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Practical drilling refusal in dense gravel.
NOTES:
1. Exploratory borings were drilled on December 1 and 4, 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 obtained by interpolation between contours shown on the site plan provided.
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
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Hepworth—Pawtok GeotoehnI al
LEGEND AND NOTES
Figure 3
Compression
Compression
0
1
2
3
0
1
2
3
4
5
6
7
8
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 3.9 percent
Dry Density = 93 pcf
Sample of: Very Sandy Silt and Clay
From: Boring 2 at 2 % Feet
-----....1
0,1
1
Compression
upon
wetting
0.1
114 522A
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1.0 10
APPLIED PRESSURE - ksf
Hepworth—Pawlak Geotechnleal
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 4
Moisture Content = 4.7 percent
Dry Density = 103 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 2 Y Feet
----------------'---c:-7�>�
a
N.
.X.)
��,,
No movement
-upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 3.9 percent
Dry Density = 93 pcf
Sample of: Very Sandy Silt and Clay
From: Boring 2 at 2 % Feet
-----....1
0,1
1
Compression
upon
wetting
0.1
114 522A
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1.0 10
APPLIED PRESSURE - ksf
Hepworth—Pawlak Geotechnleal
SWELL -CONSOLIDATION TEST RESULTS
100
Figure 4
1 Moisture Content =
•
r
r N CO
uoissaidwoo
to co ao
0
0
1-
Y
0
APPLIED PRESSURE - ksf
l..L
SWELL -CONSOLIDATION TEST RESULTS
LV
N
r
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 114 522A
SAMPLE LOCATION
BORING DEPTH
NATURAL
MOISTURE 1 NATURAL
CONTENT DRY DENSITY
(%) (pal
4.7 T-103
4.9 92
GRADATION
GRAVEL SAND
(%) (%0)
PERCENT
PASSING NO.
200 SIEVE
76
ATTERBERG LIMITS
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
UNCONFINED
COMPRESSIVE
STRENGTH
(PSF)
SOIL OR
BEDROCK TYPE
Sandy Silty Clay
1
Sandy Silt and Clay
2
21h
3.9
93
50
4
Very Sandy Silt and Clay
5
7.5
87
10
1.4
17
Sandy Silt and Clay
Silty Sand and Gravel
J