HomeMy WebLinkAboutSubsoil Study for Foundation Design~tech
HEP\.VORTH-PJ\\VLAI< GEOTECHNICAL
SUBSOIL STUDY
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 25, PINYON MESA
SAGE MEADOW ROAD
GARFIELD COUNTY, COLORADO
JOB NO. 115 028A
FEBRUARY 12, 2015
PREPARED FOR:
SCOTT DILLARD
21 COUNTY ROAD 126
GLENWOOD SPRINGS, COLORADO 81601
Sfottdillardrcalto1·f(i'gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .......................................................................... - 1 -
PROPOSED CONSTRUCTION .................................................................................. :-1 -
SITE CONDITIONS .................................................................................................... - 2 -
SUBSIDENCE POTENTIAL ...................................................................................... :-2 -
FIELD EXPLORATION .............................................................................................. -2 -
SUBSURFACE CONDITIONS ................................................................................... :-3 -
FOUNDATION BEARING CONDITIONS ................................................................. -4 -
DESIGN RECOl\11'v1'ENDATIONS ............................................................................... -4 -
FOUND A TIO NS ...................................................................................................... -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 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 25, Pinyon Mesa, Sage Meadow Road, 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 Scott Dillard, dated January 29, 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 detennine 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 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 minorwith·cut depths between about 5 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.
Job No. II S 028A
-2 -
SITE CONDITIONS
The site was vacant at the time of our field exploration and covered with patches of snow.
The front, north part of the lot had been stripped of vegetation, possibly during the
subdivision development. The site slopes gently down to the southwest with on the order
of 5 feet of elevation difference across the ~ssumed building footprint. Vegetation
consists of grass and weeds with sage brush beginning about 35 feet into the lot from
Sage Meadow Road.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa
development. These rocks arc 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 Evaporitc 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 25 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 .
FIBLD EXPLORATION
The field exploration for the project was conducted on February 2, 2015. One
exploratory boring was drilled at the location shown on Figure 1 to evaluate the
Job No. 115 028A
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 Geotcchnical, Inc.
Samples of the subsurface materials were taken with a 2-inch I.D. spoon sampler. The
sampler was driven into the subsoils at various depths with blows from a 140 pound
hammer falling 30 inches. This test is similar to the standard penetration test described
by ASTM Method D-1586. The penetration resistance values arc 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 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 38 feet of
stiff, porous sandy silt and clay with scattered gravel interlayered with medium dense,
silty sand and gravel including vesicular basalt fragments down to a depth of about 38
feet where weathered siltstone/claystone bedrock was encountered to the drilled depth of
41 feel
Laboratory testing perfonned 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 of the silt
and clay soils, presented on Figures 4 and 5, indicate low compressibility under light
loading at existing low moisture content, a low collapse potential (settlement under
constant load) and 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 were
slightly moist and the weathered bedrock was moist.
Iob No. llS 028A
FOUNDATION BEARING CONDITIONS
The sandy silt and clay soils encountered at typical shallow foundation depth tend to
settle when they become wetted. The interlayered silty sand and gravel soils would be
expected to have lower compressibility potential and relatively minor settlement potential
impact on a shallow foundation if deep wetting were to occur. A shallow foundation
placed on the upper 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
perfonnance 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 I 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 11 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 underlying
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 perfonnance of a shallow spread
Job No. 115 028A
-5 -
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
compacted back to design bearing level but to a depth of at least 5 feet below footing
bearing level. We recommend the basement area footing grade be sub-excavated at least
2 feet 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 a minimum 5 feet of compacted structural fill of the
garage and at least 2 feet of compacted structural fill of 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 settlement of about 1 inch could occur if the silt and
clay soils below the bearing level become wetted. A VJ increase in the
allowable bearing pressure can be taken for toe pressure of eccentrically
loaded (basement wall) footings .
2) The footings should have a minimum width of24 inches for continuous
walJs 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
Job No. 11 S 028A
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design grades should then be moistened and compacted. Structural fill
should consist oflow permeable soil (such as the on-site silt and 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 * 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 backfi11 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 backftll 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
lob No. 115 028A
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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 liinit 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 sub grade 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.
Job No . 11 S 028A
-8 -
All fill materials for support of floor slabs should be compacted to at least 95% of
maximwn standard Proctor density at a moisture content near optimum. Required fi1l can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAJN SYSTEM
Although free water was not encountered during our exploration, it has been our
experience in the area that Jocal 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 1 Yi feet deep. An impervious
membrane such as a 20 mil PVC liner sbould be placed beneath the drain gravel in a
trough shape and attached to the foundation wall with mastic to prevent wetting of the
bearing soiJs.
SURFACE DRAINAGE
It will be critical to the building perfonnance to keep the bearing soils dry. The following
drainage precautions should be observed during construction and maintained at all times
after the residence bas been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
Job No. I IS 028A
-9-
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 exteri9r 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 IO 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 of3%.
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 geotecbnical
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 expJoratory 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
Job No. l 15 028A
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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 hns 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,
Reviewed by:
Daniel E. Hardin, P .E.
SLP/ksw
Job No. I IS 028A
SAGE MEADOW ROAD
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LOT26 I I LOT24
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~UILDING SETBACK LINE J -------
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~ UTILITY EASEMENT
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\~ LOT30 LOT31
APPROXIMATE SCALE
1' :::s 30'
115 028A ~ch LOCATION OF EXPLORATORY BORING Figure 1
Heaworih-Pawlak Geat•chnlcal
0
5
10
15
ii) 20
Q)
LL.
I
:S a.
8
25
30
35
40
115 028A
BOAING1
15/12
15/12
12/12
WC=6.5
0Da91
1~12
WC•6.5
00=98
-200=87
21/12
wc ... 4.s
00·92
·200•52
7l'12
59/12
wc .. 11.9
00•121
NOTE: Explanation of symbols Is shown on Figure 3.
~ LOG OF EXPLORATORY BORING
HllJ'WCRn+PAWLAI< GEOTltr;;HNICAS.
0
5
10
15
20 ~ .
= c. Ql a
25
30
35
40
Figure 2
LEGEND:
SILT AND CLAY (ML·CL}; sandy, scattered gravel, stiff, slightly moist, mixed brown, slightly porous and
calcareous. lnterlayered with ba~alt rragment layers described below.
SAND AND GRAVEL (SM-GM}; silty to very silly, clayey, vesicular basalt fragments, medium dense, slightly moist.
mixed brown. lnterlayered with silt and clay described above.
WEATHERED SILTSTONE/CLA YSTONE; medium hard, slightly moist, olive/gray. Eagle Valley Evaporlte.
p Relatively undisturbed drive sample; 2·inch l.D. California liner sample.
15112 Drive sample blow count; Indicates that 15 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 February 2, 2015 with a 4·inch diameter continuous flight power auger.
2. The exploratory boring location was measured approxlmately by pacing from features et the site.
3. The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth.
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
mater'ial types and transitions may be gradual.
6. No free water was encountered Jn the boring at the time of drilling. Fluctuation in water level may occur with time .
7. Laboratory Testing Results:
WC = Water Content (%}
OD= Dry Density (pcQ
-200 = Percent passing No. 200 sieve
115 028A LEGEND AND NOTES Figure 3
Moisture Content = 6.5 percent
Dry Density = 91 pcf
Sample of: Sandy Silt and Clay
From: Boring 1 at 1 o Feet
0
~ .... ... ,. H)_
1 lj \
\ -t> Compression
< r--upon ..._ ..... -... ,... wetting
~ 2
c 0
(i5
Ul 3 ~ a. \ E 0 u
4
5 \
\
6
7 \
\
8
9 \
1
'[)
10
0.1 1.0 10 100
APPLIED PRESSURE -ks!
115 028A ~
Heoworth-Powlck Geotedlnlcol
SWELL-CONSOLIDATION TEST RESULTS Figure 4
Moisture Content = 6.5 percent
Dry Density = 98 pcf
Sample of: Sandy Silt and Clay
From: Boring 1 at 15 Feet
0
f'1 J
'--
1
I '-~ Compression
\ < -~ upon
*' 2 wetting
c: \ 0 ·u;
(/) 3 ~ c. \ E
0
(.) \ 4
5 \
I )
6 \
\
7
8 \
I~
9
0 .1 1.0 10 100
APPLIED PRESSURE • l<S f
115 028A ~ch
Haoworth-Pciwtalc Goot1chnlca/
SWELL-CONSOLIDATION TEST RESULTS Figure 5
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1 Job No. 115 028A
SUMMARY OF LABORATORY TEST RES UL TS
SAMPLE LOCATION NATURAL GRADATION ATIEABERG LIMITS UNCONFINED
MOISTURE NATURAl PERCENT COMPRESSIVE SOil OR DRYOENSllY GRAVEL SANO PLASTIC BORING DEPTH CONTENT PASSING NO . UQUIOLIMIT INDEX STRENGTH BEDROCK lYPE (%) l"l 200SIEVE
(ft) (") (pcf) (%) (%) (PSFI
1 10 6.5 91 Sandy Silt and Clay
15 6.5 98 87 Sandy Silt and Clay
20 4 .5 92 52 Clayey Sandy Gravel and
Silt
Weathered 40 11.9 121 Siltstone/Claystone