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HEPWORTH - PAWLAK GEOTECHNICAL
Hepworth-Pawlak Ceorechnícal, lnc.
5020 County Road 154
Glenwood Splings, Colo¡acìo 8l601
Phone: 970"945.7988
Fax:970-945-8454
email: hpgeo@hpgeotech.com
H
STIBSOIL STUDY
FOR FO{,INDATION DESIGN
PROPOSED NESIDENCE
LOT 43, PI}I-YON MESA
GARFTELD COUNTY, COLORA,DO
JOB NO. ß7 A92l
JANUARY 7,2008
PREPARED FOR:
LINDA CORCORAN
1114 WEST LOOK DRTVE
GLENWOOD SPRINGS, COLORADO 81601
Parker 3û3-841 "?1.19 . Colorado Sprírrgs 719-633-5562 ¡ Silverthorne 970-4ó8-1989
TABLE OF CONTENTS
PURPOSE ÂND SCOPE OF STUDY..............
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION..
SUBS{/RFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ......
FOTJNDATIONS
FOTINDATION AND RETAINING WALLS.
FLOOR SLABS......
TINDERDRAIN SYSTEM.....
SURFACE DRAINAGE
LIMITATIONS .......
FIGURE 1 - LOCATICIN OF ÐGLORATORY BORING
FIGURE 2 - LOG OF BXPLORATORY BORING
FIGURE 3 . LEGEND AND NOTES
FIGURN 4 - S\ryELL-CONSOLIDATION TEST RESULTS
TABLE 1- SIIMMARY OF LABORATORY TEST RESULTS
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PI]RPOSE AND SCOPE OF'STUDY
This report presents the results of a subsoil study for a propsed residence to be located at
Lat 43, Pinyon Mes4 Garfield County, Colorado. The project site is shown on Figure l.
The purpose of the study was to develop recommendations for the foundation design.
The studywas conducted in accordance with our agreement for geotechnical engineering
services to Linda Corcoran dated December 19, 2007, We previously performed
preliminary geoteclarical engineering studies for the subdivision development and
presented our findings in reports dated November I I, 2005 and April I 0, 2006, Job No.
tos 652.
A field exploration progtam consisting of an exploratory boring was conducted to obtain
information on the subsurface conditions. Samples ofthe subsoils obtained during the
ûeld exploration were tested in the laboratory to determine their classification,
cornpressibility or swell and other engineering characteristics. The results ofthe fie1d
exploration and laboratory testing were anaþed to develop recommendations tbr
foundation tlpes, depths and allowable pressures forthe proposed building foundation.
This report summarizes the data obtained during this study and presents ow conclusions,
design recommendations and other geoteclnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Building plans for the residence had not becn deveioped at the time of our study and this
teport was prepared for purchase of the property. In general, we assumc the proposed
residence will be a one or two story structure located in the middle of the building
envelope shown on Figule 1. Ground floor could be slab-on-grade or above crawlspace.
Grading for the structure is assumed to be relatively minor with cut depths befween about
3 to I feet. We assume relatively light foundation loadings, t¡,.pical of the assurned tlpe
of construction.
Job No, 107 0921 <¡e&ecrt
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Ifbuilding loadings, location or grading plans change significantly tom those described
abovg we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subdivision is located on a relatively flat topographic bench above the Roaring Fork
River valley and below Spring Valley. The site is located at thebase of a steep northwest
facing hillside. Vegetation has been removed from the front part of the site apparurtly
during the subdivision development and the building area is vegetated with sage bruslf
grass and weeds. The ground surface is relatively flat with a gentle slope down to the
west. A scree fie1d of basalt cobbles and boulders is visible on the hillside to the north and
Eagle Valley Evaporite is exposed in Corurty Road 114 cuts above the site.
SUBSIDENCE POTENTIÀL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa
Subdivision. These rocks are a sequônce of g¡,psiferous shalg fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. There is a possibility that
massive g)lpsum deposits associoted with thc Eaglc Vallcy Evaporite underlie portiorx of
the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to
develop and can produce ¿reas of localiz".d, subsidence.
Sinkholes were not observed in the subdivision but geologically /orrng sinkholes are
locally present in the evaporite region between Glenwood Springs and Carbondale and we
are aware ofthree sinkhole collapses in this area of the Roaring Fork River valley during
the past threo years. Based on our current understanding cf the evaporite sinkhole
process, the areas in western Colorado, including the project sitg where evaporite is
shallow have the potential for sinkhole development. The risk of future grorurd
subsidence on Lot 43 threiughout the service life of the proposed residence, in our
opinion, is low; however, the owner should be made a\ryare of tlre potential for sinkhole
development. If fi¡rther investigation of possible cavities in the bedrock below the site is
desired, we should be contacted.
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T.IELD EXPLORATION
The field exploration for the project was conducted on Decemb er 21, 2007 . One
exploratory boring was drilled at the location shown on Figure 1 to evaluate the
subsurface conditions. The boring was advanced with 4 inch diameter continuous flight
augers powered by a truck-mounted CME-458 drill rig. The boring was logged by a
representative of HepwortþPawlak Geotechnical, Inc.
Samples of the subsoils were taken wílhl7a inch and 2 inch LD. spoon samplers. The
samplers were driven into the subsoils at va¡ious 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 ofthe
relative density or consistency ofthe subsoils. Depths at which the samples were taken
and the penetration resistance values a¡e shown on the Log of Exploratory Boring, Figure
2. The sarnples were retumed 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 consist of about one foot oftopsoil (root zone) overlying stiffto very stiff
sandy clay and silt. The soils contain scattered gravel zones and are slightly calcareous
and po ssibly gypsiferous
Laboratory testing performed on samples obtained &om the boring included natural
moisture content and density and percent finer than No. 200 sieve gradation analyses.
Results of swell-consolidation testing performed on relatively undisturbed drive samples,
presented on Figure 4, indicate low to moderate compressibility under conditions of
loading and wetting. The deeper clay sample showed a moderate expansion potential
when wetted. 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
slþhtlymoist.
Job No. 107 û921 cåStecrr
4
FOUNDATION BEAßING CONDITIONS
'l'he subsoils encountered on the lot generally consist of sandy clay and silt aud are
tlpically compressible when wetted under load. The deeper mainly clay soils could be
potentially expansive but are below probable bearing level and can be ignored in the
design. Lightly loaded spread footings can be used for support of the proposd residenoe
provided that some risk of settlement is acceptable to the owner. A heavily reinforced
mat foundation would help to make the structure more rigid and bett€r able to resist
differential settlement. Compacting the bearing soils to a depth of at least 3 to 4 feet
below shallow footings would help to reduce the settlement risk. Another altemative is a
deep foundation system that extends the bearing level down to dense, relatively
incompressible granular soils or bedrock. If the deep foundation alternative is selected,
relatively deep exploration will be needed to provide additional recommendations.
DESIGN RECOMMENDATIONS
FOT-INDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the
nature ofthe proposed construction, we recofirmend the building be forurded with spread
footings bearing on the natural soils or compacted fill.
The design and construction criteria presented below should be observd for a spread
footiúg foundation system.
1) Footings or a mat placed on the undisturbed natwal soils should be
designed for an allowable bearing pressure of 1,200 psf Based on
experience, we expect initial settlement of footings designed and
constructed ¡s discussed in this section will be about 1 inch or lcss. There
couiti be ad<iitionai differentiai foun<Íation settiement on the order of 1 to 2
inches ûr more depending on the depth of any subswface wetting.
Precautions should be taken to prevent post-construetion wetting of the
bearing soils.
Job No. 1O7 0921 cåBîecrr
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2) The footings should have a minimum width of 20 inches for continuous
walls and 2 feet for isolated pads.
3) The mat edges, exterior footings and footings beneath unheated areas
should be provided with adequate soil cover above their bearing eievation
for &ost protection. Placement of foundations at least 36 inches below
exterior grade is typically used in this area. Shallow frost protection cær
consist ofrigid foam insulation in the shallow mat foundation condition.
4) Foundations should be desþed to be relatively rigid with .box like,'
configuration and isolated footings should be avoided. 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.
Foundation walls acting as retaining struchres should also be designed to
resist lateral earth pressures as discussed in the "Foundation and Retaining
'Walis" section of this report.
5) Topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to r¡ndisfurbed natural soils. The exposed
soils in footing areas should then be moistened and compacted.
Additionally removed and replaced soils below footing bearing level
should be compacted to at teast 95% of standard Proctor density at near
optimum mo isture content.
6) A representative ofthe geotechnical engineer should observe the
completed excavation at the time of construction for bearing conditions,
FOI-'NDATION 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
earlh pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf
for backfill consisting ofthe on-site soils. Cantilevered retaining structures which are
separate from the residence and can be expected to deflect sufficiently to mobilize the fu|I
active earth pressure condition should be designed for a lateral earth pressure computed
Job No. 1t1 09?L e&Ftecn
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on the basis of an equivalent fluid unit weight of at lesst 45 pcf for backfill consisting of
the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surclurge 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 wail 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 ¿nd compacted to at least 90% ofthe maximum
standard Proctor density at a moisture content near or slightly above optimum Backfilt in
pavement and walkway areas should be compacted to at least 95Vo ofthe maximum
standard Proctor density. Ca¡e should bc takcn not to overcoülpact the backfill or use
large equipment near the wall, since this could cause excqssive lateral pressgre 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 røtaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure
agaihst the side of the footing. Resistance to sliding at the bottoms of the footings ôan be
calculated based on a coefficient of friction of 0.35. Passive pressure of compacted
backfill against the sides of the footings canbe calculated using an equivalent fluid unit
weight of 300 pcf The coefficient of friction and passive pressure values recommended
above assume ultimate soil strength. Suitable factors of satbty should be included in the
design to limit the strain which will occur at the ultimate strength, particularly in the case
^f*-^-j-'^ *^-:^¿^-^^ L-:!1 -t^^^J ^^-i--¿ LL- -:t -- -frl^- r- -,:,vl lJ4ùù¡vç rçùrrL¡lrlçr'. -Fur pr¡lunu ag¿r[rsr rils trqËs ol [Ile Iooturgs m resÉI ralgfal loads
should be compacted to at least 95% ofthe maximum standard Proctor density at a
nroisture conttnt near optirmrm.
JobNo. 107 A92l cå5tecr-r
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FLOOR SLABS
The natural on-site soils, exclusive of topsoil are suitable to support lightly loaded slab-
on-grade mnstruction. There is a risk of slab settlement and distress ifthe bearing soils
become wetted. To reduce the effects of some differential movement, floor slabs should
be separatd from allbearing 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 ofminus 2 :6¡ch
aggtegate with at least 50% retained on the No. 4 sieve and less than 2% passing the No.
200 sieve.
All äll materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor density at a moisture conte,nt near optimum. Required fill can
consist of the on-site soils devoid of vegetation and topsoil.
TINDERDRAIN SYSTEM
Although free water was not encountered during our expioration, it has been our
experience in mountainous areas that local perched gfoundwater can develop during times
of heavy precipitation or seâr¡onal runoff. Frozen ground during spring runoff can create
a perched condition. We recommend below-grade construction" such as retaining walls
and basement areâs, be protected Aom wetting and hydrostatic pressure buildup by an
underdrain system. A crawlspace at shallow depth should not have an underdrain systern
The drains should consist of drainpipe placed in the bottom ofthe wall backfill
surrounded above the invert level with freerdraining granular material. The drain should
be placed at eaclt level of excavation and at least I foot below lowest adjacent finish
grade and sloped at a minimum lo/o to a suitable gravity outlet or sump and pump. Free-
draining granular material used in the underdrain system should contain less than 27o
passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum
Job No. W A92l Gåeech
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size of 2 inches. The drain gravel backfilt should be at least lYz feet deep. An
impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a
trough shape and attaclred to the foundation wall with mastic to prevent wetting ofthe
bearing soils.
SURFACE DRAINAGE
The following drainage precautions should be observed drning construction and
maintained at all times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab a¡eas should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at least g5% of the maximum standard Proctor density in
pavement and slab areas and to at least 90% ofthe maximum standard
Proctor density in landscape areas.
3) The ground surface surrounding the exterior ofthe building should be
sloped to drain away from the faundation 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 are¿rs.
Free-draining wall backfill should be capped with at least 2 feet ofthe on-
sife soils to reduce surfacc water infiltration.
4) Roof dounspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at
least 10 tbet from tbundation walls. Consideration sho.uld be given to use
of xeriscape to reduce the potential for wetting of soils below the building
caused by irrigation.
f ntfT ^ FralrıLtrvttr,¿ìr tlJlìÐ
This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area at this time. 'We mrke no wfiranty cither
express or implied. The conclusions and recommendations submitted in this report arc
JobNo. 107 A92l c'e5tecrr
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based upon the data obtained from the exploratory boring drilled at the location indicated
on Figure 1, the proposed tlpe of construction and our experience in the area. Our
services do not include determining the presencg prevention or possibility ofmold or
other biological contaminants (MOBC) developing in the firture. If the client is
concemed about MOBC, then a professional in this special field ofpractice should be
consulted. ûur findings include interpolation and extrapolation ofthe subsurface
conditions identified at the exploratory boring and variations inthe subsurface conditions
may not become evident until excavation is performed. If conditions encountered during
cnnstruction appear different from those described in this re,port, we should be notiûed so
that re-evaluation of thp recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. V/e
are not responsible for teehnical interpretations by others of our information. As the
project evolves, we should provide continued consultation and ñeld sen¡ices dwing
construction to review ald rnonitor the irnplementation of our recommendations, and to
veriff that the recomrnendations have been appropriately inteqpreted. Significant desþ
changes may require additional anaþis or modifications to the recommendations
presented hErein. We recommend on-site observation of excavations and foundation
bearing strata and testing of structural ñ11 by a representative ofthe geotechnical
engineer.
Respectfu lly Submitted,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Louis E. Eller
Reviewed by:
Steven L. Pawlak, P.E.
LEE/vad
JobNo. lO7 0921 cåFtecrr
APPROXIMATE SCALE
1" = 30'
CLIFFHOSE WAY
BENCH MAFK: GROUND AT PROPERTY
CORNER; ELEV. = 100.0', ASSUMEÐ,
t--1
LOT44 a
BORING 1
LOT 42
t_J
LOT 43
107 0921 LOCATION OF EXPLORATORY BORING Figure 1
BORING 1
ELEV.:113.8'
LOT 43
0 0
1ôt12
5 16112
WC=6.9
DD=102
5
10 23112 10
15 2?/12
wc:7.7
DD:115
't5
o)(¡)
TL
I
o-(¡)o 20 17112 20
o)
c)u-
r
o
n)Ê
25 25
30 24/12
WC:4,7
DD:'114
-200:63
30
35 35
NOTE: Ëxplanalion of symbols is shown on Figure 3
cåttecrrHâDsôrth-Paslok Gcot¡chnlaol
LOG OF EXPLORATORY BORING Figure 2107 A921
LEGEND:
ú
F
i
n TOPSOIL; organic sandy silt and clay, firm, slightly moist, brown (root zone).
CLAY AND SILT (CL-ML); sandy, scattered gravel, stiff to very stifi, slightly moisl, brown and light brown, slightly
calareous.
Relatively undisturbed drive sample; 2-inch l.D. California liner sample.
Drive sarrrple; standard penetration test (SPI, 1 3/8 ínch LD, split spoon sanrple, ASTM-1586.
Drive sample blow count; indicates that I0 blows of a 140 pound hammer falling 30 inches weretvlt¿ required to drive the California or SPT sampler 12 inches.
NOTES:
1. The exþloratory boring was drilled on December 21,2007 with a 4-inch diameter conlinuous flight power auger,
2. The exploratory boring location was measured approximately by pacing from leatures shown on the site plan
provided.
3. The exploratory boring elevalion was measured by instrument level and refers to the Bench Mark shown on Figure 1
4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the
melhod used.
5. The lines between materials shown on lhe expioratory boring log represent the approximate boundaries between
material types and transitions may be gradual.
6. No lree water was encountered in the boring at the time of drillíng, Fluctuation in water level may occur with time,
7 Laboratory Testing Results:
WC = Water Content (%)
DD : Dry Density (pcfl
-200 : Percent passing No. 200 sieve
107 4921 LEGEND AND NOTES Figure 3
0
1
às
o'ø
an
Eo-
Eo
2
3
4
0.1 1.0 10 100
APPLIED PRESSURE - KSf
*t
2
àSco'6c
ru
x
LU
c
'ı
a,oa
Eo(_)
1
0
1
¿
10
APPLIED - ksf
Moisture Content = 6,9
Dry DensitY: 102
Sample of: Sandy Clay and Silt
From: Boring 1 at 5 Feet
percent
pcf
(
No movement
upon
wetting
\
\
i
Moisture Content : 7.7
Dry DensitY: 1't5
Sample of: Sandy Silty Clay
From: Boring 1 at 15 Feet
percent
pcf
Expansion
upon
wetting
\\
0.1 1.0 100
Figure 4SWELL-CONSOLI DATION TEST RESULTS1t7 0921
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job t{o. 107 0921
sor_ oR
BEÐROCKTYPE
Sady Clay and Silt
Sandy Silty Clay
Sandy Clay and Silt
UNæNFINED
COMPRESSI\iE
SIRENGTH
(PSF)
ATTERBERG LIMITS
PLASTIC
INDEX
(o/o)
UQUTD
TIMIT
l%)
PERCENT
PASSTNG
NO.200
SIE/E
63
GRADAÎON
SAND
(o/q\
GRAVFL
(a/r)
NATIJRAL
MOISTURE
CONTENT
NATURAL
DRY
DENSITY
t02
115
tl4
6.9
7.7
4.7
SAMPLE LOCATION
DEPIH
ifr)
5
l5
30
BORTNG
I