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January 8, 2008 A fghre
Ronald Friemel
1301 Delores Way
Carbondale, Colorado I I 6?3
Job No. 107 0890
Subject:Subsoil Study for Foundation Design and Percolation Test, proposed
Residence, Lot 34, Filing 4, West Bank Mesa" Dol*res Circle, Garfield
Cnunty, Colorado
Dear Mr, Friernel:
As rsquested, Hepworth-Pawlak Geotcchnical, Inc. performe.d a subscil study and
pcrcolation test for foundation and septic disposal designs at the subject site. The study
was cnnducted in accordance with our agteement fbr geotechnical engineering services to
tr'ou dated Decemher 4, 2007. The data CIbtained and our recommendations based on the
proposed construction and subsurface conditions enccuntered are presented in this report.
An evsluation of potential geologic hazard impacts tr: preiposed development cfthe lot is
beyond the scope of this study.
Propased Construction: The proposed residence witl be a i sfory, "systems built"
structure with a walkout basement level located in the eastern parf trf the lot as shown on
Figure 1. Ground floors are proposed to be slatr-on*grade. Cut depths are expected to
range between about 3 to I feet. Foundation loadings &re assumsd to be relatively light
and typical of the proposeel type of construction. The septic disposal system is preiposecl
to be located to the west and downhill ofthe proposed residence.
trf builcling conditions or foundation loadings are significantly rlifferent *om those
describeel above, we should be notified to re-evaluate the recommen*lations presented in
this repart.
P**
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Site Conditions: The lot was vacant and covered with *irout 6 inches of snow at the time
eif our field exploration. Dolores Circle borders the lot on the east and south sid*s as
shown on Figure l. The adjacent j.ots are developed with single family residencas. The
lot is located near the top of a broad drainage area. The ground surface is rclatively flnt
with a moderat"e to strong slope dolrn to the west, There is about 20 tbet of elevation
difference across the building tbotprint. Vegetation c.onsists of weeds and sage bnrsh.
Subsurface Conditions: The subsurface contlitions at the site were evaluated lry
excavating two exploratory pits in the praposed building area and one profile pit in the
septic disposal area at the approxirnate locaticrns shown on Figure 1. The lags af the pits
are presented on Figure 2. The subsoils consist of 1 fact of topsoil {root zoneJ averlying
sandy silt and clay down to the uraximum pit depth of g feet. lt has been our experience
that the fine grained soils are on the order of 15 to 20 feet deep and overlie gravel soils in
this area of West Bank Mesa. Results of swell-consolidation telting performed on
relatively undisturhed samples, presented on Figure 3, generally indicate low
compressibility under existing moisture conditions and lighf k:ading, anil a lew c6llapse
potential {ssttlement under a cCInstnnt load} after wetting- The sample from Fit I at 4 feet
chnrvefl e rninnr cwnonoi^* nn+-*+i^l ^$-- ..,.-r+.i*., ''I"L^ ^^*-1^- ^1-- ,r- - rvrry*rrorvu Pvrlr^rrcrr qrlur wvrlu15'. r rlE llilllFrs-\ itl$u sr!^$ws$ moogralg tt}
high compressibility with increased loading alter wetting. Na fiee water wss observeel in
the pits at the time of excavation and the soils were slightly moist to maist.
Foundation Recommendationr: Considering the subsoil eonciitions encounterect in th*
exploratory pits and the nature of the proposed constructi*n: w* recomrnerul spread
fuotings placed on the undisturbed natural soil designed for an allowable soil bearing
pressurs of 1,200 psf for suppnrt of the propo.sed resitlence. The clay soils tend t*+9fi@ergc0uldbeIingh0rmoreofpost.constructiondifferential
foundntion settlement ifthe bearing soils are wettsd. Preeautiq:ns should be taken to
prevent post-consiruction wetting of=the bearing soils. Footings shtruld be a minimum
width nf 18 ilrehes f,or continur:us walls and 2 feet fcrr columns. Loose and disturbed soilr
encountered at the foundation bearing level within ths excavation should be removed *nri
the footing bearing level extended dnwn to undisturbed natural sails. Exteriar f.ootings
JobNo,1070890
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should be provided with adequate cover abeive their bearing elevations for frost
protection. Flaeement of footings at least 36 inches below the exterior grade is typieally
used in this area. Continuous foundation walls should be reinforced top and bottom to
span lccal anomalies such ae by assuming an unsupported length of at least l2 feet.
Foundation nnd Retaining Walls: Ferundatian walls and retaining structures which are
laterally supported and can be expected to undergo only a slight arnount of deflection
should be designed for a lateral earth pressurs campufed on the basis of an equivalent
fluid unit weight of at least 55 pcf for backfill consisting of the on-site soils" Cantilevered
retaining struotures which are separate from the residence and ean be expected to deflect
sutfrciently to mobilize the full aative earfh pressure condition should be designed for a
lateral earth pressure computed on the basis of an eq*ivalent fluid unit weight of at least
45 pcf for backfill consisting of the on*site soils.
Al1 foundation and retaining strucfures should be designecl fCIr appnrpriate hydrostatic and
surcharge pressure$ such as adjacent footings, traffic, construction materials antl
equipment. The pressures recommsnded above nssume drained ccnditisns behind the
walls and a horiznntal backfill surface. The buildup of water behind a wall or an upward
sloping backtill surface will increase the l*teral pressurs imposed an a foundatir:n wall or
retaining structure. An underdrain should be provided to prevent hydrostatic pressurs
buildup behind walls.
Backfill slrould be placed in uuiform lifts and ceimpacled to at least g0% of the maximum
standard Procfor density at near optirnum moisture content. Backfill in pavement and
walkway areas should be c.ompacted to at leasf g5% ofthe maximum standard Proetor
density. Care should be taken not tCI overcompact fhe backfill or use large equipment
rrear the wall, since this could cttuse excessive lateral pressure on the wall. Some
settiement of deep fuundation wall backfill shosld be expected, eyen if the matsrial is
placed correctly, and could result in distress to facilities eonstructed *n the backfill.
JobNo" 1070S90
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The lateral resistance of tbundation or retaining wall tbotings will be a combination of the
slitling resistatce ofthe footing on the foundation materials and passive earth pressure
against the side of the ftroting. Resistance to sliding at the bsttorns of the footings can bs
calculated based on a coefficient of ftic.tion of 0.30. Passive pressure of compacted
backfill against the sides of the footings can be caleulated using an equivalent fluid unit
weight of 300 pcf The coefficient of fiiction and passive prs$sur* values receimmended
above assume ultimate soil strength. Suitable factors ofsafety should be includd in the
design to limit the strain rarhich will occur at thc ultimate strength, particularly in the case
ofpassive resistance. Fill placed against the sides ofthe footings to resist lateral loads
should be compacted to at least 95% of the maximum standard Proctor d.ensity at a
moisture content near cptimurn
Floor Slabs: The natural on-site soils, exclusive of topscil, are suitable to supporl lightly
luaded slab-on-grade ccnstruction. 'I'he clay sails are typically c.cmpressible and there is
a risk of slab settlement and distress, especiaily if the subgrade soils are wetted. To
reduce the effects ofsome differential movement, floeir slabs should be se,pmated fr6m *ll
beming walls and colurnns with expansion joints which allow unrt*strained vertical
movement, Floor slab cr:ntrol jc'ints should be used to reduce damage due t* thrinkage
cracking' The requirements for joint spacing ancJ slab reinforcement shauld be
established by the designer based on experienee antl the intenderi slab use. A minimum 4
inch lay*r of free-draining gravel should be placed beneath basement level slabe to
facilitate drainage. This material should consist ofminus 2 inch aggregate with less than
50% passing ths Na. 4 sieve and less than z% passing the l*{o. 200 sieve.
All lill materials for support of flonr slabs should be cnmp*ctsri tn at Isast g5.o4 of
rnaxirrum siandard Proctor donsity nt ncar aptimum rnoisfure cuntent. Required fill cau
consist ofthe on-site soils devoid nf vegetation, tcpsoil and oversized rock.
Underdrain $ysteml Although &ee water was nct encouritere{i during our exploration, it
has been our experienoe in the area that locat perched groundwater can develop during
tinres of heavy precipitation or seaconal runoff Frozen ground during spring runoffean
Job No. 107 0890
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create a perched eondition. We recomrnend below-Sade oonstructionn $uch as retaining
walls, crawlspace and base.ment areas: be prntected ftom wetting and hydrostatic pressure
buildup by an underdrain system.
The drains should consist ofdrainpipe placed in thebottom ofthe wall backfill
surrounded above the invert level with free-draining granular material. The drain shoukl
be placed at eaeh level of excavation and at least I foot below lowest adjacent finish
grade and sloped at a minimum 1% to a suitable gravity outlet. Free-draining granular
material used in the underdrain system should contain less than 2% passing the No. 200
sieve, less thsn 50% passing the No. 4 sieve and have a maximum size of2 inches. The
drain gravel backfill should be at lenst lX feet deep. An itnperviaus rnembrane, such as
20 or 30 mil FVC, shnuld be place.d bene*th the drain gravel in a trough shnpe and
affached to the foundation wall with mastic to prevent wetting of the bearing soils.
Surface Drninage: The foilowing drainage precautions should be observect during
construction and maintained at all tirnes after the residence has been cornpleted:
1) Inundation ofthe foundation excavations and underslab areas should be
avoieled during construction.
2) Exterior backfi.ll slrould be adjusted to nesr aptimum moisture and
compacted to at least 95% ofthe maximum stanclard Practar density in
pavement and slab areas and to at least g0% of the maxirn*m standard
Prbctor density in iandscape nress. Free-drnining wall bnckfill should be
capped with at least 2 fbet ofthe on-site soils to re<luce surface water
infiltration.
3) The gr-ound surface surrounding the exterior *f fhe building sheiuld be
slopetl to rlrain arvay from the fou*dation in all directfu:ns. We
recommend a minimum slope of l? inches in the first l0 feet in unpaved
areas ancl a minimum slope af 3 inchec in the first 10 feet in pavement and
walkway area$. A swale m*y be needed uphill to clirect surface runoff
around the residence.
JobNo^ 1070890
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4) Roof downspouts and drains should discharge well beyond the limits cf al}
s)
hackfill.
Landseaping which requires regular heavy irrigation should be located at
least 10 fbet from the building. Conxideratian should be given to the use
of xeriscape to limit potential wetting of soils below the builcling caused
by irrigation"
Percolation Testing: Percolation tests were conducled on December 14, 200? to
evaluate the feasibility af an infiltratiou septic dispt:sa} system at the site. One pr<lfile pit
antl three percolation holes were dug at the locatinns shown on Figure l. The test h6les
(nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow
backhoe pits and soaked with water one day pricr to testing. The soils exposed in the
percolation holes are similar to those exposed in the Profile Pit shswn on Figure Z and
consist ofsandy silty clay down to the bottom pit depth of 8 feet below the existing
ground surface. The percolation test results are presented. in Table 1. Percolatian test
results indicated an infi.ltration rate of 10 minutes per inch. Based cn the subsurface
conditions encountered and the percolatinn te,st results, the tested asea shoulel be suitabie
for a conventinnal infiltration septic disp+sal system" S/e resomfilenrt the s3,'st*m be
oversized due to the clay soils and possible slower porcolation rates than testd.
Limitations: This study has been conducted irr accordance with generally accepted
geotechnical engineering principlex and practices in this area at this time. We make no
warranty either expressetl or i.mplied. Th* conclusions and recammendatinns submitted
in this report are based upon the datn obtained from the exploratory pits excavated at the
loeations indicated on Figure l, the proposed type of construction and our experience in
the area. Our services do not include determining the presence, prevention or Bossibility
of mold or other biological contaminants (MOHC) developing in the future. If fhe client
.:^ -^-^^*-^J --t-...- , r '^n,\lrt u(,uv$rlr$u uo{.}ur lvlLJ$L} tnen a proresslonal m tfus specral tield of practice shouid be
consulted' Our finclings include interpolation and exfrapolation ofthe subs*rface
conditions identifisd at thc exploratory pits ancl variatiCIns in the subsurfhce cCInditio*s
may not become evident until exeavation is performed. If conditions enc"ountered during
JobNo" l0? 0890
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construstion appear different &om those described in this report, we sho*ld be notified at
once so re-evaluation ofthe resommendations may be made.
This report has been prepared for the exclusive use by our client for design purpases. We
are not responsible for technieal interpretations by others of our information. As the
project evolves. we sht:uld provide continued consultation and field services during
construction to review and monitor the implementation of our reoonmendations, and to
veri$ that the recommendations harre been appropriately interpreted. Significant design
changes may require additional analysis or modifioations to the recgmmendations
presented herein. We recommend on-site observatisn of excevations and foundaticn
bearing strata and testing of structural fill by a representative ofthe geotechnical
engineer"
If you have any questions, or ifwe rnay be of further assistancq please call our office.
Respectfu lly Submitted,
HEPWORTH - PAWLAK CAL, INC.
Trevor L, Knell, P.E.
Reviewed bv:
4
Steven L" Pawlak, P.E.
TLl#vad
attaghments Figure I * Locaticrus of Hxploratory Pits and P*rcolation Test Holes
Figure 2 - Logs ofExploratory Pits
Figure 3 * Swell-Consolidation'fest Resulfs
Table I * Fermrlation Test Results
cc:Hepworth-Pawlak Geote*hnical, Inc. * Attn: Jascx Decm
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107 0890 ffi OF EXPLOHATORY PITS AND
PERCOLATION TE$T HOLES
LOCATION$
FIGURE 1
PIT 1
ELEV.*991'
PIT2
ELEV,=987'
PROFILE PIT
ELEV.=980'
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TOFSO|L; reot zone
SILT ANn CLAY {MLCL); sandy, m€dium stiff to stiff, slighily moist to moist, brown, perous, stighfiy
calcareOus, I ow plasticity.
Relatively undisturbed Z-inch diameter liner sampfe
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1. Exploratory pits were excsvated on Dscernber 13, 2002 with a John Oeere B10G backhoe.
2' Locations of exploretory pits were deterrnined in the field by the client and measilred epproxirnfft6ly by pacing fromfeatures shown on the site plan provided.
3, Elevations of exploratory pits were obtained by interpolation between csntours shCIwn on the site plan provided. pitlogs are drawn to depth"
4. The exploratCIry pit locations and elevations should he considered accurate only to the degree irnplied by the methodu$ed,
5. The lines between materials shown on theBxploratory pit logs represent the approximate boundaries betweenmaterial types and transitions may be gradual.
6 No free water was encounterf,d in the pits at the time of excavating. Fluctuation in water levet may occur with time.
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7. Labnratory Testing Results:
WC : Water Content (% )
DD * Dry Density ( pcf )-200 = Fercent passing No. 200 sieve
107 0s90 LOG$ OF HXFLONATOHY PITS FIGURE 2
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TABLE 1
PTRCOTATION TEST RESULTS
ioB N0. 107 0890
Note: Fercolation test holes were hand dug in the bottom of backhoe pits and soaked on
December LL, 20A7. Percolation tests were conducted on December 12, 2A0y. The
average percolation rates were based on the last three readings of each test. The
percolation holes were protected from frost cvernight by rigid fsam insulation.
HOLE NO HOLE DEPTH
0NcHEs)
LENGTH OF
INTFRVAL
(MrN)
WATER
DEPTH AT
START OF
iNTERVAL
(INCHFS)
WATER
DEFTH AT
ENP OF
INTERVAL
iINCHES)
SROP IN
WATER
LEVEL
(rNcHEs)
AVFRAGE
PERCOLATION
RATE
(MIN./rNcH)
P-t 57 15 9?/4 7I/s zrh
10
7W 1Vz L3/q
57:4 LVz
4 21h tlk
2V2 1 IV2
P-2 54 15 9V2 71/4 zw
10
7V4 53/a.LVi
9Vq 41/+t1h
4V+2tfe LVz
P-3 46 15 LO31+8%2
10
831+7W LVz
7l/a 53/q ltz
53/*4V+Lrlz