HomeMy WebLinkAboutSubsoils Report for Foundation Designffi CTLITHOMP$ON
GEOTEGHNICAL ENGINEERING INVESTIGATION
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SANTE SHOP AND ADU BUILDING
{660 COUNTY ROAD 259
GARFIELD COUNTY, COLORADO
Prepared fon
DAVID SANTE
1660 County Road 259
Rifle, CO 81650
CTLIT Project No. GS06773.000-120
August 9, 2023
CTLlThomgson. lnc.
Denver. Fort Cotlins, Coloradq Sprinqs, glg[S1ggd$!!i!!9g,, Pueblo, Surnrnit Countv - Colorado
Ghevenne, tlffoming and Bozema,0, Montana
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Table of Csntents
pRoposED coNsTRUeTloN ...,"...,........ .,
GEOLOGIC CONDITIONS AND HMARDS.
$uB$uRFAGE coNDlTroNs.........,.
?
3
3
Excavations
Subexcavation and'strustural Fill.. ".............
Foundation lAlall Backfi ll
FOUNDATION
4
5
6
GEOTEC,HNICAL RISK .........".......... 10
FIGURE 1 *VIC]NIW MAP
FIGURE 2 - AERIAL PHOTOGMPH
FIGURE 3 - $UMMARY LOGS OIi EXPLOMTORY BORINGS
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I:- SUMMARY OF LABORATORY TESTING
DAVID 9ANTE
{6e0 CoUNTY'ROAD 259
crllT PRoJEcT N'O. GS06773.000,1 20
ffi
SCOPE
CTllThompson, lnc. (CTLIT) has completed a geotechnicalengineering investi-
gation regarding a new shop and ADU building proposed on your property at 1660
County Road 259 in Garfield County, Colorado. We conducted this investigation to
evaluate subsurtace conditions at the site and provide geotechnicalengineering rec-
ommendations for the proposed b-uilding construction. The scope of our investigation
was set forth in our Proposal No. GS 23-0026. Our report was pr€pared from data de-
veloped from our field exploration, laboratory testing, engineering analysis, and our ex-
perience with similar conditions. This report includes a description of subsurface condi-
tions found in our exploratory borings and provides geotechnical engineering recom-
mendations for design and construction of the foundation and floor system, and details
influeneed by the subsoils. A summary of our conclusions is below.
SUMMARY OF CONCLU$IONS
Our exploratory borings drilled at the site encountered surficial layers of
drive surface gravel and natural clayey sand above weathered bedr:sck
underlain by interbedded sandstone and claystone bedrock to the maxi-
mum explored depth of 19 feet. The weathered bedrock was very stiff and
the bedrock was medium hard to hard. Free groundwater was not found in
the borings at the time of drilling.
Based on our field,and laboratory data, and our geotechnical engineer'ing
experience, the weathered bedrock has potentialfor low lo moderate vol-
umechange when wetted under building loads.
We judge that a footing foundation is appropriate for the building, provided
the natural soil and'weathered bedrock are subex.cavated from. below foot-
ings to a depth of at least 3 feet gnd rqplececlas properly-compacted.
stiucturalfillFootings can ne su ctural
fiil.
4.Tc enhanco potential floor slab performance, we recomrnend subexcava-
tion of the natural soil and weathered bedrock to a depth of at least 2 feet
and replacenrent with properly-compacted, structu ral fill.
DAVID SANTE
{660 CSUNTY ROAO 259
1
2,
3,
CTLIT PROJEcT NO,. GS06773,000;1 20
Page 'l of t{
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SITE CONDITIONS
The Sante property is addressed as 1660 County Road 259 in earfieH County,
Colorado. A vicinity map $howirlg the location of the property is incfuded ae Figure 1.
The propoeed building location is nearthe grade transitisn from eomparratively genfls
gradee to steeper hillside slop.es, The prop,osed shop b-uilding, is ahout 10.0 feet to tfi6
north-northeast of the existing residense. An aerialphotrgraph of thesite is.showno:ft
Figure 2. Ground,surfece in the proposed building area generalty stopes down to the
south at grades visually estimated at 5 to 10 percent. Vegetation in this areaiE predom-
inantly sparse g{dss€$; sage, and seub oak. Aphotograph of the site taken durlngour
subsurface inveetigation :is belolrr.
Looking north with drill rig at the TH-1 location
PROPOSED CONSTRUCTION
Plane are tro construct fl pro'tnonufactured two-story,,steel'frame building. The
building will be utilized as a shop with an auxiliary dwelling unit (ADU) on the upper lev-
bAYID sANTE.
1080 couNTY ROAD l5S
CTLIT PROJECT NO. Ggq6?7S.009.12O
eage,l of i,,I
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el. The building'will consist of pre-engineer"ed steel roof trusses and columns and metal
roofing and siding. TWo large bay doors are planned. Lower levelfloors will be slabs-on-
grade. The building footprint will be 40 feet by 50 feet. Maximum foundation excavation
deBths of about 5 feet are expected" Fill placement below the building is not anticipated.
We expect maximum vertical interior and exterior column building loads of less than 20
kips. Horizontal reaction loads are expected to be less than 5 kips.
GEOLOGIC CONDITIONS AND I"IAZARDS
We reviewed geologic mapping by the U,S. Geological Survey (USGS) for the
area of the property titled, tGeologic Map of the Silt Quadrangle, Garfield County, Colo-
rado" by Shroba, R,R. and Scott, R.B. dated ,200L. Wasatch Formation bedrock {Tws)
underlies the site and is near the ground surface above the site. The bedrock is de-
scribed as intervals of thick claystone, mudstone, and siltstone,interbedded with less
abundant intervals of sandstone. Soils above weathered bedrock and bedrock are
sho-wn. on the rnapping as sheetwash colluvium (Qsw). The eolluvium soils on this site
are mos-tly clayey sand and sandy clay deposited by water and gravity and derived from
bedrock of the Wasatch Formation above the site. No significant geologic hazards were
identified that would preclude the planned construction. The soils encountered in our
exptoratory borings are consistent with the geologic,mapping.
SUBSURFACE CONDITIONS
Subsurface conditions w€re investigated by drilling two exploratory borings (TH-f
and TH-2) on July 10,2023. The borings were drilled at the approximate locations
shown on Figure 2. Drilling operations wer€ directed by our representative, who logged
subsurface conditions encountered and obtained representative samples of the soils,
Graptric logs of subsuface conditions'found in the exploratory borings are included as
Figu-re 3.
Drvo $*lrg
1660 COTJNTY ROAQ 25e
cTLIT PROJEcT NO, G50.6773;000-l 20
Page 3 of 1,1
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Ourexploratory borings drilled at the site encountered surficial layers of drive
surface gravel and natural clayey sand and sandy clay above weathered bedrock un-
derlain by sandstone and claystone bedrock to the marimnm erplored depth of 1,9 feet.
The weathered bedrock was very stiff and the bedrockwas medium hard to hard. Free
groundwafer wa$ not found in the borings. The borings were hackfilled immediately afier.
exploratory drilling operations were completed.
$amples of the soils obtained frorn our borings were returned to our laboratory
for perlinent teeting. One-dimeneional swell-consolidation testing on a $ample of clay-
stone bedrock swelled ,0,8 percent when wetted under an apptied load of 1,000 Fsf. En-
gineering index testing performed on a sample from a depth:of '15 feet exhibited a liQuid
limit of 32 percent and plasticity index of 16 percent with 72 percent silt and ctay (pass-
ing the No" 200 sieve). A sample of soil tested had a water-soluble su.lfate content of
0.07 percent. Swell-consolidation test results are shown on Figure 4. Laboratory testing
is summarized on Table l.
SITE EARTI.IWORK
Exoavations
Maximum foundation excavation depths of about 5 feet are anticipated. Our sub-
surface investigation indicates that excavations at the site.can be accomplished using
conventional, heavy-duty excavating eq uipment..
Sides of excavations need to be slcped: or retained to meet local, state, and fed-
eral safety regulationq, The subsoils at the site will likely classiftT as Type B or Type C
soils based on OSHA standards governing excavations. From a ntrench" safely stand-
point, temporary slopes deeper than S feet that are not retained should be no steeper
tha.n1to1(horizontaltovertical)inTypeBsoilsandl.ShtolvinTypeCsoils.Con-
tractors are responsible for.determining the actual OSHA soil type when excavations
OAVID $ANTE
1660 COUNTY ROAD 259
cTLIT PROJECI NO. GS08773.000-{20
Fage4of11,
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are made and for maintaining safe excavations. Contractors should identify the soils en-
countered in excavations and ensure that OSHA standards are met.
Free groundwater was nol found in our exploratory borings. We do not believe
excavations to construct the proposed building Will encounter a free groundwater table
$ u,bexcavetion.and Structu ral: Fil I
Based on our field and taboratory data, and our geolechnical engineering experi-
ence in the area the natural soils and weathered bedrock have potentialfot'low to mod-
erate volume change under building loads.
We recommend the shop/ADU building foundation and slab-on-grade floor be
supported on strueturalfill placed on the weathered bedrock or bedrock. Footing foun-
dations.should be supported,oF o minirnum thickness of 3 feet of structural fill. We rec-
ommend a minimum 2 feet thickness:of structural fill below floor slabs, The subexcava-
tion process should extend at least 1 foot beyond the edges of the building perimeter.
The on-site'soiland weathered bedrock can be reused as structuralfill, provided
it is free oJ rock$ largerthan S inches in diameter, organic matter, and debris. A positive
alternative would be to use imported ,GDOT Class 6 aggregate base course as structur-
al fil!, Struclural'fill'constructed with Class 6 aggregate base course would be less sus-
ceptible to differential consolidation if soils below the building are wetted. The,structural.
fiilsoii shoul{ be moisture-conditioned to within 2 percent of optimum moisture csntent,
placed in loose tifts o'f I inches thick or less, and compacted to at least 98 percent of
standard Proctor (ASTM D 69S) maximum dry densig. Moisture content and density of
structurai fill shou,ld be checked by a representative of our firm during:placement.
DAVIO SANYE
t66O COUNTY ROAD i5'
cTL|T PROJECT NOi GS08?73.000-120
Page 5 of 11
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Foundation Wall Backfil|
Proper placement and compaction of foundation wall backfil! soil is important to
reduce infiltration of sudace water and $etflement frorn consslidatiOn of .backfill. This is
especially impor'tant for backfill areas that will support exterior oonorete flatwsr.k. The
soils excavated from the site can be used as backfill, prrovided they are free of rocks
larger than 3-inches in diameter, organics, and debris.
Backfill soil should be placed in loose lifts of approximately 10 inches thick or
less, moisture-oonditioned, and compacted. The backfill should be compacted to at
leasl 95 percent of slandard Proctor (ASTM D 098) maximum dry density" Moislure con-
tent and density of the backfill should be,checked during placement by a representative
of our firm.
FOUNDATION
We judge that the building can be constructed on a footing fouhdation. Our sub-
surfaee information and experience indicate the weathered bedrock and bedroek atthis
site have potential for low to moderate votume ohange w-hen wetted under building
loads, To. mitigate potential difterential movement from consolidation and/or heave of
soils below the building we recommend subexcavation of the natural soil and weathered,
bedroak to a depth of at least 3 feet. Proposed bottom of footing elev,ations can be re.
attained with properly-compacted, struotural fill in accordance with the SuLexcavation
and $tructural Fill section.
Recommended design and construction criteria.for footing foundations are pre-
sented below. These criteria were developed based on our analysis of field and labora-
tory data, as well as our engineering experience.
'1 Footings should be supported on a minimum 3 feet thickness of properly-
compacted, structuralfill in accordance with the Subexcavation and Struc-,
tural Fillsection.
DAVID SANT€
1660 COUNTY ROAD 259
CTLIT PROJECT NO. GS08773.000".t20
Fage 6 of tl
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2. Footings on the structural for a maximum net allowa-
ble soil bearing p ressure weight of backfill soils above
footings can be neg re calculation
A friction factor sf 0.40 be used to calculate resistance to sliding be-
tween conCreto footings and the s.oils.
Continuous wallfootings should have a minimum width of 16 inches.
Foundations for isolated columns should have minimum dimensions of 24
inches by 2:A inches. Larger sizes may be requlred, depending upon foun-
dation loads,
Grade,beams:and foundation walls should be well-reinforced. We recom-
mend reinforcement sufficient to span an unsupported distance of at least
12 feet.
The soils under,exlerior footings should be protected from freezing. We
recommend the bottom of footings be constructed at a depth of at least 36
inches below finished exterior grades. The Garfield County building de-
partrnent should be consulted regarding frost protection requirements.
SLAB.ON,GRADE CONSTRUCTION
Lower level floors in the.building are planned as slabs-on-grade. Exterior con-
crete flatwork is also anticipated. To enhance potentialfloor slab performance, we rec-
ommend subexcavation of the natural soil and wea.thered bedrock below the interior
floor slabs to a depth of at least 2 feet. Planned slab,elevations can be re-attained,with
proper:ly-compacted, struetural fill in accordance with recommendations in the, $qhgxca:
vatioh and SliUctufal Fill section.:We recommend the subexcavation process extend to
a depth of at least 12 inches below exterior slabs.
Based. u^n our analyois of field and laboratory data, as well as our engineering
experience, we recommend the following precautions for slab-on-grade construction at
this site.
Slabs should be separated from wallfootings and column pads with slip
joints, which allow free veftical movement of the slabs.
3.
4.
;$,
o_
1
OAVID SAI'ITE'
1860 COUNTY ROAD 259
cTLIT PROJECT NO. €505773.000-1ZO
ean be
for beari
3,000 psf.
Page 7 of 1l
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underslab:plumbfng should be pressure tested fs,r leaks before the slabs
are constructed. Plumbing and utilities which pass through slabs should
be isolated from the slabs with sleeves and provided with flexible cou-
nlings to slab supported appliances.
3. Exterior patioand porch slabs should be isolated from the building. The$e
slabs should be well-reinforced to function as independent units.
4. Frequent controljoints should be provided, in accordance with American
Conerete lnstituto (ACU reeommendations, to reduce problems associated
with shrinkage and curling.
5. The lnternational Building code (lBc) may require a vapor retarder be
placed between the base course or subgrade soils and concrete slab-qn-
grade floors. The merits of installation of a vapor retarder below floor slabs
depend on the sensitivity:of floor coverings and,building to moistur.e. A
properly installed vapor retarder (10 mil minimunr) is more beneficial be-
low eoncrete slab-on-grade floors where floor coverings will be sensitiveto
moisture.
BELOW-GRADF C ON STRUCTION
We understand the building will not include below-grade areas, such as a base-
ment oi crawl space. lf construction plans evslve to include below-grade areas, we
.should be informed so that we can provide recommendations for lateral earth pressures
and subsurface drainaEe systems.
SURFACE DRAINAGE
Surfaeedrainage is eriticalto the performahqe of buildlng foundations, floor
slabs,. and concrete flatwork. Site grading should be designed and constructed to rapidly
convey surface water away from the building. Proper surface drainage. and ,irrigation
practices can help control the amount of surface water that penetrates to found,ation
levels,and contributes to settlement and/Or heave of soils that support fqundations,
slabs, and other structuree. Positive drainage away from the foundation and avoidance
of irrigation near the foundatian also help to avoid excessive wetting of backfill soils,
which can lead to increased backfill settlement and possibly higher lateral earth pres-
DAVIO SANTT.
'r660 couNTy RbAo zsg
cTr"lT PRoJECT NO, G806773.00Q-l 20
Page I of {{
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sure$, due to increased weight and reduced strength sf the backfill. We recommend the
following precautions.
The ground surface surrounding the exterior of the building should be
sloped to rapidly convey surface water away from the building in alldirec-
tions, We recommend a constructed slope of at least 12 inches in the first
10 feet (10 percent) in landscaped areas around the building.
Backfill around the foundation walls shoufd be moisture-tr:eated and com-
pacted pursuant to recommendations in the Foundation Wall Back{illsec-
tion,
1
2
3.
4.
bAVID SANXE
1660 COUNTy ROAD t59
CTLIT PRo.TEGT NO. 6S06773,000-{ 20
CONCRETE
Concrete in contact with soil can be subject to sulfate attack. One sample of the
soilfrom our exploratory borings that was tested contained 0.07 percent water-soluble
:sulfates (see Table ,l). For this level of sulfate concentration, ACI 332-08, "eode.Re-
quirements for Residential Concrete", indicates there are no s-pecial cement require-
ments for Eulfate resistance in concrete that is in contact with the subsoils.
ln our expetience; superficial damage may occur to the eXpo$€d surfaces of
highfy perrneable concrete, even when suhate levels are relatively low. To control this
risk and to resist freeze-thaw deterioration, the water-to-cementitious materials ratio
shoufd not exceed 0.50 for concrete in contact with ssils that are likely to stay moist due
We recommerrd that the building be provided with roof gutters and down-
spouts, The downspouts should discharge well beyond the limits of all
backfill. Splash blqcks and/or extensions should be provided at all down-
spouts so water discharges onto the ground beyond the backfill. We gen-
erally recommend against burial of downspout discharge pipes.
Landscaping should be carefully designed and maintained to minfmize ir-
rigation. Plants placed close to foundation walls should be limited to those
with low moisture requirements. lrrigated grass should not be located with-
in 5 feet of the foundation. Sprinklers should not discharge within 5 feet of
fsundatipns, Plastic sheeting should not be placed beneath landscaped
a.reas adjacent to foundation walls or grade beams, Geotextile fabric will
inhibit weed growth yet stillallow naturalevaporation to occur.
Page,9 of l1
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to surface drainage or high-water tables, Concrete should have a total air content of 6
percent +/. 1.5 percent"
CONSTRUCTION OB$ERVATIONS
We recommend that CTLIT be relained to provide construction ob.servation and
materials testing services for the project. This would allow us the opportunity to verify
whether subsurface conditions are consistent with those found during this investigation.
lf othere perform these observations, they must accept responsibility to judge whelher
the recommendations in this report remain appropriate.. lt is also beneficialto projects"
from economic and practicalstandpoints, when there is,continuity,between engineering
consultation and the construction observation and materials testing phases.
GEOTHCHNICAL RISK
The concept of risk is an important aspect of any geotechnicalevatuatjon. The
primary reason for this is that the analytical methods used to develop geotechnical rec-
ommendations do not comprise an exact,scierrce: W€ never have complete knowledge
of oubeurfaco conditiong, Our analysia must be tempered with engineering judgnrerrt
and experience. Therefore, the recommendations presented in any geotechnicaleval-
uation should not be considered risk-free. \Ne cannot provide a guarantee that the inter-
acJion between the soils and the proposed structure will lead to performance as desired
or intended:. Our recommendations represent ourJudgment of those measuresthat ar-e
necessary to increase the chances that the structure will perform satisfactorily. lt is criti-
ealthat all recommendations in this report are followed.
LruNATIONS
This report was prepared for the exclusive use of the client. The info.rmation,
conclusions, and recommendations presented herein are hased upon consideratlon of
many factors including, but not limited to, the type of stfuctures proposed, the geologic
DAVID SANTE'
{660 COUNTY ROAD 259
cTLIT PROJEeT NO. GS06773;000.120
Page 10 of ,! 1
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setting, and the subsurface conditions encountered. The conclusions and recommenda-
tions contained in the report are not valid for use by others. Standards of practice con.
tinuously change in geotechnical engineering. The recommendations provided in this
report are appropriate for about three ye.ars. lf the proposed storage building is not con-
structed within three years, we should be contacted to determine if we should update
this report.
Our exploratory borings provide a reasonable characterization of subsurface
eonditions at the site. Variations in subsurface conditions not indicated by the borings
will occur, We should be provided with architectural plans, as they are further devel-
oped, so we can provide geotechnical/geo-structural engineering input.
This investigation was conducted in a manner consistent with that level of care
and skill ordinarily exercised,by geotechnical engineers currently practicing under simi.
lar conditions in the locality of this project. No warranty, express or implied, is made. lf
we can be of further service in discussing the contents of this report, please call.
CTLITHOMPSON,, tNC Reviewed by:
i mEcil i nq {Acfl th ompsan. corn
NAVID SANTE
{660 COUNTY ROAD:259
,cTLlT PROJECT NO. GS0€773,000-1 20
Page 1t of 11
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0 1,300 5,000ErrElE:r!NOTE:
slctit"gt 1' - 3,000'
DAVItrgAilfIE
umsgNwRoADgle
PFO.IECT NO" .GS06773.O6b-t m
SATELLTTE N'AGE FROM GOOGTE EARTTI
(DATED oCTOBER, ls, 2022J
'Vlclnity
Map FIG. 1
LTGIND:
TH_1 APPROXIMATE LOCATION :OF
I EXPLOMTORY BORING
APPROXIMATE LOCATION OF
-
PROPERTY BOUNDARY
NOTH:
ffi
50 ,BO
scALE :l' - 60'
DAVID SAN|E
SATELLITE IMAGE FROM GOOGLE EARTH
(DATED oCTOBER B, 2A22)
Aerial
Photograph"t660 ccxjNw RctAD 259
Cnrtr PRO.JECT NO. G8O677S.OOO-1 20 FlG. ,2
TH.1
DAVID $ANTE
1660 COUNTY ROAE 259
GRIT PROJEQT:NO, Gg0s?73.00&1 20
GRAVEL DRIVE $URFACE
SAND, CLAYEYT MEDIUM DHNSETO DENSF, MQIST,
TAN, GRAY. ($C)
WEATHERED SANDSTONE AND CLAYSTONE
BEDROCK, MED{UM HARD,GRAY, TAN.
$ANDSTONE ANO CIAYSTONE BEOROrcK, MEDIUid
HARETO HARD, GRAY, TAN.
THE SYMBOL 2?/f2.INDICATE9 22 BLOWS OF A
140-FOUND FIAMMERFALLING 3q INGHES WERE
REQUIRED TO DRIVEA 2,s-INCH O.D.
CALIFORNII\:BARFEL $AMPLER 12 INCHES.
THE SYMBOL 50'12 INDICATES 60 EtOWg OF A
l4O.POUNP I.IAMMER FALLING 30 INCHES WERE
REQUIR€D TO DRIVE A 2.o.INGI{ O.D. SPLIT-SPOON
SAMFLER 1? INCHE$,
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TH-2
LEGEND:
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10
15
F
UJull!
IF
:O.
IJJo
20
-iF- n'ronnres pmcrrcAl AuGER REFusAL.
NOT,EST
1. B(PLQRATORY BOBINGS WERE' DRILLED WITH 4.INCH
DIAMETER SOLID"STEM AUGERAND A
TRAGI("MOUNTED DRILL RIG ON JULY 10.2023.
?, GROUNDWATERWA$ NOTENCOUNTEREDIN THE
BORINGS THE PAY]OF DRILIINC.
3. THESE LOGS ARE.SUBJECT TO THE EXpt At\tATtONSi
LI}IIITATION$, ANO CONCLUSIONS INTHIS REFORT.
$ummary Logs of
Explorat6ry 'Y
Botings
FIG.3
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7
6
4
.t
2
0
.2
"3
zg*
az
dcx-r
EJ
L_€z^6-o6
uttuEt
o"
Eoo
'E
0.1
Somple
From TH-z AT 10 FEET
DRYUNITWEIGHT=
MOISTURE CONTENT=
100
122 PCF***63._o/o
10
APPLIED PRESSURE . KSF
Af ELAY.$TONE
DAYIE SANTE
1660CO[NTY ROAD 259
CTLIT PROJECT NOt GS06f74,000-120
$well-Consolidation
Test Results
t\
(
r
\\
- EXPANSION UNDER CONSTANT
PRESSURE DU.E TO WETTING
1,0
Ftc.4
TABLE I
SUMIMRY OF LABORATORY TESTING
CTLIT PROJECT NO. cS06773.000,120
ffi
TH.2
TH.2
TH,2
EXPLORATORY
SORING
15
10
5
DEPTH
(FEET)
5.7
8.9
5.5
MOISTURE
CONTENT
(olol
1
DRY
DENSITY
(PCF)
22
32
LIOUID
LIMIT
(%l
16
PLASTICITY
INDEX
(o/o't
0.8
*SWELL
(o/ol
0.07
SOLUBLE
SULFATES
{%\
72
49
PASSING
NO.200
SIEVE
(%)
CLAYSTONE BEDRCCK
CLAYSTONE BEDRCICK
WEATHERHD CLAY€}TONE
DESCRIP lON
* SWELL MEASURED UNDER 1,OOO PsFAPPLISD PRESSURE.Page 1 of 1