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University of Pretoria
GEOLOGICAL PROCESSES AND
MATERIALS
SGM210
22 April 2014
COLLAPSIBLESOILS
Thanks
to
Mr Ken
Schwartz
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COLLAPSIBLESOILS
Soil
that
can
withstand
relatively
large
imposed
stresses
with
small
settlements
at
lowinsitumoisturecontent.
Exhibitsadecreaseinvolumeandassociated
settlementwithnoincreaseinappliedstressif
wetting
up
occurs
Thechangeinvolumeisassociatedwith
change
in
soil
structure
COLLAPSESETTLEMENT
Additional
settlement
that
occurs
due
to
the
wetting
up
of
a
partially
saturated
soil
which
has
a
collapsible
fabric
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COLLAPSE SETTLEMENT
Thebasicconceptofadditionalsettlementduetocollapseofsoilfabric
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CONDITIONSFORCOLLAPSETOOCCUR
1. Thesoilmusthaveacollapsiblefabric.
Mayoccurinanyopentexturedsilty orsandysoilwithalowdensity.
Transportedsoils
Quartzrichrockssuchasgranite orfelspathic sandstone haveundergonechemicalweathering toproduceleachedresidualsoils.
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CONDITIONSFORCOLLAPSETOOCCUR
2.
Thesoilmustbepartiallysaturated
Criticalvalueofdegreeofsaturationabove
which
collapse
will
not
occur
Collapsesettlementwillnotoccurinsoils
which
are
below
the
water
table
CONDITIONSFORCOLLAPSETOOCCUR
3.
Theremustbeanincreaseinmoisture
content.
Thisisthemaintriggeringmechanism.
Rateofcollapseisdependentonrateof
increase
in
moisture
content
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4.Theincreaseinappliedstressmustbegreaterthan
theoverburdenpressure.
Becausethenaturalgroundwasstable
regardlessoftheinsitumoisturecontent
Maynotbetrueinaridareaswhichhardly
ever
get
rain.
CONDITIONSFORCOLLAPSETOOCCUR
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BUILDINGS
Collapsesettlementcantakeplacemanyyears
after
construction.
Settlementoflargemagnitudecanoccureven
under
lightly
loaded
structures.
Collapsesettlementisgenerallylocalized
(differentialsettlement)
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ROADS,AIRFIELDS,RAILWAYS
Collapseofsubgrade
Vibration(dynamicforces)maybesufficientto
cause
collapse
Negative
bulking
factor
in
cut
to
fill
Low density = danger
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EARTHDAMS,RESERVOIRS
EARTHDAMS,RESERVOIRS
Constructionproblemsduetonegative
bulking
Leakagethroughthewallduetodamagefrom
collapse
settlement
Overtoppingofthewall
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DISTRIBUTION
Transportedsoils
Soils
which
have
been
moved
by
a
natural
agency
(water,
wind,
gravity).
Residualsoils
Soilsdevelopedbyweatheringprocesses
TRANSPORTEDSOILS
Hillwash
Soils
of
mixed
origin
Aeolian
Gulleywash
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RESIDUALSOILSN.B. GRANITESOFTHEBASEMENTCOMPLEX
Johannesburg/Pretoriagraniteinlier
Tzaneen
WhiteRiver
Nelspruit
Mbabane
OTHERRESIDUALSOILSResidual quartziteMagaliesburg quartzite
Pretoria Group.
Rustenburg Very loose silty fine sand
Residual felsiteRooiberg Group
Bushveld Complex
Witbank Silty sand
Residual basaltSoutpansberg Group
Limpopo Province Deeply weathered layer of
clayey silt with a low density
Residual graniteCape Granite Suite
Stellenbosch
Constantia
Clayey silt with a low density
Residual sandstoneEcca Group
Karoo Sequence
Mpumalanga Silty sand
Residual DiabaseSill intrusive into shale of the
Pretoria Group
Gauteng
West Rand
Loose clayey ad sandy silt with a
low density
Heave/collapse behaviour
Residual CalcareniteBerea Formation
Kwazulu Natal coast
Mozambique coast
Red or orange silty sand
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POTENTIALLYCOLLAPSIBLE
UNLESS
PROVEN
OTHERWISE
HILLWASH
AEOLIAN
SOILSOFMIXEDORIGIN
RESIDUALGRANITE
BEREA
RED
FIELDIDENTIFICATION
Correctinterpretationofthesoilprofile
Moisturecontent
Looseoropenfabric
Origin
Crackinganddistortionofexistingbuildings
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LABORATORYTESTING
Indexpropertytests
Thedoubleoedometertest
Thesingleoedometertest
Thecollapsepotentialtest.
DOUBLEOEDOMETERTEST
Thenormallyconsolidatedcase
Outside scope of course
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DOUBLEOEDOMETERTEST
Theover
consolidated
case
Outside scope of course
THESINGLEOEDOMETERTEST
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THECOLLAPSEPOTENTIALTEST
THECOLLAPSEPOTENTIALTEST(howbigisyourproblem?)
Collapse potential Severity of the problem
0% to 1% No problem
1% to 5% Moderate trouble
5% to 10% Trouble
10% to 20% Severe trouble
>20% Very severe trouble
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INSITUTESTS
Directmeasurementofcollapse
Anymeasurementofsoilstiffnessneedsto
provide
a
comparison
of
stiffness
at
in
situ
moisturecontentandinasaturatedcondition:
Platebearing
test
PLATEBEARINGTEST
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PLATEBEARINGTESTRESULT
INSITUTESTSIndirectmeasurementofthepotentialof
collapse:
Measurementofinsitudensity
The
lower
the
density,
the
higher
the
potential
collapse
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ENGINEERINGSOLUTIONS
(whatcan
we
do
man?)
Precludethetriggeringmechanism
Designforcollapsesettlement
Piledandpierfoundations
Improvementbydensification
DESIGNFORCOLLAPSE
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PILEDANDPIERFOUNDATIONS
Transferloadsthrough thecollapsiblesoils
Shallowprofile:Piers
Deepprofile:Piles
Negativeskinfriction???(Downdragalongpileshafts
assoilscollapse.)
PILEDANDPIERFOUNDATIONS
Foundingwithin thecollapsiblesoilprofile
Assesstheriskofanincreaseinsoilmoisturecontent
at
depth
Designforthelowermodulusvalueifthereisan
increaseinmoisturecontent.
Theadvantagesofadrivencastinsitupile
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Adrivencastinsitupile
IMPROVEMENTBYDENSIFICATION
Removalandrecompaction
Depth Degreeofcompaction
Sitecontrol
Sitedrainage
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SURFACEROLLING
IMPACTROLLER
EFFECTOFIMPACTROLLINGONDENSITY
(DENSITY)
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EFFECTOFIMPACTROLLINGPENETROMETERRESULT
SURFACEROLLING
VIBRATINGROLLER
Improvement
up
to
1,1m Waterrequiredtobreakdownbonds
Ifwaterisnotavailablethenmechanical
breakdownisanalternative
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DYNAMICCOMPACTION
DYNAMICCOMPACTION
INSA
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DYNAMICCOMPACTION
INSA
RAPIDIMPACT COMPACTION
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OTHERSOLUTIONS
Precollapse
Compactiongrouting
Chemicalgrouting
HOUSES
NHBRC(forinterestonly)
Class Settlement Design
C 10mm Compaction below
individual footings
Soil raft
Deep strip footings
Cellular raft
Piled or pier foundations
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STATEOFTHEART
THETOOLSAREAVALIABLE
Identifycollapsiblesoils
Predictthebehaviour
Providesolutions
PROBLEMSSTILLOCCUR!!!!
Butwhy?
Constructionwascarriedoutbeforethe
collapse
phenomenon
was
identified
No
geotechnical
investigation
was
carried
out
Theinvestigationdidnotcorrectlyidentify
potentially
collapsible
soils
Therecommendationsgivenhavebeen
ignored