9. Collapsible Soils

7/26/2019 9. Collapsible Soils

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1

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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2.

Thesoilmustbepartiallysaturated

Criticalvalueofdegreeofsaturationabove

which

collapse

will

not

occur

Collapsesettlementwillnotoccurinsoils

which

are

below

the

water

table


3.

Theremustbeanincreaseinmoisture

content.

Thisisthemaintriggeringmechanism.

Rateofcollapseisdependentonrateof

increase

in

moisture

content


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4.Theincreaseinappliedstressmustbegreaterthan

theoverburdenpressure.

Becausethenaturalgroundwasstable

regardlessoftheinsitumoisturecontent

Maynotbetrueinaridareaswhichhardly

ever

get

rain.



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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

Documents

9. Collapsible Soils