StabiliSation of Sulfate-bearing SoilS · Stabilisation of Sulfate-Bearing Soils Guidelines for the...
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STABILISATION OF SULFATE-BEARING SOILS GUIDELINES FOR BEST PRACTICE
Transcript of StabiliSation of Sulfate-bearing SoilS · Stabilisation of Sulfate-Bearing Soils Guidelines for the...
StabiliSation of Sulfate-bearing SoilS
G u i d e l i n e s f o r B e s t P r a c t i c e
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Stabilisation of Sulfate-Bearing Soils
Guidelines for the stabilisation of sulfate-bearing soil
Contents
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1 Thesulfateheavemechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Sulfatesandsulfidesinclay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Siteassessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Testingforsulfatesandsulfidesinsoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5 Triggerlevelsforsulfates/sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6 Choiceofbindertominimiseriskofsuftate/sulfidedisruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7 Laboratorytestingofstabilisedmixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8 Goodconstructionpracticeforthestabilisationofclays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10 Appendix:testingofsoilsforsulfatesandsulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1 - The sulfate heave mechanism
Summary
The potential for sulfate-induced expansion in stabilised soils is well documented and understood. To avoid problems, adequate site investigation is needed prior to stabilisation and, in particular, it is essential to test for sulfides as well as sulfates.
These guidelines for the stabilisation of sulfate bearing clays are based on current knowledge and experience and aim to:
• Explainthemechanismsthatcausesulfateheave;
• Recommendmethodsforsamplingandtestingforsulfatesandsulfides;
• Describemeasurestominimisetheriskofsulfate-relateddisruption .
Thispublicationisanupdateoftheoriginalversion,whichwaspublishedin2005
1 The sulfate heave mechanismSincethelate1950s,ithasbeenwellknown[1]thatsulfatescancauseexpansionproblemsin
soilsstabilisedwithcalcium-basedstabilisers(e .g .limeandPortlandcement) .Inthepresenceof
water(particularlyexcesswater),thereactionofcalcium(fromlimeorcement),alumina(aprimary
constituentofclay)andsulfateproducescalcium-aluminate-sulfate-hydrateminerals .Thesehavevery
largeexpansionpotential,insomecasesashighas250% .Oneofthesemineralsisettringite,which
holdsverylargequantitiesofwaterwithinitsstructure .Duringitsformation,veryhighswellpressures
candevelopwithdisruptiveincreasesinvolume .
Sulfides,especiallypyrites,alsoconstituteariskfactorforsoilstabilisation .Thereport[2]fromthe
ThaumasiteExpertGroup,setuptoinvestigatesulfate-relateddeteriorationinconcrete,highlightedthe
extenttowhichdisturbanceofasoilcaninducepyritestooxidiseandsignificantlyincreasethesulfate
level .Thisoxidationisacceleratedduringsoilstabilisationoperationsbyboththepulverisationprocess
andtheuseoflimeand/orcement,whichincreasesthepHlevelandtherebydecreasesthechemical
stabilityofthepyrites[3] .
2 Sulfates and sulfides in claySulfatesand/orsulfidesarefoundinmanyUKsedimentarystrata,includingancientmarinedeposits
suchascarboniferousshales,claysandglacialtills,aswellasrecentestuarinesiltsandclays .Inmost
affectedstrata,thetopmetreorsoofundisturbedgroundisnormallylowinsulfate,duetoleachingby
rainfall .Highlevelsofsulfateareoftenfoundaroundtreerootsandatthebaseoftheweatheredzone .
Theirlocationisdependentontheweatheringhistoryandthegroundwaterflow .
3 Site assessmentTheimportanceofreviewingallreadilyavailableinformationpriortocommencementoftherelatively
morecostlyintrusivesamplingandtesting,cannotbeoveremphasised .Aphasedinvestigationwill
usuallybethemostefficientapproach .Thelevelofdetailandtimespentoneachstagecanbevaried
dependingonthescaleandnatureoftheproject .
The sampling and testing strategy should be based on a number of factors, including:
• Informationalreadyavailable;
• Anyrisksidentifiedfromtheimmediatelyavailableinformation;
• Theconsequencesofinadequateperformance(differentcriteriamaybeappropriateaccordingto
theapplication) .
4
Stabilisation of Sulfate-Bearing Soils
It is often appropriate to target specific areas for sampling and testing, for example:
• Geologicalstratawithpotentiallyhighsulfateand/orsulfidecontents;
• Locationsofhighsulfateconcentrations(hotspots)resultingfromnaturalweatheringofthesoil
profile[4] .Particularcareshouldbetakentovisuallyidentifyanyweatheringzoneswithinsoil
layers .
Inthecaseofsoilstobestabilisedinsitu,itisimportantthattestsareconductedonsamplestaken
fromlevelsthatequateaccuratelytothemateriallikelytobestabilised .Forcuttings,samplesshould
notonlybefromalocationwithinthestratumequivalenttotheproposedstabilisedlayerbutalsotoat
least500mmbelowthebaseofthislayer .Materialtobemovedbeforestabilisationmaypresentless
riskbecausemixingofmaterialduringexcavationandplacementislikelytodilutesulfatehotspots .
Diggingtrialpitsoftenprovidesacosteffectivemeanstobothvisuallyassessthegroundconditions
andprovidegoodqualitysamplesforsubsequenttesting .Whensamplingfromdeeperlevelswithin
thesoil(forexamplefromproposedcuttings)orwheresiteaccessislimited,windowsamplingand/or
boreholescanbeused .Itmaybeprudenttotakemoresamplesthanareinitiallyrequried,toallowfor
subsequenttargetedtesting .
Considerationshouldalsobegiventothepotentialforgroundwatertobringinsulfatesfromoutside
ofthestabilisedground .Testingthegroundwater(ifpresent)isusefulasanadditionalpointertothe
overallconditions,butshouldnotbeusedinisolation .AnSO4concentrationinthegroundwaterin
excessof0 .4g/l,theupperlimitof‘DesignSulfateClass1’forgroundwaterinBRESpecialDigest1[5],
indicatesthepresenceofsignificantlevelsofsulfate,butalowerconcentrationwouldnotnecessarily
guaranteetheabsenceofsufficientsulfatetocauseaproblem .
4 Testing for sulfate and sulfide in soilsTherearealargenumberoftestmethodsforthedeterminationofsulfate/sulfideinsoils(seeAppendix) .
TRLReport447Sulfate specification for structural backfills[6]reviewedtheseandrecommendedtest
methods .Thesetakeadvantageofadvancesininstrumentationandarequickerandlessexpensive
thanthehistorical‘wet-chemistry’methods .TheTRL447testmethodswereintroducedintoHighway
AgencySpecificationsinNovember2003andBritpavegenerallyrecommendsthemasthepreferred
methods .
To assess a soil for its potential for sulfate-induced expansion, it is helpful to know the following:
• Theacid-solublesulfate(AS)content,whichisameasureoftheimmediately-availablesulfate(use
TRL447TestNo .2) .
• Thetotalpotentialsulfate(TPS)content,whichisthetotalsulfatethatwouldbecomeavailableif
allthesulfideconvertedtosulfate(useTRLTestNo .4) .TPS(expressedas%SO4)iscalculatedby
multiplyingthetotalsulfur(TS)value(%S)by3 .
Thevalueofasoil’stotalpotentialsulfateshouldalwaysbegreaterthanthatfortheacid-soluble
sulfate(ifnot,questionthetesting!) .Thedifferenceprovidesameasureofthepresenceofmaterials
thatcontainsulfur,butinaformotherthansulfate .Ifthesulfurispresentassulfide(e .g .pyrite)thenit
canreadilyconverttosulfateandcontributetoheave .However,sulfurinorganicorothermatterthat
isunlikelytoconvertnaturallytosulfateisnotconsideredariskfactorwithrespecttoheave .Whilst
itispossibletoanalysefurthertodistinguishbetweentheseformsofsulfur,thenecessarytestsare
complexandexpensive .MeasuringTPSisthereforeaconservativeapproach,whichmaysignificantly
overestimatethepotentialforsulfateheave .WherethereisalargedifferencebetweenTPSandAS,itis
usuallybeneficialtounderstandthereason .Thismayrequiregeologicalexpertiseorfurthertesting .
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5 - Trigger levels for sulfate / sulfide
5 Trigger levels for sulfate/sulfideSulfate-induceddisruptionisnotnormallyconsideredarisk,wheretheTPSlevelinthesoilis
consistentlybelow0 .25%SO4 .However,siteinvestigationsmaymissisolatedhotspotsandbefore
totallydiscountingtherisk,considerationshouldbegiventowhetherthesiteanditsgeologicalstrata
havepotentialforhighsulfate/sulfideconcentrations .Figure1,reproducedfromBRESD1[5],showsthe
locationofsulfate-bearingstrataintheUKandTable1summarisesinformationprovidedinAppendixB
ofHighwaysAgencyAdviceNoteHA74/07Treatment of fill and capping materials using either lime or
cement or both[7]aboutstratawiththepotentialforhighsulfate/sulfide .
IntheUnitedStates,theNationalLimeAssociationhaspublishedaTechnicalMemorandum[8],setting
outaProtocolforlimestabilisationofclaysoilscontainingsolublesulfates .Thissuggeststhatwhere
thetotallevelofsolublesulfatesisbelow0 .3%SO4thepotentialforaharmfulreactionislowand
shouldnotbeofsignificantconcern .Abovethislevelthepotentialforsomelocaliseddistressdueto
seamsofhighersulfateconcentrationnotdetectedintestingisconsidereda‘factoflife’ .TheNational
LimeAssociationProtocoldoesnotcoverthepossibilityofsulfidesbeingpresentandoxidisingto
sulfates .
AstheTPSincreasesabove0 .25%SO4,theriskwillincreasebutthereisnosimplerelationship
betweensulfate/sulfidecontentandexpansion .ThisisreflectedinHA74/07[7],whichsuggeststhat
thelimitsforacceptabilityofsulfide/sulfatecontenthavetobedeterminedonanindividualsite-by-site
basis .Thisinvolvesobtainingsoilsamplesfromdifferentpartsofthesite,whichhavedifferentsulfide/
sulfatelevels .Thesesamplesaremixedwiththechosenbinderandsubjectedtoswell-testingtoenable
themeasuredswellingtoberelatedtothesulfate/sulfidecontent .HA74/07alsosuggeststhatthe
water-solublesulfatecontentinthegroundwatershouldnotexceed1,500mg/l,inordertocomplywith
thelimitsforwatersolublesulfatewithin500mmofcementitiousmaterials .
WheretheTPSexceeds1 .0%SO4,stabilisationshouldonlybecarriedoutwithextremecaution .
Theriskofswellingshouldbeamajorfactorinthechoiceofbinderandthereshouldbeextensive
laboratoryswelltestingtoprovethesuitabilityofthechosenbinder .
Table 1: Geological strata with a potential for high sulfate/sulfide
AlluviumBembridgebedsSandgatebedsWoolwichandReadingBedsGaultClayLondonClayKimmeridgeClayOxfordClayWealdclayBlueLiasMiddle&LowerLias
UpperFuller’sEarthMerciaMudstoneEdlingtonFormationPortsmouthSandSherwoodSandstoneNurslingSandWhitecliffesandCarboniferousshalesStonesfieldslatePeatQuaternarydepositssuchasglacialtill*
*Quaternarydepositsareunlikelytocontainhighsulfate/sulfidelevelsbecauseconditionsduringdepositionwerenotanaerobic,butsomedepositionofsulfatesmayhaveoccurredduringformation .
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Stabilisation of Sulfate-Bearing Soils
Figure 1: Principal sulfate- and sulfide-bearing strata in England and Wales
Northoftheindicatedline,muchofthesestrataarecoveredbyglacialdepositswhich,ifpartiallyderivedfromtheindicatedstrata,mayalsocontainsulfidesandsulfates
WealdClay
LondonClay
KimmeridgeClayOxfordClayLowerLiasClay
MerciaMudstone(KeuperMarl)Limitofmainareasofglacialdeposits
6 Choice of binder to minimise the risk of sulfate/sulfide disruptionWhereithasbeenestablishedthattheTPScontentinasoilisbelow0 .25%SO4,thereisminimalriskof
sulfate-induceddisruptionandlimeand/orcementcansafelybeused .WheretheTPSisgreaterthan
0 .25%SO4,stabilisationwithlimeand/orPortlandcementmayinvolveariskofsulfateexpansionandit
wouldbeprudenttoconsidertheuseofotherbindersincombinationwithlime .
Theuseofsulfate-resistingPortlandcementdoesnotreducesulfate-swellinginstabilisedclays[1] .
However,significantlyenhancedresistancecanbeachievedbyusinggroundgranulatedblastfurnace
slag(ggbs)incombinationwithlime[9] .HighwaysAgencyAdviceNoteHA74/07suggeststhat
lime+ggbsisapreferredoptionwheretherearesignificantlevelsofsulfatesorsulfidespresentinthe
soil .Furtherdetailontheuseofggbstocombatsulfatedisruptioncanbefoundatwww .ukcsma .co .uk .
CombinationsoflimeandggbshavebeensuccessfullyusedonmanyUKsites,withclaysthatcontained
upto1%ASandupto2%TPS .Theproportionofggbsshouldbeatleastequaltothatofthelime(by
weight,andassuminguseofquicklime,CaO) .Increasingthisproportionimprovestheresistanceto
expansion,andtypicallyforhighresistance,aratioof3:1ggbstolimemightbechosen[10] .
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7 - Laboratory swell-testing of stabilised mixtures
Laboratorytests[11]suggestthatflyashmayalsobecapableofreducingthesulfateheaveinlime
treatedclays .Inthesetests,greaterproportionsofflyashwereneededtocontrolswellingthanwith
ggbs,typicallyratiosofflyashtoquick-limeof3:1to8:1 .Coarseflyashwasfoundtobemoreeffective
atpreventingswellingthanfineflyash .Additionally,flyasheswithhigherlevelsofsulfatewerefound
tobelesseffectiveatpreventingsulfateheave .Furtherdetailscanbefoundonwww .ukqaa .org .uk–
TechnicalDatasheet6 .5
It should be noted that there may be other approaches that are potentially beneficial, for example:
• increasedmellowingperiods .
• asecondstagelimeadditionfollowinganextendedmellowingperiod .
Extensivelaboratorytestingshouldbecarriedouttoverifythesuitabilityoftheseoptions .
7 Laboratory swell-testing of stabilised mixturesWheretheTPSislessthan0 .25%SO4,thechoiceofthebinder(s)willbedeterminedbyfactorsother
thansulfate/sulfidecontentofthesoil,andswelltestingmaynotbeconsideredessential .Where
theTPSisabove0 .25%SO4,theriskofsulfide/sulfateswellingmustbeafactorindeterminingthe
binder(s)selectedfortest .Swelltestsareessential,particularlywhentheASisalsoabove0 .25%SO4 .
WheretheTPSexceeds1 .0%SO4,stabilisationshouldbecarriedoutonlywithextremecautionandthe
riskofswellingmustbeamajorfactorinthechoiceofbinder .Extensivelaboratorytesting,including
swelltests,mustbeemployed .Extendingtheimmersiontestingto56daysmaybeappropriatein
situationswheresignificantsulfideispresent,becausethetimerequiredforsulfidetooxidisemay
delayexpansionsuchthatitisnotcompletewithin28days .
DetailedguidanceonlaboratorytestingcanbefoundinHA74/07[7] .HA74/07recommendsthatswell
testingshouldbecarriedoutinaccordancewithBSEN13286-47[12]onsoakedspecimensprepared
andtestedinaCBRmould .Therecommendedlimitsarethattheaveragedegreeofswellingshould
belessthan5mm(4%),withnoindividualtestspecimenswellingmorethan10mm(8%) .HA74/07
emphasisestheimportanceofcontinuingmeasurementsforatleast28daystoensurethatallswelling
hasceased(whichnormallyoccurswithin14days) .Itstatesthatifswellingisstilloccurringafter28
daysbutisstillbelowthislimit,somesubjectiveassessmentofchangesintherateofswellmaybe
necessary:alternativelytheswellingtestperiodcouldbeextendedto56daysprovidedtherateofswell
wasdeclining .
Therearedifferingviewsonthemostappropriatetestforsulfate/sulfideswelling .Ithasbeensuggested
[13]thatthattheconditionsforoxidationofpyriteand/orthedevelopmentofswellrealisedinthefield,
arenotfullyreplicatedusingtheBSEN13286-47CBRswelltestbecauseboththeprotectionoffered
bytheCBRmouldandthemethodofsoakingemployed,hindertheaccessofwaterandair .Alternative
testshavebeenproposed,Theuseofunconfinedspecimens,manufacturedinProctororMCVmoulds,
fullyimmersedinwater,mightbeamoreaccurateandrobustmethodbutthereexistsnostandard
testmethod .TheEuropeanStandardBSEN13286-49[14],specifiesanunconfinedswelltest,with
thematerialcompletelyimmersedinwaterat40°C:thevolumetricexpansionismeasuredratherthan
thelinearswell .ThistestiscalledupinBSEN14227-11:2006[15]forsoiltreatedbylimewhereitis
recommendedthatthevolumetricexpansionshouldnotexceed5%(wherethisisgreaterthan5%but
doesnotexceed10%,itissuggestedthattheuseofthemixturemightbepossiblesubjecttofurther
evaluation) .HA74/07notesthatbecausetheBSEN13286-49testisundertakenat40°C,itaccelerates
certainchemicalreactionsbutdoesnotreplicatereactionswhichoccuratlowertemperatures .HA
74/07alsosuggeststhatthetestmaybetooharshandunnecessarilyrestricttheuseofstabilised
materialsforcappingintheUK .
8
Stabilisation of Sulfate-Bearing Soils
8 Good construction practice for the stabilisation of claysIrrespectiveofwhethersulfatesarepresentornot,sufficientwatermustbeavailabletofullyhydrate
thequicklimeasanythatremainsunhydratedinawellcompactedlayerisapotentialsourceoffuture
expansionwhenithydratestoCa(OH)2 .Toensurefullhydration,thetargetMCVduringmellowingshould
be10,butnotexceeding12 .Atfinalcompaction,theMCVshouldalsonotexceed12 .
Where sulfate/sulfide is considered a risk, it is advisable to:
• Avoidtheuseofsoilfromareasknowntohavehighsulfatecontents .
• Mixsoilstodiluteanyhighconcentrationsofsulfate,e .g .over-digtheformationatthebottomof
acutandreplacewithothersoil,orattheveryleastwiththesamesoilthathasbeenexcavated,
maybestockpiledandthenre-laid .
• EnsuregoodcompactionbyusingtheMCVsrecommendedabove .Thiswillachieveadenselayer
withminimalairvoidstherebyreducingthepotentialforwateringress .
• Employdrainagesystemsthatminimisethepotentialforsubsequentwaterentryintothelayerfrom
eithertheside,beloworabove .
• Applyaneffectivesealcoatfollowingfinalcompaction .Thiswillprovidesomebarriertowater,but
moreimportantlywillreducetheriskofdrying-shrinkagecracks,whichwouldalloweasyingressof
water .
Whereasoilisknown,orsuspected,tocontainminorquantitiesofsulfate,butthelevelisconsidered
lowenoughtobeacceptableforlime-onlystabilisation,itisbeneficialtoencouragetheformation
ofanyexpansivesulfatecompoundspriortocompaction,asanyexpansivemineralsalreadyformed
beforeplacementandcompactionwillnotcausedisruption .Thiscanbedonebydelayingthefinal
compactionandprovidingadequatewaterduringmellowing-thelongerthemellowingperiodand
thehighertheambienttemperature,thebetter .Lessthan48hourswouldprobablybeineffectivefor
thispurpose .Anadequateamountofwateristypically3to5percentagepointsabovetheoptimum,
sayanMCVof10 .Asusual,secondstagemixingandcompactionwithoutfurtherstabiliseraddition
shouldfollowmellowing .Ifanextendedmellowingperiodisenvisaged,thisshouldbereflectedinthe
proceduresusedforthelaboratorytesting .
Wherelimewithggbsisused,theggbsisnormallymixedinseparatelyfromthelimeatthesecond
stage .Withclaysoils,thetimebetweenthetwomixingsneedonlybesufficientforthelimetomodify
andmellowtheclaysuchthatitlosesitscohesivityandcanbemixedintimatelywiththeggbs .
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9 - References
9 References
1 SHERWOOD,PT .Theeffectofsulphatesoncement-andlime-stabilisedsoils .Roads and
Road Construction,40(470),pp .34–40,(1962) .
2 DEPARTMENTOFENVIRONMENT,TRANSPORTANDTHEREGIONS .Thethaumasiteform
ofsulfateattack:Risks,diagnosis,remedialworksandguidanceonnewconstruction .
ReportoftheThaumasiteExpertGroup .Publisher:UKDepartmentoftheEnvironment,
TransportandtheRegions,London(1999) .
3 HIGGINS,DD,THOMAS,B,&KINUTHIA,JMPyriteoxidation,expansionofstabilisedclay
andtheeffectofggbs,4thEuropeanSymposiumonthePerformance of Bituminous and
Hydraulic Materials in Pavements,UniversityofNottingham,UK(2002) .
4 SHERWOOD,PT .Thestabilisationwithcementofweatheredandsulphate-bearingclays .
Geotechnique,December1957 .
5 BUILDINGRESEARCHESTABLISHMENT .Concrete in aggressive ground .BRESpecial
Digest1,Garston,BRE(2005) .
6 REID,JM,CZEREWKO,MA&CRIPPSJC .Sulfate specification for structural backfills .
TRLReport447,Crowthorne,UK(2001) .
7 THEHIGHWAYSAGENCY .DesignManualforRoadsandBridges .Volume4,Section1,Part
6,HA74/07Treatment of fill and capping materials using either lime or cement or both
(2007) .
8 NATIONALLIMEASSOCIATION .Guidelines for stabilisation of soils containing sulfates .
Texas,USA,www .lime .org,(2000) .
9 HIGGINS,DD .&KENNEDY,J .Lime+ground granulated blastfurnace slag stabilisation
of boulder clay on the A421 Tingewick Bypass .Paperpresentedtothe3rdEuropean
SymposiumonthePerformanceandDurabilityofBituminousMaterialsandHydraulic
StabilisedComposites .Leeds,UK(1999) .
10 HIGGINS,DD,Soil Stabilisation with Ground Granulated Blastfurnace Slag,Cementitious
SlagMakersAssociation,www .ukcsma .co .uk2005 .
11 McCarthyMJ,CsetenyiLJ,SachdevaA,andJonesMR:RoleofFlyAshinthe
MitigationofSwellinginLimeStabilisedSulfate-BearingSoils,WorldofCoalAsh,
Lexington,Kentucky,May2009,[http://whocares .caer .uky .edu/wasp/AshSymposium/
AshLibraryAgenda .asp]
12 BSEN13286-47:2004Unbound and hydraulically bound mixtures-Part 47: Test method
for the determination of the California bearing ratio, immediate bearing index and linear
swelling,London,BSI .
13 NOTMAN,C,Review of Swell Testing Procedures for Stabilised Soils,Highways
ConsultancyGroup/HighwaysResearchGroupTaskReference:260(387)MTSC,Highways
Agency(2008)
14 BSEN13286–49:2004,Unbound and hydraulically bound mixtures–Part49 .
Acceleratedswellingtestforsoiltreatedbylimeand/orhydraulicbinders .London,BSI .
15 BSEN14227-11:2006,Hydraulically bound mixtures-Specifications-Part 11: Soil treated
by Lime,LondonBSI .
10
Stabilisation of Sulfate-Bearing Soils
10 Appendix
Testing of soils for sulfates and sulfidesThis note is intended for those not familiar with the detail of the chemical testing of soils. It provides background information on the test methods (including those in TRL Report 447) that are commonly used for measuring sulfate and sulfide.
Sulfurmaybepresentinsoilsassulfate(SO4)e .g .gypsum(CaSO4 .2H2Oorassulfide(S),e .g .pyrite
(FeS2) .Itiswellestablishedthatthepresenceofexcessivegypsumorothersulfatescancause
stabilisedsoilstoexpandduetoettringiteformation .However,sulfidessuchaspyritecanalsobe
disruptivebecausetheyhavethepotentialtooxidisetosulfatesandsubsequentlycauseexpansion .
Consequentlywhentestingsoilsforsoilstabilisation,itisimportanttoestablishthecontentofboth
sulfateandsulfide .
Historically,testingofsoilsforsulfatehasbeencarriedoutinaccordancewithBS1377,Part3:1990,
Soils for civil engineering purposes: Chemical and electro-chemical tests .BS1377doesnotcontain
atestforsulfide,andthishasgenerallybeencarriedoutusingthetestmethodincludedinBS1047
1983,Specification for air-cooled blast furnace slag aggregate for use in construction .BS1047was
withdrawninJune2004,butanessentiallyidenticaltestmethodfordeterminationoftotalsulfur
contentappearsinBSEN1744-1:2009,Tests for chemical properties of aggregates, Part 1: Chemical
analysis .
TRLReportTRL447Sulfate specification for structural backfillsreviewedthemethodsforanalysing
soilsforsulfateandsulfideandproposednewtestmethods,whichtakeadvantageofadvancesin
instrumentationandarequickerandless-expensivethanthehistorical‘wet-chemistry’methods .The
TRL447testmethodsarenowbeingcalledupinHighwayAgencySpecificationsandtheiruseis
recommendedinpreferencetotheoldermethods .HoweverresultsobtainedwiththeBS1377andBS
EN1744-1methodscanstillbeusedsincesulfate/sulfidedistributionswithinasitetendbevariable
andanydifferencebetweenaTRL447testresultandthatobtainedusingthehistoricalequivalentis
unlikelytobeofcriticalimportance .
TRL447introducedseveralnewacronyms/abbreviations,whichinclude:
WSS water-solublesulfur determinedin2:1waterextractandexpressedas%Sinthesample
WS water-solublesulfate expressedasg/lSO4inthe2:1waterextract
ASS acid-solublesulfur expressedas%Sinthesample
AS acid-solublesulfate expressedas%SO4inthesample
TS totalsulfur expressedas%Sinthesample
TPS totalpotentialsulfate ‘TPS’istheTScontent,convertedto%SO4
OS oxidisablesulfide ‘OS’istheTScontentlessanysulfurpresentassulfate
11
10 - Appendix
Thefollowingtableoutlinesthebasisofthemorecommonlyusedtestsandindicatestheirusefulnessandlimitationsinregardtoassessingthepotentialfor
sulfatedisruptionofstabilisedsoil .
Test
Water soluble sulfates
2:1 water:soil extract
Water soluble sulfates
10:1 water:soil extract
Acid soluble sulfatesor ‘total sulfate’ Total sulfur
Examplesoftestmethods
BS1377Part3orTRL447TestNo .1:Watersolublesulfur(WSS)
TexasDofTmethod:Tex-620-J
BS1377Part3orTRL447TestNo .2:Acid-solublesulfur(ASS)
BSEN1744-1orTRLTestNo .4:Totalsulfur(TS)
Methodofextraction
Thewatersolublesulfates(WS)areextractedbyshakingdistilledwaterwiththesoilintheratio2:1water:soilpartsbyweightfor16hours .
Thewatersolublesulfatesareextractedbyshakinghot(‘nearboiling’)distilledwaterwiththesoilintheratio10:1partsbyweightfor24hours .
Thesulfateisextractedusinghydrochloricacid .
Variousmethodsofextractionareemployedbuttheaimofallmethodsistoextractandmeasureallthesulfur,irrespectiveofwhetheritispresentassulfateorsulfide .
Howtheresultsareexpressed
BS1377resultsareexpressedasgSO3/l*inthewaterextractTRRL447resultsareexpressedas%S*inthesoilsample(WSS)andalsoasgSO4/l(WS)inthewaterextract .
Theresultisexpressedas%SO4inthesoilsample .
BS1377resultsareexpressedas%SO3*inthesoil .TRRL447resultsareexpressedas%S*inthesoilsample(ASS)andalsoas%SO4(AS)inthesoilsample .
BSEN1744-1resultsareexpressedas%Sinthesoilsample .TRL447resultsareexpressedas%S(TS),as%SO4(TPS)andalsoas%SO4(OS),inthesoilsample .
Comments Thistestisoflimiteduseforassessingthegypsumcontentofsoils,becausecalciumsulfateisonlysparinglysoluble(1 .5gSO4/lwater) .Ifthegypsumcontentofthesoilisgreaterthan0 .3%SO4,anyexcesswillnotbeextractedormeasured .
Byusingalargerwater:soilratioitispossibletoextractmoregypsum .Aratioof10:1willbeeffectiveinextractinggypsumuptoacontentinthesoilof1 .5%SO4 .
Theacidextractsallthesulfatepresentinthesoil,possiblyincludingsomethatisnotsolubleinwater .However,anyslight‘overestimate’isunlikelytobesignificant .Thismethodwillnotdetectsulfidepresentinthesoil .Sulfideshavethepotentialtoconverttosulfatesandcausesulfateexpansion .
Thismethodmeasuresallthesulfides/sulfates .Conversionoftotalsulfurtosulfatecalculatestheworst-casepotentialforsulfatebutmaysomewhatoverestimatetheriskbecausechemicalspeciessuchasorganicsulfides,whichareunlikelytocontributetosulfateexpansion,aredetected .
*ToconvertSO3toSO4,multiplyby1 .2;toconvertStoSO4,multiplyby3 .
BP/51 .Firstpublished2005 .Fullyrevised2011 .©Britpave .
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