contents · • clay fill where the water will influence the foundation or where ... where...

CHAPTER 5: Foundations

CONTENTS

5.1 GROUND IMPROVEMENT

5.2 FOUNDATIONS NEAR TREES

5.3 STRIP AND MASS FILLED FOUNDATIONS

5.4 PILED FOUNDATIONS

5.5 RAFT FOUNDATIONS

FUNCTIONAL REQUIREMENT

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Workmanship

i. AllworkmanshipmustbewithindefinedtolerancesasdefinedinChapter 1 of this Manual.

ii. Allworktobecarriedoutbyatechnicallycompetentpersoninaworkmanlike manner.

iii.Groundimprovementschemesshouldbeappropriatelytestedtoconfirmthatthecompletedworksmeetsdesignspecifications.ThetestingregimemustbeagreedwiththeSiteAuditSurveyorpriortocommencementofwork.

iv. TheDevelopershallensurethatadequatequalitycontrolproceduresareinplace.Thequalitycontrolmustidentifythatsiteworkmeetsthedesignintention.Allproceduresshouldbeauditableandavailableforinspection

Materials

i. Allmaterialsshouldbestoredcorrectlyinamannerwhichwillnotcausedamageordeteriorationoftheproduct.

ii. Allmaterials,productsandbuildingsystemsshallbeappropriateandsuitable for their intended purpose.

iii.Thestructureshall,unlessspecificallyagreedotherwisewiththewarrantyprovider,havealifeofnotlessthan60years.Individualcomponentsandassemblies,notintegraltothestructure,mayhavealesserdurabilitybutnotinanycircumstanceslessthan15years

Design

i. Designandspecificationsshallprovideaclearindicationofthedesignintentanddemonstrateasatisfactorylevelofperformance.

ii. Siteinvestigationbyanappropriatelyqualifiedpersonshouldbeprovidedandprovidethefollowinginformation.Theinvestigationmustbeendorsedbythespecialistfoundationscontractor.

• Depthoforiginalsoiltypesbelowthestructure.• Detailsofanyfilledgroundanditssuitabilitytoacceptground

improvementstechniques.• Gasgenerationorspontaneouscombustionfromground

conditions.

iii.StructuralelementsoutsidetheparametersofApprovedDocumentA(EnglandandWales),Section1(Scotland)andTechnicalBookletD(NorthernIreland)mustbesupportedbystructuralcalculationsprovidedbyasuitablyqualifiedexpert.

iv. ThegroundimprovementworksmustmeettherelevantBuildingRegulationsandotherstatutoryrequirements,BritishStandardsandEuro-Codes.

5.1 GROUND IMPROVEMENT

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5.1.1 Introduction

Ground improvement enables sites with poor loadbearingcapacitytobestrengthenedsothattheloadingsoftheproposedbuildingcanadequatelybesupportedoffsuitablefoundations.TheguidancedetailedwillbeacceptedasasatisfactorymethodofmeetingtheFunctionalRequirements.

5.1.2 Limitations of guidance

Thefollowingsituationsarebeyondthescopeoftheguidanceinthissection:

• Wheretheoriginalgroundorsub-stratais unstableorwillcontinuetosettle.• Siteswithsoftclayswithalowbearingcapacity (30kN/m2undrained).• Filledgroundwherehighlevelsofvoidsare anticipated.• Clayfillwherethewaterwillinfluencethe foundationorwherecollapsemayoccur.

Eachdevelopmentsitehasitsownspecificcharacteristicsandwhereconditionsdonotclearlyfallwithintheguidancegiven,clarificationshouldbesoughtfromtheSiteAuditSurveyororasuitablyqualifiedandexperiencedexpert.

5.1.3 Vibratory ground improvement

5.1.3.1 Introduction

Vibrodisplacementusingvibrostonecolumnsisamethodofenhancinggroundbearingcapacityandlimitingsettlement.Typicalapplicationsinclude

supportoffoundations,slabs,hardstandings,pavements,tanksorembankments.Softsoilscanbereinforcedtoachieveimprovedspecificationrequirementwhilstslopescanbetreatedtopreventslipfailure.Bothnaturalsoilsandmadegroundcanbeimproved.

5.1.3.2 Vibratory techniques

Thevibratoryprocessisappliedtoweaknaturalsoilsandfilledgroundwithaviewtoimprovetheloadbearingcapacityandprovideanadequatebearingstratumforthebuilding’sfoundations.

TherearetwovibratorytechniquescommonlyusedintheUK.Theseareknownasthe‘drybottomfeed’and‘drytopfeed’methodsandathirdtechniquelessfrequentlyusedintheUKisknownasthe‘wetbottomfeed’method.

Dry bottom feed method

Thedrybottomfeedmethodisusedinweakersoilconditionsorwherethereisahighwatertableandtheboreholeisliabletocollapsebetweenvibroflotinsertions.Thevibroflotpenetratesusingitsmass,airflushandvibration,butatdesigndepththestoneisintroducedviaahopperintoapipefixedtothesideofthevibroflot.Thestone,usuallyof40mmsize,exitsthepipeatthetipofthevibroflotandintothebottomoftheborehole.Thestoneisthencompactedintothesurroundingsoilbyrepeatedwithdrawalandinsertionofthevibroflot.

Dry top feed method

Inthedrytopfeedmethodthevibroflotpenetratestheweaksoilorfillagainusingitsmass,airflushandvibrationtoformaborehole.Oncerefusalordesigndepthisreachedthevibroflotisremovedandstonefillisintroducedintothebore,the‘charge’istypically500-800mmdeep.Thevibroflotisre-insertedand‘packs’thestoneintothesurroundingstrata.Successivechargesofstoneareaddedandcompactedbringingthecolumnuptoworkinglevel.Typicallythestonegradingis40-75mm.

Figure 1: Vibratory techniques - dry bottom feed method

Figure 2: Vibratory techniques - dry top feed method


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passesoverthem.Itshouldalsobeestablishedatanearlystagewhetherthesitehaspreviouslycontainedanybuildingsorstructures,andwhethertheyhavebeencompletelyremovedincludingbasementwalls,floorslabsetc.Thepresenceandextentofanyexistingorredundantservicesanddrainsshouldbeinvestigatedandtheassociatedbackfilltotheexcavations.AlsotheeffectthatanyproposedSustainableDrainageSystem(SuDS)mayhaveonthegroundconditionsshouldbeidentified.

TheEngineershouldsupervisethesiteinvestigation,takinganaccountofthefindingsofthedeskstudyandfirstlyestablishwhetherthereareanycontaminatedsubstancesorgasespresent.Datashouldbegatheredbyasuitablemethodforcomparisonwiththesiteposttreatment.Investigationsshouldbemadeintothepresence,levelandnatureofanygroundwater,andifitislikelytoriseandcauseheaveorcollapsebysaturation.

Theextentofanyareasofmadeupgroundonthesiteshouldbeestablished,including:

• Theproportions,compactionanddistributionofconstituentmaterialsthroughoutitsdepth.

• Thegradingandparticlesizedistributionoffillmaterials.

• Thepotentialforgasgenerationfromfillmaterialsandtheriskofcombustionofnaturaldeposits.

TheappointedSpecialistContractorshouldbesatisfiedthatthesiteinvestigationreportprovidesadequateandrepresentativeinformationinorder

Wet bottom feed method

Wherethegroundcontainsfinesandsilts,waterjettingfromthetipofthevibroflotisusedtoremoveloosematerialsandformacavityforchargesofstonetobeaddedtoreplaceanddensifythesoftground.Thecarbonfootprintofthisactivityisgenerallylessthanwithcomparablepilingsolutions.

Figure 3: Vibratory techniques – wet bottom feed method

5.1.3.3 Suitability of ground conditions

Throughtheprocessofasiteinvestigationitshouldfirstlybeestablishedbytheappointedengineerorsuitablyqualifiedspecialistthatthegroundiscapableofbeingimprovedbyavibratorygroundimprovementtechnique.Thesiteinvestigationshoulddeterminethedepthsandproperties of the natural materials under the site, includingthepresenceofcavities,mines,rocksorsoilswhichmaydissolveorerodewhenwater

todesignthegroundimprovement.TheresultsoftheinvestigationshouldbepresentedtotheSiteAuditSurveyorpriortothecommencementofthework.

TheDevelopershallobtainwrittenconfirmationfromtheEngineerandSpecialistContractorthatthesiteissuitablefortheproposedgroundimprovementsystemandthatalldetrimentalfactorsassociatedwiththesiteandtheproposeddevelopmenthavebeentakenaccountof.ThisistobemadeavailabletotheSiteAuditSurveyorpriortothecommencementofanyworkonthesite.

Site workmanship

Thespecialistcontractorshouldappointanengineertosupervisethevibratoryfoundationworksatalltimesandensurethat:

• Therequireddepthandbearingcapacityofstonecolumnsareachieved.

• Thestonecolumnsarecorrectlylocatedbeneath the proposed foundation and in accordancewithdesigndrawings.

Figure 4: The intersection of adjacent reinforced concrete strips


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5.1.4 Engineered Fill

5.1.4.1 Design of engineered fill

Carefulselectionofthematerialandcontrolledplacementshouldensurethattheengineeredfillformsanadequatefoundationmaterial,however,insomecircumstances,significantgroundmovementscanoccur.

Figure 5: Typical engineered fill construction

Engineeredfillswhichareusedtoproducesuitablyshapedlandformsforstructuresshouldbeconstructedtohighstandardstominimisetheriskofgroundmovementscausingdamagetopropertybuilt on shallow foundations.

Indesigningandspecifyingafillwhichistoformafoundationforbuildings,thefollowingtechnicalrequirementsshouldbeestablished:

i. Awellconstructedexcavation,safelyexecuted,withallsoftandhardspotsremoved,andmadereasonablydryandwelldrained.

ii. Soundfillwithoutundesirablematerialandcapableofcompactionasspecified,providedwithstarterandcappinglayersasnecessary.

iii. Placementandcompactiontoensurethattheperformanceofthefillwillmeetrequiredcriteriaasafoundationfill.

iv. Appropriatemonitoring.TheDesignermustensurethatallworkcanbecarriedoutsafelyasrequiredbytheHealthandSafetyExecutiveConstructionDesignandManagementRegulations.

5.1.4.2 Fill selection

Fillshouldbeclearlycategorisedintomaterialwhichmayandmaynotbeused:unsuitablefill,generalfill,restrictedfillandspecialfill.Fillmaterials must not present an environmental or healthhazard.

Unsuitablefillshouldnotbeusedatanylocationon the site.

Generalfillisallmaterialexceptthatwhichisunsuitable,restrictedorspecialandisnormallythemajorityofthematerialused.Itmayincludenaturalsoilsaswellassomewasteproducts.


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Restrictedfillismaterialthatwouldbegeneralfillexceptthatitcontainsmineralshostiletothebuiltenvironment.Itcanincludenaturalmaterialssuchaspyriticshales,sulfate-bearingclaysandwastematerialssuchasburntcollierydiscardandsteelslag.Itsuseisprecludedwheregroundwatercouldrisetotheundersideofthedeepestfoundationorwhereitisrejectedforpollutionreasons.Forsomedevelopmentssuchashousingwithgardens,restrictedfillswouldincludefillswhichareharmfultopeople.

Specialfillishighqualitymaterialsuchaswellgradednaturalsandsandgravels,crushedrockorcleandemolitionrubble.Itsusewilloftenhavetobereservedforspecificallydefinedpurposessuchasacappinglayerorbackfilltoretainingwalls.Wherepossiblethough,granularsoilsshouldbeusedasgeneralfillsincethesematerialsdrainreadilyandconsolidatequickly.Thesmallerthepredominantparticlesize,thelongermaybethetimerequiredforconsolidationundertheself-weightofthefill.

5.1.4.3 End product criteria

Thegreatestthreatstosuccessfulin-serviceperformanceare:

• Collapsesettlementduetoinundationofdryorinadequatelycompactedfills.

• Excessiveconsolidationsettlementofwetcompressiblefill.

• Heaveorsettlementofclayfillsduetoclimaticchangesorvegetation.

Thesegroundmovementsdependonmoisturemovement,sobyreducingthevoidsinafill,theopportunitiesforexcessivein-servicemovementsshouldberestricted.Amaximumallowableair-voidscontentof5%isasuitablecriterionformostclayfills.However,specifyinga5%air-voidscontentisinsufficientasthisvaluemayeasilybeachievedbyaddingwatertothefillwithoutincreasingcompactiveeffort.Asuitablealternativecontrolmethodistospecifyaminimumacceptabledensityasaproportionofthemaximumdrydensitymeasuredinastandardlaboratorycompactiontest.Limitsonmoisturecontentarerequiredalso.Ifthefillistoowet,therecouldbeexcessiveconsolidationsettlementandifthefillweretoodry,itmightbevulnerabletocollapsecompression.

5.1.4.4 Placing engineered fill

Asuccessfulengineeredfillrequiresnotonlyanappropriatespecificationbutalsoadequatecontrolduringplacement.AlltheworkmustbecarriedoutwithdueregardtosafetyasrequiredbytheConstructionDesignandManagementRegulations.

5.1.4.5 Site preparation and disposition of fill

Thesiteshouldbeclearedofalltopsoilandotherunsuitable material.

Softspotsandhardspots,suchasderelictfoundations should be removed, also ponds and surfacewaterfromdepressions.Removingwaterbypumpingmaybenecessarywhenfillingsomeexcavationsbelowthegroundwaterlevel.

Whenavarietyofmaterialtypesareusedasfilltheyshouldbedepositedinhorizontallayersacrossthesite.Ifthereisonlyalimitedamountofgoodgranularmaterial,itwillbebesttouseitinlayersinterspersedbetweenlayersofpoorercohesivefill.

Thefillthicknessesshouldbereasonablyconstantbeneathastructuretominimisedifferentialsettlement.

Feather-edgesresultinginfoundationssetpartlyonfillandpartlyonnaturalgroundshouldbeavoidedandthesiteworkedinsuchawaythatstructuresarelocatedeitherdirectlyonnaturalgroundordirectlyoverfillofaconsistentthickness.

Iffillistobeplacedoverslopingnaturalground,somesteppingofthegroundmaybenecessary.Constructionoverthefaceofaquarryoranopencastmininghighwallshouldbeavoided.

Specialmeasuresmayhavetobetakenbyprovidingflexibleconnectionsforservicesatthelocationofhighwallsandbythickeningconstructionforserviceandestateroads.

Ifthenaturalgroundonwhichthefillrestsissoftandcompressible(forexample,layersofpeatorsoftclay),thesurfaceofthefillmaysettleappreciablyandunevenlyasaresultoftheweightofthefillconsolidatingthesoftlayersbelow.Thissettlementwill,ofcourse,beadditionaltothatresultingfromthecompressionofthefillitself.Sensitivestructuresmaywarrantasurface(orcapping)layerformedfromspecialfillcompacted


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tomoreonerousstandardsthantheunderlyingfill.Thisshouldhelptoensurethatdifferentialsettlementsufferedbythestructureisminimised.Wherelandscapedorothernon-loadbearingareasformpartofadevelopment,theyneedlesscompactionthantheloadbearingareas.ThereshouldbeatransitionzonearoundtheloadbearingareaasshowninFigure1.

5.1.4.6 Fill placement

Fillshouldbeplacedinhorizontallayerswitheachlayerbeingseparatelycompacted.Foragivenitemofplant,compactionperformancewillbedeterminedbyfilllayerthickness,fillmoisturecontentandthenumberofpassesofthecompactionequipment.Therearethough,otherfactorssuchastheneedtoavoidexcessivehandling.Wheneverpossible,sitetrialsshouldbeundertakentodeterminethecorrectcriteria.SomegeneralinformationaboutplacingfillsisgiveninBS6031.

Eachlayershouldbeofathicknessthatallowsthecompactiveenergytospreadthroughoutthelayer,producingthespecifiedfilldensityandlowair-voidscontent.Looselayershavingathicknessgreaterthan250mmareunlikelytobesatisfactoryforearthfillscompactedtosupportlow-risestructures.Itmaybenecessarytouselayersof200mmorless.Moisturecontentatthetimeofplacingafillisfundamentaltosubsequentperformance,particularlywherethefillcontainsalargeproportionoffinegrainedcohesivematerial.Ifthefillistoodrythereisthepossibilityofheaveorcollapsesettlement.Ifitistoowetthereisthepossibilityofinsufficientstrengthandhigh

compressibility.Itwillbedifficulttoachieveanair-voidscontentof5%orsmallerwhenthemoisturecontentislow.Inthesamewaythattheadditionoftoomuchwatercandetractfromtheperformanceofengineeredfill,soilcanbeover-compacted.Granularsoilsandcohesivesoilsdrierthanoptimum,whenrolledexcessively,becomeover-stressedandwhatshouldhavebeenafirmcompactedsurface,becomesaloosetilth.Thisshouldbeavoidedwheneverpossible.Whereafillcontainingalargeproportionoffinegrainedcohesivematerial(forexample,clay)isused,fillingduringwetweathershouldbeavoided.

5.1.4.7 Quality control

Qualitycontrolproceduresshouldbeimplementedtoensurecompliancewiththespecification.Thenatureofthecontrolprocedurewilldependonthetypeofspecificationadopted.Theendproductspecificationrequiresanappropriatetypeandquantityoftestingofthefillduringplacementtoensurethatthedesiredendproductisbeingachieved.Dependinguponthetypeofcontract,qualitycontrolmaybetheresponsibilityoftheengineerorofthecontractorworkingunderthesupervisionoftheengineer.Controlparametersshouldbethesameasthosedeterminedduringthesiteinvestigationstage.Bothdesignandcontrolparametersmustbereproducible,atermwhichdenotestherangewithinwhichmeasurementsmadeonthesamefillbydifferentoperatorsusingdifferentequipmentshouldagree.Thefollowingarethemostsignificantcontrolparameters:

• Moisturecontentinrespectofanoptimum

moisturecontentestablishedatthesiteinvestigationstage.

• Drydensityinrespectofthealreadyestablishedmaximumdrydensity.

• Air-voidscontent,whichdependsonmoisturecontentanddrydensity.

• Undrainedshearstrength,whichisanalternativetomonitoringmoisturecontentanddrydensityforclayfills.

Thelaboratorycompactiontestsandtheassociatedfieldcontroltestsaresuitableforawiderangeoffilltypes,andformthemostgenerallyapplicablemethodofapproach.Forcohesivesoils,undrainedshearstrengthformsanalternativebasisforspecificationandcontroltesting.However,differentmethodsofmeasuringtheundrainedshearstrength,suchastheunconfinedcompressiontestandthevanetest,cangivesignificantlydifferentvalues.Themeasuredvalueofcohesioncanbesensitivetoadetailedtestprocedure,suchastherateofshearing.

Itisimportantthatthemethodoftestingshouldbecloselyspecified.Whereacohesivefillcontainsgravel,itmaynotbepossibletoobtainsufficientlyundisturbedsamplesforstrengthtests.Onlargersites,thepossibilityofemployingin-situmethodssuchastheconepenetrometer(BS1377:Part9)couldbeconsidered.

Smallsitesaregenerallymoredifficulttoworkthanlargesites,asfinishedworkmaybedamagedmoreeasilyinconfinedworkingareasanddeficienciesinsitepreparationusuallyreflectmorereadilyinpoorerqualitycompactionthanonlarger


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sites.Consequently,itisnecessarytotestmorefrequentlyonasmallsitethanonalargeone.

AsuggestedminimumtestfrequencyispresentedinFigure6.However,eachsiteshouldbejudgedonitsownmeritswithcarefulnotebeingtakenofanyproblemsrevealedduringsiteinvestigation.Inveryvariableordifficultconditionsmorefrequenttestingmayberequired.Testsinvisuallydoubtful areas and re-tests of failed areas should becarriedoutadditionallytothoserecommendedinFigure6

Figure 6: Number of tests

Moderncompactioncontrolrequireslaboratoryandfieldtestingduringthesiteinvestigation,andduringandpossiblyfollowingtheearthworks.Theresultsofthisworkmustberecorded,collatedandpresentedsothatthequalityoftheoperationcanbedemonstrated.Therequireddocumentationincludes:

• Summaryofthespecificationrequirementsandtheendproductintermsoftheselected

geotechnicalparametersforthevariousfills(basedonsiteinvestigationinformation).

• Listoftherequiredsuitabilitytests,oneformtobecompletedforeachborrowpitunderinvestigation.

• Suitabilitytestresultsforeachborrowpit.• Listoftherequiredcontroltests.• Resultsofthecontroltestsoneachfilltypeor

layerorarea,asappropriate.• Alistofpost-compactionmonitoring

requirements.• Theresultsofpost-compactionmonitoring.All

completedformsshouldbesignedanddatedbythepersonresponsibleandalistpreparedofanyrequiredactionorremedialworktobecarriedout.

5.1.4.8 Monitoring of fill performance

Monitoringprovidesacheckonperformanceofthefillaftercompactionandisparticularlyimportantwherevulnerablestructuresaretobebuiltorfoundationloadingisunusuallylarge.Itisalsorequiredwherethefillisrelativelydeeporsubstantialgroundwaterrisewithinthefillisexpected.

Monitoring techniques include:

• Surfacelevellingstationstomeasurethesettlementofthefillsurface.

• Magnetextensometerstomeasurethesettlementofincrementaldepthsoffill.

• Standpipepiezometerstomeasuretheriseinthegroundwatertableinthefillafterplacement.

• Loadtestsfordirectestimationofsettlementofsurfacelayersproducedbyloadings.

Surfacelevellingstationsareeasytoinstallandveryeffective.Byopticallevellingofthestations,measurementcanbemadeofthetotalverticalmovementofthefilluponwhichtheyresttogetherwithanymovementoftheunderlyingnaturalground,althoughthisisunlikelytobelargeifallsoftmaterialhasbeenremovedpriortocompaction.Levellingstationsshouldbesufficientlyrobusttoresistdamageduetoconstructiontraffic.Aroundheadedboltcastintoaonemetreconcretecubeset300mmintothefillhasbeenfoundtobeeffective.

Magnetextensometersareunlikelytobenecessaryinshallow-depthfill.Standpipesorpiezometerswillbeofadvantageifthereisreasontosuspectthatgroundwaterwillriseintothefillatanytimeinthefuturewithconsequentsettlement.

5.1.5 Testing

Testingiscarriedouttoconfirmthatthegroundimprovementworksmeetsthedesigncriteria.Thetestsareusuallycompletedtodeterminethegroundbearingcapacity.

TheEngineershallrequiretheSpecialistContractortoverifythatthegroundtreatmenthasbeencompletedtoasatisfactorystandard.

Thiswillusuallyincludecarryingoutsuitabletestingtoestablishthedegreeofgroundimprovement,itsloadbearingcharacteristicsandsettlementpotential.Thesetestsmayinclude:


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5.1.5.1 Plate tests

Thistestwillnotdeterminethedesignbutwillallow for an assessment to be made of the workmanshiponthestonecolumns.Platetestsshouldbecarriedoutonstonecolumnsortreatedgroundatafrequencyofatleastonetestperdayperrig.

5.1.5.2 Mini zone tests

Aminizonetestcanbeusedasalimitedsubstituteforzonetests.Thetestshouldbeappliedtoatleasttwostonecolumnsandtheareaoffoundationwhichtheysupport.Tobeuseful,minizonetestsshouldbecontinuedforsufficienttimetoestablishthepresenceofcreepbehaviour.

5.1.5.3 Zone tests

Anisolatedpadorstripfootingisusedanduptoeightstonecolumnsandtheinterveninggroundcanbetested.Loadings,whichshouldsimulatethedwellingloadsareheldfor24hoursatpre-determinedstagestoexaminecreepbehaviour.

5.1.5.4 In-situ tests

Wherevibrationwillimprovethegrounditselfe.g.granularmaterials,thenin-situtestingisappropriate.Theimprovementcanbeassessedwhenthetestresultsarecomparedwiththein-situtestresultsrecordedduringthepre-treatmentinvestigation.

5.1.5.5 Trial pits

Trialpitscanbeexcavatedaroundtrialstonecolumnstoprovethattheyarefullyformedandtotherequireddepthanddiameter.Thisisadestructivetestandallowanceshouldbemadeaccordingly.

OncompletionofthetreatmenttheEngineershouldsatisfyhimselfthatthetreatedgroundhasachievedtheanticipatedconditionassumedinhisdesignandconfirmthisinwritingtotheSiteAuditSurveyor

5.1.6 Fill materials

Thefollowingmaterialsrequiretestingtoensuretheirsuitabilityforuseasfilltosupportstructuralfoundationsandslabsorasbackfilltoassociatedtrenches:

• Acidwastes• Reactivematerials• Materialsthatincludesulphates(e.g.Ggypsum)• Organicmaterials• Toxicmaterials• Materialsthatcausenoxiousfumes,rot,undue

settlementordamagetosurroundingmaterials

Thesamplestestedshouldbecarriedoutbyasuitablyqualifiedpersonanditmaybenecessarytotakeanumberofsamplestoexactlyidentifythematerialcharacteristicsofthefill.

5.1.7 Sources of fill material

Wherethematerialisofastableanduniformtypefromonesourcethetestingregimemaybereduced,howeverifmaterialisvariable,orfromanumberofsourcesregularinspectionsand/ortestingmayberequired.

RecycledaggregateorotherbuildingmaterialssuchascrushedbrickshouldonlybeusedfollowinganinspectionbytheSiteAuditSurveyor.Collieryshaleandanyotherresiduefrommineralextractionorindustrialprocessbi-productsshouldonlybeusedwithspecialistapproval.

5.1.8 Suitable foundations for sites with improved ground

Foundationsonsiteswithimprovedgroundshouldbeeitherofareinforcedstriporrafttype.BothfoundationswillrequirefulldesignbyaStructuralEngineer.FunctionalRequirement5.2Foundationsneartreesmustbemetwherefoundationsbearingoncohesivesoilsorcohesivefillmaterialiftreesarenearby.

5.1.9 Relevant British Standards and Guidance Documents

RelevantBritishStandards,CodesofPracticeandauthoritativedocumentsinclude:

• BS10175Investigationofpotentiallycontaminatedsites-CodeofPractice

• BSEN1991Actionsonstructures• BSEN14731Executionofspecialgeotechnical

works.Groundtreatmentbydeepvibration


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• BSEN1997-1Generalrules• BSEN1997-2Groundinvestigationandtesting• BSENISO14688Geotechnicalinvestigation

andtesting–Identificationandclassification of soil

• BSENISO14689Geotechnicalinvestigationandtesting–Identificationandclassification ofrock

• BSENISO22476Geotechnicalinvestigationandtesting-Fieldtesting

• BR391Specifyingvibrostonecolumns• ICESpecificationforgroundtreatment


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iii.Stripfoundationsshouldbeofasuitabledepthinordertoachieveasatisfactorylevelofperformance.

Materials



iii.Thestructureshall,unlessspecificallyagreedotherwisewiththewarrantyprovider,havealifeofnotlessthan60years.Individualcomponentsandassemblies,notintegraltothestructure,mayhavealesserdurabilitybutnotinanycircumstanceslessthan15years.

Design


ii. Foundationtypeanddepthmustbesuitabletoresistmovementfromtheinfluenceofnearbytrees.

iii.StructuralelementsoutsidetheparametersofApprovedDocumentA(EnglandandWales)Section1(Scotland)andTechnicalBookletD(NorthernIreland)mustbesupportedbystructuralcalculationsprovidedbyasuitablyqualifiedexpert.

iv. ThedesignandconstructionofthefoundationsmustmeettherelevantBuildingRegulationsandotherstatutoryrequirements,BritishStandardsandEuro-Codes.

5.2 FOUNDATIONS NEAR TREES


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5.2.1 Introduction

Thissectiongivesguidanceonfoundationdesignwhenbuildingneartrees.ThedetailsdescribedinChapter5willbeacceptedasasatisfactorymethodofmeetingtheFunctionalRequirements.


Thefollowingsituationsarebeyondthescopeoftheguidanceinthissectionandwillrequireasitespecificassessmentbyasuitablyqualifiedandexperiencedexpert.

• Foundationswithdepthsgreaterthan2.5mwithintheinfluenceoftrees

• Groundwithaslopeofgreaterthan1:7• Manmadeslopessuchasembankmentsand

cuttings• Underpinning• Engineeredfoundationdesigns

Eachdevelopmentsitehasitsownspecificcharacteristicsandwereconditionsdonotclearlyfallwithintheguidancegiven,clarificationshouldbesoughtfromtheSiteAuditSurveyororasuitablyqualifiedexpert.

5.2.3 The nature of the problem

Therootsofallvegetationtakewaterfromthesoiltomakegoodthewaterlostfromtheleaves.Ifthesoilcontainsclayitwillshrinkasitisdried,orswellifitisallowedtorehydrate.Iftheshrinkingorswellingextendsbelowthefoundations,thefoundationswillsubsideorheaverespectively.Ifthemovementsareinexcessofthosethatcanbe

toleratedbythebuilding,damageislikelytooccur.

Althoughallvegetationcancausesoildrying,therootsoftreesextenddeeperandfurtherandarethusparticularlypronetocausedamage.Largeshrubscanalsoroottoconsiderabledepthsbuttheirinfluenceismorelocalised.Herbaceousplantsandgrasscanalsocausesoildryingdownto1.5mandrequiresomeprecautions.

Damageisbestavoidedbyincreasingfoundationdepthtobelowthelevelweresignificantchangesinmoisturecontentarelikelytooccur.Thissectiondefinesthedepthsthatarerequired,andthemostsuitabletypesoffoundations.Theextentofsoildryingcanbevariableandunpredictable.Ifallriskofdamageistobeavoidedtherequireddepthswouldbepunitive;insteadtheguidanceseekstominimisetheriskbystrikingabalancebetweentheextentofriskandthecostsofincreasingfoundationdepth.

Theextentofriskdependson:

i. Thesoil.ii. Thepotentialforthetreespeciestocause

soildrying.iii. Thesizeofthetree.iv. Theproximityofthetreetothefoundations.v. Thelikelyclimaticconditionsinthelocality.

Thesefactorsareconsideredingreaterdetailinthefollowingsections.

5.2.4 The soil

Soilsmaybebroadlyclassifiedintotwotypes:

i. Cohesivesoilscomprisedmainlyofclayorfinesiltparticles.Whenmoisttheyareplasticandcanbemoulded,andwillremainintactifplacedintowater.Astheybecomedriertheywillbecomestiffer,andwilleventuallycrumbleifdriedbeyondacertainpoint.Itisthesesoilsthatcanpotentiallycauseproblems.

ii. Noncohesivesoils,comprisedmainlyofsandorwithonlyaproportionofclayorsilt,theycannotbemouldedandwillbreakupifplacedinwater.Theyarenotsubjecttosignificantswellingorshrinkage.

Theclaycomponentofcohesivesoilscanvarywidely;veryfewsoilsarepureclay,buttheycontainvaryingquantitiesofsandorsilt.Claysoilsaredefinedbytheirparticlesize(lessthan2microns),anditisonlytheseclayparticlesthatwillshrinkorswell.Theparticlesaremadeupofacomplexmolecularlatticestructurethatiscapableofabsorbingwaterintothelattice.Astheyabsorbwatertheparticleswillswell,andviceversa.Therearemanydifferenttypesofclaywithdifferentmolecularstructures,allofwhichhavedifferentswellingcharacteristics.Theextentofswellingandshrinkagethatcanoccurwillthereforedependonthetypeofclayparticlesandtheproportionofclay,asopposedtosiltorsand,within the soil.

Thepotentialofthesoiltoswellorshrinkcanbedeterminedbysimpleteststodeterminetheirplasticlimit(themoisturecontentbelowwhichit


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changesfrombeingplasticandmouldable,andstartstocrumble),andtheliquidlimit(themoisturecontentabovewhichitchangesfrombeingplastic,andstartstoflowlikealiquid).TheplasticandliquidlimitscanbedeterminedbysimplelaboratorytestsinaccordancewithBS1377.Thedifferencebetweentheplasticandliquidlimitsistheplasticityindex,thehighertheplasticityindex,thegreaterthepotentialvolumechange.

5.2.5 Potential of the tree species to cause soil drying

Treespeciesdifferintheabilityoftheirrootstogrowandexploittheavailablewaterinacohesivesoil,particularlyifithashighclaycontent.Thisiscommonlyreferredtoastheir‘waterdemand’.Speciessuchasoak,poplarandeucalyptusaredeemedashighwaterdemandastheyareparticularlyefficientatexploitingclaysoils,rootingtoconsiderabledepth.Afewspeciesonlyrarelycausedamageandaredeemedoflowwaterdemand,whilstthemajorityfallintothemoderatecategory.

Hardwoodspeciestendtohaveabroad-spreadingrootsystem,extendingconsiderabledistanceslaterallyaswellastodepth.Bycontrast,theinfluenceofmostconifersismorelocalisedbutjustasdeep.Afewspecies(ofbothhardwoodsandconifers)haveintermediatecharacteristics.Theguidancetakesaccountofthedifferentpatternsofrooting,butitmustbeemphasisedthatthedistributionofrootscanbevariableandsotheguidanceshouldnotbetakenasindicatinga‘zoneofinfluence’ofatree.

5.2.6 Size of tree

Theamountofwatertakenbytherootsrelatestotheleafareaandthevigourofthetree.Withopengrowntrees,heightisusuallyconsideredtobethebestindicatorofleafarea.Thegreatestwateruptakeoccursasthetreereachesmaturity,andso‘matureheight’isthedeterminingfactor.Individualtreeswithinagrouporrowwillhaveasmallerleafarea,butastheycompetewitheachotherthematureheightoftheindividualtreesremainstherelevantfactor.

Althoughsometreesaremanagedaspollardsorsubjecttoperiodicreductiontocontroltheirsize,unlesssuchtreatmentcanbeassuredinthefuture,matureheightshouldbeused.

5.2.7 Proximity

Thecloserthetree,thedeeperthepotentialinfluence.Theguidanceindicatestherequiredfoundationdepthatanydistance.Thepartsofthefoundationsclosesttothetreerequirethegreatestdepth,butifpreferredcanbesteppeddownformore distant parts.

5.2.8 Likely climatic conditions

Weatherconditionsplayamajorroleindeterminingtheextentofsoildrying.Hotsunnyweatherwillincreasetherateofwateruptakebytheroots,whereasrainfallduringthesummercanrestorethewaterthathasbeentaken.AsthehottestanddriestconditionstendtobeinSouthEastEngland,ithasthegreatestrisk.Forotherpartsofthecountrytheguidanceallowsfor

reducingtherequiredfoundationdepthwheretherisk is less.

5.2.9 Information required for determining the foundation depth

5.2.9.1 Establishing ground conditions

TheBritishGeologicalSurveywebsite(http://www.bgs.ac.uk)canindicatethelikelysoilconditionsinanylocality.Enterthepostcodetolocatethesite.Leftclickonthelocationbringsupaboxthatshowsthebedrockgeologyandthesuperficialdeposits(ifpresent).Thenameofthebedrockorsuperficialdepositsisoftensufficienttoindicateprobablesoilconditions(e.g.LondonClay,orPlateauGravel),butifnotclickingonthenamewillbringupfurtherdetails.

Unlessthereisclearevidencethatacohesivesoilisnotpresent,siteinvestigationswillberequiredtodeterminethesoiltypetoatleastthedepthofpotentialinfluenceofadjacenttrees.Usuallytrialholesareanacceptablemethodtodeterminingthesoilstrata,butspecialistsiteinvestigationreportsare preferred if available.

Soilsamplesshouldbetakenfromatleasttwodepthsat1.5mand2.5m(orthebaseofthetrialhole,whicheveristheshallower)andsenttoasoillaboratoryfordeterminationofplasticandliquidlimit(andthusplasticityindex).Inaddition,moisturecontentofthesamplesisusuallydetermined.

Thehighestvalueofplasticityindexshouldbeusedfordeterminingfoundationdepth.


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Londonderry

Enniskillen

Dungannon

Belfast

Domoch

Inverness

Aberdeen

Perth

Stirling

EdinburghDunbar

Glasgow

Ayr

Stranraer

Carlisle

Newcastle

AlstonWorkington Middlesbrough

DarlingtonWhitby

Ripon

YorkHebden Bridge

Doncaster GrimsbySkelmersdale

ManchesterSheffield

LincolnSkegnessMacclesfieldChester

ShrewsburyStafford

LeicesterLlanidloes

Llandrindod WellsHay-on-Wye

Gloucester

Swansea

Weston-super-Mare

WatchetBarnstaple

Taunton

ExeterSidmouth

Poole

CardiffBristol

Luton

Colchester

Peterborough Norwich

NorthamptonCambridge

Welwyn Garden City

London

Newbury Staines

Aldershot Gatwick

DoverSouthampton

Brighton

Eastbourne

Swindon

Brecon

Birmingham

Nottingham

Bangor

Bala

Colwyn Bay

Hull

Dundee

5.2.9.2 Identification of trees.

ManyLocalAuthoritieswillrequireaTreeSurveyandArboriculturalMethodStatementaspartoftheplanningapplication.Thiswillusuallyservetoidentifyallrelevanttreesbothonandoffsite.Ifatreesurveyisnotavailable,assistanceontreeidentificationofallofthecommonertreescanbeobtainedfromvariouswebsites:

Natural History Museum

http://www.nhm.ac.uk/nature-online/british-natural-history/urban-tree-survey/identify-trees/tree-key/index.html

Royal Botanic Gardens, Kew

http://apps.kew.org/trees/?page_id=17ScienceandPlantsforSchool(particularlyusefulforwinteridentification,usingtwigcharacteristics)http://www-saps.plantsci.cam.ac.uk/trees/index.htm

Ifatreecannotbeidentified,itmustbeassumedtohavehighwaterdemand(deeprooting).

5.2.9.3 Size of tree.

ThematureheightofcommonertreespeciesisobtainedfromTable4inAppendixA

Matureheightshouldbeusedunless:

i. Anarboriculturalreportisobtained,indicatingthatalesserheightisappropriatefortheconditionsofthesite.

ii. Assurancecanbeprovidedthatthetreewillbemaintainedatalesserheightatalltimesinfuture.

5.2.9.4 Proximity of tree.

Measurementshouldbetakenfromthecentreofthetrunktothenearestpartofthefoundations.Ifpreferred,foundationsdepthscanbesteppeddownatgreaterdistances,inaccordancewithparagraph5.5.4,bymeasurementtootherlocationsaroundthebuilding.

5.2.9.5 Climatic conditions.

DeterminefromthemapinFigure7whetherthedepthoffoundationscanbereducedfortherelevantsitelocation.

5.2.10 Determining foundation depth

5.2.10.1 Foundation depth calculator

Foundationdepthcanbedeterminedusingthefoundationdepthcalculatorwhichcanbe found at www.premierguarantee.co.uk/foundationcalculator.

Thedepthoffoundationisdeterminedbyinputting;

i. Plasticityindexofsoil(seesection5.2.1)ii. Waterdemandofthetree(seetable4).iii. Matureheightofthetree(seetable4),or

alternativevaluesbeingused(seesection5.2.3).iv. Distanceofrelevanttreetonearestpartof

foundations,anddistanceselsewhereifsteppingfoundations(seesection5.2.4)

v. Allowanceforclimaticconditions(seesection5.2.5). Figure 7: Allowable reductions for geographical location

Figuresindicatethereductioninmetreswhichcanbeappliedtothefoundation depths obtained from the relevanttables1.09-1.11

NOTE:

Theminimumfoundationdepthforafoundationonaclaysoil,wherenotreesarepresentorhavebeenremovedwithin3yearsis

• 0.9m(900mm)forlowtomediumvolumechangepotentialsoils;• 1.0m(1000mm)forhighvolumechangepotentialsoils(PIvalue>40%);or• inaccordancewithBuildingControlpolicy,whicheverisgreater.

0.35metres

0.25metres

0.15metres

0.05metres

Useactualdepthtablesobtainedfrom1.09-1.11


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5.2.10.2 Foundation depths to allow for future tree planting.

Wherethereisalandscapeplanspecifyingfuturetreeplanting,foundationdepthsshouldbecalculatedonthebasisoftheproposedspeciesoftreeanditsproximity.Ifnospecieshasbeenspecified,theyshouldbeassumedtobemoderatewater demand.

Evenifnotreeplantinghasbeenspecified,itisadvisable to allow for reasonable future tree or shrubplantingorforthegrowthofself-seededtreesorshrubs,asshownincolumntwoofTable1.

Ifthebuildingdesignorlocationissuchthatnotreeplantingislikelyatanytimeinthefuture,minimumfoundationdepthsasshownincolumnthreeofTable1shouldbeused.

Plasticity index

Depth to allow for reasonable future tree/shrub planting (m)

Minimum depth if no future tree/shrub planting likely (m)

>40 1.5 1.0

20-40 1.25 0.9

10-20 1.0 0.75

Table 1: Minimum foundation depths.

5.2.11 Woodlands, groups or rows with mixed species of trees.

Foundationdepthshouldbedeterminedonthebasisoftheindividualtreethatrequiresthegreatestdepth.

5.2.12 Foundation design

5.2.12.1 Depths in excess of 2.5m

Wheretherequiredfoundationdepths(asdeterminedinSection5.3)areinexcessof2.5m,foundationsmustbedesignedbyasuitableexpert,i.e.aCharteredStructuralEngineer,takingaccountofthelikelymovementofthesoilonthefoundationsandsubstructure.Shortboredpileswithgroundbeamsarerecommendedandmaywellprovetobethemosteconomicalformofconstruction.Shortboredpilesareanessentialrequirementfordepthsinexcessof3.0m.

5.2.12.2 Foundation depths less than 2.5m

Conventionalstripfoundationsmaybeconstructedpracticallyandeconomicallytoamaximumdepthof2.5m.

Trenchfillfoundationsarelikelytobemosteconomicatdepthsbelow1.5mbutcanbeeconomictodepthsupto2.5m.

Forfoundationdepthsinexcessof2m,shortboredpileswithgroundbeamsarerecommended.Allpiledesignsshouldbeundertakenbyasuitableexpert,i.e.aCharteredStructuralEngineer.

Structuralraftfoundationsaregenerallynotacceptedasasuitablefoundationonsiteswithahighriskofshrinkage/heaveduetoadjacenttrees.

5.2.12.3 Heave precautions.

Allowancemustbemadefortheprobabilitythatanyexistingtreeislikelytodiesometimeduringthelifeofthebuilding.Ifthetreehasdriedthesoilpriortothefoundationsbeinglaid,whenitdies(orbecomesovermature)thesoilwillrehydrateandswell,causingupwardorlateralheavemovementofthefoundations.Severingrootswithinthefootprintofabuildingfoundationwillalsoallowthesoiltorehydrate. Iffoundationdepthisgreaterthan1m,aproprietarycompressiblematerialmustbeplacedonallinsidesurfacesoftheperipheralfoundationstoallowforlateralsoilswelling,asshowninFigures8-10.Materialisnotrequiredoninternalfoundations(asswellingpressuresarelikelytobesimilaronbothsides).Thematerialmustbecapableofcompressingtoallowforlateralswellinginaccordancewithcolumn3ofTable2. Groundbearingslabsshouldnotbeusedifthefoundationdepthisgreaterthan1.5m.Underthesecircumstancesasuspendedfloorslabshouldbeused,incorporatingeitheravoidoraproprietarycompressiblematerialontheunderside.ThethicknessofthevoidshouldbeinaccordancewithTable2,orifacompressiblematerialisuseditshouldbecapableofcompressingtoprovideavoidofthisthickness.


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Plasticity index of soil

Required foundation depth (m)

Thickness of lateral compressible material (mm)

Thickness of void on underside of edge beam and floor slabs (mm)

>40 >2.5 Engineerdesign2.0–2.5m 35mm 100mm

1.5–2.0m 25mm 75mm

20–40 >2.5 Engineerdesign

2.0–2.5m 25mm 75mm

1.5–2.0m - 50mm

<20 2.0–2.5m - 50mm

<2.0m Nospecialprecautions.

Table 2: Minimum thickness of compressible material

AtypicalfoundationdesignstoallowforheaveareshowninFigures8-10.

Figure 8: Heave protection-section through a typical mass filled foundation

Figure 9: Plan of heave protection to a mass filled foundation

Figure 10: Heave protection section-pile and beam foundation


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5.2.13 Special situations

5.2.13.1 Trees removed prior to construction.

Iftreeshavebeenremovedpriortoconstruction,precautionsmustbetakenagainstpotentialrehydrationandswellingofthesoil.Iftheyhavebeen removed within 12 months of the foundations beinglaid,designshouldbeinaccordancewithsection5.4asifthetreewasstillpresent.Iftheheightoftheformertreesisknown,thedepthshouldbedeterminedusingactualheight.Iftheidentityisnotknown,itshouldbeassumedtobeofhighwaterdemand,andifheightisnotknown,itshouldbeassumed20m.

IftreeshavebeenremovedmorethanIyearpriortoconstruction,precautionsshouldbeinaccordancewithTable3.

Plasticity index

Time since tree felled (years)

Thickness of lateral compressible material

Thickness of void below slab (mm)

>40 2-3 35mm 100mm

4-5 25 75

20-40 2-3 25 75

Table 3: Minimum thickness of compressible material where trees have been removed

5.2.13.2 Sloping sites

Iftheslopeisgreaterthan1:7,foundationsshouldbeengineerdesigned(seesection5.1.2).

Forslopeslessthan1:7,distanceshouldbemeasureddowntheangleoftheslope.Ifthereisaretainingwall,includetheheightoftheretainingwallinthedistance.

Figure 11: Measuring foundation distance on sloping sites.

5.2.13.3 Changes in level

Changesingroundlevel(eitherraisingorloweringsoillevels)beneaththebranchspreadofthetreecandamagethetreeandshouldbeavoidedifpossible.

Ifgroundlevelsarealteredinproximitytoexistingtrees that are to remain, foundation depth should bedeterminedonthebasisofthematureheightofthetreeandoriginalgroundlevel.

Ifgroundlevelsarealteredinproximitytotreesthat are to be removed, foundation depth should

bedeterminedonthebasisoftheexistingheightofthetreeandoriginalgroundlevel.

5.2.13.4 Varying foundation depths.

Asfoundationdepthdependsontheproximityofthetree,thedepthcanbereducedinstepswithincreasingdistance.Stepsshouldbeinaccordancewithsection5.3ofthisManual.

5.2.13.5 Protection for drains

InadditiontotherequirementsofChapter9ofthisManual,drainageneartreesshouldincorporateadditional provisions where there is a volume changepotentialwithintheground,theprovisionsinclude:

i. Increasedfallstocaterforanygroundmovement.

ii. Deeperandwiderbackfillofgranularmaterial.iii. Adrainagesystemthatcapableofmovement

thatmayoccurduetoheaveandshrinkage.Drainagepipesshouldnotbeencasedinconcrete.

iv. Additionalclearanceisrequiredwheredrainspassthroughthestructureofabuildingtoallowfor additional movement.

5.2.14 Made up ground

Landorgroundcreatedbyfillinginalowareawithnon-originalsoilsorotherfillsmaterial.Often,suchcreatedlandisnotsuitableforbuildingwithouttheuseofspecialistfoundations.Ifthereishighclaycontentwithinthemadeupground,specialistfoundationsmayrequireadditional


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heaveprotection.Itisalsoimportanttoestablishthedepthofthemadeupgroundbecauseifitisarelativelyshallowdepth,theoriginalsoilbelowmaybecohesiveandbewithinthezoneofinfluenceofthetree.

5.2.15 Strip or trench fill foundations in non- shrinkable soils overlying shrinkable soils

Ifnonshrinkablesoilssuchassandandgravelsoverlieshrinkableclays,increasedfoundationdepthsarenotrequiredifthedepthofthenon-shrinkablesoilisgreaterthan0.8ofthedepthwhichwouldberequiredfortheunderlyingshrinkablesoil.SeeFigures12and13forfurtherclarification.

Appendix A Mature height of trees

Broad leafed tree H Conifer H

High water demand

Elm, English

Ulmusprocera

24 Cypress,Lawson

Chamaecyparislawsoniana

18

Elm,Wych Ulmusglabra 18 Cypress,Leyland

XCupressocyparisleylandii

20

Gum tree EucalyptusSpp.

24 Cypress,Monterey

Cupressus macrocarpa

20

Hawthorn Crataegusmonogyna

10 Cypress,smooth

Cupressus glabra

15

Oak,English

Quercusrobur 24

Oak,Holm Quercusilex 16

Oak,Red Quercusrubra 20

Oak,Turkey Quercuscerris

24

Poplar, Hybridblack

Populusxeuramericana

28

Poplar, Grey

Populus canescens

18

Willow,crack

Salixfragilis 24

Willow,white

Salixalba 24

Willow,Weeping

Salixalba‘Tristis’

16

Whitebeam Sorbusaria 14


Intermediate water demand

Elm, Wheatley

Ulmuscarpinifolia‘Sarniensis’

20 Cedar Cedrus spp. 20

Lime Tiliaspp. 24 Cypress,Italian

Cupressus sempervirens

12

Oak,Fastigiate

Quercusrobur ‘Fastigiata’

20 Wellingtonia Sequoiadendrongiganteum

24

Poplar,Lombardy

Populusnigra‘Italica’

25 Westernredcedar

Thujaplicata 18

Poplar, aspen

Populus tremula

18

Figure 12: Foundation depth required to be taken down using foundation calculator and plasticity index of underlying clay.

Figure 13: Foundation depth to be suitable for the non- shrinkable soil and ground conditions. No additional depth required due to trees.


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Moderate water demand

Acacia,False

Robinia pseudoacacia

16 Dawnredwood

Metasequoiaglyptostroboides

16

Apple Malus spp. 10 Douglasfir Pseudotsugamenziesii

18

Ash Fraxinusspp. 24 Fir Abies spp 18

Beech Fagussylvatica

20 Hemlock Tsugaheterophylla

16

Cherry,Japanese

Prunus serrulata

9 Juniper Juniperus communis

6

Cherry,Fruite

Prunus cerasus

12 Larch Larixspp. 16

Cherryplum

Prunus cerasifera

10 Maidenhair tree

Ginkgobiloba 16

Cherry,Wild

Prunus avium 16 Monkeypuzzle

Auracariaauracana

14

Chestnut, Horse

Aesculushippocastanum

20 Pine Pinus spp 16

Chestnut, sweet

Castanea sativa 18 Spruce Piceaspp 16

Maple, Japanese

Acerpalmatum

8 Yew Taxusbaccata 12

Maple, Norway

Acerplatanoides

18

Mountain ash

Sorbusaucuparia

10

Pear Pyrusspp. 12

Plane Platanus spp. 22

Plum Prunus domestica

12

Sycamore Acerpseudoplatanus

20


Low water demand

Birch Betulaspp. 14

Elder Sambucusnigra

10

Fig Ficuscarica 8

Hazel Corylusavellana

8

Holly Ilexaquifolium 12

Honeylocust

Gledistsia triacanathos

14

Hornbeam Carpinus betulus

16

Indianbean tree

Catalpa bignonioides

16

Laburnum Laburnumspp. 12

Magnolia Magnoliaspp 10

Mulberry Morus spp. 12

Sweetgum

Liquidambarstyraciflua

14

TreeofHeaven

Ailanthus altissima

20

Tuliptree Liriodendrontulipifera

18

Walnut Juglansregia 16

Table 4: Water demand (rooting depth) and mature heights (M) of common trees


Technical Manual TS

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Workmanship


ii.Allworktobecarriedoutbyatechnicallycompetentpersoninaworkmanlike manner.

iii.Stripfoundationsshouldbeofasuitabledepthinordertoachieveasatisfactorylevelofperformance.

Materials




Design


ii.StructuralelementsoutsidetheparametersofApprovedDocumentA(EnglandandWales),Section1(Scotland)andTechnicalBookletD(NorthernIreland)mustbesupportedbystructuralcalculationsprovidedbyasuitablyqualifiedexpert.

iii.StripfoundationsmustmeettherelevantBuildingRegulationsandotherstatutoryrequirements,BritishStandardsandEuro-Codes.

5.3 STRIP AND MASS FILL FOUNDATIONS


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5.3.1 Introduction

Stripandmassfillfoundationsareusuallythemostsimplisticandcosteffectivefoundationforlowrisebuildingsonoriginalground,theguidanceinsection5.3,providesdetailsofhowtomeetthefunctionalrequirements.


Thefollowingsituationsarebeyondthescopeoftheguidanceinthissection:

• Traditionalstripandmassfilledfoundationsforbuildingsotherthandwellings

• Dwellingsgreaterthanthreestoreys• Foundationsonfilledground• Stripandmassfilledfoundationswhere

foundationdepthsexceed2.5m

5.3.3 Design

Stripandmassfillfoundationsshallbedesignedtoensurethatthebuildingisappropriatelysupportedatalltimeswithoutexcessivesettlement.ThisfoundationtypeshouldonlybearontooriginalgroundunlessthefoundationhasbeendesignedbyaStructuralEngineerandappropriatelyreinforced.Itisthereforeimportantthatsiteconditionsareappropriatelyassessedpriortothebuildingdesign.FurtherguidanceforgroundconditionassessmentcanbefoundinChapter4SiteInvestigation.

5.3.4 Minimum foundation dimensions

Stripfoundationsshouldbe600mmminimumwidthforexternalwalls.Forsingleleafinternalwallsupto150mmthick,foundationsmaybereducedinwidthto450mm.Minimumthicknessofstripfoundationsshouldbe150mm.Foundationsshouldbesituatedcentrallybelowthewall.

Figure 14: Typical strip foundation

5.3.5 Foundation depths

Thedepthofallfoundationsshouldbedeterminedbyspecificsiteconditions.Allfoundationsmustbearontovirginstablesub-soiland,exceptwherestripfoundationsarefoundedonrock,thestripfoundation should have a minimum depth of 450mm,measuredfromfinishedgroundlevel,

totheirundersidetoavoidtheactionoffrost.Thisdepthhowever,willcommonlyneedtobeincreasedinareassubjecttolongperiodsoffrostor in order that loads are transferred to suitable ground.Wheretreesaresituatedclosetoaproposedbuildingfoundedonaclaysoil,thefoundationdepth/designwillbeaffectedandfurtherguidanceisavailableinsection5.2.

Inclaysoilswithaplasticityindexgreaterthanorequalto10%,stripfoundationsshouldbetakentoadepthwhereanticipatedgroundmovementwillnotimpairthestabilityofanypartofthebuildingtakingintoaccountoftheinfluenceofvegetationandtreesonoradjacenttothesite.Thedepthtotheundersideoffoundationsonclaysoilsshouldnotbelessthan750mmmeasuredfromfinishedgroundlevel,depthsmayneedtobeincreasedinorderthatloadsaretransferredtosuitableground.Table1givesdetailsofminimumfoundationdepths.

Modified plasticity index

Volume change potential 40 % and greater

Minimum foundation depth (m)

40%andgreater High 1.0

>20%-<40% Medium 0.9*

<20% Low 0.75*

*Ifthemodifiedplasticityindexisnotconfirmedminimumfoundation depths should be 1m.

Table 5: Minimum foundation depths


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Figure 15: Typical mass fill foundation

5.3.6 Setting out foundations

Theaccuracyofsettingoutfoundationsshouldbecheckedbyasetcontrolledtrenchmeasurements,includingtheirlocationrelativetositebordersandneighbouringbuildings.Levelsshouldbecheckedagainstbenchmarks,whereappropriate.Inparticular,forexcavationscheck:

• Trenchwidths• Trenchlengths• Lengthofdiagonalsbetweenexternalcorners

Wallsshouldbelocatedcentrallyuponthefoundation,unlessspecificallydesignedotherwise.AnydiscrepancyindimensionsshouldbereportedpromptlytotheDesigner.Resultingvariations

shouldbedistributedtoallconcernedwithsiteworks,includingtheSiteAuditSurveyor.

5.3.7 Excavations

Excavationshouldbetoadepththatgivessufficientbearingandprotectionfromfrostdamage.Toavoiddamagecausedbyfrost,thedepthofthefoundation(s)infrostsusceptiblegroundshouldbeataminimum450mmbelowgroundlevel.Ifthefinishedgroundlevelwillbeabovetheexistinggroundlevelthen,incoldconditionswhenfreezingisexpected,thefoundationdepthshouldbecalculatedfromtheexisting,notfinished,groundlevel.

Wheretrenchfillfoundationsareinexcessof2.5mdepth,theymustbedesignedbyaCharteredStructuralEngineerinaccordancewithcurrentBritishStandardsandCodesofPractice.Fortrenchfill,itisimperativetocheckthatthefinishedfoundationleveliscorrectandhorizontal.ItwillbedifficulttoadjustfordiscrepanciesinthesmallnumberofbrickcoursesbetweenfoundationandDPClevel.

Priortoconcretingexcavationsshouldbe‘bottomedout’toremoveanydebriswhichmayhavefallenintothetrench,theexcavationsshouldbe free from water and if it has been left open for alongperiodoftime,furtherexcavationmayberequiredtoanon-weatheredstrata.

Please note: ItisimportantthatHealthandSafetyobligationsaremetandthatexcavationsareappropriatelysupportedtopreventcollapse.

5.3.8 Reinforcing

Stripandtrenchfillfoundationsshouldbereinforcedwherenecessary,tosuitlocalisedgroundconditions.Reinforcement,ifneeded,shouldbecleanandfreefromlooserustandshouldalsobeplacedcorrectly.Bars,ofanappropriatesize,shouldbeappropriatelysupportedtoguaranteethattheyare75mmabovethebaseofthefoundationorasindicatedinthedesign.Theyshouldbesecuredatlapsandcrossings.Ifindoubtaboutanysoftspots,theengineer’sadviceshouldbetakenpriortoplacingtheconcrete.

5.3.9 Foundation Joints

Ifconstructionjointsarenecessary,theyshouldnotbepositionedwithin2mofacornerorjunctioninthefoundation.Allshutteringshouldberemovedbeforeworkprogressesbeyondtheconstructionjoint.

Figure 16: Using reinforcement bars across a joint


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5.3.10 Steps in foundations

StepsinfoundationsmustnotbeofagreaterdimensionthanthethicknessofthefoundationWherefoundationsarestepped(onelevation)theyshouldoverlapbytwicetheheightofthestep,bythedimensionofthefoundation,or300mm–whicheveristhegreater.AsshowninFigure17.

Figure 17: Steps in foundations

5.3.11 Standards referred to

• BS8004Codeofpracticeforfoundations.• BS59501Structuraluseofsteelworkin

buildings.• BS6399LoadingsforBuildings.• BS8103StructuralDesignoflowrisebuildings.• BS8110StructuralUseofConcrete.


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ii.Allworktobecarriedoutbyatechnicallycompetentpersoninaworkmanlike manner.

iii.Pilefoundationsschemesmustbetestedtoconfirmthattheinstallationmeetsthedesignrequirements.

Materials


ii.Allmaterials,productsandbuildingsystemsshallbeappropriateandsuitable for their intended purpose.


Design


ii.Piledfoundationdesignsmustbesupportedbystructuralcalculationsprovidedbyasuitablyqualifiedexpert.Calculationsforfullpiling

systemsmustbeprovidedbyorendorsedbythepilingmanufacturer.

iii.PiledfoundationsmustmeettherelevantBuildingRegulationsandotherstatutoryrequirements,BritishStandardsandEuro-Codes.

5.4 PILED FOUNDATIONS


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5.4.5 Choosing the right piled solution

Thechoiceofpilingsystemtosupportthestructurewilldependentirelyuponthegroundconditionsandthereforeitisimportanttohavetheappropriatesiteinvestigationworkscarriedouttodeterminedepthsoffilledground,bearingcapacityofsoils,soiltypeandanyexistingworkingsorservicesthatmayclashwithpilelocations.AnalysisofthesiteinvestigationreportshouldbecompletedbyaspecialistpilingcontractorandStructuralEngineerastheyarebestplacedtodesignthemosteconomicpilingsystem.

Pilesareparticularlyappropriateforheavesites(treesremoved)forwhichtheyarestronglyrecommended.

Pilelayoutscanbereadilydesignedtoaccommodateanindividualplot.Agooddesignwillseektoachievecostsavingsinfoundationexcavationandmaterialsbytheincorporationoflargegroundbeamspansbetweenpilesandasmall number of piles.

ThePilingContractorshouldtakecaretoensurethatthepilesareinsertedverticallyandpiletopsarecorrectlyalignedtosupportthefoundationbeams.Anacceptableleveloftoleranceisapiletobeoffsetinplanfromtheoreticalpositionbynomorethan75mmandverticalalignmentshouldbenoworsethan1minevery75m(1:75).

5.4.1 Introduction

Pilesareusedtotransferloadsfrombuildingstothesupportingground,andareusedinawiderangeofapplicationswhereconventionalstripfootingsareinappropriate.Theyareparticularlyusedwheresoftorloosesoilsoverlaystrongsoilsorrocksatdepthsthatcanbereachedconvenientlybydrivingorboring.Theyareoftenthemosteconomictypeoffoundationwhenveryheavyloadsmustbesupportedorupliftforcesneedtoberesisted.Largepilesareextremelyusefulforlimitingthesettlementsoflargestructuresondeepstiffclays,smallerversionscanprovideappropriatefoundationsforhousesandothersmallbuildingsonstiffclaysliabletoshrinkageandswelling.Thetechniquehasbeeninuseformanyyears.


Thefollowingsituationsarebeyondthescopeoftheguidanceinthissection.

• Innovativefoundationsystemsthatdonothavethirdpartyapprovaloraccreditation.

• PilingsystemswherethestructuraldesignisnotendorsedbythepilingSpecialistContractor.

5.4.3 Pile classification

Pilesofmanydifferenttypesandmethodsofinstallation have been developed to suit the wide varietyofsoils.Pilesgenerallyfallintotwomaintypes:

• Boredanddug,includingshortboredandsecant(replacementpiles).

• Drivenandjackedpiles,steel,concreteandtimber(displacementpiles).

Figure 18: The range of piling types (BRE publication)

5.4.4 How piling systems work

Therearebasicallytwogroupingsofpilesinthewaythattheytransferloadstotheground:

• End-bearingpilesderivethegreaterpartoftheirsupportfrombearingforcesatthebase.Theyactlargelyascolumnstransferringloadsthroughsoftdeposits,usuallytodensegranularsoilorrockatthefootofthepile.

• Frictionpiles,ontheotherhand,developmostoftheirsupportfromfrictionbetweentheshaftandthesoil,usuallyfirmclay.


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5.4.6 Ground beams

Pilesshouldbecappedwithanappropriategroundbeamsystem.Thereshouldbeadequateconnectionsbetweenthebeamandthepiletoensurethattheloadsaretransmittedeffectivelyorthatthebeamsareadequatelyrestrainedtothepiletoresistupliftonsitesthataresusceptibletoheave.Allexternal,internal,partitionandpartywallscanbeaccommodatedusingthissystem.

TheringbeamanditsconnectionsshouldbepartofthepiledfoundationdesignandshouldbesupportedbystructuralcalculationsprovidedbyaStructuralEngineer.

5.4.7 Pile construction records

Pileconstructionrecordsshouldbemadeavailableforallpilesinstalled.Therecordsshouldincludethefollowinginformation:

• Piletype(driventube,ContinuousFlightAuger(CFA),augerboredetc).

• Piledimensions(diameterorwidth/breadth).• Piledepth.• Drivingrecordsfromdrivenpiles–including

hammertype,weight,dropheight,sets,hammerefficiency.

• Pileverticalityconfirmation–thisshouldnotbemorethan1:75fromvertical.

• ForCFAandconcretescrewpiles,weshouldbegiventhecomputeroutputforconcretevolumeandrig.Performance–viewatypicalCFAoutputforawellconstructedpile.

5.4.8 Testing

Piled foundation installation should be appropriatelytestedtoensurethattheinstalledfoundationsmeetthedesignrequirement.Atestingplanshouldbeagreedatdesignstagethatisrepresentativeofthecomplexityofthepilingsystem.FurtherguidanceisavailablefromTheFederationofPilingSpecialistswhohasproducedthe‘HandbookonPileLoadTesting.’Sampletestingofarateofatleastonepileperhundredisusuallydeemedasanacceptableleveloftesting,howeveradditionaltestsmayberequiredonsiteswithmoreunstablegroundorwhereworkmanshiphas been an issue.

5.4.9 Test methods

TheEngineershallrequiretheSpecialistContractortoverifythatthepilingworkshavebeencompletedtoasatisfactorystandard.Thiswillusuallyincludecarryingoutsuitabletesting

Figure 19: Pile and beam detail


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toestablishthedegreeofgroundimprovement,itsloadbearingcharacteristicsandsettlementpotential.Thesetestsmayinclude:

5.4.9.1 Dynamic tests

Alsoknownaslargestraintesting,thistechniqueismostcommonlyusedforassessingthedynamicpilecapacityofdrivenpiles.Fordynamictestsondrivenpilestodeterminethedynamicpilecapacity,aminimumnumberof1testof1%oftheworkingpilesshouldbesought,whicheveristhegreater.Furthertestsmayberequiredifanomalous results are obtained.

5.4.9.2 Integrity tests

Alsoknownaslowstraintesting,therearetwotypesoftestswhichareusedsolelyforassessingpileintegrity:

• Crossholesoniclogging• Pulseecho

Forintegritytestingofcontinuousflightaugerpiles,100%ofthepilesshouldbetested.

5.4.9.3 Negative skin friction

Wherepilespassthroughgroundthatmayconsolidateorchangeinvolume(e.g.duetoachangeinwatertableorloadingduetoraisingoflevels)theeffectsofnegativeskinfrictionshouldbetakenintoaccount.Thecapacityofthepiletoresisttheadditionalcompressiveandtensilestressesshouldbecheckedatcriticalcrosssections.

5.4.10 Standards referred to:

• BS8004Codeofpracticeforfoundations.• BS5950.1Structuraluseofsteelworkin

buildings.• BS6399Loadingsforbuildings.• BS8103Structuraldesignoflowrisebuildings.• BS8110Structuraluseofconcrete


Technical Manual TS

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Workmanship



Materials




Design


ii. Raftfoundationdesignsmustbesupportedbystructuralcalculationsprovidedbyasuitablyqualifiedexpert.

iii.RaftfoundationdesignandconstructionmustmeettherelevantBuildingRegulationsandotherstatutoryrequirements.BritishStandardsandEuro-Codes.

5.5 RAFT FOUNDATIONS


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Guidance

5.5.1 Introduction

Araftfoundationconsistsofareinforcedconcreteslab,whosethicknessandstiffnessaredesignedtospreadtheappliedwallandcolumnloadsoveralargearea.Fordomesticapplications,raftsareoftenbuiltwiththickenedperimeterstoprovideprotectionagainstfrostheave,inwhichcasetheyareeffectivelytrenchfillfoundationswithintegralgroundbearingfloorslabs.Downstandedgebeamsalsoservetostiffenthewholeofthefoundationstructure.

Raftsareusedwhereitisnecessarytolimittheloadappliedtotheunderlyingsoilortoreducetheeffectsofdifferentialfoundationmovementsasaresultofvariablesoilconditionsorvariationsinloading.


Thefollowingsituationsarebeyondthescopeoftheguidanceinthissection.

Raftsarenotconsideredasanacceptedmethodoffoundationswherethegroundconditionsaresusceptibletoheaveorshrinkagee.g.wheretreesare present or have been removed.

5.5.3 Materials

Materials and workmanship should meet the requirementssetoutinChapters1and2ofthisManual.

5.5.4 Ground conditions

Raftfoundationsareusuallydesignedforsiteswithgroundconditionswithlowgroundbearingcapacityorwheretherearepocketsoffilledground.ItisthereforeimportanttocompleteasuitablesiteinvestigationmeetingtherequirementsofChapter4ofthismanualtoascertainthebearingcapacityandsuitabilityoftheground.

5.5.5 Structural design

StructuralcalculationsshouldbeprovidedbyasuitablyqualifiedStructuralEngineer,confirmingthattheraftdesignissuitableforbearingontotheground,andthatthegroundbearingcapacitysafelysupportsthestructure.Thedesignshouldprovidesufficientinformationtoensurecorrectinstallationoftheraftanditsreinforcing.Theminimumrecommendedinformationisasfollows;

• Plansanddetailsoftheproposedraftshowingreinforcingpositionsetc.

• Structuralcalculationsconfirmingthattheraftissuitable for the proposed loads applied

• Abarschedule,tobeusedbythesteelreinforcingsupplierandinstaller.

5.5.6 Ducts and sleeving

Anyservicepenetrationsthatpassthroughtheraftshouldbeappropriatelysleevedtoprotecttheserviceduct.Serviceductpositionsshouldbeplannedandindicatedondrawingstopreventreinforcingbarsfrombeencut,unlessthestructuraldesignhascateredforthis.

5.5.7 Damp proof membranes (DPM), damp proof courses (DPC) and floor finishes

Theraftfoundationandthejunctionwiththewallshouldbeappropriatelyconstructedtoresistgroundmoisturepenetration.ADPMcanbeplacedbeneaththeraft,wrappedaroundtheexternaltoeandlappedintotheinternalDPC,however,thisdetailcanbedifficulttoimplementonsiteandcommonly,puncturingofthemembranecanoccurwhenplacingreinforcing.ThepreferredmethodistoplacetheDPMontopoftheraftslabbeneaththefloorinsulationorscreedasindicatedinFigure20.

Figure 20 Typical raft foundation design


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5.5.8 Standards referred to:

• BS8004Codeofpracticeforfoundations.• BS5950:1Structuraluseofsteelworkin

buildings.• BS6399Loadingsforbuildings.• BS8103Structuraldesignoflowrisebuildings.• BS8110Structuraluseofconcrete.