Underpinning and Repair of Subsidence Damage

8/12/2019 Underpinning and Repair of Subsidence Damage

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'DER'1nd repair of subsidence damageBY JEFFERYSMITHCHARTERED CIVILENGINEERPUBLISHED BYTHE CAMDENCONSULTANCY

The prospectof subsidencecan haunt the building owner- like the fear of aserious illness

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ACTION CHECKLISTfor underpinning a ypical 3bedroomhouse

OWNER CONSULTANTChecksigns ofbuildingmovement cracks etc.)Ifsubsidence suspected askprofessionalforconfirmation - Contact Insurance Co.askaboutconsultant for investigationObtain quotation from consultant or investigationAppointconsultant or investigationPreparepreliminaryreport nd submitto nsuranceIfneighbourspropertyis nvolvedlet hem knowMonitor active movementof he buildingPrepare inal report for owner and submitto nsuranceConsidercarrying out thework with ownerand insurers Send invoice forconsultant'snvestigation feetoinsurersObtainestimateof ees/timecharges romconsultant Appointconsultant or specialistcontractor) orunderpinnIngBriefconsultantonworkingconditions (such asavailabilityof accessandadditional repairsoralterations required)Obtainapproval to ncrease mortgagetocover nsurance excessCheckapprovals required(eg neigljbours, treepreservation orders,listed building approval)Preparedrawingsandspecificationforunderpinning schemeArrangepartywall agreement fnecessaryContactpotential enderersandpreparelist for approvalObtain insurer'sapproval ofproposedworkand tender listand details ofhowpayments aretobemadetothecontractorObtain insurer'sapprovalofcostof urniturestorageand rent ofalternative accommodationSendoutdocuments and receive tenders;reporton tendersWithrecommendations toowner and insurersApprovetender and sign contractSubmit BuildingNoticeto LocalAuthorityClear areas affectedbytheworkand ifnecessary movetoalternative accommodationPrepareconditionsurvey ofbuildings, walls, terracesand fencesadjacent to thework andagree accuracywithneighboursObtain andapprove contractor'sdetailed work programme ridcheckarrangementswithneighboursInspect underpinningworks and issue nsurerswith interimcertificates for paymentMonitor conditionofbuildingfor4-8weeksafter completionofunderpinningand beforestartofsuperstructure epairsInspectand approve completion ofworks and issue penultimatecertificateto nsurers for payment ==Obtain invoiceforconsultant's fees and submitfor paymentby insurerInspectafter maintenance periodand issue final certificateObtain excess from mortgagees and use (with final paymentby installers)tocomplete payments tocontractor


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UNDERPINNINGnd repair of subsidencedamageBY JEFFERYSMITHCHARTERED CIVIL ENGINEERPUBLISHED BYTHECAMDENCONSULTANCY2ST. MARTINSBAYHAM STREETLONDON NWI OBD1994EDITION ISBN1 898521 00X


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III

In 1905 part of Winchester Cathedral wasfoundto be subsiding and, as a result,itwasleaning outwardstoa dangerous degree.Aneminent engineer called; investigationbegan.Theproblem was foundto bea bed ofpeat through which the foundation waspunching (see page 12). Underpinningwasprescribed - but how?Excavationthroughthepeat would lead to ground water rushing infrom the gravel bedsbelow. Thefirststageofunderpinning would have to be doneunderwater. William Walker, the diver,wascalledonto do thework, using bag after bagofunderwaterconcrete osealthe gapswherethe water came In so that traditionalunderpinning could proceed. From time totime the eminent engineer would don a divingsuit and gurgle down to inspect diver Bill'swork. Satisfaction waajronounced, Diver Billhad to stay for sixrs since it was foundthat ust about all ofWinchester Cathedralneeded underpinning. Once the Cathedral hadbeen put right the Archbishop of Canterburyledaservice of hanksgiving, mentioning diverBill andthe Kingshook his hand.Photograph: John Crook/Dean andChapterofWinchester

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Billthediver


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CONTENTSChapter I: Introduction.How this book can helpWHATISSUBSIDENCE AND WHO ISAFFECTEDThebuildingsandtypes ofdamage thataredealt with IThe peoplewho haveto tacklesubsidenceproblems 1WHAT HAPPENS N ATYPICAL CASE OFSUBSIDENCEHow thepartiesinvolvedcan protect heirinterests IThe sequence ofeventsand the ime required2: preliminarystages,carrying out heworkESSENTIAL PRECAUTIONSActionsthatcan betaken3: toavoidsubsidence and similardamage,tominimisetheeffectofsubsidence ifitoccurs, o avoidrecurrentproblemsWHERE TO STARTSources ofadvice4: generaladvice,

independent dvice, advicefrom specialistcontractorsThe client's role5

Chapter 2: Identifyingthecauses of foundationfailure and checking safetyRECOGNISING THE DISTINCTION BETWEENSETTLEMENTAND SUBSIDENCESettlement6:flexible buildings,brittlebuildings, engineered buildings,parts thatarevulnerable todifferential settlementSubsidence6: defects hatcanbe mistakenforsubsidenceRECOGNISINGTHE CONDITIONSTHAT AREPRONETOGROUND MOVEMENTVulnerable soil types 11: clay, peatand riverdeposits, madegroundOtherconditions thatcan leadtosubsidence14: leakingdrains,mining subsidence, mineshaftcollapse,swallow holesRECOGNISINGTHE DAMAGE THATTREESCAN CAUSE

Damage caused bytree roots hemselves t5Removalofgroundmoisturebytree roots15Recovery ofgroundmoisture afterremovalof trees 17

INVESTIGATING FOUNDATION FAILURETheevidencethatwillberequired17Movement investigation 17Substructureand soil investigation21Decidingwhethersubsidence has occurred 21

SEEINGTHAT THE BUILDINGIS SAFETypes ofdamage thatcould leadto collapse22Ways tosecurethestability of hebuilding22

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Chapter 3: Deciding hemethodof epair Chapter 4: Makingan insurance claimDECISIONSTHATHAVE TO BE MADE 25HOW THE SUBSTRUCTURE CANBEREPAIRED 25Theprinciplesofunderpinning25Types ofunderpinning25: raditional, massconcrete,pierandbeam, piling, otherspecialisedmethods.Precautions to betakenduringunderpinning29Extent ofunderpinning30: assessing howmuch of hebuildingto underpin, effectofpartialunderpinningHOW THE SUPERSTRUCTURE CANBEREPAIRED31The principlesofsuperstructure epair31Typesofsuperstructure epair31: stitchbonding, RCbeamor lintelbonding, resinbonding, strappingand tyingASSESSING THE COST OF REPAIRS33

Investigationcosts33Underpinningcosts 33Superstructhre epair costs34Othercosts,feesand expenses toallowfor 34

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WHYMAKEACLAIM? 38WHAT IS COVERED BY INSURANCEPOLICIES35Insurance ofhouses and other buildings35NHBCinsurance 35Theexcess on an insurance policy35

Liability for damage to neighbouringproperty36Specific imitationsofcover 36: heave,landslip,floor slabs, bettermentUninsured buildings37FOLLOWINGTHE CLAIMS PROCEDURE 37Notifyingthe insurers37The insurer's nitial response 39The formalagreements37The methodofpayment, V.A.T. 38NEGOTIATINGFOR MAXIMUMREIMBURSEMENT 9How disputes over liabilitycan arIse 39:treeroots, external works and outhuildings,adjacentproperties, eaking drains,extentofdamageDealingwiththe insurer'srepresentatives40Partial underpinningand insurance 40Alternative accommodation 41DISPUTES WITHINSURANCE COMPANIES 41

Appeal to the company'smanagement 1Otherappeal and arbitration procedures41CONTINUING INSURANCE COVER AFTERUNDERPINNING 41Effectofpremium policieson rates41

Conditionsimposed afterunderpinning41WHO PAYS FOR DAMAGE CAUSEDBY TREES? 42Trees on thesamesite as the building42Trees onadjacentsites and in thepavement42


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Chapter 5:Theapprovals thatareneeded Chapter 6: Getting theworkdoneBUILDING CONTROL APPROVAL44Buildingnotice ordepositofplans44Inspectionsandcompletion certificates 44Underpinning defined asamaterialalteration44

BuildingControl's iabilityfor defectivefoundations 45Fees for BuildingControl45APPROVAL OFNEIGHBOURING OWNERS48

Partywall agreements45: in London,outside London.

OTHER APPROVALS47Treepreservation orders47Planning and historic building approval 47Noise (Control ofPollutionAct)47Rubbish skip license47Access47Protection ofunderground services 48

SELECTION OF CONSULTANTS49Experience49References49PREPARING THE DOCUMENTATION SOThe drawings 50The specification50The prelims and preamble50OBTAINING COMPETITIVE TENDERS50Selectionofcontractors50

Invitingtenders 53Standardformsofcontract54The contractagreement54SUPERVISING THE WORKANDAGREEINGVARIATIONS55

Supervisionof thework55Variationsto thecontract 55Method ofpayment55

COMPLETION OF THE CONTRACT55GUARANTEES AND WARRANTIES 56Valueofaguarantee56Insurance backed guarantees56

Negligenceclaims56RECORDS57

References 57Useful addresses57Subject index 58


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FIGURES1 (a-c) Typical crackpatternscaused bydefectsother than subsidence7-82(a-cI)Typicalcrackingdue tosubsidence 9-103 Mainareas whereshrinkable claysoccur 114 How afoundation can punch througha ayerofgoodsoil when there isa softer layerbeneath 125 Plasticityindex 136 Dessicationofsoil by a tree 137 Precautions to betaken to protectunderpinningwhengroundheave is apossibility188 Techniquesfor monitoring thewidthandextentofcracks 199 Method ofrecording crack urveyfindings 2010 Examplesofsoil investigationtechniques 2211 Typesofunderpinning 29, (a) traditional,

(b) massconcrete, (c) pierandbeam,(d) pileandbeam12 Strengthening superstructureadjacent ounderpinning topreventdistortion3013 Underpinningasteppedfooting 3014 Superstructure trengtheningmethods3215 Typicalinsurance policywordings3816 Crosssectionofrootsunderthe

microscope3917 Application of InnerLondonparty walllegislationtogroundunder a partywall4618 Contract rawing showing partialunderpinningofa house 5119 Partsofa specificationand schedule of atesfor underpinning 52

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TABLES- I Action check listfor underpinninga ypical3bedroomhouse2 Sequenceofeventsandtime requiredforunderpinningofathreebedroomhouse 33 Plasticityindex and shrinkageclassification134 Tree types- water demands, eightsandrisks tobuildings 165(a-b)Damageand movementclasification23246 Depths ofdifferent ypes ofunderpinning 257 Typicalcostsfor underpinninga 3 bedroomhouse in 1993,348 Underpinningconsultants 49


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Chapter1: INTRODUCTION- HOWTHIS BOOKLET CAN HELPWhat is subsidenceand whois affected?Thebuildingsand typesofdamagethat are dealtwith'Subsidence' ofa building is thefailureof ts foundations dueto the groundon which itis builtsubsiding extensivelyor unevenly. Underpinning' describes thevarious methodsofsupporting he buildingwhere a foundationhas failed orwhere foundationsneedtobe strengthenedto akean increased load, for example if extra storeysare to be addedor if loadsare concentrated bywidening openings in ground and basementstoreys.This bookletlooks atsubsidence ofdomestic scale buildingsandatthe repairsneededwhenthese buildingshave sufferedsubsidence. It s not a treatise. Whatit attempts isa summary of adviceand current practice thatcould be relevantand helpfulwhensubsidence is suspected. Basic issues are sometimes repeated in the text-for hebenefitof hosewho 'dip' into the book.Thepeoplewho have to tackle subsidenceproblemsThe information given here is intended both forthe building owner who has becomeaware, because ofcracksorbadlyfittingdoorsthattheremay have been subsidence,andfor the professional called in to advise, perhapswith little previous experienceofsubsidence.The ownermaynotwish to be drawn in to the technicaldetails but ageneral understanding of he options will allow the issuestobe squarely faced and thenecessary actions to be taken. Professionals will be familiar with the generalprocedures involvedbut may nothave recentexperienceofan underpinning contract.This booklet will refreshtheir memories and most will find somenuggetsof nforma-tion that can help avoid mistakes orsave time.WHAT HAPPENS IN A TYPICALCASE OF SUBSIDENCELike serious illness,subsidence is something anybuildingownercanjustifiably feelafraid of. If it is extensive it is likelyto disrupttheuseofthe building- at leastforashort ime. In extreme, andfortunatelyrarecases, itcan even lead todamage hatcannot be repairedat a reasonable cost and,just possibly,to demolition ofthe entirebuilding. Subsidence is costlyto deal with andwill require ime and energybeing spenton thingsthatnormally make no demands.Howthe parties involvedcanprotect heir interestsKnowingthe issues and howthe problems can be resolved, which theyalmostalwayscan be, is reassuring. Thereare manywaysin which the parties involvedin a case ofsubsidence can protect their interests.Whether affected as owner, neighbour, tenant,purchaserorprofessional advisor certainissues should alwaysbe considered bythoseinvolvedbefore repairsare started and, ifthingsdo go wrongat any stage,thereareoftenways of imitingthe damage andrecovering the situation.

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Underpinning: Chapter 1 Introduction How this book can help

Key issues are discussedineachchapterofthisbookletChapter 1: sayshowtogetadvicewithoutunnecessary expense;Chapter 2: looks atwhether underpinning is really needed;Chapter3: describes the techniques thatare now available;Chapter4: isaboutthe all importantquestion of insurance claimsand appeals;Chapter 5: liststhe officialapprovals thatwill (or may) be needed;Chapter 6: deals with the work, with contractconditions and with guarantees.Thesequenceofevents and thetime requiredPreliminary stages: In an underpinningoperation he preliminarystages are likely omove slowly. They will take anything from six monthsto two or threeyears. Evenwhen owners and professionalsact promptly it takes time to see what needs to bedone, to arrange thefinance andtosetup a contractfor the work. Delays are particu-larly annoyingwhentheyhold up other repairand redecoration workorpossiblythesale ofthe property. One waytocut down on unnecessary delays is tokeepan actioncheck list (an example isgivenon the insideofthe front cover) andfollow itthroughtoensure letters have arrived and someone deals with the nextstep in a reasonable time.Threegoldenrules forgetting quicker resultsare1. Speakto someone on the phone beforesending thema letter -you can hen bemore sure thatthe letter is going to the right personand very oftentheywill give

youa goodidea what to say in the letter.2. Sendletters by fax followed bya copysent hrough the post. Thefaxwill gettherefasterand its arrival willbe recorded. The copywillserve as a reminder.3. When writingto chaseaction, spellout the sequence ofeventsthathave led to theletterhaving to be sent, in casepreviouslettershavegoneastrayor staffhave changed.CarryingouttheworkThe underpinningoperation tselfis not likely o be protracted.The contractwillpenalisea contractorwho is responsible for delays. Table2 givesa reasonable timescaleforthe partialunderpinningofa threebedroom house. The most likely delay to theworkprogramme s an unnecessarily long interval between the underpinning work andsuperstructurerepairs, particularly ifthey are underseparatecontracts.Theremayalsobe delaysbecause excavationreveals the needfor additional work andfinance forthatwork has tobeapproved. The worst hing that is remotely likelyto occur is thatthe work s stopped in the middle, eitherbecause all the necessary approvals have notbeen obtained from the authorities andthe neighbours orbecause the contractorgoesout ofbusiness. Aperformancebondcould be considered for underpinning contractswheresuch delaysare likelyto lead toheavy expenses (a performancebond is aninsurancethatcovers the cost ifthe contractorfails to carryout the contract);alterna-tively building insurers may undertaketomeetexceptionalcostsresulting from delays.

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Underpinning:Chapter1 Introduction How this book canhelp

TABLE 2Sequence ofeventsand the timerequiredforunderpinning ofa three bedroomhouseEVENTS MONTHS0 3 6 12 15 18 21Contact insurance company andappointconsultant UInitialinvestigationand consultantsreport.Insurers o beasked to accept liability.Consultant monitorsmovementReporton movement nd remedial solutionsConsultant preparesdrawings andspecificationConsultant obtains tendersSub-structure epairs(underpinning)Periodallowed for initial settlementafter repairsSuperstructureepairsESSENTIALPRECAUTIONSTo avoid subsidence: this bookletdoesnotdealwith the precautions againstsubsidence thatare built into the foundation design ofnewbuildingsbut therearecertainelementary precautions that ownersof existing buildingsshould be awareofthatwill reduce the riskofsubsidence.1. Iftrees (particularlycertain species of ree) are allowedtogrow largeandclose tobuildingson claysoils there is an increased riskofsubsidence. Treesthat hreaten tocausesubsidence should be cut back rather than cutdown and ownersof reeson

adjacent and, including he highway authority, can be asked to reduce the bulk ofthreateningtrees as they can be held responsible for the damage they do.(see chapters2 and4)2. Alterations toexisting buildings, even internal alterations, can lead to aconcentration of load that eventuallycauses subsidence.3. Ifground levelsare loweredit may be necessary to underpin beforehand as exposedorshallower foundations are morevulnerableto subsidence.Normally buildingcontrol approval is needed foranyalterationthataffects loading. Ifadditional foundations are needed theywill have been a condition ofBuildingRegulation approval, however minor changes may havebeen madeornewheavy equip-mentmay have been installedwithoutanyonerealisingthatsectionsofthefoundationswere being overloaded. As a general rule the load on a soundexisting foundationshould not be increased by more than 10% and anyalteration hat affects the loads onan existing loadbearing structure should go beforebuilding control and if necessary bechecked by a structuralengineer. (In a case, suchasan agricultural building,whichisexempted from buildingcontrol an engineershould be asked for approval).Tominimisethe effectofsubsidenceifitoccurs:this bookletdeals with the repairofbuildingsthathave suffered from subsidence. Where subsidence is progressive thesooner it is dealt with the better. Superstructuredamage resulting from subsidence can

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Underpinning:Chapter1 Introduction How this bican help

be moreexpensiveto repairthan the underpinning ofthe failed foundations. It isnotwiseto treat subsidenceexperimentally- by makinga repairthatmayor may notdothe ob. Ifa foundation fails for a second time the whole ofthe superstructurerepairsmay have to be repeated.Chapter 4 dealswith insurance, It is an essential precaution tohave a good up-to-dateinsurance policywith a companythatsteers through and pays for all the necessaryrepairs. Non-domesticbuildingsare oftennot insured for subsidence. Outbuildingsandboundarywalls, etc. are sometimes not insured forsubsidence.To avoid recurrentproblems:besides continuingto take the necessary precautions toavoid subside)ice,such as cutting backtrees, and making sure that a really effectiveremedial scheme is implementedthe most importantprecaution to take is to ensurethat a full insurance cover, including coverforsubsidence, is reinstatedafter the build-inghas been repaired. Ifpossiblean undertaking on future nsurance should beobtained beforeanyofferofpaymenton a subsidence claim is accepted.WHERETO STARTSources ofadviceThree sorts of advicemaybe needed. (a) General advice in the form ofpublications,listsoffirmsand standard procedures.(b) Independentadvice in the form ofprofessional expertise or the experience of hose who havealreadysufferedsubsidencedamage. (c) Specialistadvicefromthose involved ina particular typeof repairwork.(a)General advice: is availablefrom the Building Research Establishment who havepublicationson underpinning (seereferences) Tel: Watford (0923) 894040. The Local Authority BuildingControl Department whomayhave knowledge ofother subsidencerepair In thearea. TheNational House-BuildingCouncilfor housesbuiltby heir members Tel:0494434477. The InstitutionofCivil Engineers, particularly for large caleproblemsconcerningsoils, eg. landslip.Tel: 071 2227722. The Royal InstitutionofChartered Surveyors cangive namesofmembers inanyareaTel:071 222 7000. The Royal Institute ofBritish Architectscangivenamesofarchitects inanyarea.Tel:071 5805533. The InstitutionofStructural Engineers cangivenames ofeiiiheers nanyarea. Tel:071 2354535 The House-BuildersFederation cangivenamesof ocalcontiactors.Tel:071 5805588 TheBritish Cement Assodation foradvice on concrete oundfin designTel:Fulmer (0753)662727.(b) tndependentadvice:When choosinga professional personto go to forindependent advicegivepreference to hose who have both previous experience ofunderpinning contractsand goodreferencesfrom the clients forwhom theyworked onthose contracts. Ifthe ob is small andthe repair work has no particularaestheticimplicationsitcan be handled bya buildingsurveyor. For larger obs andworkon listedbuildings you will probably need both an architectand a structuralengineer; howeverinsurers are reluctant o payfor the services ofmore than one consultant. If thereisnot much superstructure epair, orwhereitis largely structural,a structuralengineeris likelytobe able to handle the ob.You maychoose an individual consultant, with whomyou feel an affinity. Alternativelyyou maywant the backingofa largeconsultancybehind the person youdeal with.

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Underpinning:Chapter Introduction How this bookcan help

The consultant's termsofengagementshould be in writingand it should be madequiteclear what fee willbe paid andwhat other advice will be needed. The insurance willpayreasonable professional fees butthey may have views on the appointment ofconsultants, particularly if more than one consultant s involved.Some insurers insiston appointing the consultant. Notall suitably qualifiedconsultants belong to theprofessional institutions listed above.(C)Advicefromspecialistcontractors:Firms hatspecialisein underpinningandrepairwork can often givemore informed advicethan independent consultants. Thedrawback isthat heyare notina position to setup competitivetenderingor to advisebetweenthe offers madebyfirmsusingdifferenttechniques. Also theymay not knowmuch aboutsuperstructure repair (particularlyifa historic building is involved).Discussion with specialistfirms beforespecificationsand drawings are preparedcanopen the way to the inclusionofalternative solutions. The Federation ofPilingSpecialistscan recommend firmswith relevantexperienceand equipment.Tel: 071 831 7581The BritishTelecom ellow Pages irectory istsfirms that claim to specialise nunderpinning. Although the local names reuseful care mustbe taken to ensurethatfirmsonsuch ists are capableofdoing the ob.Take up references. Arrange forIndependent assessment of competitive tendersand never hand over money n advanceof he workbeing done.The client's roleAppointingconsultants or specialistsand leavingthem togeton with it is likelyto leadtoproblems. However conscientioussuchpeople are they cannot reada client's mind.Theyshould alwayswork to a written brief from the client. Itneed be no more than aletter in the firstplace but it should makeclear: The amounts,methods and times ofpayment What he personor firm is requiredtodo What ime is availablefor the workand what special conditionsare involved Whatinsurances are requiredThe initial briefcan be developedby the clientand consultantorspecialisttogether.The client can say what useofthe buildingand access is required duringthe work. Theindependent consultantmaybeasked to negotiatethe insurance claim on the building.The specialistshould beabletosayhow long each phase ofthe work is likelyto take.As the workgoesaheadthe client should make sure tokeepwell informed aboutprogress, majordecisionsand costs- withoutwasting eople's time with unnecessaryinterference.

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Chapter2: IDENTIFYINGTHE CAUSES OF FOUNDATIONFAILUREANDCHECKING SAFETY

Recognisingthedistinction betweensettlement andsubsidenceSettlementSome movement ofthe groundon whicha buildingisbuilt is to be expected and this iscalled settlement.The building can cope with this. There is an inter-actionbetweenabuilding and thesoilwhich supportsit.Slightsettlementofthe soil is normal andshould not damage he building.FlexiblebuildingsEven when an old buildinghas distorted noticeablyit may still be quitesound.Pre-1914buildingswere usuallybuilt with lime mortar, whichdampnesscauses tore-hydrate, allowingthe limemortar to 'heal' and create newbonding acrosscracks.The quaintness ofsome oldbuildingsis a resultof his "flexibility" as, for example,whenthe masonry ofa terrace ofcottagesrolls up and down.Brittle buildingsModern brickwork s laid in stronger cement mortar however this isbrittle.Foundations have therefore to be more rigid anddeeper. Moreoveritisnow recognised(thoughonlyaftermany failures) that suitable superstructureointinghas tobeprovidedso that hermal andothermovement, including movement duetominorfoundation settlement, do not causecracking.Engineered buildingsLarge, engineer designed buildingsare beyond the scope of his bookletbut some ofthe techniques derivingfrom such buildingshave been applied tomore modeststructures. Thereare many housesthat use engineer-designedstructuralelementsincluding timberor steelframes, precastconcretepanels and foundations. Such housescan look very much like more traditionally built housing but may need special solutionsthat takeaccountofthe original design principles.SubsidenceParts that are vulnerable to subsidence: Whereas a wholebuildingcan settle in onepieceand not be damaged,subsidence occurs when one partofa buildingsettles muchmore than other parts andasa resultthe superstructurecracks It is quite common forsomeparts ofthe houseto be lesswell founded han the rest ofthe house, inparticular,suchthingsas porches, bay-windowsorextensions. These parts can breakawayfrom the bodyofa building which is itselfunaffected.

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Underpinning: Chapter 2 Identifyingthecauses offoundation failure and checking safety

Defectsthatcan be mistakenfor subsidence: Crackingin brickworkcaused bysubsidence usuallyfollows distinctivepatterns. However cracking caused by certainsuperstructuredefects can sometimes be mistaken forsubsidence. Lackofwallmovementoints, spreading ofrafters, failure of lintels over openings and sulphatedamageto mortar and insufficientmovementjoints are instances. Figure 1 showstypical crackpatterns caused by some common defects other than subsidence.Figure 2 shows typical cracking dueto subsidence.

FIGURE laInternal cracking not caused by subsidence

Crackingon the ineofa flue

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Crackingwhereplaster bridgesfrom a wallbackground toa causedbyactionoflintel background fluegassesonmortar


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Underpinning: Chapter 2 Identifying hecauses o1Tundationfailure and checking safety

FIGURE lbExternal crackingnot caused by subsidence

Long-standingdifferentialsettlementcrackbetweenmain buildingandextension

FIGURE IcLackofwall movementjoints

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Opening upofjointsatevery4th course n brickworkcausedbycorrosionofwail ies

Failureofbrickarchcausescracking boveopening

Spreading ofrooftimbers causingcracksat oof evel

Tiltingof intelcausingbrickcrackingatopening

Generalcrackingofjoints indamp

masonrycausedbysulphateaction on mortar

Whileclaybrickwork endstoexpandsoonafterconstruction,calcium silicatebrickworkcontractsandwill causecrackingunlesssuitablemovementointsare

provided-modernclaybrickwork lsorequiresmovementointsbut heycan bemorewidelyspacedapartdepending on thebrick ypeandwail ocation

Constantwidthvertical cracksdue to nitial expansion and,toa esserextent, on-going hermalmovement ina terraceof ousesone ortwo endhouses canbeaffectedmore than theothrs)


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Underpinning:Chapter2 Identifyingthecauses of oundation failure andcheckingsafety

FIGURE 2aInternal cracking caused by subsidence

Figure2bTypical crackpatternsdueto subsidence

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Verticalcracksbesidebaywindow.The tree rootshavedesiccated claybeneaththeshallow oundations of hebay

Diagonal cracks,' (wideratthe top) on" both cornerfacades.Treerootshavedesiccated clay

beneath the foundation atthecorner


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Underpinning: Chapter 2 Identifying hecausesofundation ailure andchecking safety

Figure ZcSaggingmode- groundunder centre has subsided

Crack in thepartywall(orrear facadecracked n thesamewayas the front)

Widergapsinroofcoverings(and/orcracks in endwall)

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Cracksare widertowardsthe top

// Cracksare wider/ owards thebottomFigure 2dHogging mode - ground under the end

Endsofbuildinghavesubsided(or thegroundunderthepartywallhasheaved)


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Underpinning;Chapter2 Identifyingthecauses of foundation failure andcheckingsafety

RECOGNISING THE CONDITIONSTHAT ARE PRONETO GROUND MOVEMENTVulnerable soiltypesAlmostall soils will compress underheavy point loadsbut even when not overloadedcertain typesofsoil can move and damageabuildingClayThe soil thatcausesmostsubsidence damageis clay.Itaccounts forover halfof all underpinning inBritain. Inprolonged periods ofdryweather clayshrinks. Certainclaysare 'highlyshrinkable',for example Londonclay. Otherscause fewer problems.Figure3 shows the main areasofthe countrywhereshrink-able claysoccur. Howeverlocal knowledge is morereliablethan a generalisedmap. Where clayisthecause ofdamage thelocal authorityBuildingControl departmentshould be able toconfirm othercases of subsidencein the area.

Certain soils near Sunderland and Shrewsbury can bevulnerable, as can softsoils inthe SomersetLevels, the Fens, and near the Thames Estuary and the FirthsofForthand Clyde.FIGURE 3Main areasofthe UKwhereshrinkable claysoccur (shown dark)

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Underpinning: Chapter 2 Identifyinghe causesoffoundation failure and checking safety

Peatand river deposits:Soft soilssuchas peat andcertain riverdepositsare pronetosubsidenceand, although normallyavoidedasthe base to support a foundation, theymay be concealedbelowa layerofdifferentsoil that isnotstrong enough tocarry theweightof he building over the peat below.Figure 4 shows howa foundation canpunch througha layer ofgoodsoil whenthereis a softer layerunderneath.Onlyan investigationofthe soil well below the foundation level will reveal the presenceofsoftstrata.

FIGURE 4Howa foundation can punch througha layerofgoodsoilwhen there is a softer layer beneath

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Previouspositionof roundfloorwindows showndotted

Layer of oftsoilbetween twolayersof irmsoil

By wideningtheopeningsonthegroundfloor theloadhas beenconcentrated nthecentralpierand tsfoundationhaspunchedthroughto thesofterlayerof oilbeneath


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Underpinning: Chapter 2 Identifyingthecauses of oundation failure and checkingsafety

SOME OF ELEMENTARYSOIL MECHANICSFOR THOSE WHO ARE INTERESTEDFinegrained soils were formedbydeposition of oil particles from suspension inwater.In theprocess ofsettlement,consolidationanddrying out hematerial passed hroughseveral well-definedstages:1, suspension in iquid 2,viscous liquid3,plastic solid 4,semi-plasticsolid5,solidThe threeimportantboundaries between thestagesshown inFigure5were demonstratedbytheSwedish soil scientistAtterberg (afterwhom somesoil testsarenamed). They occurat the moisturecontent hat orms he lower imitof heliquid, plastic and semisolidstatesFIGURE 5 PlasticityindexLL =The liquid limit' and is theminimum moisturecontentatwhich thesoil will flowunder tsownweight. (Inthestandard estagroove inthesoil will close when a sample is apped.)PL = Theplasticimit' andistheminimum moisturecontentatwhich thesoil canberolled into a3mm thread withoutbreaking.SL = Theshrinkageimit' andis hemoisturecontentatwhich further loss ofmoisturedoes notcausea decrease in thevolumeof hesoil.P1 = The plasticityndex'and s hedifference -between the liquid andplastic limitsshrinkable soilscontain morethan35%of ineparticles (silt andclay)andhave aplasticity index ofmore than 10%.Table 3 shows theplasticityindex figures forsoils classifiedashaving low, medium andhighshrinkage potential and theNHBC minimum foundation depthsforeach classificationTable3 Plasticity index andshrinkage classification

Plasticity index (P1) classificationofshrinkage potential NHBC equired minimumfoundation depthfor newbuildingsGreater han 40% High 1.00 m

20-40% Medium 0.90m10-20% Low 0.75m

00.51.01.52.02.53.0

Figure 6 hows the howthemoisture profileof oil atdifferent depthscanbe ffected bya ree removing moisture romthesoil.Dryingofa shrinkable soil below tsequilibriummoisturecontentmaycauseit o shrinkenough to damage anybuilding itsupports.There is nodirectcorrelation between moisture contentofa soil andthe oading permittedbeneatha foundation. Aclaysoil hat isdessicated tobelow ts quilibrium moisture ontentwill beharder hanthesameclayat heequilibriummoisture ontent.Butit s theclay atequilibrium moisture content hat isused to establish thepermitted oading.Typicallya stiff claycansupport 100-200kNlm2 (1 -2 ons/sq.ft.)

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Plastic LL.Semi-solid

PLSL

Moisture contentof oil +

Greaterdepthsrequired invicinityof reesOrwhere reesare to beplantedFIGURE 6 Desiccationofsoil by a tree

Depthbelowground nmetres

10% 20% 30% 10% 50% Moistureprofileof oildesiccated by he reeDifference in themoisturecontents of he oil

Moistureprofileof'normal soil'


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Underpinning:Chapter2 Identifyingthecauses of oundation failure and checkingsafety

Made ground:Sometimes buildingsare founded on 'madeground'-material used toreplace excavatedsoil orto raiseground levels. Unless the madegroundisofa stableand uniform nature the buildingswalls or floorslabsmaysubside. It is common touseeither piling(or insome situations raft oundations) where a building is tobe builtonmadeground.Otherconditions hat canleadto subsidenceA whole rangeofconditions can cause local weaknesses n the groundsupportingabuilding.Sloping ground:Southfacing clayslopes are particularlyproneto shrinkage as hotsun dries outexposed soil. North facingorovershadowed slopes are less susceptible.Buildings on sloping groundrequiresuitably designed foundations or theymay besubject to landslipwhenthe buildingand the surrounding oil startto slide down theslope together.Leakingdrains: Leaks from drains orwater supplies can washoutfine material fromsoils, weakening them and causing the foundations they support to subside. Thissubsidence can further damagepipes and increase leaks, creating a chickenand eggsituation. Was itthe subsidencethat cracked the pipe orwas it the leak that caused thesubsidence?Acommon way to checkdrainsnowadays is the closedcircuittelevisioncamera. Moretraditionalchecks includeair pressureorsmoketests. Simply pluggingthe drain at ts lower end and fillingwith water can be usedtocheck for leakage.Miningsubsidence: In coal miningareas significantsettlementcan occur as a miningface isworked and passesundera building.This movement takes the form ofa 'ripple'ofdifferentialsettlement.Severe crackingofbuildings often resultsbutonce the rippleofground movementhas passed the buildings above can be repairedon the assumptionthat little further movement will occur.British Coal,which was formally the National CoalBoard, is legally responsible forminingsubsidence damage (f-lead officeTel. 071 235 2020).Whether heownersofprivatisedcoal mines will have equal responsibility remains tobe seen. The lackofresponsibilityfor pollution fromprivateCornish tin mines is notanencouraging precedent.Mine shaftcollapse: Unlike the progressive subsidencethatfollows the working ofacoalface, the collapseofoldmine shafts and workings is usually disastrous for hebuildingsabove andcannot be dealtwith by normal underpinningtechniques. Miningengineers can cap shafts to securethe ground aboveforother uses butthis was notalwaysdone in the past. In this specialistarea the Institution ofMiningand Metallurgycan suggestindependent engineers to approach foradvicewherethe mine was formetal ores (Tel: 071 580 3802).The Institution ofMining Engineers can provideasimilar service in relation to coalworkings (Tel. Doncaster (0302) 320486).

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Underpinning:Chapter2 Identifyingthecauses of oundation failure and checking safety

Swallow holes: Swallow holes can occur in chalk and limestone. Underground voids inthese rocksare enlarged bythe action ofwaterand migrate' towards the surface.Buildings above become vulnerable to subsidence - oreven collapse.Similar probiemscan be man-made, forexample,where quarrying is carried out underground (as itwas,around Bath). Specialist techniques are necessary to locate such voids. 'Ground radar'which detectschanges in the densityof the soil is one such echnique. Specialisttechniques are alsoneeded to remedy the situation oncea void has been located, forexample,voidsorcaverns can be filledwith large quantities ofgrout (liquidcementanda filling medium). However environmentalists are concerned thatthis could be harmful.Effect ofweather conditionsWhere subsidence is caused by the shrinking ofclaysoil, weather conditions andorientation can dramaticallyincrease subsidence, particularlyofexposed parts ofabuilding.This type ofsubsidence becomes much commoner afterdroughtsorafter aseries ofdrier than averageyears.RECOGNISING THE DAMAGE THATTREES CAN CAUSEDamagecausedbytree rootsthemselvesTree roots can sometimes causephysicaldamage to oundations. However modernconcretefoundations are noteasily penetrated by roots and aretherefore much lesssusceptible. Rootsmay alsodamageor partially blockdrains and consequential leakagecan weakensurroundingground.Removal ofgroundmoisturebytree rootsTrees dovery much more harmby removing moisture from the ground underfoundations than they do by penetratingstructures anddrains.Claysoils are mainlyaffected. Treesremove moisturefrom the clay, causing ittoshrink ocally wheretheroots encroach on soil below foundations. Since differential groundmovement isthemost disruptive movement, trees are responsible fora high proportionofsubsidence.Table4 shows the typesoftreethatextractmost waterfrom the groundand thedistance from the tree within which damagemay be caused.SmallerplantsBushesup to five metres high are not likelytocause harm.The ladywho asked"gardener'sQuestionTime" on BBCRadio4, if daffodilswere safe was told 'yes'

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Underpinning: Chapter 2 Identifying he causesofundation ailure and checkingsafety

Table 4Tree types -waterdemands,eightsand isks obuildingsbasedon NI-IBC guidance for newhousing withauthor'samendmentsWaterdemand Broad leaved trees Conifers

Species Mature Species Matureheight (m) height (m)HIGH Elm(Eucalyptus)OakPoplarWillow

18-2416-2425-2816-24

Lawson'sCypressMontereyCypress

1820

MEDIUM AlderAppleAsh(Blackthorn)Cherry Japaneseand orchard)Cherry wild)(FalseAcacia)Hawthorn(Honey Locust)HornbeamHorse Chestnut(Laurel)LimeMapleMountainAshPlanePearPlumSycamore(Walnut)Whitebeam

1892389

1718101417208221811261210221812

(Cedar)LeylandCypress(Douglas Fir)(Pine)(Spruce)Yew

202420201812

LOW Beech(Holly)Birch(Magnolia)(Mulberry)

2012149

Notes:I Insufficientinformation is availableabout trees indicated in brackets, for adefinite classification.2 In highlyshrinkable clay ground, trees may be responsible for damage to buildingswithin the followingdistances of heircurrentheight: Highwater demand trees:1.25 x height Mediumwater demand trees: 0.75x height Lowwaterdemand trees: 0.50 x heightTreesposemore risk if they haveyet to reachtheir mature height (see table).3 The deeper the foundation is the less riska tree poses.page 16


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Underpinning: Chapter 2 Identifying he causes of oundation ailureandcheckingsafety

Recovery ofgroundmoisture after removalof treesAvoiding tree rootproblems would be a lot easier han it is ifremoval of rees wouldautomaticallypreventthe damagetrees can causeRemoval is, however, usuallynot theanswerto subsidence caused by tree roots on claysoil. Iftrees are removed, or iftheydie, the moisture content inthe claywithin the rootarea is slowly restored, causing theclayto swell and thegroundto heave'.The resulting damage tobuildingscan be muchmoreextensive andexpensivetoputright than any damage caused by the tree extract-ingmoisture rom the ground. It may be possible to remove a tree that isyoungerthanthe building (provided the cracks ithas caused inthe buildinghave notbeen filled)because the ground should only recover to the original level. But with an older treethebestthatcan be done to reduce its adverseeffects is to progressivelycut it back tolimit itsfoliage andtherefore reduce the moistureit needs from the ground. Anextremely severeinitialcuttingbackisbestavoided because itmay producethe sameground heave problems thatwould have resulted from removing the tree altogetherand itcould kill the treeIt has beenfoundthatthe groundnear toa tree maybe dessicated downto adepth of4m. Theshrinking ofthe dessicated ground lowers the surface evel which rises againwhen a tree is removed, and the clayrecovers ts moisture.An idea ofthe groundheave thatcan occur when a tree is removed, is given bythe case ofupwardmovementin excess of150mmwhichwas recorded as taking place over 25 yearsafter removal ofa tree (though the firsthalf ofsuch movement is likelytohave beencompleted withinten years).The removal of reesand resultantground heave can cause other problems besidesbuilding subsidence,forexample the slope ofgrounddrainscan be alteredorevenreversed. Such badlyaffected drainswill need replacing. Precautions to dealwithunderpinning where ground heave is a possibilityare shown in Figure 7.INVESTIGATING FOUNDATION FAILURETheevidence thatwillbe requiredInsurerswillusuallyrequireevidenceofcurrent (ie. continuing) movement beforetheywill accepta subsidenceclaim. Engineers are likelyto require information abouttheground beforedesigning remedial works such as underpinning. Both call formethodical investigation.Movement investigationThe movement investigation most often relevant n suspected subsidence situations isthe recording ofcracks andtheirdevelopment. A simple technique thatproducesusefuldata andrequiresno special skills is to carefullynotepositions, approximate widthsandthe extentofcracksdirectlyon wall surfacesand if possibleonto elevationaldrawingsofthe wall drawn to scale. Each note should be dated, then as cracks extend,a pictureofthe movementchangescan be seen. For moreaccurate measurementofcrackwidthsthe various techniques illustrated in Figure 8 are available.(includinga methodof recording cracks).

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Clayheaves asmoisture contentincreases to normal

Reinforced concretegroundbeamsurroundedbycrenellatedpolystyrenebefore itis castso thatsubsequentdayheavedoesnotcause damage

II Crenellated liningsonlystrictlynecessary f wellingday could causedamageto otherconstruction

Crenellatedpolystyrene heet, up to 100mm thick, linesexcavation before the con-crete ispoured andallows dayto swell into the voidsinsteadofthrusting the wholeunderpinning sidewayspage18

Underpinning:Chapter2 Identifyingthe-causes of3bundation failure and checkingsafety

FIGURE 7Precautions to be taken to protect underpinning when ground heave is a possibility

jITTTUnderpinning

V


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Underpinning:Chapter2 Identifyingthecauses of foundation failure andcheckingsafety

FIGURE 8Techniquesfor monitoring crackwidths

Glass tell-tale: if tcracks movementhasoccurred-buthowmuch? (vulnerable todamage n somelocations)

Slidingperspextell-tales: readingsare aken on agridso that theycan show both horizontal andverticalmovementbutonlymovementgreater han 0.5mm.

A current condition survey in a case ofsuspected subsidence may need to includetherecording, on plans ofthe building, of: crackwidths and positions; out-of-plumbness, distortion and bulging ofwalls; the slope offloors, sills etc.; the extent and conditionofpavingand drainage, andtheposition, size and speciesofexistingtrees(or trees knownto have been removed)

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0

Vernier callipers (readingspossible to0.005mmbutsuchprecision isnotneededfor this task).Digitalreadingcalipersareavailable

t.qMeasurements of x'and y'takenwithcallipersacross theshaftsofbrass screws(oracross studs fixed withadhesive usinga'Demec'gauge whichincorporatesadialreadinggauge)

Recording theextentofcracking on an insidewall urface


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Underpinning: Chapter 2 Identifying he causesof oundation ailure andchecking safety

FIGURE 9Methodof recording cracksurvey findings

An assessment of he building'scondition andtheneedforunderpinning requiresaweighing up of hese measurementsand detaIls. Safe limits dependon many factors.Assessingthe significance ofcracks isdealtwith in Figures 1 and 2, assessing bulgesandout-of-plumbness (or lean) is a matter ofjudgementbased on experience since,except for freestanding gardenorboundary walls, mostwalls are provided with somedegree of lateral restraintby the floorsorbeams that hey support. In thesecircumstances bulges or lean of less than one third ofthe wall's thickness may requireno specialaction, Indeed they maynot indicaterecentmovement ordistortion - thebuildingmay havebeen builtout-of-truein the firstplace, ormoved soonafter building.Where there appearstohave been subsidence it is importanttoestablish whethermovement s continuing (or is likelytocontinue underdryconditions). It is thereforenecessary tomonitorcracks, etc. over a period,oftenof welve months.Monitoringneedstobe done systematicallyand some monitoring procedures are time consuming.Thejudgementofan experienced professional in consultation with the insurance com-pany's loss adjustorcan determine what monitoring is necessary. BRE Digests251,343 and 344 consider this issuein detail.page20


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Underpinning: Chapter Identifyingthecauses offoundation failure andcheckingsafety

Substructure and soil investigationFindingout about sub-structure andgroundconditions is moredifficultthan recordingthe development ofcracks. Not even the bestengineers have 'X-ray eyes'. The below-groundinvestigation requiresavarietyof echniques someofwhich need specialistcontractors.Two investigation methods anda description ofthe datathey can provideare shown in Figure 10.Other echniques are less intrusive than those illustrated, handaugers andcompact power augerscan beusedwithin a building to collect soil samplesand test the strength ofthe soil several metresbelow foundation levels.Samplesofsoil andtree roots taken duringa ground investigation can be sent to alaboratory o provide useful information for the design ofremedial works and also tosupport insurance claims. Laboratory testswill show whether a clay is highly shrinkableand whetherthe rootsthathave desiccated a clay soil have come froma particularspeciesoftree. Ifthis happens tobe the typeoftree maturingon a neighbours land orin a public pavement, the insurancecompany maywish to recover costs from theneighbour (or his insurance company)or the local authority.BRE are pioneering new(and cheaper) ways tomeasure he moisture contentofsoils which maketestingapracticalproposition - even for small underpinningjobs.Otheruseful information can be gleaned from desk studiesofmany differentsourcedocuments, such as: geology maps, aerial photographs, local authorityBuilding Control departments, local history libraries, recollectionsof local people, recordsofprevious owners, British Coal (in the case ofmining areas).Buta walk-round examination ofnearbybuildings in the areawould normally be thestarting pointbeforeembarking on more costly investigations.

Decidingwhether subsidencehas occurredIn thefinal analysisthis question willbe answeredby the engineer or surveyorinvestigating the situation(possibly in conjunctionwith insurers).Ifhowevercrackinganddisruption ofpavings looks like subsidence and ifanyother factors often associatedwith subsidence apply,for instance ifthe underlying ground is shrinkable clayANDdrought conditions have prevailed OR if a large tree maybe to blame, ORifa drainmay be leaking then the problem could be subsidence. Ifit s, then other buildings inthevicinitymay be affected. To findout if this is so, a telephone call to the localauthoritybuilding control department may be useful.Although they have no obligationto help, unless buildingwork is already underwayorsoontostart, building controlofficers can often be very helpful.

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Underpinning: Chapter 2 Identifying he causesof oundation ailure andcheckinq safety

FIGURE 10Examplesofsoil investigationtechniques

Types ofdamage that could leadto collapseItwillbe superstructuredamagewhich leadsto investigatingthe sub-structure.Unlessdamage to the superstructure s slightthe question mayarise: is the buildingsafe? Inthe short erm the answer is likely o be yes. Most housing has reservesofstrengthdesigned into itand these takeover fora time, Ifnothing is done and movementcontinues it is possiblethatsomuch movement willoccur in the walls thatthe bearinglengths offloor oists, lintelsand beams are gradually lost. Withoutbearings they willcollapseandsections ofwall mayfall with them. However,long beforethis stage isreached, gapsin walls are likelytobe sowide and extensive thatthe building is nolongerweathertightorhabitable.WaystosecurethestabilityofthebuildingShould horizontal gaps occur in brickworkbed-joints a moreserious situationhasdevelopedand more urgent attentionwillbe calledfor. The gapsshould be packed withhardwood (ie non-shrinkable)wedgesas a matter ofurgency.This is because supportofthe construction above has been lost. Such wedges willneed to be resetfrequentlyso thatsupport is maintained while the buildingmoves.Another more urgentsituation couldaffectperipheral parts ofthe superstructuresuchas baywindows orporches. These are much morevulnerable since they could fall awayfrom the building ifvertical cracks develop nextto them.page22

Othertypesofauger canbeusedfrom insideabuilding

Trialpitexcavated toexaminethesoil beneathfoundations ,. Shellauger droppedfromfromarig(soil samplesarecollectedfromahollowcore)

SEEINGTHAT THE BUILDING ISSAFE


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Underpinning: Chapter 2 Identifyingthecauses offoundation failure andcheckingsafety

ClassificationofdamageAttempts to classifydamagehave been made. In 1981 BRE published Digest251'Assessment ofdamage in low-rise buildings'and this includeda 6 categories ofdamagefrom 'negligible' to 'very severe'. Some controversy resultedfrom thesedescriptions - for instance ownerscould not accept hat a 5mm crackshould be classedas 'slight'. This Digestwas revised in 1990with the controversial categorydescriptionsremoved. In fact an engineerwould never assess severity ofdamagesolely on the basisofcrackwidth butwould consider the situation in its context. The preamble to thistable now recognisesthis more fully.Table5a is based on the originalBRE digest together with furthercomments on itfroma 1991 BRE publication.Table Sb is a suggestedwayofassessing active move-ment. Takentogether the two parts of able5 allow a comparativeclassificationofdamage thatnotonly takes accountofthe visibledamage ata particular time butalsodevelopmentofthe damageover a period,If thetablesare usedto describe damage inschedules, etc. it is suggested hat the degree ofstatic damageshould be given beforethe active movement description andthatboth should refer to the worstdegree ofdamage in the area being considered.Example:"moderatestaticdamage, severeactive movement"Table SDamage andmovement lassification

Sa degreeofstaticdamage Descriptionofdamage(easeof epair in bold type) ApproximatecrackwidthNegligible Hairlinecracks only up to0.1mmVeryslight Fine cracks easilytreated duringnormaldecoration;

perhaps isolatedslight racturing nbuilding; cracksrarelyvisible in external brickworkup to 1mm

Slight Crackseasilyfilled; redecoration robably required;recurrentcracks can be maskedbysuitablelinings;cracks notnecessarilyvisibleexternally;someexternalrepointingmayberequired o ensure weather ightness;doors and windows may stickslightly

up to5mm

Moderate Cracks requiresomeopening upand canbepatchedbyamason; repointingofexternalbrickwork and possiblyasmall amountofbrickwork tobereplaced; doorsandwindows ticking; service pipesmayfracture;weathertightness soften impaired

5-15mm(or anumberofcracks up to 3mm)

Severe Extensiverepairwork involvingbreaking-out andreplacing sectionsofwalls,especiallyoverdoorsandwindows;door and window ramesdistorted; loorslopingnoticeably;some lossofbearing n beams;service pipesdisrupted

15-25mm(butalsodependsonnumberofcracks)Verysevere Thisrequiresamajor repair ob nvolvingpartialorcomplete rebuilding; beams lose bearing, wallsleanbadlyandrequireshoring;windowsbrokenbydistortion;dangerof nstability

usuallymore than25mm (butalsodependson numberofcracks)

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Underpinning: Chapter 2 Identifying he causesof foufldation failure andchecking safety

Sbdegreeofactive movement Description ofdamage- Approximateincrease in crackwidth peryear

Negligible No ncrease indamage nilVery slight Increase in solated slightfracturThg that sbarelynoticeable upto 0.2mmSlight Newcracks arenotnecessarilyvLThleexternally; doors upto 0.6mmModerate

and windowsmay start osticlc5lightlyMarked increase in fracturing; doorsand windowsmaystickmore;new ractures nservice pipes; often newweather-tightness problems

0.6 to2mm

Severe Extensivenew fracturing;window and door framesbecome distorted; loorstartstoslope noticeably; some lossofbearing in ointsandbeams (could therefore be a risk n usingsome partsof he building)

2mm o6mm

Verysevere Widespread new racturing;beams ose bearing;walls ean badly (and requiresioting)windowsbreakwithdistortion;buildingcanbecome unstable(and thereforedangerous touse)

over 6mm

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Chapter3:DECIDING THE METHOD OFREPAIRDECISIONSTHAT HAVE TO BE MADERepair ofbuildings damaged by subsidence is best considered in two parts: repairofthe substructure,eg underpinning, and superstructure repairs,eg repairofcracks in wallsand redecorationIn some cases superstructure repair alone willbe enough. In others substructurerepairwillbe needed (andwill be followed by superstructurerepair).HOW THE SUBSTRUCTURE CAN BE REPAIREDThe principles ofunderpinningA basic principleof he underpinning process is that supportto construction aboveisremoved in shortlengthsand new support inserted. At anyone time supportshouldnot be removed from morethan, say, one fifth ofthe buildingorwall being under-pinned. In this waythe structure willgenerally be adequately supportedfor shortperi-ods during the underpinning operation.(In effect lengthsofwall being underpinned'arch' over onto adjacent walls). However occasionally, for instance if ground is softorthe buildingweak, specialsupport systems may be neededtoprovide interimsupport.Often thesewill be removed whenthe underpinning is installedbut therecan be sav-ings ifthe supportsare designed to be 'sacrificial',that s ifthey are leftpermanentlyembedded in the underpinning - sinceworkwill then abletoproceed morequickly.Types ofunderpinningA range ofmethods is available. Most render the sub-structureadequate by extendingit downwards to ground that issufficientlystrongand stable. The increase in depth canbe anything from half a metre to over seven metres. Table6 shows the depthscom-monly usedfor the differentmethods.TABLE 6Depthsofdifferent ypesofunderpinning

Typeofunderpinning Maximum depthTraditional 2.5mMass concrete 3.0 mPier and beam 4.0 mPiled 15.0m

Notes: Depths shown are approximate. The foundation typesmay be usedto differentdepthsfor specificconditions. Formation levels(i.e. foundationbottoms) need tobe inspected by a local authoritybuildingcontrol officer. Excavationsfor underpinningare usuallyfilled immediatelywith mass concrete but ifleftexposed they should be sealed with a layerof"blinding"concrete.

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Underpinning:Chapter3 Deciding hemethodof e5fr

Traditional: Lengthsup to I .2m are excavated below the existing foundation, anewfoundation laid and the gapbetweeninfilled with brickwork ightup to the oldfoundation. Figure 11(a) shows this. This methodis not often used nowadays sincebrickwork is expensive, but instances ofit maybe found. Mass concreteunderpinning isthe modernequivalent.Mass concrete: Again lengths up to I 2m are excavated. Concrete is castto some75mm below the original foundation. When it has setandafterallowingtime for thegreater partof its initialshrinkage (usually1 - 3 days), the gap under the oldfoundation is packed tightwith drymix concrete. Figure 11(b)shows his.Pierandbeam:This methodisfrequentlyemployedby specialist underpinning firms.Reinforcedconcretebeams (ie madeofconcrete in which a cageofsteel bars isembedded) are cast inshortlengths usually in the wall ustbelowgroundor floor level.Sacrificialacks support the superstructureduring while the beamsare cast. Massconcretepiers are builtbelow the beam endstaking the loads downto suitably stableground. Figure 11 illustrates this method.Piling:Variousmethods ofpilingsuitable for domestic scale underpinning have beendevelopedby specialistfirms. Piles usedfor underpinning may be bored andwill notneed casingthrough loose ground. Alternatively, small diameterpiles can be powerdriven through it. Sometimes the pilessupport reinforced concretebeamsmuch as inpier and beamunderpinning. However suchmethods usuallyneed additional shortlengths ofbeam so thata pilingrigcan be positioned alongside walls. Pilingrigs smallenough to betaken into houses are the pride ofsome specialists.The mini-pilessuchrigs createcan avoid the needfor groundbeams. A more recent and highly engineeredmethod is the piled raft.This method is carried out internally. Nibsare cast intopocketscutin thewalls, therebysupporting them. The original foundation is then leftredundant.Fig 11(d) illustrates various pilingmethods.Otherspecialised methods: Liquidcementgroutcan be pumped nto porous groundsuch as loose sandsor gravels, either beneathfoundations or below internal concretefloor slabs. In thisway groundcontainingvotds can be made moresolid. Knowing theextent and effectivenessofthe grouting can be difficult.There have beencases ofgroutescaping andblockingsewers.Othermethods of soil stabilisation existbut theyare not relevant o most housing applications. Such specialisedmethodswillbeengineer-designed.

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Underpinning: Chapter 3 Decidingthemethodof epair

FIGURE 11(a)Traditionalbrickunderpinning

FIGURE 11(b)Mass concreteunderpinning

Underpinning toextend(perhapsto500mm)belowroot/eta. Steelreinforcement maybeusedtodowel sectionsofunderpinning together

poorground

stableground

Sequence formassconcreteunderpinning.All 'pins' No.1 tobeeffectivebeforepins2arestarted, likewisepins3, then4

page27

Originalwall withsteppedbrickspreadersontoexisting oncretefooting (perhapsa'clinker concrete')

underpinningbrickworkonnewconcretefooting

75mmgap betweenexisting ootingand underpinning dry packedwith1:3cement/sand


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Underpinning:Chapter3 Deciding hemethod of re1Th

Reinforcedconcreteground beamsinstalledoncepilesare cast

page28

Reinforced oncretegroundbeambuiltnshort lengths

FIGURE 11(d)Pile andbeamunderpinning

Pairofpilesunderacantilevered beamavoids the need oborepiles inside thebuilding Reinforced oredpiles250-350mm in

diameterinstalled irst

Oneof hispairof ilesisboredfrom withinthe building(cantileveredpiles canbe used to avoid his)

FIGURE 11(c)Pier andbeamunderpinning

Gappierandbeam packedwith cementand sandmix

\Mass concrete piers formed nexcavationandcast


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Photoofpier andbeamunderpinning

Underpinning:Chapter3 Decidingthemethod ofrepair

Precautionstobe takenduring underpinningThereis alwaysa possibilitythatunderpinning will aggravate damage orevencausecollapse.Precautions should be taken to reduce the risks,for example: Before work is started it may be necessary to strengthenthe building temporarilt.Openingsfordoors andwindows thatare closely adjacent mayneed tobe speciallysupported using needle beams andadjustable props. Figure 12 showsanothermethodofsupporting openings. During the work the sides ofexcavations will generally need temporary supporttostop them fallingin. (In verystiffclaythis maynot be necessary). If the ground isparticularlysoftor the watertable is encountered,methods based on deepexcava-tion may not be feasible. More practicable solutions would then be shallow but wideconcreteunderpinning or piling. During underpinning some monitoring ofthe structure aboveis essential. At aminimum, a regularvisual inspection should be carried out. In somecases precisereadings of level and plumb should be taken regularlyso that anydistortions, causedbythe changing support conditions, can be assessed.

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Underpinning:Chapter3 Deciding themethod ofreifr

FIGURE 12Strengthening ofsuperstructureadjacentto underpinningtopreventdistortion

FIGURE 13Underpinning ofa steppedfooting

ExtentofunderpinningAssessing how much ofthebuilding o underpin: In most cases onlypartof abuildingis underpinned.The ustification for this may be that tree roots have causedshrinkage ofthe claysoil in a limitedarea orbecausea peripheral part, eg a baywindow, hadfoundations thatwere shallower and proved less effective than those ofthe restofthe house, orbecause partofthe house was built on a softspot'. Wherethe existingfootingsarestepped, underpinning may need tobe carried through at thelower level to avoida pointofweakness (seeFigure 13)The effect ofpartial underpinning:In engineeringterms a terrace ofhousesis onebuilding, but in termsofownership it ismany. Evenifitwere desirable to underpin the whole terrace, some owner may objector not have the money or insurance- coverfor it. So underpinning at single housesalone is common and constitutes partialunderpinning in the samewayasunderpinningpartofa detached or semi-detached house. Once underpinned the foundations ofasingle house ina terrace maythen be 'harder' than those ofthe houseson either side.page30

Bracedheavy imber framesatwindow anddooropenings withvulnerablearchbrickworkwedged

Lengthofhigherfooting underpinnedatlowerdepths,approx. Immaybeenough, butif heoriginalfootingispooragreater engthmaybecalled or

Wheresubsidence hasloosenedbrickarches, orwhereadjacentopenings mean oadsonfoundations arechentrated, thentemporary trengtheningmaybe calledforduringtheunderpinning Operation


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Underpinning:Chapter3 Deciding themethod of repair

Insteadofall moving together asa terrace the underpinningmay anchor one house andcause cracking in the neighbour's house. The adjoiningownersmay reasonably expectthe owner who underpinned tomake good hese cracks. However the instirancepolicyofthe owner who underpinned might not cover damageto the neighbour's house, inwhichcase itwould be sensibletogivethe adjoiningowner an opportunity onotifyhisinsurance company beforeworksstart. Even ifthe neighbour's insurance companyaccepts the claimthey may not be preparedtopaytheexcess. There is no easy answerto his. The best that can be achievedistouse transitional shallower underpinningdepths at the end ofeach stretchofwall underpinned andhope to demonstratesuc-cessfully hatreasonable care had been taken to avoid damaging theneighbour's house.HOW THE SUPERSTRUCTURECAN BE REPAIREDTheprinciplesofsuperstructurerepairIn most cases following underpinning it is necessary to restore full strength androbustnessto the superstructure.In cases wheresubsidence is slight superstructurerepair alone may wellbe adequate, thoughthis would onlybe so ifthere were reasonswhyfurther damaging ground movementwas unlikely: forexampleafter four dryyears, ifwetweather is likelyto restore volume to claythathas shrunkonly a verysmall amount.Itis normal for the buildingto bed downslightlyontonewunderpinning and somechanges in crackwidths and extentmay then occur.A period of ime must be allowed,perhapsfourtoeightweeks,for this to happen before thesuperstructure repair can betackled.Types ofsuperstructure repairThe followingare some of hemethods ofsuperstructure repair usedsinglyormoreoften in combination.Stitch bonding:Cracked bricks and brickworkcloseto fracturesare cut outandreplaced. Thenew matching bricks are tightlypacked uptooriginalbrickworkabove.RC beam or lintel bonding: Slots are cut out ofthe brickwork, at perhaps1.2mcentres, acrossverticalornear verticalfractures. Short lengthsofrc beamor lintel arethen cast in tocreatea length ofbondwell away from thefracture position. Stitchbonding is usedbetween lintel bonds.Resin bonding:Exceptwhere brickwork s shattered this is often a viable alternativeto stitch bonding. Different techniques are available. In one the fracture position ischased out and cleanedout. (An industrial vacuum cleaner can be usedto extract thedustand small lumps ofmortar). Nipples arefixed over the fractureperhaps100mmto 300mm apartand the gapsbetweenare sealed with a resin mortar. Usinga specialgun or injectionpump,epoxy or other liquid resin is pumped n. Indicationofhowfar itpenetratesmay occur when it emergesfrom an adjacent nipple. Aftera time the resinsolidifies.Different consistency resins are available. Some are less viscousthan water,that is they flow moreeasily.Some are thixotropic, thatis they ellwhen not in motionand beforehardening.

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Resinbonding is notappropriatein all cases. For instance parts ofsash windowboxescould fill with resin which solidifies. Resin injectioncan only work in cavity walls ifcar-ried out ina ery controlled way. Uncontrolled, the resin easilyenters the cavity, andmay bridge it inplaces allowingdampness o crossto the inner leaf. Resin is expensiveso lettingit escape is costly.Strappingand ying: On occasionit is appropriateto ie small extensions such as baywindows andporchesback to the main building.Steel straps may be installed, and theyshould at leastbe galvanised, but are bettermadeofstainless steel. Theseneed tobesuitably fixed, for example by beingwell screwed to oistsat oneend and tightlyembedded in brickworkat the other. Ifsubsidence affects a main wall and iftheelevation ison the move and not tied into the buildingas a whole, it may be necessaryto install steel tiesbetween it andeach flooratcentresof 1 .2m to 2m. A methodoftenseen in oldbuildings is thetie-bar andwall plate. l1No opposite walls are tied in in thisway. Alternatively he internal endofa tie-bar may be fixed in some other way. Aplateon a walldoes not necessarily meanadequate tying is now provided. The anchoragemay be insufficient, orcorrosion may haveweakened he bar betweenthe plate andthe wall. Fig 14 shows various superstructurestrengtheningmethods.FIGURE 14Superstructurestrengtheningmethods

Underpinning:Chapter3 Deciding themethod ofrefr

Newbrickstitched ntoreplacecrackedbrickstitchinglintel

STITCHBONDING AND LINTEL BONDING

Metalnipplefixedwith


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Underpinning:Chapter3 Deciding themethod ofrepair

ASSESSINGTHE COST OF REPAIRS

InvestigationcostsTypical current costsfor initialinvestigation ofsuspected subsidence are given in Table7(a). Apart from the amounts due as insurance excess these costsshould berecoverable throughthe building's insurance policyif a claim is accepted.Underpinning costsClearlyvarious factors havetobe considered when a contractoroffers a priceto do anywork. For example, workwill cost more ifaccess is difficult. Howevera ball park'guidance table 7(b) indicateslikelycostsformassconcreteunderpinning in heavy clayatvarious depths. (At 2.5m to 3m depth, pier and beam underpinning rates areprobablythe same).SuperstructurerepaircostsCosts ofsuperstructurerepairare less easy tostate since theyare more relatedtoaccess, protection, and subsequent e-decoration. Table7(c) gives current rates forthreecommon typesof repair.Other costs must be allowedfor in addition to the investigation and repair.Figure 7(d) givesan indicationofwhat these amountto.Costs quoted are exclusive ofsite 'on-costs'.In other words,thebuilder might charge500 on top ofthe costsofwork to setup his site, maintain itand leave it idy.Contractors ike to work in empty rooms where theyare not restrictedor interruptedand their prices are likelyto reflectthis. If thereis a significantamountofwork to bedone from insidea houseit is usuallybetter forthe furniture to be stored andfor heoccupants tomove outtemporarily. (Itshould be possible to recover rentofotheraccommodation and all expenses associatedwith moving, togetherwith consequentialcosts suchas moreexpensivetravel to work, from the insurance company).Itisoften convenientto have other workdoneat the same time as underpinning (toavoid two periods ofdisruption). It is common to have redecoration done by adecoratorrather than bythe underpinning contractor. The advantage is thatcosts canbe reduced as the underpinning contractor'spercentagewill notbe addedto thedecorator's price - but bear in mind that theremaybe disputesabout responsibilitybetweentwo contractors and alsothatthe insurers will not payfor"betterment".Theyare only obligedto restore the buildingto its conditionbefore subsidencetookplace.It is advisable to commission a consulting engineer, architector surveyor to specify andoverseethe underpinning works. This is because thewholeofthe work, not ust theunderpinning element, should be in the handsofa consultantwhoacts impartially.Hisinvolvementwillusuallybe a continuation ofthe investigation process. Ifa consultant semployed, his fees during a remedial contractwillbe extra to the contractor'spriceandin the rangeof 12 to 15%. Insurersshould paythe consultant's fees in addition to thecost ofthe workbut it is prudenttoget the insurance company'sagreementbeforeaconsultant s appointed.

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Underpinning:Chapter3 DecidingthemethodofreTh

Fees willalsobe charged by the local authorityfor inspectingtheworkand for permitsto placeskips or the like in the publichighway.These costswill best be covered bywriting hem intothe contractdocuments sothattheyare charged by the contractor asa partofthefinal contractsum.Table7Typicalcostsforunderpinning 3 edroomhouse 9937a__Investigation stage CostAninitialengineer's eport

Ground investigation (1 trial pitand 1 borehole+soil esting)Finalengineer's eport taking accountof nitialreport nd investigationandofvisitsfor monitoring purposes) 250-4007b Type ofunderpinningwork Depth Cost permetreMass concrete 2.5m 300-400Pierandbeam 3.Om 350-450Pileand beam l0.Om 400-6007c Superstructurerepairworks Cost

Rebuilddamaged acing brickwork 45 -55persq.metreResin injectionofbrickwork 40 -50persq.metreReplacedamagedplaster 25 -530 persqmetre

7d Other cost items CostAlternativeaccommodation(ifnecessary because ofwork)Contract preliminaries(setting up,clearingand maintainingthesite)Decoration

_____ 500-1000Oftaseparate contract(ifmaincontractor stoberesponsible fordecoration anextrachargewillbemade)Associatedrepairs. If heopportunity staken torenewitehispartlydamaged byunderpInnInginsurersmayconsider this "betterment"and refusetopayProfessionalfees Usuauy 10%- 15% of he costof hework

Buildingcontrol fees Miiiiium28 (but ee chapter 5)VAT ispayable (unless thework ispartofa programmeofwork for which listed buildingconsenthasbeenobtained)

5150-5300350-450

Full costofmoving, furniturestorageand rent(withinlimits setby insurers)

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Chapter4: MAKING AN INSURANCE CLAIMWHY MAKEACLAIM?There is a view that insurers have largely caused the escalation of insurance claimsforunderpinning. They may be reluctantto insure a house which shows any signsofsubsi-dence. Thereforeownersclaimfor underpinning however slightthe subsidence hasbeenbecause theyknowthat, if hey decide to sell, itwill be hard to find a purchaserfor a housethat shows signs of subsidence damageand so may be difficultto insure.WHAT IS COVERED BY INSURANCE POLICIES?Insurance ofhouses andother buildingsInsurance is a contract underwhich the insurerpromises, in return forthe paymentofpremiums, that ifthe insured suffers loss causedbya specified peril, then the insurerwillmakegood his loss, It is a principle of insurance law that a personshould notprofitfromaccidentaldamageto his propertyso insurerswillonly paythe value ofwhat hasactuallybeen lost. When underpinning is necessary the end product may well beabetter foundation han the foundation thatexisted previously. In this senseimprovementmay haveoccurred. However, sincean underpinned housecouldbe considered tobe ofless value on the market than an identical buildingnotaffectedby subsidence, insurersdo notclaim thatan increase in value hasoccurred. It is simplyaceptedthata loss invalue has been averted.Most houses are insured since it is a requirementofthe buildingsociety orbankwhichprovides the mortgageand most housesare mortgaged. If thereis no mortgageeit isequally necessaryforthe owner tokeep it fully insured. If it is under-insured anyclaimwill bemetat beston a pro-ratabasis. That is, ifthesum insuredis only halfthe costofrebuilding the insurance company may only payhalf ofany repairs costsclaimed lessthe costofexcess. To ensurea building is adequately insured a valuation should beobtained either from a surveyor orpossiblyfrom oneor two local estate agents.Aqualifiedvaluerwillbe able, for a fee, to give a written valuation that can be used inanyfuturedisputeover insurance claims.NHBCInsuranceMost privately builthouses are covered for the first 10 years byan insurance whichtheirbuilders obtain by being membersofthe National House BuildersCouncil (NHBC).Standardsofconstruction are laid down by NHBCwho send inspectors to see that hestandardsare followed on site, Ifthere is a structuralfailure inthebuildingduring thefirst2years the builder is liable butifhefails to remedy the damage, or ifthe failureoccurs in years3 to 10, then the NHBC insurance operates.New houses do need to beseparately nsured againstsuchthingsas fire and storm damage from the outset,andafter 10yearsa full insurance is needed.The excesson an insurance policyThe normal household insurance policies specifyan "excess" on certaintypesofclaim.The excess varies with the typeofclaim. Subsidence claimsusuallycarrythe highestexcess, usuallyof1000. Theyare oneofmostexpensivetypesofclaimthat scommonly madeby householders. The excess mustbe paid bythe householder.

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Underpinning: Chapter 4 Making a insurance claim

If he householder doesnothave money available he mortgageescan oftenarrange forthe mortgagors o be increasedtocover the paymentof he excess, Ifyouare not cur-rently"mortgageable", for example ifyou are unemployed, other help maybe available.The DHS "Social fund" may agreeto special payments ortoa loan. Anotherpossibilityis toapproach the local authorityEnvironmental Health Officer. He may declare thebuilding"unfitfor habitation" and as a resulta mandatory grantmay be released. This,however is a long-shotandwill only apply ifdamageis particularlyserious (forexampleifdamp proofcourses are broken and the building becomes excessivelydamp).There are sometimes disputes about a claim covering aspectsofdamage thatappearedat different imes or thatarose from differentcauses. The insurance company maythen want to treat the claimas two ormoreseparate claimsand the householder willthen havetopaytwo ormoreexcesses.Liabilityfordamage to neighbouringpropertyLiabilityfordamage to a neighbour's propertymay be covered either bythe buildinginsurance policy or the contentspolicy. If the propertyis occupiedby someone otherthan the owner then subsidence damageshould be covered underthecontents policy.Inthe case of subsidencedamagecaused by a tree the lawholds that the occupier isliablefor such damage,since the occupier is presentwhile the ownermay, beelsewhere and therefore notaware ofthe extentofthe treesgrowth.If, on the other hand, the ground beneath both houses hassubsided and a tree isnotinvolved both ownersshould notify under their buildingspolicies and ifthe claims areaccepted they should both expect to payan excess.Specific limitationsofcoverHeave: Sinceabout 1980 most policieshave included heave. If the 1987hurricaneblew atree downand, as a result, clay ground heaved and damageda nearbyhouseitshould be covered. Heave is less common than subsidence but, because ofthe damageheave does to the superstructure, he cost ofremedial measures is likelyto be greater;there is thereforean even greater need for insurance cover.Landsllp:Landslip is covered by somepolicies. It is a relativelyunusualsituationwhichoccurs on sloping sites whenthe groundsurrounding the building moves down theslope andthe building is damaged, destroyed, displaced,orburied. Landslip is beyondthe scopeof hispublication.Floor slabs: Many policiesexclude subsidence ofconcretefloor slabsunless walls havesubsided too, If he walls have notsubsided it is likelythatthe subsidence ofthefloorhas beencaused by defective excavationorbackfillingbelow the slab.Betterment: When remedialwork s carried out it often makes sensetobuild tomodern standards rather than to replace the existing construction. For examplethermal insulation can be fitted relatively cheaplytowalls, roofsorfloors slabs iftheyhave to be replaced (insulationmay in fact become a requirementunder newbuilding

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Underpinning: Chapter 4 Making a insuranceclaim

regulations where roofs or floors are replaced) . The insurance company is notobligedtopayforan improved standardofconstruction howeversensibleitmaybe to makethe improvement. Such improvements may be described as betterment; the buildingownerwill then have to payfor them.Uninsured buildingsInsurance ofchurches and farm or othercommercialbuildingsseldom ncludessubsidence cover, but theremaybeother insurance policiesin operation o cover thecosts ofa shutdownofthe business, which willhelp with the costofvacating premisesto allow underpinning to go ahead.FOLLOWINGTHE CLAIMS PROCEDURENotifyingthe insurersInsurers should be notified as soonassubsidence occurs for whicha claimmaybemade, Ifthereare doubts astowhetherthere is enough evidenceto supportasubsidence claim, an engineer,architector surveyor can be askedtogive an opinion.Theywill probably be preparedtogive an initial verbal assessmentfora nominal fee.The LocalAuthority Building Control departmentmay alsobe able tohelp, especially fthe area is particularlyproneto subsidence.The insurers initial responseThe insurerswillgenerally require a surveyor or engineerto report. Usually thistakesthe form ofan initialreportwhichsets out the circumstancesso that insurers and theloss adjustor, who is appointed o acton their behalf, can consider the situation.Ifit sclear cutthey may then accept hat remedial work, possibly including underpinning, iscalled for. In othersituations heymay requiremore investigationsto establish firmerproof.These will probably entail groundinvestigationsfollowed by laboratory ests.Alternatively (and possibly additionally)theyare likelyto requiremonitoring of hebuilding,forperhapsa year, to see how the situation developsand whether themovement acrosscracks is progressive (whichwould probably require underpinning)orwhether it hasstopped (whenunderpinning mightnot be necessary andonlysuperstructuralworkrequired).The formalagreementsBefore the repairwork starts the insurance company may require the ownerof hebuildingto formallyagree thatthe workspecifiedwillmeet the claimand when thework isfinished, beforethe lastpayment s madeto the contractor, the ownermay beasked to formally agreethatnofurtherclaimwill be madefor additional repairsrelating to he damagethat had occurred. These are important agreementsandshouldbe considered by the ownertogether with any consultants that have beenappointed.The method ofpaymentWhen a consultant s employed to supervisethework, he mayseeka mandatefrom theowner instructing the insurance companytopaythe contractordirect, as soonas heyreceivea certificate from the consultantsaying the work has been satisfactorilycompleted. In largercontractsinterim certificates are likelyto be issued for payment

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each month.Alternative methods ofpayment may involve a directpaymentto thecontractorWhen the,owneragreesthatthework has been completed orpayment othe owner when the specificationand price havebeen agreed.V.A.T.:When an owner, for examplea propertycompany, is registeredforVAT theinsurance companywillnot paythe VAT on the buildingcontractbutwill expect theowner to payand to claim reimbursement romCustoms and Excise. If the building islisted as a historic buildingand historic building consent hasbeengivenwith specificreference to he underpinningthen the underpinning work is zero rated forVAT and nopaymentneeds to be made.Whenthe underpinning is for an owner who isnotregisteredfor VAT the insurance willpaythe VAT.

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FIGURE 15Typical insurance policy wording"Escapeofwater clause"if his results nwash-outofmaterials under oundationsandsubsequentdamage, a claimmaybemetunderthisclausewithan excesslessthan25

"Subsidenceclause"thisnowadaysinvolvesanexcess of1,000more


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NEGOTIATING FORMAXIMUMREIMBURSEMENTOnce the findingsofanyspecialist investigations are to hand the investigating engineer,architectorsurveyor can make an assessmentofall aspects ncluding he substructuresuperstructureinteraction and submita final report ustifying the claim. Most housebuildings policiescover subsidence but the details ofhow it is covered canvary. In anycasetheywill not cover the excess. To what extentsubsidence insurance exists dependson the policy wording. Figure 15 gives two specimen policy wordings.

FIGURE 16Cross section of tree roots underthe microscope(longitudinalsections are alsorequired to confirm identification).

How disputes over liability canariseTree roots: Ifsoils laboratory estsshowdesiccated clayto somedepth beneath thefoundation ofa damaged building andtree root identificationtests (see figure 16)show that the rootlets froma trial hole are live andofthe samespecies as a nearbytree thiswill usuallybe sufficient evidencetodemonstratesubsidenceand a need forunderpinning. (Note that tree roots can die and regeneratein a few weeks- so deadrootsmay alsobe presentedasevidence). If, however, the thickness ofdesiccated claybeneaththe footing is much less, say300mm, then it maybe reasonable for heinsurers to require cuttingbackofthe tree canopyand the carrying out ofsuperstruc-ture repairsonlyon the assumption thatfurther shrinkage ofthe clay can be preventedand the superstructure repairswillnot beat risk.External worksandoutbuildings:Some policies cover damagetoall manner ofexternalbuildings, even swimming pools, but only ifthe house is alsodamaged.

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Photo:Dr.R.D.Macleod. TreeRoot nvestigations td.ASH BIRCH PINE


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Underpinning:Chapter4 Makinga insurance claim

Adjacent properties:Often policiesspeakofsubsidenceofthe groundon which thehouse is built. Does this mean the ground belowthe house? Whatifsubsidence underterrace house Acauses damagetoadjacent houseB's superstructure? In somecasesinsurerswill waive such arcane considerations andwill consider house B as beingsubject to subsidenceofthe groundon which it is builteven whenthe evidenceis thatthe ground thatcaused the problem was actuallyunder houseA. However in this caseownerB is likely o havetomeet the excess.Leakingdrains: If investigationsindicatethatdrains are leakingtogether with theassociated riskofgranular material thatsupportsfoundations beingwashedawaythenthe claim may be met under the 'escape ofwater' clause ofthe policy. In this case amuch lower excess, perhaps 25 or less, will be payable. However the causeof hedamagethe leakwill need to be repaireddirectly.Extentofdamage:The insurance policywillcoverdamage caused bysubsidence butnot damage resulting from neglected maintenance orpoor durabilityofsuperstructurematerials. Therecan be all sorts ofargumentsin this area. When a building is openedup weaknesses, corrosion and rotmay be revealed. Normallythese must be dealt withatthe owner'sexpense butitis sometimes possible to convince he insurer'srepresentative hatconsequentialdamagehas occurred as a result ofsubsidence cracksletting water into the structure. It has been knownfor insurance companiesto pay partofthe cost ofa new roof wherethe flashings have beendisturbed by the movementofthe walls below.Dealingwiththe insurer'srepresentativesInsurerswill choose someone to negotiate on theirbehalf. This may be a loss adjustor,though some companiesappointsurveyors orengineers or a suitably qualifiedmemberof heir own staff. Loss adjustorsusually pride hemselves as being impartially placedbetweenthe insured and the insurer. It is probablythe bestpolicytotreat hem as such(whileprudently ensuring that they are presentedwith the mostfavourable case).Partialunderpinningand insuranceAshas been said in Chapter 3 underpinningDften has to be partial. Moreover,sinceinsurance is todo with alIeviating the effectofa peril', the loss adjustorsmay onlyacceptpartial underpinning,sayatone endofa partywall. The engineerorsurveyormaysee things rather differently.Withoutnecessarily being over-defensivehe orshemay believethat the correctsolution is to underpin the whole length ofa wall. Thecompromise calledfor in this situation maywell be thatthe engineer puts on recordthe opinion that the 'correct'solution istounderpin the whole wall. The insurersmaystill onlyacceptpartial underpinning. Ifso, the engineer can establish that ifmoredamageoccurs because too little underpinning was done, then the insurers will meetthe extra costsof urther remedialwork. If his isagreed, itwill alsobe reasonable toestablish thatthiswill constitute a continuationof he sameclaim and that a furtherexcesswill not be charged. Itmay alsobe appropriate to seek an undertakingthatthe pol-icywill notbe terminated by

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Underpinning and Repair of Subsidence Damage

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