More than 1000 million worth of subsidenceclaims, involving cracking and distortion inhomes, were submitted to UK insurance firmsduring the period 1989 1991. Much of thedamage arose from foundation movementcaused by problems such as subsidence andheave of the ground, and landslip. Much of themoney was spent on underpinning thefoundations of houses and blocks of flats.Underpinning usually involves deepeningfoundations; it may sometimes involveproviding a new, different form of foundation.

Most insurance companies include cover aspart of their general building insurance. Thiswas first introduced in 1971, since when therehas been a substantial growth in claims. Eachyear many families will experience the anxietyand disruption of the underpinning process.Unless their home is newand covered by a warrantyscheme, they also have tomeet the excess on theirinsurance policy, usuallybetween 500 and 1000,and may incur additionalexpenses arising from theemployment of professionaladvisers. If they are notinsured, they willthemselves have to meet thefull costs of remedialmeasures. This could be20,000 or substantiallymore in severe cases.Underpinning accounts forabout half of this, the restbeing the cost of repairs to

cracked walls, professional fees, redecorationand temporary accommodation while repairwork is carried out.

Why has this growth in underpinningoccurred? What indeed is underpinning andwhy is it carried out at such expense? Toanswer these and other questions, the BuildingResearch Establishment commissioned anation-wide survey of underpinning. Thefindings of this survey and technical issuesconcerning underpinning are presented in aBRE Report, Foundation movement andremedial underpinning in low-rise buildings.This Digest presents a more general discussionand is intended to give backgroundinformation on the causes of foundationmovement, and when and why underpinningmay be necessary.

Figure 1 Pier-and-beam underpinning

Underpinning

Digest 352Revised 1993

CI/SfB (B2d)

BRE Digest

Concise reviews of building technology

Building Research Establishment

Technical enquiries to:BRE Advisory ServiceGarston, Watford, WD2 7JRTel: 01923 664664 Fax: 01923 664098

Many owners will be unaware of damage to their homesuntil they try to sell. Then the surveyor for the purchaser,or more often his mortgage company, reports a problemwith cracks or floor slab subsidence and the purchaser isdeterred from buying unless a favourable engineers reportcan be obtained. The vendor will, therefore, seek advice,usually from his insurer and will, in most instances, lodgea claim.

The majority of subsidence insurance policies are verysimilar, particularly those of the large insurers, but therecan be differences between policies. For example, someexclude damage to patios, drives and floor slabs whenexternal walls are not damaged by the same cause and atthe same time; others exclude damage caused by structuralalterations or faulty workmanship, or the use of defectivebuilding materials.

Most mortgage companies require to be informed whendamage has occurred. Then they pass the case to theirnominated insurance company who will normally appointa loss adjuster. Some loss adjusters have sufficientexpertise to investigate and diagnose the cause of thedamage; more commonly, the home owner making theclaim will be required to demonstrate that the claim islegitimate by obtaining professional, technical advice onwhat has happened, to decide if the problem is serious andto recommend appropriate remedial measures. Therequired expertise is more likely to be available fromchartered civil and structural engineers, or charteredbuilding surveyors. Some of these professionals will bespecialist foundation contractors.

If a property is covered by a guarantee such as the 10-year National House Building Council scheme, the ownermay approach the registered builder if the damage isdiscovered during the first two years, or the NHBCdirectly if discovery is in years 2 to 10. The Foundation15 guarantee scheme, operated by Municipal Mutual,covers a period of 15 years after construction.

Other sources of advice include the Building ControlOfficer at the local authority, a solicitor, the CitizensAdvice Bureau or similar organisations; the house ownermay directly approach an underpinning contractor.

It is important to appreciate that, with many insurers, theburden of proving a valid subsidence claim rests with thehome owners; ultimately, therefore, they must appointtheir own adviser to investigate and determine the cause ofdamage to the home. A lengthy process follows: lodging aclaim, dealing with a loss adjuster, appointing aprofessional to investigate, having the claim accepted,appointing a contractor to carry out remedial work andmaking good the damage. The time interval between thefirst discovery of damage and the completion of the repairto the building is unlikely to be less than a year and, insome cases, may be considerably longer. This long periodcan be especially disconcerting when the sale of aproperty is urgently required. Before embarking on a longprocess of putting right the problems and restoring thebuilding to its condition before the damage occurred, it isuseful to have an understanding of why damage occursand to consider carefully how much repair work to carryout.

SEVERITY OF MOVEMENT AND CRACKINGFoundation movement can result from many causes, themost common being shrinking and swelling of clay soils.From examination of underpinning practice reported in theBRE survey and in earlier studies by BRE of damagearising from clay soil shrinkage and swelling during andafter severe drought conditions, the conclusion emergesthat the degree of movement and consequent cracking thatcause concern to home owners is rarely of structuralsignificance. However, the potential for movement tocontinue and for damage to worsen substantially is notalways fully considered.

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HOW TO RESPOND WHEN DAMAGE IS DISCOVERED352

Cracking and distortion can occur for a wide variety ofreasons; only a few of these will require significant workto remove the problem, or modification of the foundationsby underpinning to stop foundation movement. The morecommon causes of cracking are given here. This list is notexhaustive. Other documents recommended for furtherreading also discuss causes of cracking; they are listed atthe end of this Digest. Except for foundation movement,the causes of cracking all stem from problems in thestructure of the building and only extremely rarely dothese problems require major repairs.

THERMAL EFFECTSDiffering amounts of expansion and contraction indifferent building materials may cause small cracks.

CREEPMany building materials deform slowly under load overmany years. This very slow movement may eventuallycause small cracks.

MOISTURE MOVEMENTSQuite noticeable cracks will sometimes appear in newhouses as walls dry out. The introduction of centralheating in old properties may cause drying shrinkagecracks. In either case, the cracks are not significant andmay be readily repaired.

MOVEMENT DUE TO LOADINGMinor cracking can result from increases in load, forexample when heavy items are stored in lofts causingceilings to sag, or when structural alterations are made to ahouse.

PHYSICAL AND CHEMICAL CHANGESAn example is severe frost, which can occasionallyexacerbate small, moisture-filled external cracks.Occasionally, the formation of sulphates in mortar cancrack walls.

VIBRATIONVery occasionally, severe vibration from heavy traffic orconstruction plant may induce small increases in existingcracks, or start small cracks where high stresses areconcentrated.

POOR DESIGN DETAILING AND WORKMANSHIPThis can cause cracking in newly-completed buildings;inferior quality materials may eventually bring aboutdamage.

FOUNDATION MOVEMENTl Settlement: downward movement brought about by the

pressure from a foundation causing compression of theground. It is the differential settlement of one part of abuilding relative to another that produces distortion andcracking; this may result from several factors, such asvariations in the shape and size of foundations (egpartial basements) or, more usually, variations in soilstrength or compressibility beneath the building.Adding an extension can sometimes result in cracksoccurring where it joins the original structure. Thiscracking is caused by the initial settlement of theextension and is not normally a continuing problem.

l Subsidence: downward movement of the groundbrought about by activity in the ground, including:l Mining.l Compression of loose, man-made fill owing to its

self-weight, or to the ingress of water.l Removal of water from the ground, for example by

pumping or, more commonly, the drying effects oftrees on clay soils, especially in dry, summerweather.

l Excavation that removes lateral support fromadjacent ground and, thereby, from the foundations.

l Soil erosion owing to natural underground waterflow or leaking drains.

l Collapse of the ground surface above naturalcavities formed below, such as solution cavities inchalk (swallow holes).

l Heave: this is upward movement of the ground. It iscaused either by the removal of load (for example in adeep excavation) or by an increase in the water contentof the ground that can occur when trees, that have driedthe soil, are removed; the soil swells as it becomeswetter. Very occasionally, in particular circumstancesand in extremely cold weather, freezing of the groundcan cause heave. Very rarely, floor slabs have beencaused to heave by the swelling of the under-floorfilling material, such as certain burnt colliery shalesthat are prone to chemical expansion in the presence ofmoisture. Insurance policies usually cover subsidenceor heave only of the site and so would not apply to thisproblem.

l Landslip: although major, severe landslipping is a veryrare event in the UK, long-term downhill movementson slopes sometimes cause damage in certain areas.

WHAT CAUSES CRACKING AND DISTORTION IN WALLS?

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352

Underpinning usually involves the extension of theexisting foundations downwards to reach stiffer or morestable ground. There are several methods:

Mass concrete Figure 2Pier-and-beam Figure 3Pile-and-beam Figure 4Pile Figure 5

The mass concrete method accounts for over 40% of allunderpinning work, followed at 30% by pier-and-beam.

The principal reasons for choosing a particular method ofunderpinning concern the extent of the underpinningrequired, the nature of the ground and the cost, includingthe degree of disturbance to the building occupants.

Whatever method is used, underpinning is applied eitherto part or to the whole of the foundation. Which isappropriate will depend on the scale of the problem andthe ground conditions. Partial underpinning is employedwhen only localised foundation movements have occurredand where the stabilising of only part of the foundationwill not have a detrimental influence on the remainder. Anexample is where a house has been built partly over thesite of an old, infilled pond. Most of the foundation isperfectly stable so that underpinning only the part over theold pond will be satisfactory. The vast bulk ofunderpinning projects involve partial work only. Totalunderpinning may be performed when an entire buildingis settling, perhaps on soft, peaty ground, or when anadditional storey is to be added to that building, requiringall foundations to be extended down to stiffer ground.

352WHAT IS UNDERPINNING?

Pier-and-beam underpinningA reinforced-concrete beam is installed in the wall, either justabove the existing footing or replacing it, and discrete pits areexcavated at intervals beneath the modified wall. After fillingthe pits with concrete to form piers, pinning-up is carried outbetween the concrete and the base of the beam or existingfoundation.

Figure 3

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Mass concrete underpinningThis involves excavating systematically in adjoining baysbeneath the existing foundation, filling the excavations withconcrete and then pinning-up the small gap between the oldand new foundation. The sequence is indicated by thenumbers on the drawing above.

Figure 2

352

Figure 5

Figure 4

5

Pile-and-beam underpinningPiles are installed beside the walls and beams areinserted, both within the walls and through them,to join the tops of the piles.

Pile underpinningVertical piles installed either side of the wall are connectedat their heads by a needle that passes transversely throughthe wall (View A). A variant of this type, in which acantilever is used instead of the needle, removes thenecessity for piling inside the building (View B).

The use of inclined mini-piles, installed through holesdrilled in existing foundations, is becoming more widely-used (View C). Small, vertical mini-piles can be installedthrough pre-drilled holes to underpin a subsiding floor slab(View D).

See Digest 313 for more details on mini-piling.

VIEW A THROUGH THE WALL

VIEW CINCLINED MINI-PILES

VIEW DFLOOR SLAB MINI-PILES

VIEW B CANTILEVERED

Underpinning is usually necessary only if furtherdamaging foundation movements need to be stopped. It istherefore important to find out if movement is continuing.However, the survey revealed that a substantial number ofunderpinning operations were carried out principally toensure that a house sale could proceed, rather than toeliminate continuing movement. It was apparent thatmany owners expect their house to be underpinned ifcracking has occurred, regardless of its severity, cause andwhether or not it is worsening.

Deciding if damage is likely to get worse may be difficult.Establishing the exact cause of movement may requireexpensive exploration and soil testing. Demonstrating thatmovement is continuing will involve time-consumingmonitoring. The attitudes of insurers to monitoring varies:some will not pay for it unless a valid claim forsubsidence is subsequently proved, others offer to payeven if no valid claim results, arguing that by so doingthey reduce the chances of paying for unnecessaryunderpinning.

In these circumstances, it is not surprising thatunderpinning is often chosen as the easiest, quickest andsafest option when doubt exits. However, a decision tounderpin can bring about its own problems; this isexplained in Difficulties encountered.

Guidance on the likelihood of progressive foundationmovements is given in Digest 251. This also containsadvice on determining the severity of a case of damage; itsuggests that underpinning is necessary only beyond acertain level of damage (Category 2) and where there isevidence of continuing movement.

However, in certain, usually obvious circumstances, theanticipated worsening of damage to Category 2 andbeyond may warrant underpinning to arrest movement.For higher levels of damage, underpinning may berequired to strengthen a fractured foundation, even if theprimary cause of damage has ceased or been removed.

In the majority of cases, a professional adviser willrecommend whether or not underpinning is needed(although the ultimate decision may rest with an insurer)but it is important for the home owner to appreciate thatunderpinning need not normally be the consequence of thediscovery of cracks.

In certain, rare circumstances underpinning may beinappropriate despite continuing subsidence; an exampleis where deep-seated movements caused by miningactivity are taking place.

A high incidence of cases of floor slab subsidence hasbeen reported in certain areas of the UK. In part, this iscaused by particular ground conditions, like the presenceof a layer of slightly organic soil just below the topsoillayer. However, in some localities it has become commonknowledge that floors may have moved, so that somesurveyors have become particularly diligent in looking forgaps below skirting boards or for evidence of hollownessbeneath the slab.

In some cases, floors have been completely replaced inorder to bring about a house sale, when all that hasactually occurred is minor movement that has sincestopped. If continuous floor movement is evident,underpinning with mini-piles may be appropriate.

352

WHEN IS UNDERPINNING NEEDED?

6

The nation-wide survey commissioned by BRE foundthat about 50% of all underpinning is carried out inLondon and the Home Counties. Figure 6 shows thedistribution with the UK divided into 12 arbitrary areas.There are two main reasons for this distribution:

l In the UK, the major cause of damaging foundationmovement in houses and other low-rise buildings isfound to be shrinking and swelling of clay soils,exacerbated by the presence of large trees. The claysoils that are most prone to volume changes arefound in south-east England where there is also ahigh density of major tree-cover, including streettrees. The south-east generally experiences lessrainfall than other areas.

l People take a more serious view of cracking themore valuable is their home. There has been adiscernible heightening in awareness of minorcracking in houses throughout the UK.

Underpinning can be obtained in several ways:l From a chartered civil or structural engineer who will

design a suitable scheme and supervise the contractorwho performs the underpinning.

l Directly from a specialist underpinning contractor:there are many of these currently in operation and mostoffer a variety of methods to fit individualcircumstances.

l From a normal building contractor; much partial, massconcrete underpinning can be carried out by builders,though it is advisable, firstly, to appoint an engineer orbuilding surveyor to ascertain that the underpinning isstrictly necessary and, secondly, to ensure it issufficiently extensive and correctly applied.

There has recently been an increasing trend for insurancecompanies and their loss adjusters to employ consultingengineers, with expertise in foundation engineering, toinvestigate claims and, where appropriate, design andsupervise underpinning work. This has led to betterassessments of the need for underpinning and is acontributing factor in the fall in the total cost ofunderpinning throughout the UK (Figure 7). However, thevery prolonged drought from 1989 to 1992 producedmany cases of subsidence damage; this is reflected in theenormous increase in claims shown in the figure.

A valuable source of information on foundation specialistswho will be able to advise on underpinning matters is theGeotechnical Directory of the United Kingdom.

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WHO DESIGNS AND CONSTRUCTS UNDERPINNING?

WHERE IS UNDERPINNING MOST CARRIED OUT?

Figure 6

Distribution of underpinning projects in 12 areas throughout the UK

Annual value of insurance claims for subsidence and heave damage todwellings.Based on payments made by the major insurance companies followingthe mild drought of 1984.

Figure 7

THE ROLE OF BUILDING REGULATIONS INUNDERPINNINGLord Denning defined underpinning as an extension of abuilding in a vertical downward direction. It, therefore,constitutes a material alteration and is subject to controlunder the Building Regulations. Applications to underpinmust be submitted for approval and inspection of the work.Some local authorities, particularly in the south ofEngland, require more information on underpinningproposals than others. It is perceived that, in givingapproval, some liability may attach to the approvingauthority. It has, therefore, to be satisfied the work willprevent any recurrence of damage that constitutes a threatto the health and safety of the building occupants and willnot cause instability to any other building such as tothreaten the health or safety of its occupants.

Remedial measures that do not constitute a materialalteration do not require Building Control approval. Suchmeasures would include the repair of drains, bonding ofcracked brickwork, forms of structural tying abovefoundation level and the removal of trees held to beresponsible for subsidence damage. Complications mayarise with tree removal if the tree is protected by apreservation order. Digest 298 deals with the influence oftrees on foundations in clay soils.

GUARANTEES OF UNDERPINNING WORKSMany specialist underpinning contractors offer guaranteeslasting for 20 or 25 years. Mass concrete underpinningwork carried out by local builders is probably not coveredby any form of guarantee. Having the underpinningdesigned and supervised by a suitably qualifiedprofessional should provide assurance of satisfactoryperformance.

DIFFICULTIES ENCOUNTEREDA particularly difficult problem sometimes arises whensubstantial underpinning is deemed necessary to one of apair of semi-detached houses or to one of a terrace ofhouses. Since the Building Regulations state that: Thebuilding shall be so constructed ... without causing ...movement of the ground, as will impair the stability of anypart of another building, local authority building controlstaff have been known to reject applications because theyperceive a significant risk that damage will occur in theadjoining property as a result of the underpinning. In thesecases, the matter is usually resolved only by calling ingeotechnical experts to assess the risks.

352

FURTHER READINGFoundation movement and remedial underpinning in low-risebuildings. BRE Report, 1991.

Geotechnical Directory of the United Kingdom. British Geotechnical Society; Institution of Civil Engineers;Westminster, London.

BRE Digests240 Low-rise buildings on shrinkable clay soils: Part 1241 Low-rise buildings on shrinkable clay soils. Part 2242 Low-rise buildings on shrinkable clay soils: Part 3251 Assessment of damage in low-rise buildings276 Hardcore298 The influence of trees on house foundations in clay soils313 Mini-piling for low-rise buildings343 Simple measuring and monitoring of movement in low-rise

buildings. Part 1: cracks344 Simple measuring and monitoring of movements in low-rise

buildings. Part 2: settlement, heave and out-of-plumb359 Repairing brick and block masonry361 Why do buildings crack?

ISBN 0 85125 576 0 Copyright BRE 1993Republished on CD-ROM 1997, with permission of BuildingResearch Establishment Ltd,

by Construction ResearchCommunications Ltd, 151 Rosebery AvenueLondon, EC1R 4QX

Applications to republish all orany part of this publication shouldbe made to ConstructionResearch Communications Ltd, PO Box 202, Watford, Herts, WD2 7QG

Anyone wishing to use theinformation given in thispublication should satisfythemselves that it is not out ofdate, for example with referenceto the Building Regulations

Technical enquiries to: BRE Advisory ServiceGarston, Watford, WD2 7JR Telephone 01923 664664 Facsimile 01923 664098

8

Underpinning has, occasionally, not prevented therecurrence of movements or the reappearance of smallcracks. The reason for this failure of the underpinning isusually poor design. For example, the underpinning has notbeen taken to a sufficient depth, construction practice hasbeen poor, the nature of the movements has been wronglydiagnosed or an insufficient length of the building has beenunderpinned.

REINSTATEMENT OF DAMAGEIf underpinning or other remedial measures are necessaryto arrest a case of foundation movement, making goodcracks or repairing other damage should not be attempteduntil these measures are complete. Advice on repairingcracked brickwork is given under Further reading.