Building Research Establishment

Digest 251Revised 1995

CI/SfB (X)

BBRREE DDiiggeessttConcise reviews of building technology

Assessment of damage in low-rise buildingswith particular reference to progressive foundation movement

This Digest discusses the assessment and classification of visible damageresulting from structural distortion. The assessment is based on adescription of work considered necessary to repair the building fabric;classification into six categories is recommended, taking into account thenature, location and type of damage.

The most common causes of damage are discussed. It is concluded thatfor damage of Category 2 or less, cracking may result from acombination of causes which are difficult to identify and the cost andeffort involved in carrying out an identification would bedisproportionate to the scale of the damage, except for circumstanceswhere the movement is likely to be progressive. It is rare for damage toprogress beyond Category 2; when it does ground movement is usuallythe cause. The various causes of ground movement giving rise to damageare described briefly and emphasis is placed on the identification ofconditions where the movement might lead to progressive deterioration.

This Digest will assist building professionals, property valuers andinsurance advisors both in putting building damage into its trueperspective and in determining necessary action, either in the form ofseeking expert advice or in recommending simple repairs.

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

Fig 1 Annual value of insurance claims for subsidence and heavedamage to housing

Note the substantial increase following 1989 and 1990 – dry years

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72 74 76 78 80 82 84 86 88 90 92 94

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The severe droughts of 1975/6 and 1989/90 brought to thepublic’s attention the fact that low-rise buildings are susceptibleto cracking of the materials from which the buildings areconstructed. That widespread publicity and some alarm weregenerated reflects not so much the severity of the damage as thegeneral ignorance of both causes of damage and the amountswhich masonry structures can tolerate. It is important to realisethat very few buildings, if any, exist without some form ofdamage. How much can be tolerated depends on a number offactors: the type of building, the function it is to perform, thelocation and nature of the damage, the expectations of the userand the cost of repair work in relation to the value of thebuilding. Many of the ways in which cracking can be producedin buildings are discussed in Digests 359 and 361.

One of the many causes of damage is foundation movementresulting especially from the drying shrinkage of clay subsoil.This is not a new phenomenon and has been experienced manytimes in the past. However, the 1975/76 drought, whilstadmittedly being more extensive than previous dry spells,

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REPORTING DAMAGEThe reporting of damage is frequently less than satisfactoryfor assessing the severity and cause of damage. Statementslike ‘extensive cracking to interior walls’ are often the onlydescription of damage given for a property. Furthermore,the subjective judgement of individuals on the seriousnessof damage varies considerably, so that properties withsimilar levels of damage brought about by similar eventsmay undergo vastly different degrees of remedial work.

A prerequisite for the objective classification of damage ina building is a thorough, well-documented survey. Asuggested procedure for carrying out such a survey consistsof:

On a sketch of each damaged wall, draw the position anddirection of any cracksDistinguish where possible between tensile cracks,compressive cracks (indicated by small flakes of bricksqueezed from the surface and by localised crushing) andshear cracks (indicated by relative movement along a crackof points on opposite side of it). Note the direction of anycrack taper, crack widths, and the frequency of cracks ifthey are too numerous to record individually. If bothexternal and internal crack patterns are plotted (as full andbroken lines respectively) on the same elevation drawings,the mode of distortion and cause of movement can be better

understood. An example of such a plot is shown in Fig 2.Generally, cracks produced by foundation movement arenot widely distributed throughout a building, but tend to beconcentrated in areas where maximum structural distortionand structural weak points coincide. In these areas, cracksare usually few in number so that recording of crack densityis not onerous. Photographs provide a useful record ofcrack patterns and density; Figs 3 and 4 are typical cases.Any movement on one side of a crack in relation to theother in a direction out of the plane of the wall will alsohelp to identify the mode of distortion (see Digests 343 and344).

Fig 2 Crack plotting on building elevation

initiated a spate of damage claims on insurance companiesout of all proportion to its severity. Indeed, cases ofdamage, whilst being geographically more widespread,were no more severe than have been identified by theBuilding Research Establishment in previous dry spells,such as the dry summers of 1946 and 1947. The growth inclaims for subsidence damage from 1971, when insuranceagainst subsidence damage became widely available, isshown in Fig 1 which was produced from data supplied byleading household insurers and the Association of BritishInsurers.

In a two-year period following the end of the drought inAugust 1976, BRE examined 90 properties which suffereddamage during and soon after the drought. These werecases specifically brought to the attention of BRE, largelyby professionals, and probably they represent the worse endof the spectrum of subsidence damage. In addition, caserecords supplied by NHBC (20) and major householdinsurers (30) were examined. The ‘worst’ examples wererequested. The overall sample size was, therefore, 140.

Examining the results of this survey it was apparent thatone single factor had been responsible for the massiveincrease in damage claims: when house insurance cover hadbeen enhanced in 1971 by insurance companies toindemnify against damage caused by ground subsidence, noqualifications had been placed on the amount of damageoccurring. In consequence, many cases of damage hithertoregarded as of no great importance had become the subjectof insurance claims. In addition, houses with cracks which

would once have been disregarded were being significantlydevalued unless expensive remedial measures were carriedout.

It also became apparent that extensions to existingbuildings and structural protrusions, such as bay windowsand porches, were especially vulnerable to slight crackingwhere they joined the main structure. As will be discussedlater, damage to extensions, usually ascribed to clayshrinkage, may well result from other causes.

The over-sensitivity of the housing market has also affectednew construction. Local authority building inspectors arenow much more cautious and it is evident that unnecessarilylarge sums of money are being spent on new foundations forno apparent reason other than in an attempt to prevent smallcracks which, when they occur, may be totally unconnectedwith foundation movement. It is known, for example, that 3 mdeep trench foundations have been used in a number oflocations and even 5 m deep trench foundations have beenreported for clay soils where 1 m deep trench foundationswould have been regarded as adequate in the past.

During the course of the investigations it was concludedthat inadequate attention was paid to describing buildingdamage and that the essential first step in any assessmentshould be to ensure that all visible damage is properlyrecorded and classified in terms of an objective, widelyaccepted scale. The main purpose of this Digest is todiscuss the recording and classification of damage to enablerational decisions to be made on such questions as severityof damage, its cause and appropriate remedial measures.

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Fig 3 Category 2 subsidence damagestuds are used to monitor crack width

Fig 4 Category 3 damagemovements predominantly rotational and horizontal

Try to determine the approximate age of the cracksThis can be done by questioning the occupants on the dateof discovery and by examining the fracture surfaces,particularly of external cracks, for signs of age. Forinstance, recent cracks in brickwork have a cleanappearance, whereas older cracks show signs of dirtaccumulation.

Where possible, measure or estimate the magnitude of anydistortion and movement of the buildingExamples are tilt and bulge of walls, slope of floors and slipon damp-proof course (Digest 344). The plotting of suchvalues on a drawing of the building can be very helpful;Fig 5 shows a plot of the results of plumbing of walls. Aseries of level readings on a course of brickwork nearground level can provide an indication of both the directionof vertical movement and the part of the structure where itis concentrated. Figure 6 shows an example of a plot ofrelative levels around a building. Of course, care isnecessary in interpreting such measures as the brick coursesmay not have been horizontal at the time of construction.Ideally, a series of measurements at different dates shouldbe taken to show if movements are continuing. While thiswill not usually be practicable for full surveys of level andverticality (Digests 344, 386), it may be possible to obtain aseries of measurements with time of the widths of cracks(Digest 343). Many cases of alleged subsidence damagetake considerable time to be resolved so that such a seriesof measurements, over say six months to a year, may wellbe practicable and of immense value in determining cause.Evidence of building distortion is often hidden in lofts; theends of roof joists and purlins should be examined toestablish the extent of any movement. Movement in roofscan also be detected by observing gaps between tiles.

Fig 5 Verticality plot for a pair of semi-detached houses Fig 6 Level plot for a pair of semi-detached houses

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Describe how the serviceability of the building has beenimpairedFor example, doors and windows jamming, window panescracked, draughts and rainwater penetrating cracks, andservice pipes fractured.

Give a thorough description of the materials of walls andfinishes and their condition, especially that of mortarInformation of this type can often enable the identificationof causes other than ground movement, for exampleshrinkage of concrete products, or differential thermalexpansion of dissimilar materials, as well as assisting in theselection of suitable methods of structural repair.

Record details of the constructionThis can have a very significant effect, on both degree andlocation of structural cracking. Every effort should be madeto establish basic information about the structure, forexample whether it has solid or cavity walls and the way inwhich the floor has been constructed. Whether or not thefloor slab has been carried off the inner leaf, for example,or is floating (see Fig 7) can affect significantly theresponse of the structure to foundation movement, the wayin which damage may occur and the form of remedialworks.

Where the cause of damage is believed to be foundationmovement, additional factors may need to be considereddepending on the scale of the damage sustainedIt may be necessary to carry out a thorough examination ofthe foundations in the area of most movement and also todetermine the nature of the underlying ground (seeAssessing the possibility of severe damage due toprogressive ground movement). In both these cases it maybe necessary to call for the services of a suitably qualifiedcivil or structural engineer.

Fig 7 Alternative forms of floor construction

CLASSIFICATION OF DAMAGEThree broad categories of damage should initially beconsidered: ‘aesthetic’, ‘serviceability’ and ‘stability’.The first comprises damage which affects only theappearance of the property. The second includescracking and distortion which impair theweathertightness or other function of the wall (eg soundinsulation of a party wall may be degraded), fracturingof service pipes and jamming of doors and windows. Inthe third category are cases where there is anunacceptable risk that some part of the structure willcollapse unless preventative action is taken. Very oftenwhen damage is described, no distinction is madebetween these three categories, making it impossible togauge the severity of the problem.

It is only a short step from the three, generaldescriptions of damage to the more detailedclassification shown in Table 1; this defines sixcategories of damage, numbered 0 to 5 in increasingseverity.

The classification is based on the ease of repair ofvisible damage to the building fabric and structure andhas been derived from a number of previousstudies(1–5). In order to classify visible damage it is,therefore, necessary when carrying out the survey toassess what type of work would be required to repairthe damage both externally and internally. Thefollowing points should be noted:

● The classification applies only to brick or blockwork andis not intended to apply to reinforced concrete elements.

● The classification relates only to visible damage at agiven time and not its cause or possible progressionwhich should be considered separately.

● Great care must be taken to ensure that the classificationof damage is not based solely on crack width since thisfactor alone can produce a misleading concept of the truescale of the damage. It is the ease of repair of the damagewhich is the key factor in determining the overallcategory of damage for the whole building.

● It must be emphasised that Table 1 relates to visibledamage and more stringent criteria may be necessarywhere damage may lead to corrosion, penetration orleakage of harmful liquids and gases or structural failure.

For most cases, Categories 0, 1 and 2 can be taken torepresent ‘aesthetic’ damage, Categories 3 and 4‘serviceability’ damage and Category 5 ‘stability’ damage.However, these relationships will not always exist sincelocalised effects, such as the instability of an arch over adoorway, may influence the categorisation. Judgement isalways required in ascribing an appropriate category to agiven situation.

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CAUSES OF DAMAGESo far, only the reporting of damage and its classificationhave been discussed. However, it is the cause of thedamage and whether it will be progressive that is of greatconcern. There are many causes of damage brought aboutby differential movements. Broadly, they can be dividedinto those associated with the structure itself, and thoseassociated with the ground beneath the structure.

Causes associated with the structure include suchitems as:● material shrinkage and creep;● corrosion or decay;● differential thermal movements in dissimilar

materials;● poor detail design or workmanship.

It is rare for damage due to such causes to exceed or todeteriorate beyond Category 2 in Table 1, except perhapsvery locally in a building.

Causes associated with the ground include:● ground subsidence and heave due to volume changes

in clay soils;● settlement and heave of floor slabs on unsuitable or

poorly-compacted in-fill beneath the slab;● instability of sloping ground;● movement due to consolidation of poor ground or

made-ground;● mining subsidence;● movement caused by nearby excavations;● chemical attack on foundation concrete or erosion of

fine soil particles due to the passage of water, forexample from a leaking pipe.

Also included is differential settlement induced byunequal foundation pressures arising from such factors asextensions added to existing buildings or concentrationsof load, for example under chimneys. Damage from thesecauses can fall within any of the categories described inTable 1.

Thus Category 2 damage can result from a variety ofcauses which are frequently very difficult to identify.Indeed, at this level damage may result from acombination of the above factors. If damage exceedsCategory 2 it is often much easier to identify the cause(which is frequently associated with ground movement)and hence prescribe a suitable remedy.

Category 2 defines the stage above which repair workrequires the services of a builder. For domesticdwellings, which constitute the majority of cases,damage at or below Category 2 does not normally justifyremedial work other than the restoration of theappearance of the building. For the cause of damage atthis level to be accurately identified it may be necessaryto conduct detailed examinations of the structure, itsmaterials, the foundations and the local clear groundconditions. Consequently, unless there are clearindications that damage is progressing to a higher level itmay be expensive and inappropriate to carry outextensive work for what amounts to aesthetic damage.

Cases of progressive damage are rare and since they aremainly associated with ground movement a briefdiscussion of the factors involved follows. It should,however, be emphasised that it is a highly specialisedtopic usually requiring the advice of an expert structuralor civil engineer. The aim of this discussion is to giveadvice on when to call in such an expert.

Table 1 Classification of visible damage to walls withparticular reference to ease of repair ofplaster and brickwork or masonryCrack width is one factor in assessing category of damageand should not be used on its own as a direct measure of it.

Category Description of typical damageof damage Ease of repair in italic type

0 Hairline cracks of less than about 0.1 mm which areclassed as negligible. No action required.

1 Fine cracks which can be treated easily using normaldecoration. Damage generally restricted to internal wallfinishes; cracks rarely visible in external brickwork.Typical crack widths up to 1 mm.

2 Cracks easily filled. Recurrent cracks can be masked bysuitable linings. Cracks not necessarily visible externally;some external repointing may be required to ensureweather-tightness. Doors and windows may stick slightlyand require easing and adjusting. Typical crack widthsup to 5 mm.

3 Cracks which require some opening up and can bepatched by a mason. Repointing of external brickworkand possibly a small amount of brickwork to be replaced.Doors and windows sticking. Service pipes may fracture.Weather-tightness often impaired. Typical crack widthsare 5 to 15 mm, or several of, say, 3 mm.

4 Extensive damage which requires breaking-out andreplacing sections of walls, especially over doors andwindows. Windows and door frames distorted, floorsloping noticeably*. Walls leaning or bulgingnoticeably*, some loss of bearing in beams. Service pipesdisrupted. Typical crack widths are 15 to 25 mm, but alsodepends on number of cracks.

5 Structural damage which requires a major repair job,involving partial or complete rebuilding. Beams losebearing, walls lean badly and require shoring. Windowsbroken with distortion. Danger of instability. Typical crackwidths are greater than 25 mm, but depends on numberof cracks.

* Local deviation of slope, from the horizontal or vertical, of more than1/100 will normally be clearly visible. Overall deviations in excess of1/150 are undesirable.

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ASSESSING THE POSSIBILITY OF SEVEREDAMAGE DUE TO PROGRESSIVE GROUNDMOVEMENTTwo questions frequently confront those examining adamaged property: is the damage due to foundationmovement, and will it get progressively worse?

Damage due to foundation movementIt is essential to carry out a thorough survey, described inReporting damage, from which records of damage anddistortion should be seen to be reasonably consistent withfoundation movement. The following observations wouldindicate such movement:

● Cracks usually show externally and internally and mayextend through the dpc and down into the foundation,though this is unlikely for Category 2 damage or less.

● Crack tapers should be consistent with differentialfoundation movement; see, for example, Fig 8.

● Crack patterns should be reasonably consistent withobserved or measured distortions (see Reporting damageand Fig 8).

● Floors slope, walls tilt, window and door openingsdistort producing uneven clearance and jamming.

The survey summarised in Figs 2, 5 and 6 points tofoundation settlement at the near corner of the building.

Progressive movementIf the damage is reasonably consistent with foundationmovement it is then necessary to assess the likelihood of itbecoming progressively worse. The main causes offoundation movement are listed in Causes of damage; somecomments on the more common ones are:

Clay soilsA major factor discussed in detail in Shrinkage and heavedue to clay soils.

Floor slab movement (See Table 2) This is a common cause of damage and results frominappropriate or poorly compacted under-floor fill. Damageis generally confined to the slab and the junctions betweenit and external walls and also to internal partitions carriedon the slab. The problem usually manifests itself early inthe life of the building although it may not be discovered atthe time it occurs; if not serious at this stage it is unlikely togive rise to long-term progressive deterioration.

Chemical attack Attack by sulphates (Digest 363) or acid substances in thenatural ground is rarely, if ever, sufficiently destructive tocause significant damage to the shallow foundations of low-rise buildings since these are usually sited abovegroundwater level. However, attack by aggressive chemicalcompounds in fill material (Digest 276) has resulted innumerous cases of disintegration and expansion of groundbearing slabs and foundation brickwork, particularly whereburnt colliery shale has been used as under-floor filling.How far any damage will progress depends on the depthand nature of the fill material.

Instability of sloping ground Ground movement due to slope instability usually results incracking of roads, garden walls, services etc as well as thebuilding itself. If progressive movement of sloping groundis thought to be the cause of damage the advice of asuitably qualified civil or structural engineer should beobtained.

Consolidation of poor ground or made ground This problem usually manifests itself within the first tenyears of the life of the building, in the form of progressivedamage. If it is suspected as the cause of the damage theadvice of an expert civil or structural engineer should beobtained. It is helpful to assemble all readily obtainableinformation about the history of the site such as thegeological map and current and old Ordnance Survey maps.Aerial photographs are particularly valuable in revealingprevious uses of the site. Local residents can often provideuseful historical information about past activities on or nearthe site. These sources of information are easily obtainableand can be extremely valuable in assessing whether the sitehas been in-filled or whether soft marshy conditionsprevailed earlier (Digests 318 and 348). If the availableevidence points to a risk of progressive settlements, it willusually be necessary to carry out a ground exploration withtrial pits and/or borings located in the areas of greatestmovement.

Fig 8 Crack patterns associated with different modes of distortion

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Category of

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Description of typical damage

Hairline cracks between floor and skirtings

Settlement of the floor slab, either at a corner oralong a short wall, or possibly uniformly, suchthat a gap opens up below skirting boards whichcan be masked by resetting skirting boards. Nocracks in walls. No cracks in floor slab,although there may be negligible cracks in floorscreed and finish. Slab reasonably level.

Larger gaps below skirting boards, someobvious but limited local settlement leading to aslight slope of floor slab; gaps can be masked byresetting skirting boards and some localrescreeding may be necessary. Fine cracksappear in internal partition walls which needsome re-decoration; slight distortion in doorframes so some ‘jamming’ may occur,necessitating adjustment of doors. No cracks infloor slab although there may be very slightcracks in floor screed and finish. Slabreasonably level.

Significant gaps below skirting boards withareas of floor, especially at corners or ends,where local settlements may have caused slightcracking of floor slab. Sloping of floor in theseareas is clearly visible; (slope approximately1 in 150). Some disruption to drain, plumbing orheating pipes may occur. Damage to internalwalls is more widespread with some crackfilling or replastering of partitions necessary.Doors may have to be refitted. Inspectionreveals some voids below slab with poor orloosely compacted fill.

Large, localised gaps below skirting boards;possibly some cracks in floor slab with sharpfall to edge of slab; (slope approximately1 in 500 or more). Inspection reveals voidsexceeding 50 mm below slab and/or poor orloose fill likely to settle further. Local breaking-out, part refilling and relaying of floor slab orgrouting of fill may be necessary; damage tointernal partitions may require replacement ofsome bricks or blocks or relining of studpartitions.

Either very large, overall floor settlement withlarge movement of walls and damage atjunctions extending up into 1st floor area, withpossible damage to exterior walls, or largedifferential settlements across floor slab. Voidsexceeding 75mm below slab and/or very poor orvery loose fill likely to settle further. Risk ofinstability. Most or all of floor slab requiresbreaking out and relaying or grouting of fill;internal partitions need replacement.

Approximate(a) crack width(b) gap *

(a) NA(b) up to 1 mm

(a) NA(b) up to 6 mm

(a) up to 1 mm(b) up to 13 mm

(a) up to 5 mm(b) up to 19 mm

(a) 5 to 15 mmbut may alsodepend onnumber ofcracks

(b) up to 25 mm

(a) Usuallygreater than15 mmbut dependson number ofcracks

(b) greater than25 mm

Mining subsidence In districts where mining subsidence is anticipated, itis advisable to seek the help of experts in miningsubsidence.

Lowering the water table by the action of brinepumping can also cause ground subsidence.

Soil erosionIn silty, sandy soils a leaking water or drain pipe can,over many years, bring out localised subsidencedamage by slowly washing fine particles from the soil.Pressure and acoustic testing can readily identify thesource of the problem and repairs of the broken pipewill stop the subsidence.

Shrinkage and heave due to clay soils(Digests 240, 241 and 242)This is the major cause of damage due to foundationmovement, but in the majority of cases the damage isonly of Category 2 magnitude or less. (Of the 140cases described earlier, about 70% fell withinCategory 2 or less). If clay subsoil is suspected ofbeing the cause of damage the following three distinctsituations must be recognised:

1 open ground away from major vegetation;

2 buildings near existing trees;

3 buildings on sites newly cleared of trees.

In all these situations the following investigations arerecommended:

● Record the damage;

● Establish that it is consistent with foundationmovements;

● Investigate the subsoil by means of trial pitswhich must reveal the depth of foundations, thepresence of clay and, in the case of (2) and (3)above, the presence of fine roots below thefoundations;

● Ascertain the history of the site (includingprevious vegetation), building and dates ofobserved movement and damage;

● Record the position, species and approximate ageof all nearby trees and shrubs;

● Record the position and condition of nearbydrains;

● Monitor.

Open ground For foundations less than about 1 m deep in clay soils,seasonal foundation movements take place which maygive rise to slight cracks which open and closeseasonally. Though sometimes unsightly, these cracksare easily masked.

* ‘Gap’ refers to space — usually between the skirting andfinished floor — caused by settlement after makingappropriate allowance for discrepancy in building,shrinkage, normal bedding and the like.

Table 2 Classification of visible damage caused by groundfloor slab settlement

The classification below attempts to quantify the assessment of floor slabsettlement damage in a way similar to that for superstructure damage, givenin Table 1. It has not yet been used extensively to determine its applicability. Itshould be noted that the categorisation may be qualified by the possibility ofprogression to a higher category; this should arise only when examination hasrevealed the presence of voids or areas of loosely compacted fill (ordegradable material) beneath the floor slab such that more settlement can beexpected.

251Buildings near existing trees (Digest 298) Where the action of existing trees has been identified as theprincipal cause of foundation subsidence, care andexperience are required in assessing the likelihood ofprogressive movement and hence in planning remedialaction.

The following factors should be noted:

● Where the trees have reached or are close to maturity,although seasonal shrinkage and swelling movementscan be anticipated, larger movements are likely to occuronly in exceptional spells of dry weather. Felling of suchtrees can lead to worse damage due to swelling of theclay. Tree pruning may offer an acceptable way ofreducing the influence of the tree

● If the trees are still far from mature there is a possibilitythat progressive foundation movement will take placegiving rise to increasingly severe damage. Specialisedadvice on remedial action should be obtained and it maybe necessary to consider pruning or felling the trees thatare positively identified as the cause of the movement.Small alterations in the environmental conditions such asthe repair of a leaking drain can lead to further rootgrowth and the possible effects should be considered.

For buildings damaged by shrinkage of clay the likelihoodof recurrence of damage (not necessarily progressive) invery dry weather may depend on whether or not structuraldamage extends down into the foundations. If a foundationis completely fractured, a hinge can develop at the point offracture which may produce recurrence of structuraldamage in the future. However, such a foundation fractureis unlikely to occur for damage of Category 2 or less.

Building on sites recently cleared of trees (Digest 298)When trees and shrubs are removed from a shrinkable claysite, any deep zone of desiccated clay induced by the rootswill tend to absorb moisture and the ground will swell.Sometimes the swelling may continue for many years. Ifdamage is consistent with foundation movement, there is arisk of progressive movement if there is evidence ofprevious large vegetation on the site and trial pits revealdesiccated clay with fine roots beneath the foundation;specialist advice from a structural or civil engineer shouldbe obtained.

Despite the qualifications mentioned above, in the light oflong experience at BRE over several droughts and theexamination of the 140 cases arising from the 1975/76drought, it can be said that progressive subsidence damageis most uncommon. Only seven of the cases were regardedas involving progression extending beyond the duration ofthe drought; of these, three or four involved tree removalwhich has been clearly identified as a situation whereprogression can occur. Consequently, any precipitate actionby interested parties on a case of subsidence damage due toclay shrinkage, whether this action be in the form ofremoval of suspect trees or underpinning of the affectedarea of the structure, would be unwise for levels of damageof no more than Category 2. The only certain way ofconfirming the progression of damage is to take a series ofmeasurements with time.

AcknowledgementThe Building Research Establishment is grateful toProfessor J B Burland, Imperial College of Science andTechnology, for his valuable contribution to this Digest.

References1 Tomlinson, M J; Driscoll, R M C; Burland, J B.

Foundations for low-rise dwellings. The Structural Engineer,1978, Part A, 56 A (6), 161 - 173.

2 Burland, J B; Broms, B; De Mello, V B F. Behaviour offoundations and structures: State of the Art Report. Proc 9thInt Conf on Soil Mech and Found Eng. Session II. Tokyo.1977.

3 Jennings, J E; Kerrich, J E. The heaving of buildings andthe associated economic consequences, with particularreference to the Orange Free State Goldfields. The CivilEngineer in South Africa, 1962, Vol 5, No 5, 122.

4 National Coal Board. Subsidence Engineers’ Handbook,1975.

5 Institution of Structural Engineers. Structure – soilinteraction: A State of the Art Report, 1977.

Other 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 3276 Hardcore298 The influence of trees on house foundations in clay soils313 Mini-piling for low-rise buildings315 Choosing piles for new construction318 Site investigation for low-rise building: desk studies343 Simple measuring and monitoring of movement in low-rise

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

buildings. Part 2: settlement, heave and out-of-plumb348 Site investigation for low-rise building: the walk-over survey352 Underpinning359 Repairing brick and block masonry361 Why do buildings crack?363 Sulphate and acid resistance of concrete in the ground386 Monitoring building and ground movement by precise

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ISBN 1 86081 045 4© Copyright BRE 1995Republished on CD-ROM 1997, with permission of BuildingResearch Establishment Ltd,

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