BRE Digest 352 Underpinning

BRE Digest

Concise reviews of building technology

Digest 352 Revised 1993

CI/SfB (B2d)

Underpinning More than £1000 million worth of subsidence claims, involving cracking and distortion in homes, were submitted to UK insurance firms during the period 1989 –1991. Much of the damage arose from foundation movement caused by problems such as subsidence and heave of the ground, and landslip. Much of the money was spent on underpinning the foundations of houses and blocks of flats. Underpinning usually involves deepening foundations; it may sometimes involve providing a new, different form of foundation.

cracked walls, professional fees, redecoration and temporary accommodation while repair work is carried out. Why has this growth in underpinning occurred? What indeed is underpinning and why is it carried out at such expense? To answer these and other questions, the Building Research Establishment commissioned a nation-wide survey of underpinning. The findings of this survey and technical issues concerning underpinning are presented in a BRE Report, Foundation movement and remedial underpinning in low-rise buildings. This Digest presents a more general discussion and is intended to give background information on the causes of foundation movement, and when and why underpinning may be necessary.

Most insurance companies include cover as part of their general building insurance. This was first introduced in 1971, since when there has been a substantial growth in claims. Each year many families will experience the anxiety and disruption of the underpinning process. Unless their home is new Figure 1 Pier-and-beam underpinning and covered by a warranty scheme, they also have to meet the excess on their insurance policy, usually between £500 and £1000, and may incur additional expenses arising from the employment of professional advisers. If they are not insured, they will themselves have to meet the full costs of remedial measures. This could be £20,000 or substantially more in severe cases. Underpinning accounts for about half of this, the rest being the cost of repairs to

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Technical enquiries to: BRE Advisory Service Garston, Watford, WD2 7JR Tel: 01923 664664 Fax: 01923 664098

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HOW TO RESPOND WHEN DAMAGE IS DISCOVERED Many owners will be unaware of damage to their homes until they try to sell. Then the surveyor for the purchaser, or more often his mortgage company, reports a problem with cracks or floor slab subsidence and the purchaser is deterred from buying unless a favourable engineer’s report can be obtained. The vendor will, therefore, seek advice, usually from his insurer and will, in most instances, lodge a claim. The majority of subsidence insurance policies are very similar, particularly those of the large insurers, but there can be differences between policies. For example, some exclude damage to patios, drives and floor slabs when external walls are not damaged by the same cause and at the same time; others exclude damage caused by structural alterations or faulty workmanship, or the use of defective building materials. Most mortgage companies require to be informed when damage has occurred. Then they pass the case to their nominated insurance company who will normally appoint a loss adjuster. Some loss adjusters have sufficient expertise to investigate and diagnose the cause of the damage; more commonly, the home owner making the claim will be required to demonstrate that the claim is legitimate by obtaining professional, technical advice on what has happened, to decide if the problem is serious and to recommend appropriate remedial measures. The required expertise is more likely to be available from chartered civil and structural engineers, or chartered building surveyors. Some of these professionals will be specialist foundation contractors. If a property is covered by a guarantee such as the 10year National House Building Council scheme, the owner may approach the registered builder if the damage is discovered during the first two years, or the NHBC directly if discovery is in years 2 to 10. The ‘Foundation 15’ guarantee scheme, operated by Municipal Mutual, covers a period of 15 years after construction.

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Other sources of advice include the Building Control Officer at the local authority, a solicitor, the Citizens’ Advice Bureau or similar organisations; the house owner may directly approach an underpinning contractor. It is important to appreciate that, with many insurers, the burden of proving a valid subsidence claim rests with the home owners; ultimately, therefore, they must appoint their own adviser to investigate and determine the cause of damage to the home. A lengthy process follows: lodging a claim, dealing with a loss adjuster, appointing a professional to investigate, having the claim accepted, appointing a contractor to carry out remedial work and making good the damage. The time interval between the first discovery of damage and the completion of the repair to the building is unlikely to be less than a year and, in some cases, may be considerably longer. This long period can be especially disconcerting when the sale of a property is urgently required. Before embarking on a long process of putting right the problems and restoring the building to its condition before the damage occurred, it is useful to have an understanding of why damage occurs and to consider carefully how much repair work to carry out.

SEVERITY OF MOVEMENT AND CRACKING Foundation movement can result from many causes, the most common being shrinking and swelling of clay soils. From examination of underpinning practice reported in the BRE survey and in earlier studies by BRE of damage arising from clay soil shrinkage and swelling during and after severe drought conditions, the conclusion emerges that the degree of movement and consequent cracking that cause concern to home owners is rarely of structural significance. However, the potential for movement to continue and for damage to worsen substantially is not always fully considered.

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WHAT CAUSES CRACKING AND DISTORTION IN WALLS? Cracking and distortion can occur for a wide variety of reasons; only a few of these will require significant work to remove the problem, or modification of the foundations by underpinning to stop foundation movement. The more common causes of cracking are given here. This list is not exhaustive. Other documents recommended for further reading also discuss causes of cracking; they are listed at the end of this Digest. Except for foundation movement, the causes of cracking all stem from problems in the structure of the building and only extremely rarely do these problems require major repairs.

FOUNDATION MOVEMENT ● Settlement: downward movement brought about by the pressure from a foundation causing compression of the ground. It is the differential settlement of one part of a building relative to another that produces distortion and cracking; this may result from several factors, such as variations in the shape and size of foundations (eg partial basements) or, more usually, variations in soil strength or compressibility beneath the building. Adding an extension can sometimes result in cracks occurring where it joins the original structure. This cracking is caused by the initial settlement of the extension and is not normally a continuing problem. ●

THERMAL EFFECTS Differing amounts of expansion and contraction in different building materials may cause small cracks. CREEP Many building materials deform slowly under load over many years. This very slow movement may eventually cause small cracks. MOISTURE MOVEMENTS Quite noticeable cracks will sometimes appear in new houses as walls dry out. The introduction of central heating in old properties may cause drying shrinkage cracks. In either case, the cracks are not significant and may be readily repaired. MOVEMENT DUE TO LOADING Minor cracking can result from increases in load, for example when heavy items are stored in lofts causing ceilings to sag, or when structural alterations are made to a house.

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Mining.

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Compression of loose, man-made fill owing to its self-weight, or to the ingress of water.

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Removal of water from the ground, for example by pumping or, more commonly, the drying effects of trees on clay soils, especially in dry, summer weather.

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

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Soil erosion owing to natural underground water flow or leaking drains.

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Collapse of the ground surface above natural cavities formed below, such as solution cavities in chalk (‘swallow holes’).

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Heave: this is upward movement of the ground. It is caused either by the removal of load (for example in a deep excavation) or by an increase in the water content of the ground that can occur when trees, that have dried the soil, are removed; the soil swells as it becomes wetter. Very occasionally, in particular circumstances and in extremely cold weather, freezing of the ground can cause heave. Very rarely, floor slabs have been caused to heave by the swelling of the under-floor filling material, such as certain burnt colliery shales that are prone to chemical expansion in the presence of moisture. Insurance policies usually cover subsidence or heave only of the site and so would not apply to this problem.

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Landslip: although major, severe landslipping is a very rare event in the UK, long-term downhill movements on slopes sometimes cause damage in certain areas.

PHYSICAL AND CHEMICAL CHANGES An example is severe frost, which can occasionally exacerbate small, moisture-filled external cracks. Occasionally, the formation of sulphates in mortar can crack walls. VIBRATION Very occasionally, severe vibration from heavy traffic or construction plant may induce small increases in existing cracks, or start small cracks where high stresses are concentrated. POOR DESIGN DETAILING AND WORKMANSHIP This can cause cracking in newly-completed buildings; inferior quality materials may eventually bring about damage.

Subsidence: downward movement of the ground brought about by activity in the ground, including:

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WHAT IS UNDERPINNING? Underpinning usually involves the extension of the existing foundations downwards to reach stiffer or more stable ground. There are several methods: Mass concrete Pier-and-beam Pile-and-beam Pile

Figure 2 Figure 3 Figure 4 Figure 5

The mass concrete method accounts for over 40% of all underpinning work, followed at 30% by pier-and-beam. The principal reasons for choosing a particular method of underpinning concern the extent of the underpinning required, the nature of the ground and the cost, including the degree of disturbance to the building occupants.

Whatever method is used, underpinning is applied either to part or to the whole of the foundation. Which is appropriate will depend on the scale of the problem and the ground conditions. Partial underpinning is employed when only localised foundation movements have occurred and where the stabilising of only part of the foundation will not have a detrimental influence on the remainder. An example is where a house has been built partly over the site of an old, infilled pond. Most of the foundation is perfectly stable so that underpinning only the part over the old pond will be satisfactory. The vast bulk of underpinning projects involve partial work only. Total underpinning may be performed when an entire building is settling, perhaps on soft, peaty ground, or when an additional storey is to be added to that building, requiring all foundations to be extended down to stiffer ground.

Figure 2

Mass concrete underpinning This involves excavating systematically in adjoining bays beneath the existing foundation, filling the excavations with concrete and then pinning-up the small gap between the old and new foundation. The sequence is indicated by the numbers on the drawing above.

Figure 3

Pier-and-beam underpinning A reinforced-concrete beam is installed in the wall, either just above the existing footing or replacing it, and discrete pits are excavated at intervals beneath the modified wall. After filling the pits with concrete to form piers, pinning-up is carried out between the concrete and the base of the beam or existing foundation.

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Figure 4

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

Figure 5

VIEW ‘A’ THROUGH THE WALL

VIEW ‘B’ CANTILEVERED

VIEW ‘C’ INCLINED MINI-PILES

Pile underpinning Vertical piles installed either side of the wall are connected at their heads by a needle that passes transversely through the wall (View A). A variant of this type, in which a cantilever is used instead of the needle, removes the necessity for piling inside the building (View B). The use of inclined mini-piles, installed through holes drilled in existing foundations, is becoming more widelyused (View C). Small, vertical mini-piles can be installed through pre-drilled holes to underpin a subsiding floor slab (View D). See Digest 313 for more details on mini-piling.

VIEW ‘D’ FLOOR SLAB MINI-PILES

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WHEN IS UNDERPINNING NEEDED? Underpinning is usually necessary only if further damaging foundation movements need to be stopped. It is therefore important to find out if movement is continuing. However, the survey revealed that a substantial number of underpinning operations were carried out principally to ensure that a house sale could proceed, rather than to eliminate continuing movement. It was apparent that many owners expect their house to be underpinned if cracking has occurred, regardless of its severity, cause and whether or not it is worsening. Deciding if damage is likely to get worse may be difficult. Establishing the exact cause of movement may require expensive exploration and soil testing. Demonstrating that movement is continuing will involve time-consuming monitoring. The attitudes of insurers to monitoring varies: some will not pay for it unless a valid claim for subsidence is subsequently proved, others offer to pay even if no valid claim results, arguing that by so doing they reduce the chances of paying for unnecessary underpinning. In these circumstances, it is not surprising that underpinning is often chosen as the easiest, quickest and safest option when doubt exits. However, a decision to underpin can bring about its own problems; this is explained in Difficulties encountered. Guidance on the likelihood of progressive foundation movements is given in Digest 251. This also contains advice on determining the severity of a case of damage; it suggests that underpinning is necessary only beyond a certain level of damage (Category 2) and where there is evidence of continuing movement.

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However, in certain, usually obvious circumstances, the anticipated worsening of damage to Category 2 and beyond may warrant underpinning to arrest movement. For higher levels of damage, underpinning may be required to strengthen a fractured foundation, even if the primary cause of damage has ceased or been removed. In the majority of cases, a professional adviser will recommend 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 that underpinning need not normally be the consequence of the discovery of cracks. In certain, rare circumstances underpinning may be inappropriate despite continuing subsidence; an example is where deep-seated movements caused by mining activity are taking place. A high incidence of cases of floor slab subsidence has been reported in certain areas of the UK. In part, this is caused by particular ground conditions, like the presence of a layer of slightly organic soil just below the topsoil layer. However, in some localities it has become ‘common knowledge’ that floors may have moved, so that some surveyors have become particularly diligent in looking for gaps below skirting boards or for evidence of hollowness beneath the slab. In some cases, floors have been completely replaced in order to bring about a house sale, when all that has actually occurred is minor movement that has since stopped. If continuous floor movement is evident, underpinning with mini-piles may be appropriate.

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WHERE IS UNDERPINNING MOST CARRIED OUT? The nation-wide survey commissioned by BRE found that about 50% of all underpinning is carried out in London and the Home Counties. Figure 6 shows the distribution with the UK divided into 12 arbitrary areas. There are two main reasons for this distribution: ●

In the UK, the major cause of damaging foundation movement in houses and other low-rise buildings is found to be shrinking and swelling of clay soils, exacerbated by the presence of large trees. The clay soils that are most prone to volume changes are found in south-east England where there is also a high density of major tree-cover, including street trees. The south-east generally experiences less rainfall than other areas.

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People take a more serious view of cracking the more valuable is their home. There has been a discernible heightening in awareness of minor cracking in houses throughout the UK.

Figure 6

Distribution of underpinning projects in 12 areas throughout the UK

WHO DESIGNS AND CONSTRUCTS UNDERPINNING? Underpinning can be obtained in several ways: ●

From a chartered civil or structural engineer who will design a suitable scheme and supervise the contractor who performs the underpinning.

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Directly from a specialist underpinning contractor: there are many of these currently in operation and most offer a variety of methods to fit individual circumstances.

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From a normal building contractor; much partial, mass concrete underpinning can be carried out by builders, though it is advisable, firstly, to appoint an engineer or building surveyor to ascertain that the underpinning is strictly necessary and, secondly, to ensure it is sufficiently extensive and correctly applied.

There has recently been an increasing trend for insurance companies and their loss adjusters to employ consulting engineers, with expertise in foundation engineering, to investigate claims and, where appropriate, design and supervise underpinning work. This has led to better assessments of the need for underpinning and is a contributing factor in the fall in the total cost of underpinning throughout the UK (Figure 7). However, the very prolonged drought from 1989 to 1992 produced many cases of subsidence damage; this is reflected in the enormous increase in claims shown in the figure.

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

Figure 7 Annual value of insurance claims for subsidence and heave damage to dwellings. Based on payments made by the major insurance companies following the mild drought of 1984.

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352 THE ROLE OF BUILDING REGULATIONS IN UNDERPINNING Lord Denning defined underpinning as “an extension of a building in a vertical downward direction”. It, therefore, constitutes a ‘material alteration’ and is subject to control under the Building Regulations. Applications to underpin must be submitted for approval and inspection of the work. Some local authorities, particularly in the south of England, require more information on underpinning proposals than others. It is perceived that, in giving approval, some liability may attach to the approving authority. It has, therefore, to be satisfied the work will prevent any recurrence of damage that constitutes a threat to the health and safety of the building occupants and will not cause instability to any other building such as to threaten the health or safety of its occupants.

Underpinning has, occasionally, not prevented the recurrence of movements or the reappearance of small cracks. The reason for this ‘failure’ of the underpinning is usually poor design. For example, the underpinning has not been taken to a sufficient depth, construction practice has been poor, the nature of the movements has been wrongly diagnosed or an insufficient length of the building has been underpinned. REINSTATEMENT OF DAMAGE If underpinning or other remedial measures are necessary to arrest a case of foundation movement, making good cracks or repairing other damage should not be attempted until these measures are complete. Advice on repairing cracked brickwork is given under Further reading.

Remedial measures that do not constitute a ‘material alteration’ do not require Building Control approval. Such measures would include the repair of drains, bonding of cracked brickwork, forms of structural tying above foundation level and the removal of trees held to be responsible for subsidence damage. Complications may arise with tree removal if the tree is protected by a preservation order. Digest 298 deals with the influence of trees on foundations in clay soils. GUARANTEES OF UNDERPINNING WORKS Many specialist underpinning contractors offer guarantees lasting for 20 or 25 years. Mass concrete underpinning work carried out by local builders is probably not covered by any form of guarantee. Having the underpinning designed and supervised by a suitably qualified professional should provide assurance of satisfactory performance. DIFFICULTIES ENCOUNTERED A particularly difficult problem sometimes arises when substantial underpinning is deemed necessary to one of a pair of semi-detached houses or to one of a terrace of houses. Since the Building Regulations state that: The building shall be so constructed ... without causing ... movement of the ground, as will impair the stability of any part of another building, local authority building control staff have been known to reject applications because they perceive a significant risk that damage will occur in the adjoining property as a result of the underpinning. In these cases, the matter is usually resolved only by calling in geotechnical experts to assess the risks.

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

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by Construction Research Communications Ltd, 151 Rosebery Avenue London, EC1R 4QX

FURTHER READING Foundation movement and remedial underpinning in low-rise buildings. BRE Report, 1991. Geotechnical Directory of the United Kingdom. British Geotechnical Society; Institution of Civil Engineers; Westminster, London. BRE Digests 240 Low-rise buildings on shrinkable clay soils: Part 1 241 Low-rise buildings on shrinkable clay soils. Part 2 242 Low-rise buildings on shrinkable clay soils: Part 3 251 Assessment of damage in low-rise buildings 276 Hardcore 298 The influence of trees on house foundations in clay soils 313 Mini-piling for low-rise buildings 343 Simple measuring and monitoring of movement in low-rise buildings. Part 1: cracks 344 Simple measuring and monitoring of movements in low-rise buildings. Part 2: settlement, heave and out-of-plumb 359 Repairing brick and block masonry 361 Why do buildings crack?

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Anyone wishing to use the information given in this publication should satisfy themselves that it is not out of date, for example with reference to the Building Regulations

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