In what circumstances is underpinning necessary and how may a suitable method be selected?
Underpinning can be defined as the construction of a new foundation which enables a load, bearing on an existing footing, to be carried at a lower depth. It is important to appreciate that underpinning is a potentially dangerous operation. Apart from stopping the progressive movement of a foundation, underpinning may be needed for other reasons. Underpinning may be undertaken so that the load-bearing capacity of the foundations can be improved. Building work adjoining a foundation, such as the formation of a basement leading to loss of lateral support, may also result in the need to deepen foundations.
In 1991 a Building Research Establishment report estimated that, during a period of some 15 years, the hitherto small-scale market for remedial underpinning had developed into an £80m pa. Remedial underpinning in the UK is mainly carried out to domestic and other low-rise buildings. Each year up to 14,000 homes were thought to be disturbed by such work. The drought of 1975-76 led to extensive shrinkage of clay soils and much subsidence damage. BRE comments that this resulted not only in a considerable increase in subsidence insurance claims but also increased the sensitivity of surveyors acting for building societies and estate agents to cracks in houses.
While subject to debate, it is probable that some remedial underpinning now undertaken is not needed purely for engineering reasons. BRE suggests a number of reasons for this occurrence: pressure for a sale to proceed; insurers and loss adjusters seeking a solution with no further risk; consultants’ conservative attitudes, protecting professional indemnity insurance; a conservative and commercial policy by specialist contractors and a defensive position adopted by some local authorities in the implementation of building control.
Greater awareness of cracks in properties and tolerance of them has resulted in owners becoming anxious when damage is slight, even when it is not likely to affect structural stability. Many surveyors will have encountered this anxiety, to perhaps a more limited extent, in the past. Both owners and potential buyers of domestic property expect freedom from cracking. BRE feels that expectation of this is likely to continue to rise. The amount of the investment in the property, its age and locality affect the level of expectation.
Underpinning is generally required only if further foundation movements causing damage need to be prevented. Its necessity or not should be based upon the probability of damage progressing to an extent that affects the serviceability of the building. At this point cracking and distortion affect wall functions such as weather tightness, fracture of service pipes and jamming of doors and windows. There is not, as yet, an unacceptable danger of collapse of some part of the structure (see broad categories of damage in BRE Digest 251).
In most cases, owners, prospective purchasers and mortgage valuation surveyors are concerned about what may be called aesthetic damage. A house sale may be put at risk by cracks 3mm wide or perhaps less. In reality, completely uncracked buildings are rare, which helps to put this development into perspective.
Reasons for structural cracking are numerous, but it is essential that a correct interpretation of causes should be made by inspection and assessment. Clay shrinkage and heave have been found to be the commonest cause of conditions requiring remedial underpinning. A diagnosis is frequently based only on an inspection of cracking. If the damage is comparatively severe this may be acceptable. However, BRE feels that this is not advisable where damage is Category 2 or less (see BRE Digest 251 for categories).
Investigation
Insight into subsoil conditions is needed if the possibility of progressive foundation movement is to be considered. Apart from desk studies, experience and local knowledge, an actual ground investigation is required. Confirmation of the reason for damage would be provided by a trial pits and, possibly, boreholes.
While crack surveys and site investigations should indicate the causes of damage, they may not prove that progressive movement is taking place. Probably the most useful technique for this purpose is to monitor levels, for example along the line of the damp-proof course. Such monitoring requires a surveyor’s level and specially installed points. A level survey, along a damp-proof course, to estimate the amount of settlement or heave which has already occurred, is best achieved using a water level (see BRE Digest 344). Even when level monitoring is adopted the results may not be conclusive.
Specialist laboratory testing techniques are available for assessing the depth and degree of soil desiccation. These can be used for determining future heave. If the heave is likely to be slight, underpinning may not be required.
Underpinning is not the only method used for remedial purposes in cases of progressive foundation movement. For example, the cause may be removed. Though an apparently simple solution, this approach may not prove so in practice. The repair of a leaking drain, for example, may not prevent further movement because cavities could have formed in the subsoil which may affect the foundations for a number of years (see also “Tree roots and foundation damage”). Structural strengthening of the building is another alternative and may include resin bonding, brick stitching and corseting with a reinforced concrete beam at ground level. The subsoil may be strengthened by grouting, used as permanent underpinning or in conjunction with more normal methods.
Underpinning is the commonest way of halting progressive foundation movement. The general sequence of operations involves, prior to underpinning, the possible strengthening of the building by repairs to walling. Shoring may also be necessary to provide support. Removing loads from the foundation could possibly be in stages by “needling” passed through the wall to obtain temporary support. After the new foundations are formed and pinned up, any temporary support is removed.
Much of the underpinning carried out in this country is only partial in extent, although there is debate about its effectiveness. Where engineering practice is adequate, partial underpinning should be a success. Potential legal and insurance problems exist if semi-detached or terraced properties are underpinned, however.
Methods
Underpinning can be classified in two broad categories: “continuous”, which employs mass concrete; and “discontinuous”, in which either piers or piles are used.
The mass concrete form transfers the load from the existing foundation to its new bearing (Fig 1). In common with most forms of underpinning, success is especially linked to effective pinning up of the new and old work so that settlement is avoided. A gap of 75mm to 150mm is generally left between the new and old construction, which is filled later by ramming a mortar with low moisture content into the space.
The underpinning is carried out in narrow bays known as “legs” distributed equally along the length of the wall. The width of each leg can, with safety, generally be 1m to 1.5m. The number of legs which can be under construction simultaneously should generally be limited so that not more than 25% of the length of the structure to be underpinned is unsupported at one time. Careful underpinning may mean that disturbance to the building is minimised.
BRE estimates that mass concrete is used for more than 40% of the underpinning carried out. It is technically the easiest to construct and is well understood. The process is slow, however, and much hand excavation and concrete is required. High costs of excavation and the limited loadings generated by low-rise buildings result in mass concrete generally being used only up to depths of approximately 2m. In addition, it is appropriate only on clay where shrinkage or heave is fairly slight and an unaffected stratum exists within the 2m limit. Steps will be needed to isolate the underpinning from the effects of heave, such as a lining of thick polystyrene. A high water table could prevent the use of such underpinning, especially if the excavation is in poor ground conditions.
About 30% of underpinning employs a “pier and beam” technique: a reinforced concrete beam is cast in the wall just above or replacing the existing footing (Fig 2). The beam spans piers consisting of mass concrete poured into pits bearing at a suitable depth. Generally the beam is formed first. A series of “stools” are installed in holes cut along the proposed line of the beam. Stools made of steel and precast concrete are subsequently cast into the beam. If insufficient support for the stools can be obtained, reinforced concrete foundation pads bearing on the ground can be used.
After the stools are placed, the remaining brickwork or existing foundations are removed and the beam is formed. The top of the beam is pinned up to the brickwork above. The piers can then be excavated, concreted and pinned up.
Beam and underpinning is suitable for most ground conditions and is safer for the operatives than mass concrete methods. High labour costs, owing to hand excavation of piers, limit the depth to which it is generally used to under 4m. A high water table would lead to the consideration of an alternative method. It is particularly suitable for use in clay which is subject to shrinkage and heave. By using a polystyrene lining to the sides of the piers the building may be isolated from volume changes in the clay.
Other methods used for underpinning lowrise buildings were found by the BRE almost exclusively to involve piling. The systems may be a beam and pile method or simply entail piling. The beam and pile method is similar to beam and pier underpinning (Fig 3).
Beam and pile is applicable where hand excavation to form piers is no longer economical, or at shallower depths where adverse ground conditions exist and hand excavation is precluded. It is suitable for almost all circumstances, but is most economical at depths over 4m to 6m. Beams are constructed in a similar way to those for beam and pier underpinning, but project at corners, junctions and other appropriate points. The extended portions act as caps to the tops of the piles. Piles used to form intermediate supports act through a needlecapping beam where access is available to both sides of the wall. Alternatively, a cantilevered pile cap is used so that internal access is not needed. Piles are usually 150mm to 400mm in diameter for low-rise buildings and are generally augured or bored and cast in situ. Smaller diameter piles may be driven. The system is safe, provided that the beams rather than the piles are constructed first.
Underpinning with piles (Fig 4) does not involve extensive beams: instead, transverse needles or cantilevered pile caps are used to transmit the loads direct to the piles. Alternatively, the piles may be installed through the existing foundations. Spans between piles are small (usually 1m or less) in low-rise buildings and small diameter or mini-piles are appropriate. Typical pile diameters are 75mm to 150mm: they are formed by driven steel castings filled with cement grout or are augured and cast in situ.
Piles placed through existing foundations are formed at an angle and cast into the footing. The piles are placed alternately each side of the foundation at 1m centres, or less, dependent on the state of the footing and subsoil. Success depends on the capacity of the existing foundation to deal with the forces from the alternately sited piles.
The main application of these systems is generally where satisfactory ground exists at depths of less than 3m to 4m. Owing to the slenderness of the piles, they are not very suitable where clay shrinkage or heave exists or where the strength of the structure adjacent to the footing level is low. Labour content is small and piling is thus comparatively cheap.
Acknowledgement
The author is grateful for the help received from M S Crilly, BRE.
References
BRE Report (1991) Foundation movement and remedial underpinning in low-rise buildings Boden J B and Driscoll RMC “House foundations – a review of the effect of clay soil volume change on design and performance”, Municipal Engineer Vol August 4 1987 pp181213.
BRE Digests
251: Assessment of damage in low-rise buildings
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 movement in low-rise buildings. Part 2: settlement, heave and out-of-plumb.
352: Underpinning.
Note
A guide is to be published early in the new year by the Institution of Structural Engineers which will seek to encourage more objective assessments of the significance of subsidence damage.
