Underpinning Rationale

A major repair of a building's substructure is termed underpinning. It is a term to describe where the existing foundation of a building is both excavated and supported, so that it transfers the loads to a lower bearing stratum. A fundamental principle of this process is that support to the construction above is temporarily removed in short lengths and new support installed. To ensure stability of the structure support should not be removed from more than about one-fifth of the building or wall being underpinned at any one time (Smith, 1994).

There are five main reasons why underpinning may be required to an existing building:

(i) to arrest excessive subsidence or progressive settlement;

(ii) to increase a building's load-bearing capacity;

(iii) to provide new or deeper basement accommodation;

(iv) to replace deteriorated or potentially defective substructure work;

(v) to make buildings with shallow foundations more 'future-proof' against ground erosion and flooding.

Any of these reasons could require underpinning to a building undergoing adaptation. This is especially the case with older buildings of load-bearing masonry construction on soils with low or poor bearing capacity. Such buildings may also require underpinning as part of a conversion to another use, because of the poor condition of their foundations. For example, farm buildings were often built with shallow foundations (i.e. less than 600 mm deep). The local authority may require underpinning of the foundations as part of proposed the change of use to such properties.

Partial underpinning is likely to be required to a building being adapted that requires repairs to the substructure of some but not all of its main walls. In such a case there is an increased risk of differential settlement in adjoining structures, and renewed localized settlement owing to the extra load imposed on the soil.

In rarer cases the whole building may require underpinning. This is usually restricted to low-rise detached dwellings.

In any event, underpinning can add substantially to the adaptation costs. As a rule of thumb, depending on the type and depth of underpinning used, at current prices it can cost between £600 and £1000 per linear metre to underpin a low-rise property (Smith, 1994).

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