Discontinuous Underpinning.

Where the existing foundation has reasonable spanning capability it is sometimes possible to excavate and install piers in mass concrete or concrete and brick at a spacing  to suit the spanning capability of the original foundation.

The area of the base of this underpinning needs to be  capable of distributing the ground pressure from vertical and horizontal loading into the sub-strata without allowable limits being exceeded (see Fig. 15.13 for typical details).

Fig. 15.13 Typical discontinuous underpinning.

In other situations where good ground exists but the foundation is not capable of spanning, a pier and underpinned beam can be used, the beam being inserted in sections in a similar manner to that of the mass concrete underpinning.

This operation tends to be more tedious and more time  consuming, but where excavations are deep it can prove economic (see Fig. 15.14). As with continuous underpinning, the engineer must carefully consider the risk to site personnel before specifying these techniques.

Fig. 15.14 Typical pier and beam underpinning.

It is particularly useful for foundation jacking where subsidence or settlement requires re-levelling, the jacks being inserted between the soffit of the beam and the top of the piers. In some cases, particularly where the building to be underpinned forms part of the new construction, piles can be inserted on either side of the structure to support needle beams inserted through the existing structure to bear onto the piles. This is particularly useful where a basement extension is to be added to an existing building; the piles form the basement columns and the beams the framework for the ground floor structure. Typical pile beam under-pinning is shown in Fig. 15.15.

Fig. 15.15 Typical pile and beam underpinning.

Temporary lowering of the water-table by sump-pumping for underpinning operations requires careful consideration relative to the effect on new and existing foundations. As previously discussed, there is a danger that soils such as fine sands may suffer from loss of fines and may cause settlement of adjoining structures. There is also the possibility that in certain soils when the dewatering process stops, running sand or clay softening may occur. It is therefore important under these circumstances that the effects of  the temporary works and methods of construction are considered at design stage.

There are numerous ingenious piling systems available which minimize disruption of the existing structure, while maximizing economy and practicality of construction and a reputable specialist contractor should be approached at an early stage where appropriate.