Continuous Underpinning.

In this part the authors have considered only underpinning of existing buildings adjacent to new developments
and not underpinning required due to structural settlement or subsidence, which is a separate subject beyond the scope of this book.

All foundation types may require underpinning when development takes place alongside or under an existing structure. The possible combinations of ground conditions, foundation details and levels is endless and complex. The basic methods and principles are quite simple. Where a new foundation or structure is to be constructed with  its foundation soffit below that of an adjoining foundation, underpinning is usually necessary. The exception to this is where the adjoining building is built upon a substantial ground strata such as hard rock.

The ‘traditional’ method of underpinning described below has been used extensively and effectively in the past and  is included here for completeness. However, the risks to site personnel involved in working beneath a temporarily supported section of superstructure of sometimes dubious integrity must be weighed against other underpinning options such as the use of mini-piles and needle beams  (see Fig. 15.15), which are now easily available and equally effective.

Fig. 15.15 Typical pile and beam underpinning.

Traditional underpinning is generally carried out in sequenced construction and in short lengths (commonly  1.0 to 1.2 m). The sequence is arranged to allow limited undermining of the structure at any one time. The limit of this undermining is dependent upon the structure’s capability of spanning over the undermined section and the  stability of the short section of unrestrained earth. In some cases beam underpinning may be provided to help the structure to span over greater distances. Typical underpinning is shown in Fig. 15.11.

plan
Fig. 15.11 Typical continuous underpinning.

The simplest and most common form of underpinning is  to remove a series of short lengths of sub-soils from below the adjoining building in a sequenced operation. As each section is excavated it is replaced immediately with mass concrete, which is allowed adequate time for curing prior  to the construction of the adjoining section. The top of the concrete is either cast with a pressure head so that it rises  to the underside of the foundation, or is cast low to allow wedging with dry pack or slate. Figure 15.12 gives a typical example of mass concrete underpinning.

Fig. 15.12 Construction methods for mass concrete underpinning.

In the authors’ opinion the preferred method of construction is to cast whenever possible with a pressure head. Concrete shrinks, and so theoretically this method encourages some slight settlement as the building above follows this shrinkage downwards. However, in the authors’ experience such settlement is usually negligible and is offset by the following advantages of the pressure head method:

(1) The underpinning is completed in one operation, rather than waiting up to seven days before dry packing. Also since concrete continues to shrink for weeks, even months, the logic of dry packing is inconsistent.
(2) The workmanship of the dry packing process is often of poor quality due to the difficulty of the technique. This requires increased supervision, and slows the whole sequence down even further.

It is rarely necessary to mechanically key mass concrete underpinning across the joints and the majority of mass concrete underpinning will perform successfully without  a key. The need for keying depends upon the requirement for vertical shear and/or tensile strength across the face, neither of which is usually necessary.