Underpinning

Introduction 

Underpinning is the transference of the load of a building to a lower stratum of the ground. This is done for several reasons: 

• To permit the construction of a new basement within or adjacent to the existing foundations which are above the level of the new basement.
• To provide a new foundation at a lower level than the existing, where the existing has failed due to, for example, shrinking clay or subsidence.
• To increase the load bearing capacity of the existing foundation.
• In extreme cases, to allow the whole building to be mechanically moved by installing a ground beam.

There are a number of ways to carry out the underpinning operation; the choice depends upon the stability of the existing structure. Except in the most simple cases and where the structure is in good condition, the work will be carried out by a specialist contractor. This contractor will employ stabilizing techniques such as the drilling of ground anchors and/or the construction of ground beams.

For a straightforward operation on a stable structure the contractor will price the work described in the bills of quantities. The constants used will be based upon those for the ground work, in situ concrete and masonry sections. 

Shoring (C30) 

It may be necessary for the estimator to price an item in the bills of quantities for strutting to window and door openings as well as for temporary shoring to walls. The extent of this work will be described as an item and it will be necessary for the estimator to have this temporary structural work designed before pricing. 

Underpinning (D50) 

The excavation work in underpinning is in two parts: the preliminary trench from the existing surface level to the base of the foundation, and the underpinning pits below the existing foundation. The preliminary trench will be priced in an identical manner to that described for hand excavation in Chapter 5.

The underpinning pits are the excavations below the existing foundation level undertaken by hand on a section of foundation normally 1 m to 1.5 m long. The working on such small sections, in part below the existing foundation, results in an addition to the hand excavation constant of 100 per cent. The bringing of the soil to the surface in the number of lifts 

calculated by dividing the total depth to the base of the underpinning by 1.5 m, as in Chapter 5, is calculated at the given constants plus 50 per cent. 

Where underpinning is carried out by working from one side only, the earthwork support to the back face of the excavation under the existing foundation is left in. This section of earthwork support is commonly in precast concrete planks, each with a hole in the top to allow for grouting any void left behind the earthwork support.

For all remaining items in the underpinning section, i.e. groundwork, in situ concrete and masonry, the constants used are those of the appropriate chapter plus 50 per cent.