It has been accepted practice for many years to use small bored piles for the foundations of low-rise structures on clay soils that are likely to shrink or swell with changes in moisture content. Design methods and installation techniques have been developed to produce foundations that are economic and that generally give good service. These are described in Digests 241 and 242.

In the last decade, piles less than 300 mm dia (and frequently less than 150 mm dia) and known as 'mini-piles', 'micro-piles' or 'pin-piles', have been increasingly used for remedial and unusual operations. Many of these piles have been installed through the substructures of historic buildings found to be deteriorating or settling unduly. Remedial works of this type need extensive site investigation and careful design. The works are normally undertaken by specialist civil engineering contractors and are outside the scope of this digest. However, in many recent cases, mini-piles have been used to underpin walls and stabilise and repair cast-in-situ floor slabs of low-rise buildings. They have also been used in the construction of new walls and floors on weak soils and fills and shrinkable clays. These piles are being installed by small specialist contractors, some of whom may not be familiar with the principles of foundation behaviour or the particular problems of pile design and construction.

The digest has been prepared to give guidance to those concerned with the design, supervision and approval of remedial works and new foundations using mini-piles in low-rise buildings.

8 pages.