Different underpinning methods are mentioned briefly in the following sections. The choice of method depends on the ground conditions and the required foundation depth. 1. Mass Concrete Underpinning Method (Pit Method) Mass concrete underpinning method is the traditional method of underpinning, as it has been followed by centuries.
Underpinning By Cantilever Needle Beam Method Figure-2 represents the arrangement of cantilever pit method of underpinning, which is an extension of pit method. If the foundation has to be extended only to one side and the plan possess a stronger interior column, this method can be used for underpinning.
The piles are connected by concrete or steel needles, which penetrate through the walls. These beams also act as pile caps. This method is effective in clayey soil and in waterlogged areas. The existing foundation is very much relieved of the load. Fig 3 illustrates the pile method of underpinning.
Pre-test method of underpinning Whatever be the types of underpinning method selected for strengthening the foundation, all of them follow a similar idea of extending the existing foundation either lengthwise or breadthwise and to be laid over a stronger soil stratum.
Lagging consisting of wood, steel or precast concrete panels is inserted behind the front pile flanges as the excavation continues. Additionally, contact lagging or shotcrete may be applied. The lagging efficiently resists the load of the retained soil and transfers it to the piles.
Pit Method – In this method the length of the foundation to be underpinned is divided into sections of 1.2 to 1.5 m lengths as shown in Fig.1 below. For each section a hole is made on the wall above the plinth level and a needle is inserted in the hole.