Underpinning is a method of increasing the depth of an existing foundation by constructing new footings beneath the existing footings. One may also underpin to reinforce an existing foundation or to add a below grade or partly below grade entrance to an existing house. Simply speaking, underpinning is a way for homeowners to lower their basement floor and thus, increase ceiling height. The most common method is described below.
Mass Concrete Underpinning Method (Pit Method)
Mass concrete underpinning method is the traditional method of underpinning, as it has been followed by centuries. The method involves extending the old foundation till it reaches a stable stratum.
The soil below the existing foundation is excavated in a controlled manner through stages or pins. When strata suitable is reached, the excavation is filled with concrete and kept for curing, before next excavation starts. In order to transfer the load from old foundation to new one, a new pin is provided by means of placing dry sand-cement pack. This is a low-cost method suitable for the shallow foundation.
The process of underpinning is not for the inexperienced. There have been a number of times that underpinning jobs have gone awry, resulting in the loss of the home and damage to surrounding homes. Companies with experience and a history of successful underpinning jobs is essential.