Underpinning is required when the depth of an excavation will be deeper than the adjacent footings of an existing building and there is not sufficient clearance to install a caisson wall.
Slots or panels 4’ (1200mm) wide by 4’ (1200 deep) are excavated below the existing footings. Forms are installed and braced at each slot and concrete is poured below the footing. The following day dry pack is installed between the top of concrete and underside of footing to ensure that shrinkage of concrete has not resulted in any voids. The slots are excavated in a specific sequence to ensure that only every third or fourth panel is open.
The soil must have sufficient cohesion and stability to retain a vertical face until the forms can be placed and concrete poured. Underpinning can be braced by tiebacks or rakers if required to resist lateral loads.