Underpinning is a technique used to lower an existing basement or to reinforce an existing foundation when adding an additional storey. Underpinning should only be done by a licensed contractor.
As shown below, underpinning is done in stages. The footing is divided into 3' sections and numbered 1,2,3,4....To avoid structural damage, sections marked 1 are excavated, poured and grouted first. When the grout has cured on sections 1, excavation for sections 2 can begin.