Underpinning refers to the process or art of repairing the existing foundation of a building or structure. Underpinning is commonly referred to as foundation repair and there are several types. In order to choose the proper underpinning method, examine and understand the architecture of the existing structure, the foundation supports and the exact problem with the foundation.


Underpinning, or repairing a foundation, is necessary in situations in which the existing foundation is unable to support the weight of the structure or is not stable enough. Also, when nearby construction and excavation has affected the soil support system of the existing foundation, underpinning is also necessary. Additional causes for underpinning include changes in the function of the structure and weight supported by the structure.


Several underpinning methods repair and provide support for an existing foundation. The types include various systems of piers and piles, such as bracket piles, helical piers, screw piles, hand-dug piers, jacked piles and mini-piles. The types of piers and piles differ according to the installation process, other materials used for underpinning and the layout of the piers or piles.


Before underpinning, foundation technicians must set up a temporary support system so they can work safely underneath the existing structure and foundation. They make temporary support with steel beams inserted at angles underneath the edges of the foundation. Once the support system is in place, technicians drill, dig holes under the structure, and construct the new support system or foundation. When the underpinning is complete, the final step involves removing the temporary support system.


During the underpinning process, the piles and piers are built to extend to stable soil, or soil resistant to compacting and settling, so the repaired foundation nor the structure it supports should not move.


Underpinning is expensive, regardless of the type, often more than $5,000 for materials and work, depending upon the type of underpinning.