In construction or renovation, underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:
Underpinning may be accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of agrout, including expanding urethane-based engineered structural resins.
Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.
Through semantic change the word underpinning has evolved to encompass all abstract concepts that serve as a foundation.