In construction, underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:

• The original foundation is simply not strong or stable enough.
• The usage of the structure has changed.
• The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during planning.
• The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
• It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.

Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on more supportive soil stratum or distributes its load across a greater area. Use of micropiles [1] and jet grouting are common methods in underpinning. An alternative to underpinning is the weakening of the soil by the introduction of a grout. All of these processes are generally expensive and elaborate.

Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.

Through semantic change the word underpinning has become to encompass all abstract concepts that serve as a foundation.

Gallery

Sketch of a cast in situ RC cantilever needle beam on micro piers or piles. Access to inside not needed.	Sketch of a standard needle beam on micro piers or piles. Inside access needed.	Sketch of concrete pier underpinning with the possibility of raising foundations.	Sketch showing the traditional method of needling a wall to reduce the weight on the foundations during underpinning work.
Underpinning the foundations of a railway bridge using a timber a box crib to support the bridge. A completed concrete pad underpinning can be seen at the bottom right.]

External links

• An illustrated glossary of the terms used in temporary types of construction work. Formwork, scaffolding etc.
• - Association of Specialist Underpinning Contractors