underpinning

Underpinning the foundations of a railway bridge. A completed concrete pad underpinning can be seen at the bottom right.

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.

http://www.asuc.org.uk - Association of Specialist Underpinning Contractors