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Underpinning

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:

  • 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 design.
  • The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
  • To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).
  • It is more economical, due to land price or otherwise, to work on the present structure’s foundation than to build a new one.
  • Earthquake, flood, drought or other natural causes have caused the structure to move, thereby requiring stabilisation of foundation soils and/or footings.

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[1] 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.

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