Underpinning is a method of construction that sees the depth of the foundations to a building being increased. The soil beneath the existing foundation is excavated and is replaced with foundation material, normally concrete, in phases.
Reasons for underpinning are generally:
- The existing foundations of the building have moved – this is caused by poor soil or changes to the soil conditions (e.g. subsidence has occurred).
- There has been a decision to add another storey to the building, either above or below ground level, and the depth of the existing foundations is inadequate to support the modified building or load (weight) of it.