Underpinning is a construction technique used to create new foundation beneath an existing structure. The new foundations may be required to improve the existing foundations or to re-locate them at an increased depth. They may also be necessary to create a retaining structure beneath an existing foundation to allow the lowering of the ground within the building; to create a basement, for example.
Underpinning involves the excavation of ground from below an existing foundation which will, potentially, leave the foundation and excavated soil faces unsupported for a short period of time. Depending on the condition of the foundation and the nature of the soil, temporary support may be required to either or both. The excavation is carried out along a relatively short length of foundation, normally up to 1m. In this way the foundation experiences a loss of support over a relatively short length in comparison with its whole length. The foundation and supported wall is assessed for its ability to ‘span’ or ‘arch’ across the excavation in the temporary condition as is the soil assessed to sustain the increase in load at the edges of the excavation. This assessment must be done by an appropriately experienced and qualified person, preferably a civil engineer or structural engineer.
Once the excavation has been made safe in the temporary condition, concrete is cast into the hole up to the underside of the original foundation. The process is repeated in a prescribed sequence around the building until it is totally re-supported on deeper, stronger foundations at an increased depth.