What is Underpinning?

In construction, underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure.

Reason for Underpinning

Underpinning may be necessary for a variety of reasons:

• The original foundation is simply not strong or stable enough, e.g. due to decay of wooden piles under the foundation.
• The usage of the structure has changed.
• The properties of the soil supporting the foundation may have changed or was mischaracterized during planning.
• The construction of nearby structures necessitates the excavation of soil supporting existing foundations.

How does it work?

Underpinning is accomplished by digging underneath shallow footings and extending the foundation by pouring concrete to extend the reach of the foundations in depth or in breadth so it either rests on a stronger soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout.
Subsidence usually occurs in lowrise and mediumrise buildings which usually have shallow foundations or cellars up to about 3m deep. Subsidence and settlement are taken as interchangeable terms, meaning the sinking of ground on which a structure is founded. Ground can also rise, in which case the movement is known as "heave". Depending on the cause of subsidence or heave, horizontal stretching or squeezing of the ground can accompany the vertical movement.
When movement is, or is likely to become excessive, so that the use or safety of the building is compromised, this is when underpinning is generally the best solution.