Foundation Underpinning

There are many different situations where a structure may need to be underpinned. Lists several underpinning possibilities:

• To support a structure that is sinking or tilting due to ground subsidence or instability of the superstructure
• As a safeguard against possible settlement of a structure when excavating close to and below its foundation level
• To support a structure while making alterations to its foundation or main supporting members
• To enable the foundations to be deepened for structural reasons, for example to construct a basement beneath a building
• To increase the width of a foundation to permit heavier loads to be carried, for example, when increasing the story height of a building
• To enable a building to be moved bodily to a new site Tomlinson (1986) also indicates that each underpinning project is unique and requires highly skilled personnel, and therefore it should only be attempted by experienced firms.

Because each job is different, individual consideration of the most economical and safest scheme is required for each project. Common methods of underpinning include the construction of continuous strip foundations, piers, and piles. To facilitate the underpinning process, the ground can be temporarily stabilized by freezing the ground or by injecting grout or chemicals into the soil.