<强>托换是这样的结构的基础上的临时支撑体，而永久性修复可以对它进行。由于用于托底要求通常在现有的基础的故障或折衷前提，它包括安装比那些目前存在的结构更能够基础元件。一种用于托底需要可以通过非计划的或过度soil settlement, particularly over a structure’s long service life. Underpinning could be required if the foundation soil’s strength characteristics were over-estimated, or the original foundation’s depth and dimensions were under-designed for the loads actually experienced. Underpinning also may be required for an existing structure’s foundation if adjacent site work will disturb it. An example would be an unforeseen excavation project adjacent to a historical building which has only a shallow foundation.