Underpinning is the temporary support of a structure’s foundation while permanent repairs can be made to it. Since the requirement for underpinning is normally predicated on an existing foundation’s failure or compromise, it involves installing more capable foundation elements than those which currently exist for a structure. A need for underpinning could be caused by unplanned or excessive 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.
Individual footing and wall footing foundations are common designs which receive underpinning. These foundations are shallower to begin with, and the proposed solution is often to construct a new footing which is deeper or wider than the original, in order to increase the foundation’s strength and stability. In this application, an underpinning pit of nominally 6 foot square dimension is dug underneath an existing wall footing or individual footing, to the depth required for the new foundation design. If calculations indicate that the existing foundation cannot bridge even this small gap in continuous support, temporary shoring is installed to allow for an underpinning pit to be excavated. Temporary shoring across an underpinning pit is commonly a perpendicular beam placed under the exterior foundation wall, or a “needle beam” actually placed through the wall. This shoring beam is then jacked in place with multiple ground supports, carrying the building loads in lieu of the foundation at that immediate pit location. A section of new wall footing foundation, or a new individual footing, can be constructed in the pit. The building loads are then transferred to the new footing by lowering the jacks on the shoring beam. This process can be repeated in sequence around the perimeter of the foundation, where necessary.
If excavation pits are not practical, steel beams or pipes might be jacked into position near the existing foundation, using the weight of the building as leverage. The beams or pipes serve as new pile foundation supports. The jacks are lowered after they’ve been replaced with constructed pile caps that connect these new piles to the existing footings. Underpinning is a customized practice and is therefore labor intensive, and is typically a more expensive endeavor compared to the unit installation cost of an original foundation element. More complex and deeper foundation systems that require underpinning will each be uniquely engineered.