Underpinning is a process that systematically extends the foundation of a structure to a deeper elevation. This system is frequently utilized in retrofit work where proposed excavations would otherwise undermine nearly structures. In addition to traditional pit underpinning, foundations can also be underpinned with drilled micropiles or grouting. Depending upon the extent of the adjacent excavations, additional items of support such as tiebacks, bracing, and/or shoring may also be required to provide supplementary support to the underpinning piers.

Pit underpinning requires that a series of pits be excavated under the existing foundation(s) requiring support. Each pit is excavated by hand and supported with rings of timber lagging. Once subgrade is reached, each pit is filled with concrete (usually unreinforced) to within three (3) inches of the underside of the existing foundation. The gap is filled on the following day with a mortar cement mixture known as “dry-pack,” which allows the foundation load to be transferred directly to the new underpinning pier. This process is continued along the existing foundation until it is fully supported to the required depth.

Due to the nature of most retrofit projects, structures which require underpinning are often delicate, and the composition and condition of their foundations are seldom known before construction begins. Therefore, underpinning can often become “design-build” work, requiring constant monitoring of construction progress and evaluation of design parameters to ensure the affected structures are adequately supported. Phoenix’s experienced construction and design personnel are an integral part of any successful underpinning project, as they bring both knowledge and flexibility to the design and construction teams.

Typical Applications for Underpinning

Foundation Support
Retrofit of Existing Structures