What is Underpinning?

Definition of Underpinning in Construction

Process that is used to stabilize an existing structure, if an adjacent structure is being erected alongside, and the foundation of the new structure is lower in elevation, than the existing structure. This is a specific procedure that is outlined by the structural engineer. Normally the procedure is based upon a sequencing of excavation, formwork and placement of concrete in a sequencing of locations identified by the structural engineer. The intent of a proper underpinning project, is to excavate enough of the depth beneath the existing foundation, to reach below the newly designed footing excavation, without endangering the stability of the existing structure. By careful, planned, and sequenced staging of the underpinning, the existing structure remains supported, and additional concrete is placed to allow the excavation of the new foundation to the new depth. Underpinning is a specific technique, and must be performed by professional, experienced contractors. The use of hydro excavation equipment to excavate the areas to be underpinned is a viable option. The hydro excavator will not produce any vibrations or heavy excavating forces that could cause a compromise in the existing foundations being underpinned. In addition, the use of a hydro excavator, will allow an accurate excavation pattern, that is consistent with the needs of the underpinning contractor. Precise limits to the excavation are possible with a hydro excavator and the ability to use hot water, if the material is frozen, is especially positive. Underpinning is a surgical technique that must be performed by experienced contractors that have performed the operation in the past.