Underpinning is a system or method that is installed to help carry the load from a man-made structure (i.e. building, wall) that is already in place, that is adjacent to a proposed deep excavation. Underpinning usually needs to be considered when this existing structure is within a zone of influence to the proposed excavated face, roughly defined at a 30-degree angle drawn from the toe of excavation to the bottom of existing footing edge. Therefore, if the existing structure is sufficiently far enough away as to not be impacted by this imaginary 30-degree angled line, then underpinning may not be required.
Underpinning methods often include an array or series of support components. Each support component is usually attached or connected to the footing or foundation system of the existing structure. The length/depth of each underpinning support component should extend some depth below the bottom of proposed excavation to transfer the structure load to an elevation below the excavated face. These lengths/depths will vary from project to project based on existing structure loads, soils type, etc.
Underpinning is often categorized as either an active system or a passive system. The active underpinning system is attached or connected to the foundation and is installed before the proposed excavation takes place. The passive system essentially modifying the proposed excavation shoring system sufficiently to retain the soil and the structure load together, and not necessarily attaching to the footing system. The premise of a passive underpinning system is if the soil is retained and not allowed to move too much, then the existing structure foundation is still serving its original purpose.