What to Consider When Excavating Next to a Neighbor's Building

The key to a successful excavation and underpinning project is to get underpinning and excavation specialists involved early in the project planning stage. Small changes to the project upfront and foreknowledge of permit and legal requirements often save enormous time and money later.

Underpinning is the direct support of an existing building foundation. It provides the opportunity to preload (i.e., jacking) to limit settlement and improve poor or non-existent foundations benefiting the neighbor while at the same time providing support of the excavation without encroaching on the new building. The alternative to underpinning is typically shoring to support the excavation face indirectly supporting the neighbor’s building. This shoring is less desirable since thicker, stiffer structural elements are needed and lateral movement can cause more distress to the neighbor.

With favorable ground or rock and light surcharges from the existing buildings, open cuts or earth shoring can be used without underpinning. It may also be feasible to modify the planned new building, limit the excavation, or use sequence construction to reduce or eliminate the need for underpinning. A professional engineer and/or engineering geologist are needed to determine if open cuts are possible, or if underpinning or shoring is required. If open cuts are not feasible, underpinning or shoring will be needed.

The most widely used underpinning systems are hand-dug concrete piers and slant drilled steel piles which are installed directly under the neighbor’s building foundation from the existing, adjacent ground surface. Selecting the correct type of underpinning is also critical since each system has its advantages given the anticipated soil and groundwater conditions, consequences of damage to the adjacent improvements, occupant safety, and legal obligations.

Legal considerations also impact the need to underpin and involve neighbor or other third parties (i.e., Caltrans, BART, PG&E). California Civil Code Section 832 and local ordinances address support of adjacent land and underpinning. It may be determined that underpinning is the responsibility of neighbors or other parties rather than the owner. In either case, competent legal advice is critical and negotiations with neighbors or third parties should start early to avoid drawn out delay. Passing the underpinning responsibility to the contractor often creates the worst case since the contractor still needs the neighbor’s permission to do the underpinning work, contributing to the last minute permit scramble and delay of the project.

Finally underpinning requires careful attention to site and project constraints and construction procedures as well as anticipation and avoidance of the many potentially disastrous problems. The bottom line is to obtain knowledgeable help early in the project.

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