Underpinning is used to transfer the load from an exiting structure’s current foundation level to a deeper level in the ground. There are a number of reasons why this may be required. Often an excavation for a new structure will occur within feet or inches of an existing foundation, and the bottom of the new excavation will be lower than the bottom of the existing foundation. To prevent damage to the existing structure, its foundation is extended to an elevation below the bottom elevation of the proposed excavation using underpinning.
Another use for underpinning is to mitigate settlement of existing foundations. Underpinning can be used to transfer the foundation load to a deeper and more competent soil stratum. This same method of load transfer can be used if there is a desire to add dead load to an existing structure in excess of the capacity of the existing foundations.
Schnabel has decades of experience in the design and construction of hand-dug piers, bracket piles, micropiles and other systems used to underpin structures. More recently, Schnabel has also used jet grouting for underpinning. Schnabel’s design\build approach to underpinning that includes close communication between our experienced engineers and field staff, combined with regular monitoring, has helped us to keep the movements of some very sensitive structures well within acceptable limits.