Pit Underpinning Pit underpinning involves the strategic undermining of an existing foundation and replacing the soil with concrete. When done segmentally, a new foundation is created underneath the existing one. It is done to provide frost protection to a very shallow foundation. In addition, pit underpinning is done to facilitate nearby excavations that would extend underneath an existing foundation. This process is commonly used to stabilize foundations that do not extend below the frost line. In addition, pit underpinning is commonly used to deepen a foundation when nearby excavation would disrupt a structure.