Conventional Concrete Pit Underpinning

The traditional concrete pit underpinning method is nearly 100 years old, and the protocol has not changed since its inception. This method may be applied when there is a need to strengthen a structure’s existing foundations and those foundations are at a shallow depth.

Conventional concrete pit underpinning is accomplished from small, wood-sheeted pits constructed beneath the foundation to be supported, typically accessed via an approach pit. Engineers can choose from a number of above-grade underpinning techniques during the design phase of the project. Nevertheless, there are circumstances – such as when new construction abuts tight against an existing building – where the conventional concrete underpinning method is often the best or only viable option.

Installation

Pit underpinning is a manual operation primarily for use in dry ground. The technique should not be attempted in wet ground unless a proper dewatering program is in place.

An approach pit is dug to a depth of five feet below the bottom of the footing to be underpinned. Sheeted pits, typically 36 inches wide, are then excavated below the footing to a competent bearing stratum. When the pit is completed, a form is placed across the approach pit and the underpinning pit is concreted to within several inches of the underside of the footing and allowed to set overnight. The gap is then dry-packed with stiff mortar to accomplish the load transfer. Pits are excavated and completed in a strategic order to form a continuous wall. The final result is basically a foundation built underneath the existing foundation.

Applications

  • To increase the load-carrying capacity of an existing structure

Advantages & Limitations

  • Simplicity of the engineering
  • Continuity of the structure's uses during construction
  • Can be performed externally or from within a structure
  • Difficult to accomplish in cobbly, bouldery or extremely dense ground