Conventional Concrete (Pit) Underpinning
Conventional concrete underpinning is accomplished from small, sheeted and braced pits constructed beneath the foundation to be supported, typically accessed via an approach pit. Following excavation to the desired level, the pit is filled with concrete to within three inches of the foundation underside. Load transfer is accomplished by dry-packing the gap with stiff mortar.
These days, 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.
Micropiles are small diameter (4- to 12-inches in diameter), reinforced load transfer elements capable of supporting design loads in excess of 200 tons. Micropiles can be installed through almost any type of subsurface condition, from sands to clay to bouldery soils to rock. Micropiling is accomplished using low-vibratory drilling techniques, a critical consideration when working around sensitive structures. The micropiles can readily be installed in confined space and low headroom situations. Often installation can be accomplished without disrupting normal facility operations.
Vertical and/or inclined micropiles can be installed to provide additional foundation support needed to withstand increased structural loading, underpin existing shallow foundations during adjacent new construction, arrest ongoing settlement, and for seismic retrofit.
Soil nailing involves the installation of closely spaced reinforcing bars (nails) in the face of the excavation or slope. A structural concrete fascia bridges the nails and supports the soil/rock between them, creating a reinforced zone that acts as a retention system. Soil nails are passive elements that become tensioned when the soil/rock deflects laterally as excavation depth increases. In general, soil nailing can be used in any soil where a vertical cut on the order of four to six feet can remain stable for at least 24 hours.
While primarily used as an economic, method of excavation support, the soil nailed wall can also fulfill an underpinning requirement and is advantageous when the excavation depth requires the underpinning system to be laterally supported.
Helical piles are structural underpinning elements primarily used for foundation settlement control. The piles are hydraulically screwed in the ground until the required torque is reached to achieve pile capacity, and attached by means of a bracket to the adjacent distressed footing.
Bracket piles are typically used in conjunction with earth retention work to support and/or stabilize existing foundations adjacent to a proposed excavation. The new piles are installed in pre-drilled holes alongside the exposed wall or column footings. A bracket is mounted to the underside of the footing and attached perpendicular to the new pile to transfer the load below excavation subgrade.