Underpinning is required when an existing building foundation is undermined by excavation for a new building or other structure. Typically a series of concrete piers are installed in relatively narrow sections directly beneath the existing foundation as excavation proceeds. The underpin sections must be able both to function as a new foundation—spreading the building load to the soil—and as a retaining wall, resisting lateral soil loads. Where the dead weight of the concrete alone is not enough to ensure this lateral stability, additional supports such as tiebacks may be used.
ln addition to engineering analysis, the procedural aspects of underpinning construction are highly critical. Work must be planed out carefully in stages so that any movement of the existing structure is prevented. Protocols include specific, detailed sequencing of operations, lab testing of each concrete pour, and survey monitoring of the existing building.
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