underpinning

Underpinning refers to a technical process used to strengthen and stabilise the foundation of a structure. It involves extending the foundation depth or increasing its bearing area so that loads are transferred to more stable soil or rock. Underpinning is typically required when foundations have settled, when adjacent excavations undermine support, when soil conditions are poor, or when a building is being altered or expanded.

Common causes include soil variation, moisture changes, or defective original design. It may be necessary for repairs to foundations damaged by settlement, or to support additional loads from extensions, excavation works nearby, or changes in use that increase weight on the structure.

Methods vary, including traditional mass concrete underpinning, where existing footings are underpinned in sections by replacing them with reinforced concrete piers extending to stable strata. Piled underpinning uses steel or concrete piles driven or drilled to deeper soil, connected to the existing foundation with a pile cap. Drilled or bored underpinning (micro-piles or continuous piles) creates deep foundations under the existing structure. Resin-injection underpinning uses chemical grouts to fill voids and relevel or stabilise small areas. In some projects, tensioned anchors or struts may be used to stabilise retaining walls during underpinning works.

Planning and execution require a structural engineer's design, careful assessment of soils and loads, temporary supports, and compliance with local building codes. Work is typically carried out by specialist underpinning contractors and monitored for movement and safety. Underpinning can be costly and time-consuming and involves ground disturbance, but it provides long-term stability and enables repairs or redevelopment without replacing the entire foundation.

See also: foundation repair, basement underpinning, piling.