Underpinning | Chalkline

Underpinning is the technique for extending an existing footing deeper or wider to support added load (a new storey, a wall extension) or to remedy settlement that has cracked the building above. It is engineer-designed, specialist-installed work, and typically far more expensive per square metre than the original footing.

Three common methods are used on residential jobs:

Mass concrete underpinning: short sections of footing (1.0 to 1.5 m) are exposed in a “hit-and-miss” sequence, the soil is excavated to a deeper bearing stratum, and a mass concrete pad is cast in. Lowest cost; suits stable drained soil.
Screw pile underpinning: a helical steel pile is screwed into the ground beside the footing and a bracket transfers load to the pile. Works in wet or unstable soils where excavation is impractical; capacity is verified by installation torque.
Resin injection underpinning: a geopolymer or polyurethane resin is injected through ports under the footing, expanding to fill soil voids and re-level the footing. Minimal excavation; only effective where the failure is in fill or weak shallow layers.

Underpinning falls under AS 2870:2011 (residential slabs and footings) for the design pathway, called up by NCC 2022 Volume Two H1D4. Excavation deeper than 1.5 m is High-Risk Construction Work under WHS regulations: a SWMS is mandatory before work starts, and the contractor must hold the relevant licence and competencies.

Underpinning is never the builder’s DIY scope. An engineer designs the solution from a geotechnical report; a specialist underpinning contractor installs. The builder’s role is to scope the work, get the engineer in, and coordinate around the works.

Also known as: footing extension, footing remediation, foundation support.

Category: Footings / remedial work / structural repair.

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