Underpinning
/ ˌʌndəˈpɪnɪŋ /
Also known as: foundation underpinning, mass concrete underpinning, mini-pile underpinning
Definition
Underpinning is the process of strengthening and extending an existing building's foundations deeper into more competent ground. It is carried out when foundations have failed (subsidence), are threatened by nearby excavation, or need deepening to construct a basement below. The traditional mass concrete bay method excavates beneath the existing footing in alternating bays and fills each with concrete. Mini-pile underpinning uses small piles drilled through or beside the existing footing. Always requires structural engineering design and Building Regulations approval.
In practice
In the traditional bay method, the work proceeds in alternating sections to avoid undermining the whole foundation at once - typically in 1-1.2m wide bays, working in a pattern (e.g. bays 1, 3, 5... then 2, 4, 6...). Each bay is hand-excavated below the existing footing to the required depth, shored if necessary, inspected by Building Control, then filled with concrete to within 75mm of the underside of the existing footing. The final 75mm is packed solid with dry concrete mortar and left to cure before the next sequence begins. The alternating pattern ensures the load is always supported by completed bays while new bays are being excavated.
The cause of the original movement must be identified and resolved before underpinning - simply deepening the foundations will not stop ongoing subsidence if the cause (a leaking drain washing away soil, or an active tree root desiccating clay) is still present. A CCTV drain survey and dendrochronological assessment are often required before an insurer will agree to fund underpinning following a subsidence claim. After underpinning, a period of monitoring is typically required before the property is sold, to confirm stability.
Building Regulations
Underpinning falls under Approved Document A (Structure) and requires full Building Regulations approval. A structural engineer must design the underpinning scheme based on ground investigation data. Building Control must inspect at each stage - particularly the open excavation before concrete is poured and the packing of the final gap. A Completion Certificate on underpinning is essential for the property records. Neighbouring properties must be notified under the Party Wall etc. Act 1996 before underpinning within 3-6m of adjacent structures.
Full Building Regulations guidance