Underpinning design

There are several underpinning methods and the choice is driven by the ground, the depth of firm strata, access, and the load. Traditional mass concrete underpinning, taking the foundation down in short, sequenced bays, suits shallow firm ground and localised movement. Beam and base underpinning spreads load onto pads or a ground beam where continuous excavation is impractical. Piled underpinning, with a beam picking up the wall and transferring load to piles, is used where firm ground is deep, where clay heave is a risk, or where excavation next to the wall would be unsafe.

The design covers the new foundation capacity to the Eurocodes, the bay sequence and dig depths, the reinforcement and the dry-pack detail where the new work meets the underside of the existing wall, and the interface with drains and services. Crucially it covers temporary stability, because underpinning removes support from part of a loadbearing wall while a bay is open. We set out the hit-and-miss sequence and any needling or propping so the building stays safe throughout.

Underpinning is as much a construction method problem as a foundation calculation, so we design it to be built safely. The sequencing that keeps the wall supported, the working space each bay needs, the treatment of nearby foundations and party walls, and the ground movement the works themselves cause all go into the design and the method statement. Where the works fall near a boundary, the party wall process and the neighbour's foundation are part of the assessment.

We coordinate with the contractor, building control and, on insurance claims, the loss adjuster, so the scheme is agreed before digging starts and the details answer site questions rather than raise them. This design work is delivered across the UK, and for European projects we prepare the calculations, sequence and details and coordinate the design with local ground data and a site visit where the foundation must be seen.