Underpinning

Frequently asked questions

Why is underpinning done in short sections?

Because the wall above has to span the gap while the section below it is open, and masonry can only arch so far. Excavating the full length under a foundation removes its support entirely and the wall comes down; excavating a metre of it leaves a short opening that the wall bridges by arching over, provided the masonry is sound and the opening is short enough. So the work proceeds in numbered bays in a HIT AND MISS sequence — dig bay one, pour it, let it gain strength, pack the gap to the underside of the old foundation, and only then open a bay that is not adjacent. The sequence and the bay width are the design, not an efficiency choice, and the two ways it goes wrong are opening too many bays at once because the ground looked stable, and packing the new concrete to the old foundation carelessly. The pack matters as much as the pour: a bay that has not been dry-packed tight to the underside carries nothing until the wall has settled onto it, which is settlement the job was meant to stop.

Is underpinning the right answer to subsidence?

Often not, and the question worth asking first is what moved the ground rather than how to reach deeper. In shrinkable clay the usual cause is moisture change, and much of that is vegetation: a tree drawing water out in dry summers shrinks the clay under the footing and the building settles seasonally. Removing or managing the tree can stop the movement without touching the foundation at all — though removal has its own risk, because clay that has been kept dry for decades swells as it rehydrates and the ground HEAVES, which damages a building in the opposite direction and is harder to remedy. Other causes point elsewhere entirely: a leaking drain washing fines out of the ground is repaired by fixing the drain, and historic movement that has stopped needs monitoring rather than works. This is why competent practice is to monitor and diagnose before designing, and why underpinning part of a building is a specific decision — an underpinned section sits on different ground from the rest and can move differently, putting a crack at the junction between old and new.

What is the neighbour's position when the work is at a party wall?

In England and Wales it is a statutory one, and starting without it is the most common procedural failure on this kind of job. Excavating within defined distances of a neighbouring building and to a depth below its foundations triggers a notice requirement under party wall legislation, regardless of whether the work is on the boundary, and the neighbour has rights to a schedule of condition, to appoint a surveyor at the building owner's cost, and to an award setting out how the work proceeds. The practical effects on a programme are real: notice periods run in weeks, the award has to be in place before work starts, and work begun without one can be stopped. Other markets handle the same physical risk through permits, notification and insurance rather than a dedicated statute, so an American or Australian programme brought to a British site regularly omits a step that cannot be compressed. The physical caution is the same everywhere: excavating below a neighbour's foundation undermines it, and the sequence protects their building as much as the one being worked on.