Method · checked 29 October 2026
Mass concrete underpinning
The traditional method: dig under the old footing in short bays and fill each with concrete, down to firmer ground.
Facts on this page checked against each source on 29 October 2026. Each one links to its source. How we check
Mass concrete underpinning digs out the ground under an existing foundation in short sections, called bays, and fills each one with concrete. The wall ends up resting deeper, on firmer ground. It is one of the simplest forms of remedial underpinning at shallow depths, and is sometimes called the traditional method (Wikipedia).
How it's done
- The engineer designs it.The depth, the width and the order of the bays are worked out from a survey and engineering calculations (Wikipedia).
- A bay is dug.A short length of ground is dug out alongside the wall and under the existing footing.
- It is filled with concrete.Mass concrete goes into the bay, under the old footing and down to the firmer ground.
- The next bay follows, in sequence.The bays are dug in a planned order so the wall above stays supported. Wikipedia notes cases where walls collapsed because lateral support was inadequate (Wikipedia).
- The cause is fixed and the ground made good.Where drains caused the movement, new uPVC drains with sealed joints go in, laid in a gravel surround (Irish Examiner, 2021).
- The cracks are repaired last.After a period of six months to a year for any settling to finish (Irish Examiner, 2021).
Where it fits, and where it doesn't
- Shallow firm ground. Mass concrete is the simple option when firm ground is not far below the existing footing (Wikipedia). When it is a long way down, mini-piles are the usual alternative.
- Common in the UK data. BRE's subsidence database found mass concrete the most common underpinning, often applied to only part of a building (Wikipedia).
- Partial underpinning has a catch. On ground with low bearing capacity, underpinning only part of a house can increase the risk of the two parts moving differently (Wikipedia). Ask the engineer why the line stops where it does.
- It is excavation. Digging under foundations on poor ground is the kind of work the HSA lists as a "particular risk", which means project supervisors must be appointed (HSA).
What it means for you
It means digging along and under the walls being underpinned, so expect excavation, concrete and spoil to remove on site. Whether any floors inside are lifted depends on the design. For a semi-detached or terraced house, work to a wall shared with a neighbour is a party structure question under the Land and Conveyancing Law Reform Act 2009: you must make good any damage and pay the neighbour's reasonable costs of professional advice, and if they refuse, the District Court can make a works order (s. 44, s. 45).
Questions to ask before you agree
- How deep does each bay go, and what did the ground investigation find?
- How many bays, and in what order?
- Which walls are being underpinned, and why does it stop where it does?
- What happens to the drains?
- Who is the PSDP and the PSCS?
- Is making good the garden, paths and floors in the price?
- When are the cracks repaired, and by whom?
- Will the engineer inspect and sign off the work?
Questions people ask
What is mass concrete underpinning?
Digging out the ground under an existing foundation in short bays and filling each with concrete, so the wall rests deeper, on firmer ground.
Why is it done in bays?
So the wall above stays supported while the ground under it is dug out. The engineer plans the order. Wikipedia notes walls have collapsed where lateral support was inadequate.
Does mass concrete cost less than piling?
We can't say: no Irish contractor we could find publishes prices for either. It is described as the simplest method at shallow depths, but the price depends on depth, length and access.
Can just one corner be underpinned?
Partial underpinning is common, but on ground with low bearing capacity it can increase the risk of the underpinned and unpinned parts moving differently. The engineer decides.
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Get it looked at properly
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