How the two differ in kind
Both methods answer the same question: the foundation is bearing on something that will not carry it, or is not deep enough to be beyond seasonal movement, and the load has to reach something better. Mass underpinning does it by extending the existing footing DOWNWARD in short sections of mass concrete. Mini piling does it by installing small-diameter piles alongside or through the foundation and transferring the load onto them with a needle beam or a pile cap.
Mass underpinning is a hand operation with a rule attached, and the rule is the whole method. Only a limited proportion of the wall may be unsupported at any moment — conventionally a bay of around a metre, with no two adjacent bays open together — so the work proceeds in a deliberate non-adjacent sequence, each bay excavated, poured, and left to gain strength before its neighbours are touched. Ignore the sequence and you have not underpinned the wall, you have undermined it.
The detail that decides whether it worked is the one nobody sees afterwards. Concrete poured against the underside of the existing footing shrinks away from it as it cures and leaves the wall unsupported on a new foundation. The gap is closed by DRY PACKING — a semi-dry mix rammed hard into the last few centimetres — and a bay that was poured full and left is a bay that carries nothing until the wall has settled onto it.
The factors that actually differ
Show
| Mass concrete underpinning | Mini piles with a needle or cap | |
|---|---|---|
| Depth it reaches | Practical to a limited depth per bay before shoring, water and spoil handling take over — commonly a metre or two, occasionally more in stages. | Whatever depth the competent stratum is at. Depth is a rig and a schedule question rather than a limit. |
| What it needs on site | Hands, a barrow route and somewhere for spoil. No plant, no headroom beyond a person, no access route for machinery. | A rig, which needs headroom — mini rigs work in a couple of metres, but not in nothing — and a route to get it there. |
| The sequencing rule | The method IS the sequence: short non-adjacent bays, each completed and cured before its neighbours. It sets the programme and cannot be compressed by adding labour. | Piles are installed without removing support from the wall, so the sequencing constraint largely disappears. The critical stage is the transfer, not the installation. |
| Groundwater | A serious problem. An open bay below the water table needs dewatering or it will not stay open, and the concrete is placed into conditions nobody wanted. | Largely tolerant of it, which is often the deciding factor on a wet site. |
| Neighbours and party walls | Excavation right against the boundary, with all the notice, survey and liability that carries — and it removes support from ground the neighbour is also standing on. | Less excavation, but vibration from driven types is its own neighbour problem. Bored or screwed types avoid it. |
| How the load gets transferred | Straight down, in line with the wall. The load path is unchanged; the foundation is simply lower. | Through a needle beam or cap onto piles that are usually offset from the wall's line — which introduces an ECCENTRICITY and a moment that the cap has to be designed for. |
| The detail that decides success | Dry packing the gap under the existing footing. Poured and left, the bay carries nothing until the wall drops onto it. | The connection between the existing structure and the pile cap, and whether the piles were installed where the design put them. |
| Shape of the cost | Labour and time, scaling steeply with depth and with anything that complicates an open bay. Cheap to start, expensive to go deep. | Mobilisation of a rig is a fixed cost that a small job carries badly, then a rate per pile that is relatively insensitive to depth. |
Which one, and when
Choose mass concrete underpinning when…
- The competent ground is shallow — the footing simply does not reach it.
- There is no access for plant at all: a cellar, a terrace mid-row, a back garden reached through the house.
- The work is a short length rather than a whole elevation, so a rig's mobilisation would dominate the cost.
- The ground is dry and stands well enough for a bay to stay open safely.
Choose mini piles with a needle or cap when…
- The competent stratum is deep, or the ground is soft to a depth no hand-dug bay will reach.
- There is groundwater, which makes an open bay a dewatering job before it is a concreting one.
- The length is substantial, so the rig's mobilisation is spread across enough piles to be worth it.
- Excavation alongside is impossible: a boundary, a service run, a neighbouring structure that cannot lose support.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Is underpinning the right answer to subsidence at all?
- Frequently it is not, and this is the most useful thing on the page. Most domestic subsidence in shrinkable clay has a CAUSE — a tree drawing moisture from beneath the foundation, or a leaking drain washing fines out of the ground — and the movement is seasonal rather than progressive. Remove the cause and the ground recovers, sometimes over a season or two, and the building stops moving without anything being built. Underpinning a house while the tree stays is an expensive way to make part of the building move differently from the rest, which can crack it in a new place. The correct order is investigation, then cause, then monitoring, and only then remedial works — and the decision belongs to a structural engineer with the site investigation in front of them rather than to a calculator.
- What is dry packing and why does everyone insist on it?
- The gap between the top of the new mass concrete and the underside of the existing footing — the last few centimetres — is filled with a very low-slump mortar rammed hard into the space rather than poured. The reason is shrinkage: ordinary concrete placed against the soffit pulls away from it as it cures and leaves a void, so the wall is sitting above its new foundation rather than on it, and it settles onto it later. A semi-dry mix rammed in place has almost no shrinkage and is in contact from the moment it is packed. It is also the operation most likely to be rushed, because it happens at the end of a bay in an awkward position, and it is invisible the moment the bay is backfilled.
- How wide should the bays be and in what order?
- Conventionally around a metre, with a strict rule that no two adjacent bays are open at the same time and that a proportion of the wall's length is unsupported at any moment — a quarter is a common working limit. The order is deliberately non-adjacent, so the wall always spans over an open bay onto ground or completed work either side. That is what makes the programme long: each bay has to be excavated, poured, left to gain enough strength to be loaded, and dry-packed before its neighbours can start. Widening the bays or opening them in sequence to go faster is the classic failure, and it fails by the wall cracking or dropping rather than by anything looking wrong while the work is under way.
- Do mini piles avoid the sequencing problem entirely?
- The installation does, because nothing is removed from under the wall — the piles go in alongside or through the existing foundation while it stays fully supported. The transfer does not. Loading the wall onto the piles means installing needle beams or a cap and then bringing them into bearing, and that is a designed operation with its own sequence and its own tolerances. It also introduces something mass underpinning does not: because the piles are usually offset from the wall's centre line, the load arrives eccentrically and the cap has to resist the resulting moment. A pile group in the right place with a cap that cannot handle the eccentricity has moved the problem rather than solved it.
- Will it stop the cracks?
- It stops the movement that is causing them, which is not the same thing. Existing cracks do not close when the foundation is stabilised; they are repaired afterwards, and the repair is normally deferred for a period of monitoring so that it is not done while the building is still moving. If the underlying cause has not been dealt with — the tree, the drain, the seasonal shrinkage — then cracks can reopen in the same place or appear at the boundary between the underpinned length and the part that was left, which is a new hinge the works created. Monitoring before, during and after is not an optional extra on this kind of work; it is the evidence that the repair can be made.
- Is this ever a DIY job?
- No, and the reason is the sequence rather than the skill. The consequence of getting a bay wrong is not a poor finish, it is a wall dropping or a partial collapse with people underneath it, and the sequence has to be designed against the specific wall, its loads and the ground. It is notifiable work in every jurisdiction this site covers, it needs a structural engineer's design and building control involvement, and it usually needs a party wall agreement where a boundary is close. The calculators here size a bay's excavation volume and estimate a pile's shaft capacity — quantities for a conversation with an engineer, not a method statement.