Underpinning advice
Concrete underpinning, step by step
Published
Ask what “concrete underpinning” actually involves and most people picture a trench dug along the base of the house, filled with concrete, done. That mental picture is wrong in one specific and important way: the trench is never continuous. Mass concrete underpinning — the oldest and still one of the most common underpinning methods used in Melbourne — is carried out in short, deliberately staggered sections called bays, and the order those bays are dug in is not administrative detail. It’s the thing that keeps the house standing while the work happens underneath it.
This article walks through what that sequence actually looks like on site, from the first excavation to the final pour. For how mass concrete compares against bored piers, screw piles and resin injection, see underpinning methods compared. For what the whole job costs, see what underpinning costs in Melbourne — this article doesn’t introduce new figures, it explains the process behind the numbers already published there and on the underpinning service page.
What mass concrete underpinning actually is
The existing footing — old brick, unreinforced concrete, or a shallow strip footing that was never adequate for the ground it sits in — is extended downward to competent ground by building a new concrete footing beneath it, in sections. Each section, or bay, is excavated by hand or with a small machine down to ground that can actually carry the load, then filled with mass concrete that bears the wall above once it’s cured. Once every bay along the affected run is done, the wall is transferring its weight to the new, deeper concrete instead of the old shallow footing.
It’s the method most suited to shallow competent ground — commonly under about two metres — which is why it turns up most often under older brick veneer and strip-footed homes on established blocks rather than out on deep engineered fill, where bored piers or screw piles are usually the better tool. More on that trade-off further down.
The sequence, bay by bay
1. Engineering design first. Before anyone picks up a shovel, an engineer sets the bay width (commonly 900mm to 1.2m), the depth each bay needs to reach, and — critically — the sequence bays are excavated in. This is what a building permit is issued against, and it’s the drawing you should ask to see before agreeing to a price.
2. The bays are numbered, not adjacent. A typical run along one wall might be split into bays numbered 1 through 8. The crew doesn’t dig 1, then 2, then 3 in order. They dig 1, then skip to 4, then 7 — leaving at least one, usually two, undisturbed sections of original footing between any two open excavations at a time. Only once the first round has cured does the crew come back for the gaps.
3. Excavating one bay. The bay is dug down past the underside of the existing footing to the depth the engineer specified, undercutting the old footing slightly so the new concrete keys into it. In tight-access sites this is entirely hand work; where a small machine can reach, it speeds this step up but the bay width and sequence don’t change.
4. Reinforcement and formwork. Steel starter bars are typically dowelled into the underside of the existing footing so the old and new concrete act together rather than as two separate elements stacked on top of each other. Formwork keeps the pour contained and the finished face vertical.
5. The pour. Mass concrete — plain or lightly reinforced, depending on the design — is placed into the bay and finished up hard against the underside of the old footing, often with a dry-pack mortar pressed into the last few centimetres by hand to close any gap and make sure load actually transfers rather than leaving an air pocket.
6. Curing before the next bay opens. This is the step that makes mass concrete slower than bored piers or screw piles. Each bay needs to gain enough strength before the crew opens an adjacent one, because an adjacent open excavation removes support from both sides of the just-poured bay while it’s still green. On a typical residential job this means a bay poured on a Monday isn’t followed by digging next to it until later in the week.
7. The second round fills the gaps. Once the first round of bays has cured, the crew returns for the sections left between them — 2, 5 and 8 in the example above — repeating the same excavate, reinforce, pour and cure sequence. By the time this round is done, the entire run has been replaced without ever having more than a short, isolated section of the original footing missing at once.
8. Backfill and reinstatement. Once the final bays have cured, the excavations are backfilled, compacted, and whatever sat above them — paving, garden beds, a path — is made good.
Why the sequence is the whole point
The engineering reason for staggering bays isn’t caution for its own sake — it’s what stops the wall above from losing its support. A footing under a loaded wall is continuously carrying that load along its length. Remove a long continuous section at once and the wall has nothing to bear on across that whole run, which is exactly the mechanism that causes an underpinning job to fail rather than fix the house. Keeping the gaps between open bays means the wall is always resting on intact footing either side of any excavation, even while roughly a third of the run is open at any given moment. It’s a slower, more disciplined way to do the job, and the sequence drawing is the evidence that the discipline was actually applied — not just described.
Where mass concrete suits, and where it doesn’t
Mass concrete works well where competent ground is shallow and reasonably predictable — that describes a lot of Melbourne’s older, established suburbs. In Hoppers Crossing, for example, the shallow strip footings on 1980s and 90s brick veneer sit on ground where a two-metre bay reaches something solid, and the smaller job that results reflects it. The same is broadly true in the older, shallower-footed streets of Werribee, where mass concrete is a genuine option alongside bored piers.
It stops being the right tool once competent ground drops away. Through the deep clay and engineered fill under estates like Melton, Tarneit and Caroline Springs, piers commonly need to reach three to four and a half metres — hand-excavating a bay that deep, safely, stops being practical, and bored concrete piers or screw piles take over instead. This is exactly why the geotechnical report comes before the method is chosen, not after: it tells you which category your site is actually in.
What a sequence drawing should show you
If you’re comparing quotes for mass concrete underpinning, ask to see the bay layout and sequence, not just a total price. It should show bay numbers, bay width, the excavation order, and the specified depth for each one. A quote that names a lump sum with no bay drawing hasn’t been engineered yet — it’s a guess dressed up as a number, and the same warning applies here as anywhere else in this trade: the diagnosis and the design come before the price, not after it.
Not sure whether your home needs mass concrete, bored piers or something else entirely? Book a free on-site assessment — we’ll take floor levels, check what depth competent ground actually sits at, and recommend the method that fits, not the one we happen to have equipment for.
Common questions
Why can't a contractor just dig a trench along the whole footing and pour it in one go?
Because the existing footing is doing real structural work right up until the moment it's replaced, and a continuous trench removes its support all at once along the whole run. The wall above has nothing left to bear on for however long that trench stays open, which is how underpinning causes the very collapse it's meant to prevent. Working in short, staggered bays means the footing either side of any open bay is still intact and still carrying load, so the wall is never unsupported along its length — only in short, isolated sections at a time. It's slower than a trench, and that's the point.
Can I stay in the house while mass concrete underpinning is happening?
Almost always, yes. The work happens outside and underneath, one short bay at a time, so there's no point where a whole wall or room is without support. You'll get noise and vibration from excavation, and dust if bays are being dug close to the house, but there's no structural reason to move out. The exception is a house already carrying heavy cracking or a distorted floor, where the owner and the engineer might agree to vacate a specific room during the bays nearest it, purely so no one is standing under plaster that could shed while a bay beneath it is briefly open.
How do I know if my house needs mass concrete underpinning rather than bored piers or screw piles?
Depth to competent ground is the deciding factor, and it's an engineering answer, not a preference. Mass concrete suits shallow competent ground — broadly under two metres — which is common under older strip footings on established suburban blocks. Once competent ground sits much deeper, which is routine on the deep clay and engineered fill of Melbourne's growth corridor estates, excavating a two, three or four-metre bay by hand becomes impractical and bored piers or screw piles take over. A geotechnical report settles which zone your site falls into before anyone commits to a method.
Think this is happening at your place?
We assess before we quote — floor levels measured, cracks mapped, and a straight answer on whether you need work at all. Read more aboutunderpinning & foundation repair, or get in touch.