Who This Guide Is For — and What It Isn't About
This is for homeowners, owner-builders and small builders at the footings stage of an extension, deck, carport or rear addition, who have a structural drawing with a pier schedule on it and want to understand what the excavation crew is actually doing.
It is deliberately not about the platform. If you want the bigger picture on renovation earthworks — how extension digs differ from new builds, sequencing, permits and what to send for a quote — that is covered in extension site cuts. This post is about the holes themselves: how they are drilled, what stops them, and why the engineer signs off before the concrete truck is booked.
Why Extensions Use Piers at All
Most of Melbourne sits on reactive clay that swells and shrinks with seasonal moisture. A shallow footing rides that movement; a pier founded deeper, below the layer most affected by wetting and drying, largely does not. On an extension there is a second reason that matters even more: the existing house has already settled and reached equilibrium. If your new structure moves differently to the old one, the join between them cracks. Piers are how engineers control that differential movement.
Trees complicate it further. An established tree near the new footings changes soil moisture for years, and its influence zone can push pier depths deeper or change the design entirely. If there is a big gum or a row of established trees near your extension, expect the engineer to have accounted for it.
Reading the Pier Schedule
The pier schedule on your structural drawings is the instruction set for the whole exercise. It will typically nominate the diameter, the depth, the spacing along the beam line, and — most importantly — the founding criteria. That last one is usually written as a minimum depth or a specified penetration into suitable founding material, whichever is deeper.
That wording is why nobody can tell you the final depth in advance. The drilling reveals what is down there, and the criteria decide when to stop. A crew that treats the nominated depth as a fixed number and stops there regardless of what the auger is bringing up is not following the drawing.
Two other details worth knowing: the set-out comes off the beam line, not off the fence or the existing wall, so it needs to be marked accurately before drilling starts; and there is usually a tolerance on position. Drift too far and the reinforcement cage no longer sits where the beam design assumed.
How the Holes Actually Get Dug
On residential extensions the common approach is an auger mounted on an excavator. The machine positions over each mark, the auger drills to depth, and the spoil comes up as a ring around the hole. It is quick when conditions are good.
Machine size is dictated by access rather than preference. On a rear addition with a narrow side path, a small excavator with an auger is often the only thing that fits, which slows the rate of work without changing the outcome. How that choice gets made on constrained sites is covered in our tight access guide.
Some holes cannot be machine drilled at all. Piers tight against the existing building, or where a service runs nearby, get finished by hand. That is slow, careful work and it should be identified at quote stage rather than discovered on the day.
Sequencing matters too. Beam trenches or strip footings are usually excavated in the same visit, and the order is chosen so the machine is not driving over holes it has already drilled or over ground it still needs to work from.
What Goes Wrong on Pier Jobs
Almost everything on this list is normal and manageable, provided it is expected:
- Hole collapse. Sand, loose old fill or gravel lenses will not stand up. Casing the hole, or tightening the drill-to-pour sequence, is the usual answer
- Water in the hole. Perched water above a clay layer seeps in steadily. It may be pumped, or the engineer may nominate a pouring method that suits — what it cannot be is ignored
- Rock before design depth. Sometimes excellent news, sometimes a change of equipment. Either way it is the engineer's call, and it is a variation. See what rock does to a quote
- Deep soft fill. On older blocks where a previous owner filled a hollow, piers chase suitable material further down and the schedule depth blows out
- Buried services and old footings. A pier mark that lands on a sewer branch or a former footing has to be resolved with the engineer, not shifted on a hunch. Lodge a Before You Dig enquiry first — our service locating guide explains why plans alone are not enough near an existing house
- Tree roots. Significant roots may not be cuttable if the tree is protected, which can force a design change
The pattern in all of them is the same: the hole tells you something the drawings did not, and someone qualified decides what to do about it. A crew that keeps drilling and says nothing is the actual risk.
A pier is only as good as the bottom of the hole. Everything above it is just concrete.
Pads, Strip Footings and Screw Piles
Bored piers are one option among several, and your engineer chooses. Strip footings and beam trenches are excavated as continuous trenches along load-bearing lines — related work to service trenching in technique, though to a completely different specification. Pad footings are isolated square or rectangular excavations under posts and columns, common on decks, carports and verandah additions.
Screw piles come up as an alternative where drilling and spoil handling are genuinely impractical, or where the program cannot accommodate curing time. They are installed by specialist contractors rather than as part of an earthmoving scope, so if your engineer nominates them, that is a separate trade — worth knowing before you assume your excavation quote covers it.
Inspections and Hold Points
This is the part that protects you. Before reinforcement and concrete go in, pier and footing excavations are normally inspected — by your engineer, the building surveyor, or both depending on the permit conditions. The purpose is to confirm the holes reached the specified depth, found suitable material, and are clean.
Practical habits that make the inspection painless:
- Record the depth of every hole as it is drilled, against the schedule
- Photograph the excavations before anything goes in them
- Clean loose spoil off the bottom rather than leaving it under the concrete
- Keep any written direction from the engineer where depths changed on the day
- Do not pour or backfill through a hold point, no matter how tight the program is
Spoil From Pier and Footing Work
Pier spoil is a small volume that behaves badly. It arrives as rings of sticky clay around each hole, spread across the working area, and on a rear-yard job it has to be barrowed or shuttled out through the same narrow access the machine came in through. Beam trench spoil adds to it.
It still has to leave lawfully, and on older blocks the material may have old fill through it, which affects where it can go. Our spoil removal guide covers that side, and soil removal explains how we bundle cartage with the dig rather than leaving piles for someone else.
When to Call a Contractor
- As soon as you have structural drawings with a pier or footing schedule
- Before locking a concrete pour date, so drilling, inspection and pour sit in the right order
- If your side access is narrow and you need to know what machine can reach the pier line
- If there are established trees, an old fill area or unknown services near the footings
- If a quote prices piers as a flat figure with no allowance for deeper holes or rock
We dig platform cuts, pier holes and beam trenches with our own machines and operators across Melton, Caroline Springs, Werribee, Tarneit, Craigieburn and greater Melbourne — one crew, not a hire machine plus a driver you organise separately. Send the structural plan and a photo of the side access. English, Arabic or Turkish on site if that helps.
FAQs
What is the difference between a site cut and pier holes on an extension?
The site cut creates the level platform your new floor sits on — bulk material off, levels trimmed, spoil away. Pier and footing excavation is the separate task of drilling or digging individual holes and trenches to the depths on the structural drawings so the footings can be poured. They are usually done by the same crew on the same visit, but they are priced and inspected differently.
How deep do bored piers go on a Melbourne extension?
Whatever your structural engineer nominates, which is why the pier schedule matters more than any rule of thumb. On reactive clay the depth is driven by reaching material below the zone affected by seasonal moisture change, and the schedule often says a minimum depth or a set penetration into suitable founding material, whichever is deeper. That means the final depth is only confirmed while drilling.
What happens if a pier hole collapses or fills with water?
Both are normal problems with normal solutions, but they need the engineer's input rather than improvisation. Collapsing holes in sand or loose fill are usually handled with casing or by drilling and pouring in a tighter sequence. Water seeping in may be pumped, or the engineer may allow a particular pouring method. What is not acceptable is pouring concrete into a hole full of slurry and collapsed spoil.
Can pier holes be drilled where there is only narrow side access?
Usually yes. An auger on a small excavator gets down side paths that surprise people, and holes close to the existing building are often finished by hand. What narrow access changes is the rate of work and sometimes the machine's reach, so the set-out needs checking against what the machine can physically get to before the job is booked.
What if we hit rock before the design depth?
Stop and get the engineer's direction. Rock may be perfectly acceptable founding material, in which case the pier may be shortened with approval, or the design may require a specified penetration into it, which needs different equipment. Either way it is a documented decision, and it is also a variation, so it should be covered by a rock clause in your quote.
Can pier holes be left open overnight?
It is best avoided. Open holes collapse, fill with rain and are a genuine hazard on a residential site with neighbours and pets nearby. Where holes must stay open for inspection, they should be covered and barricaded, and the ideal sequence is drilling, inspection and pouring close together.