Concrete underpinning is the most established method for stabilising a Wagga home whose footings have dropped or are no longer bearing on competent soil. The approach involves excavating a series of pits beneath the existing footing and filling each with concrete, effectively extending the footing downward until it reaches a stable bearing stratum. It has been used on Australian clay soils for decades and remains the most commonly specified method for older brick and double-brick homes where significant bearing capacity is needed. This guide explains how it actually works, including the sequencing that most homeowners have never heard explained, and when concrete underpinning is the right choice versus an alternative. Start by checking whether your home is a candidate with our does my house need underpinning checklist.
The step-by-step: excavate, pour, cure, repeat
Concrete underpinning is not a single continuous operation. It is a series of discrete, carefully sequenced stages that together build a new load path from the existing footing down to stable ground.
At each pin location, the sequence is:
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Excavation of the pit: Workers manually excavate a pit beneath and beside the existing footing to the designed depth. The pit is typically 600-900 mm wide and extends down until it reaches the specified bearing stratum, which might be 1 to 3 metres below ground level depending on site conditions.
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Formation of the concrete base: The base of the pit is cleared, levelled and sometimes lightly compacted or blinded with a thin concrete layer to provide a clean working surface.
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Pour the concrete: Concrete is poured to fill the pit, with reinforcement bars typically included to tie the new concrete mass to the existing footing. The concrete is compacted to eliminate voids.
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Cure time: The concrete is left to cure, typically for three to seven days before load is applied. During this time the pit is protected and the new concrete gains strength.
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Grouting the contact zone: The gap between the top of the new concrete pin and the underside of the existing footing, which cannot be fully filled during pouring, is usually grouted under pressure after curing to ensure the load transfer is complete and continuous.
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Reinstatement: Soil is replaced around the new pin, the pit is backfilled, and the site is progressively made good as each section is completed.
This full sequence is repeated for each pin across the building. A typical underpinning job might involve six to twenty or more individual pins depending on the house size and the extent of movement.
The hop-scotch sequence and why it matters
The most commonly asked question about concrete underpinning, and the aspect least often explained, is why the pits are excavated in a specific non-consecutive order rather than working along the wall from one end to the other.
The answer is structural stability during the work. The existing footing is carrying the load of the building. If you excavate under consecutive sections of footing, you are removing support progressively from a continuous run, which can cause the wall above to distort or crack under its own weight before the new underpins are in place.
The industry-standard approach is to work in a checkerboard or hop-scotch sequence: pin 1, 3, 5, 7 are excavated, poured and cured before pins 2, 4, 6, 8 are started. This means at any point in time, no more than every alternate section of footing is unsupported, and the completed, cured pins on either side provide lateral stability to the section being worked on.
Getting the sequence wrong is not just inefficient; it can cause progressive damage to the structure being repaired. This is why concrete underpinning is not a job for unlicensed operators and why the methodology should be part of any professional quote conversation. Our underpinning service covers how this is managed on site.
When concrete underpinning is the right choice
Concrete underpinning suits a wide range of residential foundation problems and is the go-to choice in several specific situations:
Double-brick and heavy masonry construction: Heavier structures need more bearing capacity per pin than lightweight construction. Concrete underpinning’s high load per underpin makes it well-suited to the double-brick homes common in older Wagga suburbs like Turvey Park and Kooringal.
Deeper bearing stratum required: Where competent soil is found at 1.5 metres or deeper, concrete underpinning can be excavated to whatever depth is needed, whereas resin injection is limited to shallower void-filling applications. Screw piles can also reach depth, but the cost comparison changes with depth.
Moderate to significant movement requiring high structural stiffness: Where a footing needs to be locked in position with high stiffness, mass concrete provides this without relying on ongoing mechanical connections.
Sites with poor access for drilling equipment: Concrete underpinning can be done entirely manually in confined spaces where machinery cannot operate. This matters for some Wagga homes with limited side access or complex site geometry.
Cost-sensitive jobs with uncomplicated geometry: On a simple single-storey home with straightforward access and moderate movement, traditional concrete underpinning is often the most cost-effective option on a per-pin basis.
When it isn’t, and what to use instead
Concrete underpinning is not always the best tool. Situations where alternatives are worth considering:
- Where the time required for concrete curing extends the job beyond what the homeowner can manage (screw piles can be load-bearing the same day they are installed)
- Where the site is extremely confined and spoil removal from manual excavation creates significant logistical problems
- Where the movement is primarily at a slab-on-ground with voiding beneath rather than a dropped strip footing (resin injection may be more appropriate)
- Where the movement is mild and the footing only needs marginal supplementary support rather than full replacement of bearing
See our comparison of screw pile versus concrete underpinning and the resin injection underpinning guide for when those alternatives are typically preferred.
Cost drivers specific to concrete underpinning
| Cost factor | What it drives | Notes |
|---|---|---|
| Number of pins | Primary cost variable | Set by the engineering spec based on load and soil |
| Excavation depth | Labour and concrete volume | Deeper bearing stratum means more cost per pin |
| Access difficulty | Labour multiplier | Side-access-only, low sub-floor, or internal underpinning all add cost |
| Spoil removal | Disposal cost | Manual excavation produces a significant volume of soil |
| Cure time | Project duration | Longer cure time can extend the project program |
| Grouting requirement | Materials and labour | Contact grouting is often required for effective load transfer |
| Reinstatement scope | Paths, gardens, internal finishes | Often underestimated in preliminary budgets |
For realistic cost ranges and how to read a quote, see our underpinning cost guide. For the full picture of how quotes are structured, see how underpinning quotes are built.
How long a concrete underpin lasts
A correctly designed and executed concrete underpin is a permanent structural element. The new concrete, when poured to adequate strength and cured properly, does not degrade or lose capacity under the conditions found in residential footings. The bearing capacity at depth below the zone of soil moisture fluctuation is stable, and the load path from the footing through the concrete pin to that stratum is enduring.
In practice, the service life of a concrete underpin should be measured in decades rather than years. The limiting factor over time is not the underpin itself but whether the causes of movement, usually moisture imbalance, are also addressed. An underpin that is installed under excellent technique but without addressing the drainage or tree issues that caused the original movement will be doing its job correctly, but the remainder of the footing that was not underpinned may continue to move.
This is why a good underpinning job includes professional advice on addressing the root cause and not just the symptom. Our foundation inspections service includes an assessment of contributing moisture causes alongside the structural evaluation. Project timelines are covered in how long does underpinning take.
FAQs
Does concrete underpinning fully fix the problem?
Concrete underpinning addresses the failing footing bearing capacity and prevents further settlement at the underpinned locations. It does not automatically close cracks in walls, restore floors to a perfectly level state, or address any moisture or drainage issue that caused the movement. After underpinning, a monitoring period typically follows before final cosmetic repairs are done. The cause of movement should also be addressed to prevent a different section of footing from eventually showing the same problem.
How many pins does my house need?
That is determined by the engineering assessment and varies with the load on each section of footing, the depth to competent soil, the footing dimensions and the extent of the movement. A rule of thumb often cited is that pins are typically spaced every 1.5 to 2.5 metres along the affected footing run, but the actual spec depends on the site. A quote without an engineering basis for the pin count and spacing is a concern.
Can I stay in my house during concrete underpinning?
In most cases, yes. The work is done section by section and the house maintains structural integrity throughout. There will be noise, dust, limited access to parts of the garden or sub-floor during active work, and some disruption to paths and paving. Some homeowners prefer to stay elsewhere for the most active period, particularly with young children or if sub-floor access requires the floor to be lifted.
What is the difference between concrete underpinning and screw pile underpinning?
Both install new bearing capacity below the existing footing. Concrete underpinning uses hand-excavated pits filled with cast-in-place concrete. Screw pile underpinning installs pre-fabricated steel helical piers that are screwed into the ground with a machine. Screw piles are faster and create less mess; concrete underpins can be done entirely by hand in confined spaces and tend to cost less per pin in straightforward applications. See screw pile vs concrete underpinning for the full comparison.
Do I need council approval for concrete underpinning in Wagga?
Often no, for like-for-like repair work. But approval requirements depend on the scope of work, your zoning, any heritage overlays and whether the work involves significant alteration rather than repair. Confirm with Wagga Wagga City Council or your contractor before starting. More detail on this is available in does my house need underpinning.