Underpinning methods explained: mass concrete, screw piles and resin injection
The main ways a footing is strengthened or re-supported in Australia, how each method works, and the situations each one suits.
Underpinning means extending or strengthening an existing footing so it reaches more stable ground or spreads its load better. It is the standard fix once an engineer has confirmed that footings have moved and will keep moving. There is no single best method; the right one depends on the soil, the depth to stable ground, access to the site and how much movement has occurred. These are the three families you will be quoted.
Mass concrete (pit) underpinning
The traditional method, and still common for many homes. Working in small, carefully sequenced sections so the wall is never left unsupported, the crew excavates a pit beneath the existing footing down to firm soil and fills it with concrete, building a new, deeper footing in stages. It is well understood, needs no specialised plant, and suits sites where stable ground is not too far down. The trade-offs are that it is labour intensive, involves excavation around the house, and takes longer than the alternatives.
Screw piles and piers
Steel screw piles, also called helical piers, are shafts with a helical plate that are wound into the ground with machinery until they reach a load-bearing stratum or a set resistance. Brackets then transfer the footing's load onto the piles. Because they are driven rather than dug, they reach deeper stable ground than mass concrete pits, install relatively quickly and with less spoil, and can often be tested for capacity as they go in. They suit deeper problems and sites where the good ground is well below the surface, though machine access to the footing is needed.
Resin injection
The newest family, and the least invasive. An expanding structural resin is injected through small tubes into the soil beneath the footing or slab. As it expands it compacts weak soil, fills voids and can gently re-level the structure, all without excavation. It is quick, clean and well suited to slab-on-ground homes, re-levelling floors and filling voids left by eroded or poorly compacted soil. It is not a cure for every case, deep-seated movement or heavily loaded footings may still need piles, so it works best where an engineer confirms the ground can be improved in place.
Whichever method is proposed, the decision should follow a structural engineer's diagnosis, and the quote should specify the method, the number of piers or injection points, the design load and any monitoring afterwards. Two quotes using different methods can differ a lot in price and disruption, which is exactly why understanding the options pays off.
Common questions
Which underpinning method is best?
There is no universal best method. Mass concrete suits shallower stable ground and tight-access sites, screw piles reach deeper load-bearing soil quickly, and resin injection re-levels slabs and fills voids with minimal disruption. The right choice depends on your soil, the depth to firm ground and the type of footing, which is why an engineer's assessment comes first.
Is resin injection as strong as concrete underpinning?
For the right conditions it can be an effective, engineered solution, and it is far less invasive. But it improves the soil rather than installing a deep structural pier, so for deep-seated movement or heavily loaded footings, piled or mass-concrete underpinning may still be required. A structural engineer should confirm the method suits your case.
Sources
- CSIRO, Foundation Maintenance and Footing Performance: A Homeowner's Guide (BTF-18)
- Standards Australia, AS 2870 Residential slabs and footings
General information only. Confirm details for your property with a licensed structural engineer.
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