Neither method is better for every home. Underpinning suits footings that need support carried down to deeper, stronger ground. Injection suits voids, loose fill, and soft soil near the footing, where the ground can be improved in place. The right choice depends on what caused the movement, the soil type, how deep the good-bearing ground is, the load on the footing, and site access.
“Geopolymer injection” is mostly a product name for injected ground-improvement resin. Below, we explain what the term really means, how each method works, when each one is the wrong choice and how to compare quotes with confidence.
What does “geopolymer injection” actually mean?
In science, a geopolymer is an inorganic binder made from aluminium and silicon-rich materials. In foundation repair, “geopolymer injection” is a trade name some suppliers use for expanding resins injected under footings. It is not a separate engineering category. Judge any injection product on its technical data sheet and the engineer’s design, not its name.
Researchers describe geopolymers as inorganic polymers formed when an alkaline solution reacts with aluminosilicate materials such as fly ash, blast-furnace slag, or metakaolin (a 2023 peer-reviewed review of geopolymer materials). CSIRO describes them as binding agents that can replace cement in concrete. That is a concrete and cement story, not a foam that lifts houses.
The expanding resin used for ground injection is well documented. A 2021 review of soil injection using expanding polyurethane resin describes two liquid components that are mixed and pumped through small holes. They react, foam and expand in the soil, fill gaps and can lift the structure in controlled steps.
Some suppliers sell proprietary resins under a “geopolymer” label. We have not seen independently published data on what those branded products contain, so we make no claims about them. If a quote names a product, ask for its technical data sheet. That sheet should state the chemistry, expansion, strength and curing behaviour. Then ask your engineer whether it suits your soil.
For homeowners, the practical comparison is the same either way: underpinning versus injection-based ground improvement. Our guide to underpinning vs resin injection covers that decision in full.
How does underpinning work?
An underpinning extends or deepens a footing so that the building’s load is carried down to firmer ground. The two common residential forms are mass concrete underpinning, dug and poured in short sections, and screw piles, which are steel shafts driven into the ground to a bearing layer and bracketed to the footing.
With mass concrete underpinning, crews dig under the footing in short, staggered sections so the wall is never left unsupported. Each section is filled with concrete, cured and packed tightly to the old footing before the next section is dug. It adds a new, deeper footing that bypasses weak or moving soil.
Screw piles reach deeper ground than hand excavation can. They need machine access and an engineer’s check on capacity, and they only start carrying load once they are connected to the footing.
Underpinning is the right tool when the footing itself is too shallow or too small, or when it sits on ground that can’t carry the load. Our underpinning service covers both methods, and the guide to which method to use compares beams and bases, piers, and other systems.
How does injection-based ground improvement work?
Injection improves the ground the footing already sits on. Small holes are drilled through the slab or beside the footing. Expanding resin is pumped in, filling voids, packing loose soil, and lifting the structure in small, monitored steps. There is no open excavation.
The research review cited above reports that the resin starts to harden within seconds and reaches its final properties within about 15 to 30 minutes. Injection happens in short shots, with levels checked between each one, to avoid lifting too far.
The injection works close to the footing. It is best at filling voids, firming up soft or loose zones and re-levelling a sound slab or footing that has dropped. It does not form a new footing, nor does it carry loads down to deeper ground the way a pile does.
Our resin injection page explains how we plan and carry out this work. If the problem is only a gap under a slab, void filling may be enough.
Underpinning vs geopolymer injection: how do they compare?
The key difference is where support comes from. Underpinning moves the load down to deeper ground through new concrete or piles. Injection strengthens the existing ground under the footing. That single difference drives depth, disruption, time, access and cost, as the table below shows.
| Factor | Underpinning (mass concrete or screw piles) | Injection (“geopolymer” or resin) |
| How it works | New concrete or piles carry the load to deeper ground | Expanding resin fills voids and firms the soil under the footing |
| Best suited to | Shallow or undersized footings; weak ground near the surface | Voids, loose fill, soft zones; a sound footing that has dropped |
| Depth | Down to a stronger bearing layer set by the engineer | Close to the footing; does not reach deep bearing ground |
| Disruption | Excavation, spoil, machinery for piles | Small drill holes; little mess |
| Time on site | Longer, staged digging, pouring and curing | Shorter; R&R’s resin page says some jobs take as little as two days |
| Curing | Concrete must cure between stages | Resin hardens within minutes (research review) |
| Access | Room to dig or bring in pile machinery | Small equipment; works in tight spaces |
| Main limits | Disruptive, slower, reactive clay needs careful sequencing | Not a new footing; soil type matters; not for failed footings |
| Cost drivers | Number of sections or piles, depth, access, and engineering | Number of injection points, resin volume, access, and engineering |
When is each method the wrong choice?
Underpinning is the wrong choice when the problem is a void or soft patch that injection could fix with far less disruption. Injection is the wrong choice when the footing has failed or the load must reach deeper ground. Using either for the wrong cause wastes money, and the movement returns.
Injection is usually the wrong choice when:
- The footing is cracked through, undersized or structurally failed
- loads need to reach firm ground well below the footing
- The cause is a fault that injection does not fix, such as a leaking pipe or blocked drain that keeps wetting the soil
- The engineer is not confident the resin will behave as needed in that soil (the 2021 review notes limited field evidence in some soils, such as peat)
Underpinning is usually the wrong choice when:
- The footing is sound, and the ground beneath has simply lost support through voids or erosion
- Access for digging or pile machinery is poor, and injection can do the job
- Staged digging beside a footing on reactive clay could add risk without fixing the cause
In both cases, fix the cause first. CSIRO’s guide to foundation maintenance explains how leaks, poor drainage and trees change soil moisture and move footings on reactive clay. If the leak keeps running, neither repair will hold.
Why should an engineering assessment come first?
The method should follow the diagnosis, not the other way round. An engineer works out why the building moved, what the soil is and how deep good ground sits. Only then can anyone say whether underpinning, injection, or a mix of both is right.
A good assessment usually covers:
- a floor-level survey to map how much and where the building has moved
- crack mapping and monitoring over time
- the soil type and site class, often from borehole testing
- checks on plumbing, stormwater and nearby trees
- the footing’s size, depth and condition
Our guide to diagnosing an unlevel building walks through these tests. You can check that an engineer is registered on the Engineers Australia National Engineering Register.
What does each method cost?
Costs depend on the method, the soil, how far the building has moved and how much work is needed. Our Melbourne underpinning page states that most underpinning jobs in Melbourne cost $5,000 to $30,000. Injection is often cheaper than underpinning for suitable problems, but only a site visit can price your job.
| Job size (R&R’s published Melbourne guide) | Typical range |
| Minor (one wall or small area) | $5,000 to $10,000 |
| Moderate (larger section of the house) | $10,000 to $30,000 or more |
These ranges come from our Melbourne underpinning page and cover underpinning methods generally. They are a guide, not a quote.
The main cost drivers are:
- the number of piers, sections or injection points
- depth to good bearing ground
- site access
- soil classification
- whether engineering design and testing are needed
When you compare quotes, check that each one is solving the same diagnosed problem. A cheap injection quote is not a saving if your footing needs underpinning, and a large underpinning quote may be more than you need if the issue is a void.
How do you check a contractor’s licence?
Ask for the contractor’s licence number and check it on the state register before you sign. Foundation repair is a type of building work, and each state sets its own licensing rules. A licensed contractor should give you the number without hesitation.
- NSW: a contractor licence is needed for residential building work over $5,000 in labour and materials, and for some specialist work at any value. See NSW licence categories of work.
- Victoria: Use the Building and Plumbing Commission’s Find and Check a Practitioner tool.
- Queensland: search the QBCC licensee register.
- ACT: check the builders’ list through Access Canberra.
What does Raise & Relevel actually do?
We carry out resin injection, traditional concrete and mass underpinning, screw piling, slab jacking, void filling and specialised grouting. We do not sell a product called “geopolymer”. Our injection service is resin injection, and we recommend it only where the diagnosis shows it suits the ground.
Because we do both underpinning and injection, we have no reason to push one over the other. Some homes need one method, some need the other, and some need a mix, such as injection along one wall and piles at a heavily loaded corner. If restumping is needed, that is outside our scope.
Our resin injection page states a 100-year warranty on our resin products and a 50-year warranty on our artistry. Ask us for the full warranty terms with your quote.
If you are also weighing a lift-only option for a sunken slab, see our comparison of slab jacking and underpinning.
We hold building licences in NSW (377694C), Victoria (CDB-U 73507), Queensland (15300551) and the ACT (20211579).
Frequently asked questions
Is geopolymer injection the same as resin injection?
For homeowners, both terms describe injecting an expanding material under a footing to fill voids and firm up soil. “Geopolymer” in science means an inorganic, cement-like binder, so the label on injection products is a trade name. Ask for the technical data sheet and let your engineer judge the product.
Is injection as permanent as underpinning?
Injection can provide a lasting result when the cause is fixed and the soil is suitable. It does not create a new footing, so it cannot serve as an underpinning when the footing has failed or when loads must reach deeper ground. Your engineer should explain the expected life of the repair.
Can an injection fix a house on reactive clay?
Sometimes. Injection can fill voids and re-level a sound footing on clay, and it avoids digging beside a footing that is already moving. But clay keeps shrinking and swelling with moisture, so leaks, drainage, and trees must be addressed,d too. An engineer decides if injection suits your site.
Which is cheaper, underpinning or injection?
Injection is often cheaper for voids and minor settlement, because there is less digging and less time on site. Underpinning costs more but solves problems that injection cannot. Our Melbourne underpinning page states that most jobs fall between $5,000 and $30,000. Only an on-site assessment can determine your home’s price.
Do I need to move out during work?
Most injection jobs let you stay home, because the work uses small drill holes and little equipment. Underpinning can be more disruptive, with digging, noise and machinery, and larger jobs may affect access to parts of the house. Your contractor should explain the plan before work starts.
Get the right repair for your home.
If you have cracks, sloping floors or sticking doors, and you are unsure whether you need underpinning or injection, start with a proper assessment. We will look at the cause, the soil and the footing, then recommend the method that fits. Book a foundation assessment or call 1300 053 835.
The short answer: “geopolymer injection” is a product label, not a separate engineering method. The real choice is between underpinning and injection, and the right one depends on the diagnosis. Fix the cause, get an engineer’s view, and check the licence of whoever quotes you.
