Cracks in your walls, sinking floors, and doors that jam for no apparent reason, these are more than just cosmetic annoyances. They can be the first signs of a deeper structural issue: foundation problems. When left unaddressed, these symptoms can lead to costly and sometimes dangerous outcomes.
Underpinning is a proven method used to stabilise and re-support foundations affected by subsidence or movement. Acting early is critical; the longer you wait, the more extensive (and expensive) the damage can become.
This guide will walk you through two common underpinning methods: traditional concrete underpinning and modern resin injection, like the one we use here at Resinject, to help you make an informed decision for your home.
Understanding Foundation Problems
Foundation movement or subsidence can be caused by several factors, including:
- Soil shrinkage and swelling: Expansive clay soils contract when dry and swell when wet, putting pressure on foundations.
- Tree roots: Large trees near a home can draw moisture from the soil, leading to uneven settling.
- Moisture variation: Poor drainage, leaking pipes, or extreme weather changes can alter soil stability.
When these shifts occur, the foundation may sink or become uneven, leading to structural damage. That’s where underpinning comes in, strengthening and re-stabilising the foundation by extending its support.
Early warning signs include:
- Cracks in brickwork or internal walls
- Gaps around windows or doors
- Uneven floors
- Doors and windows that stick or won’t close
- Hollow sounding floors
Catching these signs early allows for a more straightforward, less costly repair.
How Each Method Solves the Problem Differently
The two approaches deal with foundation movement from opposite directions. Concrete underpinning builds new physical support by excavating beneath the existing footing and casting deeper piers or footings that transfer load to firmer material further down.
Resin injection works on the ground itself. Polyurethane resin is injected into the soil beneath or alongside the affected foundation, where it expands, fills voids and densifies loose or softened material. That soil densification restores load-bearing capacity, and where the structure and site allow, it also permits controlled re-levelling.
The distinction matters because cracked brickwork and sloping floors are symptoms, not causes. Resin injection targets the weak ground driving the movement rather than the visible damage above it.
The process runs: assess, drill small access points, inject RSJ170 resin, allow the resin to expand and consolidate the soil, monitor movement, then re-support or re-level. Injection depth is set by site conditions and geotechnical information, typically somewhere between 800mm and 6.5 metres.
When Concrete Underpinning Might Still be the Right Choice
Resin injection is not a one-size-fits-all solution. The right method depends on the cause of movement, soil conditions, footing design and engineering requirements.
Traditional underpinning is still specified in several situations:
- The existing footing has deteriorated, cracked through or needs substantial reconstruction.
- Excavation is already practical because the property is undergoing a major renovation or extension.
- The problem extends beyond weak ground and calls for a different structural repair altogether.
Our assessments are carried out by licensed builders and structural consultants, and if resin isn’t the right answer for your site, we’ll tell you.
Concrete Underpinning vs Resin Injection: A Decision Table
| Consideration | Concrete Underpinning | Resin Injection (Resinject) |
|---|---|---|
| How the method works | New concrete piers or footings built beneath the existing foundation | Expanding resin injected to consolidate soil and restore support |
| Access required | Room for machinery, spoil removal and open excavations | Small injection points, workable in tight or internal areas |
| Excavation | Usually requires excavation beneath or alongside the foundation | No major excavation required |
| Typical duration | Often weeks, depending on scope and concrete curing | Many residential works finish within a day, subject to site conditions |
| Disruption | High, with noise, spoil and restricted site areas | Minimal, with most equipment kept outside the building |
| Reactive clay and variable soils | Design constraints apply under AS2870 | Resin formulations selected to suit clay, sand and unstable fill |
| Void filling and soil densification | Not the purpose of the method | Fills voids and compacts surrounding soil as it expands |
| Controlled re-levelling | Limited | Possible on suitable structures, monitored by laser level |
| Impact on gardens, driveways and interiors | Reinstatement of surfaces is often needed afterwards | Surfaces largely undisturbed |
| Labour and equipment | Labour intensive, heavy plant on site | Small crew, specialised injection equipment |
| Cost drivers | Excavation, labour hours, reinstatement, extended timeframes | Volume of resin, depth, number of injection points |
| When it’s recommended | Where deeper footings or footing reconstruction are specified | Where weak, loose or voided ground is causing the movement |
Every foundation repair should begin with an assessment of the cause of movement. Timeframes, costs and suitability vary by site.
What Affects the Best Underpinning Method for Your Property?
Two houses in the same street can need different solutions. These are the factors that shape the recommendation:
- Cause of movement: reactive clay, erosion, leaking pipes, poorly compacted fill, tree roots, washouts or shifting groundwater.
- Depth of weak ground: whether instability sits directly beneath the footing or extends well below it.
- Condition of the structure: crack pattern, footing type, slab behaviour and how far the damage has spread.
- Site access: whether excavators and spoil trucks can reach the affected area safely.
- Downtime tolerance: a warehouse floor, strata complex or shopfront has different constraints to a vacant house.
- Engineering advice: whether a consultant calls for ground improvement, new footings, or additional structural work.
It’s also worth separating two different goals. Stabilisation prevents further movement by improving ground support beneath the structure. Re-levelling goes a step further, lifting the structure back towards its original position, and it’s only attempted where the building and surrounding conditions allow controlled lifting.
What to Expect During Resin Underpinning
Work starts with an on-site inspection of the affected area and the visible signs of movement, followed by a plan setting out injection locations and depths. Small injection points are then created, and RSJ170 resin is delivered at controlled depths beneath the footing.
As the resin expands it fills voids, compacts the surrounding soil and improves support. Where lifting is part of the scope, precision laser monitoring tracks movement in real time so the structure rises evenly. The resin is non-toxic and Australian made, and our systems are ISO 9001, 14001 and 45001 certified.
Sites are left clean, with surrounding surfaces largely untouched. In all residential cases, occupants remain at home while works are completed.
Which Method Suits Which Scenario
- Stepped brick cracks, jammed doors and sloping floors: where subsidence traces back to changing soil conditions and the footing itself is sound, resin injection is usually the first option considered.
- Sunken driveway, pool surround or warehouse slab over a void: void filling and slab jacking can consolidate the ground and re-level the concrete with little operational downtime.
- Building needing new structural support below the footing: resin injection and concrete underpinning can be specified.
- Movement driven by leaking plumbing or poor drainage: fix the water source alongside the ground remediation, or the problem returns.
These are examples only. The method is confirmed after a site-specific assessment.
Comparing Quotes Fairly
Price alone is a poor comparison between the two methods, because the scopes of work differ. A concrete quote carries excavation, spoil removal, extended labour and reinstatement of gardens or paving. A resin quote reflects resin volume, injection depth and point count.
What moves the number either way: how many areas are affected and how large they are, how deep the instability runs, whether void filling, slab re-levelling or crack repairs are also needed, access constraints such as internal work areas or narrow side passages, and any engineering inspection or reporting required. Our guide to underpinning costs breaks this down further.
For suitable projects, resin injection reduces labour, excavation, reinstatement and downtime costs. But the most cost-effective option is always the method that addresses the underlying cause of movement.
Long-Term Performance of Resin Underpinning
RSJ170 is engineered to remain dimensionally stable in the ground and resist shrinkage over time, and it’s backed by a 50-year product warranty against shrinkage, subject to warranty terms.
Durability also depends on diagnosis. Resin improves soil support, so if a leaking service, blocked drain or ongoing erosion continues to remove or soften material, that source needs fixing too. Treat resin injection as one part of a remediation plan where water management or structural repairs are also in play.
Why Property Owners Choose Resinject for Chemical Underpinning
- 100+ years of combined experience across ground engineering and construction, applied to assessment and delivery.
- Over 14,000 buildings remediated, spanning residential slabs through to mining and civil infrastructure.
- Australian-engineered RSJ170 resin, formulated for local soil conditions and tested to Australian standards.
- Licensed builders and structural consultants overseeing every assessment and site works.
- ISO 9001, 14001 and 45001 certification covering quality, environmental and safety management.
- 50-year product warranty against shrinkage for long-term confidence in the repair.
Frequently Asked Questions
Resin injection is a form of underpinning, commonly called chemical or resin underpinning. Compared with the traditional concrete method, it’s the stronger option when the problem is weak, loose or voided ground and you need minimal excavation, faster completion and less disruption. Where footing reconstruction are required, concrete underpinning remains the appropriate choice.
Correctly designed and installed resin injection is intended as a long-term ground improvement solution, and Resinject backs the product with a 50-year warranty against shrinkage. Future ground behaviour still depends on site conditions, so unresolved drainage faults, plumbing leaks or continuing erosion should be rectified alongside the repair.
It depends on what’s causing the movement. Drainage repairs, leak rectification, void filling, slab re-levelling or targeted crack repairs may resolve the issue on their own. Where the foundation genuinely needs extra support, resin injection is the non-invasive alternative to traditional concrete underpinning on suitable sites.
There’s no single best method for every property. Soil type, the cause and depth of movement, site access, footing design, damage extent and engineering requirements all shape the answer. Resin injection is frequently preferred because it stabilises ground with minimal excavation and disruption, while concrete underpinning still has its place in specific situations.
