Polymer Underpinning in Australia - How injection restores AS2870 footing performance
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Polymer (Geo-Polyurethane) Underpinning in Australia
In the vast and varied landscapes of Australia, where reactive clay soils dominate much of the terrain, foundation issues are a common challenge for homeowners and builders alike. Cracked slabs? Cracked brick walls? Cracked plasterboard walls? From cracking walls to uneven floors, subsidence can wreak havoc on structures, leading to costly repairs if not addressed promptly. If you require a quick fix in a short period of time, such as when when you wish to sell the property, maybe this can be the best solution. Fortunately, there is an innovative polymer-based solutions that not only stabilize foundations but also lift and re-level them with minimal disruption. In this blog, we'll explore how polymer injection is transforming underpinning, its role in jacking up existing footings, houses, and pavements, its compliance with AS2870, and why it's become the go-to choice across the country.
Understanding Polymer Underpinning: A Modern Approach to Soil Stabilization
Polymer underpinning, often referred to as resin injection or chemical underpinning, involves injecting expanding polyurethane resins deep into the ground beneath a structure's foundation. These resins, specially formulated for geotechnical applications, expand upon injection to fill voids, compact weak soils, and provide a stable base. Unlike traditional methods that require extensive excavation and concrete pouring, polymer injection uses a patented keyhole delivery system—small holes drilled through the slab or footing allow precise application without tearing up your yard or driveway.
This technique is particularly effective for addressing subsidence caused by reactive soils, which swell and shrink with moisture changes. In Australia, where soils like black earth in Queensland or red clays in Victoria are prevalent, polymer resins mimic natural soil behavior while offering superior strength, capable of supporting up to 50 tonnes per square meter.
Jacking, re-levelling and stabilising footings, slabs, houses and pavements—while staying aligned with AS 2870
If you work in residential or light commercial buildings in Australia, you’ve seen it: cracked plasterboard, cracked bricks, cracked door frames, “bouncy” or out-of-level floors, and driveway slabs that suddenly become trip hazards. Much of that comes back to ground movement—particularly reactive clays and moisture variation—conditions that AS 2870 is fundamentally written to address (through site classification, footing performance and movement management).
Over the last few decades, polymer injection underpinning (geo-polyurethane / resin injection) has become a mainstream remediation option because it can stabilise and re-level with far less disruption than traditional mass-concrete underpinning. For many homeowners and asset owners, it’s the practical middle ground between “do nothing and watch cracks grow” and “excavate the entire perimeter footing system.”
This blog explains how polymer underpinning works, where it fits with AS 2870 / NCC expectations, and why it’s become so popular in Australia—while being honest about limitations and when engineers should recommend alternatives.
1) What “polymer underpinning” actually is
Polymer underpinning (often called chemical underpinning, resin injection, slab lifting, or ground improvement) involves injecting an expanding structural resin beneath or adjacent to a footing/slab. As the material reacts and expands, it can:
- Densify loose soils and fill voids
- Improve bearing support locally under a slab/footing
- Lift and re-level slabs/footings incrementally (controlled jacking effect)
- Reduce ongoing differential movement by re-establishing uniform support
In Australia, resin injection has been widely used for decades (at least since the 1990s), largely because it is fast and comparatively non-invasive. Though bare in mind some engineers also note the method is sometimes used without adequate diagnosis, which can reduce success on some sites long term.
2) Typical applications: footings, houses, pavements, industrial slabs
Polymer injection is commonly specified for:
A) Residential houses (strip footings, slabs-on-ground)
- Localised re-support of footing/slab zones affected by loss of founding support
- Controlled re-levelling to reduce serviceability issues (doors/windows binding, internal cracking)
B) Pavements and external flatwork (driveways, pathways, patios)
- Trip hazard removal via lift and support re-establishment
- Rapid return-to-service (important for retail/industrial access)
C) Commercial/industrial slabs
- Re-levelling differential settlement areas (often around trenches, services, or poorly compacted fills)
- Minimising downtime (some providers report very rapid curing/return-to-service)
3) Why polymer underpinning is so popular in Australia
(i) Minimal disruption compared with traditional underpinning
Traditional underpinning often requires excavation in short bays, temporary support, spoil removal, access constraints, and a longer program. Polymer injection is typically “keyhole” drilling with small diameter holes, limited excavation, and rapid reinstatement.
(ii) Speed: fast stabilisation and fast re-use
Many resin injection providers market “same day” outcomes; cost and schedule discussions in the industry often point to completion within a day for certain scopes, which drives homeowner adoption. Especially when you wish to sell a property, at a real estate auction, this quick fix can certainly help.
(iii) Reactive clay context: differential movement risk with rigid underpins
A key concept in AS 2870 practice is that you’re dealing with movement potential—often not uniform across the footprint. Industry commentary notes that in reactive clay conditions, traditional rigid underpins can create differential movement between the stiffened zones and the rest of the structure, and that AS 2870 generally warns against traditional underpinning approaches where reactive clays are the underlying driver.
(As always, engineers should read and apply the Standard directly to the scenario; don’t rely on marketing summaries.)
(iv) Precision control during lift
Polymer injection can be performed with incremental lift monitoring (laser levels/survey points) allowing small adjustments—especially valuable when re-levelling slabs where you’re aiming to reduce serviceability issues rather than “force” the building back to perfect level.
(v) Strong market maturity and contractor availability
There are now multiple established Australian providers and broad consumer awareness, which tends to reinforce adoption. For example, Urathane Solutions positions its system as non-invasive resin injection and cites a long operating history and a large number of repairs completed.
4) Where AS 2870 and the NCC fit in (what “compliance” really means)
First: AS 2870 is primarily about designing slabs and footings, not “how to inject resin”
AS 2870 focuses on:
- Site classification (reactivity, expected ground movement)
- Footing/slab performance criteria
- Design detailing and construction considerations to manage movement
The NCC then references Australian Standards (including AS 2870) as part of satisfying Deemed-to-Satisfy pathways for housing in relevant schedules/jurisdictions.
So how do you align a remediation method with AS 2870 intent?
For underpinning/remediation, “compliance” typically means demonstrating that the proposed works:
- Are based on an appropriate diagnosis of movement mechanism (not just treating symptoms)
- Are suitable for the site’s AS 2870 classification and movement characteristics
- Achieve a performance outcome consistent with a stable footing system (serviceability and safety expectations)
- Include verification (survey/monitoring/QA) that the intended performance has been achieved
In practice, that looks like an engineered process:
Step 1 — Identify the mechanism (don’t skip this)
Common mechanisms include:
- Reactive clay shrink/swell from moisture variation
- Loss of support from washout, drainage leaks, plumbing failures
- Poorly compacted fill, soft spots, voiding
- Edge heave or localised subsidence
- Tree influence, poor surface drainage, altered site levels
Polymer injection is often excellent for void filling/densification + local re-support, but if the driver is ongoing moisture imbalance (e.g., drainage not fixed), movement can continue.
Step 2 — Map the building response (serviceability-focused)
Record:
- Crack patterns (width, direction, location)
- Floor levels and differential settlement “contours”
- Door/window racking observations
- External slab/pavement profile
Step 3 — Engineer a scope that matches AS 2870 movement reality
In reactive clays, the objective is usually:
- Reduce differential movement and restore uniform support
- Avoid creating a “stiff island” that increases curvature elsewhere
- Combine ground improvement with moisture management measures (surface drainage, plumbing repairs, tree management, control joints as appropriate)
Step 4 — Specify QA/verification that is defensible
A good engineering scope for polymer underpinning normally includes:
- Injection plan layout (grid/lines, depths, target zones)
- Lift tolerances (max lift per pass, max total lift, stop criteria)
- Real-time monitoring method (laser level, dial gauges, survey control)
- Post-works level survey and photographic records
- Any required waiting/monitoring period before cosmetic crack repairs
5) How polymer injection “jacks” a slab/footing (engineering concept)
Polyurethane systems expand and generate pressure as they react. When injected beneath a slab/footing:
- The expanding resin initially migrates into voids and weaker zones
- As confinement increases, pressure rises and the slab/footing can lift
- Lift is controlled by staged injection, sequencing, and real-time level monitoring
From an engineering perspective, you’re trying to achieve:
- Improved contact/support under the foundation element
- Reduced differential settlement (serviceability improvement)
- A stable condition that does not rely on “ongoing lift” to maintain performance
6) Where polymer underpinning is a great choice (and where it isn’t)
Great fit
- Localised loss of support (voids, washout, soft spots)
- Slab re-levelling where downtime/disruption must be minimal
- External pavements and approach slabs
- Many residential serviceability remediations where controlled lift is feasible
Caution / may not be appropriate alone
- Deep-seated bearing failures requiring piles/piers
- Significant uncontrolled drainage issues not addressed
- Highly variable ground that needs deeper founding solutions
- Structures where lift poses unacceptable risk to services/finishes
Engineers should also acknowledge industry debate: resin injection can be effective, but outcomes depend heavily on diagnosis, method control, workmanship and whether the root cause is treated, not just the symptom.
Ensuring Compliance with AS2870: The Australian Standard for Reliable Foundations
AS2870, the Australian Standard for Residential Slabs and Footings, is the benchmark for designing and constructing footings that withstand soil movement over a building's 50-year design life. It classifies sites based on soil reactivity (from A for stable to P for problem sites) and mandates performance criteria to limit damage from foundation shifts.
Polymer underpinning aligns seamlessly with AS2870 by treating the underlying soil issues that cause non-compliance, such as excessive moisture variations or voids. When applied correctly:
- It stabilizes reactive clays, reducing swell-shrink cycles that AS2870 aims to mitigate through proper site classification and drainage.
- Engineered resins meet or exceed the standard's requirements for load-bearing capacity and durability, often backed by geotechnical reports and certifications.
- The method avoids the pitfalls of traditional underpinning, like altering site drainage, which could violate AS2870's emphasis on maintaining stable moisture conditions.
We worked with specialist contractor such as Urathane Solutions, to review and include full engineering oversight compliance documentation, ensuring your repair not only fixes the problem but also upholds the highest Australian standards.
Why polymer underpinning fits AS 2870 logic for slabs & strip footings
1. Most AS 2870 slab failures are serviceability, not collapse
Typical defects:
- Cracks
- Out-of-level floors
- Door/window racking
- Local slab edge settlement
These usually result from:
- Moisture change in reactive clay
- Washout or plumbing leaks
- Poorly compacted fill
- Edge moisture imbalance
Polymer injection addresses support loss, which is exactly what AS 2870 is trying to manage.
2. Polymer injection restores uniform bearing, not rigidity
A critical AS 2870 principle is:
- Uniform support is better than rigid local restraint
Traditional mass-concrete underpinning can:
- Create stiff “hard points”
- Increase curvature and cracking elsewhere in reactive soils
Polymer underpinning:
- Densifies soil
- Fills voids
- Re-establishes contact beneath the slab/footing
- Reduces differential movement, not just vertical displacement
This aligns very well with AS 2870’s movement-management philosophy.
3. It does NOT change the footing design assumptions
Importantly:
- Polymer underpinning does not alter slab geometry
- Does not change load paths
- Does not increase imposed loads
- Does not convert the footing system into something else (e.g. piles)
It simply restores the founding conditions that the original AS 2870 design assumed.
When polymer underpinning would NOT be considered appropriate
Even for slabs and strip footings, engineers should be cautious if:
- The house was never AS 2870-compliant (e.g. undocumented fill, uncontrolled construction. Rare, but can happen to very old house)
- There is deep-seated bearing failure beyond shallow founding depth
- Ongoing moisture problems are not rectified
- The slab requires structural strengthening, not just re-support
- Excessive lift would risk damaging services or finishes
In these cases, polymer injection alone may not satisfy performance expectations.
If your home, slab, driveway, or pavement was originally constructed to AS 2870 and is now showing signs of movement such as cracking, uneven floors, or settlement, polymer injection underpinning offers a proven, non-invasive way to restore support and improve performance without the disruption of traditional underpinning. When correctly assessed and engineered, this method aligns with the intent of AS 2870 by re-establishing uniform footing support and reducing differential movement, particularly in Australia’s reactive soil conditions. If you’re unsure whether polymer underpinning is suitable for your property, an engineering assessment is the critical first step. Contact us or someone like Urathane Solutions to discuss your site conditions and find out whether polymer injection is an appropriate, compliant, and cost-effective solution for your underpinning needs.
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