Resin Injection Underpinning Adelaide: How It Works, Where It Suits and Where It Does Not

• What it is: a ground improvement method, also called chemical underpinning, that injects expanding resin, a polyurethane product, into the soil beneath a footing or slab to fill voids, compact loose ground and re-level a settled structure.
• How it differs from traditional underpinning: there is no trench under the foundation. The work goes in through small drilled holes, so the excavation, spoil and concrete of traditional underpinning are avoided.
• Where it may suit: foundations that settled because of compaction or voids, sites with restricted access, internal floors, and older stone or shallow foundations where excavation is difficult.
• Where it may not: deeper movement, variable soil, and sites where moisture keeps changing the clay. One Adelaide engineering firm says resin is not always suitable for deeper movements or highly variable soil profiles.
• Reactive clay: a University of Newcastle paper calls the method a viable alternative to underpinning, then warns that resin-filled cracks cannot close when the clay is rewetted.
• How long it lasts: published figures run from 8 to 15 years to 10 to 20 years, and none comes from an independent long-term study.
• Cost: this page gives no figures. Published ranges, as general guides, are set out under underpinning cost.
• Next step: send photos of the cracks with a short note on the home, and an assessment can follow.

Resin injection underpinning treats the ground instead of adding a new footing, which is the opposite of concrete underpinning, where new foundations are poured beneath the old ones. A two-part polyurethane resin, an expanding resin, is pumped into the soil through small holes, expands as it reacts, and fills voids and cracks while pressing the surrounding soil together. Because the material is a chemical product rather than concrete or steel, the same work is called chemical underpinning, and Mainmark's own page says chemical underpinning is "also referred to as resin injection". Some businesses add terms such as polymer injection or ground consolidation. Resin underpinning and chemical underpinning are the same technique under two names, and resin underpinning is the term most Adelaide business pages use.

The Newcastle paper describes the chemistry. The resin forms from a polyol and an isocyanate mixed in set proportions, expands "in less than 10 seconds" and hardens "within one minute" in the laboratory, reaches about forty times its starting volume when free to expand, and can push with pressures of up to 10 MPa when confined. Once injected, the authors treat it as stable, since it is degraded only by ultraviolet light and a few solvents that should not be found under a home. Those are laboratory figures for unconfined resin. In soil, the resin follows the weakest path, fills and enlarges existing cracks, and sets in layers, so the result is a mixed material of soil and resin and not a clean block.

Two jobs are done by the same injection. Compaction and void filling improve the bearing capacity of the soil under the foundation. Where the structure and the site allow, further injection pushes the foundation up, which is the re-levelling that traditional underpinning rarely achieves. An engineer decides which of those two jobs your home needs, and the underpinning Adelaide overview explains how the other methods compare.

Most enquiries about chemical underpinning start with foundation movement. A home shows cracked brick walls, sticking doors, a sinking corner or a floor that has begun to slope, and the owner wants to know whether chemical underpinning could stabilise the foundations without excavation. The same signs can come from several causes, and the cause matters more than the method.

Published pages list the usual causes of subsidence and settlement. Soil shrinkage after a dry spell or flooding removes moisture from the clay under a footing. Erosion from a defective drain washes fine material out from beneath a slab. Heavy traffic can set loose soil vibrating, and earlier land use, including poorly compacted fill, can leave voids. On a reactive clay site the soil swells and shrinks with the weather, so the same foundations can show one pattern of movement in summer and another in winter. To follow that cycle step by step, see house underpinning.

Resin injection underpinning is aimed at one of these problems: weak or voided ground under a footing or slab. It does not repair the foundations themselves, it does not dry out wet soil and it does not remove a tree. A footing that dropped because of a void is a natural candidate, and footings over voids are where most injection starts, because the resin fills the void and improves the bearing capacity of the soil beneath it. A footing that moves every season because the clay swells and shrinks is a different problem, and in that case foundation repair starts with water and roots. A sunken slab is treated differently again, since injection can often be placed through the slab itself.

Resin underpinning is also used well beyond houses, on driveways, loading docks, roads and bridge approaches, according to business pages. This page stays with homes and small buildings, and large slabs are covered under commercial underpinning. Whatever the home, the first question is the same: why did the ground move, and has that cause stopped?

The words owners use for foundation trouble are loose, and chemical underpinning only addresses some of what they describe. Subsidence is the downward movement of the ground under a home. Settlement is the downward movement of the foundation or slab that follows it. A sinking floor, a sinking corner or sinking foundations all describe the result, and the result is the same whether the ground compacted, washed out or dried and shrank.

That difference matters, because the Newcastle paper says resin suits settlement caused by consolidation or compaction, which is irreversible, and raises concerns where the volume changes can reverse, as they do in reactive clay. Subsidence from a void is a job for void filling and compaction. Subsidence from seasonal shrinkage is a job for moisture control first. Settlement from a footing that was too shallow to begin with is a job for an engineer, who may favour traditional underpinning or piles to carry the load to firm ground.

A foundation assessment should therefore sort the problem before choosing a method. The table below shows how published pages and engineers treat the usual cases, and where the answer is not chemical underpinning. It is a guide to the questions, not a diagnosis, and the cause of the movement on your site is what an engineer's inspection will establish.

Inside the soil, chemical underpinning does three separate jobs, and a good quote says which of them the work relies on.

Void filling comes first. Voids form where soil has washed out, shrunk away or been poorly compacted, and a footing above a void has nothing to bear on. Resin that fills the void and sets gives the foundation something to rest on again.

Compaction and densification come second. As the polyurethane resin expands under confinement it presses the surrounding soil together, which raises the bearing capacity and the load bearing ability of loose or softened ground. The Newcastle paper reports pressures of up to 10 MPa when the resin is confined. Expansion and densification are separate effects: expansion fills space, and densification firms soil. Densification, the squeezing of loose soil into a denser mass, including fill beneath a slab, is why providers call the method ground improvement: the aim is better ground, not a new structural member.

Lift is the third job and the most visible. As more resin is injected, its expansion pushes against the foundation above, which is how a sunken corner or slab is re-levelled. Lifting needs control, because over-lifting is a risk the Newcastle authors raise for reactive clay. Laser levels and a stop point set by the engineer manage it, and a floor level survey before and after gives a record.

Void filling and compaction are the stabilising jobs. Re-levelling and lifting are separate decisions, and a quote that lumps void filling, compaction, re-levelling and lifting into one line has not said which it relies on. Void filling alone may stop further settlement where a void caused the problem, while lifting is optional and depends on what the engineer wants the finished levels to be. Careful lifting raises a sunken footing in small steps, and re-levelling by injection can be stopped at any point. The resin cures within minutes, expansion is nearly complete in seconds in laboratory tests, and the short cure time is one reason injection is done in staged shots rather than one pour.

None of these jobs adds a structural element tied into the foundation. That is the main difference from piles and concrete piers, and it is why engineers describe chemical underpinning as ground treatment while concrete underpinning is structural. Mainmark describes the resin as inert and says it remains in the ground to continue supporting the structure for many years, which is a business description. Whether ground treatment is enough for a given building is the question the engineer's report answers, and the underpinning Adelaide overview explains the structural methods in turn.

Resin injection, or chemical underpinning, may suit a footing that settled because the ground under it compacted or developed voids. Resin underpinning of that kind treats the cause, which is weak ground, and not the visible cracks. The Newcastle paper makes exactly that distinction, saying the approach is well suited where volume changes come from consolidation or compaction, since those changes are irreversible and the resin does not have to cope with the soil swelling back.

It may also suit sites where excavation is hard. OB Engineering, an Adelaide firm, calls resin injection less disruptive and quicker, which makes it suitable where access is restricted, such as beneath internal floors. A narrow side passage, a floor that cannot be lifted or footings that cannot be reached by a pile rig are the usual reasons an engineer considers it.

Older stone and brick foundations are a third case, and heritage buildings are the usual example. Brick walls over a stone footing may be fragile, and tree roots or a leaking pipe near an old stone wall are worth ruling out before any injection. MFS Engineering notes that underpinning heritage houses on shallow bluestone foundations is difficult and sometimes impossible, and in some cases injection may be the only viable option. Urathane Solutions describes staged injection and propping to stabilise a stone building in St Peters. For a listed building the heritage authority may have a say in the method, so mention the heritage listing to the engineer at the start.

Finally, a small, local drop in one of several footings can be a good candidate. A corner of a home that has sunk, or a patch of slab over a void, calls for less than a whole-perimeter solution. To tell a local problem from a whole-building one, see house underpinning.

The same sources that explain the benefits also list limits, and a decision should weigh both.

• Deeper movement and variable soil. OB Engineering says resin injection is not always suitable for deeper movements or highly variable soil profiles. The resin goes only where the layout puts it, and a profile that changes metre by metre is hard to treat evenly.
• Moisture that keeps changing. MFS Engineering says injecting resin below foundations will not prevent further movement if the soil is continually exposed to moisture fluctuations. A leaking pipe, poor drainage or a thirsty tree is a cause that resin does not remove.
• Uneven stiffness. The same firm warns that differential movement can become more pronounced if treated soil is firmer than the reactive soil under other foundations. Concrete piers raise a similar concern, and Magryn says adding a rigid support point can move cracking elsewhere.
• A footing that needs rebuilding. Resinject's own comparison lists a deteriorated or cracked-through footing, a renovation where excavation is already practical, and problems beyond weak ground as cases where concrete underpinning is still specified. Traditional underpinning of that kind is excavation-led and is chosen when the footing itself must be rebuilt.
• It cannot be undone. MFS Engineering and a rival Adelaide business both say resin cannot be reversed. The rival adds that concrete piers cannot be installed where resin has gone in. Treat that as one party's view.
• Internal drilling. Where injection has to go through the floor, holes can leave permanent marks or break older floor finishes, and that repair belongs in the scope. Injecting from inside brings less noise and dust than excavation, though drilling is not silent.
• A leaking pipe or drain. A leaking pipe under or beside a footing can wash out soil and soften it, and one rival says resin can enter cracked pipes. Locate pipes and test drains before injection, and repair any leaking service first.

Some of these voices sell the alternative, so none is neutral, and the disadvantages they list should be read with that in mind. Foundation Solutions sells screw piles, and its pages carry criticisms whose wording overlaps the MFS page, so they count as one voice. Urathane Solutions says that if cracks are hairline and stable, or the damage is in render or plaster, you probably do not need underpinning at all, and that a leaking pipe or poor drainage needs fixing first.

Failure of chemical underpinning is discussed more openly by engineers than by providers. The failure modes in the sources fall into four groups.

The first is a cause that was never fixed. A leak, a failed stormwater pipe or a tree keeps changing the moisture in the clay, and the foundation moves again. Every source mentions it, from MFS Engineering and OB Engineering to the resin providers themselves, and it is the main reason a repair is called temporary and not permanent.

The second is a mismatch between the movement and the method. If the settlement is deeper than the resin reaches, or the soil profile is highly variable, the injection can miss the layer that is moving. OB Engineering names deeper movements and variable profiles as cases where resin is not always suitable.

The third is uneven stiffness. MFS Engineering warns that if treated soil becomes firmer than the reactive soil under other parts of the home, differential movement can become more pronounced, and says that in some cases there was little or no difference in floor levels afterwards.

The fourth is behaviour in the clay itself. The Newcastle paper's concern about resin-filled shrinkage cracks and enhanced swelling is a possible failure mechanism, not an observed one. Foundation Solutions, which sells screw piles, goes further and says resin does not reach stable ground or connect to the foundation, but that is a competitor's argument and the paper does not test it.

Spotting failure early depends on records. Level readings before and after, dated crack marks and photographs let an engineer say whether the foundation is still moving. If a repair has failed, ask for an engineering report before a second repair, since MFS Engineering warns that a failed attempt can reduce the chance of success for later remediation. The disadvantages are real, they differ from site to site, and so are the benefits, and which outweighs the other depends on the site.

Most Adelaide houses with footing trouble sit on reactive clay, so the question for most owners is how resin behaves in clay that swells and shrinks. The Newcastle paper, by Olivier Buzzi, Stephen Fityus and Yasumasa Sasaki, is the clearest Australian treatment found in preparing this page, and it needs careful reading.

What it found. The resin follows the weakest path, fills and widens cracks in the clay, and can enter cracks as fine as 0.1 millimetre. At a reactive clay test site near Newcastle, air-permeability tests found the injected zone about 40 to 55 times less permeable than untreated soil at the depths tested. Water therefore has a harder time entering cracked clay, and a storm is less likely to wet it quickly.

What it warned. When clay shrinks it cracks, and resin fills those cracks. The paper says future wetting then cannot close them, which "could lead to enhanced vertical swelling that could over-lift the remediated foundation". Whether a treated foundation over-lifts depends on how much the clay swells, how strong the resin is under that pressure and the load on the foundation, and the authors call this a highly complex problem.

What it did not settle. The paper is a conceptual analysis from 2007. It says very little data was available on resin in expansive soil, concludes that resin injection can likely remediate deflected foundations under certain conditions, and promises a method for deciding what those conditions are. Mainmark Uretek helped fund the research, so read it as informed and not impartial.

The sources then disagree. Mainmark says AS 2870, the Australian standard for residential slabs and foundations, advises that traditional underpinning should generally be avoided on reactive clay. Magryn Engineering says concrete piers are particularly suitable for highly reactive clay with large swell-shrink potential. The standard itself was not read for this page, so it claims neither, and the engineer's report on your soil should settle it for your home.

Published lifespans for resin underpinning do not agree. MFS Engineering gives a range from 8 to 15 years, longer in ideal conditions, and calls resin injection underpinning a medium-term and not a permanent solution. A figure of 10 to 20 years circulates in search summaries, drawn from a forum thread and a UK site, with the same condition that the root cause must be fixed. Urathane Solutions says cured polyurethane resin is water resistant and does not break down in soil. Mainmark says concrete underpins built from sound concrete should last more than 50 years, which sets a benchmark for concrete and not for resin, and is several decades longer than the resin figures above.

Three things stand out. None of these is an independent long-term study of houses in Adelaide, and the MFS figure comes from a page whose wording overlaps a pile contractor's. The Newcastle paper does not give a lifespan, and says the resin itself is chemically stable under a home. And every source agrees on the main condition: if the cause of the movement is not fixed, the foundation can move again.

Some providers and a UK site say correctly applied resin can last for decades, and engineers question that for reactive clay. Permanent is the word owners use, and a permanent repair should be read as one that stays put in this ground, not one that lasts forever. Permanent or temporary therefore depends less on the resin than on the ground. A permanent result is more likely when the cause is fixed. A footing whose settlement came from a one-off event, such as a leak that has been repaired, is a better candidate for lasting results than one in clay that keeps swelling and shrinking. Ask any provider to explain the disadvantages of resin for your site, and what they promise in writing, what it covers and what it excludes, and compare that with the engineer's view of your soil.

A resin injection underpinning Adelaide quote is only as good as its assumptions, and the questions below turn a brochure into a scope.

• Did an engineer recommend it? A report from a structural engineer, and a geotechnical investigation where the soil is uncertain, should come before a method is chosen, and a geotechnical report should be kept with the assessment. The assessment should name the cause of the movement, the soil and the target levels, and the inspection should be recorded, since a geotechnical test and an engineering assessment are what a later buyer or insurer may ask to see. One resin business argues the ground is never certain, which is a reason to test it.
• Why resin for this building? Ask why resin underpinning suits this soil and this movement, and what concrete underpinning or piles would involve.
• What depth and how many points? The layout should state the injection depths and the number of points it assumes, since published depths range widely.
• How is the job priced and stopped? One business prices by the kilogram of resin with a contingency, another by a lump sum. Ask what happens if more resin is needed, and who decides when to stop.
• How will levels be monitored? Look for laser levels, a floor level survey or similar records before and after.
• Where will holes be drilled? If injection goes through floors, ask how they will be repaired.
• What if the movement continues? Ask what is promised in writing, and what the engineer will do next.
• Where are the pipes? Ask how drains and services will be found, since a leak under a footing can change the result.

Licence checks, contracts and questions that suit any underpinning method are covered under underpinning contractors, and why quotes differ is explained under underpinning cost. When you are ready, contact us to start an enquiry, and read about us to see who answers it.

Specialist businesses describe resin injection as quick. Mainmark says work is usually completed within one or two days, and MFS Engineering says typically less than two days. Urathane Solutions says most houses take days rather than weeks, and describes a two-storey project with 386 injection rods that took two weeks. Larger and more complex jobs therefore run longer, and the quote should give the expected time. Traditional underpinning and concrete underpinning both need excavation under the footing, and concrete underpinning also waits for the concrete to cure, which is why published pages give weeks for concrete underpinning and days for resin.

Minimal disruption is the other selling point. Compared with the excavation, spoil and concrete of traditional underpinning, drilling small holes in a non-invasive process makes less mess, noise and dust, and published pages say owners can usually stay in the home. Concrete underpinning needs excavation beside and beneath the foundation, and the disruption to the garden and paving is greater. These are business descriptions, so ask where equipment will stand, which footings it will reach, which rooms need clearing and whether floors will be drilled from inside.

Access can still matter. A narrow passage limits where equipment can go, and footings behind a fence or paving are the hardest to reach, and injection from inside needs the floor covering lifted or drilled. The engineer's layout should show where each hole goes, so you can see in advance what the job will touch.

For resin injection underpinning Adelaide owners ask about, whether it or any chemical underpinning needs approval in South Australia could not be verified from a primary source, so put the question to the council or a certifier before any booking. Foundation Solutions, a Queensland business, argues that resin is a soil treatment and not building work, and that it does not appear to require a building application in that state. That is a statement about Queensland, and it does not decide the question here.

The same caution applies to building indemnity insurance. How the South Australian rules work for domestic building work above a value threshold where approval is needed is explained under underpinning contractors. Whether a resin job falls within them depends on how the work is classified, which is a question for the council and the contractor.

Insurance for underpinning under your own policy is a separate matter, covered on the underpinning Adelaide overview. Keep every underpinning report and quote together, because an insurer or a later buyer may ask for them.

The work is not finished when the resin sets. Cracks that have already formed are not repaired by the injection, and OB Engineering says they usually need separate repair, such as sealants or replastering and repainting, once stability has been achieved. If moisture, tree roots or leaking pipes are not addressed, it adds, new cracks may appear even after underpinning.

Watching the foundation is the other half, and monitoring does not stop when the crew leaves. A floor level survey repeated a year later, and again after a wet winter and a dry summer, shows whether the foundation is stable. MFS Engineering suggests professional inspections every two to three years, drainage control around footings and stable soil moisture. Poor drainage is the cause most often missed, and drainage faults are quick to check. Dated pencil marks at the ends of each crack and photographs from the same spot each season give you your own record, and an engineer can use them to say whether movement has stopped.

Then deal with the causes. Fix leaks and downpipes, keep water away from the foundations and think hard about large trees close to the home. Those steps are covered under foundation repair, and the signs worth monitoring are listed under house underpinning.