Polyurethane resin injection is marketed as fast, non-invasive and cheap. Early in our journey we looked hard at whether it belonged in our toolkit. For the reactive-clay soils that dominate South East Queensland, we concluded it did not. Here is the reasoning, so you can make an informed choice for your own home.
What resin actually does
Resin injection pumps an expanding polyurethane foam into the ground beneath a slab. The foam fills voids and can, in the right conditions, lift a slab. Its job is essentially to fill gaps in the soil. It does not do what a foundation is meant to do: permanently carry the load of the building on stable ground, regardless of how the soil above behaves.
The problem with reactive clays
Reactive clay swells when wet and shrinks when dry, sometimes dramatically, and it does this season after season. That is the zone of influence: the upper soil layers that never stop moving. Resin injected into this zone stays within the very material that is causing the problem. When the clay next shrinks or swells, the movement returns, the cracks reopen, and the homeowner is back where they started, often after significant expense.
Four reasons it fell short for us
- It does not reach stable ground. Resin fills voids but does not establish a connection to competent, load-bearing strata.
- It does not connect to the footing. There is no mechanical lock to the foundation, so it cannot stabilise or lift with precision or permanence.
- It reacts poorly in reactive clays. In clay-rich or black soils, resin can expand unevenly or break down over time.
- It is hard to monitor or certify. There is no torque data, no engineering design and often no load testing, making results difficult to verify.
What we chose instead
We exclusively use screw pile underpinning because it secures the structure below the zone of influence, on stable strata, and mechanically connects to the footing so the home can be lifted and re-levelled with precision. Every pile is torque-monitored and the solution is engineered and certifiable. We have been called in to rectify a number of failed resin injections, which only reinforced the decision. If a previous resin repair has not held, an honest, engineered assessment is the right place to start.