Resin injection has a marketing problem. It is sold to homeowners as the easy alternative to underpinning, and to engineers as a niche tool for specific ground conditions. Both are partly right. The honest answer to the headline question is that resin injection is sometimes the correct engineering solution, sometimes wholly wrong, and the difference is decided by what is actually failing. For the alternatives, see mass concrete underpinning and mini piling.
What resin injection actually does
Geopolymer resin is injected into the soil beneath a foundation through small ports. It expands, fills voids, increases the density of granular soil, and in many cases lifts the foundation back to level. It treats the soil, it does not strengthen the foundation itself. The full method write-up is on the resin injection service page.
When it works well
- Sandy or silty soils that have lost density (often from leaking drains).
- Settlement of ground-bearing slabs that need re-levelling without breaking out.
- Voids beneath foundations caused by washouts.
- Properties where access for traditional underpinning would be very difficult.
When it is the wrong tool
- The foundation itself has cracked or rotated, there is nothing to inject around.
- Active clay shrinkage near a tree, the resin will not stop the tree drinking next summer.
- Heave (clay swelling), adding more pressure makes it worse.
- Deep failure where the bearing layer is many metres down.
How insurers and lenders view it
The technology is mature and accepted by most major UK insurers when specified by a competent engineer with appropriate guarantees. Mortgage lenders will generally accept a property with a resin injection history provided documentation is in order. Treat any contractor who is vague about insurer acceptance with caution. The wider toolbox is laid out in underpinning alternatives.
The two questions to answer first
Before agreeing to resin injection, two questions decide it honestly. First: what is the soil? Second: is the failure in the foundation, or in the soil under it? A trial pit and a soil log should give you the answer in a day, and it is the cheapest money you will spend on the whole job.