GCL pond lining in Coventry

Where a clay liner earns its place

Inside a GCL is a thin bed of sodium bentonite, stitched between two fabrics. Buried under soil and soaked, that clay expands until water barely passes through it, and it keeps that ability: a small hole made later by a root or a sharp stone is closed by clay pushing into it. For a wildlife pond on the edge of a nature area or a planted SuDS feature on a new Coventry estate, that soft, soil-covered bed is often exactly what the ecologist wants.

It is not the answer for every pond. GCLs need confinement under soil or water, prefer gentle bank slopes, and must suit the water that will sit on them. If a design has steep sides, exposed edges or aggressive contents, a welded membrane is the better choice, and we would say so.

Checking the site first

Before we recommend a clay liner we look at the profile, the cover the design allows for and the source of the water. Runoff from roads and roofs, a brook, or groundwater each behaves differently against bentonite, and the suitability check is there so nobody finds out after handover.

Then the formation. Your groundworker trims and compacts it; we inspect it for sharp stones, ruts and soft patches and give it our sign-off before any roll is unwound. Whatever is under a clay liner shows through in how well it works.

Laying, lapping and detailing

The rolls overlap by a wide margin, and each lap is dressed with bentonite, as paste or loose granules, before the next roll covers it. Nothing on a GCL is welded: the joint closes because the clay in the overlap swells. Inlet pipes, outlet structures and the top of the bank each take their own collar of paste, and our records list each lap and detail as it is finished.

On a park pond with a timber dipping platform or a stone headwall, those fixed structures take most of the time. A neat paste collar at each one is what keeps the water level steady in the years after.

• Suitability check on profile, cover and water chemistry
• Formation inspected and signed off before laying
• Laps closed with paste or granules
• Every pipe, headwall and edge given its own detail
• QA records stating the cover depth required

Cover follows straight after

After handover, the soil goes on to the depth our records state, and only then is water let in. That order matters. Bentonite that meets rain while it is still uncovered swells in the wrong places and loses its seal, so on Coventry jobs we agree the covering programme before laying starts and unroll only as much as can be buried that day or the next.