GCL pond lining in Hampshire

Clay beds in chalk country

Hampshire has a long habit of clay-lined ponds. The downland dewponds were puddled by hand, and plenty of village and farm ponds were sealed the same way. A GCL does that job in a factory-made sheet: a core of sodium bentonite stitched between two fabric layers, which takes up water and closes into a very tight barrier.

That suits the kind of schemes the county asks for. Wildlife ponds and wetland features in new housing, ponds restored on estates and nature land, and planted basins where a visible black membrane would look wrong all benefit from a bed that behaves like natural clay once it is covered.

Chalk water and the suitability check

Water from the chalk is hard. Calcium in the water that hydrates a sodium bentonite liner is one of the things that can change how the clay performs over the years, so our first step on a Hampshire job is to ask what will fill the pond: rain, a borehole, a chalk stream feed or runoff from roofs and roads.

A GCL also needs confinement to seal, which means a soil or stone cover over it, and it prefers gentle side slopes. If the water, the profile or the use don't suit it, we say so and recommend a welded membrane instead, which we also install.

• What water will fill and top up the pond
• Side slopes gentle enough to hold a cover layer
• A formation free of sharp flints and debris
• A contractor ready to place cover promptly after we hand over

Laying it

Your groundworker prepares the formation, and on chalk that means picking out the flints and getting a smooth, compacted surface. No roll is opened until we have walked that surface and signed it off. Each panel then overlaps its neighbour by a wide margin, and each of those laps is dressed with bentonite, as paste or granules. There are no welds in a GCL; the overlaps and the clay do the work.

Pipes, inlets, edges and any structure in the pond get the same paste or granules, and every detail is ticked off on the QA record as it is finished. When we leave, the records state how deep the cover over the GCL must be; your contractor spreads that layer and only then lets the pond fill.