GCL pond lining around the New Forest

A clay bed on free-draining ground

Much of the land around the Forest, from the plateau near Fordingbridge to the sandy country south of Ringwood, drains far too quickly to keep water in a hollow. What a GCL adds is the clay that the ground is missing, as a sheet of sodium bentonite carried in fabric that tightens into a seal once it is wet and held down.

Owners and land managers here often want a pond that reads as part of the heath and woodland, with soft margins, plants and no visible plastic. A covered GCL gives exactly that, and should a root or a hoof nick it, the clay expands to plug the damage.

When it isn't the right liner

A GCL needs a cover of soil or stone to seal, gentle side slopes to hold that cover, and water that suits the clay. Steep banks, a pond that will be drawn down hard, or contents that would attack the bentonite all point to a welded membrane instead.

High groundwater in the valley bottoms is the other thing to raise. Water rising from below can wet the clay before the cover is on, or push against the liner when the pond is low. Your designer decides how that is handled; we check the plan suits a clay liner before recommending one.

Wet weather matters too. A Forest winter can leave the formation soft and puddled for weeks, and a GCL laid onto standing water starts to swell unconfined. Spring and summer are usually the easier seasons to lay one here.

• Confirm the water source and use
• Check the slopes will carry the cover
• Inspect and sign off the prepared formation
• Lay, overlap and seal, then record each lap

Laying and handover

The basin is dug and trimmed by your contractor, smooth and compacted with no roots or stones left proud. With that surface signed off, the rolls go out side by side with a broad overlap at each join, and paste or granules of bentonite are worked into the lap. Edges and any pipes get the same treatment; nothing on a GCL is welded.

Every lap and detail goes on our record sheet, which also states how much cover the liner must have. Your contractor places the cover layer straight away, before the pond is filled, because clay that hydrates with nothing on top of it is ruined.