GCL pond lining in Manchester

Why not just puddle the local clay?

Glacial till looks like it should hold water, and in places it does. But it is a mix of clay, sand lenses and boulders laid down unevenly by ice, and a pond cut into it can find a sandy seam and drain through it. Puddling clay by hand is slow and hard to check.

A GCL is sodium bentonite locked between two geotextile sheets at the factory, so its thickness is the same everywhere. Wetted once it is buried, the clay expands into a near-watertight layer, and it presses into small punctures if a root or stone finds the liner later. The pond still has a clay bed, which suits planting and wildlife.

Where it fits around the city

The best candidates are gentle-sided basins that will be covered in soil or water: wildlife ponds in country parks along the Mersey valley, wetland features on housing schemes, and amenity water where a black membrane edge would look out of place. Steep banks, exposed edges or aggressive contents point back to a welded membrane instead.

• Wildlife ponds on the urban fringe
• Shallow wetlands on new estates
• Amenity water in parks and grounds
• Restorations wanting a clay finish

Laying it on stony till

Manchester's till throws up cobbles, and a GCL can only perform as well as the bed it lies on. Your contractor trims and rolls the formation. We walk every metre of it before the first roll goes down, and we won't start until the cobbles and sharp debris have gone.

Rolls go down with wide laps. Bentonite paste or granules goes into each lap, and around each pipe, edge and structure, and our QA sheet logs every one of them while the job is under way. There is no welding on a clay liner; the laps carry the seal, so they get the care a weld would.

Cover follows straight after

The clay needs weight on it to seal. When we finish, you receive our QA sheets along with the minimum soil depth the chosen product requires, and placing that soil, then hydrating the liner, is your contractor's very next task. In a Manchester shower, a GCL left bare can take on water and swell before it is confined, which ruins it, so the cover crew should be standing by.

The finished pond looks and behaves like a traditional clay-bottomed one. Marginal planting can root straight into the cover soil, and the bed settles into a natural colour within a season.