GCL pond lining in Birmingham

Why clay suits some city ponds

An urban wildlife pond takes knocks a rural one never sees: roots from mature park trees, rubbish thrown in, people wading out to fetch a ball. The sodium bentonite at the heart of a GCL swells when wet and presses into small holes, so a minor puncture closes up instead of draining the water away.

It also looks and behaves right. Under its soil cover the bed acts like the puddled-clay ponds farms and estates relied on for centuries, which makes a GCL a good match for nature reserves, school grounds and the damper corners of the city's parks.

When we'd steer you to membrane

We install welded membranes as well, so we'll say when a clay liner is the wrong call. A GCL needs confining under soil and water, gentle side slopes, and water chemistry that won't strip the bentonite of its swelling.

• Steep banks or edges left exposed to the air
• Ponds that will be drained and left dry for long spells
• Water fed by runoff from roads, yards or contaminated land
• Sites where plant has to track across the finished lining

How a Birmingham GCL install runs

Your groundworker digs and trims the basin. We walk it before a single roll goes down, looking for soft spots, ruts and the brick ends and stones that turn up in made ground around the inner city. Anything that would press into the clay layer gets flagged then, not after the panels are down.

Rolls are run out with the overlap width the manufacturer sets, and every lap is then dressed by hand with bentonite paste or granules. Around pipes, headwalls and outlet structures, the liner is packed round and collared using that same clay. Because a GCL seals by swelling rather than by welding, the lap log we keep as we go is the record of how well it was built.

Then timing takes over. The liner must be covered before it gets wet, so we agree the hand-back with your contractor and the cover soil goes on straight afterwards. A clay liner rained on before it is confined swells with nothing to push against and is spoilt.

Ponds where a clay bed earns its keep

School grounds, community orchards, the wetter edges of big parks and reserve scrapes along the Cole and Rea valleys all suit a liner that ends up looking like a plain clay pond. Once planted, nobody walking past would know a geosynthetic was there.