GCL pond lining in Dorset

A clay seal for chalk and sand

Neither the chalk of the Dorset Downs nor the sandy heaths of the Poole Basin can keep water in a hollow without help. A GCL places a thin layer of sodium bentonite between two geotextiles, and once it is wetted and confined the clay swells and closes up into a layer that water barely passes. Once covered, the bed looks and behaves like a traditional clay pond, which is why GCLs suit estate lakes, wildlife ponds and landscape schemes where a black membrane edge would look wrong.

Bentonite also seals small punctures as it swells. On a site with flints in the formation or tree roots near the margins, that is a useful margin of safety, though it is no excuse for running machines over the liner.

Ponds that suit it in Dorset

The best candidates are clean-water ponds with gentle banks: a restored dew pond on the downs above Cerne Abbas or Cranborne, a scrape on farmland in the Frome valley, a lake on an estate near Sherborne, or a landscape pond on a new housing scheme around Dorchester where the planting runs right down to the water.

Heathland ponds sit on acid sand that holds nothing. A GCL there gives a natural bed that plants and invertebrates can colonise, so the pond reads as part of the heath rather than as something built into it.

Checking that GCL is right first

Before recommending a GCL we check the application suits a clay liner. Three points matter most in Dorset.

• Water source: water off chalk carries a lot of dissolved calcium, which can affect how sodium bentonite performs over time, so we look at what will fill the pond
• Slopes: a GCL needs a cover layer to confine it, so it suits gentle banks; steep sides or exposed edges usually point to a welded membrane instead
• Contents: clean water for amenity or wildlife is a good fit; aggressive liquids are not

How a GCL goes in

Your groundworker forms the pond. Before a roll is opened, we inspect that formation and sign it off: smooth, compacted and free of sharp flints. GCL has no welded seams. Rolls go out with wide overlaps, and each join is closed by working in bentonite, as a paste or as dry granules. Inlets, outlets, headwalls and the pond edge get the same bentonite treatment, and we log every overlap and detail on the day it is made.

Products come from the main manufacturers, among them NAUE Bentofix, CETCO Bentomat and Solmax, alongside UK-made liners from Geosyn and Wrekin. We fit what your specification names or suggest one that suits the pond.

Cover goes on promptly

A GCL only seals when it is confined and hydrated. Our handover tells your contractor how much soil must go over the GCL and includes the QA records; they then place that cover and wet it. That should follow straight after us: bentonite that gets wet with nothing on top of it swells loose and loses its seal for good, so on an open downland site we plan our laying around the cover crew and the forecast.