GCL pond lining in Northamptonshire

Wildlife ponds on the county's new estates

Housing around Northampton, Kettering, Wellingborough and Rushden often comes with ecological conditions attached: ponds for great crested newts, scrapes for wading birds and wet features in the public open space. Those ponds are expected to look and behave like old field ponds, with plants rooting into the bed and soft margins.

A GCL suits that brief. Sodium bentonite carried between two geotextiles takes up water and expands into a dense, near-impermeable sheet, and once buried under soil it gives a planted, natural finish. If roots or a stray spade nick it, the clay swells into the hole.

Hard water from the limestone

One thing we look at closely in this county is water chemistry. Water that has run through the Jurassic limestone and ironstone is hard and rich in calcium, and over time calcium can swap into sodium bentonite and loosen the seal. The specification, the cover depth and the source of water filling the pond all bear on whether a GCL is the right choice.

That is why the first step is a suitability check. Where the chemistry or the profile is wrong for clay, we install a welded membrane instead and tell you why.

How the liner goes in

Your contractor shapes the pond with gentle sides and compacts a smooth formation. We inspect it, pick out stones and roots, and sign it off before any rolls are opened. The panels are laid with wide overlaps, and every lap gets a bed of bentonite paste or granules. Outlet pipes, a dipping platform post or an inlet headwall each get their own bentonite collar.

Our sheets note each lap and detail on the day it is laid, and they come to you with the minimum soil depth this product calls for. Covering is your contractor's next job, followed by wetting the liner, and it must not wait: bentonite soaked by rain before the soil is on swells loose and is spoiled.

Ecology and programme

Newt ponds are often tied to licence timings and seasonal windows. Tell us those dates when you book and we will plan the laying so the GCL is down and handed back in time for the cover to follow straight away. If the pond has to be finished before newts move in for the spring, that date drives everything else on the programme.