Pond and lake lining in Northamptonshire

Why some Northamptonshire ponds leak

The flooded gravel workings strung along the Nene from Northampton past Wellingborough and Thrapston hold water because they sit in the river's own groundwater. Climb out of the valley and things change. The Northampton Sand ironstone and the limestone around Corby, Kettering and Oundle are fissured and porous, and land that was quarried for ironstone and then put back is loose and uneven.

A clay-puddled pond on that kind of ground often weeps through its banks or base. A welded liner removes the guesswork about what the soil will do.

Ponds we line here

The jobs in this county are varied. Each one gets a liner chosen for its use and size:

• Coarse fishing lakes and angling pools along the river valleys
• Irrigation reservoirs on arable farms in the east of the county
• Ornamental lakes and water features on country estates
• Balancing and amenity ponds on housing at the edge of Northampton and the market towns
• Relining of older ponds that have started to lose water

Picking the membrane

Smaller garden and wildlife ponds usually suit EPDM, which drapes well over shelves and curves. Larger lakes and reservoirs are normally HDPE or LLDPE in welded panels, with HDPE favoured where the sheet will be exposed to sunlight along a long waterline. PP is useful where the shape is awkward and needs a lot of hand detailing. If a geosynthetic clay liner suits the job better, we will say so.

From formation to handover

You or your contractor excavate and trim the shape. We walk the formation first, because sharp ironstone fragments and limestone rubble are common here, and a geotextile underlay goes down to keep them off the liner. Panels are then rolled out and joined with a wedge welder on the long seams and an extrusion welder around pipes, outlets and corners.

Round the top of the bank, the sheet's margin drops into a trench that is then backfilled, so neither wind nor the weight of water can pull it down the slope. On every Northamptonshire pond we cut and peel a trial weld before the main seams, and the handover records list each seam and detail we formed.