Car park lining in Northamptonshire

When the ground can't take the water

Lias and boulder clay drain too slowly for an unlined permeable pavement to empty between storms. Elsewhere in the county the problem is what is in the ground: made ground from ironstone working and old industry around Corby, Kettering and Wellingborough can carry contamination that rainwater should not be pushed through.

Either way, the drainage drawings will show a membrane under the aggregate. Rain then fills the voids in the stone below the bays and drains away through outlet pipes rather than into the ground.

Typical sites

Most of our car park lining here is on commercial ground, much of it built in recent years along the county's main roads:

• Retail parks and supermarket car parks in the larger towns
• Staff and visitor parking at warehouses on the logistics estates
• Business parks and offices on the fringes of Northampton
• Schools, health centres and community buildings with new parking

How we line it

Digging and trimming to the designed gradients is your groundworker's work. When it is done we walk the formation, checking that it runs towards each outlet and is free of sharp stone, which on Northamptonshire brash can mean a lot of picking. Non-woven protection fleece covers the floor and the side walls, and the impermeable sheet is then welded over it panel by panel.

Each outlet, overflow and any pipe crossing the lined area is welded or clamped into the sheet, and the perimeter is turned up and fixed at the height shown on the drawing. Another layer of fleece guards the upper face. Before the main seams we weld and peel a test strip and log the result, and that record comes to you at handover so the sub-base and paving can follow straight on.

Adding crates beneath

Some car parks need more storage than the sub-base can give. The designer then adds a geocellular tank under part of the parking. Because we install both the lining and the crates, the two are built as one system and recorded in one set of documents.