Car park lining in Wolverhampton

Why the east side lines its car parks

Permeable paving is a good SuDS surface, but it assumes the ground below can safely take the water. On made ground, ash and old foundry fill, letting rain percolate through can mobilise what is buried there. The answer is a welded impermeable liner under the sub-base, with the water stored in the stone and let out to the sewer or a brook at a controlled rate.

The same applies on the Coal Measures clays, which simply won't take water fast enough. Retail parks, supermarkets, hospital and campus car parks and the yards of new industrial units in the east of the city are typical.

And on the sandstone?

West of the centre the ground often drains well, and an unlined permeable car park may be acceptable. The sandstone is a principal aquifer, though, so where a car park carries heavy traffic or sits close to a borehole, your engineer may still call for a liner to protect the groundwater. Their design decides. Where it is lined, our method matches the east side; only the reason differs.

Lining the formation

The groundworker excavates to the design falls and hands over a trimmed formation. Our first job is to check it: level, firm, no sharp stones, and falling to every outlet. A non-woven protection fleece goes down next, spread over the floor of the dig and lapped up its edges.

The membrane follows, unrolled sheet by sheet and fused at every joint, with a trial weld logged before the first production seam. Every outlet, overflow pipe and duct crossing is sealed through the sheet, and its perimeter rises to the height the drawing gives. Over the top goes a second geotextile, and that is where our work stops and the stone layer begins.

• Protection geotextile under and over the liner
• LLDPE, HDPE or PP to the specification
• Welded joints, never tape
• Sealed outlets and penetrations
• Weld test records and handover documentation

Campuses, hospitals and retail parks

Car parks serving the university, the hospital and the retail parks around the ring road can rarely close entirely. The usual answer is to rebuild them in blocks, with each block fenced, dug, lined, filled and reopened before the next starts. We fit our visits to that rhythm and join each new block to the one before with a welded seam.

Combined with crates

If the stone alone falls short of the volume the engineer needs, a crate tank is often added under the parking. Lining and crates can both be ours, recorded as one system.