Car park lining in Preston

Permeable paving on glacial till

Block paving with open joints lets rain straight through the surface, but on the boulder clay that covers much of Preston the water then has to go somewhere other than down. The usual answer is a lined system: the stone sub-base becomes the storage, a membrane beneath keeps the water in, and outlets release it to the drainage at the rate the design allows.

Lining also makes sense where the ground is contaminated, where the parking sits close to foundations, or where the water table rises high near the Ribble. Older parts of the city still drain to combined sewers, which is one reason a design will throttle what leaves a car park.

The car parks in question

Around Preston that means supermarket and retail parking, staff and visitor spaces that come with hospital and college expansion, business parks off the M6 and M55, and the parking courts on new housing in the north-west of the city. Each sub-base has to hold water under traffic for years without losing it into the ground.

Layer by layer

Your groundworker cuts the formation and grades it to the falls. Before any fleece goes down we walk it: is it flat where it should be, clear of sharp stone, and running towards the outlets?

• A protective non-woven layer rolled over the floor and turned up the walls of the excavation.
• The impermeable sheet, in LLDPE, HDPE or PP as specified, joined panel to panel by heat welding.
• A trial weld run and tested before the main seams, with the result written into the records.
• Every outlet, overflow and service duct sealed through the sheet, and the perimeter raised to the height shown in the design.
• A second fleece over the top, so the stone bears on fabric rather than on the membrane.

Crates under the bays

With the top fleece laid, the weld records are yours and the formation goes back to the groundworker for stone and blocks.

Some Preston schemes need more storage than a stone sub-base can give. Then crates go in under the bays as well, and because we build those too, the tank and the lining share one set of weld records and one handover rather than two jobs that meet at an outlet.