Lined permeable car parks in Cambridgeshire

Three reasons to seal the sub-base here

Cambridgeshire produces lined car parks for different reasons in different places. On the Fens the water table is too close to the surface for the paving to drain downwards. On the gault clay through the middle of the county water barely moves into the ground. And on the chalk south of Cambridge, which supplies drinking water, runoff from parking areas may not be allowed to infiltrate because of the risk from oil and fuel.

In each case the designer specifies an impermeable liner. The aggregate sub-base stores the rain under the bays and lets it out through controlled outlets.

Where we line

The sites vary in size and use:

• Car parks on science and business parks around Cambridge
• Retail and leisure parking in Peterborough, Huntingdon and St Neots
• Shared parking courts on new settlements
• Car parks at offices and depots along the A14 and A1(M)

Sequence on site

Your groundworker excavates to the design falls and trims the formation. We inspect it for level, falls and sharp stones, including flints on chalk sites, before laying a non-woven geotextile across the base and up the edges. The membrane goes on top in panels, joined by hot-wedge welding on the spot, and a trial seam from each Cambridgeshire job is peel-tested and written up.

We seal each outlet, overflow and service pipe through the liner and turn the membrane up the side walls to the height on the drawing. A top layer of fleece then shields it from the stone to come, and our weld log travels with the handover so the aggregate can go in the next day if your programme allows.

When the sub-base needs help

On a car park with a big catchment, the sub-base may not hold the full design storm. The designer then adds attenuation crates under the parking. We build those too, so the lining and tank come from one installer with one record set.