Car park lining in the Black Country

Why infiltration is off the table

Permeable paving is meant to let water through. On an old foundry or works site, letting it carry on into the ground could move contamination towards the canals, the brooks or the groundwater. Over shallow workings it could wash out fill. And Coal Measures clay won't take it in any case.

So the engineer specifies a lined system: the paving and sub-base stay permeable, but an impermeable membrane beneath turns the sub-base into storage, released to the drainage at a controlled rate. On contaminated plots the membrane also keeps the ground's water out of the drainage.

Retail parks, trade estates and hospitals

The big car parks here serve retail and leisure around Merry Hill and the town centres, trade and industrial estates across the four boroughs, hospitals, colleges and supermarkets. Smaller jobs include housing courtyards and parking for schools and surgeries.

Big retail sites tend to be built in phases with shops trading alongside, so the lining is often laid one block of bays at a time, each one welded, logged and covered before the next is opened up.

Lining step by step

Digging and shaping the formation is the groundworker's job, and so is the stone and paving once we've finished. Between the two, our sequence runs like this.

• Formation walked for falls, proud slag and brick before we lay
• Cushion fleece across the dig floor, lapped up each wall
• Impermeable sheet laid out and joined with a heat welder
• Trial weld peeled, checked and logged on the day
• Outlets, overflows and duct crossings sealed into the sheet
• Top fleece spread to shield it from the stone

Brownfield formations

Formations on reclaimed land are rarely uniform. Soft spots, old foundations and debris turn up as the dig goes down, so we look at the formation before each stage of lining and won't lay over anything that could puncture the membrane. If it needs more trimming or a blinding layer, we say so there and then.

Slag and furnace waste are common in Black Country fill, and they can be sharp and hard. That's one reason we put fleece on both faces of the sheet, and why a formation that looks fine from the edge still gets walked before we lay.

When gas protection is on the same job

A retail or trade unit on old industrial ground may need a gas membrane under its floor as well as a lined car park outside. We install both, so the two membranes can be planned together, and the outlet and service details where they meet are thought through once rather than twice.