Attenuation tanks for the Black Country

Building on old industrial ground

Most new building in the Black Country goes on land that has been used before: foundries, works yards, canal-side wharves and the sites of old pits. That leaves made ground of uncertain make-up, shallow coal and limestone workings in places, and contamination that the site investigation has to deal with.

On ground like that, pushing surface water into the soil is rarely an option. Infiltration can wash contaminants towards groundwater, and water running into old workings is the last thing a geotechnical engineer wants. The usual answer is to collect the runoff, hold it in a lined tank and let it out to the sewer or a watercourse at a set rate.

Why the lining matters more here

On a clean greenfield site the membrane's job is mainly to keep the stored water in. On a Black Country brownfield site it also keeps the ground's water out. A taped joint that lets go lets polluted groundwater into the drainage system, or lets stored water out into made ground that was never meant to be wetted.

That is why we heat-weld every seam and never tape one, and why a test weld goes on record before any fill is placed. Where the site also needs a gas or hydrocarbon membrane under the buildings, we can install that as well, so one team is answerable for both layers.

• 300gsm protective fleece against sharp fill and brick in the formation
• Impermeable membrane with heat-welded seams
• Crates at 95–97% void, built to the designed volume
• Sealed inlets, outlets and flow-control connections

The sites we see

Black Country jobs are often industrial and trade units, retail parks and housing on reclaimed land, with the tank under a yard or an estate road. Formations can be uneven where old foundations and obstructions have been dug out, so we check the excavation before we start and say clearly if it needs blinding or more trimming.

The area drains to two different rivers. The Tame takes much of the east towards the Trent, while the Stour and its brooks take the south and west towards the Severn. Your drainage engineer sets the discharge rate for the outfall; we build the tank to match the drawings.

Canal-side plots are common here too. A tank close to a canal or a culverted brook has to stay watertight in both directions, and that comes down to the quality of the welds and the care taken around each pipe connection.