Attenuation tank installation in Leicestershire

Clay ground, controlled discharge

Infiltration tests across most of the county come back slow or flat. When that happens, the drainage engineer turns to storage: a crate tank wrapped in fleece and a welded membrane, holding the storm and letting it go through a flow control to a brook, the Soar or its tributaries, or the sewer.

The county's low, rolling land has a practical effect. Outfalls are often shallow, which pushes designs towards tanks that are wide rather than deep. Crates suit that well: they build to any plan shape at a modest height and can take highway loading under a yard.

Big roofs and new neighbourhoods

The M1, M69, A5 and A42 have made Leicestershire one of the country's main distribution areas. Warehouse roofs at Lutterworth, Hinckley and around East Midlands Airport shed a lot of water in a storm, and the tank under the service yard has to hold it.

Housing is the other big strand: urban extensions on the edge of Leicester, and estates growing out from Loughborough, Coalville, Melton Mowbray and Market Harborough. Those tanks tend to sit under open space or estate roads and are often built in phases as the groundworker opens each section.

Perched water in boulder clay

Glacial till often holds pockets of trapped water that seep into an open excavation, especially after rain. The groundworker keeps the dig pumped dry while we lay and weld, because hot-wedge welding only works on a dry sheet. We check the formation is level and firm and say so on the day if it has softened.

Sand blinding is common on these sites, laid by the groundworker where the clay is uneven or stony. It gives the fleece a smooth bed and the first layer of crates a true base.

Close to home at Corby

Just over the county line from Market Harborough, the new Lidl at Corby sits over a 673m³ tank of ours: 33.6m by 12m in plan, 1.67m from base to top. Its seams were all heat-fused and pressure-tested before it was covered. A Leicestershire tank is handed over with the same QA pack.