Attenuation tank installation in Derbyshire

Mudstone, coal measures and four rivers

Derby and the south of the county are largely Mercia Mudstone, a red clay that holds water rather than passing it on. North-east Derbyshire, from Chesterfield through Clay Cross to Bolsover, is Coal Measures country: bands of mudstone and seatearth with old workings beneath. On either ground, infiltration testing tends to come back poor, and the drainage engineer turns to sealed storage instead.

Runoff from those sites ends up in the Derwent, the Rother, the Erewash or the Trent itself. Planning conditions set the rate a new development may discharge. A crate tank wrapped in welded membrane holds back the excess during a storm, and a flow control meters it out no faster than the condition allows.

Approved Document H still asks for infiltration to be tried first, so a Derbyshire drainage strategy usually starts with test pits. On the clays, the results rarely leave the engineer any choice.

Weld by weld at Chesterfield

On the edge of Chesterfield we built a 970m³ tank for a new wood recycling plant. It sits beneath a future car park that will carry HGVs, so the crates were heavy-duty units, and the outlets were raised 400mm off the tank floor to keep a standing reserve of water for the plant's fire plan.

Partway through, the dig exposed a water main nobody had marked. We reworked the layout on site and built the tank in two stages around it without losing volume. The membrane arrived in 3m rolls, half our usual width, which doubled the seam count; each weld was numbered, pressure-tested and logged, and the job was handed over in five days.

The sites we see across the county

Derbyshire tanks tend to fall into a few kinds of job, and each brings its own constraints on access, loading and shape.

• Distribution and industrial units near the M1 at South Normanton and Alfreton, with tanks under service yards
• Housing on the edges of Derby, Ilkeston, Swadlincote and Chesterfield, where storage often sits under public open space
• Employment land along the A50 and A38 corridors
• Sloping plots in Matlock, Belper and Ashbourne, where a long, stepped tank follows the fall of the site

Our part of the job

The engineer behind the drainage strategy decides how big the tank is, how fast it may empty and what traffic it must carry. Your groundworker opens the excavation and refills it afterwards. Between those two moments the tank is ours: a look at the formation first, then 300gsm fleece, then the liner with every seam heat-welded, then the crates to the drawing, and finally the pipes in and out, flow control included.

We weld a trial seam on the day and log its result before the wrap closes over the crates. That log goes into the handover file with the as-built details, ready for building control or a warranty inspector.