Attenuation tank installation in Birmingham

Why Birmingham sites store water rather than soak it away

Much of Birmingham sits on clay. Where the ground is mudstone or boulder clay, water has nowhere to go, and an infiltration test usually confirms what the engineer already suspected. Where there is better-draining sandstone, a dense city often leaves no room for the clearances a soakaway needs from foundations and boundaries.

So the surface water goes to the sewer or a watercourse, and the planning condition caps how fast it can get there. The Rea, the Tame and the Cole all carry the city's runoff, and none of them has room for a new development to add its peak flow. A sealed attenuation tank holds the storm and lets it out at the agreed rate through a flow control.

City-centre plots and suburban schemes

Birmingham work splits into two kinds of job. In and around the centre, regeneration sites are hemmed in by roads, neighbours and live services, and the tank often goes under a car park, a service yard or a podium. Out in the suburbs it is housing, schools and community buildings, where there is more space but a landscaped or trafficked surface on top.

Both suit a crate tank. Crates build to the shape of the hole, so an awkward footprint or a long, narrow trench is no problem. We built a 115m³ tank at Swanhurst School in Birmingham as a single long run alongside the site, wrapped and finished ready for the groundworker to backfill.

• Car parks, service yards and access roads over the tank
• Long, narrow runs along boundaries where space is short
• School and public-building sites with term-time access limits
• Housing estates with tanks under shared open space

Getting on and off a city site

From our Tamworth base, Birmingham is a short run in on the A38 or the M6, which lets us fit around the delivery windows and loading restrictions that come with city work. We agree a drop time for the crates and membrane, keep the materials inside the site boundary, and work to the groundworker's programme so the dig doesn't stand open any longer than it must.

The first thing we look at is the formation. Uneven ground, loose spoil or a base that wants blinding gets raised on the day, while it can still be put right, not once a tank sits on it.

What you get at handover

Every Birmingham tank is wrapped in protective fleece and an impermeable membrane with heat-welded seams, never tape. The crates, at 95–97% void, are stacked to the storage volume and load rating the drainage drawings set out. Each Birmingham job includes a test weld we put on record, and the QA and handover file we leave is the one building control and the adopting authority will ask to see.