Attenuation tank installation in Northamptonshire

Roofs and yards between the M1 and the A14

The county sits where two of England's busiest freight routes cross, and that shows in the kind of development we see. Warehouses around Northampton, Daventry, Kettering and Corby put acres of roof and lorry yard on to land that used to be fields, and the runoff from them arrives all at once in a storm.

Underneath, much of that land is Lias clay or glacial boulder clay, so letting the water soak in is rarely an option. The design stores it instead and meters it out to a sewer or to the Nene, the Ise or a smaller brook at whatever rate the planning condition allows.

Loading under lorry parks

A tank beneath an HGV turning area carries very different loads from one under a landscaped verge. Your engineer picks the crate type and loading class to suit the surface above; we build to that class and don't swap in something lighter to save time.

On larger industrial plots the tank often runs under the service yard in one deep block, with the flow control chamber at the downstream end. We set out the connections to the drainage drawing so the inlet and outlet levels match what the designer intended.

• Service yards and HGV parking over the storage
• Retail car parks where the tank sits beneath the bays
• Estate roads on new housing at the edges of Northampton and Kettering

One block at Corby

For the new Lidl store at Corby we built a single Rigofill SD tank of roughly 673m³, measuring 33.6 by 12 metres on plan and 1.67 metres from base to top. A 400m² floor takes a lot of LLDPE, so the panels went down in a planned sequence that kept every seam reachable for testing.

NW300 geotextile protects the membrane on both faces, and the inlet and outlet were sealed into the wrap before we handed the excavation back to the groundworks team.

Our part of the job

We don't design the tank or dig the hole. Once the groundworker has formed the excavation, we inspect the formation and raise anything that would compromise the liner, such as loose stone or standing water. Then the fleece goes down, the 0.75mm LLDPE is heat-welded panel by panel, and the crates are built up layer on layer to the designed volume.

Every Northamptonshire tank gets a test weld, logged with the date and the welder's settings, and none of its joints are taped. The QA file goes to you with the as-built details for building control and the warranty provider.