The new 3,500 sq multistorey car park at North Manchester General Hospital has a 100mm-deep plastic mat with holes in it across its footprint, which has capacity to store 300 cubic metres of water. Rainwater collected in the voids of the mat is then released into the surface water drainage system in a controlled way to prevent flooding.

SEL Environmental’s Formavoid system, developed and manufactured in the UK, has also the need to excavate and move hundreds of tonnes of earth and foundation aggregate. The conventional approach would have been to install a large, deep attenuation tank, negotiating existing underground pipes and cables.

SEL Environmental installed the Formavoid sustainable drainage system (SuDS) within the subbase for main contractor Morgan Sindall Construction, which has built the 964-space car park as part of £36.5m of enabling works ahead of the wider rebuilding of the hospital estate.

Drainage engineers at the Lanes Manchester depot have used a recycler jet vac tanker to clean the 18 silt traps in the giant water tank prior to handover to the NHS.

The Formavoid system is manufactured in Lancashire from 100% recycled plastic. It integrates within an open-graded subbase to enhance the void ratio of aggregate, improve stability and reduce construction depths.

SEL Environmental  managing director Andy Shuttleworth said: “One lorryload of our Formavoid system replaces 37 lorryloads of foundation material, so that’s a lot of truck movements and material saved.

“Those savings can be added to the resource, time, energy and cost savings made by not having to construct a large attenuation tank, and contributes to a significant reduction in the project’s carbon footprint. So, we were very pleased to have been able to present this solution.

“Once our tank is in place, the only task necessary is to clean the silt traps so the water can flow freely through the system. It was good that Lanes could use its sustainable water jetting system to help us do this.”

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