Attenuation tank enables county council to enlarge park and ride site by 10%.

A “simple, green and quick to install” stormwater management system by European drainage specialists Graf UK has enabled Cambridge County Council to meet increased demand for its park and ride sites.

Nearly, 29,000 of Graf’s EcoBloc Maxx geocellular modules have replaced an attenuation pond in a corner of one of the city’s five park and ride sites, increasing its size by 10% and allowing for an additional 200 parking spaces.

The main contractor, Skanska, had been challenged to find an attenuation system with the structural ability to form a car park directly above the pond at Trumpington that could hold sufficient volumes of surface water to drain the entire site, which is surrounded by housing.

Graf UK’s EcoBloc Maxx system of geocellular modules feature a reservoir volume of up to 96% – three times the storage volume of a standard gravel infiltration ditch. One module takes the place of around 376kg (0.376 tonne) of gravel or a 24m (78ft) length of 110mm drainage pipe.

Installed over six weeks – and two weeks ahead of schedule – by a team of up to 10 from Graf UK, the crates, which are manufactured from 100% recyclable plastic, were offloaded and lowered into the excavation by forklift and telehandler, then carried to the point of assembly.

This 100m long by 72m wide and up to 4m deep excavation, Graf UK’s biggest to date, was prepared by Skanska’s groundworks team. A geotextile fleece was laid onto the compacted gravel base to protect the geomembrane liner which was then laid and wrapped around the tank to make the entire 6,500m3 structure watertight.

The completed tank was then backfilled with 0.5m of granular material and 1-2m of site-won material before the sub-base and finished tarmac were installed.

Designed for a 100-year storm event, excess water is stored in the tank and released slowly at the rate of 15l/sec – via a flow control manhole into the River Cam.

Skanska project manager Dan Wood said: “This project was challenging due to the size of the site and potential impact on neighbours, so we sheet piled around the tank to reduce its footprint and the side force on it.  The Graf system itself is very simple and quick to put together and because it’s made from recycled materials it’s very environmentally friendly. It’s also green in the way in which it can be stacked for storage and transportation which reduced the number of vehicle movements required.”

He added: “The actual tank work was completed two weeks ahead of schedule as the Graf team put it in more quickly than they originally thought.”

The project was due for completion in March but due to the Covid lockdown was delayed by two months.

Graf UK technical manager Mark Caswell said: “This was a challenging project for us due to the tight design parameters, including the size and footprint of the tank, with numerous internal corners required to follow the layout of the sheet piling.”

EcoBloc Maxx modules are BBA-approved and have a design life in excess of 50 years.

The Trumpington Park and Ride scheme has helped Skanska to a shortlisting in the Environment and Sustainability category in the 2020 Highways Awards, the winners for which will be announced on January 28th, 2021.