GRAF UK stormwater system trumps the schedule at Trumpington

Posted by Callum Vallance-Poole, on September 19, 2024.

EcoBloc Maxx stormwater tank for Trumpington

GRAF UK provided a stormwater management solution that enabled Cambridge County Council to expand its Trumpington Park and Ride site by 10%, accommodating an additional 200 parking spaces. The project, spearheaded by Skanska, replaced an existing attenuation pond with GRAF UK’s EcoBloc Maxx geocellular modules.

With increased demand for park and ride facilities in Cambridge, the County Council needed to enlarge one of its five sites. The challenge was to implement a stormwater attenuation system robust enough to support a car park while effectively managing surface water.

GRAF UK’s EcoBloc Maxx system, comprising nearly 29,000 geocellular modules, was chosen for its high storage capacity and structural strength. These modules offer a reservoir volume of up to 96%, providing three times the storage capacity of a standard gravel infiltration trench. Each module replaces approximately 376 kg of gravel or a 24m length of 110mm drainage pipe.

Over six weeks, a team of up to ten from GRAF UK installed the EcoBloc Maxx system two weeks ahead of schedule. The installation process involved offloading the crates and lowering them into the prepared excavation using forklifts and telehandlers. The 100m by 72m excavation, prepared by Skanska, was lined with a geotextile fleece to protect the geomembrane liner, ensuring the 6,500m³ tank was watertight. The tank was then backfilled with granular material and site-won material before the sub-base and tarmac were laid.

The project presented several challenges, including tight design parameters and the need to minimise the impact on neighbouring areas. To address these, Skanska implemented sheet piling around the tank to reduce its footprint and manage side forces. The stackable design of the EcoBloc Maxx modules facilitated efficient storage and transportation, reducing vehicle movements and contributing to the project’s environmental friendliness.

Designed to handle a 100-year storm event, the EcoBloc Maxx system stores excess water and releases it slowly into the River Cam at a controlled rate of 15 litres per second. The project, originally scheduled for completion in March, was delayed by two months due to the COVID lockdown but still finished ahead of the revised timeline.

Dan Wood, Skanska’s project manager, praised the simplicity and efficiency of the GRAF system, noting its environmental benefits and the speed of installation. The project was completed two weeks ahead of schedule, demonstrating the effectiveness of the collaboration between GRAF UK and Skanska.

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