Walthamstow Stadium

Located on the site of the former greyhound stadium, adjacent to the River Ching, the site features both contaminated land at shallow depths and a high water table. Prior to the redevelopment, surface water discharge was unrestricted into the river, however, as part of this project, the Environment Agency stated that there was the need to ‘make space for water’ and reduce the existing urban stormwater run-off by 80%. This allows the site to be capable of handling rainfall in the event of a ‘one in 100’ year storm, in line with climate change planning.

Working closely with consultant engineers, MLM, and Quadrant Construction, Polypipe Civils & Green Urbanisation [CGU] designed a suitable solution using the high-strength Permavoid geocellular system to form a sub-base replacement system beneath 4,500 square metres of permeable paving. This will provide 1,500m³ of water attenuation to meet the requirements previously agreed with the Environment Agency during the pre-planning stage.

Installed by groundworker Tamdown, the design features 150mm deep Permavoid cells, with Permafilter geotextile laid on top between the cells and permeable paving. The Permafilter acts as a barrier to capture and treat surface water run-off at source from the permeable paving above, before entering the tank. The sides and bottom of the tank are wrapped in a geomembrane to allow for the stormwater to be attenuated, before discharging at a rate agreed with the Environment Agency into the river with the use of flow control devices.

The Permavoid cells boast a 95% void fill ratio and incorporate a unique jointing mechanism to form an interlocking ‘tensile raft’. Despite being light in weight, the innate strength of the cells allows them to support structural loads across heavily trafficked areas, making them suitable to withstand the compressive and dynamic loads produced by vehicles at the site.

In areas of hard standing without permeable paving, Polypipe CGU’s Permachannel and Permavoid Biomat have also been installed to capture, treat and attenuate surface water run-off.

Permachannel acts as both a surface water collection point and treatment system that intercepts silt and oil with a zero gradient at pavement level. Water is then discharged from the side of each Permachannel into Permavoid Biomat cells, which are wrapped in a geomembrane to provide attenuation. Permavoid Biomat comprises of a tri-laminate low density composite, the biomat floats on the water to intercept and treat any residual emulsified oils that may still be present in the surface water run-off from the road above, allowing only treated water to be discharged into the local watercourse.

Sean Robinson, Permavoid Project Manager at Polypipe CGU, said: “this project provided the opportunity to showcase why Permavoid works so well in sub-base replacement projects. The system’s 95% void radio meant a design that meant the Environment Agency’s requirements was possible, even with the high water table and the site’s contaminated ground conditions.

“Providing a water management solution at shallow depths can bring a whole host of advantages, not least decreasing installation times and eliminating the need for the removal of large amounts of contaminated excavated materials from site, which is why we are increasingly called upon to provide solutions of this nature.”