High strength needed
The design featured 150mm deep Permavoid cells, with Permafilter geotextile laid on top between the cells and the 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 the bottom of the tank were wrapped in a geomembrane to allow for stormwater to be attenuated, before discharging at a rate set by the Environment Agency into the river with the use of flow control devices. In areas of hard standing, without permeable paving, Permachannel and Permavoid Biomat were installed to capture, treat and attenuate surface water run-off.
Despite being so lightweight, the strength of the Permavoid 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.