Located in the small Essex market town of Coggeshall, the development featured a high water table at a depth of 0.5m, which was likely due to hydraulic connectivity from a nearby river. Polypipe CGU utilised its popular Permavoid geocellular solution to provide a suitable sub-base replacement system.
The void ratio of Permavoid cells provided ideal coverage levels for the expected rainfall levels within the footprint of the development’s car park area.
Capable of attenuating 110m³ of stormwater, Polypipe CGU supplied Permavoid cells that were 150mm deep to create a high-strength structural raft. The system sits beneath permeable paving in place of the traditional aggregate sub-base.
Located beneath the site’s car parking facility, the system specified was required to withstand heavy loads from HGVs and cars during day-to-day retail and residential use. Permavoid’s 95% void fill ration made it an ideal solution for stormwater storage, with its high vertical and lateral strength making the tank strong and durable to withstand daily heavy loads.
Using Permavoid at shallow depths meant that necessary excavation was greatly reduced and the lightweight, interlocking system made a positive impact on the speed of installation.
To avoid any future pollutants potentially entering the local watercourse, Polypipe CGU provided its Permafilter Geotextile beneath permeable pavement and above the tank in order to trap oils and break them down through biodegradation.
Jim Rudd, Director at MLM, said: “With varied cover levels and a high water table, there were a number of engineering challenges to overcome on this site. Thankfully, Permavoid offered us the chance to engineer a solution strong enough to cope with the site’s necessary HGV traffic without compromising on stormwater storage or depth levels.
“The interlocking nature of Permavoid cells also meant that we could configure a system that made best use of the space available on site, providing ideal coverage levels for expected rainfall within the car park area.”