The school buildings at Thomas Bennett Secondary School are nearing the end of their life and will be demolished when a brand new complex, commissioned by West Sussex County Council and now rising on an adjacent site, is completed. The new school will consist of eight separate, large, flat-roofed buildings with an extensive paved car parking area around. The combination of a clay underlay to the soil and a vast rainwater run-off from both roofs and car park meant that an effective ground drainage system was essential to prevent ponding even in showery weather and potential flooding in storm-water conditions.



The site Groundworker, LF Nugent Group based in nearby Handcross, decided in consultation with manufacturers Polypipe Civils that an attenuation system based on Polystorm Modular Water Storage Units would provide the perfect answer to the ground drainage problem. Each Polystorm unit, measuring one metre long x 0.5metre wide x 0.4 metres deep, weighs only 9 kilograms yet has a storage capacity of 190 litres [a 95% voiding ratio] and, with a minimum strength of 40 tonnes per sq.m., will withstand any normal loading when installed. The modular construction means that the units can be interlinked to form a rigid storage tank of any shape required, whether in narrow strips or, as in the case of Thomas Bennett School, in very sizable rectangular areas. When the entire tank is wrapped in an impermeable geomembrane it forms an attenuation unit that will absorb vast quantities of rainwater in storm conditions for subsequent release into sewer pipes within set discharge limits.

At Crawley LF Nugent excavated two large areas one 40m x 8m, the other 35m x 8m to a depth of 2.7m and installed a total of 7,200 Polystorm units, all purchased through builders merchants A R Hendricks of Thurrock which were then encased in non-porous membrane. Discharge into the tanks is achieved via a total of seven 300mm diameter uPVC pipes the drained water is then pumped into the existing storm water sewer system. Despite the size of the area the work, excavation and installation, was completed in two weeks and the site workers were impressed by both the simplicity of the Polystorm system and the co-operation received from Polypipe Civils from the design stage upwards.

Staff and parents at Thomas Bennett School in the future will never guess that their cars are parked on a huge underground storm reservoir.