Silent Valley, which runs up from the Welsh town of Ebbw Vale, was restored and turned into a nature reserve managed by the Blaenau Gwent County Council and Gwent Wildlife Trust. The once post-industrial mining valley, contaminated with metal recovery waste has been cleaned up and the upper valley turned into a modern domestic landfill facility which is capped off and landscaped to become part of the nature reserve. Silent Valley is now one of the most beautiful natural environments in Gwent.
With the finished landfill profiles incorporating 80m long 1 in 2.5 slopes the challenge was to achieve a stable cap without any reprofiling or use of geogrid reinforcement.
The aim was to install a multi-layered syn-thetic cap comprising a 1mm LLDPE textured geomembrane with a drainage capability above the membrane and a gas venting capa-bility below.
The challenge was to provide sufficient drain-age capacity on both interfaces and adequate interface shear strength to withstand the dead load of the 0.5m cover soil and live load of the construction traffic whilst placing the backfill material on this critical slope.
The required slope stability was not possible to achieve with standard geocomposites typically used for the landfill cap drainage. A geocomposite with enhanced performance was required for this 1 in 2.5 slope application.
Geocomposite drains have long been used as alternative to crushed stone. ABG is well known as a pioneer of cuspated drainage and in 1995 was the first to introduce a 4.4m wide drainage composite suitable for wide scale use in environmental projects. Since then, many millions of sqm of Pozidrain have been used on landfill and mine capping projects to provide sub-surface drainage above and below the barrier to improve stability. It has become clear that the interface shear strength is of critical im-portance. As slopes have become ever more steep, geomembrane texture and spikes have improved the barrier/geocomposite interface shear strength to the ex-tent that it is the cover soil to geocomposite interface that is often the limiting factor. ABG has invented and patented the Pozi G with a unique lattice core that has proven to give in excess of 20% improvement to the cover soil interface. Shown is an example project that used Pozi G on a 1:2.5 slope. Consideration of the inter-face friction was needed for the construction phase allowing for plant tracking up the slope to spread cover. Just like the original Pozidrain, Pozi G has MD and CMD flow which ABG believe is fundamental to true equivalence to crushed stone and for a realistic factor of safety. The ability for the water to flow in all directions equally guards against any possibility that a longitudinal flow channel is inadvertently compromised on site, as the water can simply detour around that zone and continue safely to the outlet pipe.
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