Articles & Case Studies


Posted: Thursday 11th December 2014

Hydraulic asphalt engineering experts WALO UK have started lining the 11th cell at Westmill Landfill site in Hertfordshire using Dense Asphaltic Concrete (DAC). This is the largest of the cells WALO UK has lined at Westmill since 2003 and once completed, the site, operated by Biffa Waste Services Ltd, will have an overall storage capacity of around 2.5 million cubic metres

Founded in 1939, WALO's Dam and Landfill department celebrates its 75th anniversary this year and is a global specialist in the field of hydraulic asphalt engineering, providing bituminous lining systems wherever an impermeable barrier is needed.

The area to be lined in Westmill's 11th cell, with over 13,000 tonnes of asphalt, consists of a basal surface area of 18,400 sq. m, with slopes of 9,300 sq. m. The longest slope length to be lined measures 60 m with an incline of 1:2. WALO UK is also constructing intercell bunds, measuring two metres high, into adjacent existing asphalt lined cells, which will cover an area of 3,600 sq. m., again with a slope incline of 1:2.

The lining system itself has been formulated in WALO's Zurich laboratory to a design mix specific to this project. It comprises a 300mm mineral attenuation layer; a 200mm sub-base material layer, a 60mm asphaltic binder layer; a 80mm DAC layer and a mastic seal coat to the base, side slopes and bund areas. The asphalt is being placed by hand on the intercell bunds and by paver and winch on the base and slopes. Once the lining works are completed, a leachate collection and removal system will be installed.

The 38 hectare Westmill site is located in a working sand and gravel pit near Ware in Hertfordshire and every year receives up to 350,000 tonnes of municipal waste from the south east region of the UK. Being located on a chalk aquifer, Westmill is a particularly environmentally challenging site so protection of the groundwater is of the utmost importance when it comes to selecting a lining system. Using DAC, WALO has created a highly robust landfill barrier lining system, which is impermeable to leachate and methane, found in most solid waste landfill deposits.

David Wilson, Managing Director of WALO UK Ltd, said:

"Our DAC lining system has been tried and tested with great success at Westmill over the last 11 years and has proved to be the most practical and effective impermeable barrier system possible for this site. Clearly, with a chalk substrate such as this, environmental issues are incredibly important and our DAC with its robust, flexible and non-toxic properties has proved to be the perfect solution to contain the waste while safeguarding the groundwater."

WALO UK's lining work on the 11th Westmill cell started in the middle of June and is scheduled to last ten weeks.

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