Harthorpe Viaduct | West Coast Mainline, Moffat, Dumfriesshire

September 2017

Contractors seeking an urgent solution when large voids were found within the structure of landmark railway viaduct turned to Leca® LWA as the lightweight solution. The unexpected voids were discovered behind the abutments of a Victorian bridge during a major £8 million replacement project at Harthorpe Viaduct on the West Coast Main Line near Moffat, Dumfries & Galloway.

By using the Leca® LWA 55m³ blowing vehicle, Skanska were able to deliver the material pneumatically from ground level to the top of the old viaduct abutment. Placing a full load of 10-20mm Leca® LWA to fill five voids up to 10 metres in depth and up to 1.5 metres wide, in a single day, allowed work on the main project to continue simultaneously, keeping delays to a minimum. The use of Leca® LWA meant that there was no requirement for stockpiles of material on site, no waste and no need to compact the fill. Skanska's design manager, Spencer Williams commencted: "Additional unexpected voids were discovered on site very late in the programme. We chose to use Leca® LWA because it could be delivered pneumatically to the top of the main contract on Harthope Viaduct, a well-known structure known locally as the 'hole in the wall', involved the replacement of the 100 metre long, six span structure that carried the West Coast Mainline to Glasgow. The project involved completely replacing the bridge - which originally dated back to 1848 and 1875 - including construction of a new central pier; conversion to two spans; strengthening of existing support structures and piling work. Skanska slid the new 18m superstructure into place during a five day blockade of the line. Leca® LWA is a lightweight expanded clay aggregate formed by heating and firing natural glacial clay in a rotary kiln at up to 1150°C. This process transforms the clay into lightweight ceramic granules that have a hard ceramic surface and porous core. The material is extremely light with a bulk density of a fraction of traditional fill material thus reducing loading on underground voids and abutments. The 10-20mm Leca® LWA has an average density after comp;action of just 300kg/m³, 85% lighter than traditional crushwd rock or gravel fill.

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