The Discontinuance of Sunnyhurst Hey and Improvements to Earnsdale Reservoirs, Darwen, Lancashire

Sunnyhurst Hey and Earnsdale Reservoirs are adjacent reservoirs situated on the north-western edge of Darwen Moor, some 1.5 km west of Darwen town centre. Earnsdale Reservoir was constructed in 1863 and impounds 433Ml with a 300 m long, 24 m high embankment. A history of seepage and settlement and a more recent stability assessment, that indicated an insufficient factor of safety (FoS) on global stability, required mitigation measures. As a result a unique solution was formulated that incorporated a geogrid reinforced earth berm with a basket/rock facing, constructed on a geogrid reinforced load transfer platform that spanned between a number of deep soil mixed foundation cells and a filter that covers the downstream face of the dam. These were substantially completed in 2015 and form the main focus of the paper.Sunnyhurst Hey reservoir was constructed in 1875 and formerly impounded 436Ml. The embankment, of 855 m in length, suffered from a history of seepage, damp areas and soft ground at the toe. During investigations in 2008 the embankment was found to have no “core” and no “cut-off”, contrary to historical information. Given the extent of the mitigation measures that would be required to bring the reservoir to an acceptable safety standard it was decided to permanently discontinue the reservoir.