Grassholme Reservoir: northern embankment stabilisation works

Grassholme Reservoir is a large impounding reservoir commissioned in 1914 in the Lunedale Valley of County Durham. The most recent statutory inspection in 2016 resulted in a ‘Measures in the Interests of Safety’ (MIOS) recommendation to investigate and rectify a suspected unstable embankment slope between the reservoir and an adjacent watercourse. In November 2021, the stability works were completed to the satisfaction of the Qualified Civil Engineer and the MIOS recommendation discharged. A total of around 20,000 m³ of selected clay fill was used to construct a reinforced earth slope with an interlocking concrete block wall at the toe. As a consequence of stabilising the northern embankment, the adjacent Carl Beck was diverted through a buried pipe until realigned in spring 2022. The aim of this paper is to summarise and illustrate the various complexities associated with stabilising the northern embankment and realigning the existing watercourse, such as mitigating the loss of ancient woodland, managing complex ecological constraints and mitigating land access delays from the offset.