Rockfall Barriers

Knockan Crag in northwest Scotland is internationally recognised as a landmark site for thrust tectonics and holds designation as a Site of Special Scientific Interest (SSSI). Located approximately 13 miles north of Ullapool, this remote and exposed location is subject to some of the most severe weather conditions in the UK, creating significant challenges for slope stability and infrastructure protection. At the site of the “Rock Room” geology visitor centre, the terrain is dominated by a steep rock slope rising 20 to 30 metres with an inclination of 75 to 85 degrees. In 2008, a major rockfall incident occurred when several large boulders, measuring up to 2 x 1. 5 x 1 metre, detached from the slope and fell towards the visitor centre. The rocks narrowly missed the structure, highlighting a critical safety risk to visitors and staff. Given the increasing risk of rockfall hazards in this high-exposure environment, Scottish Natural Heritage (SNH) required a robust and reliable rockfall protection solution to safeguard the visitor centre while preserving access to this important geological attraction.

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The Shetland HVDC link, approved in 2020 after more than a decade of development, is a landmark project that connects Shetland to the Great Britain transmission network for the first time. This critical energy infrastructure unlocks the island’s renewable energy potential, supports the 443MW Viking Wind Farm, and contributes to the UK’s net-zero carbon targets. As part of the project, a 320/132kV substation and HVDC converter station were constructed at Kergord. The works required significant bedrock excavation, resulting in a 6-metre-high exposed rockface formed within the Weisdale Limestone Member. Following excavation, sections of the rockface showed signs of instability, including ravelling and loose material. This created a high rockfall risk to workers operating in the adjacent storage yard at the base of the slope. Without mitigation, falling debris posed a serious safety hazard, making effective rockfall protection essential to maintain safe construction operations.

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Approximately half a mile north of Denby Dale station, a single-track railway line exits Cumberworth Tunnel into a 14-metre-deep cutting formed within lower coal measure geology. The cutting comprises interbedded semi-competent sandstone alongside weak, friable siltstones and mudstones, creating a geologically variable and inherently unstable slope environment. The cutting slopes are steep, ranging between 45 and 55 degrees, with exposed rock outcrops measuring between 1. 0 and 2. 0 metres in height. Over more than 160 years of exposure, natural weathering processes have significantly degraded the strength and cohesion of the rock mass. This deterioration has increased the likelihood of slope instability and rockfall, posing a direct threat to railway infrastructure, operational reliability, and passenger safety. Recognising the growing geotechnical risk, Network Rail commissioned URS to undertake a detailed site assessment. The objective was to evaluate slope stability conditions and develop an effective long-term rockfall protection solution for this critical section of railway.

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