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Cliff Hanger

Detecting rockfall above one of Britain’s most famous stretches required a fresh, innovative solution.
Olivia Gagan reports.

Installing a rockfall monitoring system on two sections of cliff face near Dawlish, close to Exeter in south west England, was important not just to preserve the structure of the cliff, but also to the efficiency of an iconic railway line.

The Exeter to Plymouth mainline runs along the cliff base. Known as the “Riviera Line” owing to the English seaside resorts it connects to, the line was designed by Isambard Kingdom Brunel and opened in 1846.

Busy line

The single-track, broad gauge railway still carries heavy tourist traffic from London, the Midlands and the north.

With so much traffic, closures and speed restrictions can be a massive disruption to a wide range of journeys.

The 40m high red sandstone cliffs along this particular stretch of the line are subject to erosion, making falling rocks a potential risk to passing trains.

For some time, Network Rail has managed the risk by carrying out regular cliff face inspections, with temporary speed restrictions imposed if required.

Technical solution

Network Rail engaged Carillion and Tony Gee & Partners to develop a technical solution to the problem, and it was decided that rock netting, reinforced by vertical wire cables, should be installed over the cliff face to hold the rocks in place. Load measurement specialist Strainstall was contracted to provide a rockfall monitoring system for the netting.

All 57 of the vertical cables were instrumented with a 25t load shackle, which measures tensile load, and an accelerometer. These in turn were connected to a datalogger. Rope access-trained engineers installed the system directly onto the cliff netting.

The accelerometer is used to detect any vibration in the netting on the cliff, which alerts the system to a potential rockfall

The accelerometer is used to detect any vibration in the netting, which alerts the system to a potential rockfall.

The load on the shackle responds to increased load in the vertical restraint cables, confirming whether or not the netting is supporting a new rockfall and the magnitude of the load.

Five data loggers are linked to a site computer, which in turn links to Network Rail’s Exeter signalling centre via broadband.

This enables the signalling team to see when changes have been recorded, allowing them to take the appropriate actions based on Network Rail protocol.

Should the broadband connection fail, the signallers are notified and a GSM backup connection is automatically used.

The rock fall monitoring means that temporary speed restrictions or line closures are only imposed when there is a measurable risk to trains. Previously blanket restrictions were applied when periodic inspections warned that a rockfall could be imminent.

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