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Network Rail unearths forgotten landslip images

Network Rail has released images of a historic landslip at Folkestone Warren, Kent.

The images were found in a filing cabinet during an office move by route asset manager for geotech, drainage and off-track Derek Butcher.

The original photographs of The Great Fall, a severe landslip at Folkestone Warren in December 1915, show train derailments and continued rail movement following the incident.

At the time the railway line to Dover moved 50m towards the coastline and 1.5M³m of chalk fell into the sea following weeks of heavy rain.

The line remained closed until 11 August 1919 as the First World War delayed its reopening.

Butcher said: “All landslides are activated by rainfall… [that location] has quite complex geology formed of chalk overlying clay. The water from the rainfall percolates through the chalk and sits on top of the clay and saturates it and leads to landslips. The chalk can’t sit in a stable manner on the clay when it’s that wet.”

The kink in the line remains visible today.

“We believe the train pictured was alerted to the landslip by the signal box at Folkestone Junction and was slowed down and found itself part on and part off the landslip. They were able to evacuate passengers who walked through the tunnel to Folkestone Junction station and were taken away from there.

“There was a significant amount of movement following the train stopping… That’s why it looks so horrific.”

Landslips have been a major feature of the line since it opened in 1844. In 1877, two people died when part of the Martello Tunnel was destroyed. The line remained closed for three months afterwards.

The last major movement was recorded in 1939 but Butcher said some ground movement had forced Network Rail to implement speed restrictions on the line in recent years. The need to take such precautions typically follows a very wet winter.

Butcher added: “The landslip [at Folkestone Warren] is still active… There are a number of ways that movement is controlled. Firstly, we monitor the location extensively with settlement points on a monthly basis.”

Network Rail use light-detecting and ranging (LIDAR) technology, a laser scanning technique to record points on the landscape. The data helps us keep track of which locations are moving.

Read GE’s feature about the monitoring in place at Folkestone Warren here.

Other techniques include boring holes in the ground to drain water, and building walls and other structures designed to stop the landslip from moving.

Meanwhile, Derek and his team regularly walk over locations at risk, comparing photographs to see whether the ground has moved.

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