The coastal railway line between Folkestone and Dover is one of the most stunning in Britain, but it is also one of the most challenging to operate – reputed to be the most expensive stretch to maintain in the whole country.

It is built through a landscape that is continually at threat of landslides and ground movement, as Rhys Griffiths discovered when he visited the site with Network Rail to learn more about the challenges of keeping this tricky track open…

We are standing on a small chalk ridge overlooking the Warren, a site of special scientific interest just outside Folkestone, with cliffs to our right and the English Channel to our left.

Birdsong is the only soundtrack until the rhythmic rattle of an approaching train fills the air as it passes along the line below, which bisects this stunning coastal landscape.

It’s hard not to marvel at the feat of engineering which, almost 200 years ago, saw this railway line driven through the chalk cliffs between two port towns. It is also easy to wonder at the wisdom of constructing the line through what is to this day an active landslide.

We have come to meet Derek Butcher, principal geotechnical engineer at Network Rail, who is closely acquainted with this stretch of the county’s rail network. He is open in his acknowledgment that, were a railway between Folkestone and Dover being built today, you would be unlikely to choose this route for your tracks.

“The railway here was opened in 1844,” Mr Butcher explains, as we survey the scene from our vantage point above the tracks.

“It is 180 years old, which actually only makes it 20 years newer than the Stockton to Darlington railway line, which is the first UK railway line.

“This makes it incredibly difficult to manage. It's an old piece of line, constructed not in the ways that we would construct railway lines today. It's been constructed through a chalk landscape. It runs next door to the sea between Folkestone and Dover, which makes it quite challenging to run and maintain.”

A 3km stretch of the coast at the Warren is formed of what is known by the experts as a rotational landslide, described by the British Geological Survey as one of the largest anywhere on the English coast. Here a layer of chalk sits over a layer of gault clay, with the chalk from the ‘high cliff’ prone to slide down to form the ‘sea cliff’ nearest the water’s edge.

The risk of a landslide grows when the ground absorbs rainwater. And since England has only recently emerged from its wettest 18-month period on record, the potential for further landslides is clearly increased.

To understand the potentially catastrophic consequences of a major landslide on the railway, you only have to look back into the history books for examples of previous cliff collapses on this stretch.

“We’ve seen a number of instances of quite large landslips,” Mr Butcher says.

“There were large landslips in 1877 and 1915. The 1877 landslip killed two people. The 1915 landslip meant that the line was closed for three years. I think some of that was due to the war effort going on at the time.

“But certainly the railway was completely destroyed with the landslip and had to be rebuilt from the base level upwards.”

The 1915 landslide, which became known as the Great Fall, was so severe that it caused the railway line to shift 50m towards the sea as a section of the chalk cliffs collapsed after weeks of heavy rain. The kink in the line caused by the sudden shifting of the ground remains visible to this day, proving the severity of the cliff fall.

Mr Butcher discovered dramatic photographs of the aftermath of the landslide in a filing cabinet while moving offices some years back, and they show just how much damage can be caused to the railway by sudden shifting of the landscape around it.

"We believe the train pictured was alerted to the landslip by the signal box at Folkestone Junction and was slowed down,” Mr Butcher said at the time the photographs were unearthed.

“It found itself part on and part off the landslip. They were able to evacuate passengers who walked through the tunnel to Folkestone Junction station. There was a significant amount of movement following the train stopping. That’s why it looks so horrific."

The challenge posed by the rotational landslide at the Warren is two-fold. Firstly, it is active and ongoing. Secondly, it is of a scale that means a complete engineering solution to prevent the movement is impossible. This leaves Network Rail with a job which focuses on a combination of monitoring and maintenance.

“We've got an extensive monitoring regime for the Warren and the railway line itself,” Mr Butcher explains.

“We use something called Lidar, light detection and ranging, which is a laser scanning system which operates either from the air or from the ground.

“That allows us to determine how much the railway line and associated cliffs are moving on a day-to-day basis.

“We have some remote systems as well, which were installed into the ground, some inclinometers which monitor vertical movement, and some extensometers at the top of the cliff, which monitor horizontal movement.

“By looking at all those different changes we can determine which areas are moving, by how much, whether it's increasing over time, and what we need to do with that information.”

The recent spell of prolonged wet weather has not only affected the railway line through the Warren, where a speed restriction is in place following ground movement, but also the cliffs further along the coast in Folkestone itself. The town’s Road of Remembrance is set to remain closed all year after sections of the cliff there fell away.

Wetter winters and drier summers, driven in part by climate change, pose a challenge to Network Rail when it comes to maintaining the entire rail system. But while in other locations engineering works can significantly reduce the chances of landslides impacting the railways, things are more tricky here at the Warren.

“One of the issues with this particular landslip is its depth of movement,” Mr Butcher explains.

“The movement that occurs here is about 70 metres below ground level, so that's really different to other locations that we might have stabilised during the winter, which are a very shallow amount of movement, typically two to three metres worth of movement.

“There we will intervene with sheet piles. We can install sheet piles to about 10 metres depth, typically - sometimes beyond that to 20 metres. But we would never be able to install piling down to 70 metres.

“It's just beyond the range at which those conventional techniques are able to be installed.

“So typically at Folkestone we are looking at a different approach. That regular maintenance and monitoring approach seems to have stood the test of time down at the Warren.”

Network Rail plans to address the line between Folkestone and Dover this summer, adding fresh ballast to the track bed to return the tracks to their previous position prior to the most recent ground movement.

This should then allow the existing speed restriction affecting services between the towns to be lifted. However, this work will not be able to be carried out until the chalk and clay have dried out sufficiently and the experts are convinced we have seen the last of the ground movement in the area.

In the event that the worst should happen, and a significant cliff collapse or ground movement is detected, sensors would relay that information to railway controllers in real-time – allowing them to immediately halt trains in the area of the potential danger.

With the Folkestone to Dover line’s 200th anniversary coming 20 years from now, let us hope we do not see a repeat of the terrible drama of the ‘Great Fall’.

This stretch of the railway may be expensive to maintain, but it is a scenic delight and a feat of engineering which should be enjoyed by generations to come.