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Rail: Watching and waiting

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Network Rail plans to use remote monitoring more extensively in control period 6 and all eyes are currently on a site in Kent that is putting this concept to the test.

Probably one of the few people hoping for a long, wet summer is Network Rail route asset manager for geotech, drainage and off-track Derek Butcher. But he only wants it to rain on a specific part of the Kent coast – Folkestone Warren, to be exact.

In 2016 Network Rail worked with Costain, Socotec and Getec to install remote monitoring at two high risk locations along the railway between Folkestone and Dover in a bid to gain certainty about landslide risks. Two years on and a good baseline of data is being built up and Butcher believes it will soon be able to help Network Rail forecast ground movement trends based on weather information.

2016 instrumentation installation

2016 instrumentation installation

The instrumentation system was installed in 2016

“Weather conditions are specific to the local area,” he says. “The data we’re gathering here will give some insight into how microclimates impact on ground movement. We’re probably five years away from being able to use that information though.

“We’re comparing the data here with other weather centres at Dover and Biggin Hill.”

The monitoring system, which was part of a £1M investment that included laser surveys, was installed in two locations – within Folkestone Warren itself and at the High Cliff above the railway line.

“Previous monitoring has focused on surface levels, but the new instrumentation is giving us a clearer picture of what is happening at depth from the rotational failures within the Warren,” says Butcher.

According to Getec general manager Justin Roffey, what makes the instrumentation and monitoring different from other systems he has worked on is that it is more detailed and looks at more aspects of the ground movement.

“It is looking at the overall cause of the ground movement as well as the impact on the railway,” he says.

At Folkestone Warren a slip cuts across the route of the railway and this is the section that presents the highest risk to the railway, which is why the instrumentation focuses on this section.

“At the Warren, two boreholes were installed to 70m – the first was fully cored and logged, while the second was open holed,” says Roffey.

“One has a Shape Accel Array (SAA) installed through the full depth and anchored in the Folkestone Beds and the other has two vibrating wires and four piezometers installed.

“The SAA is showing movement on two slip surfaces – the first in the top of the Gault and the second within the Sulphur beds of the Greensand.

“Two of the piezometers are installed in the Chalk rubble and two at lower levels.

“The upper one in the Chalk is showing lots of fluctuation with rainfall and the lower ones show a tidal influence.”

Butcher believes that the SAA is the only one installed on a Network Rail site. “The SAA can monitor movement over a greater range than conventional inclinometers, so is a longer term investment,” adds Roffey.

The strategy for the High Cliff is different and the instrumentation there is formed by two 8m long horizontal tilt arrays placed in trenches.

“The ground movement at the top is a first-time failure,” says Warren Geotechnical Associates director Colin Warren, who has been advising Network Rail. “There are conjugate joint features in the Chalk that suggest that the failure could be sudden and catastrophic.”

Roffey explains: “At the top of the complex at High Cliff we have two trenches across the backscar to monitor for lateral movement. Movement so far has been very small – in the region of 0.2mm.”

Network Rail senior asset manager (geotech) Manos Tsoukalas, who is working with Butcher on the project, adds: “We are hoping that any movement trends picked up by the instrumentation will flag up a warning ahead of a failure.”

The trigger levels are currently set at 5mm, 10mm and 20mm for the trenches in the High Cliff and the SAA trigger levels are set at 10mm and 20mm. “We’re still watching the trends though,” says Butcher.

Tsoukalas adds: “The trigger levels are currently more like baselines and will be refined.”

folkestone warren diagram

folkestone warren diagram

At present the system issues alerts by mobile phone if the trigger levels are breached, but plans are underway to directly link the alerts to the signal box at Folkestone East later this year.

“The system will use a signal booster at the Warren to send the data wirelessly through the tunnel to the signal box,” explains Roffey.

The ultimate aim of the work is for Network Rail to manage the risk that a train emerging into the landslide complex from either the Martello Tunnel at the western end or Abbots Cliff Tunnel at the other end could be affected by ground movement or landslide debris.

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Normally the line carries 50 train services a day including high speed services.

“We are looking at what protocol to set for when the alarm goes off,” says Butcher. “We need to consider whether a train is sent through at caution or if there is preset speed to send a train through.”

When it comes to managing the risk, there is also the effect of data collection frequency to consider, which is a careful balance of having regular readings with battery life of the data logger.

“The piezometer and vibrating wires are connected to a data logger that records every 15 minutes and uploads data every three hours,” says Roffey.

Data is everything and the more Network Rail has, the better the understanding it will have of the ground movement, hence Butcher’s hopes for more rain.

“At the moment we have a watching brief and hoping for high rainfall to help understand how this influences ground movement,” explains Butcher.

“We know that the Gault moves more when there has been high rainfall levels, but at the moment it is unclear how this affects movement in the Chalk. We need to understand the differential movement.”

We have seen an increase in pore pressure over the winter and it will be interesting to see how this changes over the summer months.

“Heavy rainfall at the start of May showed a clear correlation with a spike in the piezometer,” says Roffey. “There was 50mm of rainfall and a clear change in pore pressure. We are currently analysing the data to understand if the movement we’re seeing is a trend triggered by rainfall.”

While the immediate concern is about the safety to operate the rail line, Warren is also considering what the data could mean for improving the stability of the ground. “The data could help us understand if there is a need for more drainage,” he says.

Whatever the outcome for Folkestone Warren in terms of early warning or stabilisation solutions, the scheme is likely to be a pioneer for Network Rail’s control period 6 (CP6), which runs from 2019 to 2024.

“Greater use of remote monitoring is a key element of CP6 and the work at Folkestone Warren is a real learning opportunity, as remote monitoring elsewhere is based on surface movement,” says Butcher.

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Deflection in the sea wall shows the ground movement

Long Term issues

Ground movement of the landslide has led to many closures since the line was first opened in 1844, the longest being four years. Catastrophic failure still remains a real risk.

After the rail line was built there were some small slope failures but larger movements occurred in 1876 and 1896, causing disruption to the tracks. The 1896 failure is recorded to have cracked the Martello Tunnel, resulting in part of the tunnel being demolished and the portal being rebuilt further west to prevent a future landslide shearing the structure.

In 1915 a major failure buried track under up to 20m of debris and creating a debris flow 70m out to sea. A train was already on the section when the failure occurred but fortunately there were no serious injuries. This damage took four years to repair.

Drainage was installed in the 1950s and stabilisation work then focused on constructing sea defences, including a concrete toe-weighting apron, coastal protection and adits for drainage, but the area has continued to experience stability problems.

The landslide complex within the Chalk and Gault Clay deposits is 1.7km, so stabilisation is not an option and neither is re-routing the rail line. Normally the line carries 50 train services a day including high speed services.

Various monitoring systems have been installed at the site in the past but none has given clear detail about ground movement at depth.“In the 1980s the risk was managed by basic surface monitoring,” says Butcher. “We dabbled with electrolevels and tiltmeters but none of them proved to be long-term solutions as the wires were often broken by tamping operations. We still carry out expert walkovers once a year as well as track monitoring and earthworks monitoring, but it is more of a watching brief.”

In the last 10 years, £10M has been invested at Folkestone Warren with much of the money spent on maintaining and improving the rock revetment and the toe weighting apron to minimise movement in the coastal margins.

Network Rail came to the end of a 10-year management plan for the landslide complex last year and started to look at how the issue could be better managed or monitored. This has led to the current phase of work.

 

 

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