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Slope engineering: Eye spy a landslide

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A trackside landslip in south east London in February has not only proved the worth of instrumentation investment, but also provides a perfect case study.

In the early hours of Monday 11 February trains were stopped running between Bexleyheath and Barnehurst because of a landslip onto the track heading out of London. Nothing new there – it had happened to other locations in the same cutting in 2010, 2014 and 2016. But what makes this slip different is that Network Rail knew the moment it happened and was expecting it because of the remote monitoring system installed in 2016.

“It’s a busy line,” explains Network Rail senior asset engineer (geotech) for the Kent Area - South East Route Manos Tsoukalas. “We knew that there was movement in the area as the monitoring system had recorded movement in the days before.”

This stretch of the track opened in 1895 and is made up of the Harwich Formation with bands of sandy silt, gravel and clay. Network rail has installed remote condition monitored (RCM) Senceive Flat Mesh triaxial tilt sensors, along with 3G cameras after previous ground movement closed the line.

Costain survey manager Rob Bagnall who looks after the monitoring explains that there were around 200 nodes installed along the track after the slip in September 2016 with seven cameras covering the majority of the cutting.

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Barnehurst img 2759

A 20m wide landslip moved around 300t of debris, knocking out several nodes

“The nodes were installed 5m apart about a metre above the toe of the slope alongside the track and we are able to access the readings in real time remotely. Anything outside the tolerances and we are notified by text and email immediately,” says Bagnall.

The nodes communicate with each other, and when one node senses movement they all “wake up” and all the nodes associated with that particular network give a reading, including the camera which takes a photograph too.

It was the slow, gradual movement of one particular node in the days before the 11 February that alerted the team to a possible problems ahead.

The tolerances for the Senceive system are set at green for 10mm movement, amber for 30mm, red for 60mm and black for movement of 90mm.

The red alert for the one node on Thursday 7 February, was followed by a black alert for the same node on the Friday 8 February. The team met and decided to visit the site and install further monitoring the following day.

Bagnall explains: “Because of the earlier alerts and that we are able to remotely monitor the site, we visited on the Saturday 9 February and installed another node a metre from the top of the slope directly above the node that had recorded movement.”

Watching from home, Bagnall adds: “I had been watching remotely all day on the Sunday and had my phone next to bed when I went to bed that night. I was expecting to be woken up.”

A 20m wide landslip did happen at around 3am over a period of 30 minutes and moving around 300t of debris onto the track while knocking out several nodes, including the node installed the previous day, in the process.

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barnehurst cam 4 190205 to 190211

Images taken from the camera closest to the landslip 

“Readings are taken every five minutes normally, but when movement is detected the readings increase in frequency to 2.5 minutes after an amber trigger and one minute after red trigger, explains Bagnall. “We had incredible accurate information of exactly what had happened, when it had happened and while it was happening.”

Tsoukalas adds: “I received a call at 3am on the Monday morning that a large slip had occurred and the track needed to be closed.”

On his journey, a Whatsapp group created after the black movement trigger, was already in action. Images from a camera nearest to the ground movement and readings from the tilt sensors were already being shared so that everyone was aware of the current situation.

Fast forward 90 mins and Tsoukalas was onsite with the Network Rail mobile operations manager and the local delivery unit permanent way team.

“The failed mass was still on the move when I carried out my inspection in the dark and you could clearly hear the trees creaking as they were moving,” explains Tsoukalas. “I banned anyone from getting on the failed mass and the initial plan to remove leaning trees was aborted until it was safe to do so.”

Remedial work led by Bam Nuttall started later that same day and the cutting was cleared as well as the debris that had fallen on the track.

“We were lucky,” said Tsoukalas. “The slip was close to a station so we could access the site along the track pretty easily, and as it happened Nuttall was working a few hundred metres away so we could use that workforce and we were able to replicate the design of previous remedial work.”

A 45m king post wall was installed with reinforced concrete planks and will be backfilled at later date. In addition, 82 extra nodes and two additional cameras were installed by Costain and Senceive along the site – some to replace the ones damaged in the slip and some to add an additional row further up the slope. This brought the total number of nodes in the area to 282 with nine cameras. The additional equipment was installed when the team had track possession.

The track reopened on Monday 18 February – only seven days after the failure.

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Barnehurst finished works 3

A 45m king post wall was installed with reinforced concrete planks

Tsoukalas says: “It was only a week’s closure, which is clearly annoying for passengers, but we were able to install a permanent solution and have the track reopened within a matter of days without a speed restriction. 

“We were lucky that the slip was being monitored, we knew about the movement, were able monitor it remotely and the system worked. It was an excellent testbed for Network Rail and Senceive.”

Senceive CEO Graham Smith agrees and plans to use parts of the Barnehurst slip as a case study to demonstrate just how his firm’s monitoring can work. “Barnehurst has been important for us, it shows what we can do around intelligent sensing and demonstrates that relationship within the system between nodes,” he says.

Senceive customer service manager Barry Charlton adds: “Barnehurst was a proactive site. We know we are trying to stop something that we know is eventually going to happen. There are only a few sites that we could call proactive and that makes Barnehurst special.”

Tsoukalas says: “I look after 830km of track in Kent and we have to prioritise when we invest in monitoring but this was a priority after the previous ground movement and the monitoring worked.”

In CP6 Tsoukalas plans to install more monitoring along the rest of the track between Bexleyheath and Barnehurst and a full review of what happened at Barnehurst is already underway.

“It’s a brilliant case study,” says Smith.

Bagnall adds: “The slip has been extremely useful. We have the visual evidence, as well as the data, and the confidence in the system has paid off. It was the perfect storm.”

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