North Rode, Macclesfield, UK: Every ten minutes, a train speeds across the marsh here, travelling at 200 km per hour. Such speeds require a perfectly level track, so both the track and the 25-meter high embankment it is built on must be carefully maintained.
In late 1998, construction began on an embankment for a road running parallel to the railway. The 6-meter high embankment would be built at the toe of the railway embankment. While this offered some advantages for the road builders, it could cause settlement of the railway embankment. Halting or slowing the trains during construction was not an option, so engineers decided to monitor the embankment for vertical displacements and slow the trains only if settlement or heave occurred.
Boart Longyear UK was asked to install a track monitoring system, which included a 210-meter long string of EL beam sensors . The sensors were anchored to concrete blocks embedded in the ballast which would settle with the embankment.
A CR10 data logger provided continuous monitoring, and a modem transmitted readings from the data logger to a computer at the dispatcher's station. The computer, running MultiMon software, displayed a displacement profile of the track bed. If movements exceeded 15 mm over any 3-meter gauge length, an alarm would sound, screen symbols would change from green to red, and the engineer in charge could order the train to reduce speed.
The system proved itself on the morning of April 9th. At 9:00 the readings were normal, but at 10:00, a small change was noted by the engineers. No action was required because the movement was below the alarm threshold. However, by 11:00, movements had exceeded the 15 mm alarm threshold and the alarm was triggered. The dispatcher immediately ordered trains to reduce speed to about 3 km/h.
A 75-meter length of the railway embankment had slipped. The photograph at left shows the displaced line of beam sensors. The embankment was repaired, construction procedures were reviewed, and now the trains are running at speed again.
Thanks to Chris Rasmussen for providing this story.