UNEXPECTED SETTLEMENT beneath a railway line, triggered by Dublin Tunnel works, forced the temporary suspension of rush hour Dublin commuter train services last week.
Ground movement occurred at the start of a six month pipejacking operation under the railway line at Fairview Park, by contractor Nishimatsu Mowlem Irishenco.
Around 50, 1.2m diameter steel pipes are to be driven horizontally to form a three-sided arch 30m wide and 20m deep under the line.
The enclosed soil will then be excavated along its 52m depth to form a void through which the road tunnel will run.
The steel pipes will be supported on vertical steel ribs installed at 3m or 4m centres as the excavation proceeds with the ribs supported on piles.
The movement occurred during jacking of the second pipe through poor ground, the site of a former municipal dump.
Dublin Port Tunnel project engineer Sean Wynne told NCE that an elaborate 'early warning' alarm system was installed to detect movement of the railway line, well in advance of any risk to safety.
'The pipes are connected to each other by clutches. The 'female' clutches have an opening 250mm by 100mm into which a the male Tsection fits. We believe that a train compacted the soil into the clutch, causing settlement, ' said Wynne.
An alarm triggered by the 5.9mm movement alerted controllers to stop trains. An 'amber' alert follows movement up to 3mm, while movement over 4.5mm causes a 'red' alert, stopping trains.
Port Tunnel and Irish Rail engineers are discussing a solution. Wynne said that his team was hoping for a relaxation in the settlement allowed before trains are stopped.