Special alarms which measure subsidence failed to activate yesterday when ground close to Dublin Port Tunnel works subsided. Dublin City Council and Iarnród Éireann have launched a joint investigation into how the subsidence happened.
Commuters entering the capital from the north endured early morning chaos.
A hole of around two metres in diameter and one metre in depth appeared beneath railway tracks just south of Clontarf DART station at sometime in the early hours of yesterday morning. All train and DART passengers from 6.30 a.m. were forced to disembark at Clontarf and make their own way into the city centre.
Full train service did not restart until shortly after 10 a.m. yesterday.
The hole was around 30 metres from a hoarding which blocks the tunnel work site off from the public. The section of earth which shifted was directly beneath and to the side of one of six railway lines in the area. However, because the line itself was not displaced the alarms failed to go off.
The alarms are electronically linked to the port tunnel's base in Fairview. The base is in 24 hour telephone contact with Iarnród Éireann.
The alarms measure any vertical or horizontal shift in the earth around port tunnel works. They went off during a false alarm in February.
At that time the Dublin Port Tunnel company said the equipment which triggered the alarm was so sensitive it picked up any slight vibration because of the strict safety criteria surrounding the tunnel construction.
Yesterday's subsidence was only discovered just after 6.30 a.m. by an Iarnród Éireann worker conducting a routine check on the line.
The tunnel passes beneath six railway lines - two of which are live - just three metres beneath the ground in the area where the subsidence took place. A crew of Iarnród Éireann workers excavated beneath the cavity yesterday morning. No deeper fault was found and the hole was filled. Mr Barry Kenny, a spokesman for Iarnród Éireann, said it is anxious to determine exactly why the subsidence occurred. "Any modifications that are needed will be introduced," he said. Deputy city engineer, Mr Tim Brick, conceded yesterday that there may have to be a "rethink" in the way the alarms are used in the future. Mr Brick is responsible for the Port Tunnel.
He said that while the council is worried about yesterday's incident it was still not clear whether work on the Port Tunnel had caused the subsidence.
He added that the tunnel has been completed in the area where the subsidence took place. Calls from residents in nearby Marino to stop tunnelling were "baseless", he said. "There is no comparison between tunnelling in three metres of soft earth under live railway lines and driving through solid rock 60ft below houses," he added. The tunnel works will extend under homes in Marino, Griffith Avenue and Santry in the New Year. Marino residents are particularly concerned about the possible impact of the digging on their houses.
Mr Fintan Cassidy of the Marino Development Action Group said yesterday's events "do not inspire confidence".
"It was exactly in this area in Fairview where the most research was done, where the most holes were drilled. So we would be concerned now that even there, with all they knew about it, that subsidence has happened."
Mr Cassidy's group yesterday called on work on the tunnel to be stopped pending a new safety assessment of the area.
That call was echoed by local independent TD, Mr Finian McGrath.
He said there are now "very real public safety concerns" surrounding the project. "I would be most worried that there has been slippage on the construction timetable and that the Port Tunnel Company may be progressing at the expense of safety," he said.Council engineers were last night due to begin using special radar equipment to assess whether the area around yesterday's subsidence had shifted further.
Workers from the Dublin Port Tunnel company and the council will continue their investigation on the line tonight.
