Council told tidal scouring caused viaduct to collapse
COMPREHENSIVE REPORT ON STEPS TAKEN PRESENTED TO CLLRS
Work in ongoing at the site of the collapsed viaduct.
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Wednesday September 23 2009
TIDAL scouring is believed to be the cause of the dramatic collapse of the rail viaduct across Malahide estuary, which occurred seconds after a packed train had crossed the bridge, councillors have been informed.
At last week's full council meeting, councillors were presented with a comprehensive report on what steps had been taken since the incident in August.
Analysis
An analysis carried out by Iarnród Éireann has found that the likely cause of the dramatic collapse was due to the pier being undermined by the scouring action of the tides.
'This scouring resulted from a breach of the weir which exists across the mouth of the estuary,' the report stated.
The council explained that the weir was a very important component of the viaduct.
'Its presence distributes the daily movement of the waters evenly over each opening in the viaduct, ensuring that the entire waterway is used to handle the flow thus reducing the velocity of the water.
'Minimising the velocity of the water in this fashion reduces the risk of scour thus protecting the structural integrity of the viaduct.'
However, the breach in the weir reduced the level of the top of the weir at the fourth pier. ' Water will naturally flow towards a low point, and it did so in this instance, greatly increasing the speed of the flow through one eye of the viaduct.
'The increased speed of the water resulted in local scouring around the base of one pier eventually triggering its collapse. The flow through the scoured section was equivalent to 70% of the entire flow into and out of the estuary.'
Councillors were told that the most pressing task at the site of the collapse was to repair the weir.
' This work has been progressed satisfactorily and the water level in the Broadmeadow estuary has now been stabilised at a level close to its original level,' the report stated. Work to assess the correct solution to restoring the viaduct continued while the weir replacement activities were in hand
'It is now proposed to reconstruct the damaged pier and replace the beams linking it to both adjacent piers. Work on site at this time is concentrated on preparing a replacement foundation on which to place the planned replacement pier.'
To facilitate the works, a causeway has been constructed across the estuary parallel to the viaduct, which is used to allow machinery and materials to be brought to the site.
' The question of the retention or otherwise of this causeway has not yet been decided.
'Irish Rail are being advised on this aspect of the works by Dr E McKeogh, Department of Hydrology, UCC, who is directing the construction of a physical model of the site and compiling a computerised model of the water movements.'
- Fergal MADDOCK
