1. The Malahide Railway Viaduct Group 15: Harry Griffin & Carl O’Sullivan

2. The Malahide Viaduct extends to 175m in length. It is composed of : Eight 15.85m spans and four 12.9m spans. Spans consist of precast post-tensioned concrete beams 11 masonry piers support the spans A natural stone based weir supports the piers

3. Viaduct Weir Extends for full length of viaduct Composed of large rocks and boulders Piers sit on weir Grouting layer (1.5m deep) protects weir from scouring erosion

4. Structural Collapse Date: 21 st August 2009 Collapse of Pier 4 into the Broadmeadow Estuary Collapse of Span 4 & Span 5’s post -tensioned beams Rail-line suspended in mid-air Collapse due to: Failure of engineer’s to carry out safety checks on foundations of viaduct. Undermining of weir supporting Pier 4 by scouring

5. Water flowed between viaduct’s piers Weir caused water flow to increase in velocity, raising the scour potential Hydraulic jump occurred on the east side of the viaduct due to weir Material at weir’s crest became eroded due to turbulence from jump. This caused weir to elongate and deepen over time Mode of Failure

6. Whenever the tide reached its critical point: Hydraulic jump occurred between Piers 4 & 5 Turbulence of jump caused deepening of rock weir at this area Channel was formed where water velocity increased, thus increasing scouring of the rock layer Uneven distribution of flow across the 12 spans Resulted in increased scouring of grouting layer under Pier 4

8. Piping Piping by water flow occurred beneath the grouting layer This caused Pier 4 to become undermined Further undermining of the remaining grout that supports and surrounds Pier 4 caused it to collapse Unsupported Spans 4 & 5’s concrete slabs collapsed as a result weir

9. Site of Pier 4

10. Response Since the collapse, reconstruction of the viaduct has increased its overall strength and stability. For example: All piers were strengthened with the installation of 15 piles at every pier, increasing the piers’ load bearing capacity. The collapsed spans were re-built using 12 pre-cast concrete beams. In-situ concrete infill was applied to each span. Railway tracks and handrails were re-constructed. Damaged weir around pier 4 was repaired with rock armour. The overall profile of the weir was restored and improved.