The Malahide
Railway Viaduct
Group 15: Harry Griffin & Carl O’Sullivan
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
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
Structural Collapse
Date: 21st 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
Mode of Failure
• 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
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
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
Site of Pier 4
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.