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Sunday 25 February 2018

Bridge pillars rest on manmade causeway

Paul Melia

Paul Melia

BUILT in the 1840s, the wooden Malahide viaduct was replaced 20 years later by an iron railway track supported by 12 stone piers.

But unlike today, when the piers would be piled into the bedrock to ensure maximum support, the engineers of the 1860s had them "sit" on a man-made causeway constructed of large stone blocks (known as rip rap) which rested on the seabed.

More stone was added over time to replace rocks eroded by the water (a process known as scouring). This was achieved by removing the timber decking from beneath the track and dropping stones into place, which ensured that the viaduct was stable.

But in 1967 the iron deck was replaced with a concrete structure. No longer could rocks be placed at specific points, instead they were dropped over the side and staff would manually shift them into position.

At this time, it was also decided to pump grouting into the causeway to "anchor" the supporting piers into place.

It was believed that these works would reduce the need for ongoing maintenance, particularly in relation to the unloading of rip rap.

The report notes that since this time, use of rip rap was "more limited" and appeared to be carried out to protect the piers.

It concludes that the emphasis was on maintaining the viaduct structure, and not the causeway, and "the importance of maintaining the weir profile was no longer fully appreciated".

On August 21 last, erosion of the causeway resulted in one supporting pillar -- Pier 4 -- collapsing into the sea just moments after a train passed over the viaduct.

Irish Independent

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