A MARINE-engineering expert has labelled the predicted three-month repair time on the Malahide viaduct as "nonsense", saying it will take at least twice as long as Irish Rail estimations.

The viaduct, in north Co Dublin, collapsed last month, severely disrupting services on the Dublin-Belfast rail line.

Dr Peter McCabe of McCabe, Delaney & Associates said the repair process will take at least six months, and possibly longer.

However, Irish Rail has insisted it is "confident of delivering" its three-month repairs before returning the line to operational use.

It has also ruled out the replacement of the viaduct with either a suspension or arch bridge which would cost in the region of €20m and €10m respectively according to experts. "The works [will] ensure that the existing structure will be perfectly fit for full operational use on completion of the current works. As a result there is no need for full replacement," a spokes­man said.

However, there remains a serious difference of opinion about those works and how long they would realistically take to complete.

"Drilling beneath the site is necessary to locate where the adequate foundations are under the mud," said McCabe, a marine engineer since the mid-1960s. "When the tide goes in and out of the estuary there is a big flow and that flow is undermining the random rubble foundation."

Labelling a three-month repair timeframe "nonsense", he said: "The site investigation will take about two months and then the building of new piers and sheet-piling will have to be done."

The sheet-piling process, he said, will involve a "zig-zagging" of material under the water to form a cavity around the foundations which will then be filled with cement to reinforce them.

"You can't put boulders in; they will be moved with high currents. I would say it will take a minimum of six months, maybe longer, depending on the site investigation."

According to Irish Rail, a full foundation survey is now underway on the viaduct and "significant additional foundation systems including deep piling will be utilised on all piers and abutments before the line re-opens".

Meanwhile, the company has issued tenders for the supply of bus services to transport passengers who normally avail of the train.