Irish Sea Link | Proposal for bridge and tunnel crossing via two man-made islands - International Burch University
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Irish Sea Link | Proposal for bridge and tunnel crossing via two man-made islands

A proposal for a combined bridge and tunnel crossing between Dublin and Holyhead, including two new Irish Sea islands, has been suggested as an alternative to plans for a Scotland to Northern Ireland link.

Engineer Ian Hunt, who has shared the plans with NCE, likens the proposal to Denmark’s Oresund Crossing.

Hunt formerly worked at Gifford – now owned by Ramboll – where he led the Mersey Gateway team up to the public inquiry in 2009 that allowed the bridge project to proceed.

He prepared the plans for the Wales-Ireland link back in 2019, in response to prime minister Boris Johnson’s suggestion of a bridge between Scotland and Northern Ireland.

Hunt believes the Dublin to Holyhead route is preferable because it “offers the advantage of an easy link with established infrastructure”.

His idea suggests dividing the route into three, approximately equal, lengths. Elevated bridges would be provided at either end, each approximately 40km long, with a central 45km-long tunnel section under the deepest part of the sea.

“The distance between the south of Dublin Bay and the A55 at Holyhead is approximately 130km,” he said. “A long crossing – but not impossible. Water depths in the Irish Sea can be greater than 100m but is generally shallower.”

It comes after tunnelling expert Bill Grose last month said that Grant Shapps’ suggestion of a Wales to Ireland tunnel could prove to be a “more attractive” prospect than the Scotland to Northern Ireland proposal.

According to Hunt, construction of the tunnel/bridge proposal would take 10 to 15 years and the scheme could cost around £150bn in total, with £50bn needed for each island, £30bn for each bridge, £40bn for the tunnels plus the costs at each end to tie into the existing infrastructure.

“These are massive figures,” he said, “but the benefits to local economies are likely to be equally considerable.”

Islands

The two “islands” would be formed in the middle of the Irish Sea to facilitate the transition between the high level and submerged sections of the route.

Hunt described this as “a major task – but not an impossible one”.

In order to permit tunnel construction each island would need to be “more than 5km long” but only around 400m wide. They would be formed in approximately 60m-deep water so will need “careful construction”, Hunt said.

An external “skin” of very large rock armouring (for example 2m x 2m x 2m boulders weighing in excess of 20t) backed by smaller rocks (for example 1m x 1m x 1m, weighing 2t to 3t ) could be formed into a ring around the perimeter of each island. The central core would then be filled with dredged sand, and each island raised in intervals to a final level of approximately 10m above sea level.

Work sites could then be established to enable the construction of the tunnels and the other buildings. On completion of the project, the islands could also be used to provide rest breaks for travellers as well as the necessary operating provisions for the tunnels in particular, for example ventilation, drainage and lighting systems.

Tunnels

Hunt proposes a central rail tunnel flanked by two road tunnels carrying two lanes of traffic in each direction. Between these tunnels, service tunnels would provide emergency exit if needed. These should be slightly pressurised to prevent the ingress of smoke. Egress from each service tunnel will be at either island where provision for dealing with evacuees would be provided.

Hunt added: “It is envisaged that the maximum gradient for rail is 2%. In such a case it will be necessary for the rail tunnel to be something like 4km long before it can achieve a depth of 80m below island level. This is the depth necessary to permit the tunnel boring machines to get below the rock armouring around the island.

“Highway gradients can be steeper (4%) so the tunnel lengths can be less before leaving the proximity of the island. Thus the portal can be nearer the middle of the island.”

Bridges

For the bridge sections, the enclosed bridge deck is likely to be 8m deep and 48m wide.

“It is suggested that this be a largely concrete structure formed in match-cast units 8m long. The whole to be longitudinally prestressed to provide strength and continuity,” Hunt explained.

“In order to naturally ventilate the roadways and to provide shear strength, a repeating pattern of cruciform steel elements are incorporated at mid-height. The central rail-carrying cell is flanked by reinforced concrete walls providing the necessary shear strength to the longitudinal bridge deck. These will be regularly pierced by elliptical openings facilitating ventilation and enabling emergency egress.”

The depth of the deck would allow varying structural forms catering for a variety of spans up to approximately 2500m. There would also be a need to incorporate movement joints. Foundations and supports to the elevated structures will be needed in fairly deep water. Where conventional cofferdams are not feasible, Hunt suggests that off-shore rig technology is used.

Background

Various options have been put forward for a Scotland to Northern Ireland crossing since the bridge option was first suggested.

Downing Street officials have recently suggested that three tunnels under the Irish Sea could connect in an “underground roundabout” beneath the Isle of Man. The proposal includes three starting points: at Stranraer, Heysham, near Lancaster, and one near Liverpool. Then a single tunnel would run on from the Isle of Man to Northern Ireland.

Grose has previously highlighted several challenges presented by the proposal. He added that another strength of the Wales to Ireland suggestion is that it would be “shorter in total tunnelling terms” than the Isle of Man route.

Formal proposals for an Irish Sea link between Scotland and Northern Ireland have been put forward by various groups as part of the government’s ongoing Union Connectivity Review which will look at how to boost transport infrastructure throughout Scotland, Wales, Northern Ireland and England via road, rail and air, and across the Irish Sea.

The review will be published in the summer. Its interim report concluded that further work should now be undertaken to look at a “fixed link” across the Irish Sea.

Source Irish Sea Link | Proposal for bridge and tunnel crossing via two man-made islands | New Civil Engineer

Department of Civil Engineering – International Burch University (ibu.edu.ba)