Irish Sea link | The bridge and tunnel options deemed technically feasible by ex-ICE presidents

The feasibility study – published alongside the Union Connectivity Review – found that a bridge crossing would cost around £335bn, while a tunnel would cost around £209bn.

Carried out by two former ICE presidents, Douglas Oakervee and Gordon Masterton, the study ultimately ruled that while a crossing was technically feasible the costs were unjustifiable.

In total seven potential routes were assessed, with one preferred route selected for the tunnel option and one for the bridge option.

A tunnel would stretch from Bangor in Northern Ireland to Stranraer in Scotland, while a bridge would begin further south along the coast from Bangor and link to Scotland at just south of Stranraer.

Tunnel option

The preliminary design basis for the tunnel crossing of the North Channel includes provision for:

• A rail track in each direction with continuous access platforms and overhead power lines
• Rail tunnels to accommodate freight, vehicle shuttle and passenger trains
• The transport of dangerous goods such as petroleum
• A rail service tunnel for operations, maintenance and to support emergency evacuation and incident management
• Fresh air ventilation system to assume intakes (via shafts) at approximately 40km spacing

The report concludes that although there are "very significant challenges", none are complex enough "to render the construction of a tunnel crossing of the North Channel infeasible".

A proposal for a submerged floating tunnel was, however, ruled out due to the “technical immaturity of the concept” which “poses significant risks” and an immersed tunnel option was also ruled out due to the water depth in combination with the length of the tunnel and the possibility that a large extent of the seabed excavations could be in rock.

The potential rail tunnel has been assumed to have a maximum gradient of 1%. The tunnel portals would be inland due to the topography on both sides of the channel being steeper than the tunnel gradient (see diagram below).

In addition, permanent shafts would need to be constructed as close to the shore as possible for construction and to provide permanent ventilation and emergency access and egress.

Indicative tunnel vertical alignment

The study found that a configuration similar to the Channel Tunnel was the best option. However this would have two running tunnels with a larger diameter of 9m to avoid the need for "costly" piston relief ducts. The third tunnel for operation and rescue services would also be enlarged due to lessons learnt by the Channel Tunnel operators.

Indicative tunnel cross section

It would also be necessary to build two caverns approximately a third of the way into the undersea tunnel from either end to allow the trains to crossover onto the other track.

The study explains: "This provides a degree of resilience and allows railway operations to continue during periods of maintenance and in the case of an incident. These caverns would require a different excavation method."

Finally, launch shafts for the TBMs would be located on the Scottish and Northern Irish coastlines.

"From each of these shafts, three TBMs (two for the running tunnels and one for the service tunnel) would be launched to bore the marine tunnels and meet somewhere under the North Channel," the report explains.

"A further three land tunnels would be needed from the shafts on either coast, boring inland to a portal. The locations of the launch shaft should be sited to allow sea transport of materials and equipment to minimise environmental impact and disruption to communities."

Bridge option

The following preliminary design arrangement is proposed for the bridge option:

• A multi-span suspension bridge with seven main spans of 3,750m and two side spans of 1,500m. This roughly 30km long structure would occupy most of the length of the crossing in the deepest water and be supported by eight concrete pylons and two cable anchorages founded in the seabed.
• The approaches at each end of the suspension bridge comprise multiple cable stayed spans which follow a gently curving plan alignment. Relatively modest spans of about 370m are currently proposed since this suits the planned geometry of the alignment although longer spans would be feasible.
• A conventional viaduct with short spans of about 75m over the land sections at each end completes the crossing which is about 41km long overall between the abutments.

According to the study, the main challenges for the design, construction and operation of a major bridge crossing of the North Channel are the deep water, Beaufort’s Dyke and unexploded ordnance, as well as sub-sea utilities, large shipping volumes and the geology and seabed.

It concludes that although "very significant", these challenges do not render a bridge crossing "infeasible".

However, the study adds: "The cost and complexity of constructing foundations and substructures in deep water is high, and this work would be on the critical path for the overall programme.

"Therefore, to reduce construction cost and programme length, the study has concluded that it would be necessary to minimise the number of supports in water depths greater than approximately 40m. This inevitably leads to solutions involving multiple and very long spans."

Geology and seabed conditions would significantly influence the design and location of the foundations. According to the study the conditions would vary significantly along the length of the alignment and the design of the individual support foundations would therefore need to take this into account (see below).

The study says: "There is good rock available for the major bridge foundations. The layer of overlying softer deposits on the seabed varies from zero, where there is exposed rock in places, to perhaps 50m or more. The foundation type and design would need to be able to cope with this variability."

Geological section through the North Channel approximately on the line of the bridge route

The proposed alignment of the bridge means that pylon foundations would be required in water depths up to about 165m, with the study emphasising that bridge foundations have not been constructed in these depths before.

"However, the technology for building offshore platforms in deep water has been used for many decades and is well understood," it adds. "A form of concrete gravity base structures, designed to be founded on or just below the seabed, depending on the seabed conditions has been assessed as the preferred option.

"Shallow footings allow transfer of load to the load-bearing strata. They comprise reinforced and prestressed concrete elements and usually have a very large base to ensure the bearing pressures are small. The preliminary design for the pylons anticipates that they would rise 545m above sea level (for reference, The Shard in London is 310m above ground level)."

Meanwhile, the width of the deck required for structural strength and stability can accommodate two traffic lanes plus an emergency lane, a maintenance access lane and a railway.

Finally, the vertical clearance under the bridge deck in the main navigation channels would need to comply with international shipping requirements for all ocean-going vessels. This would normally be to a height of approximately 70m above mean sea level but due to specialist traffic in the North Channel, a minimum clearance of 76m would be assumed.

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