Challenges of a Multiple Super-long Span Suspension Bridge Crossing of the Irish Sea

Constructing a 36km long bridge across the North Channel of the Irish Sea between Scotland and Northern Ireland in water up to 300m deep presents several substantial challenges. In March 2021, the authors were assigned the task of establishing whether such a crossing is feasible, as part of the UK Government's review of the connectivity between the four parts of the United Kingdom.The technical challenges are enormous and focus principally on constructing the substructures and foundations in very deep water, the main cable design and erection, the severe environmental conditions and many operational factors. The solutions that emerged from the high-level study included a suspension bridge carrying twin railway tracks and dual three-lane carriageways on a multiple steel box girder deck structure, with seven spans of 3750m supported on eight pylons rising to a height of nearly 550m above sea level. Similar concepts have been developed by COWI previously for the Gibraltar Strait Crossing and the Yemen-Djibouti Crossing, and this study was able to draw on lessons learnt from that earlier work.This paper describes the evolution of the preferred structural solution and focusses on the complex technical issues which set this project apart from any other long span bridge solution, including the deep water foundations and the design and construction of the main cables. It concludes with a discussion of the key risks and some lessons learnt for the design of such enormous structures in future.