Welcome to this themed issue of Bridge Engineering on bridges for sea crossings. Sea-crossing bridges present bridge engineers with some of their greatest challenges; bridges are exposed to harsh climatic conditions and require high standards of design and construction to achieve safe and durable structures. The capital costs of marine crossings are high, there is a wide range of potential construction risks, and important environmental and safety matters must be addressed. In spite of these difficulties and risks, experience confirms that the improved connectivity provided by many major sea crossings has brought significant economic and social benefits, which extend well beyond their locality to have regional and even national significance.
The eight papers submitted for this issue illustrate just some of the ways in which engineers are addressing the challenges, extending our capabilities and pushing back the boundaries of what can be achieved economically. There are perhaps three common threads that can be seen running through these papers; the need for a multi-disciplinary approach to take advantage of techniques developed in other fields such as offshore engineering; the increasing use of off-site modular prefabrication of components to capture the benefits of mechanised construction and reduce the risks of offshore working; and advances in design and availability of the specialist equipment that enables the transportation and erection of increasingly large prefabricated elements.
As an introduction, the paper by Mullins and Collings (2023) gives an overview of a selection of the more well-known sea crossings that have been constructed worldwide, as well as some projects currently under construction or at the planning stage. From the collected data, the authors have identified various trends and put forward proposals for benchmarking future projects to better bound initial estimates of time, cost and emissions.
We then have two papers about the Temburong Bridge (Sultan Haji Omar Ali Saifuddien Bridge) in Brunei, which is the longest sea-crossing bridge in south-east Asia. This bridge across Brunei Bay provides the first secure transportation link between two parts of Brunei without the need to pass through the adjacent country of Malaysia. The crossing was completed in early 2020 and soon proved to be a particular asset when the Covid-19 pandemic closed international borders. In the longer term, the bridge is expected to stimulate growth and investment in the less developed region of Temburong and in Brunei as a whole. The paper by Yip and Hussain (2023a) reports on the design and construction of the bridge and the strategies adopted to achieve a construction programme of under four years, as well as the construction techniques that were developed to minimise damage to environmentally sensitive swamp forest.
The subsequent paper by Taylor (2023) describes the unique lifting gantry that was designed to simultaneously erect two 50 m long, full-span, precast concrete box girder deck elements for the sea-crossing viaduct section of Temburong Bridge. The importance of innovation and development of specialist construction equipment can go underappreciated and it is interesting to note the author's comments that this gantry design was both the key to the successful bid submitted by the contractor, and to the achievement of the rapid construction programme.
Our next two papers describe recently completed sea crossings in the Pearl River Delta region of southern China – an area that is susceptible to severe weather conditions, including typhoons. The construction of both bridges was also disrupted by the Covid-19 pandemic, which provided an additional challenge for the construction teams.
In Hong Kong, on the east side of the Delta, Wang et al. (2023) describe how the concept of design for manufacture and assembly was used extensively for the Cross Bay Link Bridge, where most of the bridge elements above sea level were prefabricated or precast offsite. Possibly the most striking achievement was the full pre-assembly of the 200 m long, butterfly double arch bridge forming the central span of the viaduct. Weighing 10 000 t, the bridge superstructure was transported over 900 km by sea before installation on site.
Located on the west side of the Delta, the Jinhai Bridge is an unusual, four-pylon, cable-stayed bridge designed to carry both road and rail traffic at the same level. Ping et al. (2023) outline how the design was influenced by the requirements of the railway and how construction of the superstructure of the bridge adopted large, prefabricated elements to improve efficiency and minimise the risks associated with offshore working. Notably, each of the four pylons weighing approximately 2700 t, was prefabricated off site and erected in one piece using large floating cranes. The paper also describes the real-time monitoring system that was developed to control this key operation.
Construction of the 1915Çanakkale Bridge across the Dardanelles, Turkey (now the longest span suspension bridge in the world) began in 2017 and the bridge opened to traffic in March 2022, more than a year earlier than the original target date. In their paper, Ascaso Til et al. (2023) describe how innovations in the design and operation of the self-erecting, deck erection gantries enabled the main contractor to achieve significant savings in the deck construction programme, as well as minimising obstruction to international shipping passing through the Strait.
Our final two papers look to the future. A bridge across the mouth of Manila Bay, Philippines, has been talked about for some time, but has to overcome a demanding combination of deep water, seismicity, typhoons and ship impact. Yip and Hussain (2023b) describe how the feasibility study and preliminary design for the Bataan–Cavite Interlink Bridge adopted a multi-disciplinary approach to address the key issues, including detailed marine navigation simulations and consideration of foundation techniques developed originally by the offshore energy industry. Recently, it was announced that detailed design studies for the bridge have commenced.
A multi-disciplinary approach is also described by Xiang (2023) in his overview of the wide-ranging innovation, development and research being undertaken for fjord-crossing bridge technologies by the Norwegian Public Roads Administration. The ambition is to make the economically important E39 west coast highway ‘ferry free’, the achievement of which will require fixed crossings to some major fjords up to 3 km wide and with water depths of more than 500 m. The paper outlines some novel concepts that are being studied to overcome these exceptional site conditions, as well as the analysis techniques and model tests that support these studies.
We wish to thank everyone who has contributed to this themed issue, particularly the authors and reviewers for taking the time to share their experiences and valuable comments. We hope you find these papers to be interesting and thought-provoking and we especially invite readers to also contribute by submitting a discussion piece on the content of any of the papers. Details of how to contribute your comments can be found at the end of each paper.
The topics covered in this edition have focused on the planning, design and construction phases of bridges for a variety of sea crossings, but subsequent operation and maintenance also bring their own special challenges to ensure the availability and durability of these key items of our infrastructure. The implications of climate change are drawing increasing attention to the resilience of sea crossings, which are especially exposed to the effects of extreme weather. Many more bridges are being planned and built across the sea and this journal proposes to continue with this theme in the near future. As well as reports on new projects, we would like to widen the scope to include asset management, operation, maintenance, resilience and risk management. Please do write to us to let us have your ideas and proposals.
