An example of an immersed tube tunnel under Sydney Harbour
However, there are structures that are far more stealth, and seemingly invisible, in their construction methods which brings up the question, “How do they do that?”.
Tunnels are a prime example; the work is unseen until the day they are open to traffic. One type of tunnel that sparks people’s interest is a tunnel under water and one method to achieve this is immersed tunnelling.
This form of tunnelling is where I have spent most of my career as an engineer. I often get asked how we connect watertight boxes in a chain together, some nearly as tall as a three-story building, in waters as deep as 50 metres. This is one of the reasons why I enjoy designing immersed tunnels; the seemingly impossible nature of the task, but the imense benefit it brings to cities by connecting its people between their destinations. Its status really becomes an invisible landmark for a city.
An immersed tunnel is an underwater tunnel composed of tunnel elements, constructed elsewhere and then floated and towed to the tunnel alignment to be sunk into place and then linked together.
This type of tunnelling solution is usually favoured for harbour and sub-sea environments due to its shallow depth below the seabed, that allows shorter tunnel lengths and better connections to the road network at both ends of the subsea section. In addition, the shape of an immersed tunnel is flexible to fit different projects.
Tunnel elements are built in a casting basin or fabrication yard and classified as one of three types:
Sinking of an immersed tunnel element
With each form of tunnel structure, the tunnel elements are immersed in the water at their final resting place and joined to each other to form the tunnel. Each element can weigh up to 80 000 tonnes and be as long as two football fields.
Whilst the tunnel elements are being made or cast, a trench is dredged under the seabed where the tunnel will finally rest; a significant and sometimes contentious component of the project, with the need to carefully change the seabed structure to minimise impacts on the marine environment or the surrounding ecology in the vicinity of the tunnel site.
The construction of immersed tunnels is not a new concept, but an application in tunnelling design that has been used for more than 100 years, however not as widespread as other tunnelling methods. Today, the number of immersed tunnels that have been built worldwide is more than 150, with the majority being constructed for road and railway.
There are immersed tunnelling projects that remain significant for me; they were challenging, but they transformed cities and connected communities and commercial operations. One such project is the world’s longest immersed tunnel, at 6-kilometres, linking two man-made islands for the Hong Kong-Zhuhai-Macau Bridge project in China. The travel time of four hours for motorists was reduced to 30 minutes with the new tunnel.
I can see that tunnel infrastructure in Australia is going through a period of unprecedented change, with growing populations and emerging technologies transforming how we plan, design and build these projects. Like some of the mega tunnels across Asia and Europe, Australia needs these projects to move its people in the future.
Joining of immersed tunnel elements
The Sydney Harbour tunnel was the country’s first immersed tunnel opened in 1992 and is used by more than 90 000 vehicles each day. Engineered by Aurecon, it really is an invisible landmark between the iconic Harbour Bridge and Opera House as a submersed harbour road tunnel that connects one of the world’s largest cities. Immersed tunnels are a great solution for Australia’s cities, to be able to move people but fit within the confines of the urbanised settings.
The main advantage of immersed tunnels is that they can be considerably more cost-effective than alternative options – i.e. a bored tunnel beneath the water being crossed (if indeed this is possible at all due to other factors such as the geology and seismic activity) or a bridge that will obstruct the water view. Other advantages of immersed tunnels:
The environment and location must be right though, because there are many variables that engineers need to consider when designing immersed tunnels:
This is where marine, civil, alignment, tunnel system, geotechnical and tunnelling engineers have a critical role to play. Engineering is key to designing the right tunnel shape; round, oval or rectangular, for the water crossing’s geological profile, ventilation requirements and the volume of traffic to be handled through the tunnel.
A nearby fabrication yard is important for an immersed tunnelling project, to lower the risk of floating and towing tunnel elements. In addition, the casting basin should have sufficient depth to be able to float and tow the tunnel elements, so its formation works need to be extensively designed and built. Engineers of immersed tunnelling projects put key consideration into the topographic, geological and geotechnical aspects when selecting casting basin and fabrication yard sites.
Other key considerations include:
The cross-sectional design of immersed tunnel elements needs to be carefully calculated. During the temporary conditions, it needs to be light enough to allow it to float and tow from the fabrication basin to the tunnel side. On the other hand, the tunnel elements need to have sufficient weight to guarantee safety against uplift under their permanent location under water.
The cross-sectional design of immersed tunnels depends on the application of the tunnel and what its purpose is; highway, railway or both, or ventilation. These are the determining factors for the interior dimensions. Other issues to be considered are:
Emergency egress, ventilation, services, road drainage, road geometry (super-elevation, grades and sight distance for horizontal and vertical curvature) also play important parts in the cross-sectional design.
The design of the cross-sectional geometry is very sensitive to variations in the density of water and the construction materials; dimension inaccuracies; and the weight of temporary equipment needed for transportation, temporary installation and the permanent condition.
With the dimensions determined based on the buoyancy check, the tunnel cross-section will be checked structurally under ultimate and serviceability limited states loading conditions.
In writing this article so far, I’ve touched on the design and construction of immersed tunnels but, there’s another aspect to the role of an engineer on underground tunnels, and that is risk and safety. The variables in designing and constructing immersed tunnels require pinpoint precision during the construction of these modern marvels. It requires constant vigilance, keen adaptability and minute adjustments to changing conditions. The modern evolution of digital engineering is one platform that is helping designers to identify and minimise project risk and I’m glad to say that it’s here to stay.
Backfilling of an immersed tunnel
With the design, safety and risk considerations of immersed tunnels, it’s imperative for constructors and engineers to have timely access to accurate and highly-detailed data. Digital engineering tools can help to achieve this if used in an effective way.
Two-dimensional drawings still have a place on projects as they are used for construction, but 3D models can house sets of data for the tunnel systems designs. Models deliver value by allowing engineers to generate designs that require complex coordination between structural design and systems. Graphical interfaces enable customisable design workflow to occur simultaneously and work nodes to be connected to specific tasks and inputs.
By defining different parameters, in 3D, to represent different dimensions of tunnel cross sections, the weight of tunnel elements and ballast concrete can be determined automatically. The dimensions and weights can then be exported to buoyancy checking programmes for several iterations until the best dimensions are determined.
With the adoption modelling for the buoyancy checking of immersed tunnels, it can significantly enhance the effectiveness and efficiency of checking processes. In addition, the dimensions of the tunnel elements can be exported to the digital structural analysis modelling programme for serviceability and ultimate limited state checking.
This leap from manually generating 2D design plans to adopting ‘smart’ digital processes for immersed tunnels will also contribute to the collaboration and co-creation of constructing these modern ‘invisible’ transport networks.
Tunnels play an important role in facilitating commerce and travel between points separated by mountains, rivers, bays, and even straits. Improvements in equipment, processes, digital technology and tunnel systems are entering a whole new era with smart technology, allowing us to deliver improvements in how we design and construct immersed tunnels.
I’m proud to be contributing to the building of immersed tunnels to make travel faster, more convenient, safer and easier, for residents of some of the world’s largest cities, and I’m looking forward to embracing the future changes in the way we construct road, rail and utilities tunnels.
CK Tsang is Aurecon’s Technical Director – Infrastructure, with 20 years of experience in the industry. As a project manager and director of tunnelling, metro and highway projects, he has worked on many world-renowned tunnelling projects. These include using the world’s biggest subsea tunnel boring machine for the Tuen Mun-Chek Lap Kok Link project in Hong Kong, and the world’s longest immersed tunnel linking two man-made islands for the Hong Kong-Zhuhai-Macao Bridge project in China.
Tsang appreciates the critical role that tunnels play in the ongoing delivery and maintenance of infrastructure for our modern world. He is the author of many technical papers focused on tunnelling and transport infrastructure, and has been a part-time lecturer on tunnel and cavern engineering at the Hong Kong University of Science and Technology.