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Immersed tunnelling - under water is not a boring tunnel solution

An example of an immersed tube tunnel under Sydney Harbour

An example of an immersed tube tunnel under Sydney Harbour

In the world of heavy construction, there are many structures that take shape right in front of our eyes; a bridge, a freeway or perhaps a building are easily seen as they are constructed.

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.

Let it sink in – how immersed tube tunnels are constructed

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:

  1. Steel shell tunnel: usually in a circular-shaped section for single tube or a binocular shape for a double-tube cross-section. The steel shell structure consists of relatively thin-walled composite steel and concrete rings and provides the water barrier. The ballast concrete is placed outside the shell in pockets formed between the structural diaphragms. The shell provides the water tight environment and can be prefabricated in the fabrication yard. Being lightweight, it can be lifted and launched sideways or longitudinally to the water, and is less sensitive to foundation discontinuities and temperature deformation than concrete. The advantage of steel shell over concrete is it can be segmented with much smaller pieces where you have limitations on casting and floating.
  2. Concrete tunnels: reinforced structures in which most of the final weight is incorporated in the structural components. Concrete immersed tube tunnels are most often used for double and multiple tubes where rectangular shapes are required for internal clearance of traffic and good conformity to resistance and weight.
  3. Steel-concrete-steel sandwich tunnel: both internal and external surfaces are constructed with structural steel plates. Steel diaphragm plates and shear connectors are used between internal and external surfaces and filled with non-shrinkage self-compacting concrete, eliminating the necessity for reinforcements.

 Sinking an immersed tunnel

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 past and the future

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 an immersed tunnel

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.

In the belly of the beast – the benefits of immersed tunnels

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:

  • Shallow cover and shorted length with better connection of the road networks.
  • Tunnel cross sections are highly versatile, making them particularly suitable for wide highways and combined road/railway crossing.
  • No unforeseen ground risks as the trench is dredged and exposed.
  • Resistance to seismic activity by providing shear keys and longitudinal anchors across the immersion joints.
  • Safety of construction (for example, work in a dry dock as opposed to boring beneath a river).
  • Flexibility of profile (although this is often partly dictated by what is possible for the connecting tunnel types).

The environment and location must be right though, because there are many variables that engineers need to consider when designing immersed tunnels:

  • Weather
  • Seasonal tidal levels
  • Density of harbour bed
  • Sea current
  • Siltation of harbour bed
  • Condition of the harbour bed
  • Availability of local contractor equipment & expertise

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 equipment used to tow and sink tunnel elements
  • Harbour bed foundation type
  • Waterproofing of tunnel elements and joints (Gina gaskets and Omega seal)
  • Immersion joint and method of final closure joint
  • Marine works for dredging
  • Cofferdam for the connection to cut and cover tunnel at both sides
  • Impacts on the surrounding environment and marine life

Design of immersed tunnel cross sections

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:

  • Density of water (depth in harbour)
  • Weight of construction materials
  • Weight of temporary equipment
  • Internal facilities required
  • Standards and volume of traffic for which the tunnel is designed for
  • Integrating with existing shipping lanes and sea operations

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 an immersed tunnel

Backfilling of an immersed tunnel

Digital disruption through the 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.

About the author

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.


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