The evolution of tunnelling and delivering with modern solutions

Tunnelling and transport planning

In the late 1960s, public outcry over “the Motorway Plan” in London prevented a network of surface freeways from being built. Similar public sentiment progressively prevented the completion of freeway networks in Australian cities.

Tables 1 and 2 lists some of the major Australian tunnel projects of the past 20 years. Many of the road tunnels listed above represent the “missing link” in a freeway plan which was drafted in the 1960s and which have had to wait until the tunnel solution can be found.

The trend which is evident from these lists is towards longer road tunnels. The Airport Link project at 7km is now the longest road tunnel in Australia and one of the longest urban road tunnels in the world. (The longest road tunnel in the world is the Laerdal Tunnel in Norway at 24.5km).

Not only are we seeing these changes in Australia but New Zealand is also planning to deliver major transportation projects using tunnel solutions.

Table 1. Urban transportation tunnel projects in Australia 1990-2010.

Table 2. Planned urban transportation tunnel projects in Australia.

Restrictions on tunnel construction

While tunnel equipment and technology has improved, the public has become less tolerant of the environmental effects of tunnelling. Often tunnelling is a 24 hour activity and noise at the worksite can be a critical issue. In 1972, the contractor on the Eastern Suburbs Railway was served with an injunction preventing 24 hour operation. Since that time, many city worksites have incorporated acoustic noise sheds which also have served to control dust.

In addition to this control, spoil can be stored during night operations to then be trucked offsite during the day (Figure 3).

A particular issue in modern tunnelling has been the focus on hydrogeology. The tunnel’s effect on the groundwater has driven design solutions in Melbourne and Brisbane recently. At the EastLink tunnel in Melbourne, the need to prevent drawdown of the watertable and maintain environmental flows to Mullum Mullum Creek was critical. The topography meant that the creek ran above the tunnel itself which led to the tunnel being designed and constructed as a completely watertight structure. Design specifications required very tight allowable leakage into the tunnel once was completed.

The solution was a hybrid precast/cast-insitu lining solution (Figure 4). However, other tunnels such as Clem7 (Brisbane) and Airport Link (Brisbane) have adopted the full segmentally lined solution installed behind a TBM.

Other environmental issues which are impacting on tunnel construction include the vibration caused by both mechanical and TBM tunnelling, known as regenerative noise. This was carefully monitored at the Clem7 site and will be monitored at the Northern Link project. The vibration is temporary, with limited intensity and short term as the tunnels move quickly past each residence. Because the vibrations may induce sleep disturbance, the issue is often handled by direct consultation with the affected residents.

Finally, the issue of ventilation outlets can be a matter for public concern, as was the case at Sydney’s M5 East tunnel. The purpose of the ventilation outlets is to elevate the outflow of spent air from the tunnel’s ventilation system so that this air can be mixed with fresh air in a controlled manner. This issue is reducing with every year, as the vehicle fleet improves. With the increasing use of electric and hybrid vehicles it can be anticipated that this issue might disappear in time.

User characteristics

Revolutionary changes in urban transportation tunnels are coming from the vehicles using the tunnels and the systems which can control them. Road and rail vehicles are changing rapidly with a potential knock-on effect in terms of the safety and ventilation systems required to be fitted to the tunnels (Table 3).

Within 20 years, it is likely that a substantial reduction in the capital cost of constructing a tunnel will be made due to a reduced need to provide ventilation equipment and ducts, and due to a reduced need to provide for cross passages and emergency walkways. Further in the future it is possible that lane width could be substantially reduced and that more lanes could be retrofitted to existing tunnels.

Table 3. Changes in vehicles and systems.

Tunnelling analysis and design

Similar to the situation in tunnelling construction, no major breakthroughs have occurred in tunnelling design since the 1970s. That said, incremental advances have taken place in the tools and methods, mostly on the back of the enormous advances in computers.

In 1970, the finite element analysis method had begun to be applied to tunnelling design, although each problem took weeks to set up and run. Using finite element models is now routine and powerful packages allow the tunnel engineers to solve problems in minutes (not weeks). The models available to the designer can now incorporate all properties of materials and ground, 3D features and water flow. (See Figure 5 for some examples of models.)

However, what has not changed is that the characterisation of the ground remains dependent on an expensive and time-consuming ground investigation, and that the development of relevant and useful design models relies on experience and on well judged model inputs.

One advantage now is that each of the cities in Australia has a database of previous experience in tunnelling which allows modellers to compare their outputs with actual sites.

Tunnelling in the future

So what does the future hold for tunnels? The summary below lists a number of possible future trends, including some speculative suggestions:

• The use of TBMs and precast concrete tunnel lining support will increase; conventional tunnelling methods will reduce. Ultimately, the cost per metre will gradually reduce with time, in terms of today’s dollars. The use of robotic systems in the construction of tunnels will become routine, with less labour seen in the underground part of construction.
• Tunnel analysis will improve, particularly using the ability to reference the database of previous projects.
• New metro systems will be built in Australia with bespoke, low fire load rolling stock, automatic train control and platform screen doors.
• Rock melting, jetting or chemical breakage will not be discovered as a revolutionary new method for cutting rock.
• Environmental restrictions on tunnelling operations will continue to get tighter. Hydrogeological issues will drive more tunnels to be watertight.
• Longer road tunnels will be planned and will be built.
• Improved vehicle characteristics will allow tunnel cross sections to be reduced.
• If the vehicle fleet changes to predominately electric or hydrogen vehicles, with zero emissions, tunnels may be built with no emission ventilation system and with petrol engine vehicle banned from entering.
• If hands free driver systems become common, lane widths may be reduced in tunnels and some older tunnels may be refurbished with additional lanes.

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Contributing author & reviewer: Harry Asche