Humans began constructing tunnels with picks, shovels and manual labour. Today, tunnelling is highly mechanised, using enormous Tunnel Boring Machines (TBMs) capable of operating 24 hours a day seven days a week. Constructing tunnels is increasingly automated and the equipment used is growing in size and technical sophistication.
In the world of civil contracting and geotechnical engineering, the TBM is the equivalent of the modern commercial airliner. These mighty pieces of equipment are pushing the boundaries of what is possible and they continue to grow in diameter and capability. The largest TBM in the world measures a ‘whopping’ 19.25 metres in diameter. The machine was constructed to deliver the (now) mothballed Orlovski Tunnel project in St Petersburg. The largest TBM currently in operation measures 17.5 metres and is in operation in Seattle, Washington in the USA. While in Australasia, the 15.1 metre TBM excavating the Waterview Tunnel in Auckland, New Zealand holds the title for the biggest diameter machine.
With increases in the cutting diameter of TBMs, it is now possible to create a tunnel cavity that is capable of accommodating a double deck, three lane road design – something that would have been considered impossible 15 years ago. Is there a limit to TBM size? For the moment, it seems not.
“In the past, TBMs either dealt with the softest ground or the hardest rock but they are now capable of effectively dealing with mixed face conditions,” says Harry Asche, Unit Manager Transport Services at Aurecon.
“In mixed face conditions, the skill is in knowing the material mix entering the machine and now operators have access to equipment that allows them to automatically measure spoil coming into the tunnel. This is done via belt weighers and scanners that employ automated, real time analysis.”
“There is an increasing range of highly accurate measurement options for use on tunnelling projects. We can scan strata at the excavation face, employ highly accurate bubble detectors and manage machine performance via multiple, web-based data acquisition tools which can be reviewed on remote computers, tablet devices and even on smartphones,” adds Harry.
“Tunnel systems are entering a whole new era where smart technology is allowing us to deliver improvements in how we operate systems that use the infrastructure."
This flexibility offers improved system performance for passengers and cost benefits for owner/operators. In addition, the design specifications around such factors as fire load within the metro system can be modified, resulting in a reduction in the overall cost of design and construction of the asset and the rolling stock. Critically, the system is scalable in that it copes seamlessly with the steady growth in passenger numbers, typical of urbanised societies.
In mature markets, such as Europe, approximately 50 per cent of work undertaken on tunnels is actually refurbishment of existing systems. In the past, reconfiguring a tunnel’s capacity (e.g. changing from two to three lanes or increasing the tunnel diameter) was impossible. In Europe, there are examples of tunnels being widened using a technique that incorporates a box-like structure constructed outside of the existing tunnel lining, in which the construction engineering is undertaken, while the tunnel remains in either full or part-time use.
In some instances, tunnel boring machines have been used to undertake parallel excavation works. The capability to increase capacity has been utilised across tunnel infrastructure servicing roads, rail and utilities. The flexibility to widen tunnels results in a once fixed capacity asset being able to be reengineered to meet increased capacity requirements. This capability allows engineers to take advantage of the embodied energy in the existing structure to create a better system that uses less energy to operate, in contrast to constructing an entirely new system.
At a smaller scale, a conventional steel box constructed around an existing tunnel allows widening of tunnels to be undertaken using machines known as ‘Pipe Eaters.’ These machines are capable of excavating around existing tunnels such as sewers, while allowing water flow to continue as the sewer diameter is increased.
“Tunnel systems are entering a whole new era where smart technology is allowing us to deliver improvements in how we operate systems that use the infrastructure,” says Harry. “Within a generation, we will see a huge change in the way we use road, rail and utilities delivered via tunnels.”