Tunnel vision: Australia goes underground

Tunnel boring machine Matilda excavating the northbound tunnel tube of the North-South Bypass Tunnel in Brisbane

New roads and train lines are big winners in the current batch of Australian infrastructure spending. But with our cities ever more cramped, where can we put all that steel, concrete and bitumen? In many cases there is no option but to start digging…

When it comes to cutting through 36 kilometres of Sydney sandstone to deliver the Sydney Metro Northwest, not any old tool is up for the job. In Sydney’s case, as in many cities around Australia, giant Tunnel Boring Machines (TBMs), were deployed. They weighed in at a staggering 900 tonnes, arriving on site in 18 shipping containers and each one took a crew of more than 70 workers to assemble. With Stage One complete, they were barely out of their holes when planning for Stage Two had begun. Linking Chatswood under Sydney Harbour through to a series of new CBD stations, it will complete the first high-capacity underground train network in Australia.

Meanwhile in Melbourne, another set of TBMs will soon set about digging the twin 9km cross-city tunnels that make up Melbourne Metro. Weighing in at over 1000 tonnes, it is possible that once these giant machines are assembled and set to work, they will never see the light of day again. The Melbourne Metro Rail Authority has given the successful bidder the option to abandon them underground once drilling work is complete, such is the expense and logistics involved in their removal. They will literally dig their own grave.

Waterview tunnel project (at Mt Roskill), in Auckland. Image by Noel Jones

Finally, more TBMs are on order to dig out Brisbane’s $5.4 billion Cross River Rail project: a project renowned for its many false starts, amid hot political debate. For some years Queensland has weighed up population pressures, commute time, and the economic benefits of the project.

For now, with the project budgeted, the Cross River Rail Authority instituted, and the Queensland Infrastructure Association backing the build, not even a wall of solid granite looks likely to stop this in its tracks.

But what are these giant machines anyway? TBMs are large machines that tunnel through ground, progressively installing concrete linings to support the excavated tunnel. Excavated material is transported through the machine to the surface for removal by trucks. To find out more we spoke to the Cross River Rail Delivery Authority (CRRDA).

A view into the proposed George St Station, Brisbane

In comparison to Brisbane’s other infrastructure projects, how large does this compare?

Cross River Rail (CRR) is a 10.2 kilometre link from Dutton Park to Bowen Hills, with 5.9 kilometres of tunnel under the Brisbane River and CBD, four brand new underground inner city stations and an upgraded Exhibition station. The project will transform South East Queensland with more trains, more often and better services for commuters, as well has significant economic development opportunities over a 40 year horizon.

Could you briefly describe the terrain and soil that the tunnels are going to bore through?

Cross River Rail will be tunneled entirely through bedrock. This includes sandstone and siltstone found within the Aspley/Tingalpa formation, Neranleigh-Fernvale phyllite’s and the Brisbane volcanic tuff formation.

Not easy going then! Are there any other projects that have dug through terrain similar to what lies underneath Brisbane?

Brisbane’s Clem Jones Tunnel (CLEM7) follows a similar alignment, which passes through similar geological units and also underneath the Brisbane River.

Cross section of Brisbane’s Cross-River Tunnel

Drilling underneath a large river must bring significant risks. What’s the average water-bearing capacity of the geological layers that the tunnel will run through?

Cross River Rail’s tunnel will be located wholly within the low water-bearing rock of the Aspley/Tingalpa and Neranleigh-Fernvale beds. The extent of ground water movement will be dependent on the persistence of geological features in the rock, such as joints, fissures and faults.

How will the tunnels be constructed, and what kind of machinery will be used in the process?

It is anticipated that tunnels will be constructed using Tunnel Boring Machines north from Woolloongabba to the portal on the Exhibition line, and mined with roadheaders or drill and blast methods south from Woolloongabba to Boggo Road.

The tunnel boring machines will be custom built for the project and will be approximately seven meters in diameter. The number of machines, manufacturer, weight, power, country of origin, and other features will be determined by the successful tenderer.

The CRRDA is currently leading a tender process for our two biggest work packages: Tunnels, Stations and Development (TSD) and Rail, Integration and Systems (RIS). Invitations to submit an Expression of Interest were issued in September and this process will formally close later this month.

The Cross-River Rail Route

Presumably this tunneling will result in a consider amount of soil excavated from the construction process. What will happen to that soil?

It is anticipated that clean spoil (the soil and rocks that is removed when tunneling and excavating) will be placed at five potential sites around Brisbane, including Brisbane Airport, Port of Brisbane, Pine Mountain Quarry, Swanbank and Larapinta. Removed contaminated soil will be taken to approved disposal sites in accordance with legislative requirements.

Watch this space for more news on Brisbane’s Cross River Rail project and other city transforming projects across Australia.

Article by brickworksbp

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