At the peak of construction, nine tunnel road headers were used for the 8.5km of tunnelling which included the connecting ramps and ventilation tunnel. For the first time in NSW construction history, four of these 100 tonne machines were lowered down a 42-metre-deep vertical access shaft near the intersection of William and Bourke streets. This provided truck access for the removal of tunnel spoil, allowing 24-hour tunnelling.
A total of 680,000 cubic metres of spoil was excavated across the project. An extra ramp was constructed to link the tunnels with the Eastern Distributor so trucks could remove the spoil via the Eastern Distributor and therefore avoid city streets. The spoil was sold as sub-grade material and used for new road projects around Sydney. During peak periods, more than 5,000 tonnes of spoil was removed daily in nearly 150 truck and trailer loads. Approximately 92 per cent of the excavated material was reused on other infrastructure projects such as the F3 widening, the Bangor Bypass and rehabilitation of Kurnell’s former sand quarry.
Tunnelling began at the centre of the route in January 2003 allowing cut-and-cover construction at the portals to commence in tandem. The main tunnels were excavated by a road header, carving an 8.6 metre wide by 7.8 metre high face through the tough sandstone rock using 36 tungsten tipped picks. At the height of tunnelling activity, when all nine road headers were in use, they were each able to cut through up to 6 metres a day in a process which advanced the tunnel in 600mm lengths. The surface of the tunnel was then secured by installing rock bolts and spraying steel fibre reinforced concrete onto the excavated face in two 50 millimetre passes.
Seeing the light
The breakthrough by a 100 tonne road header at Rushcutters Bay on 16 June 2004 marked a significant project milestone. Tunnelling works had commenced on the 2.1km CBD tunnel beneath William Street on 19 May 2003 - little over a year before. The then Minister for Roads, Carl Scully, noted that this milestone was testament to the dedicated work carried out across the length of the tunnel to deliver an exceptionally complex project beneath Australia's biggest city.
At the time of the breakthrough, tunnelling activity was approximately 70 per cent complete with engineers still facing a mountain of work underground. About 120,000 cubic metres of rock still had to be excavated and 32,500 cubic metres of concrete to be poured before the electrical fit out of the tunnel could commence.
The tunnel breakthrough was unusual for Sydney as tunnelling projects usually experience a midway breakthrough underground when two machines working in opposite directions meet. The innovative technique of lowering some of the road header machines down a shaft at Bourke Street to burrow out towards either end of the project allowed the tunnelling work to start earlier. Usually tunnel entrances and exits must be completed beforehand to provide entry for the tunnelling equipment, however, on the Cross City Tunnel project, these works occurred concurrently.
Importantly, tunnelling from within also minimised the impact on the community by removing the majority of spoil underground via the Eastern Distributor instead of trucks on city and residential streets.
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