Five TBMs, the largest number for an Australian transit project, will be required to excavate the 15km of twin running tunnels for the planned 32km-long Metro City and Southwest (MC&S) program in Sydney.
Preliminary owner design carried out on behalf of the New South Wales Government specifies 4 x 6m diameter TBMs to drive the majority of the underground alignment between Chatswood and Sydenham (Fig 1), with a slurry or Mixshield for the drive under Sydney Harbour.
A mixture of roadheader and drill+blast is anticipated for excavation of the three cavern stations at Victoria Cross, Martin Place and Pitt Street. Project scope also includes two further cut and cover stations at Crows Nest and Barangaroo; and cut and cover excavation of new platforms for an interchange with the existing Sydney Central Station. Final design is likely also to include a further station at either Sydney University or Waterloo, but a final decision on this is not yet made (Table 1).
Like the Sydney Metro Northwest (formerly known as the North West Rail Link) – for which TBM tunnelling using four NFM machines wrapped up earlier this year – the construction procurement strategy for MC&S envisages a single main civils package. This will cover all of the running tunnels between Chatswood and Sydenham, portals at the north and south end of the alignment, all but one of the station excavations, and a short grade-level stretch of track between Chatswood Station and the MC&S north portal for seamless connection with Sydney Metro Northwest.
|Table 1. MC&S underground stations|
|Central Station [*]||25m||C&C|
|Sydney University [**]||Not known||Undefined|
|Waterloo [**]||Not known||Undefined|
[*]=New underground platforms for existing station
[**]=Options under investigation
RFQs for the design-build civils contract (Contract TSE) are expected to be issued in the coming weeks, ahead of a planned construction start next year (2017) and launch of the first TBMs by 2018. The contract period for excavation of the running tunnels is likely to be 36 months, and 48 months for the station excavations.
Station and tunnel fit-out, M&E, fire and safety systems, and track laying will be awarded as part of a separate design-build contract (Contract STME); design-build of the cut and cover Sydney Central station box, plus construction and fit-out of the north–south concourse (Contract CSW) is broken out of the main stations and running tunnels contract and will be awarded as part of an Early Contractor Involvement arrangement.
Upgrade of the existing 15km-long grade-level stretch of line between Sydenham and Bankstown will be developed in a later construction phase. A further extension from Bankstown to Liverpool is under study.
Geotechnical studies of Sydney Harbour along the planned alignment to the west of the Harbour Bridge, and to depths of 80m below the surface, have revealed that the rock strata sits 21m deeper than had been previously mapped in the 1970s and 1980s (Fig 2).
After completing a program of sonar and other geophysical scans to determine the level of the seabed, and the material below it, 22 targeted boreholes were drilled from two barges, and core samples of the rock taken. A further 30 probes have been conducted to classify the sediment sitting above the rock on the seabed.
The overall conclusion of the geological surveys is that the Hawkesbury Sandstone under Sydney Harbour is deeper than had first been thought. For the TBM drive to remain exclusively within the competent sandstone layer would require what the project owner describes as “unacceptable” station depths at both Barangaroo on the south side of the Harbour and Victoria Cross on the north side.
An immersed tube solution is considered feasible, but is rejected on both cost and environmental grounds. Preliminary design therefore specifies parallel TBM bored tunnels through a predominantly sandstone geology at the north and south of the alignment, but transitioning through a sedimentary layer at the centre of the Harbour.
Ground improvement measures will be undertaken in advance of TBM excavation to manage the machine’s passage through the transition zones. This will be achieved either by a program of ground freezing, or by jet grouting from surface level using barges. Both methods are aimed at facilitating easier maintenance access to the TBM prior to driving through the soft sediments. The area requiring ground treatment at each of the transition zones is estimated at 20–30m long x 35m wide x 16m deep.
Three sites that will service TBM operations are identified. Two machines will drive from the south portal just north of the existing Sydenham Station, with two more machines driving from the portal just south of Chatswood Station. In the early stages of owner design the 3km stretch between Chatswood and Crows Nest stations was to have run at surface level, but land and environmental factors mean it will now be run underground.
The machine that will excavate the Harbour crossing is expected to be serviced from the Barangaroo Station site on the south side, but the design and location of the interface with the southern drives out of Chatswood is subject to further geological studies and pre-bid industry engagement.
The majority of the alignment is expected to consist of Hawkesbury Sandstone, similar to that experienced by the Thiess/John Holland/Dragados (TJHD) JV during double shield excavation of the 15km of twin running tunnels of the Metro Northwest. The TJHD JV completed TBM excavation of the underground section between Bella Vista and Epping last month (January 2016), less than three years after being awarded the A$1.15 billion station and tunnel civils contract in June 2013.