The first section of the Metro Tunnel in Melbourne, Australia, is now complete. The first TBM, a 7.2m diameter Herrenknecht Mixshield, broke through into a 13m deep shaft at Childers Street, Kensington on 23 February 2020.

First section of Melbourne Metro Tunnel complete

Since its launch in September 2019, the TBM has travelled 1.2km from Arden Station to the western tunnel portal at Kensington – under rail lines, CityLink, Moonee Ponds Creek, North Yarra Main Sewer and the West Melbourne Terminal Station. Due to being so close to the rail corridor and the ground conditions in the area, a reception seal was used and the shaft was flooded with water for the breakthrough, to maintain the pressure supporting the ground around the TBM.

The TBM has installed 4,200 curved concrete segments to create 700 rings lining the walls of the tunnel. The segments, each weighing 4.5 tonne, are among 56,000 being manufactured at a purpose-built concrete manufacturing plant in the Melbourne suburb of Deer Park.

The TBM is one of four Herrenknecht machines of the same type and diameter to be used on the project. The second TBM is travelling on a parallel route to excavate the second tunnel from Arden to Kensington, and will break through in the coming weeks. Work began in January 2020 at the site of the new Anzac Station on the St Kilda Road site to assemble the third and fourth TBMs for the project. These are expected to be launched in mid-2020 and will break through at a 19.5m retrieval shaft at the eastern tunnel portal in South Yarra.

When complete, the Metro Tunnel will create capacity for more than a half a million extra passengers a week during peak times across the Melbourne train network.

Tunnelling has now commenced on the North Melbourne to Kensington section of the Melbourne Metro in Victoria, Australia. The PPP project, awarded to the Cross Yarra Partnership of Lendlease Engineering/John Holland/ Buoygues Construction and Capella Capital, will deliver a 9km twin-tube link from Kensington to South Yarra as part of a new end-to-end rail line from Sunbury in the west to Cranbourne/Pakenham in the south east with five new underground stations at North Melbourne, Parkville, State Library, Town Hall and Anzac (Fig 1).

First TBM being assembled

Two 7.2m diameter Herrenknecht Mixshield TBMs were assembled at the new North Melbourne Station site in mid-2019 and were launched separately in October and November 2019. The first TBM has progressed more than 250m to date, while the second has progressed about 50m. They are expected to reach the western tunnel portal at Kensington in early 2020 before being transported back to the launch site in North Melbourne where they will relaunch towards Parkville and the city center.

A further two TBMs will be launched in 2020 from the future Anzac Station in the Domain Precinct and advance towards South Yarra from where they will be transported back to the Domain work site to be reassembled in the station box to start their journey towards the central business district (CBD) and under the Yarra River.

Fig 1. Melbourne Metro extension alignment

The AU$6 billion metro project will free up space in the current City Loop subway and rail system by taking three of the busiest train lines in the city out of the Loop and running them through the new underground route, creating space for more trains, more often across the network.

Along its route, the new metro line will be up to 40m deep (Fig 2). The deepest point will be under Swanston Street, at the northern edge of the CBD, where the new tunnels pass under the existing city loop tunnels. From the station under Swanston Street at Flinders Street, the underground alignment runs under below the Yarra River before passing under the CityLink highway on its way to the new Anzac Station under St Kilda Road.

Fig 2. Station depths and TBM routes

To keep Swanston Street open while the new link is built, working access shafts are being excavated adjacent to where station entrances will be. These working shafts will transport machinery, equipment and workers underground to excavate the station caverns. Disruption on the surface will be greatly reduced, and trams will continue to travel along Swanston Street during construction.

The State Library and Town Hall stations will be built as trinocular caverns. Three overlapping tunnels will be excavated by roadheaders to create a wide open space that allows the concourse and platforms to be integrated on a single level, rather than two tunnels separated by a cross passage. The new North Melbourne, Parkville and Anzac Stations and entrances will be built by cut-and-cover.

The geology of the area is variable and includes soft soils, including Coode Island Silt, and hard basalts under the Yarra River and in some sections of the western alignment. Many sections of the tunnel alignment feature a mixed-face geology.