John Holland, the Melbourne-based engineering and construction firm, ended 2016 with a flying start on the first major Melbourne Metro Tunnel contract, a A$324-million (US$233 million) early-works package, as part of preparations for the tunnel and the new underground stations in the city’s Central Business District (CBD). The work includes excavating two massive, 35-meter-deep shafts adjacent to Swanston Street, located in the center of the Victoria state capital. The A$11-billion ($7.9 billion) Melbourne Metro Rail project will be the largest-ever single engineering project undertaken in the state of Victoria.

Stephen Shaddock, director of early works for the Melbourne Metro Rail Authority (MMRA), says, "Shaft construction is an incredibly complex job, made even more challenging by the fact we’re doing it in the middle of Melbourne’s busy Central Business District. We’re digging 35-meter-deep holes in the heart of the city."  

Shaddock says the project contractor chose tunneling methods and machinery to meet Melbourne’s complex geology. Ground conditions include basalt and siltstone bedrock and clay and silts, deposited by watercourses over a very long time.

“We have also identified a huge volume of essential services located underground along the tunnel corridor and near the station locations that we need to relocate or protect before main tunneling and station construction works get underway,” Shaddock adds. “Building the access shafts as part of the early-works package will enable us to get straight to work building the stations and tunnels underneath Swanston Street in early 2018, after the [public-private-partnership] contract is awarded. These access shafts, in the form of deep basements, will be used to transport machinery, equipment and workers underground to where the station caverns will be constructed.”

Excavation of the CBD's North Station will involve two shafts—at Franklin and A’Beckett streets—that will be covered by massive "acoustic sheds," reducing noise for neighboring residents and offices. Construction of the CBD's South Station will involve the partial demolition of the underground parking garage that sits underneath City Square.

The project passed another milestone before Christmas, when Victoria’s minister for planning, Richard Wynne, signed the Environmental Effects Statement, green-lighting the project. The Metro Tunnel will provide a single passageway through the CBD for three of the busiest train lines, freeing up space in the City Loop to run more trains more often to the suburbs and regional Victoria.

Also in the CBD, construction of the Metro Tunnel and two new stations under Swanston Street presents a range of engineering challenges, including navigating existing belowground infrastructure, including the City Loop tunnels, CityLink tunnels and major utilities.

While the majority of tunneling will be undertaken by tunnel-boring machines, it has been proposed that, between the CBD North and CBD South stations, the tunnels will be mined. This method of excavation is considered to be the most appropriate due to the expected ground conditions and the number of building basements and foundations in the CBD.

The schedule for the construction of the North and South stations and the tunnels under Swanston Street is estimated at about five years. Different station elements will take different lengths of time, depending on their size and complexity. To deliver the project, construction of the Metro Tunnel will be divided into a number of work packages and use established contract models, such as alliances and a private-public partnership.

Public-private-partnership works include the excavation and fit-out of the project’s twin, 9-kilometer tunnels and five new stations, located at Arden, Parkville, Domain, and the North and South stations. Three consortiums have been short-listed for the P3: Cross Yarra Partnership, comprising Lendlease Engineering, John Holland, Bouygues Construction and Capella Capital; Moving Melbourne Together, comprising Pacific Partnerships, CPB Contractors, Ghella, Salini Impregilo, Serco and Macquarie Capital; Continuum Victoria, comprising ACCIONA Infrastructure, Ferrovial Agroman, Honeywell, Downer EDI and Plenary Origination.

The Metro Tunnel's P3 contract is valued at up to A$6 billion ($4.3 billion), and a contract is expected to be awarded by the end of 2017, with tunneling works to commence soon after.