The Crossrail Archaeology Program is one of the most extensive archaeology programs ever undertaken in the UK. Prehistoric animal bones, Roman remains, mass plague victim graves and remnants of Britain’s industrial past are just some of the 10,000 artefacts uncovered in the massive undertaking beneath the city of London.
The program is run in conjunction with work on Europe’s largest construction project – the London Crossrail.
The London Crossrail construction project spans 100 kilometres across 40 construction sites where archaeologists are making new discoveries every day. This is in no small part thanks to the extensive excavation work required for the momentous project.
Eight 140-metre-long, 1000-tonne tunnel boring machines (TBM) were used to construct 42 kilometres of new rail tunnels 40 metres below the British capital, clearing the way for treasure troves of artefacts to be discovered.
The question now stands: what archaeological marvels will be uncovered when the Melbourne Metro Rail Authority (MMRA) undertakes a similar task and tunnels beneath the Yarra River?
The MMRA will employ TBMs like those used in the London Crossrail to dig underneath the Yarra as part of the momentous Melbourne Metro Rail Project (MMRP).
While the likes of mass plague graves and the remnants of ancient Roman life are unlikely to be unearthed during excavation work, like the London Crossrail project, MMRP is set to change the face of public transport in the city for the future.
The MMRP will deliver two nine-kilometre rail tunnels from South Kensington in the west to South Yarra in the inner east. This is part of a new Sunbury to Cranbourne-Pakenham line. Five new stations will be built at Arden, Parkville, Central Business District (CBD) North, CBD South and Domain.
The two tunnels will be constructed underneath Swanston Street and cross the Yarra River to the south, expanding the City Loop rail line.
In August, Victorian Premier Daniel Andrews and Minister for Public Transport Jacinta Allan announced that TBMs had been chosen as the preferred construction methodology to tunnel beneath the river section of the project.
MMRA Chief Executive Officer Evan Tattersall says four TBMs will be used to excavate most of the nine kilometres for the two MMRP tunnels. Two TBMs will operate in the northern section of the dig and the other two in the south.
“Building twin rail tunnels under Melbourne’s iconic Yarra River presents a number of engineering challenges,” he says. These include navigating complex geological and hydrological conditions, the need to protect the environmental significance of the river, and limited space on either side of the river for construction.
Another method the MMRA considered for the river section of the project included an immersed tube, where the tunnel segments are prefabricated, floated to the tunnel site, sunk into place and then linked together.
It also considered the cofferdam solution. A cofferdam is a temporary enclosure built around a body of water to allow the enclosed area to be pumped out, creating a dry work environment.
Mr. Tattersall explains that the MMRA chose to dig under the river using TBMs because they reduce environmental impacts on the river and the surrounding areas of the project. TBMs will also limit inconvenience to businesses, pedestrians and commuters.
The TBMs have specialised cutting heads that grind through soil and rock. The excavated material is moved from behind the cutting head and onto a conveyor belt where it is deposited behind the TBM and removed from the tunnel shaft. While the front excavates, a different part of the machinery installs the concrete lining of the tunnel.
While London’s Crossrail project saw 42 kilometres of underground tunnels dug approximately 40 metres below the bustling city, the Melbourne Metro Rail twin tunnels will vary in depth. “The distance between the top of the tunnels and the existing land surface will vary due to Melbourne’s uneven terrain,” explains Mr. Tattersall. He says that the maximum depth may be nearly 30 metres from the surface in some locations, while the tunnels will be situated around seven metres under the bed of the Yarra.
Mr. Tattersall says geotechnical investigations are currently underway to gather more data about the ground and soil conditions in the area. “Melbourne is geologically very complex,” he says.
“There are a great variety of ground conditions underneath the city including basalt and siltstone bedrock; and clay and silts that have been deposited by watercourses over time.
“The greater number of geotechnical investigations the project undertakes at this early stage, the more confidence the project team will have about conditions and the more effective we can be in delivering the project when it moves into construction,” he adds.
The MMRP is still in the early planning and development phase. Early works are expected to commence in 2017 and major construction is anticipated to begin in 2018.
“Melbourne Metro is an extremely complex project to design and build,” says Mr. Tattersall. However, he asserts that the MMRA is confident that these challenges can be addressed and overcome as Melbourne’s newest underground rail line comes to fruition. According to Mr. Tattersall examples such as London’s Crossrail project and Hong Kong’s Express Rail Link are key case studies for the MMRP to learn from.
The final location, design and construction methodology for the tunnels will be determined by the contractor, who is yet to be appointed. The MMRP is expected to be operational by 2026.