A number of different construction techniques will need to be used to successfully deliver a project as large and complex as the Metro Tunnel.
The proposed construction techniques have been, and continue to be, used to build many underground rail projects around the world and are tried and tested under a range of geological conditions and inner city urban environments.
Building the Metro Tunnel presents a number of challenges, including:
- managing disruption to residents, businesses and events
- mitigating the impacts on road traffic, pedestrians, cyclists and existing public transport and freight services
- navigating existing underground infrastructure including utilities, the City Loop and CityLink tunnels
- excavating through a range of geological conditions including rock, clay and silt
- tunnelling under two significant waterways, the Yarra River and Moonee Ponds Creek
- managing the logistical task of removing large amounts of excavated material.
Along the alignment, the depth of the Metro Tunnel will be up to 40 metres below ground level. The deepest point will be under Swanston Street, north of the CBD, where the new tunnels will pass under the existing City Loop tunnels. Although the EES considered a route of the tunnel above the CityLink tunnels, the Metro Tunnel will travel under CityLink.
We will work with the contractors (once appointed) to identify ways to reduce the unavoidable construction impacts of the work required.
Site investigations at various locations along the proposed alignment are helping to understand the existing geological conditions and confirm the location of underground services. This information will also assist in determining the preferred construction methodology for different sections of the project.
Role of contractors
The contractors appointed by the Victorian Government to build the Metro Tunnel will refine the approach that will be used to deliver the project.
Some construction methods will be prescribed, such as the use of tunnel boring machines (TBMs) to build the rail tunnel outside the CBD and for excavating underneath the Yarra River. Other aspects or approaches may be optimised or altered by the contractors. They must meet Environmental Performance Requirements, which outline the environmental outcomes the project must achieve during its design, construction and operation.
Construction for the Metro Tunnel will be divided into a number of work packages and use established contract models, such as alliances and a Public Private Partnership (PPP), to deliver the project.
Tunnel Boring Machines
Tunnel Boring Machines will be used to build the majority of the rail tunnels. Tunnel Boring Machines are large machines used in the construction of long underground tunnels. They are tailored for specific conditions and can be several metres in diameter, more than 100 metres long and weigh up to 1,000 tonnes.
The Metro Tunnel TBMs will build tunnels approximately seven metres in diameter.
To minimise impacts on the Yarra River and surrounding areas, tunnelling using TBMs has been elected as the preferred method of construction for this section of the project.
For more information, see our Tunnel Boring Machines page.
Mined cavern construction
It is proposed to use a technique known as mined cavern construction to build the Metro Tunnel's two CBD stations.
Access shafts are dug from the surface to build the caverns, in this case on land adjacent to Swanston Street where the station entrances will be. These shafts are then used to transport machinery, equipment and workers underground to the station cavern. Equipment will then be used to excavate and line the station caverns below the surface of Swanston Street.
This construction approach will minimise disruption at surface level and is commonly used overseas, particularly in constrained city environments. Under this solution, trams will continue to run up and down Swanston Street during construction and the above ground disruption to businesses, cyclists, pedestrians and cross streets will be greatly reduced compared to other construction methods.
Cut and cover
The new Arden, Parkville and Domain stations and the entrances to the tunnels will likely be built using a construction technique known as cut and cover.
Connecting tunnels to the Sunbury and Cranbourne / Pakenham rail lines in Kensington and South Yarra will use a similar approach, with excavation equipment used to dig a descending trench from ground level to a depth where the Metro Tunnel begins.
A different method of tunnelling is proposed to conduct the excavation beneath Swanston Street to construct the Metro Tunnel between the two CBD stations.
Due to the expected ground conditions and the number of building basements and foundations in the area, a mined tunnel is considered to be the most appropriate method of excavation.
The proposed mined tunnelling method uses road header machines to dig the tunnel. These machines have a rotating cutter head that is mounted on a boom. After the road headers carve out the tunnel, it is reinforced with a combination of rock bolts, lattice girders and sprayed concrete as a temporary support and then a final cast in-situ concrete liner.
Using this technique rather than 'cut and cover' reduces the impact of construction on the CBD, as the tunnel between the two new CBD stations can be built completely below the road surface. Trams, pedestrians and cyclists can continue to access the heart of the city on the surface while construction activity occurs underground.
Around two million cubic metres of excavated material (soil and rock) will be produced during the construction of the Metro Tunnel. This is expected to require more than 170,000 return truck movements from construction sites along the tunnel route during the tunnel and station construction process.
Given the importance of roads to local communities, effective traffic management plans will be developed to manage this large number of truck movements. Traffic studies have been completed as part of the EES process to assist with planning to minimise the impacts on Melbourne's road network.
The predicted daily truck movements for the Metro Tunnel will not create substantial increases in daily traffic volumes on the arterial roads proposed to be used as truck routes.
Most truck movements will occur during the day, however some activities and construction sites may require 24 hour truck movements. Mitigation measures will be put in place to minimise impacts on surrounding areas.
During construction, there will be limited opportunities to re-use excavated clean fill as part of the project. As such, it will need to be removed from construction work sites and could be potentially re-used at other locations.
Materials that cannot be re-used due to contamination will be disposed of in line with Environment Protection Authority (EPA) Victoria and WorkSafe Victoria guidelines at appropriately licenced facilities.
The timing of works on the Metro Tunnel will be determined in consultation with the project contractors, local councils and EPA Victoria, and through the planning approvals process. Under certain circumstances, above-ground works may be needed after hours, and neighbouring residents and businesses will be notified in advance of these works.
Underground tunnelling works will be expected to generally occur 24 hours a day, seven days a week. Construction sites that support tunnelling activities could also operate around the clock. Additionally, a variety of noise-reducing structures including noise walls and acoustic sheds will be used at sites required to operate 24 hours a day.
Other out of hours works may be required in order to complete work safely or to reduce disruption on the road network, such as the delivery of oversized equipment or road closures needed to support construction activities.
The below working hours are subject to approval based on appropriate management plans and noise treatments being in place.
|Above ground sections||
7am - 6pm Monday to Friday|
7am - 1pm Saturday
Sunday and Public holidays (subject to approvals)
|Stations upon placement roof slab/acoustic shed||24 hours a day, 7 days a week|
|Underground tunnelling works||24 hours a day, 7 days a week|
|Tunnel support (Arden, Domain and Fawkner Park)||24 hours a day, 7 days a week|