Tunnelling under the Yarra River
The Metro Tunnel will be constructed under the Yarra River, which is up to four metres deep in this location. The top of the twin tunnels, each around seven metres in diameter, will be approximately seven metres below the riverbed, east of the Princes Bridge.
Crossing under the Yarra River is a significant element of the project and presents a number of engineering challenges, including:
- complex geological and hydrological conditions, and
- safeguarding the environmental significance of the river,
- limited space on either side of the river for construction.
Tunnelling using Tunnel Boring Machines has been selected as the preferred method of construction of this section of the project.
Ground conditions under the Yarra River are very complex, with geotechnical investigations showing different soil types including basalts, silts and clay. More than 20 geotechnical bore holes have previously been drilled in this location to inform decision-making. In 2016 we undertook a second stage of drilling to supplement these results.
There is a layer of Fishermens Bend Silt under part of the Yarra River bed that may require stabilisation during tunnel construction. The need to stabilise this ground will depend on the detailed alignment of the tunnels in this location and the type of Tunnel Boring Machine (TBM) used.
If stabilisation is required, then grout is injected either above and ahead of the cutter head of the TBM from inside the tunnels or via bore holes from above the tunnels, using a barge on the river. The cutter head injection method will not have an impact on the Yarra River as works are undertaken below the river bed, and the barge ground treatment method can be carefully managed with a high degree of control.
At this stage, it is too early to say whether ground treatment will be required and if so, which method may be used. The appointed contractor, in consultation with the Melbourne Metro Rail Authority, will assess the need for ground stabilisation and the method used. It is also too early to determine the amount of grouting, if any, that may be required to construct the tunnels under the river.
Working with local businesses and traders
We are committed to working closely with local Yarra River businesses, traders and sporting clubs to manage and minimise the unavoidable impacts of construction. We will undertake ongoing consultation to understand their needs and how these might inform the planning and delivery of the project.
We will work closely with Parks Victoria, Heritage Victoria, the Department of Environment, Land, Water and Planning, and the Environment Protection Authority Victoria to identify measures to minimise impacts on the Yarra River, aquatic and plant life, and the surrounding area.