It will be built just seven metres below the river bed, and the government's submission to the federal government to meet national environment laws said there would be no construction work sites on the Yarra River banks. The Melbourne Metro will tunnel below the Yarra River. Credit:Penny Stephens But because of the unstable Fishermens Bend silt that sits directly beneath the Yarra River, contractors working for the government will have to inject "grout" into the ground under the river. The injection will have to fill "875 square metres in the centre of the river", and will take three months to complete, the government's submission to Canberra said.

The grout will be "cement mixed with water", the submission said. The concrete is designed, it said, to fill a gap in the stronger basalt rock beneath much of the Yarra River. The submission was made as part of approvals needed under the Environment Protection and Biodiversity Conservation Act. The concrete that will be injected underneath the river will either be done by a barge on the Yarra above, or from beneath by one of the massive tunnel boring machines that will be used to dig the Melbourne Metro. THE TUNNELLING METHOD

The government was at pains in the document to say that none of the silt or material generated by cutting into the ground beneath the Yarra would be dispersed into the river. THE BARGE METHOD Under this option, a barge would sit on the Yarra and a pipe would be drilled into the ground beneath the river. A nozzle would be inserted into the pipe and cement injected into the unstable ground beneath the Yarra. Contractors for the government's Melbourne Metro Rail Authority have drilled more than 20 geotechnical bore holes around Princes Bridge in order to understand the very complex ground conditions under the Yarra River.

Melbourne University engineer Professor Priyan Mendis​ said that underwater concreting of this type was nothing new. "Technically this quite a hard task, but it is also reasonably common this sort of work, especially in Europe," he said. Professor Mendis said that the nearby CityLink Burnley tunnel had problems when it was completed in 2000, but much had been learnt since then. "It shows there can be problems if you don't do it properly. I'm sure we won't make any of those mistakes again," he said. The Melbourne Main Sewer Replacement, a $206 million project that replaced a tunnel from West Melbourne into the main Hobsons Bay sewer, also involved construction of a tunnel through conditions similar to those in the Melbourne Metro alignment, including passing under the Yarra River. The Age asked to speak with an engineer from the Melbourne Metro Rail Authority but was told no-one was available on Monday to discuss how the grouting work detailed in its submission to the federal government could be done.

A spokesman said it was "too early to say whether ground treatment will be required and if so, which method may be used".