Large amounts of concrete will have to be injected into the ground beneath the Yarra River to make it stable enough to build the Melbourne Metro rail tunnel, documents filed with the federal environment department show.

But how will this difficult work be completed?

The geological formation beneath the Yarra consists of a variety of different materials, but there is a shelf-like layer of hard basalt rock (or newer volcanics) above where the tunnel will be built.

There is also a layer of soft Fishermens Bend Silt under part of the riverbed, and to ensure a tunnel can be built beneath this silt, it will need to be stabilised.

Under documents filed with the federal environment department, the state government has said it wants to inject concrete to stabilise approximately 875 square metres of silt in the centre of the river channel.

There are two ways of doing this work: parking a barge above and drilling down, then injecting a cement mix, or using a tunnel boring machine to stabilise the silt from below.

Either method will use a grout that will consist of cement mixed with water.

THE BARGE METHOD

Using the barge method, a rig would be set up on the Yarra River to drill boreholes and inject grout.

A 20-centimetre pipe would be submerged and a nozzle inserted into the pipe, "and with the aid of high pressure water or cement suspension cutting jets, the soil is cut and the eroded soil is mixed with the cement suspension", the submission said.

THE TUNNELLING METHOD

Under the alternative plan put forward by the government's rail authority, one of the tunnel boring machines that will build the Melbourne Metro would be used to inject the cement grout mix into the area where the silt lies, to create a very stable route for drilling.

This method of grouting would have no interaction with the river, as works would be undertaken below the surface of the river bed.

Either method is predicted by the government to take up to three months to complete.