The Government of Victoria in Australia has ordered four giant tunnel boring machines (TBMs) that will work on the A$11bn ($8.47bn) Metro Tunnel in Melbourne.

Each of these TBMs, scheduled to start arriving from early next year, will be 100m long, with a diameter of 7.2m weighing up to 1,000t.

Equipped with giant cutting heads, the machines will operate like moving factories as they advance under the city’s surface burrowing two 9km tunnels.

“The tunnels will link one side of the city to the other and their completion will represent a significant engineering feat.”

Following their arrival, two machines will be delivered to work sites at Arden and Domain each, where they will be assembled and lowered into a shaft 20m underground.

As they proceed, the TBMs will install watertight concrete lining in the new tunnels. The TBMs are expected to advance around 10m per day, and will install a total of 55,000 individual concrete segments necessary to create the two tunnels.

Victoria Minister for Public Transport Jacinta Allan said: “These machines will operate like giant underground factories as they burrow around 40 metres beneath Melbourne to build the Metro Tunnel.

“They will dig through a variety of ground conditions to link one side of the city to the other and its completion will represent a significant engineering feat.”

In the first leg of the journey, each TBM will move away from the city and be retrieved in Kensington and South Yarra, before being transported back to the starting points for their journey towards the city.

The tunnelling works are expected to be completed by 2021.

With five underground stations, the Metro Tunnel will offer a new connection from Sunbury to Pakenham, improving transportation between the city and suburbs.