The CBD's roads will have a further 438,000 trucks added to the traffic burden over the four years of Metro rail tunnel work. Credit:Craig Sillitoe Those trucks will run through the CBD every five minutes, 24 hours per day, for four years, and will impact the city's commuters, office workers, residents and retailers. According to one engineer who has worked on the planning for the project, that truck burden is only necessary due to the tight deadline placed on the project by the Andrews government. He claims the state government's decision not to use advanced tunnel boring machines (TBMs) to build the entire tunnel means 430,000 semi-trailer loads of dirt and rock will have to be excavated from underneath the CBD, loaded onto trucks, and driven through the centre of the city. "The state government has requested a mined cavern design for the section of tunnel between CBD North and CBD South stations," he said.

Tunnel boring machines will not be used on Melbourne Metro tunnel. Credit:Christian Pearson "That means we will drop road headers under the city to build the tunnels. When you use road headers, who need to take all the spoil out through vertical shafts and truck it away. The only reason I can see for doing it this way is to meet the deadline. "If you use TBMs, you can take the spoil back out through the tunnel you have built." A tunnel boring machine at work. Those road headers will be lowered underground via three giant shafts, 11 storeys deep, that will be excavated in the CBD – at Franklin Street and at A'Beckett Street, both near Melbourne Central station, and a third shaft at City Square, near Flinders Street Station.

Road headers will be lowered via those shafts and will begin the process of excavating the 2 million cubic metres of soil and rock needed to complete Metro. That's more than enough to fill the MCG to the roof, or fill 800 Olympic-sized swimming pools. The tail end of a tunnel boring machine . Credit:Paul A Blunden "Under the original plan, when we were only building a shallow tunnel of 10 metres, so there was no way we would use TBMs," the engineer told The Sunday Age. "When we moved the tunnel almost 40 metres beneath the surface, TBMs became an option, and a better option in my mind. That's because you can take the spoil back out through the tunnel, pretty much underneath the city. Instead it is going to be four years of truck hell in Melbourne." Illustration: Matt Golding

Asked why road headers would be used instead, the engineer replied: "Time". TBMs can tunnel at up to 50 metres per day. Even if they achieve that maximum tunnelling rate on the Metro project, it will take many months for the tunnel borers to reach the city from either Arden or Domain. "Using road headers means we can build the middle section of the tunnel and stations while the TBMs are approaching from each end. It shaves time off the project for sure, but the downside is the trucks." The Environmental Effects Statement for Metro reveals the CBD section of the project will require 300 truck movements per day. That equates to one truck every five minutes, 24 hours a day, 365 days a year, for four years. Put another way, it an extra 438,000 semi-trailer trips through the heart of Melbourne over four years.

No economic or environmental modelling has been done on what impact that extra traffic will have on the city. Three consortia remain in the race to build the Melbourne Metro tunnel, and all have been instructed to use the "mining cavern" technique for the CBD section of the project The reason, according to state government documents, is to "minimise the impacts on Melbourne's city centre, and specifically Swanston Street". The favourite to win the bidding process is the Cross Yarra Partnership, which includes Lendlease, John Holland and Bouygues​ Construction.

It has already won the right to build vertical shafts for each station. According to sources at Spring Street, the Cross Yarra bid is also considering an extension of the road header section of tunnel, through to Parkville. That would put even more trucks on Melbourne's roads. There are two rivals to the Cross Yarra bid. The first is the Continuum Victoria consortium comprising Acciona​, Ferrovial Agroman​, Honeywell, Downer EDI and Plenary Origination. The second is the Moving Melbourne Together consortium, which comprises Pacific Partnerships, CPB Contractors, Ghella​, Salini Impregilo​ and Serco. The early submissions from all three bidders are already in. They total more than 100,000 pages, and a final decision will be made on who gets to the build the project and how it will be built later this year.

The Sunday Age put a series of questions about the construction of Metro, and why road headers are being used to build the mid section of the tunnel, to the state government. The state government did not respond. ﻿