Environmental impact statement provides information about Great Western Highway twin tunnels' portals and path

The documents not only outline how the government proposes the twin tunnels would be dug, but it gives an idea of where they would run.

The environmental impact statement for the project shows the 11-kilometre twin tunnels - two lanes in each direction, with a speed limit of 80 kilometres an hour - would run on the non-railway line side of the existing Great Western Highway and then swap to the other side of the highway underneath central Blackheath at a depth of up to 68 metres.

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The depth would reach 99 metres on the outskirts of Blackheath heading towards Mount Victoria.

Underneath central Mount Victoria, meanwhile, the tunnels would run south of the existing Great Western Highway at a depth of up to 194 metres and would be further away from the main urban area than in Blackheath.

The tunnels would be closer to the surface - at 105 metres - on the outskirts of Mount Victoria heading towards Victoria Pass.

The EIS says the grade of 13 per cent at Victoria Pass makes it the steepest "on any classified road or freight route in NSW". It also says crashes on the

Great Western Highway have been increasing over time.

Between east of Blackheath and Little Hartley, according to the EIS, there were 64 crashes between 2016 and 2020, one of which was fatal and 65 per cent of which resulted in an injury.

The EIS says the potential for additional traffic to be attracted to the Great Western Highway (known as induced demand) by the tunnels project was considered "and it was concluded that the project would be unlikely to generate new trips that would not have otherwise occurred".

It does say, though, that "higher productivity vehicles that can transport larger freight volumes" that are not permitted to use the existing Great Western Highway would be able to use the twin tunnels.

The EIS says the duplication of the Great Western Highway between Lithgow and Katoomba - of which the twin tunnels would be one section - would "complete and realise the potential of decades of work in upgrading the Great Western Highway across the Blue Mountains".

Regardless of the release of the EIS, though, the NSW Government continues to face a financial shortfall with its Great Western Highway upgrade.

The Commonwealth Government's $2 billion for the east and west sections (either side of the proposed tunnels) of the highway upgrade remains on hold while new Infrastructure Minister Catherine King says she wants to see the overall plan for the entire project, while the twin tunnels have NSW Government money committed but not Commonwealth money.

As reported by the Western Advocate recently, the EIS says two tunnel boring machines would set off from Little Hartley to dig the twin tunnels to Blackheath.

Tunnel boring machines have been used on the first and second stages of the underground Sydney Metro rail system and are expected to be at work this year on the third stage of that project.

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