Melbourne’s West Gate Tunnel achieves final TBM breakthrough

Tunnel boring machine (TBM) Bella recently broke through in Melbourne’s inner-city suburb of South Kingsville. It has completed its 4km drive from the west side of the Maribyrnong River under Yarraville, to just east of Millers Road near the West Gate Freeway.

Over the past 15 months, CPB Contractors and John Holland (CPBJH) JV’s tunnelling team has been operating the TBM 24/7. The machine has excavated more than 700,000m³ of rock and soil, while also installing more than 15,340 individual concrete segments to create the tunnel walls.

This milestone marks the completion of the bored tunnel works on the West Gate Tunnel road scheme, which is a partnership between the Victorian Government and Transurban to give Melbourne a second freeway link between the west and the city.

The first TBM on the project, Vida, finished its 2.8km journey at Williamstown Road near the West Gate Freeway in February of this year.

Bella and Vida hold the record for the largest TBMs in the southern hemisphere, with diameters of 15.6m. Each machine also weighs 4,000t and stretches to a length of 90m.

The machines' 450t cutterheads feature 96 cutting tools, which rotated up to twice per minute during normal tunnelling operation and up to three times per minute when cutting through a hard basalt rock section and through the “bluestone” that Melbourne’s geology is known for.

TBM Bella will now be dismantled and removed piece by piece, with the large cutterhead buried on site, and other components either recycled or returned to the manufacturer, Herrenknecht.

The start and end of each tunnel is being built using the cut and cover method. This involves excavating a trench and then covering it with a concrete deck to form a roof.

The project team will now continue building the road deck and installing electrical, lighting and safety systems to prepare both tunnels for completion in 2025.

Above ground, work is  also underway to construct the ventilation system at the outbound portal and to prepare for the installation of an eel net structure.

To build the West Gate Tunnel, approximately 1.5M.m³ of rock and soil is being removed. As the project is being built in former industrial areas, testing has found low levels of the group of chemicals known as per- and polyfluoroalkyl substances (PFAS) in some of the excavated soil.

This discovery brought tunnelling preparations to a halt in 2019 and led to a dispute between Transurban and CPBJH JV over which party would be responsible for the cost of the disposal. The parties ultimately came to an agreement, with the toll road operator increasing the value of the design and construct (D&C) contract by A$3.4M (£1.8M). Tunnelling was thus able to start in March 2022.

This however caused the completion date to be postponed by three years and an increase in the project cost to at least the tune of the D&C contract value increase.

The aim of the West Gate Tunnel project is to deliver an alternative to the West Gate Bridge, providing a second river crossing. The twin tunnels – a 4km outbound tunnel and a 2.8km inbound tunnel – will run under Yarraville between the West Gate Freeway and the Maribyrnong River.