There is a reason why Sydney is one of the most popular tourist destinations in the world. But, for those who live here or simply need to get from A to B anywhere near the central business center, conditions can quickly turn less than ideal. Actually, it may be a bit of a headache to drive around the city, which is why a brand-new roadway is being built beneath the harbour to reduce traffic. When construction has already started, however, it is now happening in a way that is entirely different from the initial intentions as of late 2022, and in the world of large infrastructure, that is a massive decision. There aren’t many options if you wish to get across the harbour by road, albeit that is partially due to the high rate of automobile ownership and the dearth of public transportation. And a lot of people do that; every day, more than 200,000 trips are conducted across the Harbour Bridge alone.

Western Harbour Tunnel is the name of this $5BN (AUD) project. Unexpectedly, it turns out to be a different tunnel under Sydney Harbour, this one extending further to the west and providing a much-needed additional route around the CBD. The Western Distributor roadway, the old Harbour Tunnel, and the Harbour Bridge will all see a 35%, 20%, and 17% reduction in traffic, respectively, according to the New South Wales government. The project’s timeline increased as a result of changes made to the building approach owing to objections after construction on this tunnel began in the middle of 2022. 

Phase 1 of the construction

Project in-depth: Sydney's $5BN Tunnel U-Turn - Sheet1
Stages of construcion_©The BIM

Road header tunneling equipment is being used during Stage 1 of construction, which started in the middle of 2022. The original Harbour Tunnel was constructed using the same method in the early 1990s, and the segment that would pass beneath the water would be an immersed tube tunnel. To do this, a sizable trench would need to be dug on the bottom of the harbour, into which prefabricated tunnel segments would be dropped, and the trench would then need to be covered. At either end of the crossing, sizable temporary work sites were planned to make this all possible. They would be employed for everything from creating the tunnel segments to building the cofferdams and getting rid of waste.

Cofferdams on North and South side_©The BIM

There were worries that the dredging project would release a “cocktail of chemicals” into the harbour, that the temporary construction sites would be very disruptive, and that the project as a whole would generate a lot of noise, dust, and possibly even property damage. The reasons against it were becoming so compelling that it looked like something ought to be done about them, thus the tunnel’s construction system was altered in between phases of work.

Phase 2 of the construction

A pair of tunnel boring machines are working on the underwater section of the excavation. These machines need to be a massive 16 meters in diameter to dig holes large enough for a six-lane roadway. Compare that to the TBMs that were used to build the Sydney Metro and the West Gate Tunnel in Melbourne.

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TBM placement_©The BIM

There are difficult ground conditions to overcome as well as the need for the TBMs to dig farther than the immersed tube tunnel would have. In reality, the location’s weak geology, which can make deep tunneling problematic, was one of the reasons for not choosing the earlier approach.

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TBM under the water_©The BIM

The contractor selected for this phase of the project, ACCIONA, will utilize a Mixshield TBM because of this. The earth under Sydney Harbour is made up of a variety of materials and has high soil and water pressure, making them perfect for excavating a tremendously large hole in such an environment. Hence, switching to TBM means no longer needing dredging and eliminating the requirement for two of those building sites, as well as reducing costs.

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Mixshield TBM_©The BIM

What is ahead for Sydney’s $5BN Tunnel U-Turn?

Sydney’s $5 billion tunnel U-Turn is one of the examples of the benefits and drawbacks of a significant infrastructure project that will be able to significantly impact the city’s traffic but will be expensive, turbulent, and time-consuming to complete. However, it will significantly reduce traffic over the Harbour bridge. The new technology of the mix-shield will help in better construction which will be done underwater and will neglect the use of cofferdams which will be troublesome for the people living nearby and so it is going to be a user-friendly project. As Sydney being a tourist place it will also ease out for the local drivers to move around with the tourists in less time as it is a nine kilometer long tunnel, specifically twin tunnel, which connects the M1 Pacific Motorway at the Wahroonga to the hills of M2 Motorway at the West Pennant Hills in Sydney North.


Hetvi Jadia is an architecture student and a dancer, born and brought up in Surat, Gujarat. She started developing interest in writing since a long time.