Sydney Harbour Bridge is an iconic image of Sydney. As the sixth-longest spanning-arch bridge and tallest steel arch bridge in the world, it is a testament to Australia’s civil engineering heritage. As a main connection for Sydneysiders between the city’s CBD and the North Shore, it serves a vital purpose.
However, as Sydney’s population grew, so did the congestion on the Harbour Bridge.
By the 1980s, increasing traffic volumes saw the eight-lane structure packed out during peak travel times. The need to alleviate this congestion and provide an alternative cross-harbour route was identified in the original Sydney Orbital Network Plan.
This was what would ultimately become the four-lane, 2.3-kilometre, twin-tube Sydney Harbour Tunnel.
Due to the cost and scope of such a project, the New South Wales Government sought a proposal from the private sector to build and toll. The contract was awarded to joint venture partners Transfield and Kumagai Gumi in 1987.
The tunnel consists of twin 900-metre land tunnels on the harbour’s north shore, twin 400-metres land tunnels on the south shore and a 960-metre immersed tube structure. Construction began in 1988.
Immersed tube technology was required for the project. Eight reinforced concrete caissons were constructed in a dry dock at Port Kembla, which had to be floated into position and lowered onto the seabed.
A trench was dredged before the arrival of the concrete units, which were then submerged into place using a system of pontoons and control towers. The tunnel’s deepest point reaches 25 metres below sea level.
The land tunnels were constructed by a combination of driving and cut-and-cover techniques.
Engineers were faced with the challenge of joining the submerged tunnel with the land tunnels beneath the Opera House concourse, which had recently been refurbished.
To excavate without disturbing the concourse, engineers had to underpin the concourse with massive piers and beams so the old piers that were in the path of the tunnel could be removed.
As part of the project, the northern end pylons of the Sydney Harbour Bridge had to be altered to allow for exhaust fumes from the tunnel be ventilated out.
In August 1992, after nearly four years of construction, the Sydney Harbour Tunnel was completed and opened to traffic. It joins the Warringah Freeway at North Sydney, and the Cahill Expressway at the entrance to the Domain Tunnel, providing a crucial second crossing for the harbour.
The $750 million tunnel was a milestone project. Most notably it was the country’s first significant Build Own Operate Transfer (BOOT) project.
Today it is a vital piece of infrastructure and a significant section of Sydney’s bustling road network.