Sydney Harbour Tunnel - Construction

Construction

The tunnel is made up of three sections: twin 900-metre land tunnels on the north shore, twin 400-metre land tunnels on the south shore and a 960-metre immersed tube (IMT) structure. The tunnel falls about 55 metres from the northern entrance and about 35 metres from the southern entrance to its deepest point, 25 metres below sea level.

The IMT structure consists of eight precast concrete units. The units were constructed over 100 kilometres away in a casting basin at Port Kembla and then towed to Sydney Harbour. A trench was dredged before the arrival of the IMTs and then the IMTs were lowered into the trench by a system of pontoons and control towers. After the IMTs were in place the trenches were backfilled and then a rock armour was placed over the top to protect the units against marine hazards, such as anchors or sinking vessels.

The land tunnels were constructed by a combination of driving and cut-and-cover techniques. The total construction cost of the project was A$554.25 million, and was designed to be strong enough to withstand the impact of earthquakes and sinking ships.

One of the northern end pylons of the Sydney Harbour Bridge was altered to allow for air exhaust from the tunnel to rise over the harbour. Bradfield Park on the northern side of the bridge also has hidden air intakes behind some conspicuously placed bushes.

Fresh air is drawn by an underground ventilation station on the north shore and is pumped to all sections of the tunnel through vented ducts. The air supply uses fourteen axial flow fans, each up to 2.5 metres in diameter. The exhaust uses sixteen (eight in each northern pylon tower) fans that draw exhaust air through two underground ducts from the tunnel ventillation station and transfer the air to the top of the pylon. They can expel it at up to 1 500 m³/s – equivalent to changing all the air in the tunnel every two minutes with the capability of running in reverse in an emergency and all fans are rated for smoke extraction. Each of the fans has a duty of 53 to 103 m³/s. The testing of the fans was one of the most comprehensive ever, covering flowrate and pressure, power measurements, sound levels, bearing vibration, x-raying of all impeller components, high-temperature tests at 200 °C for 2 hours, impeller strain, and 24-hour run tests for reversals.

The tunnel was opened to the public to walk through from North Sydney (Falcon Street) to South Sydney (The Domain) on 30 August 1992. The proceeds of the tickets sold for this event were donated to the Royal Institute for Deaf and Blind Children. The tunnel opened to traffic on 31 August 1992.