For decades, Sydney’s wastewater and sewage had been mechanically treated at its three main sewage treatment plants in North Head, Bondi and Malabar, and then discharged into the sea. This caused an increasing amount of pollution at Sydney’s beaches and triggered comprehensive research into new solutions during the 1980s.

Sydney Water decided that the best option was routing the water and sewage through three tunnelled outfalls up to 4 km offshore in depths up to 80 m below the seabed, and then discharging it into the ocean through vertical outlets.

Construction on the outfall tunnels commenced in 1986. The ocean floor beneath the outfalls varied from rough bare rock to smooth sediment of varying depths. It was concluded that differing methods of excavation for the tunnelling works would be appropriate in the various rock formations.

For the offshore tunnels at Malabar and North Head, the use of tunnel boring machines (TBMs) was most ideal, while the Bondi offshore tunnel could be completed with either a TBM or roadheader excavation.

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A Robbins 133-146 TBM was converted for installation of the precast concrete segment rings at the Malabar and North Head tunnels. For advance drilling, the machine was used for driving the tunnels.

Roadheaders with a drill and blast method were also used for the decline excavation through the harder sandstones.

The Bondi outfall tunnel was also driven by a roadheader in the upper weathered sandstones, with drill and blast excavation used in the lower, hard and abrasive strata. Shotcrete lining of arch and cast in situ concrete roadway was placed on the completion of the excavation.

The choices for excavation methods were largely influenced by two major factors: the programming considerations, and the severe environmental restrictions on blasting.

The disruption caused by blasting resulted in the solution only being used in regions of extremely hard and abrasive sandstone strata, which would have been too difficult to achieve by mechanical means.

Installation of risers, diffusers and protective caissons was carried out at Malabar from September 1987 to March 1988, at Bondi from March to June in 1988 and at North Head from June to October in 1988.

The Malabar tunnel was commissioned in September 1990, the North Head tunnel in December 1990 and the Bondi tunnel in August 1991.