The Northern Sewerage Stages 1 and 2 Project involved the construction of 12.5 kilometers (8 mi) of new sewer in the northern suburbs of Melbourne, Australia. The project increases the capacity of the existing sewerage system for Melbourne and in doing so will help to protect local creeks by virtually eliminating sewage spills that can occur after heavy rain.
The two-stage project was developed jointly by Melbourne Water and Yarra Valley Water. Working with Sinclair Knight Merz, McMillen Jacobs Associates led the tunnel and shaft design on both stages of the project, and provided construction services through completion in November 2011. Stage 1 consisted of 8 kilometers (5 mi) of gravity sewer pipeline installed in a tunnel, five major work shafts, and several smaller connection manholes and pipelines. Tunnel and shaft depths vary from 15 to 60 meters (50 to 200 ft), and the pipeline’s internal diameter ranges from 1.6 to 2.5 meters (5 to 8 ft) to suit hydraulic requirements. Most of the tunneling encountered weak, interbedded siltstone and sandstone. Excavation was carried out by multiple earth pressure balance tunnel boring machines (TBMs). Initial tunnel support consists of precast segmental lining. The 4.5-kilometer-long (3 mi) Stage 2 tunnel has an internal diameter of 1.8 meters (6 ft) at depths ranging from 8 to 35 meters (25 to 115 ft), and excavation was mostly through basalt with a hard rock TBM. Shaft depths range from about 20 to 40 meters (65 to 130 ft), with the main shaft measuring 8.3 meters (27 ft) across. To meet a 100-year design life and address the potential for hydrogen-sulfide-related concrete corrosion, the carrier pipeline for both stages consists of fiber-reinforced plastic (FRP) pipe. In addition, FRP was used to line critical components of the main shafts and connection manholes.
The owners chose a unique procurement method for the Northern Sewerage project: a form of design-build that shares risks and rewards amongst the project partners based on their ability to control the work.