Desiree Willis, Technical Writer, The Robbins Company

The Robbins runner and its operating crew

A 3m (9.8ft) diameter double shield Robbins TBM has completed its first section of tunnel for a major flood control project in Melbourne, Australia. The machine is working on sections of Stage 2 of the city's five-year Northern Sewerage Project (NSP), an undertaking designed to prevent heavy seasonal rains creating the potential for hazardous wastewater overflows into valuable streams and rivers and provide for future urban growth in Melbourne's northern suburbs.

Split into two stages, the NSP involves the construction of 13km of new sewer tunnels, located along the Merri Creek Valley from Coburg to Reservoir and the Moonee Ponds Creek at Pascoe Vale. Each of the seven new tunnels is being constructed under a AUD$600 million contract by contractor John Holland using rock and soft ground EPB TBMs.

The Robbins machine's first drive completed in July, is the 2.1km (1.3 mile) tunnel drive from Newlands Road to Jukes Road. Following a short maintenance break, the machine is set to continue tunneling a further 1.1km northwards to L.E. Cotchin Reserve, where the pipeline will connect with an existing sewer system. Drill+blast and EPB tunneling are completing other drives in the total 4.5km (2.8 miles) alignment of the Stage 2 project.

Plan of the NSP Stages 1 and 2

Although both drill+blast and TBMs are being used, John Holland and Stage 2 client, Yarra Valley Water, opted for TBMs on the majority of the State 2 sections to benfit from the potential for higher advance rates as well as to reduce disturbance in urban areas. "Drill+blast operations are limited to day shift only (7am to 5:30pm)," said Dave Kristy, Community Relations Officer for the John Holland Group, "whereas TBMs allow for 24hr operations, even in built-up residential areas."

The TBM was launched from a shaft at Newlands Road, near to residential homes, requiring acoustic noise mitigation, including full sound-dampening enclosures for the three 24hr construction sites located across the project. John Holland has also installed 2.4m high hoardings around all shaft sites. To date, the Robbins machine has tunnelled beneath hundreds of residential properties without registering any concerns from the residents above.

Geology along the alignment varied from massive columnar basalt to a mixed face of weathered basalt with infilled clay seams. Roof conditions have thus far remained competent enough to allow installation of the split set rock bolts originally designed for the tunnel.

Melbourne's Northern Sewerage Project (NSP) is part of the Victorian State Government's Yarra River Action Plan. Once completed, it will provide new sewers for planned urban development in Melbourne's northern growth corridor. The wastewater system will help protect two local creeks from the damaging impact of sewage overflows after heavy rains, and ensure the prolonged health of the city's iconic Yarra River. The project spans some 13km (7.76 miles) of tunnels that are jointly delivered by Melbourne Water (Stage 1) and Yarra Valley Water (Stage 2). The project is scheduled for completion in mid-2012.