Heavy seasonal rains in Melbourne, Australia, can create the potential for hazardous wastewater overflows. Protecting the area’s valuable streams and rivers is the primary priority of the city’s five-year Northern Sewerage Project (NSP). Split into two stages, the NSP involves the construction of several new sewer tunnels, located deep beneath Melbourne’s densely populated northern suburbs. Each of the seven new tunnels is being constructed with TBMs. A 3-m (9.8-ft) diameter Robbins Double Shield TBM recently completed its first section of tunnel in July 2009. The machine was launched to bore sections of Stage 2 during the first quarter of this year.

The Robbins machine, which is being operated by local contractor John Holland, recently completed the 2.1-km (1.3-mile) long tunnel drive from Newlands Road to Jukes Road. Following a short maintenance break, the machine is set to continue tunneling a further 1.1 km (0.68 miles) northward to L.E. Cotchin Reserve, where the pipeline will connect with an existing sewer system. Drill and blast, as well as EPB tunneling operations are also under way, making a total 4.5 km (2.8 miles) of new tunnels on Stage 2 of the project.

Though both drill-and-blast and TBMs are being used, John Holland and client Yarra Valley Water opted for TBMs on the majority of the sections because of the higher potential advance rates, as well as reduced disturbance in urban areas. “Drill-and-blast operations are limited to the day shift only (7 a.m. to 5:30 p.m.), whereas TBMs allow for 24-hour operations, even in built-up residential areas,” said Dave Kristy, community relations officer for the John Holland Group.  

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 24-hour construction sites located across the project. John Holland has also installed 2.4-m (7.9-ft) high hoardings around all shaft sites. To date, the Robbins machine has tunneled beneath hundreds of residential properties without registering any concerns from the residents above.

Geology along the alignment has 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 more than 12.5 km (7.76 miles) of tunnels jointly delivered by Melbourne Water (Stage 1) and Yarra Valley Water (Stage 2). The project is scheduled for completion in mid-2012.

Desiree Willis a technical writer for The Robbins Co., in Solon, Ohio.

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