Massive sewer project pushes on

Awarded to leading contractor John Holland, the project will see 13 kilometres of new sewers tunnelled through the northern suburbs in two stages for clients Melbourne Water and Yarra Valley Water. Stage One involves construction of 8km of tunnels beneath the Merri Creek Valley, which will connect the existing sewer in Coburg to the northwestern sewer in Essendon. Stage Two is a 4.5km stretch tunnelling north from Coburg to Reservoir.The system is designed to alleviate the risk of sewage inflow into waterways during heavy rain events and to cater for the northern suburbs population expansion. It is a priority project under the state government’s Yarra River Action Plan. A series of eight shafts 18-65m deep and up to 13m diameter are being constructed as tunnel boring machine launch and retrieval points for the 13km of tunnelling. The shafts also provide for connection to existing sewer infrastructure and maintenance access for Yarra Valley Water and Melbourne Water.Construction of NSP Stage 1 began in August 2007 and Stage 2 in September 2007. The entire project is timed for completion by mid 2012. Tunnelling is expected to be completed in 2011.Key to the project is the deployment of three TBMs, with the first machine commissioned having already completed two thirds of a 1600m drive from De Chene Reserve to Carr St in Coburg.The German-built Herrenknecht earth pressure balance TBM is cutting a 3m-diameter tunnel at a depth of 19-32m, through variable geology including basalt and Silurian sedimentary formations. A second TBM, a US-built 3m-diameter Robbins double shield is being commissioned for launching at the bottom of a 32m-deep shaft constructed at Newlands Road, Coburg. This machine faces the toughest task and will use 43cm cutters to tunnel through basalt up to 270 mPa hardness.John Holland’s third TBM, a 4m-diameter Herrenknecht EPB TBM, will begin boring at a 13m-diameter shaft at Brearley Reserve, Pascoe Vale South, from a launch chamber at 60m depth. It will move about 1800m south to the Vanberg Road, Essendon shaft. When this is completed it will be moved to begin a 2930m tunnel drive from Brearley Reserve to Carr Street, Coburg North. The TBMs use a combination of rail muck cars that are hoisted from the shafts by gantry cranes and vertical conveyors. Spoli is tipped into concrete bins in preparation for loading out to road transport.All tunnels utilising EPB TBMs are being lined with 200mm-thick pre-cast concrete tunnel lining segments of about 1m long including a key segment. The TBMs feature segment erectors that allow the tunnel lining to be erected as the machine progresses.The annulus – the space between the concrete liner and the earth – is also pressure grouted at the same time. The Stage One excavated tunnels will then be lined with the final lining pipe – typically 1600-2500mm concrete pipes with HDPE liner. The Stage Two section, requiring tunnelling through basalt will see support provided by rock bolts, while a fibreglass reinforced plastic pipe (GRP) lining will be used to pipe sewage.John Holland project director Rob Muley said early stages of the project involved significant input into the design of the three TBMs. “One of the key issues for this project is to ensure we have designed and specified the TBMs to cope with the variable geology we are likely to encounter,” he said.“We have spent about a year designing the machines with particular specifications in conjunction with Herrenkencht and Robbins to suit the ground conditions.“As far as tunnelling techniques go, we have two EPB and one Double Shielded TBM, roadheaders and drill and blast in the launch chambers, and a small amount of hand mining in the connections.”

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