From the magazine, Microtunnelling, Pipe jacking, Tunnelling

Tunnelling in the Top End

Navigating cyclones, extreme heat and multi-storey residential properties alongside the pipeline alignment, the trenchless component was completed to program and without incident.

The project, which is part of Power & Water’s Larrakeyah Sewer Diversion Scheme, began in January 2011 with completion in early November. Winslow was engaged by MacMahon Contractors to construct eleven tunnel drives, ranging in lengths from 40 m to 160 m.

The route of the pipeline commenced with a crossing under Darwin’s Stuart Highway, then negotiated through multi-storey residential areas towards the existing sewer system in Darwin’s CBD.

The geology comprised predominantly porcellanite and phyllite rock, which varied from fresh to highly weathered throughout the tunnel alignment. Bands of stiff clay and granular material were evident, particularly at the downstream end of the pipeline route.

A slurry tunnel boring machine (TBM) manufactured by mts Perforator was utilised for the project, with a rock head favoured over a mixed face cutter head.

This decision proved to be well founded, with maximum advance rates of 22 m per shift achieved whilst jacking Hobas GRP jacking pipes. The Hobas pipe assisted to maintain low jacking forces, due to the exceptional frictional properties of the GRP pipe.

As the contract started during Darwin’s notorious wet season, a critical issue was the control of floodwater entering the shafts and tunnels. This wasn’t assisted by experiencing the heaviest rainfall on record and the arrival of Cyclone Carlos in February. Determined to proceed against the elements, the first two drives had been successfully completed by the start of the dry season.

Another challenge was the extremely shallow grade of the alignment, generally limited to 0.2 per cent. The experience of the operators was critical to maintain exacting tolerances in variable geology, and the survey data was monitored continuously.

On completion of the contract all drives had been completed within tolerance.

The contract had many potential problems at its outset, with deep shafts and tunnels weaving through residential and tourist areas. However by maintaining an excellent working relationship with main contractor, MacMahon, and managing the numerous site moves with minimum disruption, the contract was completed successfully.

Winslow minimised their footprint in the heart of Darwin’s CBD by ensuring effective production rates whilst liaising effectively with local residents and stakeholders to achieve a successful completion.

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