From the magazine, Microtunnelling, Tunnelling

Darwin’s CBD gets jacked up

The $A80 million project, headed by Power and Water Corporation, forms part of the Darwin Sewerage Strategy and will tunnel under the CBD. In particular, it is a key section of the diversion of an existing untreated sewage discharge into the Darwin Harbour at Larrakeyah to a wastewater treatment plant at Ludmilla – ultimately discharging through an existing outfall of East Point.

Macmahon Contractors has been awarded with the $A12 million contract to use microtunnelling in order to minimise above-ground disturbances. The tunnelling contractor for the project is Winslow.

The overall project will involve the upgrade of an existing sewage pumping station at Doctors Gully, Larrakeyah, increasing the capacity of the treatment plant at Ludmilla and increasing the capacity of the effluent rising main and outfall at East Point.

The Larrakeyah diversion stage 2 comprises of approximately 1.2 km of DN900 HOBAS pipe, of which 930 m is to be installed by pipe jacking under the Darwin CBD. The first jacks are to commence in late 2010.

Power and Water Corporation Senior Infrastructure Development Engineer Peter Lockett said “Trenchless techniques have been specified in the design and construct contract due to the deep sewer to be constructed through built-up areas and is the only method considered possible in the urban environment.

“This will minimise impacts on the residents and businesses by minimising noise, dust, construction traffic, property access interruptions and traffic management issues during construction,” Mr Lockett said.

Selecting the right pipe

There were major concerns about the rocky ground conditions for pipe jacking. These are risky ground conditions for rigid or brittle jacking pipes. For this reason, HOBAS jacking pipe was carefully considered and chosen for the project.

HOBAS jacking pipe was able to offer the added advantage of having a substantially smaller outside diameter than the other pipes that were considered. This meant that a smaller tunnel boring machine could be used, which resulted in noteworthy cost savings.

HOBAS jacking pipe is also able to be jacked without the use of packing rings (pressure rings made of timber particle board or MDF).

This is considered a major advantage given that when these rings are used for standard rigid pipe, they stay in the joint and deteriorate early in the pipes service life, leaving a gap behind. In many pipes, the rings keep the joint from closing fully and take away from the joints’available deflection during installation. Jacking without the need of packing rings is ideal.

The Northern Territory Government plans to shut down the Larrakeyah outflow by October 2011.

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