The duplicated tunnel was designed to provide a bypass main in the event of a failure or shutdown for maintenance on the 1350 mm main. It was proposed that the main would have a
900 mm mild steel cement lined pipe and a drive length of 180 m.
A Herrenknecht AVN 800 slurry shield closed face tunnelling machine was proposed to install 1200 mm steel casing pipes. The thrust shaft was rectangular of concrete construction, poured in situ, and sinking methodology was used until at the correct level below the invert of pipeline at a depth of 9.8 m.
A plug was then poured, and an entrance ring/seal and thrust block was placed for commencement of tunnelling. The receival shaft was a concrete circular caisson shaft of 3.5 m. Both shafts were raised to 1 in 50 year flood level above ground to minimise future flood risk to the tunnel.
Ground conditions along the drive varied between silty clay, clay, shale and mudstone rock. A seismic survey was undertaken prior to commencing to confirm the bottom of the river bed and the depth of the loose silty material and cover to the top of the proposed pipeline.
Due to the critical nature of the project, the works were carried out on a double shift 24 hour basis to minimise jacking tonnage along the drive. Intermediate jacking stations were also installed along the pipeline in the event of jacking tonnages becoming excessive, whereby the intermediate jacking stations could be activated.
The drive of 180 m was completed in 12 days and line and level were within 15 mm of design. A special flush joint was designed in-house for the project, suitable for the steel casing pipes and minimising the installation time associated with other jointing alternatives.
Upon completion of the drive the 900 mm carrier main was installed inside the 1200 mm casing pipe using a specially designed skid system to ensure no damage to the protective coating on the pipe. These were installed in 6 m lengths with weld joints applied as the pipe was installed. Following completion, the void between inside of the casing and carrier pipe was grouted.
The company also undertook the mainland trenching and pipeline construction works, including several tie-ins to live 1350 mm mains at either side of the river crossing and several valves and flowmeters including valve pit structures. The trenching works were within 6 m of the existing above ground 1350 main.
Environmental considerations, including treatment and disposal of acid sulphate soils, increased the complexity of the project, and environmental management plans to minimise the impact of the works on the existing wetlands and rare fauna and flora of the location were developed and adhered to.
For further information on the specialised civil engineering capabilities of MacCormick, refer to their website at www.maccormick.com.au.