The 38-metre hydrofraise machine in action.
Our $1.2 billion Central Interceptor project has entered its first phase of major construction at the pump station worksite adjoining Māngere Wastewater Treatment Plant.
The giant wastewater tunnel will run below ground from Grey Lynn, under the Manukau Harbour to the Māngere plant. It will have several link sewers and shafts along the route for collecting and transferring wastewater into the tunnel.
Central Interceptor executive programme director, Shayne Cunis says a 38-metre hydrofraise machine is now excavating an almost 50-metre deep structural support in the ground, known as a Diaphragm wall (D wall): “Five out of 29 panels have been dug, so we’re making good progress.
This is a very complex, highly specialised form of construction and we’ve sourced experts from around the world to operate this machinery.”
The hydrofraise cutterheads weigh 36 tonne and burrow slowly into the ground. Bentonite slurry (fine clay) is pumped into the hole simultaneously. Once it has reached the required depth, reinforced steel cages are lowered into the trench. Concrete is then pumped to the bottom of the trench to begin filling the D wall around the cages. As the concrete rises in the trench, the bentonite is drawn off. The desanding plant at the surface separates the bentonite from the spoil and recirculates the bentonite to be used again into the excavation.
The top of the yellow and black hydrofraise crane is visible from Māngere streets bordering the site, whose entrance is in Greenwood Rd. Ghella Abergeldie Joint Venture, the contractor building the Central Interceptor, imported the crane from Germany which is believed to be the largest ever seen in New Zealand - even bigger than the one used to build the City Rail Link.
Preparatory work at another two sites at May Road, Mt Roskill and Keith Hay Park, Hillsborough has also begun.
A giant Tunnel Boring Machine (TBM) will be used to construct the Central Interceptor. It was designed in Germany and is currently being assembled in China. It will arrive by boat towards the end of the year.