Rosedale Tunnel and Outfall Project on target

The Rosedale Tunnel and Outfall project has reached some key milestones in the last two months. The tunnelling section of the $116million project was completed this week. The completion marks the end of almost a year of tunnelling under the city from the Rosedale Wastewater Treatment Plant through to the off-shore connection in Mairangi Bay.

The tunnel runs almost entirely under road reserve at depths of between 25 and 60 metres. Behind the tunnel head, 18,000 tunnel segments have been put in place to create a strong 2.8 metre diameter lining to the tunnel.

After working 24 hours a day since January, the tunnel boring machine, Amelia Rose, has finished her journey 600 metres from shore, 30 metres below the surface, and 3km from where the tunnel started. The internal parts of the machine will be retrieved but the shield and tunnel head of Amelia Rose will be abandoned and forever form part of the permanent works.

Also in recent weeks, divers have fitted diffusers to the last few hundred metres of marine pipe. The marine pipe runs 2.1km out into the fast moving waters of the Hauraki Gulf and rather than having a single exit point for the treated effluent, the final section of marine pipe will have 58 ‘duck-bill’ diffusers. These disperse the effluent into the seawater across a greater area in smaller volumes. They are also flexible and are designed to bend if they were possibly caught on an anchor. Each one of these 58 diffusers needed to be installed by hand by a diver on the seafloor.

Chairman of the Infrastructure and Environment Committee, Ken McKay, says the past months have marked some key milestones for the project.

“Many of the high visibility aspects of the tunnel have been completed in recent weeks. Hard work and careful planning mean we are on track for completion on our target date in mid 2010.

“Many of the residents in the area have become quite involved in the progress of the Amelia Rose and have been watching with interest the progress on the signboard at Mairangi Bay. I would like to thank the residents for their ongoing interest in the project and for understanding the benefits this outfall will bring to all of the North Shore.”

However, there is still work to be done on the project before the final completion and commissioning in mid 2010. Connections need to be made from the marine pipe to the land based tunnel in Mairangi Bay and from the wastewater treatment plant to the new tunnel.

A cascade shaft is being built to control the descent of treated effluent from the surface at the treatment plant, down 45 metres to the outfall tunnel. The effluent needs to enter the tunnel with as little air as possible or the descent can cause operational problems and reduced capacity of the tunnel. Computer modelling, scale models and overseas experience shows that the best solution for ensuring the least air in the tunnel is by using a cascade shaft, lowering the water in a zig-zag movement step by step down the shaft.

There are also ongoing geotechnical investigations and final works to ensure everything is ready for commissioning in July 2010.