The tunnel is necessary to replace the 90 year-old sewer pipe that bisects Hobson Bay on Auckland’s waterfront Tamaki Drive. No deep sewer tunnel of this size or complexity has been built in New Zealand.
The $NZ118.6 million Orakei Sewer Replacement, Hobson Bay Tunnel Project (Project Hobson) will meet projected growth in the area, practically eliminate wastewater overflows into the bay and the Waitemata Harbour, open the bay for recreational purposes and improve the views from Tamaki Drive.
Watercare Project Manager Mike Sheffield said that Project Hobson began with extensive consultation with the local community and regulatory bodies. Together with the approvals and design process, the planning was completed over a period of six years.
Extensive site investigation was essential to the project, Watercare invested approximately $NZ1 million in site investigation for Project Hobson, undertaking some 60 boreholes, said Mr Sheffield.
The Sinclair Knight Merz and Connell Wagner joint venture is responsible for all permanent works design for Watercare, except for the tunnel linings.
Heavyweight TBM in NZ
The TBM machine was chosen for several reasons including the technical requirements of the job, environmental factors, and because the machine had been was specifically designed for the ground conditions, geology and location of Hobson Bay.
Watercare said the TBM is a heavyweight in the construction world and its advanced capabilities are a New Zealand first.
The TBM was manufactured by Lovat. The machine is a mixed face earth pressure balance machine, weighing 270 tonnes and measuring 75 m in length. The diameter is 4.32 m.
Due to the immense size of the TBM, it was shipped from Canada in segments and was assembled 35 m below ground in a specially prepared shaft and a back shunt tunnel.
To enable continuous tunnelling, a temporary noise enclosure was constructed. Measuring 50 m long and 25 m wide, the enclosure was insulated with two layers of acoustic lining and clad with steel.
Mr Sheffield said this enabled tunnelling to take place round the clock by keeping the external noise below the levels permitted in the resource consents.
Tunnelling started in June 2008 using innovative technology that allows the machine to complete the work in one pass. The company said that the machine drills through the rock at the boring face, while behind the cutting chamber, the tunnel is lined with pre-cast concrete segments. The gap between the segments and the rock is grouted, and ready to go.
The sealed and lined internal diameter of the tunnel measures 3.75 m. Debris from the excavation is removed using a small train system.
Obstacles overcome
Some of the challenges identified by Mr Sheffield included the construction of the pumping station. As the shaft is 38 m deep and 22 m in diameter, this component of Project Hobson required careful thought in regards to the method of excavation.
Watercare also carefully selected a tunnelling method that would be able to handle the specific conditions of the project. Other techniques considered to replace the old pipeline included a new pipeline, alternate tunnelling options or a surface pipeline.
Project Hobson: the future
McConnell Dowell operated the LOVAT EPBM, Te Kaha, breaking through in February 2009.
Since then, works have been completed to demobilise the tunnelling equipment and strip out the tunnelling services. Once this was completed, the temporary access shaft used to service the tunnel works was lined and backfilled. The whole construction area used for tunnelling is now ready to be restored to its original playing field use.
Mr Sheffield said that once the pump station is commissioned, first flows are expected early 2010, and although that commissioning date has slipped due to delays associated with offshore valve supply, the overall works – including demolition of the existing sewer pipe across Hobson Bay – will be completed in mid to late 2010, in line with the project’s original timeframe.