Oraki sewer replacement
Watercare is boring the tunnel to replace the 90 year old sewer pipe which currently bisects the bay on Auckland’s waterfront Tamaki Drive. No deep sewer tunnels of this size and complexity have been built in New Zealand.
The $NZ118.6 million Oraki Sewer Replacement, Hobson Bay Tunnel Project (Project Hobson) will see the replacement of the concrete sewer with a 3 km tunnel.
The new tunnel 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.
Implementing the GBR
Mr Sheffield told Trenchless Australasia that a Geotechnical Baseline Report (GBR) should comprise a realistic and quantative assessment of the geotechnical conditions to be encountered at a project site and forms an integral part of the conditions of contract, in particular the unforeseen conditions clause. The GBR provides both the employer and the contractor greater certainty as to the allocation of risk and also provides the employer with a better basis for obtaining and evaluating tender prices.
In tunnelling, in particular, many aspects associated with ground conditions will have a major influence on time and cost; the critical issues are cuttability, support, and all phases of spoil handling.
Extensive site investigation is therefore essential. Watercare invested approximately $NZ1 million on site investigation for Project Hobson undertaking some 60 boreholes, said Mr Sheffield.
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 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 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 at the tail end of the machine the tunnel is lined with pre-cast concrete segments. The gap between the segments and the rock is grouted, ready to go.
Once sealed and lined the internal diameter of the tunnel will measure 3.75 metres. Debris from the excavation is removed using a small train system.
The slab
The new pump station is an important component of Project Hobson. In April of this year a concrete pour required approximately 130 concrete trucks to deposit 700 m3
The concrete formed a 1.4 metre thick base slab for the new pump station on the Orakei Domain. Flygt Pumps from Sweden supplied six pumps for the pump station.
The large scale pour was necessary to ensure the pump station has enough weight to support the structure on top and to counteract the uplift from groundwater.
The station will pump waste water from some central and eastern suburbs to the Mangere Wastewater Treatment Plant for processing.
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 3 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
Works on Project Hobson began in May 2007. The new tunnel is due to be completed in 2010, following the removal of the old pipe.