Watercare Services awarded McConnell Dowell, in joint venture, the construction of the Hobson Bay Tunnel Project located in Auckland, New Zealand.
The project was to replace a 90 year old sewage pipe that crosses the Bay, on Auckland's waterfront Tamaki Drive. The tunnel offers greater sewerage capacity to cater for projected population growth in the area and prevents wastewater overflows into both the Bay and Waitemata Harbour basin. The replacement of the existing wastewater pipe also opens up the Bay for recreational boating.
The project scope consists of a 3 km long, 3.8 m internal diameter tunnel which is driven approximately 30 m below the bay floor; construction of three shafts of approximately 35 m to 40 m in depth (23 m, 8 m & 10 m internal diameters); surface reticulation/connection works; and demolition of the existing sewer pipeline that crosses the Bay.
This project uses a Tunnel Boring Machine by Lovat of Canada, and construction techniques are designed for a combined firm rock and soft substrate.
The 23 m diameter shaft includes considerable mechanical and electrical works (the supply and installation of six 1MVA pumps), while all three shafts also require a significant amount of concrete works. McConnell Dowell has worked closely with Watercare Services to develop a stakeholder management plan to ensure bay users were not impacted during the construction of the new tunnel - especially during the demolition of the existing sewer line.
Construction of the surface reticulation works has required some traffic management plans to be developed and implemented to ensure minimal impact on road users.
To minimise distruption to local residents in the construction area the construction team have also implemented innovative noise reduction measures including completely enclosing of the head house.
Tunnelling 30 m under the bay's mud and rock floor has ensured minimal environmental disturbance during construction.
The demolition of the old sewer is the subject of a detailed environmental management plan which ensures removal of the structure does not impact on the marine environment.
A huge effort has also been put into procuring equipment from a diverse range of manufacturers located throughout the world. Extensive QA practice's have been utilised to ensure that the equipment has been manufactured to standard and would be delivered to New Zealand on time. The tunnelling section of the works started on the day of the programme and was completed on 5th February 2009 (breakthrough), 17 days ahead of schedule.