Customer: Watercare Services Limited Contract type:  Design & Construct Location: Auckland, New Zealand

Fast Facts 3 km long, 3.8 m internal diameter tunnel 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), including considerable mechanical and electrical works Surface reticulation and connection works Demolition of the existing sewer pipeline 17 days ahead of schedule 75 m long TBM weighing over 200 tonnes 112 manhours per week 157 m per week progress rates 690 t less CO2 emissions every year, equivalent to removing 150 cars from the road, by using the latest TBM technology to raise the alignment 13,000 m3 capacity of sewage and water every hour

McConnell Dowell was awarded the Hobson Bay Tunnel project for the replacement of a 90 year old sewage pipe crossing Hobson Bay on Auckland's waterfront. The tunnel offers greater sewerage capacity for projected population growth in the area and prevents wastewater overflows into the bay and Waitemata Harbour basin, a popular swimming spot.  The replacement of the existing pipe also opens up the Bay for recreational boating.

Key project challenges included dealing with the unique geology of the area, operating quietly 24 hours a day on a site located in a residential area with imposed restrictions on traffic, operating noise, and visual impact.

The team used an earth pressure balance tunnel boring machine by Lovat of Canada, and construction techniques were designed for a combination of firm rock and soft substrate. The use of this type of TBM was the first of its kind in New Zealand and in the East Coast Bays Formation, as was the use of gaskets on the segmental lining. This minimised any risk of differing ground conditions on the project.

We developed an innovative lining design for water tight structures, which is of critical importance to safety both during construction and for the operational life of the asset.  

A further innovation to reduce costs and to comply with the design requirements for no permanent fixings to be left in the lining, was to use only “screw bolts” that were able to be removed from the lining after installation, without leaving any element of the bolt in the hole. This saved on the costs of remedial works.

To minimise disruption we developed an innovative noise reduction mechanism with an enclosed head house. Tunnelling 30 m under the bay’s mud and rock floor ensured minimal environmental disturbance.

A collaborative effort, particularly in key areas such as tunnel lining design, allowed us to devise an optimum design that took into account construction costs. Extensive QA practices ensured that equipment such as the TBM and overall project deliverables were delivered to standard and on time.