Developing a corrosion strategy to protect New Zealand's Largest Wastewater Asset

The Central Interceptor is a new deep tunnel sewer that will provide additional capacity in the network to meet planned population growth and development in Auckland, provide a more resilient wastewater system, and mitigate wet weather overflows in central Auckland. The tunnel is to be built between Western Springs and the Mangere Wastewater Treatment Plant (WWTP) over the next six years. The main tunnel will be approximately 13 kilometres long and up to 110 metres below the ground surface. It will cross under the Manukau Harbour approximately 15 metres below the seabed. The main tunnel will be excavated by tunnel boring machine and will have an internal diameter of 4.5 metres. In addition, two separate link sewer tunnels of 2.4 and 2.1 metres internal diameter and 1.1 and 3.2 kilometres in length respectively will be constructed by pipe jack methods. The pipe jacking drive lengths range from 300 to 960 metres between shafts.

Nine drop shafts will be constructed along the Main Tunnel alignment to provide flows to the tunnel. With a spacing of up to 4 kilometres between shafts, future access for maintenance is a recognised challenge for the project. The main tunnel and shafts are required to have a 100-year design life and therefore must be made of durable materials with an extremely low level of maintenance. To achieve this design life, a number of strategies are being employed including environmental controls, material selection, and hydraulic design.

Watercare has undertaken multiple sampling campaigns to determine the corrosion potential of the wastewater to be conveyed in the system. This has consisted of sewage sampling as well as advanced computer modelling to predict corrosion rates. Over the last two years Watercare has undertaken extensive laboratory testing of corrosion resistant materials for the highest risk elements. Concurrently, samples of these corrosion resistant materials have been exposed to the existing sewer network to replicate the in-service conditions that the Central Interceptor tunnel and shafts will experience throughout the asset’s life. Additionally, design elements such as concrete cover, embedded polyethylene liners, and the overall hydraulic performance have been considered in determining the optimum strategy. A risk assessment further assisted in defining what strategy should be applied in specific elements of the scheme.

This paper will describe the overarching strategy, field investigations, modelling, lab testing process, and results of the testing programme. With construction works to commence in 2019 on this flagship project, Watercare can proceed with confidence that the design objectives can be achieved.