ABSTRACT
Watercare is building a 16.2 km-long underground wastewater tunnel called the Central Interceptor (CI) that will run from Māngere Wastewater Treatment Plant to Point Erin, Herne Bay. It is intersected by two link sewers, which capture flows from central and west Auckland suburbs. A range of major infrastructure such as a new pump station and confluence chamber has also been built at the Māngere site. The Central Interceptor project will significantly reduce wet weather overflows into waterways and Waitematā beaches, leaving a legacy of a cleaner environment for the people of Auckland. The project also involves the excavation of 18 shafts, where 3 of the smaller shafts (less than 4.5m internal diameter) are drilled shaft with Fibre Reinforced Polymer (FRP) as their permanent lining. The paper discusses the design development, construction and monitoring aspects of the structure connecting the main tunnel and drilled shafts.
Beginning from the tender design, safety in design was a key consideration for the connection design as it requires highly risky removal of main tunnel lining, rock excavation and subsequent working under confined space. Focus on minimising the amount of work required for the tunnel construction by reducing structural steel connections and large in-situ concrete pours offers reduction of safety accident risk, improved quality, less disruption with the standard activity, as well as reducing the material used and the associated carbon footprint. Through utilization of the existing structures of drilled shaft and bored tunnel, over 90% reduction in permanent concrete volume is achieved, resulting in significant risk reduction, time saving and carbon footprint reduction for the project.
