The Central Interceptor will ensure sufficient capacity in the wastewater network to meet the planned population growth of the city. It will provide a resilient wastewater system and immediate remedy to historical wet weather discharges into local creeks and harbors.
The main tunnel is 16.2 kilometers long (10 mi), at depths up to 110 meters (361 ft) below the surface, and crosses under the Manukau Harbour. Eleven deep drop shafts divert wastewater and stormwater flows into the main tunnel. The tunnel termination point is an underground pumping station that connects to Auckland’s largest wastewater treatment plant.
Two separate link sewer tunnels, 3.2 kilometers (2 mi) and 1.1 kilometers (0.7 mi) in length, constructed by pipe jack methods, connect into the main tunnel.
Delve Underground was responsible for detailed geotechnical and structural design of the main tunnel, shafts, and diaphragm walls for the pumping station, including development of drawings, specifications, and a geotechnical baseline report. During construction, our services included construction management for the mainline tunnels and shafts, and geotechnical engineering support.
The main tunnel is lined with a one-pass, precast segmental lining that was lined with welded polyethylene sheeting to withstand the corrosive sewer environment. Ground conditions for the tunnels ranged from alluvial soils to weathered and unweathered bedrock consisting primarily of sandstones and laminated mudstones. Basalt flows were also encountered in excavations.
The use of cascade drops eliminated several deaeration shafts, thus reducing project cost and carbon footprint. For deep, small-diameter drop shafts, an innovative glass fiber reinforced plastic prefabricated lining was implemented, avoiding manned entry for construction and reducing the construction schedule. Finally, a dual-cell diaphragm shaft for the pumping station separated tunnel construction activities from the pumping station construction, thus reducing the construction critical path, as well as providing for emergency pressure relief of the tunnel.