Auckland’s New Wastewater Tunnel: Central Interceptor

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Project Update by Sean Peterfreund, PE, SE, and Shannon Goff, CPEng

Watercare’s Central Interceptor (CI) will be New Zealand’s largest wastewater tunnel, traversing under Auckland. The project includes a 14.7-kilometer-long (9.1 mi), 4.5-meter-diameter (14.8 ft ) main tunnel and two pipe-jacked link sewers totaling 4.3 kilometers (2.7 mi) and will run from Grey Lynn south to the Māngere Wastewater Treatment Plant. This $1.2 billion infrastructure project will ensure ample capacity in the wastewater network to meet projected population growth, reduce overflows into the waterways by 80%, and provide a more resilient system for the city.

CI’s main tunnel will be 15 to 110 meters (49–361 ft) below the surface, including a section under Manukau Harbour approximately 15 meters below the seabed. Nine deep drop shafts will divert wastewater and stormwater flows into the main tunnel. The tunnel termination will be a 6 m3/s (1,585 gal/s) underground pumping station, connecting to the Māngere wastewater treatment plant, Auckland’s largest.

Central Interceptor Alignment

CI Construction Sites and Timing. Link sewers will extend from main tunnel to Rawalpindi and Miranda Reserves (source: Watercare).

The main tunnel will be excavated by a 5.45-meter-diameter (17.9 ft) earth pressure balance tunnel boring machine (TBM). It will launch from the Māngere shaft site (MPS) and drive north to May Road, where it will be refurbished and relaunched to complete the northern section of the main tunnel drive. Constructed by pipe-jack methods from seven shafts, link sewers B and C will connect into the main tunnel.

Ground conditions for the main and link sewer tunnels are anticipated to range from alluvial soils to weathered and unweathered bedrock consisting primarily of sandstones and laminated mudstones, with basalt flows in isolated locations.

McMillen Jacobs Associates has been involved with Central Interceptor for its entire life cycle. In 2009, during feasibility and concept design, the design team (McMillen Jacobs, AECOM, and Jacobs Engineering, Inc.) explored alignment efficiencies and corrosion protection. We then provided detailed design of the main tunnel and shafts, including drawings, specifications, and the Geotechnical Baseline Report. Presently, we are providing on-site Construction Management Services for tunnels and shafts.

During planning and design, the team addressed several challenges, including construction of deep drop shafts in dense urban environments. The team reduced the number of these shafts with

Central Interceptor Shells

Shells dating back 3-4 million years have been unearthed by the Central Interceptor team during their work at the Māngere site (source: Watercare).

implementation of cascade-type drops and provided for nonpersonnel-entry construction methods on shafts exceeding 60 meters (197 ft) deep. Other project challenges and innovative solutions included tunneling under the Manukau Harbour, dual-cell diaphragm wall shafts for the pumping station, and design of a durable one-pass, precast segmental tunnel lining, itself lined with polyethylene sheeting to withstand corrosion.

Construction was awarded to the Ghella-Abergeldie Joint Venture (GAJV). Establishment of site works at the MPS site commenced in September 2019. In December 2019, construction by hydrofraise began on the 40-meter-deep (131 ft) MPS shaft, using 29 diaphragm wall panels measuring 1.2-meters thick (4 ft). The final panel was installed in May 2020. Excavation of this dual shaft—with 14-meter (46 ft) and 28-meter (92 ft) diameters—was completed this month.

Construction at the May Road site commenced in early 2020 with Shaft A, connecting to Link Sewer C; and Shaft B, primarily a construction shaft to facilitate the main tunnel drive. Excavation is in progress for these two 65-meter-deep (213 ft) shafts.

Enabling works for the construction of the remainder of the main tunnel and sewer shafts are also underway. The TBM will be delivered to the MPS site from Germany later this year and be assembled and tested prior to launch in April 2021. Project completion for the Central Interceptor is targeted for 2025.

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