Auckland interceptor TBM on order 23 Jan 2020

Karen Martin, TunnelTalk

Construction work on the Watercare NZ$1.2 billion central interceptor project in Auckland is well underway, and the TBM that will be used for the excavation has officially been ordered. The 14.7km long x 4.5m diameter tunnel will be excavated with a 5.45m diameter EPBM provided by Herrenknecht.

Shaft construction at the future TBM launch location
Shaft construction at the future TBM launch location

Herrenknecht was selected following a detailed multi-criteria evaluation. “A key factor in its selection was the previous experience it has had on tunnelling projects in Auckland, coupled with its proven global experience,” said Francesco Saibene, Project Director, Ghella Abergeldie JV (GAJV). “The technical requirements from the project owner Watercare specified the EPBM type, and the GAJV, together with Herrenknecht, designed a tailor-made EPBM with capability to operate at up to 9 bar.” The TBM is expected to arrive in Auckland at the end of 2020, and will be launched in the first half of 2021.

The central interceptor will run from Grey Lynn to the Māngere wastewater treatment plant (Fig 1). The Grey Lynn tunnel, a 2km long extension, was awarded to the JV in early 2019 as part of the construction contract. The extension is part of a wider strategy by Watercare to reduce overflows and improve water quality in Auckland’s western isthmus. Construction on this extension is currently scheduled to start early in 2023.

The main tunnel will be excavated at depths of between 15m and 110m and will cross the Manukau Harbour at about 15m below the seabed. “The TBM will bore through different geological formations from soil to rock, both under the water table and in open air,” said Saibene. “There are a number of challenges on this project, with the biggest challenges likely to be tunnelling under the Manukau Harbour, and then tunnelling in residential Auckland, predominantly under private property, at depths of up to 110m and potentially experiencing water pressures of up to 9bar.

“The East Coast Bay, Tauranga and Kaawa soil formations are well known and studied by Watercare and the GAJV, but there is a small possibility that it will be required to operate in basalt, and as such the cutterhead has been designed for operating in hard rock,” he added.

Fig 1. Central interceptor route including Grey Lynn extension
Fig 1. Central interceptor route including Grey Lynn extension

Most of the spoil is intended to be reused at Puketutu Island, which is an extinct volcano that had previously been excavated to produce aggregates. Watercare is rehabilitating the island to restore the original profile of the volcanic cones by utilising the biosolids from the wastewater treatment plant combined with clean fill, and the island will one day be opened to the public as a regional park.

Along the route will be 17 shafts reaching depths of up to 80m, and two link sewer tunnels of overall 4.3km, excavated with two pipe-jacking machines. Link sewer B will be 1.1km long x 2.4m i.d and link sewer C will be 3.2km x 2.1m i.d. Link sewer C construction is expected to take place in the second half of 2020; site preparation works are complete and temporary works are about to commence.

Construction started at the Māngere wastewater treatment plant and May Road sites in 2019. The Māngere site is composed of two adjacent shafts: the inlet shaft and the main pump station shaft. The TBM will launch from here using both shafts, and is expected to progress at an average rate of 16m/day. Pump station construction can start only once the TBM has advanced about 200m, when TBM operations will be executed only from the inlet shaft. The diaphragm walls for the double shaft are currently in construction. May Road is the second main site of the project, and tunnelling operations will be relocated here once the TBM arrives from its first drive from the Māngere site.

“The logistics in the Māngere launch shaft and May Road shaft are very demanding in terms of limited space and depth – up to 69m in the May Road shaft – for which GAJV studied and analysed each sequence of the TBM cycle to minimise any disruption and to give the best performance,” said Saibene.

Two more sites are being established early in 2020, with work beginning on the 80m deep shaft at Keith Hay Park on 17 January 2020, and construction at the Walmsley Park site due to start in March 2020. The project is expected to be completed in 2025.


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