Work heads underground for Watercare’s Central Interceptor project
While last year was all about setting up onsite for Watercare's Central Interceptor project, the focus has now moved underground as the team start to dig deep, ready for the arrival of the key piece of equipment that will deliver the tunnel.
Watercare’s Central Interceptor is a super-sized wastewater tunnel project that will reduce overflows of wastewater from central Auckland into the city’s waterways, helping to make them cleaner. In the older parts of central Auckland, wastewater and stormwater flow into a combined network of pipes which were designed to direct overflows into natural waterways. The Central Interceptor is a key part of a region-wide wastewater strategy which focuses on supporting population growth while protecting the environment.
The tunnel will run underground from Grey Lynn to the Mangere Wastewater Treatment Plant, with several link sewers and shafts along the route for collecting and transferring wastewater into the tunnel. Once completed, the tunnel will run for 14.7 km, making it the longest bored tunnel in New Zealand. At 4.5 m in diameter it will be high enough for a giraffe to stand inside it.
Principal contractor Ghella Abergeldie Joint Venture (GAJV) is building the Central Interceptor. GA has more than 150 years’ experience with tunnelling and wastewater projects of this size across the globe. To date, GAJV has created six construction sites across Auckland in preparation for the underground work, with key sites at Grey Lynn, May Road in Mt Roskill and at the Mangere Wastewater Treatment Plant.
Most of the construction work will be between 15 and
110 m underground, helping to reduce construction effects on residents and local roads.
TBM on its way
Watercare has commissioned a tunnel boring machine (TBM) from Herrenknecht in Germany, one of the world’s premier TBM manufacturers. The machine will dig
the Central Interceptor tunnel and lay the tunnel lining segments as it goes. The TBM has been named Hiwa-i-te-Rangi after a Matariki star, and is
190 m long and
5.47 m in diameter. It is similar to the ones used to construct Watercare’s Hobson Bay Tunnel and Waka Kotahi NZ Transport Agency’s Waterview Tunnel.
The TBM recently passed its official factory acceptance test in Germany after 10 months of being assembled at the Herrenknecht factory using parts from all over the world. After being disassembled and loaded into containers, the TBM is now en route to New Zealand onboard the cargo ship Parsifal, which is due to dock in Auckland in November.
Shayne Cunis, Central Interceptor executive programme director, says it is great news for the whole project team. “For months we’ve been watching remote video footage as the TBM was pieced together. We’re really pleased to hear Hiwa-i-te-Rangi will be here mid-November. It will begin work on the Central Interceptor tunnel early next year.”
The TBM main elements consist of a cutter head, a main drive, installed in the front shield, a middle and rear shield, the thrust cylinders, the screw conveyor and the erector. The front shield holds the hyperbaric chamber that allows workers to operate in a compressed air environment for cutter head inspections, repairs and tools replacement.
Behind the shield, the TBM is made up of 18 gantries for storing essential equipment required for the operation to progress. The TBM has a precise sequence of installation, and it will be lowered piece by piece into the main shaft for the assembly. Once tunnelling operations begin, the gantries will be added in the shaft as the machine moves forward.
The TBM of the Central Interceptor is an earth pressure balance type machine that uses thrust cylinders to advance forward by pushing off against concrete segments, and it gets its name because it uses the excavated material to balance the pressure at the tunnel face. The pressure is maintained in the cutter head by controlling the rate of extraction of spoil through the screw conveyor and the advance rate.
The shafts at Mangere are huge – the two interconnected shafts are 12 m and 26 m in diameter and will go down 42 m before the concrete base launching slab is poured. Excavation of the main shaft at Mangere is progressing well and has reached a depth of more than 30 m (as of early October). Excavated soil is being deposited at a former quarry at nearby Puketutu Island, which will eventually be returned to the people of Auckland as a park.
Elsewhere, two shafts are under construction at May Road, and work at other sites has also begun at Keith Hay Park and Haycock Avenue in Mt Roskill, as well as at Dundale Avenue and Miranda Reserve in Blockhouse Bay. The Walmsley Park site is planned to open mid-October.
Both Mangere and May Road have been declared as ‘mine sites’ and Watercare is complying with the mining operations and quarrying operations regulations, which place tighter controls and limit access to the shafts.
Watercare has its own purpose-designed training and induction centre at Mangere which was opened in late 2018. Tunnel construction teams have been undertaking simulated training at the centre in readiness for work underground. This has included practising changing the giant cutter head tools using a special replica TBM, which arrived earlier this year.
They have also been practising entering and exiting a hyperbaric chamber, which is necessary because the Central Interceptor tunnel will reach depths up to 110 m, as well as travel underneath the Manukau Harbour.
The Central Interceptor project is expected to be completed in 2025.
Watch a video of the TBM at the Herrenknecht factory at youtu.be/dTjdD5wOLlE