Auckland is the largest city in New Zealand with a population of 1.6 million. It is flanked to the north and south by the Waitemata and Manukau Harbours respectively. The original sewer network dates back to the early 1900s, with substantial extensions into its current form during the 1950s and 1960s. Most of the network in central Auckland relies on a combined sewer system, which carries a considerable volume of stormwater during rainfall events which overflows into local stormwater and watercourses. As a result, the water quality in streams and the harbours is significantly affected both during and after rainfall events.

During the 1990s, it was recognised that Auckland needed to improve its environmental performance in respect of combined sewer overflows (CSOs). The specific need for a new central interceptor was identified in studies between 2005-2008. Having responsibility for most of the combined sewer network, Watercare Services began studies in 2009 to design and obtain regulatory consent to construct a solution. This was required to:

  • Capture regionally-critical sewer overflows to facilitate treatment.
  • Provide redundancy to aging and vulnerable assets.
  • Provide increased capacity to meet growth within Auckland, Waitakere and Manukau Cities.

Concept design began in 2009 and was completed in 2011, concluding that the preferred option was a deep tunnel sewer running under the Manukau Harbour to terminate at the Mangere Wastewater Treatment Plant, where a deep pump station would lift flows into the plant for treatment. Regulatory consents were obtained between 2012 to 2015. Development and implementation of the project has included significant ongoing partnership with local Iwi groups and representatives, to ensure that Maori values are considered and incorporated into the design and construction processes.

In September 2014, Watercare engaged Jacobs, in association with McMillen Jacobs Associates and AECOM, to provide principal engineering advisory services, for which the key deliverable was the completion of detailed design in early 2018.

The project entered the procurement phase in late 2017, with a call for expressions of interest from local and international contractors. In May 2018, Watercare floated tenders to the four shortlisted consortia and awarded a modified FIDIC red 1999 Contract to the Ghella-Abergeldie Joint Venture (GAJV) in March 2019, with contract commencement in May 2019. Jacobs (including McMillen Jacobs Associates and AECOM) were engaged by Watercare to provide construction management services.

The project is substantially an employer’s design contract, however there are significant elements for which the contractor is responsible, such as the main tunnel, jacking pipes, some drop shafts and an air treatment facility (ATF).

The US$853m Central Interceptor is the largest wastewater project in New Zealand and the most significant wastewater investment in water provider Watercare’s history. Construction began in 2020 and is currently on track to be completed in 2025. The project comprises 14.6km of 4.5m ID tunnel, at depths between 20m and 110m, conveying sewer flow to a 7.2m3/s capacity pump station.

Other key components include a 1.1km-long sewer at 2.4m ID and a 3.3km-long sewer at 2.1m ID, both pipejacked. Sixteen drop shafts, mostly of the cascade type, drop flow from shallow connections to the deep link sewers and main tunnel. Inflows to the tunnel are limited via actuated gates at most of the shallow connections to the existing network. At its downstream end, the tunnel invert is at 32m depth as it enters the pump station. Figure 1 provides the general arrangement of the project.

GEOLOGY

The geology under Auckland is typically East Coast Bays Formation (ECBF), which comprises an alternating sedimentary flysch sequence of sandstones and siltstones-mudstones. The beds tend to be less than 1m thick and are usually very weak to weak rock, although some lenses of Parnell Volcaniclastic Conglomerate are expected to occur. These conglomerate units tend to be harder and can sustain open fractures resulting in higher rock-mass permeabilities.

Overlying the ECBF are two main soil units: Kaawa Formation and Tauranga Group. The Kaawa Formation is expected to typically comprise dense sands, while the Tauranga Group is characterised by soft silts, clays and pumiceous deposits.

The Auckland area has 53 volcanic cones and explosion craters, many of which are associated with basalt flows. The early design of the project went to some lengths to identify and avoid tunnelling through volcanic materials.

The vertical alignment of the main tunnel is predominantly through ECBF, with some sections of Kaawa Formation and Tauranga Group sediments near the Mangere launch shaft. Figure 2 shows the geology along the tunnel alignment.

To date, four segmentally-lined TBM tunnels have been completed in Auckland, three of which utilised Earth Pressure Balance TBMs. One of the biggest issues to date has been stickiness of remoulded material in the ECBF, coupled with spoil handling and disposal.

A Geotechnical Baseline Report (GBR) forms the contractual basis for risk allocation and assessment of unforeseeable physical conditions for the shaft and tunnel components.

DESIGN

At tender stage, the allocations of design responsibility were as follows:

  • Employer (Watercare): overall system hydraulic and pneumatic design; pump station and ancillaries; drop shafts; connection chambers and piping. This included the tunnel route vertical and horizontal alignment, diameter and gradient. The tunnel was specified as requiring a mechanically anchored polyethylene corrosion protection lining.
  • Contractor (GAJV): tunnel lining, jacking pipes, May Rd ATF.

GAJV appointed Arup as its designer for the tunnels and shafts, and appointed Beca for environmental and consenting support.

Various drop shafts were initially designed to be constructed from acid-resistant concrete (ARC) to resist microbially-induced corrosion. While some promising ARC products were available, GAJV evaluated the procurement and construction risk associated with large in-situ concrete pours and a change was agreed with Watercare to remove ARC and substitute GRP cascade drop-shafts along with chambers of ordinary concrete with PE corrosion protection linings. These elements have become the GAJV’s design responsibility.

The design has achieved a ‘Leading ISCA Design Rating’ – the highest possible rating from the Infrastructure Sustainability Council of Australia (ISCA). This was achieved by whole-of-life energy reduction through network efficiency, pump design, and shaft optimisation, among other reductions.

LAUNCH SHAFT CONSTRUCTION

The main tunnel launch shaft at Mangere Pump Station was designed by the client, and comprised a dual-cell diaphragm wall shaft, with panels approximately 50m in depth. The diaphragm wall shaft cell diameters are 14m and 28m respectively, with secondary cast in-situ internal concrete linings approximately 1m thick. The diaphragm wall panels have a minimum 6m embedment into the underlying ECBF rock and were designed to cut-off groundwater emanating from Kaawa Formation sands. The front cover of this issue of T&TI shows the Ghella-operated Bauer BC-35 hydrofraise rig used to construct the diaphragm walls.

During excavation, significant groundwater inflows were encountered at approximately 40m depth and continued to the base of the excavation, apparently associated with persistent open defects in the underlying rock, including the conglomerate member of the ECBF. A pressure relief underdrainage system was installed to facilitate base slab construction in dry conditions. At the time of TBM launch, the pump station secondary liner walls were approximately 75% complete.

TUNNEL METHOD

The specifications required the GAJV to use an EPB TBM with precast concrete segmental lining for the construction of the 14.6km-long main tunnel. The key TBM requirements included the following:

  • 9bar maximum operating pressure of the TBM, catering for hydrostatic pressures of up to 8.7bar under the high cover section of the alignment.
  • To cater for the maximum operating pressure, the TBM was required to have either two screws, or a single screw plus supplementary means to manage the maximum pressure down to routine dynamic operating pressures.
  • Airlock maximum operating pressure of 5bar, to facilitate hyperbaric cutterhead interventions.
  • The screw discharge was required to be equipped with double discharge gates.

Tunnel boring machine maker Herrenknecht manufactured the EPB TBM and delivered it to site during November 2020. The start of TBM manufacturing coincided with the outbreak of the COVID-19 pandemic. Despite minor manufacturing delays related to the transportation of TBM components from different locations within Europe, the machine was delivered on time and the launch was in progress at the time of writing this article.

The TBM has a cut diameter of 5.45m and an overall length of 190m, including trailing gantries. Predicting that normal operating face pressures will not exceed 4bar, GAJV elected to supply a positive displacement pump in lieu of the requirement for the second screw. The screw installed in the TBM is designed to manage up to 5bar of pressure; but for any over-pressure up to 9bar, the positive displacement pump will permit, at low speeds, the excavation to continue. For such cases, pressurisation of the main bearing will be required, along with activation of a spare hydraulic high-pressure circuit for the TBM thrust system, providing emergency propulsion capacity up to 48,000kN.

GAJV is constructing the main tunnel by launching the TBM at Mangere pump station and driving north. Approximately halfway through the tunnel, there will be a relocation of the tunnel operation from Mangere to the halfway point at May Rd (Figures 1 and 2). At this time, a bulkhead will be installed to allow the southern tunnel and pump station to be put into service while construction of the northern tunnel continues.

Due to space constraints in the shafts, and difficulties with constructing temporary backshunts through the permanent shaft walls and into the Kaawa sands, GAJV elected to launch the TBM at Mangere using movable thrust arrangement. The system involves a movable reaction ring jacked off threaded bars, in turn reacting off the shaft structural walls.

TUNNEL LINING AND SEGMENT DETAILS

It was a specification requirement that the main tunnel be protected from sewer corrosion by a mechanically anchored polyethylene (PE) Corrosion Protection Lining (CPL), with the concrete segment incorporating a minimum redundant sacrificial thickness equivalent to 50 years of corrosion resistance. The CPL was specified to be minimum 3mm thickness, covering the full 360° of the tunnel intrados. GAJV elected to cast the CPL directly into the precast segments, building on recent project experience by project personnel on the IDRIS programme in Doha, Qatar which used a similar system. This approach avoids the time-consuming procedure, quality and safety issues associated with a secondary cast insitu lining as was used on STEP (Abu Dhabi, UAE) and DTSS Phase 1 (Singapore). The CPL will be welded at segment joints after tunnel drive completion, with groundwater pressure relief provision incorporated in the invert area. GAJV selected Agru’s Ultragrip product for the CPL membrane.

GAJV’s segmental lining designers (Arup) designed a uniform geometry lining, but with four reinforcement types. All reinforcement types include SFRC, with one type being SFRC only, while the others incorporate varying degrees of traditional rebar reinforcing. The different lining types are designed and selected based on ground conditions. The lining makes use of cast-in anchored gaskets.

To avoid unnecessary patching to the CPL, it was specified that linings with cast-in CPL should incorporate guide rods and dowels, rather than bolts and dowels for ring erection. Standard segment erector sockets are used in lieu of vacuum pads to avoid the risk of pulling out the CPL anchorages during lifting.

The sub-contract for supplying the tunnel lining was awarded to local precaster Wilson Tunnelling, which has supplied segmental linings for all four previously completed segmentally-lined tunnel projects in Auckland.

The injection of a bi-component cement mixture through lines integrated in the tail shield, to backfill the void behind the extrados of the segment ring, will complete the lining construction process.

GROUND CONDITIONING AND SPOIL DISPOSAL

At the time of writing, GAJV anticipated using standard foam and polymer conditioning of ground in the cutterhead and screw. Extensive conditioning tests on soil samples have been performed, initially, at the foam supplier’s laboratory and then replicated at University La Sapienza in Rome, Italy.

GAJV is also planning to use supplementary lime treatment as necessary, to condition the spoil moisture contents to allowable limits for disposal at Watercare’s nearby Puketutu Island spoil disposal facility. It is planned that quicklime will be added and mixed as necessary, via pugmill, to the spoil on the surface before deposition into muck bins and eventual haulage to the spoil disposal destination. Laboratory tests were undertaken in Auckland to verify the moisture reduction effectiveness of the lime on samples of soil preconditioned with foam.

Watercare operates a landfill facility at nearby Puketutu Island, which receives the biosolids from the Mangere Wastewater Treatment Plant. The island is basically a quarried-out volcanic cone, which Watercare is re-profiling back to the original profile as part of a long-term rehabilitation project. It is planned to use as much tunnel and shaft spoil as possible to assist with daily biosolids cover and reconstruction of the Puketutu Island profile.

KEY RISKS ANTICIPATED DURING TUNNELLING

Several significant risks were anticipated during the tunnelling operation which have been mitigated to the extent possible in the design and construction planning phases of the project. These risks include:

  • Risk of fire during construction, exacerbated by the PE lining in the tunnel. Controls include substituting diesel locomotives with electric locomotives, along with detailed combustibility tests on the PE lining.
  • Some potentially high hydrostatic groundwater pressures, with associated inundation and mechanical risks. Various mechanical provisions associated with the specified operating pressures have been implemented as controls.
  • Risk of encountering basalt as either full face or mixed face; damage to cutterhead and screw. The cutterhead has been equipped with suitable armouring and wear detectors. Other potential tools include probing ahead and planned interventions.
  • Spoil consistency and moisture content and the associated suitability for disposal at Puketutu Island. Controls include lime treatment and specific conditioner testing.

Due to the nature of the geotechnical conditions and overlying development, the risk of settlement-induced damage to buildings and utilities is not considered to be a significant risk for the project.

NEXT PHASES OF THE PROJECT

At the same time as the project moves into the full-scale tunnelling phase, construction of the Mangere Pump Station will continue, as will shaft construction, pipejacking and sewer connections at the northern sites. Years 2022 and 2023 expect to see peak workforce for the project. It is hoped that the COVID-19 pandemic will have a decreasing influence on the global workforce and supply chain, which is essential for a project of this nature.