Auckland’s water utility Watercare is progressing with the development of one of New Zealand’s most ambitious tunnelling projects, the Central Interceptor. The project includes the construction of a 13 km wastewater tunnel at depths of up to 110 m underneath the city, as well as 4.4 km of link sewers using microtunnelling.
The tunnel will run between Western Springs and the Mangere Wastewater Treatment Plant, crossing Manukau Harbour approximately 15 m below the seabed. The pipeline will connect to existing wastewater networks along the route, which will divert flows and overflows into the tunnel.
After shortlisting four joint ventures to build the project, in March 2019 Watercare signed a construction contract with Ghella–Abergeldie Joint Venture (JV); tunnelling is expected to begin in late 2020.
What are your responsibilities on the Central Interceptor Project?
As the CEO of Watercare Services, so my role is to make sure the project is delivered to program and to budget. It is significant in the sense that the project is costing about NZ$1.2 billion – a substantial sum for us. It is by far the largest project we’ve ever done and possibly will ever do.
With very large projects, if you get variations or issues with program delays, the sums of money you talk about are big. It may be a small percentage, but it’s a small percentage of a big number. My job is to make sure traditional project outcomes are delivered by the program director, including scope, program, timetable and dollars.
More importantly, we’ve got to make sure the Central Interceptor resonates and engages with our customers, and that we’re a good neighbour during its construction, as we’ll be in locations throughout Auckland for a while. This tunnel has a number of shafts located in built up residential areas, so we don’t want to upset customers and stakeholders during the five years of the project.
What does the project entail and why is it required?
This infrastructure was identified as being required over a decade ago. It’s a large tunnel with an outside diameter of 5 m and inside diameter of 4.5 m. It’s going to be 13 km long and will be up to 110 m deep.
The tunnel will pick up existing combined sewer networks and take the flows to our largest wastewater treatment plant at Mangere, north of Auckland airport on the southern side of the city.
In this part of central Auckland, wastewater and stormwater flow into the same pipe. This system was designed that way at the turn of the century with the intention these pipes would carry the stormwater to a nearby stream, waterfront or coastal waterway where it would be discharged.
This was acceptable 100 years ago because dilution was considered a means of treating sewerage; however, it’s now 2019 and for the past few decades Auckland has been struggling with the decision to separate the sewers and build new stormwater pipes.
Watercare is responsible for drinking water and wastewater services in the Auckland region; however, stormwater is a council responsibility. Where there’s a combined sewer, the stormwater goes into our wastewater system and the law then requires us to address that issue.
The Central Interceptor was identified as a solution that could be installed in a short period of time and address about 80 per cent of the overflows as soon as it’s constructed. The project will also provide a decade for Auckland Council to continue with separation works.
How was the project’s design developed?
The Central Interceptor is actually the spine of a larger network, connecting to two link sewers which are substantial in themselves. They’re not as large in diameter, but they will also be constructed using trenchless technology.
The first challenge we had was the size of the pipe that we need could have been substantially smaller, less than 2 m in diameter. But we are building this tunnel in a built-up and older part of Auckland, and it’s almost impossible to follow the grade and build it using traditional trenching methods, so we had to go trenchless.
As soon as made this decision, we said, “Well, we’ve got to go deep to stay away from the uncertain layers of the geology in Auckland.”
We also needed the diameter of the pipe large enough that it would allow for good tunnelling. The advice we got was that we were therefore looking at a diameter of more than 4 m.
In the end, we decided we would go for a larger pipe diameter, with the larger capacity used to store storm flows. The Central Interceptor reduces the need to increase the capacity at the treatment plant, allowing us to transport and store, and at a diameter allowing for good tunnelling.
The two link sewers will be constructed using microtunnelling because they’re smaller in diameter, but the spine that is the Central Interceptor will be constructed using a tunnel boring machine (TBM).
What did the tendering process involve?
Initially, there was an expression of interest, before we shortlisted four consortiums, which subsequently submitted proposals. Any of the four joint venture (JV) proposals could have done the job, but at the end of the day, our criteria – in addition to price – specified experience and a culture which reflected the values of Watercare.
We were looking for a contractor that would be a partner, that would share our focus on our customers and make sure the project delivered positive outcomes. In addition to the construction of the tunnel, over the next five years the process is going to result in trained personnel, an enhanced health and safety culture for workers and subcontractors, and positively contribute to lowering carbon emissions, among other benefits.
The proposal from the Ghella-Abergeldie JV best met the criteria we detailed in the contract, as well as Watercare’s aspirations. I’m sure the other shortlisted groups would have done a fine job too, but the process and the number of people who were involved and evaluated the bids collectively decided the JV was the one for us.
After meeting the consortium’s personnel, the Ghella family and the Abergeldie personnel, we are confident we made the right decision.
Is it important to provide the public with added value when planning these largescale projects?
Absolutely, the community expects us to. It comes at a very marginal cost, if any additional cost. It’s not a question of ‘either-or’, it’s ‘and’. You know, we can build the tunnel and have other benefits because of the scale and the duration of the program.
What’s the timeline for activities?
The JV got possession of the contract and the site in May 2019. The TBM will be fabricated, shipped, assembled, disassembled and then sent to the receiver; it’s a process that’s going to take 12 months or more. In the meantime, there’s a lot of preparation work to be done such as preparing shafts for tunnelling.
Another significant undertaking is staffing, which the JV is working on while waiting for the TBM to arrive.
When will the project be completed?
The tunnel will be commissioned in stages, so we won’t wait for the entire length to be completed before we start to benefit from it.
The whole project will be completed at the end of 2024, but we will be able to get the first section of the Central Interceptor up and running by 2022 or 2023. As soon as it’s available and we see fit, we’ll start using it.
Why has Watercare been using trenchless technologies over traditional trenching in recent projects?
One of the key drivers is the fact that we’re building our infrastructure in an area that is already built up and we want to minimise disruption to our customers and roads, as much as possible. Going underground allows us to do that.
New Zealand is a very young island from a geological point of view, so it does present challenges for us, but I think the contracting fraternity has consistently come back with innovative ways of doing the work and cost-effectively. Even in the case of the Central Interceptor, the tenders have come in, the contract has been signed and we will be within our budget, which is a fantastic outcome.
Going underground means less destruction and the innovation being demonstrated by the contractors has meant we haven’t paid a huge premium. In fact, one of the water main jobs we’re doing now, where we’re avoiding going through a commercial area, is going to be cheaper using trenchless technology than if we had gone with open cut.
When we look at the cost, we don’t just look at the cost to Watercare: we look at the cost to Watercare and to the community. Cost in terms of money, businesses that may be disrupted because of the trenching work being done in the streets, the non-financial costs in terms of noise and disruption and dust and all the other things that come with construction.
We’ve found that we’ve got a lot of options when it comes to addressing the challenges of building infrastructure. We select what makes sense and trenchless technology has been very good to us.
For more information visit the Watercare website.
If you have a project you would like featured in Trenchless Australasia contact Assistant Editor Chloe Jenkins at cjenkins@gs-press.com.au