Wellington Water and McConnell Dowell Constructors Ltd have partnered together in a key infrastructure project to strengthen the Hutt Valley’s wastewater network in preparation for future growth and to reduce the risk of wastewater entering the environment in the event of a major earthquake.
Ageing infrastructure and historic under-investment in critical assets such as wastewater networks are an all too familiar tale in New Zealand’s capital city Wellington.
Just across from Wellington Harbour in the Hutt Valley there is a tale of another city where multiple water infrastructure projects are active throughout the community.
One of the more ambitious projects is being undertaken by Wellington Water and McConnell Dowell. It involves duplication of an old wastewater pipe that transfers about 90 percent of the Hutt Valley’s wastewater servicing over 150,000 people.
The installation of a new 1.2 km long by 1 m wide pressurised wastewater pipe from the Barber Grove Pumpstation to the Seaview Wastewater Treatment Plant is a fascinating challenge says Wellington Water project lead Linda Fairbrother.
“Since taking over the project, I’ve learned the value in having early contractor involvement (ECI) in place. It allows time to foster a customised approach and for greater innovation, which in this case led to innovation, utilising trenchless technology,” says Fairbrother.
In an area susceptible to earthquakes, going trenchless is advantageous, enabling the new duplication pipe to be positioned deeper into more solid layers of earth. In the event of a large earthquake, the new pipe is less likely to be compromised by ground liquefaction.
This approach is also favourable towards protecting the existing environment. A key attribute of micro tunnelling is the accuracy to excavate only the required earth to install the pipe while bypassing other underground network pipes and cables, without the need to cut or move them.
“McConnell Dowell and our design consultant Stantec, clearly had the expertise and made the most compelling tender. They acknowledged the difficult site conditions, high traffic volumes and likely impacts on the environment, community, and businesses,” Fairbrother says.
To minimise traffic congestion and to avoid reducing the busy suburban Randwick Road to one lane, micro tunnelling was the obvious choice. It meant approximately 55 per cent (600 m) of the new duplication pipe would be drilled up to 9 m below ground level to the Seaview roundabout.
The route is not direct. It involves intermittent excavations along Randwick Road about 200 m apart with the final retrieval pit at the roundabout. From there, the new pipe will be fed through existing concrete pipes (sleeves) to safely cross the local Awamutu and Waiwhetū streams to the final phase and open trenching to the Seaview Wastewater Treatment Plant.
Excavating near waterways to access existing pipe sleeves, presents challenges from a cultural and environmental perspective with the project route having to negotiate the protected Waiwhetū Artesian Aquifer.
Wellington Water engaged early with mana whenua after commissioning a cultural impact assessment to identify cultural values or interests in the area. Maoridom places great importance and emphasis on guardianship (Kaitiakitanga) of the land and environment.
“I took local iwi through the site, and they provided a statement of position in support of the project. They had legitimate concerns about the mana and spiritual wellbeing of the waterways during construction and that Wellington Water would manage contamination risks and respect mana whenua protocols around a urupā [burial ground]. For me, personally, it’s vital to have a meaningful and authentic relationship with a key stakeholder to establish goodwill and trust,” recalls Fairbrother.
Te Āti Awa were invited to provide a name for the micro tunnel borer machine and chose Te Rū Tiokaoka as this relates to Rūamoko the Maori ‘God of earthquakes’ and the journey, it will make moving through Papatūānuku [land], tunnelling residue and soil aside.
“We are honoured to receive the iwi’s blessing and thrilled about using Te Rū Tiokaoka. It’s the first time Wellington Water has used this technology, and it recently set a world record in Auckland for its tunnelling prowess, so we’re excited to have it in Wellington,” says Fairbrother.
Randwick Road is a confined site, with alluvial soils and a high-water table. These challenging geological conditions can be alleviated through micro tunnelling, which offers greater precision, reliability, reducing risks to crews and machinery in comparison to open cut trenching.
For this project, a launch pit is established with hydraulic jacks propelling the German manufactured Herrenknect AVN 1000 forward at 2.6 m intervals. The pipe sections are pushed behind the machine using a jacking frame. Excavation depths across the pipeline range from 3.5 m to 8.5 m at the Seaview roundabout.
Te Rū Tiokaoka is controlled remotely by a surface operator with laser-guided steering. This enables real-time data on location and orientation of the micro tunnel borer. It allows for the operator to adjust alignment and maintain line and grade. The friction and weight of the concrete pipes (1.2 tonnes) is mitigated through interjacking stations which provide additional thrusting capacity to negate obstacles and complete the advance of the new pipe.
Swedish Grindex Minette single phase drainage pumps, are employed to stabilise water levels in a restricted space. They are aluminium and relatively small, weighing 29 kgs with a 240 mm diameter.
Tunnel Superintendent Peter Farr says, “Water is the biggest nemesis for micro tunnelling, you have to control water, or it controls you.”
To manage the high-water table, the flows and pressure, Te Rū Tiokaoka operates in earth pressure balance (EPB) mode, providing a shield to maintain the stability of the tunnel face, adjusting the pressure inside the cutterhead chamber to achieve a balance with the water and ground pressure in front of the cutterhead. Rubber seals are installed at the entry and exit points to restrict waterflows into the shafts.
The operation site is bunded and contained, so any spillage is recovered and recycled through the slurry separation plant, minimising the risk of contamination
This plant transports waste, sediment, and solids back to the surface for screening and recirculation. Any loss of slurry in the mining operation into the gravels is absorbed, as the Bentonite is a natural clay.
One hazard to the tunnelling route is the presence of timber or fibrous materials. Given the project site is on alluvial plains there is the likelihood of unwelcome deposits such as timber, which cannot be transported through the slurry plant.
“We have modified Te Rū Tiokaoka so it can adapt accordingly to the environment. The wheels were changed and the cutting head profile from a mixed face to a soft head, this way, it can tackle the likes of timber. It’s not uncommon to hit chunks of Pohutukawa which have been washed down from the riverbed,” says Farr.
“Te Rū Tiokaoka’s journey will vary with the unpredictable ground conditions. The drive lengths are essentially short. We expect it will achieve speeds of up to 50 mm a minute and we’re hopeful it will complete around three to four pipes a day, equating to 7.8 m.”
The lingering shadow of COVID and related impacts is an ongoing challenge for this 13-month project which is due to be completed in mid-March 2023.
While final project preparations coincided with peak COVID restrictions, Wellington Water undertook early procurement to minimise the impact of cost inflations and disruptive supply chains. The delivery of the micro tunnel borer from Germany and the slurry separation plant from Australia were welcome milestones given the uncertainties of global shipping.
“The project team has learned to become more agile, there was never the danger of placing an order and forgetting about it,” says Fairbrother.
Leading a key infrastructure project
A much-discussed initiative for a generation, before Wellington Water was conceived in 2014, this $27 million dollar Hutt City Council and Upper Hutt City Council investment, will strengthen community resilience towards natural hazards and protect the wider environment.
Fairbrother describes the project as a career highlight since joining the water entity six years ago. An advocate for social responsibility and late starter to infrastructure project management, she has a passion for delivering community-oriented outcomes.
“My background is local government so there were connections and that element of shared values, and vision of leaving things better than you found them.”
Fairbrother is particularly comfortable working in a male-dominated sector where there is no ceiling to ambition and the pursuit of bigger and more complex projects.
“I’m fulfilling a personal aspiration and feel privileged to get this finally off the ground and be involved in a leadership capacity. The wider Wellington Water whanau are supportive, and we have great women role models in senior positions who nurture and encourage personal development.
“The company is growing in size and expertise, we’re not just about pipes, we’re leaning into bridges and large reservoirs as well and that’s certainly an interesting field I would welcome and embrace in the future,” says Fairbrother.
For more information visit Wellington Water.
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This article appeared in the October edition of Trenchless Australasia. Access the digital copy of the magazine here.