A new Tunnel Boring Machine (TBM) was used at Army Bay, Whangaparaoa to help install a new outfall pipeline, as part of $31 million upgrades to Watercare’s Army Bay Wastewater Treatment Plant.
The wastewater plant currently services the population of Whangaparaoa Peninsula, Orewa, Hatfields Beach and Silverdale. Operational since 1982, it treats wastewater and discharges it through a 2.8-kilometre outfall pipe that extends one kilometre out into the fast-flowing Tiri Channel.
The existing outfall pipe is ageing and not designed to cope with future growth. Work has been underway since August last year to install a replacement pipe which will be sized sufficiently to accommodate for predicted population growth. The pipe will be installed from the Watercare’s Wastewater Treatment Plant across the peninsula, within the New Zealand Defence Force-restricted land area. Treated wastewater discharge volumes at the plant will quadruple, from 350 litres per second to 1450 litres per second.
The Tunnel Boring Machine set a new world record when it reached 1496.48 metres, the longest ever stretch of pipe laid by the Direct Pipe tunnelling method, on Friday July 13 at Watercare’s Army Bay Wastewater Treatment Plant.
The state-of-the-art $6.2 million micro TBM was imported from Germany and uses a ‘direct pipe’ method of tunnelling, which means sections of steel pipe were thrust into place.
The two-kilometre route starts at the treatment plant and ends at the foreshore. A further 900 metre HDPE pipe section, with concrete footings, was then floated into place and rested on the seabed. A hydraulic jack was used to push the outfall pipe beneath the seabed off the Whangaparaoa Peninsula. There it was connected to a pipe being floated from the Firth of Thames which was sunk in place.
Watercare project manager, Dirk du Plessis, says the project is record-breaking. “This is the first time that this Direct Pipe method is being used in New Zealand and a two-kilometre thrust is a world record!
“There are many advantages to using this type of construction, which is safer for workers, produces less dust and has less impact on the environment than open-cut methods – which is particularly important as the site lies within the Shakespear wildlife sanctuary.”
The TBM has been given a name, as is the tradition in the construction industry. Earlier this year, project staff from Watercare and contractor, McConnell Dowell were given a shortlist of proposed names and then voted for their preferred option.
‘Blanche’ was the name chosen and recognises the matriarch of the Shakespear family, after whom the nearby reserve land is named.
The project is due to be completed in November.