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Outfall tunnel project sees costs rise

Outfall tunnel project sees costs rise
November 8, 2005

Continuing inflationary pressures combined with the high level of activity in the country's construction sector have boosted the estimated cost of the biggest environmental construction project ever undertaken in North Shore City.

The city's proposed new tunnel and outfall will take highly treated effluent from its existing wastewater treatment plant 2.8 kilometres out to sea, replacing an existing 1960s outfall that exits only 600 metres from shore.

The council's infrastructure and environment committee chairman, Tony Barker, says while a 2004 cost estimate indicated a $75m completed cost, the council is concerned that inflation and a busy construction sector has increased the project's estimated cost to $94m.

The project is due to start construction in 2008 with completion in 2010.

Other factors contribute to the increase, including higher tunnelling costs and an increase in the length to be tunnelled as opposed to trenching. Also, extensive ground tests last year showed that tunnelling conditions could be more difficult than first thought, and contractors have indicated that this may also increase costs, says Councillor Barker.

"It's unlikely that the project can be deferred or reduced in scope, and until expressions of interest and a final fixed price contract comes back from potential contractors, the final figure for the project is unknown," he says.

"The construction industry could remain volatile, while the exchange rate and other factors might also influence the final project cost.

"Our engineers are carefully reviewing construction options and will choose the most economical method that causes the least disruption to local residents and traffic movement. Expressions of interest are currently being sought from construction companies throughout Australasia, he says.

The current plan is to build a tunnel from the eastern end of the Rosedale wastewater treatment plant ponds 2.6 kilometres to Mairangi Bay beneath council land and roads, and then beneath the seabed to a specially-designed diffuser 2.8 kilometres into the Rangitoto Channel.

North Shore City's general manager, water services, Geoff Mason, says the outfall is designed to cope with the city's future growth and increased flow from the city's wastewater treatment plant.

"It must be completed by 2010 to comply with Auckland Regional Council consents, Mr Mason says.

"It's an essential part of our overall plan to improve water quality in the city's streams and on our beaches. This plan includes ongoing improvements and expansion of our stormwater, sewerage and treatment plant systems to cope with the city's continuing rapid growth."

The current outfall is more than 40 years old, requires frequent maintenance and will not cope with greater volumes.

Around 300 residents attended council-organised open days in late October. Engineers were on hand to explain the tunnel and outfall route, and the construction techniques that may be used. Visitors were told that, depending on the methods used, there might be some short-term disruption while construction work is carried out along the route between the city's wastewater treatment plant and Mairangi Bay, from where the outfall extends to the Rangitoto Channel.

More detailed information is available on the city's website: www.northshorecity.govt.nz.


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