Cost of North Shore tunnel project shoots up $8m

By WAYNE THOMPSON

North Shore ratepayers face paying more than $75 million for a cross-city effluent tunnel project which has struck ground stability problems.

The revised project will now cost about $8 million more than previous estimates and force council workers to dig up the road and lay a trench outside about 100 Mairangi Bay homes.

The proposed tunnel is to run 2.8km across the city to carry treated wastewater from the city's Rosedale plant to a sea pipeline off Mairangi Bay.

It is supposed to be built by 2010 to honour resource consents issued by the Auckland Regional Council for anti-pollution works.

But the North Shore City Council now wants more time before starting work, its works chairman Joel Cayford saying the council could find a better solution for its money.

Already, a change in construction methods is likely after engineers advised of difficult ground for tunnelling in the Mairangi Bay Valley sector.

They predicted that works would cause ground settlement of up to 10cm above the route.

This would affect land beneath houses, foundations and driveways and possibly water and gas pipes.

"The sucking of water away from the ground gives a subsidence risk and with people's investment in their homes, we have to make sure there's no risk involved," said Dr Cayford.

The council wanted to use alternative methods such as trenching and pipe-jacking, or thrusting, from the intersection of Maxwelton Drive and Mayfair Crescent towards the foreshore.

This method would not cause slumping on properties, said Dr Cayford.

But 80 to 135 of them would have access restricted while the berm and road outside the house was dug up and trenched.

About 7000 commuter vehicles a day would be restricted to one-lane access.

Dr Cayford said the ARC had to give some leeway on consent conditions. Methods of dealing with wastewater were improving all the time and likely to present a better and cheaper solution than the original tunnel project.

The true cost was unknown in the present inflationary climate for construction projects, he said.

Recent tender prices for projects had been double the engineers' estimates.

The ARC said concerns about settlement were raised in light of experience with de-watering problems in Auckland City with the Three Kings Quarry and the Watercare Hobson Bay sewer tunnel.

The plant's 40-year-old tunnel outfall needed to be bigger to cope with greater flows from the plant as the city's population grew.

ARC water resources investigator Alistair Smaill said a long delay in building a new tunnel would increase the risk of overflows from the Rosedale ponds into the Upper Waitemata Harbour.

Dr Cayford, who is seeking election to the ARC, said the old tunnel was a tiny pollution risk compared with leaky sewers and contaminated stormwater.