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Treatment plant tunnel construction revised

September 10, 2004

Treatment plant tunnel construction revised

Difficult tunnelling conditions have lead North Shore City Council engineers to find a better solution for a proposed new Rosedale wastewater treatment plant outfall.

North Shore City Council has decided to change the construction method for the new outfall tunnel for the Rosedale wastewater treatment plant to ensure properties near the route are protected.

The need for a new land-based effluent outfall was first identified through the Project Rosedale Resource Consent process and an ARC consent condition requires construction of a new land and sea outfall to be completed by 31 December 2010.

Investigations of the previous proposal to build a deep tunnel from the treatment plant to Mairangi Bay found the settlement of land above the tunnel on some parts of the route would be too high. The new construction method will be to build a tunnel along half of the 2.8km route, which runs entirely beneath public roads and parks, and to use trenching or pipe jacking techniques over the remainder.

Tunnelling and pipe jacking greatly reduces disruption at the surface, and the method is being used wherever possible, and particularly at busy intersections.

Trenching and pipe jacking reduces the risk of settlement but there will still be considerable traffic disruption, and a noise and dust nuisance over the 18-month construction period, which is scheduled to start in 2008, depending on the time taken to obtain the necessary variations to the consents.

Preliminary work on testing soil conditions will start soon, however, and residents are likely to see truck mounted drilling rigs working for short periods in the area between October and December this year.

The council will apply for variations to the resource consents for the project from the ARC, and the affected public will have the opportunity to participate in the statutory consultation process. It's likely that the council will consult on these variations early in 2005. Submissions from the public within the notification period will be assessed and the council will, if at all possible, resolve any issues identified.

In 1999 council engineers embarked on an investigation of alternative routes before proposing a selected route for consent purposes to councillors mid year. The existing 40-year old tunnel and treated effluent pipeline was shown to be too small to meet the needs of an increasing population and needed repairs. It has trouble maintaining a constant flow, and the location of an existing telecommunications corridor offshore from the existing discharge point prevent a new offshore outfall at the desired location.

A new treated effluent outfall route was proposed stretching 2.8km from the Rosedale Treatment Plan east to Mairangi Bay where it would discharge high quality treated effluent 2.5km out into the Rangitoto Channel. A designated tunnel route from the eastern end of the treatment ponds along Maxwelton Drive and Ramsgate Tce was confirmed and consented for by the ARC in 2002.

Further work to confirm the practicability of construction has now lead council engineers to believe the original proposal may cause unacceptable ground settlement above the tunnel in some areas. Engineers predict ground settlement directly above the previous tunnel construction method of 20mm to 30mm from the Treatment Plant to Ramsgate Terrace, rising up to 100mm in the Mairangi Bay Valley area.

Committee Chairperson Councillor Joel Cayford says the cost estimate for the project has been increased to $75m due to higher material and construction rates, and he is concerned at the escalating costs. The council will consider funding for the project when the next City Plan is prepared for 2006/07.

He says the changed method could cause understandable concern to Maxwelton Drive residents, however the council will leave no stone unturned to ensure that access to and from private property is maintained during the construction period.

He says the preferred construction method presented to council today minimizes the potential for settlement, as the eastern half of the tunnel (pipe) would still be bored through sandstone, which has a relatively low permeability, therefore reducing the draining effect on clay soils above. "

The original construction method took the western half of the tunnel near deeper alluvial material with a high water table where tunnelling would almost certainly cause the water table to drop and the clay soils above to dry and settle," he says.

The new construction method reduces the need for a deep tunnel along the entire original consented route by bringing the 2.1 metre pipe closer to the surface midway along its original path, where it would be constructed an average of four metres below the road.

From the intersection of Maxwelton Drive and Mayfair Crescent the preferred construction method would be a combination of open cut trenching, subsurface boring and pipe-jacking up to the foreshore. From there the trenched outfall extends another 2.5km to a discharge point in the Rangitoto Channel.

Extensive testing of ocean currents and water behaviour at the discharge point is currently being undertaken. This will help engineers design the most efficient way to disperse effluent at the outfall.

Cr Cayford says this route option is regarded as the best one in terms of avoiding the need to disturb private property, the potential for serious traffic and general community disruption, geotechnical conditions (settlement issues) and cost. It also presents the best opportunity to be completed by the end of 2010 with construction beginning mid 2008. "

Council will do its best to minimise disruption, however the public needs to know that this work is essential. It needs to be completed in time for the expected longer-term population growth in the northern part of the city and to meet stringent ARC consents for the ongoing discharge from the treatment plant. "

Local residents and the public will be fully informed about the project and as work proceeds, says Cr Cayford.


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