North Shore City tunnel officially opened

A new NZD 116M (USD 81M) wastewater tunnel and outfall was officially opened today in North Shore City, New Zealand. The scheme is the city’s largest infrastructure project, a key part of a long running council project to improve water quality at the city’s beaches.

The tunnel and outfall replaces a pipeline built in 1958 that carried treated effluent from Rosedale Wastewater Treatment Plant and discharged it just 600m offshore from Castor Bay. The treated effluent now goes much further out to sea.

Construction on the 3km tunnel took contractor McConnell Dowell ten months and was started in February 2009. The tunnel was bored by TBM. The shell of the boring machine remains deep inside the tunnel because of problems getting it out.

The council says the tunnel and outfall have a lifespan of at least 100 years and will cater for the city's continued growth. The 2.1km outfall pipe, constructed by joining 12-metre lengths of pipe, was fabricated in Kaiaua on the Firth of Thames. Each section, 400m to 500m long, was individually towed by tugboats about 100km to Mairangi Bay. After the pipeline was installed divers attached 58 diffuser nozzles, to disperse the treated effluent, to the last 350m of pipeline. The final stage of the project saw 24 concrete segments, each weighing 56 tonnes, installed to connect the treatment plant to the tunnel entrance. Following successful tests in mid-July the tunnel and outfall are in full use.

Treated wastewater travels 3km underground from the Rosedale plant to Mairangi Bay and then 2.1km offshore where it is dispersed deep in the Rangitoto Channel.