Tunnellers inch towards daylight

Excavating tunnels for the Orewa motorway extension is expected to take a year. Picture / Brett Phibbs

Tunnellers have bored 60m into a hill at the northern end of the $365 million motorway project past Orewa, one-fifth of their way to daylight on the other side.

A 50-tonne boring machine and crews working two shifts have taken less than two months to carve a cavern longer than an Olympic-size swimming pool into Johnstones Hill just south of Puhoi.

Transit NZ and its construction partners intend drilling 340m twin road tunnels through the hill to make it the northern gateway to Auckland, leading to a 7.5km toll motorway extension which will include three sweeping viaducts between Waiwera and Orewa.

The electrically driven tunnelling machine, known as a road-header, has been imported from Australia for what is this country's first big road-tunnel project in 30 years.

Tunnels construction manager Tony Pink, who worked on Vector's 9km electricity supply tunnel through the Auckland isthmus and on the Manapouri dam's second tailrace tunnel, yesterday reported good progress despite encountering a large ground fault early in the job.

That had required extra steel reinforcing, but although he expected to run into more faults as the work continued, he said the sandstone rock formation of the hill was generally relatively easy to extract.

Even so, the six-member tunnelling teams, mostly recruited from Australia, are moving cautiously through the hill with roof bolts and a robotic machine which sprays a concrete and steel-fibre mixture above their heads as a temporary support until the cavity can be permanently lined.

A year-long excavation phase will be followed by a year of lining the tunnels, then another six months of fitting them with lighting, traffic control systems, ventilation fans and fire suppression equipment.

Each tunnel will be large enough for two lanes of traffic, a hard shoulder and an emergency pathway, although only one northbound lane will be marked initially.

That is for traffic "calming" purposes, until State Highway 1 is widened further north, to ensure vehicles reduce speed before reaching the end of the motorway.