Tunnel vision of gateway to the North

Workers make the breakthrough at the southern end of the southbound tunnel excavation at Johnstones Hill near Waiwera. Picture / Kenny Rodger

It was hole-through time for Auckland's new northern gateway yesterday, as cheering tunnellers broke into daylight on the $365 million Orewa-to-Puhoi motorway project.

A final layer of rock shredded away like confetti as a 50-tonne boring machine corkscrewed out of the southern side of Johnstones Hill above the Waiwera River, leaving a 333m long cavity for the first of a pair of sophisticated motorway tunnels.

The tunnels are costing $35 million to dig and fit out with lighting and ventilation systems at the northern end of the 7.5km toll motorway extension from Orewa.

The project will also include three sweeping viaducts across steep and bush-clad terrain inland from the Hibiscus Coast.

Transit New Zealand and its construction partners promise that once complete in about 18 months, the twin tunnels will form an impressive northern gateway to Auckland, while leaving an important wildlife corridor largely intact on the hill between Waiwera and Puhoi.

That is in contrast to an earlier and cheaper plan, which Transit eventually ditched as environmentally unacceptable as well as physically problematic, for an open cut 60m deep through Johnstones Hill.

Although a laser guidance system on the electrically driven boring machine left little risk of navigational errors for yesterday's hole-through, Northern Gateway Alliance tunnels manager Tony Pink admitted some relief when it emerged "exactly where we wanted it to".

"And the biggest thing is that we did it with no injuries," he said.

The machine, which began its subterranean journey 5 1/2 months ago after completing a toll tunnel beneath central Sydney, will be overhauled before starting the second tunnel in a fortnight.

Conventional earth-moving machinery still has to scrape out the bottom half of the initial tunnel, to form an opening 12m wide and 9m high, for two lanes of southbound traffic.

The second tunnel - 15m to the west - will be the same size but available initially to only one lane of northbound traffic.

That is to ensure vehicles reduce speed before they reach the end of the motorway and are faced again with the old highway carrying just one lane of traffic each way, without separation by median barriers.

Traffic will have two dedicated lanes in each direction along the rest of the motorway, including a 537m viaduct being built over the Waiwera River to connect to the tunnels.

The viaduct will rise to 30m above river level, and the initial tunnel opened up yesterday will provide a haulage road to build its northern abutment.

Project officials say this will reduce the area of bush that would otherwise have to be cleared to create a construction road along the river.

SIZING THEM UP

* Twin tunnels, to be extended to 345m long after portals added. Both 12m wide and 9m high, the tunnels will be set 15m apart, although connected by an emergency passage.

Southbound traffic will have two lanes in the eastern tunnel, but its twin will provide only one lane for northbound traffic until the motorway can be extended even further north, past Puhoi.

The project is Transit's first major underground project since Wellington's 435m Terrace Tunnel was built almost 30 years ago.