New Zealand's largest roading project will soon send a mechanical mega-mole underground to dig almost 5km of motorway tunnels between Owairaka and Waterview in Auckland.
The $55 million tunnel-boring machine will be launched like a subterranean ship into the earth late this month or early in November from a deep open pit in a transformed Alan Wood Reserve at the southern end of the $1.4 billion Waterview motorway project.
Sent in 97 sections from China and reassembled in situ over more than two months, the 2800-tonne German-designed ground-eater - dubbed Alice after a naming competition for schools - will dig two 2.4km parallel tunnels between the reserve and a new three-level interchange with the Northwestern Motorway at Waterview, for which supporting piers are starting to rise above traffic.
After launching itself off a temporary concrete backstop with 56 electrically powered thrust cylinders, the machine will spend about a year underground before emerging through a portal already marked out at Waterview, and then being turned around to dig the second tunnel back to Owairaka.
It will function as an underground factory, lining the tunnels by installing 24,000 concrete segments with an automated lifting arm to form 2m-wide circular sections as it goes, while equalising the earth pressure around it to prevent surface subsidence and being kept on a precise curved track by its 16-strong crew and a laser and GPS-assisted guidance system.
The 10-tonne segments are already being churned out of a new pre-casting plant in East Tamaki, built for the Waterview project.
Burrowing through relatively soft sandstone beneath a thick layer of volcanic basalt, the boring machine's 14.46mdiameter rotating head - studded with almost 300 tungsten cutting tools - will claw out 800,000cu m of material for the two tunnels.
Spoil will be partly liquidised to a toothpaste-like consistency in a large pressurised chamber behind the cutting wheel. It will then be loaded at a rate of about 6000cu m a day on to a conveyor belt cradled by a long tail section of the 90m machine, which resembles a three-decker river steamer perched over a giant slipway, and is packed with support equipment such as compressors, transformers and pumps for its two-year underground voyage.
The conveyor will be continually extended to keep up with the machine, while reaching back to an above-ground spoil-handling depot 700m from the tunnels, near Richardson Rd in New Windsor.
From there, the spoil will be trucked below new road bridges already built at Richardson Rd and Maioro St to a mined-out quarry at Wiri, via the Southwestern Motorway.
The Transport Agency and its six-partner construction consortium estimate that will keep 50,000 truck movements off local roads.
By the time the second tunnel is completed in late 2015, the conveyor belt will be 6km long, after doubling back behind the boring machine on its return trip from Waterview.
The project also includes 2km of new surface motorway from Maioro St to the tunnels, for which a 300m approach trench has been dug since early last year, to a depth of about 27m.
Foundation work has also begun for a new pedestrian and cycling bridge over the surface motorway section, around which the park and re-aligned sections of Oakley Creek will be reinstated before the project ends over the 2016-17 summer.
Hundreds of giant piles have been sunk through sandstone to build the approach trench walls, yet to be covered by a facade to give motorists uninterrupted sight-lines into and out of the tunnels.
"Some drivers get claustrophobic - it will be a challenge to the designers to make it look seamless," said Transport Agency highways manager Tommy Parker, while gazing up at the towering walls from inside the trench.
He also promises state-of-the-art lighting in the tunnels to protect drivers' vision, as they make transitions from and to daylight.
Although drivers will not gain the same sense of height as in Auckland's far shorter Victoria Park motorway tunnel, which was built as a "cut and cover" boxed structure, the Waterview pair will be more spacious than those through Johnstones Hill on the Northern Gateway toll road as they will each carry three lanes of traffic.
Once the boring machine starts its slow voyage to Waterview, construction will begin on a 15m venting building above the southbound tunnel portal.
A services building will also be erected above the tunnels' northern portals at Waterview, although the Transport Agency is required as a condition of its resource consent to place a venting tower at that end of the project on the opposite side of Great North Rd from where motorway traffic will emerge.
That is to keep the tower away from Waterview Primary School.
Emerging traffic will rise almost immediately, on an embankment already built, to the new interchange with the Northwestern Motorway.
The interchange will rise to 20m at its highest point and rival Spaghetti Junction for complexity, providing connections to and from the east and west while also retaining existing links between the Northwestern and Great North Rd.
Although the Waterview Connection will provide a direct motorway link between the airport and downtown Auckland, only one of the northbound tunnel's three lanes will head east, leaving the main traffic flow turning west.
Mr Parker says that is because the project's main purpose is to complete Auckland's 48km western ring route between Manukau and Albany.
About $600 million will be spent on associated projects, including raising and widening the Northwestern Motorway causeway between Waterview and a new interchange to be built at Te Atatu, and also adding extra traffic lanes in the other direction - back to St Lukes.
The public are invited to sign up to get up close to Alice the tunnelling machine at an open day on Sunday, October 13. Entry will be free but registrations are required for up to 20,000 visitors to be safely accommodated between 10am and 3.30pm. Visit www.nzta.govt.nz/waterviewconnection or www.facebook.com/AliceTBM to register.