Marked by the use of an independent self-propelled culvert gantry, New Zealand's largest-ever highway project passed the halfway mark on the first drive in Auckland. The first of parallel 2.4-kilometer-long tunnel bores, when completed this fall, will mark a major milestone for the $1.2-billion Waterview Connection, the last section of a Western Ring Route.
The milestone follows a brief hiatus and successful completion of essential maintenance work on the earth-pressure tunnel-boring machine (TBM), nicknamed Alice, in May. The Herrenknecht machine, manufactured at the company’s facility in China, cost $46 million dollars to build and transport to New Zealand. Alice underwent a 3-week replacement of hundreds of steel fiber brushes installed on its shield. The brushes help form a waterproof seal as the concrete segments that line the tunnel are lifted into place.
The Well-Connected Alliance, which includes the New Zealand Transport Agency, Fletcher Construction, McConnell Dowell, Parsons Brinckerhoff, Beca Infrastructure, Tonkin & Taylor and Japanese construction company Obayashi Corp., has the design-build contract for the new six-lane, 4.8-km motorway. Sub-alliance partners are Auckland-based Wilson Tunnelling and Spanish tunnel controls specialists SICE. After construction completion, slated for 2017, the team will maintain and operate the tolled highway for 10 years.
Designed and built specifically for this project, and with a cutting head measuring 14.5 m, Alice is the tenth-largest machine of its kind ever used in the world, says the contractor. As of this month, Alice has placed 833 out of 2,404 rings, which consist of 24,040 tunnel lining segments (ten per ring) and 275,000 cu m of spoil out of a total of 800,000 cu m. It is believed to be the first boring machine to be manufactured with an independent self-propelled culvert gantry. Designed to operate some 200 m behind Alice, the arrangement represents a departure from typical practice, says Stefan Hanke, construction director with the alliance.
Each ring set features nine segments, plus a crown key. Some 2,400 inverted U-shaped precast concrete culverts, measuring 2 m long x 3.7 m wide x 2.2 m high, are being cast to run by the independent gantry under the road deck, to contain ventilation, communication, fire detection and lighting systems. Hanke believes it is a "neat solution" for keeping up productivity. During recent maintenance on Alice, project operations continued unhampered.
"It’s an innovation, and one that will help us overcome any minor problems to do with productivity," explains Hanke. "It means we can separate TBM production from culvert-placing production, and at the same time, when we turn the TBM around at the northern portal we want to manage that operation as efficiently as possible. Having a gantry bridge attached to the TBM would mean that you would have to disassemble it, then rebuild it on the return journey. It would take more time and effort. By separating the TBM and gantry operations, we are able to optimize TBM backup and overall turnaround time."
The NZ Transport Agency’s (NZTA) Group Manager for Highway and Network Operations, Tommy Parker, says replacement of the brushes had been expected sometime during Alice’s journey from the Auckland districts of Owairaka and Waterview to build the first of the twin tunnels. “Tunneling on this scale is a complicated task, and some wear and tear on the machine is inevitable. We decided it would be prudent to replace the brushes now, before we possibly encounter more onerous groundwater conditions later.”
NZTA's acting highways manager Steve Mutton says the TBM has averaged around 14 m per day. “At those rates, the machine is well on schedule to complete the first tunnel in late September.” Then Alice will be turned around over the following three months and relaunched to carry out its southbound journey early next year, building the second (northbound) tunnel. In early June, NZTA reported that the turnaround site is one step closer to being ready for the TBM’s arrival, with the completion of the base slab of the Northern Approach Trench (NAT). The base slab is the floor of the 25-m-deep trench and will provide a turnaround platform after the breakthrough at Waterview.
Weighing in at 25,000 tonnes, the TBM requires a sound foundation for this operation. The slab contains over 2,000 cu m of concrete and 150 tonnes of reinforcing steel, poured in three separate layers. Construction of the Northern Approach Trench began in early 2013, with the installation of the diaphragm trench walls followed by the excavation of more than 20,000 cu m. Current construction of the headwall will be followed by the backwall.
Construction of the two tunnels is expected to be completed at the end of 2015, when they will be fitted out with ventilation fans, communication systems, fire protection, etc. Sixteen cross-passages at 150-m intervals will connect the the twin tunnels.