Alpurt B2 tunnel gets ready in New Zealand

17 November 2005

Works continue to gather momentum for the twin 350m long road tunnels on the Alpurt B2 bypass, (T&TI, September, p8), the largest tunnel project in New Zealand on the largest infrastructure scheme in New Zealand.

T&TI has learnt that the contractor has purchased a Mitsui S200MA roadheader from Sydney's Cross City Tunnel and is refurbishing it ready for roadheader excavation to start in January next year. Tony Pink, tunnel construction manager for Northern Gateway Alliance, which is delivering the scheme, told T&TI that the size of the roadheader was determined by the available power from the local network (which could not support a larger machine) and the requirement to be able to excavate small section pilot drives if dictated by ground conditions.

The tunnels will be driven at 117m.2 face area by way of a 48m2 top heading, 60m2 bench and 9m2 invert. Geology is expected to consist of interbedded siltstone, sandstone and mudstone with strengths varying from 5-25MPa typically. Groundwater is not expected to be a problem as the alignment is above the water table.

Top heading ground support will be provided by 4m long self-drilling grouted rock bolts and steel fibre reinforced shotcrete between 50 and 100mm thick. A modified Atlas Copco 352 Rocket Boomer equipped with bolting magazines and feeds will be used to enable remote bolt installation. Bolting patterns will vary according to ground conditions and can be supplemented with 6m bolts and lattice girders. This level of additional support is only expected for fault zones, at the portals and in areas where wedges failures are likely said Pink.

A new Sika / Putzmeister PM500 shotcrete robot has been purchased for the project and selection of shotcrete additive supplier is underway with Sika and Degussa both submitting products for review. The bench excavation will be by bulldozer and ripper with trimming carried out by hydraulic hammer and milling heads mounted on excavators.

Pink confirmed the d-shaped section tunnels will have a finished height of 8.5m and a width of 12.5m. The permanent lining will be a 325mm thick cast in-situ concrete lining that is not reinforced. It will be backed by a waterproof membrane and geotextile drainage fleece and will contain polypropylene fibres for spalling control in the event of a fire.

The current programme calls for the top headings and benches to be completed late next year with lining to follow excavation and be complete by late 2007. The mechanical and electrical installations are due to be finished by early 2008 leaving plenty of float to the opening of the entire bypass in December 2008.