On 21 July, Transit New Zealand, the state highway authority, released funds for the Northern Motorway Extension (Alpurt B2) between Orewa to Puhoi to maintain the momentum of the construction works that should see the twin tunnels through Johnstone’s Hill get underway before the end of this year as planned.
The Northern Gateway Alliance is delivering the scheme and comprises Transit New Zealand, contractors Fulton Hogan and Leighton Contractors, together with engineering, environmental and design consultancies URS New Zealand, Tonkin & Taylor and Boffa Miskell. The group started work last December following release of advance funds of US$18.3M. This latest announcement sees the commitment of the balance of the full US$126.8M allocated to the scheme. The remaining funding to build Alpurt B2 will be raised through debt repaid by tolls charged for the route.
The two parallel 340m long tunnels through Johnstone’s Hill are seen as a key feature of the overall scheme. The 12m wide by 9m high tunnels will be driven at a 15m separation. A roadheader will be used for excavation through interbedded sandstone and siltstone that has a strength in the region of 30MPa., spoil will be used as fill in the earthworks on the northern approaches to the tunnels. Fully concrete lined, the tunnels will be configured to carry two lanes, a hard shoulder and a footpath.
Early plans along the alignment had called for a deep cut through the hill, but the tunnel option was eventually deemed more environmentally sensitive and aesthetically pleasing. Twin tunnels were chosen to separate opposing traffic flows for safety reasons. In addition, the tunnels will be fully lit and ventilated, as well as having a fire suppression system and a single cross passage for emergency egress.
In June, Alstom New Zealand scooped the US$7M subcontract for mechanical and electrical works for the tunnels including the design and installation of most tunnel systems.
