Boom time for motorway extension

The country's largest ever construction blast helped to clear the way for the $365 million Northern Motorway extension yesterday. Picture / Richard Robinson

Twenty tonnes of explosives ripped into a hillside on the route of the $365 million motorway extension north of Auckland yesterday, in the country's largest construction blast.

The explosives - two truckloads' worth - were drilled into a honeycomb of 200 holes about 10m deep, in a saddle of Chin Hill above the Waiwera Valley and Hatfields Beach.

Chin Hill's 155m summit, the highest point along the 7.5km motorway route between Orewa and Puhoi, will be left intact during the remaining three years of construction. But a cut to a depth of 55m into the hill's western saddle - including the 19,000cu m of rock dislodged yesterday - is expected to give motorists the feeling of driving through a ravine.

The cut is believed to be the largest in the country, and would easily swallow the 40m-high Auckland Harbour Bridge.

It will connect a 256m viaduct across the Otanerua Valley to the south - an eight-span structure due to be completed this winter - to a 515m bridge across the Waiwera River leading to twin tunnels being bored through Johnstones Hill above Puhoi.

Transit NZ's first plan was to carve a 60m cut through that hill, but engineering challenges and environmental concerns about severing an important wildlife and bush corridor led to a late design change incorporating the tunnels.