Mt Roskill dug up for road

Duncan Gibb (left) and Les Barker of Fulton Hogan on the rock to be crushed and used for drainage behind retaining walls. Picture / Glenn Jeffrey

Auckland motorway builders have lost no time scooping an impressive pile of volcanic boulders from the Mt Roskill lava field, but have plenty more to dig.

The 500cu m mound is only what has been removed for a 300m bypass off May Rd, to allow the first of six bridges to be built for Transit New Zealand's $169 million Roskill motorway extension.

Duncan Gibb, project manager for lead contractor Fulton Hogan, says another 114,000cu m of basalt will be taken from the northern toes of the mountain.

It will be ground up by a mobile crusher due to arrive from overseas next month, and will all be used in the project, mainly for drainage behind retaining walls.

The basalt spewed from the northern base of the volcano, before flowing west as molten lava, and the initial rock-pile has been dug over the past month from the opposite side of May Rd.

Transit and the Auckland Volcanic Cones Society spent 18 months improving the design of the motorway around Mt Roskill to soften the boundary between its lower slopes and an on-ramp from Dominion Rd, and a geologist is on call to inspect any discoveries such as lava caves.

A resource consent for the project requires such geological features to be recorded, and damage to them minimised "where practicable".

Fulton Hogan is having to build the sealed bypass of May Rd to five-year design-life specifications to cope with high traffic volumes, even though it will be required only until a motorway bridge is complete early next year.

Mr Gibb said his team was working to "a very, very tight" six-month programme for the bridge, in time for grocery giant Foodstuffs Auckland to build a new supermarket between the motorway and Stoddard Rd.

Foodstuffs was not prepared on Friday to say what brand of supermarket was planned, but it is believed to be a New World outlet.

The bypass road, which will be open to traffic in November, will be ripped out once the bridge is built.

A block of May Rd has been closed to southbound traffic, which is having to take a detour along streets at the foot of the mountain, but will reopen in four to five weeks once a large stormwater culvert is laid to a tributary of Oakley Creek.

The motorway will run about 6m below ground level, and the bridge will raise May Rd by about 2m.

Mr Gibb said a temporary bypass would also be constructed later in the four-year project, further east in Hayr Rd, for a second road bridge.

Cycle and pedestrian bridges will also be built at Keith Hay Park and at Ernie Pinches St, at the New Windsor end of the project, and there will be full interchanges linking the motorway to Dominion Rd and to Hillsborough Rd.

Preparatory work is also starting on a 400m off-ramp to Hillsborough Rd from the end of the Southwestern Motorway, which will have to be sunk 8m below its existing terminating intersection.

All traffic leaving or joining the motorway will be diverted to the off-ramp or to a proposed eastbound on-ramp when the realignment work starts.