Geological testing starts on Manawatū Gorge replacement route

Monday, 14 May 2018

Drone footage shows the scale of another slip that crumbled down at the Manawatū Gorge in July last year.

The long road to a replacement route for State Highway 3 between Manawatū and Hawke's Bay has begun, with testing starting on the ground where the new road will be built.

The highway used to run through the Manawatū Gorge, but has been closed for a year because of repeated rockfalls blocking the road.

The New Zealand Transport Agency announced in March its preferred route would go north of the Manawatū Gorge, but south of the Saddle Rd bypass.

The agency said construction would likely begin in 2020, as consents needed to be issued before work started.

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Workers undertake geological testing along the proposed route for the Manawatū Gorge replacement.

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But that does not mean there is nothing to do in the meantime.

Two portable geotechnical rigs have been deployed to sites along the preferred route to take core samples.

The agency's Manawatū Gorge project manager Lonnie Dalzell said the route announced in March was a preferred corridor.

The core sample work would enable more detailed designs to be created, he said.

'All the geotechnical work until now has been a desktop exercise.

'You can do as many visual inspections as you like and see how the faults have occurred over time, but you don't know what is really underground until you start drilling.

'It's about confirming the assumptions we have made up until today.'

The work also involved cone penetration testing, during which the ground was 'pushed' to see how hard it was, Dalzell said.

The core samples could give key information, such as how stable the surface was for building on, or how easy shallow rock would be to cut a channel through.

It could also reveal if there were fractures in certain layers, or if there was rock material they did not expect to find, Dalzell said.

The Manawatū Gorge has been closed to traffic since April, after large slips came down on the road.

Doing the geotechnical work before applying for resource consents would make the process much simpler, he said.

'It's important to understand as much as we can before consenting.

'If we find an area and have to realign the road, we don't want to have gone through design consenting first.'

The agency wanted to get started on the testing, the first phase in gathering information about the route, as soon as possible, due to the highway closure affecting the local economy, Dalzell said.

'We are trying to get it moving along.'

Most of the current testing sites were on the hills either side of the Tararua Range, as well as certain areas on the Woodville side, Dalzell said.

Helicopters would transport the rigs to sites inaccessible by road.

The agency appreciated the co-operation of landowners for allowing the rigs on their properties, Dalzell said.

The work is expected to take six weeks, depending on the weather.