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Omoto slip, SH7, near Greymouth – the way forward

Omoto, SH7, near Greymouth – the way forward for road and rail

Waka Kotahi NZ Transport Agency and KiwiRail geotechnical advisors and highway and rail managers have been meeting this week on the West Coast to discuss what will be done to return Omoto, east of Greymouth, to a safe highway and rail link.

The Omoto section of SH7 briefly reopened last week but has been closed since Friday night, 18 October, with significant slumpage of more than five metres that weekend after ongoing rain.

“We know that the movement is caused by groundwater deep within the slip. The next step is to see if we can drain the water from the slip surface and see if that will arrest the instability at road and rail level,” says Moira Whinham, Maintenance Contract Manager for the Transport Agency on the West Coast.

“We have a drill rig on its way and from next week will be investigating if it is possible to successfully de-water (drain) this site,” she says.

“By the week starting 11 November, we should have a better idea if the drainage strategy is succeeding and if that will allow us to provide a short-term solution to rebuild the highway and rail track.”

The continuing wet weather and saturated ground has meant that the slope has continued to move and it has not been safe to do any drilling or drainage work while the site was moving, she says.

Longer term, geotechnical advisors are working on a solution to reduce the potential for slips of this nature, which has had a significant impact upon the TranzAlpine service to the West Coast, as well as freight, and been inconvenient for local people living east of the slip site.

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“We understand the disruption and inconvenience this is causing to people, particularly residents in Kaiata and Dobson, and ask for their patience while we work to make sure we have a robust repair strategy. We are working closely with KiwiRail to make sure what we do will provide long term security for both the state highway and rail link,” says Ms Whinham.

“KiwiRail remains committed to restoring the link and resuming both our freight and passenger services to the West Coast,” says KiwiRail’s General Manager – South Island Operations Jeanine Benson. “We are working hard to allow services to resume as quickly as possible while at the same time developing a long-term remedy for the site.”

Ms Whinham thanked people for slowing down on the Taylorville detour route, particularly at the Coal Creek overbridge, which has a 30 km/h speed limit to protect it from the effects of High Productivity Motor Vehicles (large trucks) travelling on it at speed.

The landslide first became unstable and required increased monitoring with traffic management from mid-August. Prior to this recent period of significant instability, the Omoto slip site had been relatively stable since the 1980s, and easily managed by highway crews with minimal disruptions.

Web link for Omoto East landslide project page – photos and recent history https://www.nzta.govt.nz/projects/sh7-omoto-east-landslide/

Detour map: https://www.nzta.govt.nz/assets/projects/sh7-omoto-east-landslide/Taylorville-Road-Detour.pdf

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