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Epitaph Slip, South Westland – plan for future work

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The NZ Transport Agency has a $1.5 million programme of work planned for the Epitaph Slip, near Knight’s Point, north of Haast, following a recent geotechnical review.

The NZ Transport Agency has a $1.5 million programme of work planned for the Epitaph Slip, near Knight’s Point, north of Haast, following a recent geotechnical review.

The Transport Agency has been monitoring the South Westland site with instruments that measure groundwater and ground movement since 2012 when the slip occurred below the road, between the road and the Tasman Sea. At all times, the safety of road users and security of the route have been the guiding priorities around the highway’s management.

Recent and ongoing geotechnical investigation and monitoring at the Epitaph Slip site indicates that two main geological processes are occurring. “These are broadly considered to be independent of one another,” said Pete Connors, Regional Performance Manager for the Transport Agency.

 Photo courtesy Grey Star showing State Highway 6 past the slip site.

Photo courtesy Grey Star showing State Highway 6 past the slip site.

Mr Connors says that erosion is occurring to the slip face itself, below the road, and a larger geological process is generating more deep-seated movement adjacent and north of the slip.

Currently, the road itself is separated from the top of the slip by at least five metres, and this has remained relatively unchanged since October 2012.  However, the top of the slip is being eroded by ongoing weathering processes. This will eventually affect the road and will need to be managed through engineering works. “At present, this is not considered to represent an immediate risk to road users,” says Mr Connors.

A further and larger geological process has also been identified resulting in some ongoing minor road slumping, affecting a 50 - 60 metre section of road to the north and immediately adjacent to the main slip. This is believed to have been moving slowly prior to October 2012 when the original slip below the highway first occurred.  Monitoring stations positioned on the site have recorded in the order of 50 mm of settlement in the last four years. This small amount of movement has been triggered by heavy rain.

“We expect to see small amounts of slumping continuing, however this does not mean that there is an immediate risk to the highway,” says Mr Connors “This could be ongoing for several years before the site ‘settles down’.  This type of movement has been observed in several locations in South Westland.

“It is unlikely, but possible, that a major movement event could occur, which could lead to a temporary road closure. However, based on current information, the risk is assessed as no greater than any other part of the road network where similar structural mass movements are happening.  If this occurred, it is expected that the road would be closed for up to three days while a temporary route is constructed through the site.”

History of slips of this nature

Slips of the scale of Epitaph, with a 200 metre drop into the sea on one side are relatively rare events, probably occurring every 50 years or so in this type of environment. They are not easy to manage given the need to maintain a State Highway, a key lifeline link through an unstable mountainous area. However, with the right monitoring in place to identify any sudden or extreme changes within the site, the risk can be managed, Mr Connors says.

What is the Transport Agency going to do to mitigate the risk?

There are a number of ways the Transport Agency can mitigate the risk to the West Coast’s main highway, State Highway 6.

An earlier option to cut into the hillside above the road has been ruled out given the potential for this to trigger greater instability above the road.  The following work is currently in the design process with implementation planned for this financial year 2016/17:

  • Dewatering (drainage) of the landslide by the installation of horizontal drainage below the road to reduce the risk of road slumping triggered by heavy rain.
  • Surface shaping, vegetation removal and installation of horizontal drainage above the landslide, including side protection immediately above the existing headscarp.
  • Installation of electronic measuring equipment for remote monitoring of the site to ensure any significant changes are immediately recognised.
  • A retaining wall below the road to manage the longer term risks of weathering is being investigated along with possible vegetation planting below this wall to protect the face of the slip.

“This is considered to be the best way forward to mitigate the risks at this site,” said Mr Connors.  “But even with these controls in place, in a volatile environment like this the loss of the road can never be ruled out. For this reason, the Transport Agency will undertake further geological investigation of the wider area beyond the slip itself to determine possible longer term roading options should they become necessary.”

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