Contractors set to start work on clearing slips in the Manawatu Gorge video

Ben Masters/Mastersphotographic

Drone footage of the Manawatu Gorge slip.

A wider road through the Manawatu Gorge comes with a hefty price tag – an unstable rock face and mounting bills to keep the state highway open, an earth scientist says.

"Every time you strengthen and widen it [the road], it makes slips more likely," Massey University earth science senior lecturer Dr Alan Palmer said.

"People want wider, faster roads, but that's the price you pay."

A 3500 cubic metre slip is blocking State Highway 3 at the Woodville end.

A 3500 cubic metre slip is blocking State Highway 3 at the Woodville end.

Engineers are assessing a large slip in the Manawatu Gorge that plunged on to State Highway 3 on Monday to see when it is safe for contractors to start clearing it. The road is expected to be closed for three weeks.

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They say it is safe for crews to start clearing a smaller slip blocking the road.

The smaller slip at the Ashhurst end of the gorge.

The smaller slip at the Ashhurst end of the gorge.

NZ Transport Agency highway manager Ross I'Anson said 3500 cubic metres of earth fell in the larger slip, which was at the Woodville end of the gorge.

He broadly agreed with Palmer but said much work had been carried out to prevent slips.

Every time the gorge's rock face was worked on, the exposed rock was affected by weather conditions, which could lead to "new issues".

Saddle Rd is being upgraded to make it a quicker and safer alternative route to the Manawatu Gorge.

Saddle Rd is being upgraded to make it a quicker and safer alternative route to the Manawatu Gorge.

However, platforms, known as site benches, were cut into the side of the Manawatu Gorge to catch rock falls or slips.

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The site benches were designed to reduce the risk of slips hitting the state highway below, he said.

During the closure, the agency would clear site benches, he said.

I'Anson said they had to be careful about how they cleared the slips, checking there was no more loose material at the top that was going to come down.

The cost of the work clearing the latest slips is not yet known but the transport agency had already earmarked $1.1 million to install rock fall netting in the year ahead.

This netting would prevent small slips, but this could not prevent large slips, l'Anson said.

About $100,000 was also spent on road pavement maintenance each year and in the past three years the Government has spent an average of $500,000 per year on rock netting and rock barriers.

In 2011, a large slip, which closed the Manawatu Gorge road for 14 months, cost $15.8m to repair.

In 2015, there were two slips, which cost $801,000 to deal with.

Woodville's Windfarm Cafe owner Linh Le expected a slow couple of weeks at the eatery while the road was cleared.

Business dropped off in the first few weeks after the 2011 slips, although it did recover, she said.

She lost a few customers as travellers diverted around Woodville, or delayed their trips, but loyal locals helped even things out.

In 2011, Vision Manawatu, which became part of the Central Economic Development Agency last year, estimated the regional economy loses $62,000 every day the gorge is closed.

That's based on the 500 trucks and 6300 other vehicles using the gorge each day.

The estimate represents the lost productivity due to the additional 13 to 20 minutes of travel around the gorge via the Pahiatua Track or Saddle Rd, as well as extra fuel costs and expenses.

Palmerston North City Council economic adviser Peter Crawford said the estimate was still a fair indication of the economic impact of closing the road.

However, there was a bump in retail spending in the wider Manawatu economy while the gorge was closed in 2011, Crawford said.

Mainfreight Palmerston North operations manager Keri Monk said the closure would again mean extra costs for the transport company, but that wouldn't be passed on to customers unless there was a long-term shutdown.

Mainfreight drivers' feedback was the Saddle Rd was safer and in a better condition than in 2011, he said.

Monk said Mainfreight would be keeping an eye on how well the Saddle Rd handled the stresses of all the heavy trucks going through.

"It'll obviously be tested by the increased traffic over the next few weeks, and we'll see how well the infrastructure holds up."

The improvements come from the work done as part of an $8.4m upgrade under way to widen and improve Saddle Rd.

Tararua Alliance spokesman Ray Cannon said the Saddle Rd upgrade had increased the safety, accessibility and resilience of the road.

 - Stuff


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