Tuesday, August 15, 2023

Waka Kotahi contractors working hard in Murchison stabilising Dellow’s Bluff and building retaining wall for under-slip SH6 beside Buller River


Overview showing Dellow’s Bluff and under slip - Photo Waka Kotahi

Rock blasting and anchors help stabilise Dellow’s Bluff

The end is in sight for ongoing roadworks at State Highway 6, Dellow’s Bluff, near Murchison.

Waka Kotahi contractors have been working hard to reduce the risk of large boulders falling onto State Highway 6 at Dellow’s Bluff, with a combination of rock clearing and rock anchors to stabilise high-risk cliff sections next to the highway.  

In July 2022, large boulders, including one the size of a car, fell onto the road, closing it for three days, affecting the critical transport link between Nelson, the West Coast, and Christchurch.

Boulders from Dellow’s Bluff, SH6, July 2022  - Photo Waka Kotahi 

One of the boulders, nicknamed ‘Eric the egg’ due to a distinctive nodule on the rock, is now displayed outside Murchison Museum as a permanent reminder of the scale of rock fall. 

Rob Service, System Manager Top of the South, says tree roots and water getting into cracks in the cliff face caused the massive boulders to fall.

“Some parts of the bluff are particularly prone to erosion, and goats clambering over the bluff increase its instability. Inspections revealed more fractured rocks and loose material at risk of falling on the road. These had to be removed.”

“We used a range of techniques to make the cliff face safer. Scaling and blasting removed loose rocks and overhangs, and rock anchors (or bolting) helped stabilise other sections,” Mr Service says.

A challenge for the project was using helicopters to lift large equipment, such as drilling rigs, to the top of the bluff. Specialist abseiling crews also installed 280 rock anchors. 

Rob Service says the work has stabilised the most at-risk section of the cliff, and future work at Dellow’s Bluff will be prioritised under the national resilience programme. In the meantime, some safety measures will be kept at the site.

Dellow’s Bluff rock anchoring work - Photo Waka Kotahi

“We will keep the protective container wall in place for up to another 12 months. Keeping it lets us monitor the cliff’s stability under a range of conditions. Monitoring the amount and type of debris coming down, and where it comes from, helps us decide what is needed to best protect the road in the long term.” 

As well as stabilising the cliff face, crews have been working to repair an under-slip about 150 metres from the rock face on the river side of the road. 

Mr Service says sharing traffic management and resources helped both projects and minimised disruption to road users.  

“We had been running ongoing maintenance on the under slip, using asphalt fill as a temporary measure. However, after a weather event last February, we decided to get resilience work for this site done sooner.”

While work on the under slip had been planned to be finished earlier this year, Mr Service says challenging ground conditions (including a high water table and little rock to anchor into) mean the retaining wall is now expected to be finished by the end of 2023. 

Construction of under slip retaining wall  - Photo Waka Kotahi

“We’ve installed 58 vertical piles to build a 60-metre retaining wall and are now drilling 27 horizontal 15-metre anchors to complete the tiebacks. We’ve also taken the opportunity to put in more drainage to remove water and stabilise the underground conditions. This will help make the road more resilient in future heavy rain events,” Mr Service says.

He says Waka Kotahi does appreciate that the long-running project has sometimes created delays and inconvenience for road users.

“The good news is we are almost there, and, at the end of this project, this section of State Highway 6 will be a much stronger and safer stretch of road for everyone to use,” Mr Service says.

Press release Waka Kotahi NZ Transport Agency

Pigeon Post News Richmond

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