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A slope stabilisation system has been built on State Highway 1 in the Dome as a permanent repair to a major slip that closed the road after Cyclone Gabrielle in February 2023.
Waka Kotahi/NZ Transport Agency regional manager transport services Stephen Collett says that initially, a low temporary steel barrier was installed at the site to stop further debris slipping onto the road, which allowed State Highway 1 to reopen.
“In June [2023], a much larger slip occurred at the same site, overtopping the temporary barrier and spilling across the road,” Collett says.“Large boulders landed on the road, with many more unstable boulders at risk of coming down. This forced both lanes through SH1 to close.”
To re-open the road quickly and safely, NZTA contractors again erected a temporary fix, a ‘H-pile’ debris wall.
“This was always intended to be a temporary solution while a geotechnical investigation and design work was carried out for a permanent solution.”
The permanent fix includes rock anchors drilled and cemented deep into the hillside, holding tightly tensioned wire ropes and mesh to hold the hillside in place and prevent further slips. In the middle part of the slope, a draped mesh system will catch any future falling debris behind a wire rope barrier at the base.
Collett says removing debris from time-to-time is expected, such as happened recently. He says this is anticipated by the design of the system and is not a failure.
Construction of the temporary wall cost around $250,000 and its removal cost around $36,000. The permanent construction cost around $1.42 million and is expected to last for at least 100 years.
