The underslip site on SH23. A retaining wall supported by 12-metre concrete piles is being built to restore the highway. Photo / NZTA
The underslip site on SH23. A retaining wall supported by 12-metre concrete piles is being built to restore the highway. Photo / NZTA
A reinforced retaining wall supported by 12-metre concrete piles drilled deep into the hillside is being built at the site of the serious underslip which closed the main road to Raglan in February.
A temporary diversion road was opened on March 4 around the underslip and the restored State Highway 23 between Raglan and Whatawhata should reopen in May, says Waka Kotahi NZ Transport Agency.
The temporary diversion road has enabled geotechnical experts to thoroughly investigate the site and allowed NZTA to decide on the best permanent fix.
Waka Kotahi regional manager infrastructure delivery for Waikato and Bay of Plenty, Jo Wilton, says a number of options were considered.
The rig is drilling to a depth of 12m, with concrete then injected through the auger drill to create a continuous pile. Photo / NZTA
“As the ground conditions under the slope were worse than expected, it soon became clear that the best solution was to build a geosynthetic-reinforced retaining wall, which will have concrete piles drilled deep in the hillside to support it.”
The piles are known as continuous flight auger piles. At SH23 these are being drilled to a depth of 12m, with concrete then injected through the auger drill as it is slowly removed, creating a continuous pile without ever leaving an open hole.
“It’s a robust and cost-effective solution,” says Wilton. “It will deliver peace of mind to people who rely upon this crucial link between Raglan and Hamilton, providing resilience for the future.”
The rig was established on site and drilling began on March 17. It is anticipated to be finished early next week. Following that, building the retaining wall will take around another fortnight.
The temporary diversion road was built around the underslip so the road could open while a permanent fix was built. Photo / NZT
With resurfacing work, installation of guard rails and lane markings needed to complete the project, Waka Kotahi is aiming for the highway to reopen in May – though this will depend on the weather throughout the project.
Waka Kotahi says it is grateful for the generous co-operation of the landowners, which enabled work to begin on the temporary diversion road within days of the initial underslip. Wilton says once the original highway is reopened, the project crew will work on restoring the affected properties, including fencing, planting and landscaping.
The section of SH23, known as the Raglan Deviation, began to crack in late January when the ground was saturated following unprecedented rainfall.
The cracks soon turned into an underslip, which significantly worsened after Cyclone Gabrielle, with a 30m-long section of highway dropping around 2.5m.
