New Zealand: Geotechnical experts help provide a solution to restore State Highway 23 to Raglan

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The temporary diversion road has enabled geotechnical experts to investigate the underslip site and allowed NZTA to decide on the best permanent fix.

Jo Wilton, Waka Kotahi Regional Manager Infrastructure Delivery for Waikato and Bay of Plenty, says a number of options were considered.

“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,” says Waka Kotahi Regional Manager Infrastructure Delivery for Waikato and Bay of Plenty Jo Wilton.

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. It will deliver peace of mind to people who rely upon this crucial link between Raglan and Hamilton, providing resilience for the future,” says Waka Kotahi Regional Manager Infrastructure Delivery for Waikato and Bay of Plenty Jo Wilton.

The rig was established on site and drilling began last week (17 March), and is anticipated to take around 10 days.

Following that, building the retaining wall will take around another fortnight.

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 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. Ms Wilton says once the original highway is re-opened, 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.

Source: © 2023 Waka Kotahi NZ Transport Agency

Image source: © 2023 Waka Kotahi NZ Transport Agency

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