Waka Kotahi has decided upon a solution to restore State Highway 23 between Raglan and Whatawhata, which was damaged by a serious underslip in February.

The transport agency says the road is anticipated to re-open in May.

A temporary diversion road was opened on 4 March to skirt the underslip that had closed the highway; this has allowed geotechnical experts to thoroughly investigate the site, and for the best remedy to be decided.

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.

Jo Wilton, Waka Kotahi regional manager infrastructure delivery for Waikato and Bay of Plenty, said 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.”

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,” said 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 in mid-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.