SH23 Whatawhata to Raglan slip repairs

22 March 2023

The temporary diversion road that opened 4 March

Restoring State Highway 23 to Raglan

The road to Raglan is a big step closer to a permanent fix.

Waka Kotahi NZ Transport Agency has decided upon a solution to restore State Highway 23 between Raglan and Whatawhata, which was damaged by a serious underslip in February, with the anticipated re-opening 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.

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' and are being drilled to a depth of about 12m, with concrete then injected through the auger drill as it is slowly removed, creating a continuous pile without ever leaving an open hole.

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

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.

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 State Highway 23, 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.

For more details, background and photos go the project website

The auger will drill concrete around 12m deep into the hillside to support the retaining wall