Update: 6 June 2023
Why did you close SH23 between Raglan and Whatawhata?
We did everything we could to keep State Highway 23 open but a geotechnical inspection late on 1 February confirmed the risk to the travelling public had increased. Despite our best efforts to keep water out of the cracks, the fissures worsened and the underslip continued to move and grow. Geotechnic engineers carried out drilling and core penetration tests to investigate the severity of the situation and the ongoing risk, and plan how best to fix it.
Work was quickly underway on a temporary diversion road on private land, skirting the slump site, thanks to the generous co-operation of the landowners affected, and hard work from our contractors, Fulton Hogan.
The temporary diversion road opened on 4 March, putting an end to the long detour route via Ngāruawāhia that had been in place since 1 February.
Traffic returned to the original highway in the middle of May.
How long did it take to fix the original state highway?
Geotechnical investigation of the underslip began immediately, and the temporary diversion road allowed us the time we needed to thoroughly plan for the long-term restoration and resilience of SH23. The slope was stabilised using soil nails, a construction measure used to strengthen weak and unstable ground.
A solution was decided in mid-March: to build a geosynthetic-reinforced retaining wall with continuous flight auger piles.
First, the piles were drilled 12m deep into the hillside, with concrete then injected into the auger drill as it was slowly removed, creating a continuous pile without ever leaving an open hole.
The next step was to build the retaining wall, which was completed in early May.
The final stages of resurfacing the road, installation of guard rails and lane markings were delayed during the many wet days that followed, but traffic returned to the original highway in the middle of May.
The final tasks were completed over the first week of June, with the temporary 30 km/h speed restriction then lifted.
What will happen to the temporary diversion road?
Now the original highway has re-opened, the project crew will work on restoring the affected properties, including fencing, planting and landscaping.
How many vehicles on average use this section of SH23 between Whatawhata and Raglan?
7500 a day, of which 615 are heavy vehicles.
What is the cost of rebuilding the road expected to be?
The total estimate is $3.6 million, including detour costs, initial response, diversion and reinstatement of the private property.
