Update on SH25a slip repairs with Waka Kotahi Regional Manager for System Design

Section menu
  • 19 Apr 2023

Next month we’ll announce the best solution for the large slip on State Highway 25A, with a decision on a how we’ll restore the road.

We're assessing three options at once so we can quickly progress to the design phase once we've made a decision. The options are to build a new road bypassing the slip, to build a bridge over the slip, or to build a retaining wall and then rebuild the collapsed road.

A bypass well away from the slip face would need substantial digging out, removing between 300,000-400,000m³ of soft soil before we can start building a new road. We also have to factor in the ecological impact, and the lack of sites in the Coromandel where we can take the soil we've dug out. A bypass closer to the slip face is another option. We'd rebuild an embankment by cutting into the banks above and filling over the slip below. The old slip material would need to be removed—we'd have to do this in winter, which'll be factored into the programme.

We're currently assessing the design, safety, and ecological impact of a bypass.

Regarding a bridge, at this stage our investigations show a steel bridge (rather than a bridge with concrete beams) could be the fastest to build. We're looking for time-saving options such as precast deck elements and long beams that aren't too heavy. These'll reduce the number of time-consuming piles to be installed. The smaller the crane to lift the beams, the better—building a crane platform and access to the platform on weak soil will take time.

Rebuilding the road by first building a retaining wall is the final option. This week we tested soil samples to better understand the ground conditions—the results are important to help us decide if rebuilding the embankment is viable or not.

While we work through the data and choose the best option, we'd like to introduce Jess Andrew. Jess is our Regional Manager for System Design. She's playing a vital role in the repair process and in ensuring the solution stands the test of time.