The Government has committed to building a bridge over State Highway 25A after it was severely damaged during Cyclone Gabrielle and the Auckland Anniversary flooding.

Transport Minister Michael Wood announced today that the Government has taken advice that a bridge will be the fastest, most resilient way forward and will commit the funding straight away.

TCDC said three options were being explored to find a permanent fix for the main route.

Earlier, Waka Kotahi NZ Transport Agency’s regional manager of infrastructure delivery, Jo Wilton, said these options included building a bridge replacing the lost section of the road, creating a deviation that bypasses the slip site or making a retaining wall and effectively rebuilding the road from the ground up.

TCDC said there were a lot of complexities to work around such as the steep topography of the area, poor ground conditions and the need for considerable earthworks and retaining.

It said temporary staging would also not be a quick fix and would likely be in the way of the construction of a permanent solution.

Access tracks have been formed down into the SH25A slip area for the geotechnical investigation contractors. Photo / Waka Kotahi

SH25A was closed at the summit on Friday, January 27, to ensure the safety of road users, when deep cracks formed in the road after ex-Cyclone Hale. The cracked section of the road at the summit then slipped away during the Auckland Anniversary storm event.

Waka Kotahi NZ Transport Agency said earlier that geotechnical investigations were progressing well at the site of the huge slip that closed the road.

Two drilling rigs were on site and if good weather continues, the fieldwork is scheduled to be completed this week.

The surface conditions remain steep and slippery for the work crews.

Waka Kotahi said the investigations are assessing the ground conditions within the slip area and the hillside above.

“Ideally, we want to find stable and strong soils or rock that can support the weight of an embankment or structure without the risk of movement or slipping,” the agency said.

Boreholes are about 50mm in diameter and go to depths of about 20m. Photo / Supplied

The data being collected also helps to determine the amount of excavation needed, slope stability and foundation requirements for the various options being considered. These are all important factors that help determine a solution that can be built quickly and efficiently.

“We don’t know which option is most feasible yet. The right solution cannot be established until the geotech work is complete. We know the rebuild must be resilient and as timely as possible. We understand how critical this highway is for Coromandel residents, businesses, and visitors,” Wilton said.

“Boreholes are being drilled to depths of around 20m to help understand what the ground conditions are like, we’re getting data from different methods — boreholes and test pits”, says Waka Kotahi.

The slip on SH25A now measures around 110 metres across. Photo / Waka Kotahi

“Last week test pits were cut from the sites where we only need to understand the soil conditions at a reasonably shallow depth — e.g. near the side slopes of the slip. They are cut and formed by a digger, are about 2-3m square and are dug to a depth of around 3.5m. They’re good at giving a clear picture of the soil composition.”

The two rigs on site are drilling boreholes, retrieving cores of the soil from greater depths and providing more detailed soil and rock strength information. Boreholes are about 50mm in diameter and enable a core of the soil to be extracted. These are being drilled at the sites of the potential bridge and bypass options.