Coromandel Roading update: Geotech checks and slips cleared

State Highway 25A on the Coromandel Peninsula January 30. Photo / Waka Kotahi

The Coromandel’s roads, severely affected by this summer’s devasting weather, continue to undergo geotechnical investigation while roading contractors have been kept busy clearing more 200 reported slips, spending more than 56,000 combined man-hours at a cost of $5 million.

Fifty slips have been classified as significant (more than two truckloads of materials in size), by Waka Kotahi NZ Transport Agency, which reports that 30 overslips and 20 underslips are still considered “active” and in need of permanent solutions.

Heavy rainfall has damaged stormwater systems, and Waka Kotahi said it is “continuing to inspect the drains and working on the maintenance and repairs”.

Three repair options for the vital Kopu-Hikuai access route SH25A are under consideration. These include a new bypass cut north of the existing road, built around the top of the site.

A second option explores replacing the lost section of the road with a multi-span bridge complete with concrete beams as the likely option. A steel bridge, which uses lighter, easier-to-work-with beams, is also being considered.

A third option involves rebuilding the supporting ground (of the existing road) with retaining walls.

Geotechnical investigations that include drilling boreholes have begun.

Waka Kotahi says the first borehole has been drilled on the eastern side of the remaining existing roadway. It was drilled to 13.5 metres and the last 5m of that was in rock. This borehole is where the possible bridge could be built, on what would be the eastern abutment (bridge support). Having rock available to pile into the bridge abutments is ideal. The next step will be collecting further information from the western side.

A total of four boreholes will be drilled in the northern access track (above the slip site) and these are planned to be finished this week. They are the slowest to drill as they have to reach down to the proposed new road level for that option and in places, with the steep hills being worked in, this is more than 20m from the surface.

The rig is now on the western end of the northern access track. The first bore in this section has so far gone down 15.5m and will continue to 20m. No rock has been found here as yet.

To keep the momentum going, another drilling rig will be brought to the site next week to work in the lower slip area. By the time it arrives, the work on the track will have finished.

Waka Kotahi said the creation of a temporary access track for normal road traffic to use while repairs are conducted is not feasible given the steep topography, which “doesn’t allow us to build quick temporary access for regular vehicles. The access routes that have been formed for the drilling rigs have steep grades and are only viable for machinery with tracks that can maintain traction and avoid sliding”.

“We cannot install temporary staging across the face of the slip, as this will be at risk should the face move further. This would also block access for the geotech rigs to the base of the slip.”

The following roads are affected by repair work:

• SH25 Opoutere-McBeth underslip – two traffic management teams onsite (traffic management and monitoring day and night) with chip sealing completed yesterday
• SH25A Taparahi slip - two earthworks teams plus geotech on-site
• SH25 Thames Coast – team clearing piles of material and vegetation near Waikawau north
• SH25 Tairua north - one team working at Pumpkin Hill overslip site and multiple crews working between Whenuakite and Tairua, mostly carrying out drainage work (please adhere to the 50km/h speed restriction)
• SH25 Tairua Quarry – site now open with a speed restriction
• SH25 Hikuai settlement dump site – taking the material from each site and shifting as needed within the dump area
• SH25 Whiritoa – working on clearing drainage and slips along this length
• SH25 Kuaotunu wall – Geotech drilling team onsite.