NZTA works to help stabilise State Highway 4 slip at Te Oreore

The crew at Te Oreore drilling boreholes and installing groundwater monitoring devices, deep within the landslip area on State Highway 4. Photo / Supplied

Waka Kotahi NZ Transport Agency says work on stabilising the Te Oreore slip area on State Highway 4 is progressing, with extraction wells and pumps set up to remove excess water from the site.

The section of the Parapara Highway was damaged in a slip last October which was caused by water gradually building up in the soil and rock from groundwater seepage.

This increased the water pressure at the base of the landslip, reducing the soil's ability to resist the sliding movement to a point that the land slipped.

Regional transport systems manager Ross I'Anson said deep groundwater extraction wells and pumps were now set to remove groundwater from the slip area into the newly upgraded drains.

"This followed preparatory work which included creating access tracks, upgrading the drainage, drilling bore holes, taking samples deep below the road and putting in monitoring equipment. We are monitoring any land movement to check the impact of changes in groundwater levels or earthquakes," he said.

"The measures are necessary to lower the groundwater levels. The steep hill slopes and weak underlying rocks in the region means it is more susceptible to landslips from earthquakes and prolonged periods of rain."

I'Anson thanked landowner Ātihau-Whanganui and mana whenua Ngāti Rangi for their continual generosity and support throughout the process of building the temporary road and helping with options for the permanent route.

"We look forward to continuing to work with them as we move to the next phase," I'Anson said.

The work supports reconstructing the permanent road, planned to get under way in summer 2021.