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Dewatering work underway to stabilise SH4 slip

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Waka Kotahi NZ Transport Agency is making good headway on stabilising the Te Oreore slip area on State Highway 4, where a temporary road replaces the section of highway damaged in the October 2019 slip.

Regional Transport Systems Manager Ross I’Anson says deep groundwater extraction wells and pumps are now set to remove groundwater from the slip area into the newly upgraded drains, to take water away from the site.

“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.

“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,” he says.

Last year’s slip 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 landslip movement to a point that the landslip was initiated.

“We would like to thank landowner Ātihau-Whanganui and mana whenua Ngāti Rangi for their generosity and support throughout the process of building the temporary road and looking at options for the permanent route.

“We look forward to continuing to work with them as we move to the next phase,” Mr I’Anson says.

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

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

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

 

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