Motorway remedial

Remedial work on unstable land adjacent to the northern end of the new $880 million Pūhoi-to-Warkworth motorway is expected to run until October, but will not push up the price tag, according to Waka Kotahi.

“[The road building contractor] NX2 is responsible for ensuring the project meets necessary quality and safety standards, including the required maintenance standards, during the 25-year operation period at no additional cost to Waka Kotahi,” general manager of transport services Brett Gliddon said.

A Waka Kotahi spokesperson confirmed that that included the work now underway at the slip site.

An external engineers’ report released by Waka Kotahi in response to an Official Information Act (OIA) request by Radio NZ revealed that severe rainfalls in January appeared to have reactivated “a pre-historical and deepseated landslide” just south of the Pukerito roundabout at the Warkworth end of the route.

The report authored by GHD and Jacobs was dated April 4, about 10 weeks before Prime Minister Chris Hipkins officially opened the 18.5-kilometre stretch of motorway. It said “catastrophic failure” of the cut slope was unlikely during typical rainfall events. However, if not stabilised, there was a risk of landslide movement accelerating, especially if more extreme rainfall events were experienced. The authors added that stabilisation was required in the short term to minimise the risk of accelerated movements and exposure to extreme climatic, storm events and strong earthquakes.

“While works at this location are ongoing, concrete barriers are in place and a temporary traffic management plan has been instituted,” Waka Kotahi said. “These measures ensure the landslide poses no risk to the public travelling on the motorway.”

Gliddon said the motorway was built through an area with challenging geology, and with a history of slips and land movements.

“The road has been designed and constructed with these issues taken into account, in order to provide a safe and resilient route,” he said.

Highlighting the “challenging geology”, a second document released under the OIA stated that multiple landslides had occurred at various locations along the entire route of the road under construction since late 2019, continuing into late last year.