Motorway remedial work underway

Waka Kotahi says work on a slip alongside the Pūhoi-Warkworth motorway won’t add to the price tag.

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, Waka Kotahi said this week.

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

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 deep-seated landslide” just south of the Pukerito roundabout at the Warkworth end of the route.

“The slope is moving towards the carriageway,” it said.

The report said information from NX2 showed that after the Auckland anniversary storm, the slope above the road “experienced periodic episodes of comparatively rapid movement” – exceeding 30cm a week at times – “separated by short periods of relatively little movement.”

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 in mid-June.

“Cracks have been reported in concrete barriers in two separate places on the western side of the carriageway” it said, adding that there was “the potential that some landslide movement is occurring across the full width of the carriageway”.

The remedial work is being carried out by NX2, the contractor responsible for designing, constructing, and maintaining the motorway under the public-private partnership arrangement.

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

In his statement, 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.

He said the heavy rainfall events earlier this year had presented “a significant test” and that while there were some minor landscaping issues, no major slips occurred and the road demonstrated its resilience in contrast to existing roads in the region.

At the opening a month ago, Waka Kotahi made clear some work would be ongoing for several months, including earthworks at the location in question near the northern end, Gliddon said. He described that work as a “long-term design solution”.

“We worked hard to get the motorway open as soon as possible and we’re confident that the road is safe, and people are already reaping the benefits of this new world class motorway.”

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

“The 2023 Auckland anniversary rainfall event and Cyclone Gabrielle caused further soil slope failures and topsoil slips,” said the report, compiled for Waka Kotahi by a company engaged to provide a geotech landslide risk assessment, whose name was redacted.

The 300-page report assessed more than 90 cuts through slopes adjacent to the motorway, graded 51 of them as very low risk, another 26 as low risk, 14 as low to moderate risk, and two as moderate risk.

In addition to the slip near the Pukerito roundabout, another site further south was found to have a slope stability safety factor of 1.3, rather than the required 1.5.

According to Gliddon, “a second slip location was identified through the landslide risk assessment report. This section has since been reshaped to a compliant slope with a less steep gradient.”

The report found that the slope near the Warkworth end of the route moved up to 40cm between June and November last year, with movement increasing in August. A slope near the Moir Hill Bridge moved up to 190cm from May to August last year.