From woe to go - Anatomy of a slip fix-up

In our shaky, rainy isles, Kiwis are no strangers to landslides. We know from hard won experience that the largest can take months – sometimes even years - to clear and fix.

Greg says the first step in remediating slips is a geotechnical site assessment that happens as soon as possible after they come tumbling down. Technical experts then put their heads together on fix-up options. Because of varying ground conditions and slope behaviour, there’s no one size fits all.

“We work closely with the client on the feasibility of the options, make modifications if needed, agree a works approach, then arrange for contractors to carry out the work.

“The entire slope could be anchored to contain a larger failure, for example; vulnerable parts of the slope could be covered in draped mesh; we could install catch fences at the bottom of the slope. It’s about bringing the client on the options journey, having them understand the risks and costs of each.”

One of the biggest challenges in repairing landslides is constructing in a way that doesn’t pose a risk to workers’ safety. Client, contractors, and stakeholders are heavily involved throughout. From start to finish, safety is the thread that runs through all repairs. So is waste disposal, with most landslides generating an incredible volume of debris.

Last August, an area of numerous slips in Pukerua Bay cut off State Highway 59. WSP’s geotechnical team was called into action to support the Wellington Transport Alliance (WTA) emergency response team. This involved visiting the site, coming up with repair options and talking these through with the alliance team - Waka Kotahi NZ Transport Agency, Fulton Hogan, and WSP.

A decision was made that involved approximately 24,000 cubic metres of soil being removed from the location and disposed of to make the road corridor more resilient to future events.

The recovery included reprofiling the slope with terraced benches, with ‘safety by design’ for workers and post-installation maintenance works front of mind. This technique provided a safe platform for contractors to work on and reduced the likelihood of further erosion or landslides impacting that critical section of road in damaging storm or earthquake events.

Finished just weeks ago, the result is a feat of engineering. Slopes between the five-metre-wide benches have been sprayed with concrete, anchored, and penetrated with deep drains. The site has been hydroseeded to control surface erosion and bring a green aesthetic back to the slope.

As part of the Wellington Transport Alliance, WSP’s Wellington-based geotechnical engineers help with six-monthly inspections of the region’s state highway road network to identify sites with geological threats, such as landslide hazards. This helps provide a risk rating for hazard evaluation and where to remediate high-risk sites that pose a threat to safety and transport. It’s about getting ahead of the eight ball, says Greg.

Work like this is incredibly important in making the Capital’s roading corridors safe and resilient to future landslide events. Ngauranga Gorge is one such site. Carrying over 90,000 vehicles a day, it’s a vital transport link. Having it cut off or vehicles impacted by rockfalls would be a nightmare scenario.

The gorge’s steep slopes were cut back in the 1930’s to form Ngauranga Gorge. They were widened by 1950 to form the region’s first motorway to Johnsonville and the northern suburbs.

Greg says engineered slopes like these in Wellington’s greywacke rock masses relax with time. Joints open and blocks of rock tend to loosen with weathering.

“Understanding the rock mass and defects that control drop-outs allow our engineers design rockfall protection structures, such as the blanket of draped steel mesh and anchors that has just been installed over vulnerable sections of the gorge.”

Ngauranga Gorge’s new rockfall system channels debris down through the mesh and onto benches where it can be quickly and safely cleaned up. The work was done by Capital Journeys® and WTA in two stages - in 2020/21 and 2022/23 – with great success.

WSP’s geotechnical experts work in partnership with transport authorities around the country – providing much-needed expertise, helping identify high-risk complex sites that need immediate, custom-made fixes, through to lower-risk sites that may just need a simple retaining wall or gabion (caged rock) wall.

Thanks to the march of technology, the task is becoming easier. Drones have been a “gamechanger” in mapping the topography of hard-to-access sites. 3D modelling is helping identify the most appropriate and buildable types of systems to remediate unstable ground. Plus, landslide risk assessments and design standards are continually being reviewed, as engineers learn from landslide mitigation systems that haven’t performed as well as expected.

Slope monitoring systems developed by WSP Research in Petone are another innovation being used to monitor slope movement in real time with movement sensors that can be linked to site-specific weather data.

Greg says landslide callouts are becoming more frequent. It’s hardly surprising, with Government climate scientists predicting more intense bouts of rain due to climate change.

Thankfully, Aotearoa New Zealand is a world leader in landslide engineering solutions. Greg points to WSP’s geotechnical expertise in ground condition assessments, and access to the latest in slope monitoring and remediation technology – not to mention decades worth of historic landslide data that can give clues as to what’s driving slips on the transport network in the here and now.

“We’re proud to work alongside our clients to provide that vital expertise in repairing unstable slopes and keeping our communities connected and moving. Based on the start we’ve had to 2023 in the North Island, it’s shaping up to be a busy winter.”