Geotechnical assessment

Geotechnical assessment

In 2021, we commissioned engineering geologists to survey Te Āpiti and report back on the current and future risks, such as landslides and rockfalls.   

The assessment looked at the risks of landslides and rockfalls over the next 20 years and considered how the road and existing infrastructure such as bridges, retaining walls and rock fall mesh might deteriorate over time.

Different timeframes from the present through to 2041 were considered and the various sections of the Gorge were scored, depending on the risks. 

Risks for two groups of people were considered, those in vehicles and those walking or riding a bike or horse. 

Using established risk assessment methods, the team agreed a tolerable risk threshold, balancing the chance of being killed by a rockfall or landslide with the benefits of being able to access the Gorge.

It concluded that much of the Gorge, including an almost four-kilometre-long section in the middle, would expose people travelling in vehicles to an unacceptably high risk of injury or death.

Risks to people walking or riding a horse or a bike were found to be lower and of a tolerable level – but only with risk mitigation measures in place.

The assessment involved four phases:

• Stage one included using a drone to survey the area to provide footage for the team to carry out an assessment of any bridges, retaining walls or other infrastructure such as rockfill mesh. LiDAR, or ‘light detection and ranging’, also used to create a 3D representation of Te Āpiti. This allowed the team to compare new and previous information and identify additional areas of movement.
• Stage two involved looking at information that exists about the area and the landslips that have occurred there.
• In stage three, geotechnical experts visited the area to map historic and potential slips.
• Finally, stage four involved taking all the information gathered and using it to create a detailed map to identify the different risk zones.

Frequently asked questions

•  Tell me more about the walking, horse-riding or biking assessment

  The assessment found that the risks to people walking or riding a horse or bike were lower and the risk of being killed by a rockfall or landslide was similar to the chance of dying in a road crash. If a person in this group visited the Gorge once a week over their life, they had a one in 100 chance of being killed by a rockfall or landslide. Risks changed over the various time periods, showing an increase over time.

  Risk levels for cyclists are the lowest, as they are generally travelling faster than people walking or on horseback, and so are exposed to the hazards for shorter periods of time.

  It is important to note that resumption of road use in ‘tolerable’ areas would only be tolerable with risk mitigation measures in place. People are reminded that at present it is not safe to proceed beyond the locked gates at the entrances to the gorge.

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•   Why is the risk higher for cars than for cyclists?

  People in vehicles are at highest risk mainly because their lives could be put at risk in two ways from slope hazards - driving into a pile of debris (or a hole caused by a landslide) and being hit by debris as they are driving. Due to the lower speeds pedestrians and cyclists are travelling at, they are able to avoid riding/walking into a pile of debris or a hole but are still at risk from being hit by falling debris.

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•   Are there some areas along the route that are safer than others?

  Areas that could be investigated further and potentially made accessible for cyclists, horse riders and people walking were highlighted in the report. Areas that could be considered initially include the westernmost end of the route (the Manawatū River bridge) to immediately west of Kerry’s Wall, and just west of Barney’s Point to the eastern end of the route at the Woodville Intersection. The area from Kerry’s Wall to Barney’s Point could also be considered but would likely need more mitigation works.

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•   What assessment methods are used?

  Risk to vehicle occupants and people in the open (pedestrians, horse riders and cyclists) were assessed using established methods.

  Risk to people in cars was assessed using the NSW Slope Risk Analysis (NSW) method while the Australian Geomechanics Society (AGS) method was used to calculate risk to people in the open.

  The NSW method has been adopted by Waka Kotahi as its preferred risk assessment methodology for vehicle occupants. It has been applied in several locations in New Zealand to date including the Kaikōura coastline following the 2016 earthquake, and the Mt Messenger Bypass (SH3).

  The AGS quantitative life risk assessment methodology, or variations of it, has also been applied extensively throughout New Zealand.

  The risk assessment was carried out by Beca and peer reviewed by Aurecon.

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•   How do you decide what is a ‘tolerable risk’?

  New Zealanders are no strangers to taking risks while we are doing things we enjoy. We assess the risks against the benefits and decide what we’re happy to tolerate. While New Zealand has no national guidelines for life risk tolerability, guidelines from Australia have been applied in our country several times and have set a precedent. This published information helped guide our thinking.

  For pedestrians, cyclists and horse riders, the annual life risk tolerable threshold is 1 in 10,000, which means if you visited the Gorge once a week for your entire life (80 years), you would have roughly a 1 in 100 chance of dying from the slope stability hazards.

  For vehicle occupants, the tolerable risk threshold is a qualitative value of ARL3, which is the accepted Waka Kotahi level using the NSW method.

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