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 but, to be considered tolerable, risk mitigation measures would need to be put in place.

Assessment phases

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.

Assessment methods

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

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 in deciding ‘tolerable risk’.

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 NZTA as our 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.