Strengthening work in Ngaio Gorge
On 12 February 2018 we started work on the upper section of the gorge – the latest stage, after a new 42-metre-long retaining wall was built below the road about half way up the gorge during 2017.
We are also investigating the best way to strengthen the area of hillside above the road where two major slips occurred in July and August 2017.
What's happening with the slip
Most of the slip debris and rock was cleared in August so the road could re-open. Some debris has come down since then, so the road will be closed from 14-16 February during off-peak times while this happens.
New GPS technology is being installed across the two slip sites. The instruments are part of a student-led research project at Victoria University of Wellington. They
are low-cost solar or battery-powered wireless GPS sensors which use a unique, cloud-based algorithm to calculate the location of each sensor, relative to a fixed-base station. This enables daily measurements to be taken at multiple points on a landslide without the need for site visits, with no line-of-sight or cabling requirements, and no need for intervention at the site for five or more years.
Options to more permanently strengthen and protect the slope are being peer-reviewed before decisions are made on how this work will be done. This will be a big job, and planning work will take time. Strengthening work above the road is unlikely to start until sometime in the second half of 2018.
In the meantime, the wall of containers, which are filled with concrete blocks and anchored to the hillside, will remain in place, along with the 30km/h speed limit. The containers are protecting this section of road.
What's happening further up the road
A new 45-metre concrete anchored wall, with a 100 year design life, will be the main feature being built opposite the area of houses numbered around 89-93 Ngaio Gorge Road, as well as new footpaths, hand rails, and landscaped grass areas.
The work is the latest in a series of projects designed to protect the road and reduce the likelihood of slips occurring in major earthquakes and storms, as part of the long term project to improve resilience on this key transport route.
Why we are doing this work
The Ngaio Gorge Road is an essential access route linking Wellington City to the north which is why we have been progressively strengthening vulnerable sections of it in stages since 2001. It is part of our ongoing programme to strengthen key structures and transport routes and make the city more resilient.
What's being done
- Constructing a new retaining wall
- Replacing and widening the footpath
- Installing new handrails
- Landscaping grass areas
- The new wall will have 26 concrete piles over 6 metres deep
- The wall will be tied to rock with 16 anchors, each will be around 11 metres deep
The previous road strengthening projects in Ngaio Gorge have involved upgrading or replacing old retaining walls in vulnerable spots, the most recent wall was to support a section of road that has not previously been reinforced.
- Preliminary work on the new wall started Monday 12 February 2018
- Work is estimated to take up to 26 weeks, weather permitting
- Two traffic lanes will be open as usual at peak times
- Traffic will be reduced to one lane around the Ngaio Gorge Road work sites between 9am and 4pm, Monday to Friday, and on Saturdays between 8am and 4pm
- Temporary traffic lights and traffic management will be in place, so people driving should expect short delays during the day
- Safe access for pedestrians will be provided at all times
The project cost which covers the investigation, planning and construction of this project is around $1.4 - $1.5 million.