Major land slip
In October 2019 a major slip on State Highway 4 south of Raetihi destroyed a large section of the road, forcing its closure. The slip caused major disruption to iwi, the community, businesses, and tourists who frequent the road.
Major slip in October 2019.
The work required to reinstate the route is complex and significant. The slip was approximately 400 metres long and 200 metres wide – a similar size to the large slips that occurred in the Manawatū Gorge in 2011.
Geotechnical engineer inspecting the area.
In December 2019 we opened a temporary road, reconnecting State Highway 4 Raetihi to Whanganui. The temporary road runs across Ātihau-Whanganui Incorporated whenua.
SH4 light tower and solar panel.
What's happening now?
Permanent solution
In May 2020 we announced the reinstatement solution chosen to permanently replace the road affected by the slip. More detailed information on the options and chosen solution can be found in the State Highway 4 bulletin.
State Highway 4 bulletin – May 2020 [PDF, 3.3 MB]
The solution chosen for SH4 is the option that runs across a stabilised area at Te Oreore. The assessment of this option showed it scored particularly strongly with network resilience, reducing the frequency of road closures caused by natural hazards such as rockfall and washouts. It also scored positively in protecting indigenous plants and animals. This is because the option avoids disturbing any streams and will not remove any indigenous vegetation.
We are working towards construction of the new permanent road as soon as possible.
Slip area stabilisation
Stabilising work.
Following geotechnical investigations, we completed improvements to ground stability at Te Oreore by lowering the groundwater levels. This was achieved by removing water from the underlying soil and rock.
Our steps to increase stability and improve the safety of the slip area included:
- removing water from most of the slip area to keep groundwater levels down, using a series of pumping wells positioned at strategic locations along the site
- reinforcing the road subbase allowing SH4 to remain more intact during any future minor slope movement
- performing ongoing land movement and groundwater level monitoring to observe the impact on the stabilisation works or any natural hazard events, such as earthquakes
- installing a retaining wall along the slope to the north of the Te Oreore slip to improve support to the shoulder of the road.
Further works
In February 2023 we got started on some further works to ensure the safety and resilience of the route.
This work includes the construction of new retaining walls north of the main slip area and the drilling and construction of two new dewatering wells. This work is needed as a result of the heavy rain during winter which caused some ground movement at the slip site, reducing the temporary road to one lane.
The work will prevent further movement and safeguard the area so we can keep moving forward with the permanent replacement project.
