Stabilising slopes to prevent risk of slips

KiwiRail has a continuing programme of work underway to protect the network as much as we can from landslides in order to keep our services running.

We determine our slope stabilisation work programme by assessing and prioritising slip prone areas. Unfortunately, this work doesn’t remove the risk of slips entirely as they are unpredictable events.

While part of our risk assessment includes analysing rainfall prone areas, contributing influences for landslides are complex and extend well beyond just rainfall alone. In addition geotechnical assessment of slip risk area can't predict exactly where the next slip will occur. So, while we do have monitoring and mitigation in place, slips are always going to be a risk because of the nature of Wellington's terrain.

However our work to stabilise the areas we consider slip prone does reduce the risk of these areas incurring a landslides causing disruption to our services. The risk of slips is not limited to the rail network, and it affects both road and rail transport.

There are two kinds of slips that can affect our lines. Upslope slips which is the risk of rocks falling onto the track or the less common downslope slips where ground under the track might be at risk of erosion.

Over the past four years, about $10 million has been spent to mitigate risks at key areas on these lines – mainly around tunnels.

These include 3 sites on the Johnsonville line, at approximately $800,000, and 15 sites on the NIMT, where between $8m and $9m has been spent – at North-South junction in particular, but also on other areas of the line.

Stabilising slopes is not easy work, especially with our services running. We try and utilise the times when we stop or reduce services to undertake this work. Upslope stabilising work usually involves installing metal anchors deep into the rock face. These anchors hold rock fall netting in place. Once the anchors are positioned the netting is installed.