The Danger of Rockfall

Rockfall from the hills around Christchurch was a significant hazard during the Christchurch earthquake and continue to be a hazard for many living in the vicinity of the Port Hills. To learn more about rockfall hazards and mitigation techniques, William and Hamish attended an engineering geology course in the valley behind Sumner Beach, Christchurch.

The force of the Christchurch earthquakes dislodged many rocks from the hills which tumbled and bounced at terrific pace and height. The rocks originated from the fractured/weathered basalt outcrops spaced about the upper slopes in the area. As a result of the rockfall in the valley behind Sumner Beach, several homes downslope of the rockfall source were severely damaged and many houses were removed from the most hazardous rockfall zones. The processes observed during the earthquakes indicate that rocks do not always fall directly downslope and they can change direction substantially, so even if a house is not located directly below a rock outcrop, a property may still be at risk. Ground structure and dense bush were often proven not to be sufficient barriers to rockfall, with rocks able to weave around trees and jump/bounce over structures. The risk of rockfall in some areas of the hills around Christchurch was so significant that properties were 'red zoned' and rebuilding excluded.

There are many techniques that can be adopted if rockfall hazard is of concern. These include, but are not limited to the following:

1) Shifting rocks into a position where they are unlikely to roll downhill. This includes lowering rocks into holes via pulley systems, or simply digging a hole in front of a rock and rolling it into the hole.
2) Tying back fractured rock outcrops with steel rope or net systems
3) Stabilising rock faces with rock anchors (grouted or mechanical bolts etc)
4) Controlled blasting of unstable rock outcrops using explosives.

The potential for rockfall hazards should be assessed as part of a geotechnical assessment and mitigation measures can be determined to minimise the risk, if required. If you are concerned your property or future development may be at risk of rockfall or other natural hazards, KGA will be happy to help with a geotechnical hazard assessment and provide options for hazard mitigation.