Managing a ground stability hazard in a residential area

Landsliding was discovered to be the cause of longstanding periodic disruption to roads and services in a coastal residential area on the Coromandel Peninsula of New Zealand’s North Island. Investigations and monitoring of the landslide complex, which covers about 50 hectares in area, commenced in 1996 and continued through to 2011. The investigations showed both inactive and slow moving landslides in volcanic geology, and movement is maintained by high groundwater pressures and the slow removal of toe material by coastal erosion. The landslides are ancient and include shallow and deep seated components. Stabilising the main landslide feature is deemed not to be feasible due to the depth of the slide plane, and a method of assessing and managing risk to properties was developed in conjunction with landowners and the local council. The geology, ground and groundwater conditions, triggers for landslide movement, landslide effects on infrastructure, movement monitoring and risk management strategy adopted are described.