Greymouth township is located at the outlet of the Grey River on the West Coast of the South Island. Urban expansion is currently encroaching on the steep (20+°) hill country underlain by westerly dipping Nile Group limestones and massive Blue Bottom Group mudstones. This study was an engineering geological investigation and landslide hazard assessment of the Greymouth urban area. The purpose of such a study was to identify areas prone to landsliding in order to establish areas that are suitable for future urban development.

Landslides were identified from aerial photographic interpretation and engineering geological mapping at a scale of 1:10000 and were classified as translational rock slides and rock block slides, falls, rock avalanches, translational debris slides, rotational debris slumps and flows. Limited geotechnical testing was undertaken for selected rock and soil lithologies with the objective of characterising bedrock and soils in the area.

Two landslides, a translational rock slide (Stanton. Crescent Slide) and a translational debris slide (Australasian Hotel Slide), identified by the author were mapped in detail at scales of 1:1500 and 1:1000 respectively. The mode of failure for both of these landslides was determined and general failure models were developed from field investigation.

From an historical data base compiled as part of this study, it was concluded that intense or prolonged rainfall is the primary initiating agent in the development of landslides involving surficial materials. Field mapping suggests that bedding within the mudstone is an important defect along which failure takes place. Earthquakes have also been important in initiating landslides and will be in the future.

An engineering geological data base including engineering geological mapping (1:10 000), aerial photographic interpretation, geotechnical characterisation and the information that was obtained by the two site specific studies was compiled, from which landslide hazard zonation and land-use suitability maps at a scale of 1:10 000 were derived. These identify the most landslide prone areas as the coastal escarpment and other slopes underlain by mudstone and the Twelve Apostles Range. In addition, these maps provide a guide as to the geotechnical limitations to development and the level of additional site investigation that may be required in a given area.

Recent changes in government legislation has seen a growing awareness by local administering authorities of the need for sound engineering geological information on which to base land-use planning. It is anticipated that the local authority will use the information contained in this study for the administration of land-use planning in the Greymouth urban area.