Analysis of the mechanism of landsliding, Garie Road, Royal National Park, NSW

Once landslide movement is identified, it is important to employ the necessary investigation techniques to characterise the landslide type and analyse the mechanism of mass movement. Defining the geometry of the landslide slip surface and presenting this with confidence within the engineering geological model is fundamental to the development of the appropriate remedial design solution.

A large landslide developed along Garie Road, Royal National Park in March 2022 following a series of intense rain events. Tension cracking along the centreline of the road progressed quickly to large scale deformation as a slow-moving landslide developed retrogressing to the south and west. This paper presents a case study of the engineering geological and geotechnical assessment of the Garie Road landslide using various investigation techniques including drone terrain evaluation, landslide mapping, detailed core logging and instrumentation review. The data collected to analyse the landslide mechanism is presented and validated through a literature search and comparison of a similar Illawarra escarpment landslide. The comparative study was fundamental in identifying similar basal sliding in close proximity to the interface between the Bald Hill Claystone and the Bulgo Sandstone.