The Bogong Landslide, a case study into an Australian landslide in unsaturated soils

Over the course of two weeks between 28 September and 12 October 2022 a landslide estimated to have mobilised a volume of 30,000 m³ of soil occurred on north-facing slopes near Bogong Village in Victoria. Debris arising from the landslide blocked Bogong High Plains Road – the main road to one of Australia’s largest ski resorts, Falls Creek, and forced the evacuation of Bogong Village downslope of the landslide. The road needed to be re-opened before May 2023 to allow access to the resort for the 2023 ski season, it being the only road to the resort. Understanding the processes that caused the landslide became critical to informing the design and safe execution of interim works to re-open Bogong High Plains Road. Various lines of evidence including video of the landslide occurring, geomorphological evidence, investigations and analysis informed the development of an engineering geological model for the landslide. The model indicated anomalously deep extremely weathered micaceous granodiorite with a high void ratio at the sheared contact between two plutons. Perhaps owing to their northerly aspect and high evapotranspiration, the soils comprising these slopes preserve high soil suctions. A loss of soil suction arising from wetting fronts that developed over the course of three successive La Niña years was assessed to have triggered the landslide. On the basis of this model, interim works were designed to safely remove a sufficient volume of soils to re-open a single lane of the road. Following the interim works, permanent landslide mitigation was provided, the basis of which was to promote the preservation of soil suctions whilst preserving the natural landscape adjacent to Alpine National Park as far as practical. Understanding the response of the unsaturated soils to prolonged rainfall was critical to informing the design of interim and permanent landslide mitigations. This paper describes the development of the engineering geological model for the Bogong landslide and how the model then informed interim and permanent works to reopen the road and to prevent further landslides.