Slope Stability in Weathered Basalts

Tertiary basalts are widespread along the eastern Australian continental margin, extending from The Torres Strait to Melbourne in the south. In many instances, they have been extruded on, or adjacent to the Great Dividing Range, and remain prominent in the steep and elevated topography. By their nature, they weather deeply and rapidly, to form soils with undesirable engineering properties, which in the geomorphically-immature topography of the Great Dividing Range and the Great Eastern Escarpment, leads to engineering challenges for transportation infrastructure. This paper provides a description of the occurrence of these basalts around the margins of the Hunter Valley, with consideration of their control on topographic stability and geomorphic evolution. Factors such as their bedded structure, the presence of palaeosoils, and their tendency to creep are described and evaluated in the geological profiles in which they are encountered, and the engineering problems associated with them are discussed. Experiences from a recent project involving the remediation of a road, affected by basalt-hosted slope instability, are presented and discussed to provide insights into good and bad engineering practices in these geological environments.