Investigation of slope movements in a coastal sand dune in South-East Queensland

This paper presents the results of monitoring and investigation of slope movements which occurred in a coastal sand dune in South East Queensland, at a time of elevated groundwater levels. The magnitude and pattern of displacement suggest that a Factor of Safety of very close to 1 developed as a result of the rise in groundwater levels. Subsurface investigations identified the presence of a layer of former swamp deposits (sandy clay and clayey sand) beneath the dune, which has been interpreted as the layer in which shear displacement was concentrated. Finite element and limit equilibrium analyses using a Mohr-Coulomb constitutive model yield consistent results, and indicate that a n angle of friction of 11° is required for a Factor of Safety of 1 for a failure mechanism which passes through the former swamp deposits. Such a low friction angle is inconsistent with limited laboratory testing on the material. Further work is being undertaken to better understand the mechanisms which led to the development of large scale movements, i n order to better predict the potential future development of such movements.