Pre-failure behaviour of slope materials and their significance in the progressive failure of landslides

The concept of pre-failure behaviour within a landslide mass was first established by Terzaghi (1950) who noted that, for landslide failure to occur, a progressive reduction in shear strength was required at the shear surface. Similarly, observations by Varnes (1978) noted that deforming materials undergo pre-failure creep as a result of deformation of the material. These observations have been developed by others into methods for predicting slope failures and landslides. Despite these prediction methods, the mechanisms by which prefailure creep occurs remains poorly understood. This paper develops a predictive pre-failure model through detailed analysis of surface movement patterns of first-time and pre-existing landslides. The analysis includes specialist pore-pressure re-inflation tests where elevated pore pressures are simulated under a constant deviatoric stress. The results demonstrate how subtle variations in pre-failure creep are related to material deformation and local porewater pressure conditions at the shear zone. This new research contributes to the development of site-specific ground behaviour and failure prediction models for complex landslides undergoing first-time and reactivation failures.