Rainfall-induced landslides: Development of a simple screening tool based on rainfall data and unsaturated soil mechanics principles

There is now widespread acceptance that a key trigger mechanism for shallow landslides is the reduction in soil suction that occurs as a wetting front moves through an initially unsaturated soil profile. Slopes often remain stable at angles steeper than would be predicted using effective stress strength parameters. Such slopes may remain stable for many years, if not decades, with failure only being triggered when moisture conditions within the slope reach a critical threshold. Our understanding of the role of unsaturated soil mechanics in slope stability is now well developed. This paper uses this knowledge to suggest techniques that may be used to provide suitable management tools, the intention being to predict the level of risk associated with a particular slope. The technique combines statistical rainfall data, complemented by confirmation studies using numerical modelling (utilising commercially available software SVFlux and SVSlope), to provide a screening tool that takes account of antecedent conditions; a critical aspect for providing appropriate risk evaluation capability. An approximate approach,which utilizes analytical solutions, is also described. To implement the risk management strategy discussed in the paper, some form of in situ monitoring is required and two alternatives are briefly described: discrete monitoring using buried instruments; and remote sensing of soil water status.