Prediction of Landslide Potential Using Steady State Seepage in Unsaturated Soils
Karthikeyan MUTHUSAMY#+
Surbana Jurong Consultants Pte Ltd, Singapore

The stability of residual slopes is strongly influenced by climatic and hydrological changes such as precipitation, infiltration, evaporation and transpiration processes. Landslides triggered by rainfall are caused by the buildup of water pressure into the ground. Prediction of rainfall-induced landslides is problematic as it needs a full understanding of flow processes and engineering behaviour of soil within the unsaturated (or vadose) zone. It is generally accepted that the rapid rise of rainfall-induced pore-water pressure is critical to the initiation of slope failures. The loss of matric suction and the advance of the wetting front are controlled by the infiltration characteristics of the slope. An ability to model this process is critical to an accurate assessment of slope stability problems. One way to evaluate the effect of infiltration is to use an analytical formulation for the suction stress above the water table. On the basis of this analytical formulation and the classical infinite slope stability analysis, Lu and Godt (2008) proposed a first-order method for evaluating potentially unstable soil slopes subject to steady surface infiltration. In this paper, a simple practical method based on a modified form of Lu and Godt (2008) is proposed. The method evaluates an average antecedent rainfall intensity that could potentially trigger landslides above the water table.  A series of parametric studies were conducted to study the relative importance of soil properties, weathering characteristics, slope geometry, initial water table location in inducing of landslides. Several case studies are presented to illustrate the potential usefulness the proposed framework. The relationship between the predicted rainfall intensity versus observed rainfall intensity for the landslide events shows the reasonable agreement, given the significant simplifications involved, particularly the estimation of matric suction using steady state infiltration and the vague definition of observed rainfall intensity. The analytical framework proposed in this paper is capable of assessing the likelihood and precipitation conditions that induce the triggering mechanisms of landslides. This analytical framework could form part of a landslide warning system or provide a quantitative basis for landslide risk assessment.