Methodology for Calculation of Rain-Induced Slides

A large number of slides occurred in the South-Eastern part of Norway during the autumn of year 2000. The slides were triggered by extreme duration of rainfall, leading to saturation of slopes that are partly unsaturated under normal conditions. On this background, a study aiming at establishing a methodology for assessment of stability of slopes prone to rain-induced landslides was started. The suggested methodology for calculation of the effect of rainfall on slope stability is described and applied on a typical example (model slope). Meteorological data together with assumed soil properties are used to define a yearly normal infiltration function and similar functions for extreme events. The normal infiltration function is used to calculate soil water content variation through a normal year using the computer code SEEP/W (2004). Extreme events are analysed by starting calculations from normal conditions. The resulting pore pressures are used to calculate safety factors for slopes during normal and extreme events. Stability of the model slope is calculated using effective stresses based on changes in pore pressures and water content in the undrained and drained zones. Calculations have been performed for the normal year, for the 100-year situation, and for the extreme situation represented by the autumn of year 2000 for both shallow translatory slides and deep circular surfaces. The calculations have demonstrated a significant reduction in safety factors for both deep seated and shallow potential failure surfaces. A tentative conclusion from these calculations is that the safety level (margin) for climatically influenced slopes should be minimum in the order of 0.2 when considering the normal year situation (i.e. FS in a normal year > 1.2). This will ensure that stability of deep-seated shear surfaces do not drop to a critical level during a period of extreme rainfall, like the rain experienced in the South-Eastern part of Norway during the autumn of year 2000. For shallow slides, the safety margin must be evaluated based on top layer material properties.