Unsaturated Soils and Rainfall Induced Landslides

Abstract

Seepage and slope stability issues concerning infiltration in unsaturated slopes are investigated and presented. 2-D finite element analyses are used to study the effects of the different hydraulic characteristics of a fine and coarse grain soil. The influence of the saturated coefficient of permeability (ks), the air entry-value (a) and the desaturation coefficient (n) are studied. The results are showing how the changes in negative pore-water pressures in the model slope are controlled by the hydraulic properties of the soil and the initial conditions within the slope. For the coarse soil, the zero pore-water pressure surface moves gradually upslope with time. In the fine grain soil model, the rate of suction loss is nearly the same at the toe, in the middle and at the top of the slope. These different infiltration patterns are leading to different types of failure surfaces. For a coarse grain soil, slip may initiate at the foot of the slope as a consequence of positive pore pressure build-up. The fine soil model is prone to a loss of matric suction along its entire length. In this case, the infiltration pattern may lead to shallow translational type of sliding. It is also found that the geometry of the slope affects more the pore pressure distribution in a coarse grain soil than in a fine grain one. Rainfall-induced landslides in unsaturated soils are frequent in the tropical and subtropical regions of the world. However, temperate regions are also prone to this type of failure and are attracting increasing attention of the geotechnical community. In these regions, natural slopes are constantly subjected to changing environment; from dry summer period to rainy fall and from cold winter to wet spring. The majority of these slopes can be considered as being unsaturated during normal conditions. A numerical back-calculation of a landslide in the small community of Åmot in Norway is performed in this thesis. The 38 degree slope failed in November 2000 after an extremely wet fall. The landslide threatened and endangered more than 24 houses and their residents. To analyse this failure, the author determined the SWCC of the soil in the laboratory, and geophysical investigations on the slope during the summer 2004 to determine groundwater level and water contents. In the 2-D finite element analyses, the rate of infiltration was based on meteorological data for a normal year and for the year of 2000.The modelled slope is found to be stable under normal rainfall conditions but failed when subjected to the rainfall conditions of the year 2000. The calculations are showing that the failure is due to a lowering of the suction and frictional strength during the intense rainfall. The calculated time to failure is in agreement with the observed failure and shows that the event could have been predicted.