Wetting Induced Deformation of Soils Triggering Landslides in Pakistan

Shallow and deep seated landslides in natural slopes are often induced by rainfall. The cause of the failure is usually considered to be due to the decrease in effective confining stress due to a suction loss by water infiltration. However, rainwater infiltrates into the slope surface resulting in a reduction of shear strength of soil and deformation and failure may occur even without significant change in effective stress. It is thus essential to examine the deformation and failure characteristics of the soil induced by wetting. This study investigates on the wetting-induced deformations under isotropic compression condition and triaxial shearing condition using a triaxial test apparatus for unsaturated soils. Two soil samples collected from the active landslide sites from Pakistan were used to examine both the shear strength and deformation behavior during water infiltration at different levels of deviatoric stress by keeping the deviatoric stress constant during the water infiltration stage. The test results showed that, even though the deviatoric stress is kept constant during water permeation at different stress levels, the distortional deformation was exhibited due to wetting by a significant amount for both samples. The effect of water infiltration on the deformation behavior of unsaturated soil regarding the change in the degree of saturation and void ratio was also observed. There was a decrease in the void ratio for both specimens during increasing the degree of saturation at different level of deviatoric stress ratio. Under higher deviator stress, more decrease in void ratio and an increase of the degree of saturation were observed. Therefore, it can be said that deformation of soil due to water infiltration is a critical phenomenon and it should be considered while analyzing the soil behavior due to water infiltration by rainfall or rise in groundwater level, even if the slope is not failed, significant deformations may hinder the performance of natural slopes.