Laboratory characterization of unsaturated soils for slope stability studies

Many slope failures in tropical regions are caused by rainfall infiltration. These slopes are typically in unsaturated residual soils with a deep ground water table. During rain, part of the rain infiltrates into the slope and the remainder becomes runoff. Due to rainwater infiltration, matric suction decreases resulting in a reduction in shear strength of the soil, causing slope instability. In this case, the net normal and shear stresses of the soil remain essentially constant during rainwater infiltration. The reduction in shear strength of unsaturated soils due to rainfall can be measured using shearing-infiltration tests in modified direct shear or triaxial apparatus. In this paper, two laboratory tests are examined in detail: shearing-infiltration tests in modified direct shear or triaxial apparatus and measurement of high suction. For the shearing-infiltration tests, the modified direct shear, the triaxial apparatus and the respective test procedure are described in this paper. In addition, the interpretation of shearing infiltration test results is discussed in detail. The results indicate a close relationship between the decreasing matric suction and the increasing deformation rate of soil. At the initial part of the infiltration process, there is a rapid reduction in matric suction that is accompanied by little movement in the soil. When failure of the soil is imminent, the soil movement will accelerate. It is interesting to observe that in some cases failure may occur even when a significant matric suction still exists in the soil. For the measurement of high suction, the chilled mirror hygrometer is used. The accuracy of the chilled-mirror hygrometer is evaluated by measuring the components of suction, matric suction and osmotic suction, independently. The results show that at low osmotic suctions, the sum of measured matric and osmotic suctions are lower than the measured total suction. The reverse is true at high osmotic suctions. The accuracy of the chilled mirror hygrometer depends on the degree of saturation of the soil specimen and may give relative humidity readings outside its measurement accuracy. A correction function is suggested for total suction measured using the chilled mirror hygrometer so that it agrees with independently measured matric suction and osmotic suction.