Effect of changes in matric suction on slope stability in natural unsaturated soil

Abstract

Stability analyses were performed for a real engineering-geological cross-section of unsaturated Quaternary silt sediments. Unsaturated shear strength is a function of the two stress variables: net normal stress and matric suction. Constitutive equations (unsaturated shear strength - matric suction) and (angle ∝b - matric suction) were defined for these silty soils. These equations were established on the basis of primary constitutive relationships for unsaturated soils by soil-water characteristic curves (effective degree of saturation - matric suction). Soil-water characteristic curves were obtained from results of experimental tests on draining saturated soil samples under different pressures, performed for the first time in Serbia, in a 15 bar pressure plate extractor, according ASTM. Effective shear strength parameters c' and ∝' were also experimentally obtained from direct shear tests. Stability analyses were performed using the GLE method for different climate conditions, before and after rainfall, i.e. for different values of matric suction. Stability analyses were performed for the same groundwater level, too. It was confirmed that rainfall decreases the angle (pb and stability of natural unsaturated soil slopes. Decreasing the ∝b angle of unsaturated soil, due to rainfall, decreases the safety factor of the slope faster for coarse-grained soil than for finegrained soil.