Abstract: As an important mechanical parameter of the water-soil interaction in unsaturated soil, matric suction is of great significance for researching the shear strength of unsaturated soil and the stability of landslides induced by changes in rainfall or reservoir water level. In this research, the effect of rainfall-induced change in matric suction on the slip soil in limestone zone was examined via GDS triaxial test system with standard stress path. Matric suction was set different at four values and net confining pressure three levels. The curves of matric suction versus maximum shear stress, strain versus shear stress, and net confining pressure versus shear stress were obtained and further analyzed using the variable double stress principle of unsaturated soil strength theory. Results evinced that under the same matric suction condition, shear strength increased rapidly with the climbing of confining pressure at first and when shear strain reached 10%, such increase attenuated. Under the same confining pressure and unsaturated conditions, shear stress rose with the increase of matric suction. Total cohesive force is in a linear relation with matric suction. The slope of this linear relation, which also equals the related angle of matric suction, is 15.5°. When matric suction is 0 kPa, 30 kPa, 60 kPa and 90 kPa, respectively, the total cohesive force is 22.7 kPa, 33.4 kPa, 43.8 kPa, and 46.9 kPa, respectively, and the corresponding internal friction angle is 21.8°, 23.6°, 26.2°, and 24.5°. According to Fredlund and Ralaadjo's double stress strength theory of unsaturated soil, a modified formula for the shear strength of the slip soil of the studied landslide is proposed for reference.

Key words: slip soil, matric suction, weak interlayer, double stress intensity theory, net confining pressure