Analysis and Validation of Effective Stress Parameters on Shear Strength for Unsaturated Soils

To investigate the influencing factors of effective stress parameters on shear strength for unsaturated soils and quantitatively verify their values, this study systematically analyzed 264 suction-controlled triaxial shear tests and 102 direct shear tests on unsaturated soils from both domestic and international sources. This research focused on the magnitude and governing factors of effective stress parameters within the commonly encountered suction range (0-1500 kPa), quantitative verification and clear assessment of four types of effective stress parameters. The key findings are as follows: 1) Within the suction range of 25-400 kPa, more than 26.5% of triaxial tests results show effective stress parameter values exceeds the theoretical upper limit of 1, espically a maximum mearsured value of 2.63, breaking through its maximum limit of 1, which is not incidental. The reason is that the effective stress method simply attributes the influence of suction on the shear strength of unsaturated soil to friction and ignores the influence on cohesion; 2) In the suction range of 25-450 kPa, over 15.5% of triaxial tests and 23.5% of direct shear tests yielded effective stress parameter values within the range of 0 to 0.35, which is far lower than 1. The mechanism is related to the increase of the structure and internal friction angle of the undisturbed soil with the increase of suction. It shows that low suction does not necessarily correspond to high effective stress parameter values, which is also a non-random occurrence; 3) Considering ±10% relative error threshold as the accuracy criterion, there is a significant deviation of four effective stress parameters, i.e, saturation degree at failure, effective saturation degree at failure, and two published empirical equations, from the experimental values, with qualification rates below 20%. It is found that these four effective stress parameters substantially overestimate or underestimate the contribution of suction to the shear strength of unsaturated soils, which seem improper to act as effective stress parameters; 4) The effective stress parameter on shear strength depends complexly on soil type, suction magnitude, net confining pressure (or vertical net stress), and stress path, reflecting soil viscosity, pressure sensitivity, dilatation and structural properties. Due to the absence of clear patterns, direct measurement of effective stress parameters via suction-controlled triaxial drained shear or direct shear tests remains the currently feasible method.