Determination of the related strength parameters of unsaturated loess with conventional triaxial test

Abstract

The strength and the related parameters of unsaturated soil are the key issues in unsaturated soil mechanics. The triaxial or direct shear apparatus with suction control is used to measure the related parameters in the strength formulas proposed by Bishop and Fredlund. However, the technique is time-consuming for cohesive soils because it takes a long time to achieve a water–air equilibrium condition under a given suction state. In addition, the feasibility of the technique is also doubted. Lu et al. proposed the concept of suction stress and expressed the unsaturated strength as a function of suction stress which can be determined by the conventional triaxial test. The method can avoid measuring matric suction and is convenient in engineering application. In this paper, conventional triaxial CU tests with different moisture contents were utilized to obtain the effective steady state strength parameters and then the suction stress characteristic curves were obtained. To compare Lu et al.’s strength formula with Bishop’s and Fredlund’s, the soil–water characteristic curve of the same loess sample was also measured by the tensiometer. Theoretically, the three strength theories are different in physical mechanisms, but they can transform to each other mathematically. From the point of view of engineering applications, Lu et al.’s shear strength theory based on suction stress is more convenient to apply in engineering.