Effective stress interpretation of direct simple shear tests using Mohr’s circle and finite element analyses

The direct simple shear (DSS) test is often employed to assess the undrained strength of fine-grained soils or the liquefaction behavior of coarse soils. Effective strength parameters are not derived from it because the state of stress within the specimen is not considered sufficiently well defined or uniform enough to allow a rigorous interpretation of the results. In the DSS, the failure plane is not imposed, and the principal stresses are not measured, unlike in the direct shear or triaxial tests, respectively. In the past, the DSS was often dismissed because comparisons with other types of tests indicated that it produced lower effective strength parameters. However, this paper presents results of DSS with stacked rings and direct shear tests that are in good agreement with each other. A finite element analysis and Mohr’s circle representations are used to assess the internal stresses and strains in the DSS specimen and to locate the failure plane. Based on these results, a reinterpretation of other works is conducted. It is shown that the stress distribution becomes approximately uniform after the early phases of the shearing, and the inclination of the failure plane can be predicted. It is also discussed how the use of low height-to-diameter ratios may influence the results. Thus, the DSS with stacked rings may be considered suitable to determine effective strength and deformability parameters, provided an appropriate height-to-diameter ratio is employed, and the results can be expected to agree with those from other types of tests.