Abstract: Shear strength is an important mechanical parameter that determines the stability of soil and structures, such as foundations and slopes. Its value is generally obtained via direct-shear or triaxial tests. In these tests, three or four samples are typically used to measure the cohesion and internal friction angle of soil; the differences among samples will inevitably affect the accuracy of the results. Hence, this study improves the conventional direct-shear instrument so that it can be used to obtain the shear strength of a sample via repeated shears as well as the cohesion and internal friction angle of soil using the Mohr–Coulomb envelope. The results of multistage direct-shear tests for the consolidation drainage of compacted soil are compared with those of conventional direct-shear tests. The results show that the shear strength obtained from the multistage direct-shear test is slightly lower than that obtained from the conventional direct-shear test. Additionally, the error for the internal friction angle is ±2.5◦, which verifies that the multistage direct-shear scheme of consolidation drainage can be used to evaluate the shear strength of compacted soil.

Key words: multistage direct-shear test, conventional direct shear test, shear strength, Mohr-Coulomb envelope, cohesive force, internal friction angle