Abstract: The abundance of fissures is one of the important characteristics for expansive soils. The inherent fissures and fissure propagation during the loading process have a significant impact on the mechanical behavior of the soil. In order to investigate the influence of fissures on the deformation and failure mode of expansive soil, with the help of an improved true triaxial instrument, the undisturbed expansive soils with different inclination angles of inherent fissures (type I - the long axis of the sample is perpendicular to the dominant fissure direction, type II - the long axis of the sample is 45º oblique to the dominant fissure direction, and type III - the long axis of the sample is parallel to the dominant fissure direction) were used to conduct the consolidated drained plane-strain shear test. Meanwhile, the deformation was analyzed using digital image correlation (DIC) technology, focusing on the local deformation characteristics controlled by fissures. The results show that under the same confining pressure, the peak stress of the type-II fissured soil sample is the smallest, the stress−strain curve is of strain softening type, and its failure type is sliding failure. The peak stress of the type-I fissured soil sample is the largest, and the stress−strain curve is strain hardening type. The failure type of the type-I fissured sample is compression-shear failure, and the type-III fissured sample presents different failure forms due to different confining pressures. Under the same confining pressure, the inclination angle of the shear zone produced in the type-I fissured samples is smaller than that of other fissure types and basically does not change with the confining pressure. The shear zone of the type-II fissured sample develops along the original fracture surface, and its dip angle has no obvious regularity with the confining pressure. The confining pressure affects the number of shear zones developed in the type-II fissured samples. For type-III fissured samples, the confining pressure affects the development type of the shear zone, and as the confining pressure increases, the dip angle of the main shear zone decreases. Based on Roscoe’s theory, the inclination angle of the shear zone in the samples with different fissure directions is more in line with the inclination angle of the shear zone when the sample is actually damaged. This test lays a foundation for studying the anisotropic mechanical properties of fissured expansive soil.

Key words: fissured expansive soil, plane-strain test, digital image correlation, shear zone evolution