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.