Shear Properties of Fissured Clay with Different Fissure Orientations under Plane Strain Conditions

An important characteristic of some clays is their abundance of fissures. In the case study reported here, to investigate how the fissure inclination angle affects the deformation and strength of fissured clay, samples of undisturbed fissured clay with different inclination angles of its inherent fissures (0°, 45°, and 90°) were subjected to consolidated undrained plane-strain shear tests using a true triaxial apparatus. Moreover, consolidated undrained triaxial tests were carried out on samples with the same inclination angles for comparison. The results showed that compared with the triaxial state, the degree of fissure influence on samples with different fissure angles is different under plane strain, which weakens the influence of the fissure inclination angle on the soil’s mechanical behavior. Under the designed consolidation pressures, the peak stress of the 45° fissured soil samples was the smallest, with a stress–strain curve that exhibits strain softening. The 0° fissured soil samples exhibited the highest peak stress, with a stress–strain curve that exhibits strain hardening. The 90° fissured soil samples fell in between, with a stress–strain curve that exhibits a relatively stable trend. The intermediate principal stress coefficient b-value showed different trends at different fissure angles, which also reflects the influence of fissure dip angle. According to the von Mises and Lade–Duncan strength criteria, the generalized plane-strain criterion for fissured soil was obtained. The dip angle of the shear band was calculated from Mohr–Coulomb theory, and the difference between the calculated and measured dip angles was found to be small.