Variations in excavation construction periods for fissured soil transportation engineering lead to differing unloading rates, which affect the soil's mechanical properties. This study utilizes a triaxial testing system to conduct monotonic and cyclic loading undrained shear tests on undisturbed fissured samples as well as remolded samples subjected to three distinct unloading rates. The K0 consolidated samples are regarded as soil mass that undergoes no unloading during testing. The findings indicated that the initial unloading rate influences the reloading shear mechanical properties of undisturbed and remolded specimens. The effects of unloading rates differ between undisturbed and remolded soil, a discrepancy attributed to inherent fissures. Specifically, undisturbed soil exhibits significant damage at low unloading rates due to fissures, while remolded soil experiences strength augmentation due to compaction with decreased unloading rates. Similarly, unloading will cause a loss of strength. Structural disparities result in the monotonic loading strength of undisturbed specimens being higher than that of remolded ones. In contrast, remolded specimens demonstrate greater dynamic strength under cyclic loading, likely because fissures deform, diminishing overall dynamic strength. Subsequent microscopic analysis, utilizing SEM images, along with a discussion of macroscopic inherent fissures, elucidated the impact of unloading rate on soil damage mechanisms, advancing the understanding of fissured soil behavior post- unloading. The study of mechanical properties of fissured soil following varying unloading rates is crucial for comprehending its damage mechanism and determining post-unloading soil strength parameters, providing valuable insights for practical applications in soil engineering.

In geotechnical engineering, the small-strain shear modulus and its attenuation characteristics are pivotal for analyzing and evaluating soil vibration responses to various engineering construction projects. This study conducts the resonant column test on undisturbed fissured clay samples, exploring the impacts of fissure inclination and confining pressure on the shear modulus in small-strain range. Results indicated that the shear modulus and its attenuation behavior in undisturbed fissured clay are substantially affected by both the fissure inclination angle and the confining pressure. With constant confining pressure, the shear modulus increases as the fissure inclination angle grows, reaching its maximum value at a fissure angle of 90 degrees. In addition, as the confining pressure rises, there is a notable increase in the shear modulus and a corresponding reduction in the decay rate. Through the threshold strain, the elastic deformation of the specimen increases as the fissure inclination angle increases, and the confining pressure increases the ability of the fissured soil to deform at small strains elastically. Based on the acquired data, this research analyzes the relationship between the fitting parameters A and N and the fissure angle in the context of the Harding-Drnevich formula. Consequently, a mathematical model based on the fissure inclination angle and the effective confining stress was established to predict the maximum dynamic shear modulus (Gmax) and decay attributes of undisturbed fissured clay. Additionally, the study offers a comparative analysis of the maximum shear modulus and its attenuation features in clay with varied degrees of fissure development. The stiffness anisotropy is related to the orientation of particles and the normalized decay rate of the fissured clay has a certain relationship with the fissure density.

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 degrees, 45 degrees, and 90 degrees) 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 degrees fissured soil samples was the smallest, with a stress-strain curve that exhibits strain softening. The 0 degrees fissured soil samples exhibited the highest peak stress, with a stress-strain curve that exhibits strain hardening. The 90 degrees 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.