Experimental study on dynamic characteristics of saturated undisturbed expansive soil with different fissure orientations under cyclic loading

Fissure orientation is one of the important factors contributing to the anisotropy of expansive soil mechanical properties. To explore the influence of fissure inclination angle on the dynamic properties of soil, this study used undisturbed expansive soil with different fissure inclination angles (α=0°, 22.5°, 45°, 67.5° and 90°) in the Shihang Irrigation Area of Anhui Province as the research object, and the unidirectional cyclic load test was carried out on the soil by using the GCTS dynamic and static true triaxial instrument. The effects of fissure inclination, confining pressure, and different dynamic stress amplitudes on the characteristics of cumulative plastic strain, dynamic elastic modulus and damping ratio of expansive soil were studied. The test results show that as the cyclic load amplitude increases, the cumulative strain of the sample with the same fissure angle increases continuously. Among the studied five fissure angles, the α = 67.5° sample will produce greater cumulative plastic strain at each cyclic load amplitude. Under low confining pressure, the fissure angle greatly influences the critical dynamic stress amplitude of the soil, and its anisotropy characteristics are significant. As the confining pressure increases, the anisotropy of the soil weakens. With the increase of the fissure angle, the dynamic elastic modulus of the sample shows a trend of decreasing first and then increasing. When α = 67.5°, the dynamic elastic modulus of the sample is the smallest, and the dynamic elastic modulus decreases rapidly when the amplitude of the cyclic vibration load increases to the critical dynamic stress. According to the characteristics of the damping ratio changing with the number of vibrations, this paper proposes an expression model that can consider the characteristics of the attenuation rate, initial damping ratio and stable damping ratio, and analyzes the influence of dynamic stress amplitude and confining pressure on the variation of damping ratio. The research results are of great significance for revealing the anisotropy of the mechanical properties of fissured expansive soils under dynamic loads and ensuring the safe operation of railway routes and rail transit in expansive soil areas.