Microscopic Insight into the Soil Fabric During Load-Unload Correlated with Stress Waves

Soils have a complex contact network (i.e. soil fabric), which affects the mechanical characteristics (e.g. stiffness anisotropy, shear strength) and further determines the performance in geotechnical constructions. Conventional laboratory tests on soils can give macro-scale mechanical responses as a physical relevance, while the discrete element method (DEM) has been used as a numerical tool to gain insights into micro-scale mechanical characteristics such as soil fabric. In this study, laboratory element tests are conducted using spherical glass beads subjected to several cycles of load-unload reversals at selected pre-peak strain ranges, where stress waves are continuously measured during the loading process. DEM is adopted using spherical particles to simulate the equivalent load-unload reversals and wave propagations. Based on the experimental and DEM results, the compression velocity (V_p) and shear wave velocity (V_s) are found to be approximately reversible when the load-unload cycles are at low stress levels, while a significant reduction in both velocities can be observed at higher stress levels. The variation of soil fabric anisotropy during load-unload reversals is explored using DEM; the fabric anisotropy increases markedly beyond a stress ratio threshold of 1.8 and cannot be fully recovered during the subsequent unloading process. Fabric anisotropy is found to be linearly correlated with wave velocity ratio (V_p/V_s) at the pre-peak stage, although both quantities are respectively affected by the given stress states.