K0 of granular soils: a particulate approach

The coefficient of earth pressure at rest K-0 of granular soils is revisited by discrete element method simulation. The effects of confining pressure, soil density and over-consolidation ratio (OCR) on the K-0 value are investigated. The coefficients of earth pressure at rest defined in the conventional form K-0 and in the incremental form K'(0) are compared. Most importantly, the microstructure of the soils (i.e. coordination number, anisotropies of contact normal and contact force) is monitored during the tests and it is used to explore the microscopic factors affecting the K0 value of soils. The results suggest that K-0 and K'(0) generally approach a similar constant when the applied vertical stress is 6-10 times and 4 times of the initial isotropic stress, respectively. Meanwhile, the K-0 value decreases as the void ratio decreases and the vertical stress increases. The OCR has a significant effect on the K-0 value and for the same OCR the K-0 value is quite different during unloading and reloading. Better predictions are obtained by Jaky's equation and Mayne and Kulhway's equation if peak friction angle is used. The analyses indicate that the K-0 value depends on the coordination number of the soil, which is consistent with its density-dependent macroscopic behavior. The results also indicate that the K-0 value is closely related to the anisotropy coefficients of contact normala alpha(r), normal contact force alpha(n) and tangential contact force alpha(t). During the initial K-0-loading, alpha(r), alpha(n) and alpha(t) keep nearly constant and the evolution of K-0 during unloading and reloading is mainly resulted from the evolutions of alpha(n) and alpha(t). It reveals that the particle rearrangement is negligible and the soil mainly adjusts the particle contact force to resist the external load during the K-0 loading and unloading.