Experimental and DEM Examinations of $K_0$ in Sand under Different Loading Conditions

This paper reports $K_0$ of dry sand in response to different loading conditions and under secondary compression. The experiments were carried out using an oedometer and tactile pressure sensors were used to measure the horizontal stress for $K_0$ calculations. During loading, $K_0$ remains a constant as the vertical stress ${\sigma }_v^{\prime }$ increases. The corresponding discrete element method (DEM) simulations reproduced this experimental finding and suggest that the force transmission pattern is similar and the contact forces among particles are proportionally increased in both the horizontal and vertical directions to give a constant $K_0$ as ${\sigma }_v^{\prime }$ increases. $K_0$ increases as the overconsolidation ratio (OCR) increases during unloading because of the locked-in horizontal stress. Reloading makes the sample less sensitive to OCR and thus the sample behaves as if it were in a normally consolidated state where $K_0$ is a constant. Experimental results and DEM simulations demonstrate that the horizontal stress ${\sigma }_h^{\prime }$ and $K_0$ continue to increase during secondary compression on the loading path. Secondary compression is initiated by creep at particle contacts and exhibits similar responses like primary creep. The subsequent loading after secondary compression can gradually bring $K_0$ back to its initial value. During secondary compression on the unloading path, however, $K_0$ continues to decrease according to the DEM simulation results. The creep process tends to relax the locked-in horizontal stress and the sample expands in the vertical direction. Therefore, ${\sigma }_h^{\prime }$ and $K_0$ gradually decreases.