Micro mechanism and analytical model of stress distribution in loosened soil zone above active trapdoor

The formation of discontinuities within soil masses due to arching phenomena is a critical issue in geotechnical engineering, profoundly influencing the stability and integrity of underground structures. This study investigated the microscopic mechanisms and associated stress redistribution in the soil mass above an actively moving trapdoor. Utilizing the Discrete Element Method (DEM) model, the influence of varying initial soil packing densities on the induced arching phenomena is examined concerning the soil displacement mechanism and shearing configuration: transfer from inner triangular to vertical bands is observed in dense soil while only vertical shearing bands are observed in initially loose soil. To account for principal stress trajectories under differing initial density conditions, an analytical model was developed to quantify the variations in soil stress. The findings reveal a noteworthy impact of the initial soil void ratio $e$, on the pattern evolution and stress redistribution of soil arching, manifesting a passive stress limit coefficient, $K_p$, in denser soils along the central line above the trapdoor. The analytical model demonstrates robust agreement with both numerical and experimental data in soil loosening stress distribution and the load–displacement response of the trapdoor.