Macro and mesoscopic mechanical responses analysis of EPB shield under bias-load effects in soil-rock composite strata using FDM-DEM coupled method

To address the pronounced bias-load effects and the insufficient understanding of macro–meso mechanisms during EPB shield tunneling in soft–hard composite strata, this study develops an FDM–DEM coupled model to simulate shield–ground interaction. Based on this framework, the influences of geological conditions and advance rate on macroscopic mechanical responses are investigated. Meanwhile, meso -scale characteristics, such as force chain networks and fabric anisotropy, are introduced to qualitatively analyze the linkage between micro-structural evolution and macroscopic tunneling behavior. The results indicate that composite strata exhibit more pronounced responses in both over-excavation and bias potential, highlighting the strong influence of geological heterogeneity. As the advancement distance increases from 0.8 m to 8.0 m, the maximum contact force acting on the shield increases by approximately 2.1 times. Meanwhile, the number of contacts in the vertical direction decreases, leading to a reduction in the non-uniformity of contact distribution. The advance rate significantly affects meso -scale structural evolution. Higher advance rates result in a lower strong force chain ratio and higher fabric anisotropy, indicating a more unstable and reorganized internal structure. The influence of bias loading on meso -scale characteristics is mainly reflected in the alteration of their evolution trends rather than significant changes in their absolute magnitudes.