Abstract: Against the backdrop of the rapid development of urban underground engineering, tunnel construction faces challenges from complex geological conditions. The cobble-gravel stratum, a common but adverse geological formation, poses a threat to the stability of tunnel surrounding rocks and construction safety due to its weak inter-particle cohesion, sensitivity to environmental disturbances, etc. This study, based on the Nanjing Shangyuanmen River-Crossing Tunnel Project, employs a coupled FLAC3D and PFC3D modeling approach to develop a hybrid continuum-discrete numerical model to investigate the mechanical response characteristics of shield tunnel surrounding rocks under overlying cobble-gravel strata conditions, with a comparative analysis against conditions without cobble-gravel strata. The results show that the hybrid model effectively reveals the complex mechanical behavior of the cobble-gravel strata. Excavation disturbances induce a shear dilation effect in the cobble-gravel strata, exerting additional pressure on the surrounding rock mass, leading to stress redistribution. In the model with the cobble-gravel strata, both tunnel crown settlement and segment deformation are greater than those in the model without cobble-gravel strata, with the settlement range approximately twice the width of the cobble-gravel strata. Surface settlement follows a Gaussian curve, with the presence of the cobble-gravel strata slightly increasing the settlement magnitude. Furthermore, particle contact force analysis reveals that excavation disturbances weaken inter-particle interactions in the cobble-gravel strata, further confirming the existence of a shear dilation-pressure mechanism. In conclusion, the cobble-gravel strata significantly impact local mechanical responses, necessitating further checks on deformation and stress in tunnel design to ensure structural safety. This study provides a theoretical basis and technical support for the design optimization and risk control of shield tunneling in complex geological formations.

Key words: shield tunnel, cobble-gravel strata, numerical simulation, coupled continuum-discrete model, underwater