Mechanical behavior of tunnel excavation at great depth in coarse-grained soil-rock backfills: insights from FDM-DEM simulations

Tunneling through deep backfills composed of coarse-grained soil-rock mixture (SRM) poses critical engineering challenges due to their loose structure, high porosity, and large block inclusions. These deep, fractured, and highly heterogeneous fill materials exhibit complex mechanical responses, making traditional design and predictive methods insufficient. This study employs a coupled finite difference method-discrete element method (FDM-DEM) framework to investigate excavation-induced deformation and stability in deep SRM backfills. Parametric simulations quantify the effects of excavation method, burial depth, tunnel diameter, and rock block content on ground response. The study also examined the role of pipe roof support parameters, including diameter, spacing, and installation angle, in mitigating deformation and fracture propagation. Results demonstrate that the reserved core soil method (RCSM) provides enhanced stability, reducing the ultimate crown collapse by up to 0.28 m compared with the full-face method and promoting a more uniform redistribution of stress. Increasing burial depth leads to larger displacements and a transition in failure mode from basal shear to crown-dominated collapse. Higher rock block content enhances interlocking and overall stiffness while shifting deformation toward the sidewalls. Systematic optimization of the pipe roof support shows that a configuration with a 127 mm diameter, 400 mm spacing, and a 5° installation angle provides the best overall performance, reducing surface settlement by up to 41.5%, limiting grout-body cracking, and improving control of shear concentration at the vault-shoulder interface. These findings provide a quantitative and mechanism-based framework for tunnel support optimization in coarse-grained SRM deep backfills. The coupled FDM-DEM approach not only captures the multi-scale mechanical response of SRM but also bridges the gap between advanced numerical methods and empirical engineering design, thereby enhancing stability and reducing construction risks. • A hybrid FDM-DEM model was developed to simulate tunnel excavation responses in deep, loose, and highly heterogeneous coarse-grained SRM. • Impacts of excavation method, burial depth, tunnel diameter, and rock block content on deformation and stability were quantitatively evaluated. • Pipe roof support parameters, including diameter, spacing, and installation angle, were optimized to enhance control of deformation in fractured and block-rich backfills. • Practical strategies were proposed for deformation control and safe tunnel construction in deep backfill zones based on systematic parameter optimization and failure mechanism insights.