Stability analysis and fracture evolution in topographical bias tunnels within layered rock masses: insights from FDEM simulation - He Liu; Junjie Wang; Panpan Cheng; Panpan Zhu; Quansheng Liu; Tengfei Guo; Chengzhi Pu; Qiao Liao; Peng Zhang (Computers and Geotechnics, 2026)