Tunnel collapses and adaptive support at rock-soil interfaces under low-to-medium overburden in tectonically disturbed rock masses

Tunnel stability in tectonically disturbed rock masses remains particularly challenging near rock–soil interfaces, where abrupt stiffness contrasts and heterogeneous geological structures can trigger collapse mechanisms that conventional rock mass classification systems do not adequately capture. This study identifies and quantifies a geometry-controlled instability mechanism governing tunnels excavated near rock–soil interfaces under low-to-medium overburden conditions. The analysis combines 26 roof-collapse incidents recorded in three railway tunnels and one adit in Northern Thailand with geological mapping, RMR–GSI characterization, construction records, convergence monitoring, and two-dimensional finite element analyses. Results from the Phayao Tunnel demonstrate that the occurrence of collapse is governed not only by rock mass quality but also by the effective thickness of competent material available above the excavation. A normalized interface-depth parameter V/H is introduced to quantify this confinement condition. Field observations and numerical simulations show that when V/H decreases below approximately 2.5, the compressive arch above the tunnel crown becomes progressively truncated, leading to reduced confinement, increased deformation, and increased collapse susceptibility. A collapse-risk framework integrating rock mass quality, effective confinement, and interface geometry is proposed. Although additional validation is required in other geological environments, the proposed approach provides a mechanics-based framework for identifying interface-controlled instability and supporting adaptive excavation and reinforcement strategies.