Spatial control of tunnel position on rainfall-induced deformation in red-bed slope-tunnel systems

Red-bed slopes in southwestern China are susceptible to rainfall-induced deformation and instability, which can threaten the safety of underlying tunnels. To investigate how the spatial relationship between a tunnel and the potential sliding surface influences slope-tunnel deformation, three rainfall-induced physical model tests were conducted by taking an actual engineering case as a prototype. In the tests, the tunnel was positioned above the potential sliding surface (M1), intersecting the potential sliding surface (M2), and below the potential sliding surface (M3), respectively. The results indicate that the spatial relationship between the tunnel and the sliding surface is a key factor controlling the deformation behavior of the slope-tunnel system. In Test M1, deformation was primarily governed by front-edge traction and local collapse. In Test M2, the tunnel structure interrupted the continuity of the sliding surface, effectively restraining overall slope deformation. In Test M3, large-scale sliding occurred along the mudstone layer. Earth pressure and pore water pressure responses along the sliding surface developed first at the rear of the slope and then progressively propagated toward the toe. The development of surface deformation exhibited stage-dependent correspondence with fluctuations in differential stress along the sliding surface. However, the largest cumulative surface deformation corresponded to the smallest peak tunnel strain among the three tests, indicating that slope deformation is associated with stress release along the sliding surface, whereas tunnel strain is primarily governed by localized differential deformation and shear concentration. Based on the coupled evolution of stress, strain, and deformation, three typical coupled deformation modes were identified: the front-edge traction-synchronous tunnel sliding mode (M1), the sliding surface barrier-tunnel anti-sliding shear mode (M2), and the overall sliding-tunnel disturbance and squeezing mode (M3). The findings provide a reference for hazard prevention and engineering design of slope-tunnel systems in red-bed regions.