Failure Investigation and Treatments of Tunnel Entrance Collapse in Weak Diatomaceous Soil Induced by Heavy Rainfall Through Coupling Surface and Groundwater Flows

Rainfall is a great threat to the stabilities of the slope at tunnel entrances located in mountainous areas because it can reduce the shear strength of soil and, as a result, the stabilities of tunnel entrances. Although conventional rainfall-induced slope damage analyses commonly overlook the impact of runoff, it is crucial to consider it to assess slope instability accurately. To assess the factor of safety distribution about the slope of the Feifengshan Tunnel in Zhejiang Province, China, and analyze the impact of runoff on slope instabilities, this paper uses a coupling model of surface runoff, groundwater flow, as well as soil mechanics, through the shallow water equation, Richards equation, and the local safety factor (LFS) method. Furthermore, given the strain-softening characteristics of diatomaceous soil, designing and analyzing them according to peak strength may result in potential hazards. Thus, this study analyzes the distribution of the slope safety factor under different constitutive models and recommends suitable slope reinforcement measures. The study also investigates the slope reinforcement measures required to avoid the secondary collapse of tunnel entrance during rainfall, verifying numerical simulation results regarding the section's volumetric water content and surface settlement values. The findings of this study offer a dependable approach for evaluating the stability of tunnel entrance slopes and are significant for tunnel site selection and reinforcement effect assessment. The numerical results emphasize the effects of surface runoff from rainfall water distribution. Furthermore, the strain softening model has significant advantages in calculating surface settlement and tunnel slope failure.