| Summary: | Rainfall is a crucial issue affecting the entrance slope stability of mountain tunnels, as it decreases the shearing strength of soil and reduces the stability of tunnel entrance. This paper presents a case history of the collapse failure of a tunnel entrance in Yunnan Province under rainfall conditions, in which the failure mechanism and potential factors and treatment measures were discussed by field investigation, theoretical analysis, and in-situ monitoring. The analysis results show that the decrease of soil shear strength was mainly attributed to the decline of matric suction value of soil caused by the increase of soil water content. The decrease of the soil shear strength reduced the sliding resistance of the entrance slope and then triggered the collapse. Based on the results, three treatment measures to prevent a secondary tunnel entrance collapse due to rainfall are adopted, including anti-slide pile, grouting, and slope reinforcement. Combined with the field monitoring data, the effects of treatment measures were investigated. Lessons in this case study facilitate prevention and treatment of tunnel entrance constructions under rainfall conditions.
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