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.

中文翻译：

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持续降雨导致隧道入口坍塌的失效分析与处理：以案例研究

降雨是影响山区隧道入口边坡稳定性的关键问题，它降低了土壤的抗剪强度，降低了隧道入口的稳定性。本文介绍了云南省某隧道入口在降雨条件下坍塌破坏的案例，通过现场调查、理论分析和现场监测，探讨了其破坏机理、潜在因素和处理措施。分析结果表明，土壤抗剪强度降低的主要原因是土壤含水量增加导致土壤基质吸力值下降。土体抗剪强度的降低降低了入口边坡的滑动阻力，进而引发了塌方。根据结果​​，采取防滑桩、注浆、边坡加固等3项处理措施，防止因降雨造成隧道口二次坍塌。结合现场监测数据，研究处理措施的效果。本案例研究中的经验教训有助于在降雨条件下预防和处理隧道入口施工。