The construction and operation of the Jiuzhaigou County bidding section of the Jiumian Expressway is affected by debris flow disasters. It is important to scientifically and reasonably evaluate the debris flow risk in the engineering area. Therefore, according to the data of topography, meteorology, hydrology and regional geological structure of the project area, 12 influencing factors, such as regional geological disasters, loose material reserves along the gully and maximum punching amount at one time, are selected as the evaluation indexes of debris flow risk assessment. Based on the catastrophe progression method, a mutation model of debris flow risk assessment suitable for the project area was established. The debris flow risk assessment mutation model was used to evaluate the debris flow risk of 8 trenches in the project area, and the applicability of the model is verified by comparing the actual occurrence history of debris flow in the trench with the evaluation results. The analysis results show that the risk of debris flow in four gullies of Gangou and Xiaganzuo in the bid section project area of Jiuzhaigou County of Jiumian Expressway is extremely dangerous, and the debris flow risk of Shangganzuo gully is severe. These five debris flow gullies are the key objects of debris flow prevention and control during the construction and operation of Jiuzhaigou County bidding section of the Jiumian Expressway.

Debris flow accumulation has the characteristics of loose and broken, low strength, poor stability and so on. When the tunnel passes through the debris flow accumulation area, it is easy to bring major safety hazards to the construction site. Based on the engineering background of Gangou tunnel crossing debris flow accumulation area in the fourth bid of Jiumian expressway, this paper establishes a numerical calculation model, studies the large deformation characteristics of tunnel surrounding rock in debris flow area, determines the reasonable reinforcement range of tunnel surrounding rock, and puts forward the comprehensive control technology of tunnel passing through debris flow accumulation body combining surface stability and underground reinforcement based on simulation results and field experience. The numerical results show that the deformation of surrounding rock and ground surface can be effectively controlled. The rationality of the proposed control measures is verified by field monitoring, which provides a reference for surrounding rock control of tunnel under type conditions.

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