Abstract: In the tunnel excavations, especially in the construction of the portal segment, various hazards such as water inrush, mud inrush, tunnel roof collapse, and floor failure due to the adverse geological conditions, are frequently encountered due to the instability of surrounding rock mass. The factors affecting the stability of surrounding rock are complex, including physical and mechanical properties of bedrock, integrity of rock mass, structural characteristics of rock mass, geological structure, groundwater conditions, construction conditions, etc. these influencing factors often are often coupled with each other, and the superposition to a certain extent would aggravate the evolution of instability state of surrounding rock mass. This paper presents the Heiyakou tunnel as an example and combines with the previous research results. The engineering geological and hydrogeological conditions of the exit segment of the tunnel portal are presented. The paper analyzes and discusses the integrity and stability of the tunnel surrounding rock mass, in order to provide reference experience for the stability research of similar surrounding rock mass.

Key words: Surrounding Rock Mass, Geological Conditions； Influencing Factors； Instability