Stability Analysis And Evaluation Of Surrounding Rock Of Large-span Shallow Tunnel Excavated In Weak Weathering Granite

This paper takes a metro tunnel in Qingdao as the research object.The effects of the high-span ratio and the two-step construction method on the stability of surrounding rock are studied by numerical simulation.On this basis,the stability evaluation index system and risk evaluation method of tunnel surrounding rock are established,and the risk is classified.Through the research of this paper,the following conclusions are mainly drawn:(1)The high-span ratio has a great influence on the ground settlement and the arch roof settlement,followed by the bottom heaving curve,and has a small influence on the convergence of the two sides.The influence on the ground settlement is mainly concentrated on the inside of inflection point of the settlement tank.(2)As the high-span ratio decreases,the stress concentration appears above the arch roof.At the same time,with the increase of the high-span ratio,the range of the plastic zone expands continuously.And the plastic zone in front of the palm plane does not intersect the plastic zone in the radial direction of the tunnel.(3)When the two-step construction is adopted,the ground settlement is affected by the excavation division of the upper bench,excavation order of bottom bench and the curvature of the temporary support.With the increase of the pilot tunnel area,the maximum ground settlement and the center of the settlement tank are changed.The curvature of temporary support has a great influence on the maximum ground settlement and a small influence on the width of settlement tank.With the change of support curvature,the maximum principal stress around the tunnel has one intersection point,and the minimum principal stress has two intersection points.(4)When the two-step construction is adopted,the release degree of the principal stress around the tunnel gradually increases from the two sides to the arch roof,and the distribution pattern of the maximum principal stress has a certain degree of symmetry.With the progress of excavation,the stress concentration at the end of temporary support increases and then decreases,and reaches the maximum before the temporary support is removed.The excavation sequence of the bottom bench has little influence on the stress distribution around the tunnel.(5)This paper proposes a factor substitution analysis method,and determines the criterion of surrounding rock stability under this method.It also establishes an improved evaluation index system of surrounding rock stability,and puts forward a risk evaluation method suitable for tunnel and underground engineering.At the same time,the concrete evaluation procedure,calculation rule and classification standard are established,and the extension concept of proximity degree is put forward.