Numerical Simulation Study On Failure Mechanism Of Shallow Mountain Tunnel Under Strong Earthquake

Shallow mountain tunnel has certain seismic capacity,but it will still be serious damaged under high intensity earthquake.Rock surrounding the tunnel will slip and even collapse because of instability,and the tunnel lining will be damaged in large area.Therefore,it is of practical significance in engineering to study the failure mechanism of shallow mountain tunnel under strong earthquake.This paper applies FLAC 3D-finite difference software to numerical calculation,in which the seismic wave is adjusted by seismosigal software and the free field boundary is applied in the model to simulate the propagation of seismic wave at the boundary.Comparison between the numerical simulation and experiment used in the seismic dynamic response of typical tunnel section lining under high intensity earthquake shows that the calculation model,material parameters and numerical calculation method used are feasible,and the results are reliable.And failure mechanism analysis shows that z displacement of tunnel vault is maximal,resulting in the concrete of tunnel vault collapsing.And the maximum of x displacement is at the arch foot,which presses the tunnel invert and causes the floor heave.The principal stress in the area of tunnel wall and arch foot is larger than that in other area so that the concrete is easy to crack and even to spall.When the condition of surrounding rock becomes worse,the value as well as the principal stress of tunnel lining increases under earthquake,therefore the damage of tunnel is more serious.Failure mechanism analysis between different angles of tunnel and slip under strong earthquake shows that when the value of the angle decreases,both displacement and principal stress increase,and the rock surrounding the tunnel is under complex stress state with tensile stress and shear stress meanwhile the shear strain increment becomes larger.It can be seen that when the angle between tunnel and slip is 90 degree,the earthquake damage is less serious than that under other cases.The tunnel and surrounding rock near the slip server shear and tensile failure.And the shear strain increment decreases as the distance between the slip and section of tunnel and rock,the rock stays stable.Analyzing the failure mechanism of tunnel portal under strong earthquake indicates that the displacement and shear strain increment of the rock between the two holes at the entrance are larger,therefore the holes shows interaction.The effect is weakened along the longitudinal direction of the tunnel towards the tunnel body.The maximum displacement and principal stress occurs at the entrance of the tunnel,which gradually decrease along the longitudinal tunnel.When the distance is more than 50 meters,the tunnel lining shows the same mechanical characteristics as that of typical section of tunnel,which indicates that the length of seismic fortification of the tunnel entrance is generally 30 meters to 50 meters.The mechanical characteristics of the tunnel and surrounding rock in the middle section are influenced by the tunnel portal and trunk.