Seismic Response Characteristics And Collapse Analysis Of Shallow Tunnel With Soft Ground

In recent years,with the gradual development of the "One Belt and One Road" strategy,China has increased its investment in engineering construction in the field of transportation.Shallow tunnels are commonly found in practical projects.Since seismic activity is very frequent in our country,which brings great threat to the safe operation of the tunnel.Large amount of tunnel damages show that shallow tunnel is one of the most unstable areas where subjected to earthquake.In order to obtain the seismic response of the shallow tunnel with soft surrounding rock under small earthquakes and the characteristics of vibration collapse after large earthquakes,the paper takes the shallow tunnel under the condition of grade V surrounding rock as the research object,and conducts research through shaking table model test,theoretical calculation and numerical simulation.The main achievements are as follows:(1)The model test results show that the acceleration of surrounding rock increases with the decrease of distance from the surface,the ground acceleration is approximately 63% larger than vault acceleration,and the vibration of the surrounding rock near the tunnel in the plane with the same height has certain strengthening;The horizontal displacement of surrounding rock at the vault is approximately 1.23 times that at the hance,and the horizontal displacement keeps decreasing when its location moves away from the tunnel,and the displacement at each point increase with the shaking times and shaking intensity;With seismic intensity's increase,in the ground appeared a collapse area,similar with the funnel.Under seismic loading,the axial force and bending moment changes most obviously at spandrel and arch foot of tunnel,and the most cracks appear at the spandrel and the arch foot where the anti-seismic design should be strengthened to improve the seismic performance of the lining structure.(2)The calculation results of the limit equilibrium method show that the radius,depth,cohesion force and internal friction angle have a significant influence on the distribution of collapsed sliding surface of shallow tunnel,and the radius and cohesion are the two main factors affecting the stability of shallow tunnel.Therefore,reducing tunnel radius and increasing cohesion of surrounding rocks are effective ways to improve the stability of shallow tunnel.(3)Numerical calculations show that the acceleration of the surrounding rock increases in the vertical direction as it approaches the surface and decreases in the lateral direction as it moves away from the tunnel,the ground acceleration is about 1.5 times that of the vault acceleration,and acceleration at lateral 20 m is only 90% of vault acceleration,but the overall increase with the decrease of the buried depth;The surrounding rock has obvious horizontal relative displacement under earthquake action,the relative displacement increases with the increase of stratum height,decreases with the decrease of buried depth,and increases with the decrease of surrounding rock grade;The earthquake changed the initial stress state of the lining structure.Under the action of earthquake,the internal force of the lining near the spandrel and the arch foot are the most obvious,and the internal force decreases with the improvement of the surrounding rock conditions and increases with the increase of the lining stiffness.(4)The dynamic responses of shallow tunnel with soft surrounding rock in model tests and numerical simulations are generally consistent,which proves the rationality of the model tests;The subsidence area of the surrounding rock after vibration in the model test coincides with the collapse area of the surrounding rock in the theoretical calculation,which verifies that the theoretical calculation based on the principle of limit equilibrium can be applied to analyze the dynamic collapse of shallow soft rock tunnel.