Study On Movement Characteristics Of Rockfall On The Slope Of Tunnel Portal And The Mechanism Of Impacting Open Cut Tunnel

With the further development of the western development strategy in China,the construction of the railways and highways has continuously extended to mountainous areas,and the rockfall disaster of slope at the entrance of tunnels has become increasingly prominent.As the topography is steep and the rock mass is cracked on the slope of the tunnel entrance,and many potential rockfalls are on this slope,therefore,it needs huge manpower and material resources if all potential rockfalls are completely cleared from the slope.In order to ensure the safety of vehicle operation at the entrance of the tunnel,apart from the active nets,passive nets and other measures,the open cut tunnels are also built at the entrance of the tunnel to protect the slope of tunnel entrance.There are few researches on the rebound characteristics of rolling rockfall collision,and there is also no clear result about the study of rockfall impacting the arched open cut tunnels structure.In view of this,the characteristics of the rockfall movement and the mechanism of rockfall impacting the arched open cut tunnel were systematically studied in this article taking the slope of the tunnel entrance of the Chengdu-Lanzhou railway as an example.The field research,theoretical analysis,model tests,numerical simulations and other methods were carried out in this study.The main works and research achievements are as follows:(1)The calculation methods of the normal coefficient of restitution between the rockfall and slope were derived based on the Hertz contact theory,ideal elastoplastic contact theory and so on.Based on kinematics and considering the effect of the friction coefficient during the collision between the rockfall and slope,the formulas for calculating the tangential coefficient of restitution of rockfall under different states of collision were proposed.Meanwhile,the influences of the normal coefficient of restitution of rockfall,angular velocity before collision,sliding friction coefficient,rolling friction coefficient and other parameters on the tangential coefficient of restitution were discussed.(2)The effects of the mass,shape,impact angle,and falling height of rockfall and sandy soil thickness of slope on the coefficient of restitution of rolling rockfall were studied through model tests.Based on the ratio of the sandy soil thickness of slope to the diameter of rockfall,the method for calculating the coefficient of restitution of rockfall considering rockfall sizes and sandy soil thickness of slope was proposed.The results show that there has a threshold for the effect of the rockfall mass on the coefficient of restitution;the shape of rockfall has a significant effect on the coefficient of restitution of rockfall;the normal,tangential,kinematics and horizontal coefficients of restitution of the sphere rockfall are the largest,followed by the cylinder rockfall,and the cube rockfall is the smallest;as the impact angle increases,the kinematics and energy and vertical coefficients of restitution of rockfall all linearly decrease,and the tangential coefficient of restitution first decreases and then increases,while the horizontal and angular velocity coefficients of restitution first increase and then decrease,and the normal coefficients of restitution has no clear trend;as the sandy soil thickness increases,the tangential,kinematics,energy,horizontal and angular velocity coefficients of restitution of rockfall decrease,and the normal coefficient of restitution first decreases and then increases,while the vertical coefficient of restitution first increases and then decreases;the effect of the falling height on the coefficient of restitution of rockfall is more complicated,and it is closely related to the sandy soil thickness.(3)Taking the slope of one tunnel entrance as an example,based on field rockfall tests and the normal and tangential coefficients of restitution of rockfall obtained from model tests,the influences of the rockfall shape on the movement characteristics of the rockfall were studied by the numerical simulation methods,and the movement trajectory and the energy of impacting the open cut tunnel of the potential rockfall were obtained.These results can provide references for the prevention of rockfall disasters and the structural design of the open cut tunnel.The results show that the movement modes of the sphere rockfall on the slope are mainly rolling,collision and bouncing,and the velocity of the sphere rockfall gradually increases during the movement;the movement modes of the cube and cuboid rockfalls on the slope are mainly collision and bouncing,the velocities in the horizontal direction of the cube and cuboid rockfalls stepwise increase,and the velocities in the vertical direction zigzag increase;the distance from the position that the cube rockfall impacting the open cut tunnel to the slope is the largest,followed by the cuboid rockfall,and sphere rockfall is the smallest;the translational kinetic energies of different shape rockfalls have no obvious difference,and the rotational kinetic energy of the sphere rockfall is the largest.(4)The study on rockfall impacting the arched open cut tunnel structure was conducted by model test.The load distribution law of acting on arched open cut tunnel and dynamic response law of the open cut tunnel structure were clarified and acquired by changing the rockfall mass,the falling height and the backfill thickness.The comparision between the model test results and the numerical simulation results verifies the reliability of the numerical simulation method.(5)Taking the double-ear wall open cut tunnel as the research object,the rockfall impact load and the dynamic response of the open cut tunnel structure were studied by the dynamic finite element method.A method for calculating the rockfall impact force,impact compressive stress and dynamic response of the structure was proposed.The results show that as the backfill thickness increases,the impact force of rockfall does not change significantly,while the maximum impact compressive stress of rockfall decreases rapidly;The impact compressive stress of rockfall decreases with the increase of horizontal distance from the impact point of rockfall,and its shape is approximately circular truncated cone;the diffusion angle of impact compressive stress is about 50°;the vault and arch foot of the double-ear wall open cut tunnel structure are the main stress parts,and the inside of the vault and the outside of the arch foot are mainly subjected to tension,while the outside of the vault and the inside of the arch foot are mainly subjected to compression;the impact force of rockfall,impact compressive stress,and the first principal stresses of the main stressed parts of the open cut tunnel structure increase with the mass and impact velocity of rockfall.