Analysis On Risk Assessment Of Rockfall For Mountainous Tunnel Portal On High And Steep Slope

Due to strong weathering and earthquake. the Rock Mass on high and steep slope is easy to form landslides debris flows and rockfalls that endanger the safety of tunnel portal Construction and operation in western region of China. Based on the Chengdu to Lanzhou railway technology research project of China railway corporation, the rockfall hazards at mountains tunnel portal sections on high and steep slope are chosen as research object, Theoretical analysis, numerical simulation and the site investigation method are adopted in research, the rockfall motion path and characteristics, scope of rockfall impact, the foundation for setting safety netting system, rockfall hazard risk assessment methods and comprehensive prevention and control measures were studied, main results are as follows:(1) The process of rockfall collide with the slope is simulated through the ANSYS/ls-dyna software in this thesis, discusses the collision restitution coefficient variation law under the condition of different slope, rockfall radius, velocity and angle. Results show that as the condition of slope from the bedrock to loose the accumulation, the normal restitution coefficient decreases, and tangential restitution coefficient increased, the tangential restitution coefficient with the increase of incident velocity increases, the normal restitution coefficient changes smaller, incidence angle increases, the tangential restitution coefficient decreases sharply, incidence angle between 0-45°, the normal restitution coefficient has increasing trend, incidence angle between 45-90°, the normal restitution coefficient has a tendency to decrease, the influence of rockfall radius on the restitution coefficient is weak.(2) The two-dimensional motion characteristics of Chengdu to Lanzhou railway rockfall hazards are simulated via Rocfall software, predict rockfall hazard scope in combination with offset ratio, the biggest impact velocity and length reference value of shed hole or opencut tunnel structure are concluded.(3) The changes of rockfall kinetic energy on straight slope are derived by theoretical, scale factor of the kinetic energy and the initial energy is introduced, passive fence design process and formula of the straight slope are put forward to avoid simply relying on experience method.(4) The motion law of rockfall on six kinds of typical slopes is studied, according to the bounce height and kinetic energy, the setting position of passive fence is suggested.The risk classification of different slopes is based on energy dissipation ratio, the energy dissipation ratio of ladder curve slope is biggest, the risk grade of ladder curve is lowest grade 4, the risk grade of concave curve and negative terrain cure is grade 3, the risk grade of steep slow steep cure and straight slope is grade 2, the energy dissipation ratio of convex curve and slow steep slow curve is least, the risk grade is highest.(5) The evaluation grade and standard of rockfall hazard affecting factors are proposed on the basis of RHRS evaluation system, based on the AHP-fuzzy comprehensive evaluation method to establish tunnel portal section of rockfall hazard risk evaluation model, conducting detail evaluation of the rockfall hazard at the Maoxian tunnel entrances. This thesis put forward active protection to individual dangerous rock mass, and set up three passive fence to prevent the rockfall fall down to the tunnel portal section, combined active prevention measures with passive prevention measures, the results suggest the shed hole or opencut tunnel length be not less than 18 meters.