Three-Dimensional DDA Contact Model And Failure Law Investigation Of Rockfall

Rockfall is second only to landslide in geological disasters.The dangerous rock mass on the steep slope is cut by three-dimensional(3D)spatial structural planes,and forms rockfall under the external forces,such as gravity,weathering,earthquake and infiltration pressure.The deformation instability,movement,development and failure of the slope are the typical dynamic problem of large displacement and large deformation of discontinuous block system.There exists contact transformation among the blocks of complex shapes and the slope of complex terrain.Therefore,the accurate description of the rockfall movement process,and then grasp the rockfall movement laws and control methods,are a recent work of great concern to researchers.Based on the 3D discontinuous deformation analysis(3D DDA)method,combining the laboratory experiment,the campus experiment and the field experiment,and considering the 3D topography of slopes and the geometric characteristics of rock blocks,this paper quantitatively and qualitatively studies the slope instability and the rockfall movement,energy conversion,impact failure capacity and disaster-causing volume.The mechanisms and laws of slope instability and rockfall disasters are revealed,which lays a foundation for the design of rockfall protection measures and engineering disaster prevention and mitigation.The main content of this paper is as follows:(1)The contact theory based on the entrance block model is introduced into the 3D DDA.Based on the S-R decomposition theory,the 3D DDA large rotation model is improved numerically to solve the volume expansion problem caused by large rotation of 3D blocks.The contact state of the whole joint surface is determined by the sum of contact forces at each corner of the joint surface,and the contact judgment criterion of 3D DDA is improved to solve the unreasonable problem of the block movement in critical sliding state.According to the concept of coefficient of restitution,the momentum theorem and the development mode of DDA contact force,the coefficient of restitution,impulse and impact force of block collision are obtained.A 3D DDA method is developed for the stability analysis of large rock slopes and the simulation of rockfall.By using the basic motion models,including sliding,oblique projectile,freefalling,collision bouncing and rolling,the effectiveness of 3D DDA is verified.(2)Considering the inertia components of slope deformation,the formulas of dynamic limit equilibrium conditions for block instability at any position of slopes are derived.Combining with the 3D DDA method,the initiation,movement and development process of sliding and toppling are analyzed,and the mechanisms of slope toppling failure are researched.Compared with the static LEM of Hoek and Bray,the 3D DDA and dynamic LEM broaden the pure sliding condition of slope instability,but narrow the topping-sliding condition.The static LEM has overestimated the stability capacity of toppling slopes.The 3D DDA results are consistent with the observed phenomena in geological engineering.It is pointed out that the small-scale toppling failure will eventually lead to disasters in the form of rockfall.(3)The protection measures of tree barriers and platforms for rockfall based on the 3D DDA method are proposed.According to different characteristics and arrangement of trees,the barrier effect on rockfall is analyzed.The laws of platform protection against rockfall are summarized.The results show that the contacts and collisions among different entities(rock blocks,trees and slopes)are the source of kinetic energy dissipation,and cause change in the motion trajectories of blocks.In the process of the block movement,the magnitude and variation tendency of the translational kinetic energy are close to those of the total kinetic energy.The rotational kinetic energy cannot be neglected in slope protection,although it accounts for a small proportion of the total kinetic energy,because the angular velocity of a moving block has an influence on the direction of its motion trajectory.The results provide a basis for tree arrangement design during planting and platform width design.(4)A laboratory experimental platform system for block movement and a field experimental system of binocular stereo vision for rockfall are developed.Combining with the 3D DDA simulation,the instability conditions and 3D failure characteristics of block systems such as a single block,a blocky column,a single row block system and granular mixtures are studied.The lateral displacement,resting position,jump height and kinetic energy evolution of blocks in campus and field experiments are investigated,and the movement characteristics of blocks under different masses,shapes,falling heights,falling angles of blocks and different geometric characteristics of slopes are studied.The results show that the campus experiment,field experiment and 3D DDA simulation can quantitatively determine the energy,jump height,migration distance and lateral motion range of blocks,and the laws of the rockfall movement and dynamic process are summarized.(5)The whole process and phenomena of landslides and rockfall in K4580 typical project of G318 national road in Tibet are analyzed.Through the 3D DDA simulation analysis,the formation processes of the internal cavity,surface subsidence,tension crack and shear dislocation in the landslide body are displayed.The instability modes and failure processes of boulders and massive collapses in potentially dangerous rock areas are predicted.The movement range,stopping position and influence area of boulders and massive collapses under different slope conditions are evaluated,which provides a basis for the formulation of disaster prevention countermeasures for practical projects.