| Geological disasters such as landslides,collapses and debris flows are typical large deformation examples,showing flow-slip phenomena of geological granular materials.From the point of view of granular flow,the strain-softening failure of soil and the flowing failure of slope are typical phase-transition processes of granular materials from solidlike state to liquid-like state.It is difficult to investigate the complex contacts,friction and collision mechanisms of geological granular materials via insitu studies,model tests and numerical simulation based on continuum media.The discrete element method(DEM)becomes the main means of analyzing the mechanism of granular materials.However,the timesteps in the tranditional DEM codes using “soft-contact” approach is too small(<10-5 s)and the stiffness parameters lack for physical basis.The computational efficiency is also limited when facing arbitrary-shaped or high-speed particles.Our understanding is still limited in the micro-mechanisms and solid-liquid transition laws of geological granular materials.In this paper,a discrete element analysis method is established from the theory of impulse-based dynamics,and a new 3D DEM code named BLENDERREATICLE is developed.The systematic study is presented about the mechanical response of geological granular in different phases.By comparing the experimental and numerical results from the existing literature,the rationality and accuracy of the discrete element analysis method and its numerical code are verified and validated.The main results in this paper are as follows:(1)To overcome the shortages of the traditional discrete method based on the softsphere model,a discrete element analysis method is established from the theory of impulse-based dynamics,and a new DEM code named BLNEDER-REATICLE is developed.According to the method,the interactions of particles including friction and collision can be described by a unified linear complementarity problem(LCP).In the impulse-based dynamics equation,the physical quantities of particles such as length,time,and density follow the Buckingham Pi theorem.A dimensioning system based on the similarity theory is established to the LCP.Then,the explicit incremental form of the contact impulse is obtained from the calculation results of contact detection including broad phase and narrow phase.Thus,the velocity and position of the particles can be updated in the iterative process,and the contact force is also recorded at the same time.By forking the open-source programs named Blender and Bullet,BLENDERREATICLE has systematically developed pre-and post-processing modules and core calculation solver modules.The new DEM code is verified and validated for the simulation and dynamic analysis of geological granular materials.The discrete element analysis method and its numerical code follow the rigid-body assumptions and capture the real particle modeling(different shapes,sizes,etc.),when adopting large time steps and ensuring high computational efficiency.(2)BLENDER-REATICLE is adopted to study on the numerical test of the plane shear flow.On the basis of the existing jam-flow phase theory,four types of phase states are proposed,namely the jam,jam-flow,slip-flow,and flow states by analyzing the characteristics of the velocity distribution,kinetic energy distribution,contact number and force chain network.The strong force chain and the large number of contacts are the most significant characteristics of the jam state.The particles contact closely with the strong force chain,so that the granular assembly can resist a certain shear stress without irreversible deformation.In the jam-flow state,the particles in granular materials start to move,but are still constrained by the force chain.In the slip-flow state,the force chain begins to break,and an overall slip motion is observed between the granular assembly and the confining boundary.In the flow state,the force chain network is destroyed,and the number of contacts is significantly reduced.The particles gain large kinetic energy,and the collision effect is increased.On the basis of the existing theoretical models such as the linear function,the power-law function,and the inverse proportional function,a new function model was proposed with zero-order infinitesimal properties,which can accurately describe the friction law of granular materials during the jam-slip-flow phase transition.A theoretical model based on the logarithmic function is also proposed for the phase-transition law.The model can accurately reflect the quantitative relationship between the volume fraction,shear stress and normal stress of granular materials in different phase states.(3)The numerical tests of the triaxial shear have been studied.The effect of particle shape on the triaxial shear behavior is presented by analyzing the volumetric strain,critical void ratio,stress ratio,critical state line,and the distribution of the contact number and contact force in the critical state.In the mesoscale,the aspect ratio of the particles will directly change the fabric structure,leading to the scale variation of the void ratio.Therefore,the triaxial specimen shows different mechanical behaviors like mediumdense sand(aspect ratio AR=0.67)or dense sand(aspect ratio AR=0.50),while the density,volume,mass,void ratio,and number of particles are the same.A smaller apsect ratio can effectively promote the inter-locking effect among particles,so that the specimen shows a stronger ability to resist deformation.On the basis of the theoretical model for the phase transition of granular materials,the phase evolution of the triaxial shear is also analyzed:In the initial stage,the triaxial specimen is in the jam state.The general shear stress increases rapidly while the solid fraction is nearly constant.As the loading progresses,the specimens enter into a jam-flow state after the peak dilatation angle is reached.Moreover,large shear dilation is observed and the general shear stress gradually decreases.Finally,the volumetric strain,void ratio,and general shear stress tend to be stable when the critical state is reached.(4)The numerical tests of the column collapse have been studied.The evolution of the deposit morphology of the column collapse is presented by analyzing the evolution of the horizontal velocity and contact force of the particles.The effect of the initial aspect ratio of column and particle shape on collapse progress is revealed.Refer to the nonspherical particles,the initial aspect ratios of column are the main factors affecting the final deposit morphology.The particle shape,to a limited extent,can only affect the contact angles among particles.For example,the contact force distribution changes when the particles undergo more relative rotational motion causing the change of contact angles.On the basis of the theoretical model for the phase transition of granular materials,the phase evolution of the column collapse is also analyzed: After the instantaneous removal of the horizontal wall,the initial jam state of the granular assembly is destroyed.Under the gravity,granular flow is mainly falling in the z-axis direction and running out in the x-axis direction.The volume of the granular assembly expands rapidly firstly and show contraction gradually.The general shear stress first increases rapidly,then gradually decreases to zero,then increases and finally tends to be stable.In the initial acceleration(dimensionless time t = 0~2),the general shear stress is consistent with the granular flow direction.In the final deceleration stage(t =2~4),the general shear stress is opposite to the granular flow direction.In the stress relaxation stage(t > 4),the general shear stress,soild fraction tend to be stable,leading to the jam state.(5)The numerical simulations of Jiweishan landslide have been studied.The characteristics of the three stages of the Jiweishan landslide are presented by analyzing the evolution of the displacement and velocity.In the sliding-impact stage(t =0~2),the boulders near cliff fall rapidly,collide with the ground and promote the distal part of the slope sliding and shearing out.In the scraping-collision stage(t =2~10),the distal part of slope turns and accumulates under the influence of proximal bounders colliding and mountain scraping.In the flowing-accumulation stage(t =10~30),the friction interaction among bounders is enhanced,and gravity and site conditions become the dominant factors in the evolution of the landslide deposit.On the basis of the theoretical model for the phase transition of granular materials,the phase evolution of the Jiweishan landslide is also analyzed.In the sliding-impact stage(t =0~2),the general shear stress has reached the critical value.The slope faces the flowing failure erratically and enter into the flow state.In the scraping-collision stage(t =2~10),the direction of general shear stress is changing,which promotes the deceleration of boulders.In the flowingaccumulation stage(t =10~30),the kinetic energy of the landslide decreases continuously.The granular assembly finally reaches a new jam state with a stable general shear stress and a larger contact number. |
