The Combined Finite-Discrete Element Method Of Rockfill Materials And Its Macro-and Micro-Mechanical Behaviors

Rockfill materials are cohesionless frictional geomaterials, usually consisting of rock blast, gravel, etc.. The complex mechanical properties, such as nonlinearity, strain-hardening, dilatancy, effect of intermediate principal stress, time-dependent behavior etc., results from the particle interaction, particle interlocking and particle breakage. The numerical simulation methods based on continuum mechanics have difficulty in revealing the evolution of microscopic structure of rockfill materials.The combined finite-discrete method (FDEM) is aimed at problems involving transient dynamics of systems comprising a large number deformable bodies that interact with each other, and may in general fracture and fragment, thus increasing the total number of discrete elements even further. Each individual discrete element is of a general shape and size, and is modeled by a single discrete element. Each discrete paticle is discretized into finite elements to analysis deformability, fracture and fragmentation. The Munjiza-NBS algorithm with linear efficiency is adopted to eliminate couples of discrete elements that are far from each other and not in contact. In FDEM, contact interacting algorithm makes use of finite discretization of discrete elements, and combined this with the so-called potential contact force concept. An efficient ten-noded quadratic element developed in a format suitable is used to discretize the separate bodies in the combined finite-discrete element analysis.Many discrete element analyses show that the particle shape has significant effects on the macro-and microscopic mechanical properties of particle assemblies. This work proposes an algorithm to randomly generate convex polyhedron particles. The form and roundness of particles can be controlled by outsourcing ellipsoid's axial ratio and particle's vertices. Through this way, convex polyhedron particles will be closer to the actual shape of rockfill materials. This paper proposes a particle swarm algorithm MPSO based on particle migration, which improves the premature convergence problem of PSA and increases the optimization precision. Based on the MPSO-RBF, the inversion platform of rockfill microscopic parameters is established. After analyzing microscopic parameters'sensitivity, parameters will be calibrated by the indoor test results.The microscopic numerical simulation based on combined FEM/DEM can describe the mechanical properties of rockfill from the particle scale, thus avoiding the complex constitutive relation. By choosing suitable microscopic parameters, numerical tests have a good forecasting ability, which can reproduce the main rockfill mechanical properties of nonlinear, press hard, dilatancy and shear shrinkage. At the same time the numerical test is not limited by sample size. In a numerical test, various factors influencing the performances of rockfill properties can be distinguished, and the evolution process of rockfill microscopic structure will be easily monitored. Numerical tests provide a new way to research rockfill's macroscopic and microscopic mechanical properties. Microscopic numerical experiments reveal that rockfill's shear strength is derived from the friction and interlocking between the particles. The interlocking between particles can be divided into two kinds of internal friction angle components, one is due to dilatancy and the other is caused by particle arrangement and breakage. The microscopic mechanism of particles'macro shear strength is explained by Stress-Force-Fabric model. The rockfill's macro shear strength is derived from the anisotropy of fabric and contact force. To the combination of irregular shape particles of different size, the anisotropy of fabric is divided into anisotropy of contact normal vector and anisotropy of branch vector, while the anisotropy of contact force is composed of anisotropy of normal contact force and anisotropy of tangential contact force.This paper researches rockfill's deformation and strength properties under the complicated stress condition, introduces a new corner function based on the Rosce dilatancy model to reflect the effects of intermediate principal stress. After comparing the adaptabilities of Mohr-Coulomb, Drucker-Prager, Lade-Duncan and Matsuoka-Nakai criteria to describe the strength under true triaxial stress state, revised Lade-Duncan criteria is proved to be better. After a certain stress strain history, when the stress increment and the corresponding strain increment satisfy the equation, the overall instability of irreversible damage will occur even if external load on the rockfill is not changed and no external energy is input, that is dispersion failure mode.A combined continuous-discontinuous approach suitable for modeling the whole failure process of rock is proposed. According to the gelling characteristics of rock materials, rock is discretized into bulk elements and non-thickness interface elements. Damage and fracture occur in the interface elements and the Mohr-Coulomb criterion with tensile cut-off is adopted as the failure criterion. Remove the failure interface elements from the element mesh configuration. The bulk elements, previously connected by the interface elements, come into contact. By using this method to simulate the fracture, crushing and abrasion of polygonal particles, the influence of particle breakage on the macroscopic properties of the rockfill and the microscopic mechanism are studied.The subcritical crack extension caused by stress corrosion is the inherent mechanism of time-dependent properties of rock materials. Based on this understanding, the rockfill particle strength deterioration model is put forward to describe the evolution process of particle strength parameters with time. It can be seen that after the initial transient elastic-plastic deformation, particles generate a delay crushing due to stress corrosion, and macroscopically rheological deformation evolved over time is showed. The whole rheological process can be divided into three stages. The first stage is decelerating rheology. This phase rheology rate decreases with time. The second stage is stable rheology and the rheology rate is nearly constant. The third stage is accelerated rheology and the rheology rate increases rapidly with time.