Study On Multiscale Model And Macro-mesoscopic Mechanical Properties Of Rockfill

With the rapid deveolpment of high rockfill dam engineering, research on the macro-mesoscopic mechanical characteristics of rockfill is of much higher requirements. However, due to the multiscale stuctural characteristic of rockfill and its discreteness, heterogeneity, nonlinearity and anisotropy, the macro-mesoscopic deformation mechanism of this material is not clear yet. Design theories and constitutive models available currently cannot meet the requirements of engineering practice. At present, the finite element method based on continuum model is mainly used for the research of stress and deformation of rockfill. It can get the equivalent stress and deformation characteristics at the macro level, but it is difficult to reflect the evolution process on the mesoscopic scale, such as particle breakage, particle slip. Numerical experiments based on discrete element method or multiscale method coupled DEM are not subject to the size restriction, and various factors affecting the mechanical properties of rockfill can be distinguished, while the evolution of rockfill fabric in the loading process can also be conveniently monitored at the same time. The advantages of numerical experiments provide a new way to study the mesoscopic deformation mechanism and macroscopic mechanical properties and provide a theoretical basis of improving constitutive relationship of rockfill. Therefore, it is necessary to research the mechanical properties of rockfill from the perspective of macro-mesoscopic by means of numerical analysis and multiscale method.Multiscale method is an effective approach to study the complex particle system. A multiscale simulation method coupled FEM and DEM hierarchically is proposed in this paper. The key links in this method are analyzed and derived, and the corresponding computing framework is constructed. In this multiscale approach, the finite element method is used to simulate the boundary value problem. From the discrete particle aggregation embedded in each gauss integral point, the constitutive relation is extracted for the overall solution. This method can overcome the limitations that the traditional continuous method requires phenomenological assumptions in the constitutive relation and the discrete element method cannot effectively simulate the large-scale engineering problems. Meanwhile, the macroscopic response can also be associated with mesoscopic mechanism effectively.Through biaxial compression multiscale test of rockfill, macro-mesoscopic mechanical properties are systematically investigated. The macro mechanical response is related to the confining pressure and the asymmetric strain localization phenomenon is showed. Integral points which are inside, outside and at the edge of the shear zone show different local responses. In terms of mesoscopic fabric, the evolvement of coordination number, the movement of particles and the development of the contact force chain during the loading process are analyzed, and the mechanics properties of the particle system are explained in the mesoscopic level. Contact force distribution of the particle aggregation is characterized by heterogeneity and spatial anisotropy. The principal stress of particle aggregation in the shear zone is deflected, and the degree of anisotropy is more obvious. The evolution processes of anisotropic coefficients and the weights of different anisotropic sources are analyzed. The evolution of energy is also studied. With the increase of particle numbers in representative volume element, the contact normal distribution become more uniform and the degree of anisotropy become smaller, but there are some differences in the simulation of softening behavior. The numerical example fully demonstrates the multiscale method has good applying prospect in the basic characteristic research and practical engineering application of granular material.The contact mechanics model, the elastic-plastic stress and strain relations, the discrete of time-domain and integral, and other aspects of the Stochastic Granule Discontinuous Deformation (SGDD) method are briefly introduced. The rockfill random generation algorithm is developed. Taking the rockfill in the actual project as an example, particle shape is analyzed and simulated. The algorithm can generate particle aggregation with different particle shape, porosity or gradation, implementing the accurate description of the rockfill geometry and spatial distribution at the mesoscopic scale.On the basis of SGDD, a cohesive zone model is employed to simulate the fracture of quasi-brittle material. In this model, the material is discretized into bulk elements and non-thickness interface elements, and the parameters such as stiffness of the interface are deduced. If the interface cracks and failures, fracture will happen. This method does not need subdivision mesh at the crack tip. It can simulate three-dimensional particle breakage phenomenon explicitly, ensure the stability of numerical calculation and improve the computational efficiency. Damage and fracture occur in the interface elements, while bulk elements only have elastic deformation. When the stress state of an interface element reaches the failure criterion, the damage evolution model based on fracture energy is activated. The failure interface elements are removed from the element mesh configuration. The bulk elements, previously connected by the interface elements, come into contact. Numerical verifications including the Brazilian Disc test are given to demonstrate the validity of this method. The results show that this method can simulate the crack initiation and propagation effectively. Based on the analysis of particle breakage mechanism, the rockfill breakage is simulated using this method.Based on multiscale mechanics model and SGDD method, the numerical test platform of rockfill is established. It can simulate the rigid or flexible boundary conditions, provide the displacement or stress loading, and extract macro mechanics indexes of particle aggregation. After illustrating the numerical test process in detail, the size effect and the anchoring effect of rockfill are studied, respectively. Using the primary and secondary rockfill of shuibuya CFRD, considering particle breakage and size effect of particle strength, and focusing on the influence of particle strength and specimen size, the change of mechanical properties of rockfill are analyzed. On the other hand, the numerical implementation of the bolted gravel experiment is realized. Samples with different anchor spacings and different particle sizes are generated. The numerical experiment can well reflect the deformation and anchoring effect of the different anchored granular structures. Their macroscopic properties have close relationship with the evolution of mesoscopic fabric.