Study On Multiscale Numerical Analysis Methods For Failure Process Of Soil Mass

Soil is composed of numerous grains. The failure process of soil mass is the result of the gradual evolution associated with the multiscale coupling from microscopic particle, mesoscopic material to macroscopic field. The conventional soil mechanics, which is based on the hypothesis of continuum, fails to capture the contact detachment, severe particle rolling and particle crush during the failure process of soil mass. The multiscale numerical analysis methods are effective means to reveal the failure process of soil mass. For two kinds of typical failure problem, including the failure with strain localization and the failure without strain localization, study on multiscale numerical analysis methods has been carried out in this thesis. The findings are as follows:(1) For the failure problem of soil mass with strain localization, the weak discontinuous deformation of shear band is equivalently represented by the strong discontinuous deformation, and the cohesive zone model is adopted to describe the deformation property and energy dissipating mechanism for the shear band. Subsequently, the shear band element without thickness is constructed based on the augmented finite element method, and the augmented finite element method with cohesive zone model for modeling the failure process of soil mass is presented. Numerical analyses for the progressive failures of soil slopes show that the proposed method primely reproduces the propagation of shear band and effectively overcomes the mesh sensitivity. The softness of shear band, which has little influence on the shape of the slide surface, significantly reduces the ultimate bearing capacity.(2) For the failure problem of soil mass without strain localization, both the interface coupling methods based on the displacement/velocity compatibility and the force compatibility are implemented for coupling finite elements with discrete elements. Numerical examples show that it’s impossible to avoid spurious reflection in the coupling system for the interface coupling method based on the displacement/velocity compatibility. The contact between the particle and the edge of finite element is permitted to separate for the interface coupling method based on the force compatibility, thus the contact interaction and the momentum/energy transfer between the discrete elements and the finite elements can be effectively revealed.(3) The generalized bridging domain method is presented. First, the artificial boundaries and the coupling domains of distinct models are introduced based on different given weighting functions. Second, the displacement/velocity, force compatibility conditions for distinct models are enforced in the coupling domains. Utilizing different weighting functions, four typical coupling methods, the bridging domain method, the edge-to-edge coupling method, the separate domain coupling, and the separate edge coupling, are obtained. For the last two coupling methods, the finite element-discrete element and the discrete element-finite element coupling domains are nonoverlapping and the compatibility conditions are independent. In addition, an integration algorithm with multiple time steps is developed for the generalized bridging domain method. Numerical results show that the separate domain coupling and the separate edge coupling outperform other methods in avoiding the spurious reflection. For the bridging domain method and the edge-edge coupling method, the high frequency waves are transformed into low frequency waves to keep the energy conservation. However, for the separate domain coupling and the separate edge coupling, due to the filtering of high frequency waves which can not be resolved by the finite element model, the coupling system presents energy loss which decreases as the finite element domain is refined and the high frequency waves is reduced. Additionally, the proposed mutiple-time-step algorithm is very effective.(4) The multiscale numerical analysis platform for the failure process of soil mass is established through adding the codes of the finite element method and the generalized bridging domain method into the DEM open-source software Yade. The numerical result of the multiscale model is similar to that of the discrete element model. However, the computational efficiency is significantly improved.(5) The cone penetration test is modeled by the generalized bridging domain method. In the local domain of interest, the particle flow phenomenon of soil is reproduced, and the failure mechanism of soil mass for different cone tip angles and different penetration depths is revealed. The specific penetration resistance and tip resistance increase as the increase of the cone tip angle, the gravity field and the penetration velocity. The lateral friction is mainly influenced by the gravity field and the penetration velocity, while it is nearly independent of the cone tip angle.