The development of a combined finite-discrete element model for quasistatic geotechnical engineering applications

The mechanics of soil erosion around buried structures is a challenging soil-structure interaction problem. It involves progressive particle movement and interaction with an existing subsurface structure. Using standard finite element method (FEM) and continuum mechanics to simulate the soil erosion process (that involves large soil displacement and discontinuities in the soil matrix) has proven to be inefficient. Continuum methods do not allow for the realistic simulation of the gradual soil loss into the structure. Discrete element method (DEM), on the other hand, has proven to be suitable for such analysis, however, it involves large amount of calculation to simulate a real problem. This thesis is devoted to develop an efficient method that allows the analysis of the above problem considering both the micro and macro scale features of the problem.;In the combined method, the soil in the area experiencing large displacements or discontinuities is modelled using discrete elements whereas the rest is modelled using finite elements. The integration scheme used in the FEM has been investigated and modified to suit the combined finite-discrete element. Dynamic relaxation technique with automatic time step control has been developed to optimize the performance of the scheme. Finally, interface has been introduced to bridge the finite and discrete element domains. By adopting a suitable damping scheme for each domain, the numerical stability of the combined method has been maintained. The developed method has been calibrated using benchmark problems and used to investigate the effects of particle loss induced around an existing tunnel lining on the stress distribution in the lining structure. Conclusions and recommendations are made regarding the three dimensional effect of soil deterioration on the structural integrity of existing tunnels in soft ground.;The research results have been published in refereed journals and conference proceedings amounting to 4 journal papers and 2 conference papers. These papers are compiled to produce 7 chapters and 2 appendices in this manuscript-based thesis. The investigation of the effects of erosion voids around existing tunnels is first conducted using conventional finite element method (FEM) to illustrate the limitation of the FEM in modelling problems involving large deformation and discontinuities. The rest of the thesis is devoted to describe the development of the combined finite-discrete element method. A simple and effective algorithm to generate realistic initial soil conditions required for the discrete element method has been developed. Using the developed algorithm, the calibration procedure needed to determine the properties of the discrete elements is only necessary for a small initial sample (e.g. grain size distribution, internal friction angle). The final packing is then generated based on the initial sample exhibiting the same macro behaviour.