Abstract
The present PhD work deals with numerical modelling of the tamping process on railway ballast, which is a common operation in order to restore the level of the rail tracks following differential settlement under dynamic loading, in view of micromechanical analysis of physical phenomena involved in different phases of this process. A discrete element model is developped in the framework of the contact dynamics method that allows us to simulate a full cycle of the process consisting of introducing vibrating metallic arms into the ballast, compressing the ballast under the sliders and pulling out the arms. These simulations take into account the polyhedral shape of the grains which is an important element for realistic simulations of ballast. A detailed investigation of the quasistatic rheology of granular materials with polyhedral particle shapes was performed allowing us to evidence the micrormechanical origins of the shear strength of these materials closely related to the facetedness of the particles. The vibrational dynamics of the grains was studied in interaction with a vibrating intruder and under the action of horizontal vibrations of a retaining wall. A scaling of the dynamics as a function of loading parameters is proposed by dimensional analysis and validated by simulations, as well as a characteristic frequency for optimal compaction of the material. Finally, on the basis of these investigations and parametric studies, concrete recommendations are proposed in order to improve the performance of the tamping process.