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ItemInternal stability of artificial and realistic gap‐graded granular assemblies( 2021)Internal erosion is responsible for nearly half of dam failures globally and considered a major risk to safety and security of water management structures. Suffusion as one of the main mechanisms of internal erosion is defined as relocation or migration of fine particles within pre-existing pores of internally unstable soils caused by seepage forces. This research is focused on suffusion and the impact of influential parameters on internal stability potential of gap-graded granular material. Several experimental research works have been carried out so far to study suffusion, influential parameters, and its impact on the mechanical behaviour of soils. However, due to the macro-scale nature of experimental studies, micro-scale interactions between soil particles is not well investigated. In fact, macro-scale observations in physical experiments is dictated by micro-scale interactions inside granular packing such as particle displacements, contact forces, connectivity status of particles with their neighbours and pore size distribution. These micro-scale interactions are investigated by micromechanics and discrete element method (DEM) is an efficient numerical modelling tool in providing micro-scale properties of the packing of granular material. DEM has been widely used to evaluate microstructure of granular material under the impact of various influential parameters. The influence of external loadings, particle shape (i.e. spherical particles and angular particles selected from predefined library of different shapes) and qualitative relative density have been investigated in previous DEM studies. This PhD work aims to quantify internal stability status of the gap-graded granular packings under the impact of several controlling parameters. Controlling parameters investigated in this research are fine content (FC), gap-ratio (GR, size ratio between the smallest coarse particle and the largest fine particle), relative density (Dr), stress path and real soil fabric. To find the research gap, previous experimental and numerical modelling studies is reviewed first. Due to the lack of standardized method for sample production with target relative density in DEM, a new procedure based on variation of inter-particle coefficient of friction is developed. Then, packings with FC between 10% to 50%, GR between 2 to 10, and Dr in range of Dr,min to 100% are generated in DEM to assess the impact of these controlling parameters on contribution of fine particles to stress transferring mechanism and internal stability status of packings. Analysis of stress reduction factor (defined as the ratio of effective stress transmitted by fine fraction to the effective stress carried by the packing) shows that contribution of fine fraction to stress transferring mechanism of packings with GR = 4, FC = 25% and 35% improves with Dr suggesting that these packings can be classified as transitional. However, transitional zone starts at higher FC as GR increases beyond GR = 4. Variation of strong contact types under the impact of controlling parameters reveal that stress transferring mechanism is mainly dominated by strong fine-coarse and fine-fine contacts as transitional zone starts and when the packing is internally stable, respectively. In addition, DEM modelling outcome is resulted in a framework based on intergranular matrix phase diagrams to predict internal stability of the gap-graded granular soils. To investigate the impact of stress path on the roles that fine particles play in stress transferring mechanism, axial compression and extension stress paths are applied to multiple gap-graded packings. Packings with GR = 4 and FC = 10% and 15% at Dr = 100% and FC = 25% at Dr = 50% (internally unstable), FC = 25% and 35% at Dr = 100% and 50%, respectively, (transitional) and FC = 35% at Dr = 100% (internally stable) are considered here. DEM findings show a very similar impact of axial compression and extension on variation of micro-scale and macro-scale parameters. In this study, stress reduction factor, proportion of active (directly involved in force network), semi-active (provide a secondary support to the soil primary structure) and in-active (unstressed) fine particles and Z (coordination number) are considered as micro-scale and e (void ratio) as macro-scale parameters. In fact, stress reduction factor, proportion of active fine particles and Z decrease, but e increases when packings with FC = 25% and 35% at Dr = 100% undergo through axial stress paths. Although, the reverse trend is achieved for the variation of these micro- and macro-scale parameters with axial strain for the same packings at Dr = 50%, the impact of axial compression and extension stress paths is more noticeable for the packing with FC = 35% at Dr = 50%. In addition, it is interesting to note that both micro-scale and macro-scale parameters in packings with FC = 25 and 35% at Dr = 50% and 100% show a converging trend under axial extension, but they fully converge under axial compression at large strains. Proportion of semi-active and in-active fine particles increases and decreases, respectively, at large strains for packings with FC = 25 and 35% at Dr = 50%. Therefore, it is expected to observe initiation of suffusion under larger hydraulic gradient at large strains in comparison to isotropic compression for this packing. To study the impact of real soil fabric on contact network and contribution of fine particles to stress transferring mechanism, internally unstable gap-graded soil sample is scanned by micro computed tomography (micro-CT) and transferred to DEM. To enhance quality of micro-CT scans, various image filters are used, but anisotropic diffusion and non-local mean filters demonstrate an outstanding performance. DEM outputs suggest a crucial impact of coefficient of friction on porosity and coordination number of coarse particles. Findings of this study provides a better understanding of suffusion, erodibility potential of fine particles and microstructural changes in packing of granular gap-graded material under the impact of controlling parameters. Outcome of this research helps in describing underlying phenomenon of observations in physical erosion experiments. Finally, results of this study can be used in revising standard methods and guidelines which are based on empirical observations and do not consider micro-scale interactions between particles.