An Integrated DEM-FEM Simulation Framework for Preferential Flow Path Evaluation in Waste Landfill Composed of Non-Spherical Particles  
[PERSON]  
1 Material Cycles Division, National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba 305-8506, Japan  
there is no advection and only diffusion acts, which induces a tailing phenomenon, where low-concentration leachate is generated for a long period of time. Neither phenomenon can be handled by conventional theories.  
To address these issues, previous studies have used a qualitative approach. Soil tank experiments, for instance, use colored dyes as tracers to investigate the formation process of water channels through unsaturated zones [1; 2; 3]. In addition, instead of soil tank experiments, there have been studies that precisely analyze and characterize pore structures by sampling selected elements in the soil and subjecting them to X-ray CT analysis or FIB-SEM. Research has also been conducted to numerically simulate the flow that occurs within the pore structure using fluid analysis [4]. However, these techniques are difficult to apply to waste landfill layers for the following reasons: Waste is a complex of various colors; thus, dye-based tracers are inappropriate for use. The size of the object that can be analyzed is limited in X-ray CT analysis, and metals contained in the waste generate secondary radiation, which could cause noise.  
In practical work, continuum mechanics, which assumes a homogeneous pore structure, is adopted, even under the situation that water channels have not been clarified. A field of study known as soil mechanics has become established for the range of particle sizes known as soil. In the soil pore structure, the effect of water channels on the flow and transport phenomena is relatively small, and the applicability of soil mechanics has been proven by numerous verification experiments. On the other hand, the study of crushed stone and waste materials that do not fall under the definition of soil has not been fully developed. These particles have more heterogeneous pore structures than those of soil, so the effect of water channels cannot be ignored. However, the research is expected to require a considerable amount of cost and tends to be avoided. Along with the particle size varying, the variability also increases, making it difficult to explain the representativeness in experimental evaluations. Considering sensitivity to scale effects, experiments of a certain size are essential. But large-scale experiments are not practical due to the cost, resources, and materials required to conduct systematic research.  
In this study, we investigated a numerical experimental method that combines the creation of a simulated porous medium using the discrete element method (DEM) and the finite element method (FEM) to analyze the flow in the pore structure. This technique is highly dependent on computing resources, so it has only become feasible in the modern era thanks to improvements in computer performance, efficient programming environments and compilers, and advances in software. The final goal is to identify the porous characteristics related to the formation of water channels, but their parameter estimation is still the subject of ongoing research. In this study, we present the basic concept of the numerical experimental method using DEM and FEM and an example of evaluating the flow in a heterogeneous porous medium. We developed coupled DEM-FEM analysis, which, for the first time, includes non-spherical particles (thin plate-like and rod-like particles). In addition, we overcame the limitations of experimental approaches using the coupled analysis to simulate the fine internal structure of the waste layer, which, until now, has only been studied via X-ray CT. The proposed method suggests the possibility of contributing to research on water channels.