Keywords
Soil–rock mixtures, internal erosion, fines content, solid-fluid interaction, particle skeleton structure

INTRODUCTION

Soil–rock mixtures consist of rock blocks within soils composed of sand, silt, and clay. The geological origins of soil–rock mixtures vary between landslide, colluvial, alluvial–diluvial, and glacial deposits (Xu et al., 2007), which are widely distributed in the Three Gorges Reservoir area, China (Zhang et al., 2015; Shi et al., 2016). Under the action of rainfall infiltration, landslides, collapses, and other geological disasters can easily occur due to the migration of fine particles in gap-graded soil–rock mixtures (Grosta et al., 1999; Zhang et al., 2011; Johan et al., 2013; Lei et al., 2017; Fan et al., 2020). Therefore, the study of seepage-induced fine particle migration through gap-graded soils is crucial (Cheng et al., 2018; Tian et al., 2020).
Soil–rock mixtures are granular materials that can be constituted in many different ways with different types of intergrain contacts (Thevanayagam et al., 2002). The microstructure and mechanical behaviour of granular materials largely depend on the particles of the constituent materials and the particle size distributions (PSDs). Particles of different sizes exhibit a high degree of force heterogeneity. Skempton et al. (1994) found that fine content (FC) in gap-graded soils affects soil strength. Ouyang et al. (2015) presented experimental investigations on the influence of initial FC on the fabric of soils subjected to internal erosion. Lopez et al. (2016) proposed a case study of a soil structure based on previous empirical observations, which conceptually explained the effects of different components (coarse and fine particles) on the mixture behaviour. Tian et al. (2020) studied the critical FC of natural accumulation soil, beyond which the internal stability of the mix was distinctly altered. Based on the fact that fine particles in soils gradually migrate downwards and lead to subsequent slope failure during rainfall events, Yin et al. (2021) developed a 3D discrete solid–fluid sequentially coupled model to analyse changes in pore structure caused by fine particle migration. However, there are few studies on the micro process and evolution characteristics of fine particle migration of different FCs and the visual description of the infiltration process phenomena.
Owing to advancements in computer technology, numerical simulation methods have been employed to address this limitation and investigate the particle migration process with high precision (Guo and Yu, 2017; Peters et al., 2019; Wang et al., 2020). For the study of gap-graded soils, particle-size numerical test using the CFD–DEM method has become a research trend (Zou et al., 2020; Xiong et al., 2021), as it can enable a deeper understanding of the fine particle migration process and the coupling mechanism between particles and seepage flow. Zou et al. (2013) studied the particle transport mechanism in a base soil–filter system using the CFD–DEM method. Yang et al. (2019) improved the semi-resolved CFD–DEM model for seepage-induced fine particle migration. Cheng et al. (2021) revealed that unresolved CFD–DEM models predict unreasonably high critical hydraulic gradients for suffusion in gap-graded soil columns packed under gravity.
The occurrence of internal erosion of grap-graded soil–rock mixtures is the result of the interaction of water pressure, particle migration, and soil stress state. In this study, a fluid–solid coupled model of internal erosion through the coupling of CFD and DEM was developed to study the coupling effect of water pressure, particle migration, and soil stress state. Moreover, a permeability test for the stability of the gap-graded soil–rock mixtures considering particle loss was conducted to analyse the effect of different FCs, and the numerical simulation results were verified. This study focused on the particle-scale process of the seepage deformation of grap-graded soil–rock mixtures and dynamic changes in load-bearing skeletons with different FCs. This study is helpful in exploring the deformation and failure law of soil–rock mixtures under the condition of rainfall infiltration.

COUPLED CFD–DEM MODEL

The coupled CFD–DEM method used in this study included three types of formulation: DEM, CFD, and CFD–DEM coupling formulations. PFC3D was used to solve the particle system at the particle scale, and Newton’s law of motion was used to control the particle motion. Moreover, ABAQUS with the porous media seepage theory, Darcy flow model, was used as the CFD solver. The particle–fluid interaction, including the drag and pressure gradient forces, was calculated using the ABAQUS–PFC3D program developed by us. The entire fluid–solid interaction process is illustrated in Fig. 1.

2.1 Governing Equations of the Particle Phase

In the fluid–solid coupling method, the behaviour of the particle phase is controlled by Newton’s law of motion. In the PFC3D program, the governing equations of the particle phase are given by Eqs. (1) and (2) (Itasca, 2018),