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),