Investigating the effect of particle angularity on suffusion of gap-graded soil using coupled CFD-DEM

Microscopic investigation of the influence of complex stress states on internal erosion and its impacts on critical hydraulic gradients

“…There is an extra height in the CFD domain to enclose the DEM sample (as shown in Figure 7). The size of the fluid cell is equal to the largest particle diameter, and this size ratio has been verified by Qian et al 7 …”

“…Based on previous studies by Qian et al., 7,57 the gap‐graded soil specimen adopted in this paper is a mixture of two types of sands: the coarse particle whose diameter is 1.8∼2 mm and the fine particle whose diameter is 0.3∼0.33 mm. According to Fannin and Moffat, 58 the soil is classified as internally unstable if $${D_{15c}}/{d_{85f}} \ge 4$$, where$${D_{15c}}$$ is the particle diameter corresponding to 15% by mass of coarse particles, and $${d_{85f}}$$ is the particle diameter corresponding to 85% by mass of fine particles.…”

“…A non‐linear gradual increase in the hydraulic gradient is used to obtain an accurate critical hydraulic gradient (i.e., 0.1 per 0.5 s for $$i \le 0.5$$, 0.15 per 0.5 s for $$0.5 < i \le 1.1$$, 0.3 per 0.5 s for $$1.1 < i \le 2.0$$, 0.5 per 0.5 s for $$2 < i \le 10.0$$). The reason for only giving a time of 0.5 s at each hydraulic gradient level is that, based on experience from previous numerical calculations, 7–9,22 the rate of fine particle loss is fastest at the beginning. Even with an action time of only 0.5 s at each stage, there is still a clear distinction between seepage characteristics under different magnitudes of hydraulic gradients, as evidenced by the macro and micro results in Sections 5 and 6, respectively.…”

“…In contrast, the coarse‐fine contact type starts with a high interaction force, suggesting that the fine particles in c‐f contact can still bear part of the load. However, some of the c‐f contacts seem to be vulnerable as there is a fast decrease in the average force once the fluid is taken into effect (such contacts are defined as metastable structures in other studies 7,50 ). It's also observed that the evolution of c‐f contact begins to fluctuate at $$t = 1.3{\rm{s}}$$, which is in general agreement with the time point of the critical hydraulic gradient determined by the mass loss curve and the surface velocity of the fine particles.…”

“…Previous studies have shown that erosion is affected by various factors, for example, grain size distribution, 3,4 fine particle content, 2,5,6 particle shape, 7,8 and hydraulic gradient 9,10 . Conventional approaches, for example, laboratory experiments 10,11 and analytical method 4,12 based on continuum theories, have provided a phenomenological manner to successfully capture the macroscopic response of soil subjected to erosion.…”