# Experimental and DEM-CFD Coupling Investigations on the Characteristics and Mechanism of Seepage Erosion for Cohesionless Soil

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## Abstract

**:**

## 1. Introduction

## 2. Model Test on Seepage Erosion of Cohesionless Soils

#### 2.1. Material

#### 2.2. The Seepage Erosion Device

## 3. DEM-CFD Coupling Analysis of Seepage Erosion of Cohesionless Soils

#### 3.1. DEM-CFD Coupling Method

#### 3.2. DEM-CFD Model of Seepage Erosion for Cohesionless Soils

^{−5}, as a criterion for balance to ensure that the soil layer reached a stable state. Finally, the model was determined to have a total of 17,134 particles.

## 4. Results and Discussion

#### 4.1. Experimental Phenomenon and Critical Hydraulic Gradient Analysis

#### 4.2. Mesoscopic Analysis of Seepage Erosion Process and Particle Migration Law

- (1)
- Stable seepage stage

- (2)
- Fine particle upward migration stage

- (3)
- Skeleton particle-loosening stage

- (4)
- Complete failure stage

#### 4.3. Microstructural Characteristics of Soil Particles during the Seepage Erosion Process

#### 4.4. Influence and Mechanism Analysis of Seepage Erosion on the Mechanical Properties of Soil

## 5. Conclusions

- (1)
- A self-made seepage erosion device for cohesionless soil was developed, which could be used to observe the seepage erosion process, measure the critical hydraulic gradient of seepage erosion, and investigate the escaping particles during the seepage erosion process.
- (2)
- The seepage erosion process of cohesionless soil could be divided into four stages: stable seepage, fine particle upward migration, sand sample boiling, and erosion damage.
- (3)
- Mixing with coarse-grain sand could increase the critical hydraulic gradient of the fine-grain quartz sand sample because the coarse-grain sand could improve the soil structure and force chain of the fine-grain sand sample.
- (4)
- Increasing erosion ratio meant more loss of fine sand particles and an increase in pore structure between soil skeleton particles. These changes could result in the rearrangement of soil particles, i.e., a decrease in contact number and the weakening of contact force, which led to strength decrease.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 6.**Positions of the measuring balls. (Red ball is the No. 1 measuring ball, green ball is the No. 2 measuring ball).

**Figure 8.**C1 sample experimental phenomenon. (The red circle means: (

**b**) small bulges appeared and sand boiling; (

**e**) seepage channels and cracks).

**Figure 9.**C2 sample experimental phenomenon. (

**a**) Soil sample surface before seepage; (

**b**) small bulges appeared and sand boiling; (

**c**,

**d**) sand boiling intensifies jumps and spreads; (

**e**) sand sample gushed out along the cone; (

**f**,

**g**) seepage channels and cracks.

**Figure 14.**Gradation curves of particle size before and after erosion of C1 sample. (

**a**) ${C}_{sw}$ = 0.014. (

**b**) ${C}_{sw}$ = 0.020. (

**c**) ${C}_{sw}$ = 0.043.

Sand Sample | Particle Composition | Characteristic Particle Size (mm) | Grading Curve Coefficient | |||||
---|---|---|---|---|---|---|---|---|

0.075–0.5 mm | 2–4 mm | ${\mathit{d}}_{10}$ | ${\mathit{d}}_{30}$ | ${\mathit{d}}_{50}$ | ${\mathit{d}}_{60}$ | ${\mathit{C}}_{\mathit{u}}$ | ${\mathit{C}}_{\mathit{c}}$ | |

C1 | 10 | 0 | 0.13 | 0.21 | 0.30 | 0.36 | 2.73 | 0.89 |

C2 | 4 | 6 | 0.18 | 0.44 | 2.08 | 2.67 | 14.57 | 0.39 |

Computation Modules | Parameter Types (Units) | Values |
---|---|---|

Particle model | Particle density, $\rho $/(kg·m^{−3}) | 2650 |

Normal stiffness, ${k}_{n}$/(N·m^{−1}) | 3.0 × 10^{6} | |

Shear stiffness, ${k}_{s}$/(N·m^{−1}) | 2.0 × 10^{6} | |

Bond stiffness ratio, k_{1} | 1.5 | |

Friction coefficient, ${\mu}_{1}$ | 0.5 | |

Initial porosity | 0.48 | |

Wall model | Bond stiffness ratio, k_{2} | 1.5 |

Friction coefficient, ${\mu}_{2}$ | 0.3 | |

Fluid model | Fluid density, ${\rho}_{f}$/(kg·m^{−3}) | 1000 |

Fluid viscosity, ${\mu}_{f}$/(Pa·s) | 0.001 | |

Grid size, (m) | 0.03 × 0.03 × 0.04 | |

Fluid grid cells, (m) | 0.005 × 0.005 × 0.005 | |

Number of grid cells | 288 |

Erosion Rate | State | Average Particle Size ${\mathit{d}}_{50}$/mm | Fineness Modulus ${\mathit{M}}_{\mathit{x}}$ | Inhomogeneity Coefficient ${\mathit{C}}_{\mathit{u}}$ | Curvature Coefficient ${\mathit{C}}_{\mathit{c}}$ |
---|---|---|---|---|---|

C1 sample | Before erosion | 0.302 | 2.74 | 2.729 | 0.888 |

${C}_{sw}$ = 0.014 | After erosion | 0.315 | 2.78 | 2.715 | 0.915 |

Exudate particles | 0.175 | 1.99 | 2.178 | 0.988 | |

${C}_{sw}$ = 0.020 | After erosion | 0.320 | 2.78 | 2.737 | 0.934 |

Exudate particles | 0.178 | 2.04 | 1.849 | 0.971 | |

${C}_{sw}$ = 0.043 | After erosion | 0.332 | 2.82 | 2.757 | 0.979 |

Exudate particles | 0.202 | 2.26 | 2.093 | 0.984 |

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**MDPI and ACS Style**

Su, H.; Dai, D.; Zhang, T.; Yang, J.; Mu, Z.
Experimental and DEM-CFD Coupling Investigations on the Characteristics and Mechanism of Seepage Erosion for Cohesionless Soil. *Water* **2023**, *15*, 3085.
https://doi.org/10.3390/w15173085

**AMA Style**

Su H, Dai D, Zhang T, Yang J, Mu Z.
Experimental and DEM-CFD Coupling Investigations on the Characteristics and Mechanism of Seepage Erosion for Cohesionless Soil. *Water*. 2023; 15(17):3085.
https://doi.org/10.3390/w15173085

**Chicago/Turabian Style**

Su, Hui, Da Dai, Ting Zhang, Jiaqi Yang, and Zhiyong Mu.
2023. "Experimental and DEM-CFD Coupling Investigations on the Characteristics and Mechanism of Seepage Erosion for Cohesionless Soil" *Water* 15, no. 17: 3085.
https://doi.org/10.3390/w15173085