# Effect of Gradation Characteristics and Particle Morphology on Internal Erosion of Sandy Gravels: A Large-Scale Experimental Study

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## Abstract

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## 1. Introduction

## 2. Methodology

#### 2.1. Test Apparatus

#### 2.2. Test Materials

#### 2.3. Specimen Preparation

#### 2.4. Test Scheme

## 3. Test Results

#### 3.1. Test Repeatability

#### 3.2. Effect of Gap Ratio

#### 3.3. Effect of Fines Content

^{−2}cm/s to 0.3 cm/s. In contrast, the hydraulic conductivities of G8P10A and G8P40A did not show obvious changes. Figure 8b gives the relationship between fines eroded ratio and hydraulic gradient as well as the relationship between fines eroded mass and hydraulic gradient. It can be seen that the fines eroded ratio of G8P10A was significantly higher than that of G8P25A, while, due to the different fines content, their fines eroded masses were close. However, the hydraulic conductivity development of the two specimens was different during the erosion process.

#### 3.4. Effect of Coarse Particle Morphology

^{−4}cm/s to the ${k}_{\mathrm{f}}$ = 1.13 cm/s, exceeding the hydraulic conductivity of G8P25A after the hydraulic gradient exceeded 1.5. This phenomenon can be attributed to the higher fines eroded ratio. As shown in Figure 10b, the fine eroded ratio of G8P25R reached 56.78% at the end of the test, almost double the value of 32.25% of G8P25A. Therefore, the change in hydraulic properties was larger, and the erosion-induced volumetric strain was more remarkable (as shown in Figure 10c).

#### 3.5. Skeleton Stability Analysis

## 4. Conclusions

- (1)
- The gap ratio can impact both the internal stability and mechanical stability of the soil. The increase in gap ratio not only reduced the progression hydraulic gradient, but also promoted the fine particle erosion process and the consequent permeability increase and volumetric strain. Moreover, a positive correlation was shown between the gap ratio and the skeleton stability index, indicating that soil with a higher gap ratio is more likely to suffer mechanical instability during the erosion process.
- (2)
- The fines content influences the filling state of inter-granular pores and the transfer of effective stress on fine particles, thereby affecting the permeability and internal stability of the soil. The soil showed a decreasing trend in the hydraulic conductivity while an increasing trend in the progression hydraulic gradient with the increase of fines content. Additionally, it was found that the increase in permeability was not directly related to the increase in porosity but might be highly related to the change in the pore channels morphology.
- (3)
- The morphology of coarse particles is an influential factor in determining the internal erosion characteristics. The increase in roundness, sphericity, and smoothness can reduce the interlocking effect within coarse particles, which results in two consequences. First, it facilitates the movement of fine particles. The specimens with round coarse particles showed a lower progression interstitial flow velocity and a higher fines eroded ratio compared with the specimens with granular coarse particles. Second, it prompts the soil matrix to reach a denser packing state. The specimens with round coarse particles not only showed a lower porosity and permeability after the specimen preparation, but also showed a higher skeleton stability index during the erosion process.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Photo of the test materials and the mesoscopic surface view of particles ranging from 2 to 5 mm.

**Figure 3.**Variation of porosity with fines content in gap-graded soils (with coarse fraction ranging from 2 mm to 80 mm and fine fraction ranging from 0.075 mm to 0.5 mm) of gap ratio ${G}_{\mathrm{r}}$ = 8, and the red-dotted line denotes the approximate critical fines content.

**Figure 6.**Test results of the specimens in G8P25A, G10P25A, and G8P25R: (

**a**) variation of hydraulic conductivity with hydraulic gradient; (

**b**) variation of fines eroded ratio with hydraulic gradient; (

**c**) variation of volumetric strain with hydraulic gradient.

**Figure 7.**Internal erosion characteristics of the specimens with different gap ratios: (

**a**) variation of hydraulic conductivity with hydraulic gradient; (

**b**) variation of fines eroded ratio with hydraulic gradient; (

**c**) variation of volumetric strain with hydraulic gradient; (

**d**) variation of progression hydraulic gradient with gap ratio.

**Figure 8.**Internal erosion characteristics of the specimens with different fines contents: (

**a**) variation of hydraulic conductivity with hydraulic gradient; (

**b**) variation of fines eroded ratio and fines eroded mass with hydraulic gradient; (

**c**) variation of volumetric strain with hydraulic gradient; (

**d**) variation of progression hydraulic gradient with gap ratio.

**Figure 9.**Mesoscopic fabric illustration to compare the change in pore channels induced by fine particle loss in: (

**a**) soil in which fine particles under-fill the inter-granular pores; (

**b**) soil in which fine particles precisely-fill the inter-granular pores.

**Figure 10.**Internal erosion characteristics of the specimens with different coarse particle morphologies: (

**a**) variation of hydraulic conductivity with hydraulic gradient; (

**b**) variation of fines eroded ratio with hydraulic gradient; (

**c**) variation of volumetric strain with hydraulic gradient; (

**d**) progression hydraulic gradient and progression interstitial flow velocity in different specimens.

**Figure 11.**Relationship of the skeleton stability index and hydraulic gradient: (

**a**) specimens with different gap ratios; (

**b**) specimens with different fines contents and coarse particle morphologies.

**Figure 12.**Mesoscopic fabric illustration to compare the volumetric strain induced by fine particle loss in: (

**a**) soil with a lower gap ratio; (

**b**) soil with a higher gap ratio.

Particle Size Fraction (mm) | Angular Coarse Particles | Round Coarse Particles | ||
---|---|---|---|---|

Sphericity | Roundness | Sphericity | Roundness | |

80~40 | 0.59 | 0.69 | 0.71 | 0.89 |

40~20 | 0.63 | 0.78 | 0.72 | 0.89 |

20~10 | 0.58 | 0.84 | 0.76 | 0.96 |

10~5 | 0.54 | 0.89 | 0.71 | 0.97 |

5~2 | 0.55 | 0.92 | 0.63 | 0.96 |

Specimen | Gap Ratio | Fines Content (%) | Coarse Particle Morphology | C_{u} | Initial Porosity |
---|---|---|---|---|---|

${\mathit{\varphi}}_{0}$ | |||||

G4P25A | 4 | 25 | granular | 63.1 | 18.7 |

G6.7P25A | 6.7 | 25 | granular | 84.2 | 18.5 |

G8P25A | 8 | 25 | granular | 87.8 | 18.1 |

G10P25A | 10 | 25 | granular | 93.5 | 18.0 |

G8P10A | 8 | 10 | granular | 60 | 23.6 |

G8P40A | 8 | 40 | granular | 72 | 22.1 |

G8P25R | 8 | 25 | round | 87.8 | 16.1 |

Specimen | ${\mathit{k}}_{0}$ (cm/s) | ${\mathit{k}}_{\mathbf{f}}$ (cm/s) | ${\mathit{\mu}}_{\mathbf{f}}$ (%) | ${\mathit{\epsilon}}_{\mathbf{v},\mathbf{f}}$ (%) | ${\mathit{i}}_{\mathbf{p}}$ | ${\mathit{S}}_{\mathbf{s},\mathbf{f}}$ |
---|---|---|---|---|---|---|

G4P25A | 2.55 × 10^{−2} | 5.43 × 10^{−2} | 12.42 | 0.43 | 1.5 | 0.170 |

G6.7P25A | 1.69 × 10^{−2} | 0.18 | 29.78 | 1.93 | 1.5 | 0.268 |

G8P25A | 2.22 × 10^{−2} | 0.30 | 32.25 | 1.71 | 1.25 | 0.259 |

G8P25A-r | 1.65 × 10^{−2} | 0.23 | 28.34 | 1.37 | 1.0 | 0.236 |

G10P25A | 1.83 × 10^{−2} | 0.95 | 80.60 | 10.88 | 1.0 | 0.658 |

G10P25A-r | 1.58 × 10^{−2} | 0.94 | 73.08 | 10.19 | 0.8 | 0.680 |

G8P10A | 0.63 | 0.84 | 79.23 | 0.23 | 0.5 | 0.038 |

G8P40A | 4.63 × 10^{−5} | 3.64 × 10^{−5} | 0.08 | 0.06 | >3.0 | 2.391 |

G8P25R | 5.78 × 10^{−4} | 1.13 | 56.8 | 8.51 | 1.25 | 0.715 |

G8P25R-r | 3.99 × 10^{−4} | 1.05 | 59.9 | 9.31 | 1.25 | 0.741 |

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**MDPI and ACS Style**

Deng, Z.; Chen, X.; Jin, W.; Wang, G.
Effect of Gradation Characteristics and Particle Morphology on Internal Erosion of Sandy Gravels: A Large-Scale Experimental Study. *Water* **2023**, *15*, 2660.
https://doi.org/10.3390/w15142660

**AMA Style**

Deng Z, Chen X, Jin W, Wang G.
Effect of Gradation Characteristics and Particle Morphology on Internal Erosion of Sandy Gravels: A Large-Scale Experimental Study. *Water*. 2023; 15(14):2660.
https://doi.org/10.3390/w15142660

**Chicago/Turabian Style**

Deng, Zezhi, Xiangshan Chen, Wei Jin, and Gang Wang.
2023. "Effect of Gradation Characteristics and Particle Morphology on Internal Erosion of Sandy Gravels: A Large-Scale Experimental Study" *Water* 15, no. 14: 2660.
https://doi.org/10.3390/w15142660