# Analysis of the Influence of Water Level Change on the Seepage Field and Stability of a Slope Based on a Numerical Simulation Method

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## Abstract

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## 1. Introduction

## 2. Unsaturated–Unstable Seepage and Slope Stability Analysis Theory

- When k/μv < 1/10, the wetting surface in the dam still remains about 90% of the total initial water head after the reservoir water level drops.
- When k/μv > 60, the wetting surface in the dam remains below 10% of the total water head, and the wetting line in the slope is approximately a straight line, which is more consistent with the shape of the wetting line in slow-down mode, and the water level drop is regarded as a slow drop.
- When the ratio is in the range of 1/10 < k/μv < 60, the mode is the intermediate slow drop–sudden drop category.

_{1i}is the average height of the first soil strip; h

_{2i}is the average height of the second soil strip; b

_{i}is the width of the soil strip; ${p}_{wi}$ is the pore water pressure; l

_{i}is the arc length of the i-th soil strip; $\gamma $ is the natural weight of the soil; and ${\gamma}_{sat}$ is the saturated weight of the soil. The symbols in the formula are shown in the calculation diagram of the safety factor in Figure 1.

## 3. Design and Verification of Model Tests and Numerical Simulations

#### 3.1. Slope Model Test

#### 3.2. GeoStudio Numerical Simulation

#### 3.3. Comparison and Verification of Model Test and Numerical Simulation Results

## 4. Analysis of Influence of Water Level Fall on Slope Stability

#### 4.1. Analysis of Influence of Water Level Falling Speed on Slope Stability

#### 4.1.1. Influence of Falling Speed on Seepage Field

#### 4.1.2. Influence of Water Level Falling Speed on Slope Safety Factor

#### 4.2. Influence of Drop Ratio on Slope Stability

#### 4.2.1. Influence of Drop Ratio on Seepage Field of Slope

#### 4.2.2. Influence of Drop Ratio on Safety Factor of Slope

#### 4.3. Influence of Filling Materials on Slope Stability

#### 4.3.1. Influence of Permeability Coefficient on Seepage Field of Slope

#### 4.3.2. Influence of Permeability Coefficient on the Slope Safety Factor

## 5. Practical Application of Modeling Methods

## 6. Conclusions

- When the water level in front of the slope falls at different speeds, the greater this speed, the greater the downward seepage force formed by the seepage field of the slope to the slope body. When the water level in front of the slope falls at a constant speed, the penetration force is greater at the position closer to the slope.
- When the speed at which the water level before the slope falls increases, the safety factor of the slope decreases. When the water level in front of the slope falls at different speeds, the change curve of the safety factor is steeper when the speed is more, and the safety factor value of the same water level before the slope is smaller. The safety factor of the slope when the water level falls at a speed of 0.5 cm/s (as studied in this paper) has a maximum falling range of 37% during the water level change process.
- With an increase in the drop ratio, the safety factor of the slope decreases. When the drop ratio is constant, the loss of stability is worse if the initial water level is lower. Due to the difference in soil mechanical properties, under the condition of the drawdown of water levels in front of the slope, the noncohesive medium sand slope is more prone to instability failure than the cohesive silt slope.
- When this modeling method is applied to matrix suction, it is found that the effect of matrix suction increases the safety factor of the slope. When the speed of the fall of the water level in front of the slope decreases, after the water level in the front of the slope is stable, the unsaturated area formed in the slope is larger, and the influence of the matrix suction on the stability of the slope also increases.

## Supplementary Materials

## Author Contributions

## Funding

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**The layout of the model box and marking points: (

**a**) model box; (

**b**) mark point arrangement; (

**c**) instrument burial.

**Figure 4.**Micro-osmometer and embedded position map of earth pressure box in model test of medium sand slope (m).

**Figure 6.**Slope dimension diagram of an example studied after [50].

**Figure 7.**Comparison of results of different soil model tests and numerical simulation: (

**a**) medium sand; (

**b**) silt.

**Figure 8.**Comparison of water levels inside and outside of the silt slope with different deceleration slopes: (

**a**) x = 230 cm; (

**b**) x = 165 cm; and (

**c**) x = 100 cm.

**Figure 9.**Variation of the safety factor of silt slope with different influence conditions under different water levels: (

**a**) falling speed; (

**b**) time.

**Figure 10.**Changes in slope stability with time for different drop ratios after the water level stabilizes.

**Figure 11.**Changes in slope stability with time at different initial water levels after water level stabilization.

**Figure 12.**Variation of pore pressure at different points in slopes with different materials: (

**a**) x = 100 cm; (

**b**) x = 140 cm; (

**c**) x = 180 cm; (

**d**) x = 220 cm; and (

**e**) x = 250 cm.

**Figure 13.**The safety factor of medium sand slope varies with different influence conditions under different water levels: (

**a**) falling speed and (

**b**) time.

Soil | Natural Weight γ (kN/m^{3}) | Permeability Coefficient k (cm/s) | Cohesion c (kPa) | Friction Angle φ (°) |
---|---|---|---|---|

Medium sand | 19.4 | 2.4 × 10^{−2} | 0 | 32 |

Silt | 23.0 | 1.9 × 10^{−4} | 1.92 | 23 |

Filling Material | Slope Ratio (Height: Width) | Slope Height (cm) | Landing Velocity (cm/s) | Drop (cm) |
---|---|---|---|---|

Medium sand | 1:2 | 75 | 0.043 | 54 |

Silt | 1:0.7 | 90 | 0.15 | 46 |

V (cm/s) | Bishop | Janbu | Ordinay | Morgenstern–Price | ||||
---|---|---|---|---|---|---|---|---|

Yes | No | Yes | No | Yes | No | Yes | No | |

0.5 | 1.555 | 1.553 | 1.432 | 1.428 | 1.449 | 1.446 | 1.553 | 1.550 |

0.05 | 1.606 | 1.594 | 1.485 | 1.458 | 1.493 | 1.469 | 1.604 | 1.592 |

0.001 | 1.762 | 1.713 | 1.654 | 1.559 | 1.660 | 1.597 | 1.761 | 1.711 |

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**MDPI and ACS Style**

Sun, Y.; Li, Z.; Yang, K.; Wang, G.; Hu, R.
Analysis of the Influence of Water Level Change on the Seepage Field and Stability of a Slope Based on a Numerical Simulation Method. *Water* **2023**, *15*, 216.
https://doi.org/10.3390/w15020216

**AMA Style**

Sun Y, Li Z, Yang K, Wang G, Hu R.
Analysis of the Influence of Water Level Change on the Seepage Field and Stability of a Slope Based on a Numerical Simulation Method. *Water*. 2023; 15(2):216.
https://doi.org/10.3390/w15020216

**Chicago/Turabian Style**

Sun, Yongshuai, Zhihui Li, Ke Yang, Guihe Wang, and Ruilin Hu.
2023. "Analysis of the Influence of Water Level Change on the Seepage Field and Stability of a Slope Based on a Numerical Simulation Method" *Water* 15, no. 2: 216.
https://doi.org/10.3390/w15020216