# Propagation Modeling of Rainfall-Induced Landslides: A Case Study of the Shaziba Landslide in Enshi, China

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Study Areas

#### 2.1. Overview of the Landslide Area

#### 2.2. Geological Condition

^{2}in Enshi and analyzed 119 landslides, including the Shaziba landslide. They reported that about 71% of the landslides occurred in two strata: the Silurian strata and the Triassic Badong Formation strata. The Shaziba landslide is located in the Silurian strata. The geological settings in the study area provide abundant loose source material for landslide events.

#### 2.3. Rainfall Characteristics

#### 2.4. Landslide Dam

^{6}m

^{3}[16]. After the landslide, the vegetation was severely damaged, with obvious scars on the left side and sparse vegetation still on the right side (Figure 2).

#### 2.5. Failure Process of the Shaziba Landslide

^{6}m

^{3}source material on the west of the slope collapsed and flowed downward, some of which slipped into the Qingjiang River. The deformation of the slope developed continuously under the influence of heavy rainfall. Eventually, approximately 2.5 × 10

^{6}m

^{3}of source material failed and rushed into the gully at 5:30 am on 21 July 2020. As shown in Figure 2, the gully in the transportation area divided the landslide into two sections. The eastern section had a short moving distance and good integrity, while numerous sliding masses moved outward on the western section, forming a large steep slope. In addition, there were two concentrated flows in the central and western parts of the steep slope, which were the result of surface flows during heavy rainfall (Figure 8d). Under the influence of heavy rainfall and surface flows in the western gully, the sliding mass flowed into the Qingjiang River (Figure 8e,f), forming a landslide dam with a length of 60–80 m, a width of 300–350 m wide, and a height of 8–10 m. The Shaziba landslides caused serious damage to the surrounding farmland, vegetation, houses, and infrastructure.

## 3. Methodology

#### 3.1. PFC Theory

#### 3.2. Model Establishment

^{6}m

^{3}based on the field investigation. The calculation accuracy of the PFC model increases when using fine particles, while the computational efficiency decreases sharply when increasing the particle numbers. Therefore, to balance the calculation accuracy and efficiency, the Shaziba landslide was discretized into a total of 13,136 particles with a diameter of 2–2.5 m by using the Ball-Wall mode. The density of the material was 2.1 × 10

^{3}kg/m

^{3}. As shown in Figure 10, the sliding surface of the Shaziba landslide was composed of 696,129 wall elements. To capture the evolution of displacement and velocity during landslide propagation, a total of 15 monitored particles were placed in the front, middle and rear parts of the sliding body respectively (Figure 10).

#### 3.3. Calibration of Parameters

## 4. Results

#### 4.1. Kinetic Process of the Shaziba Landslide

#### 4.2. Landslide Runout Behavior

#### 4.3. The Morphology of the Landslide Deposits

## 5. Discussion

#### 5.1. Mechanism of the Landslide

#### 5.2. Influence of the Effective Modulus on Landslide Propagation

^{5}Pa, 16 × 10

^{6}Pa, 16 × 10

^{7}Pa, and 16 × 10

^{8}Pa, respectively. The final accumulation area, average velocity, and runout displacement of the landslide under different effective modulus are shown in Figure 18. When the effective modulus was set to 16 × 10

^{5}Pa, the particle system was loosely distributed in the accumulation areas. As the effective modulus increases, the distribution of particle system becomes more concentrated, and the height of the final landslide deposits increases (Figure 18a–d). Particularly, when the effective modulus increased to 16 × 10

^{8}Pa, due to the close contact of the particles, the velocity of the landslide significantly decreased. As a result, a large number of particles could not reach the river channel (Figure 18d). The results evidently show that the runout behavior of a landslide and the morphology of landslide deposits are closely related to the effective modulus in the contact model.

#### 5.3. Limitations of the Modeling Approach

## 6. Conclusions

- (1)
- The Shaziba landslide is located in an area of Silurian strata, which is prone to landslides. Due the continuous heavy rainfall, the precipitation infiltration caused slope failure and triggered the large-scale landslide. After the landslide occurred, a steep scarp was formed on the trailing edge of the landslide. The sliding mass rushed into the Qingjiang River channel. Finally, a landslide dam formed, causing significant damage to the surroundings.
- (2)
- The simulation results show that the whole process of the Shaziba landslide took approximately 1000 s. It can be divided into five stages: early accelerated deformation, disintegration at the trailing edge of the slide, runout along the main sliding surface, decelerated movement and final deposition stage. The average velocity of the landslide could reach up to 7.5 m/s, and the average displacement was approximately 1000 m. The landslide piled up along the Qingjiang River valley after the movement stopped. The thickness of the landslide deposits gradually decreased from the center to the sides. The maximum height of the landslide deposits was about 19 m. The length and width were approximately 108–127 m and 328–386 m, which is in good agreement with the field investigations.
- (3)
- The runout behavior of a landslide and the morphology of landslide deposits are closely related to the effective modulus in the contact model of the PFC3D. As the effective modulus increases, the distribution of particles becomes more concentrated, and the height of the final landslide deposits increases. However, the velocity of the landslide significantly reduces.
- (4)
- The results achieved in this study show that the PFC3D model can provide an effective tool for investigating the dynamic features of flow-like landslides and a means for mapping hazardous areas and estimating hazard intensity.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 2.**Topographic map of Shaziba landslide and surrounding area: (

**a**) top view of the Shaziba landslide and surrounding villages; (

**b**) landslide dam on the Qingjiang River.

**Figure 8.**Damages caused by the Shaziba landslide: (

**a**) cracks in the building; (

**b**) ground cracks; (

**c**) cracks near the houses; (

**d**) main scarp of the landslide; (

**e**) slip surface; (

**f**) dammed lake.

**Figure 11.**Parameters calibration: (

**a**) the model used in the biaxial compression tests, (

**b**) Mohr failure envelopes.

**Figure 12.**Dynamic process of the Shaziba landslide (velocity nephogram): (

**a**) t = 20 s; (

**b**) t = 100 s; (

**c**) t = 200 s; (

**d**) t = 300 s; (

**e**) t = 500 s; (

**f**) t = 700 s; (

**g**) = 900 s; (

**h**) t = 1000 s.

**Figure 15.**The velocity and displacement histories of the monitored particles: (

**a**) velocity history of particle ID 1-5; (

**b**) displacement history of particle ID 1-5; (

**c**) velocity history of particle ID 6-10; (

**d**) displacement history of particle ID 6-10; (

**e**) velocity history of particle ID 11-15; (

**f**) displacement history of particle ID 11-15; (

**g**) velocity history of landslide front; (

**f**) displacement history in the front, middle and rear parts; (

**h**) velocity history in the front, middle and rear parts.

**Figure 16.**The transverse and longitudinal slices of the landslide dam: (

**a**) the simulated geometry of the landslide dam, (

**b**) the transverse slice along the A-a section of the landslide dam, (

**c**) the longitudinal slice along the B-b section of the landslide dam.

**Figure 18.**Simulation results with different effective modulus of the particles: (

**a**) emod = 16 × 10

^{5}Pa; (

**b**) emod = 16 × 10

^{6}Pa; (

**c**) emod = 16 × 10

^{7}Pa; (

**d**) emod = 16 × 10

^{8}Pa.

Parameters | Values |
---|---|

Particle density (kg/m^{3}) | 2100 |

Effective modulus (Pa) | 16 × 10^{7} |

Normal-to-shear stiffness ratio (/) | 2 |

Bond effective modulus (Pa) | 16 × 10^{7} |

Bond normal-to-shear stiffness ratio (/) | 2 |

Friction coefficient (/) | 0.3 |

Cohesion (Pa) | 3 × 10^{4} |

Tensile strength (Pa) | 1 × 10^{5} |

Friction angle (°) | 30 |

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**MDPI and ACS Style**

Wei, L.; Cheng, H.; Dai, Z.
Propagation Modeling of Rainfall-Induced Landslides: A Case Study of the Shaziba Landslide in Enshi, China. *Water* **2023**, *15*, 424.
https://doi.org/10.3390/w15030424

**AMA Style**

Wei L, Cheng H, Dai Z.
Propagation Modeling of Rainfall-Induced Landslides: A Case Study of the Shaziba Landslide in Enshi, China. *Water*. 2023; 15(3):424.
https://doi.org/10.3390/w15030424

**Chicago/Turabian Style**

Wei, Li, Hualin Cheng, and Zili Dai.
2023. "Propagation Modeling of Rainfall-Induced Landslides: A Case Study of the Shaziba Landslide in Enshi, China" *Water* 15, no. 3: 424.
https://doi.org/10.3390/w15030424