# Analysis of Rainfall-Induced Landslide on Unsaturated Soil Slopes

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Study Area

#### 2.1. Regional Framework of the Study Area

#### 2.2. Geological and Geomorphological Data

^{2}, channel gradient generally less than 22 degrees, and a maximum runout distance of all debris flows of 1365 m. Individual watersheds contain up to 30 landslides and these form the main source of the debris flow development. The mobilized volume ranges from 8 to 1827 m

^{3}in each gully. The Raemian watershed (W1, study area) has the largest debris flow volume of 1827 m

^{3}, some 52% of the total landslide volume. Landslides are initiated at slope angles ranging from 16 to 44° and some 60% of all landslides occurred at slope angles greater than 30°. The morphological characteristics are summarized in Table 1. It can therefore be concluded that most debris flows will also originate at slope angles close to 30°, and this observation is supported at other sites [26]. Field surveys revealed that landslide and debris flow in the Umyeonsan region were the result of three main steps: (1) initial failure produces a shallow landslide trace caused by the transitional sliding of the loose colluvium overlying gneiss bedrock (Figure 3a). (2) with the incorporation of surface water runoff resulting from intensive rainfall, the soil mobilizes completely to form debris flows; (3) overland flow in rills is gradually concentrated in the gully, which then easily erodes the loose debris flow deposits and runs rapidly downhill in relatively narrow channels. The depth of these channels varied from 0.1 to 1.5 m, and the base of the gullies was located within the colluvial layer or along the interface between the colluvium and bedrock (Figure 3b). The gneiss bedrock, where exposed, was deeply fractured and highly weathered (Figure 3c). The intense rainfall was concentrated in steep bedrock channels with relatively thick colluvium. It caused surface runoff. The transported debris flow was deposited at the toe of the mountain. The debris flow material was comprised of various sizes of rock, soil, woody blocks, and water (Figure 3d). In general, the gneiss has thick weathering layers enriched in fine particles and clay minerals that appear to be essential for long-distance debris flow transport [27].

## 3. Methodology

#### 3.1. Laboratory and Field Tests

^{2}were investigated. A total of six circles were investigated (Figure 4). As a result, 149 woody species were recorded: all living trees (the diameter at breast height, DBH > 0.06 m) were counted, and the characteristics of vegetation communities that include species, DBH, height, and root depth were measured.

#### 3.2. Landslide Analysis

_{x}and k

_{y}are the permeability coefficients in x and y directions, respectively. H is the total water head. q is rainfall and m

_{w}is the slope of SWCC. In SEEP/W program, the permeability function of unsaturated soils is calculated from the SWCC fitted, based on the saturated permeability coefficient K

_{sat}. In this study, van Genuchten [30] SWCC model was used. Limit equilibrium analyses were conducted by using SLOPE/W to determine the safety factor of the landslide, based on the pore water pressure distributions from the transient seepage analyses. And the minimum factor of safety is determined within the range for a slope failure. Strength parameters of unsaturated soils are needed for estimating the safety factor using Bishop’s simplified method. A modified form of the Mohr–Coulomb equation must be used to describe the shear strength of an unsaturated soil (i.e., a soil with negative pore water pressures). The shear strength equation is:

_{a}is the pore air pressure and u

_{w}is the pore water pressure. ϕ

^{b}is the angle indicating the rate of increase in shear strength relative to the matric suction.

^{b}). For most analyzes the pore air pressure can be set to zero. This analysis uses ϕ

^{b}whenever the pore water pressure is negative and ϕ’ whenever the pore water pressure is positive. The internal friction angle associated with matric suction is called ϕ

^{b}and can be estimated as an alternative solution [31]. For the friction angle of unsaturated soils, Equation (3) [22] is used:

_{s}and θ

_{r}are the saturated and residual volumetric water content, respectively.

## 4. Results and Discussion

#### 4.1. Rainfall Characteristics

#### 4.2. Vegetation Characteristics

_{s}is the cohesion of the soil, c’

_{r}is the constant number of additional shear strengths by the roots of trees, D

_{s}is the depth of the unsaturated soil, D

_{w}is the depth of the wetting band, q

_{0}is the uniform load from trees, γ

_{t}is the total unit weight of the soil, γ

_{sat}is the saturated unit weight of the soil, γ

_{w}is the unit weight of water, and β is the angle of the slope.

_{r}) is considered as 1 kPa and uniform load from trees (q

_{0}) is considered as 0.253 kPa. Consequently, a slight reduction in safety factor had been found in the Raemian watershed analysis (0.01 to 0.08). This result is in accordance with the previous phenomenological approach [45]; it is well known that the self-weight, length, and strength of tree roots have a close relationship with landslides. As confirmed in this study, such results were categorized as contributing factors of the landslide.

#### 4.3. Geotechnical Characteristics

^{−4}m/s and the permeability of the weathered rock is on the order of 10

^{−5}m/s.

#### 4.4. Numerical Results

## 5. Conclusions

- The results of comprehensive investigations in the Umyeonsan region demonstrate that landslide activity is closely related primarily to rainfall, vegetation, and soil properties. From the data collected in the field, we can conclude that:
- ○
- The Umyeonsan landslides were triggered by a heavy rainfall event of a relatively high intensity (112.5 mm/h), preceded by a long period of antecedent rainfall (a total rainfall of 306.5 mm over the 16 h prior to the landslides). The average rainfall intensity of this rainstorm exceeded the rainfall intensity-duration thresholds for landslide initiation with respect to other global rainfall events.
- ○
- The roots of the vegetation were generally located at a relatively shallow depth (<1 m) in the colluvial deposits, indicating the lack of deeply penetrating roots in the Umyeonsan region. Wood was an important part of the landslide, and became concentrated at the front of the deposits.
- ○
- The colluvial soils in Umyeonsan region are classified as SM (Silty sand) and SC-CL (Clayey sand with many fine particles). As a result of the soil water characteristic curve test, the air entry value was 12 kPa, the saturated volumetric water content was 50%, and the residual volumetric water was 18%. According to the results of field measurements, the matric suction of the ground was 75 to 85 kPa in the dry season and approximately 20 kPa in the wet season.

- From numerical simulations of slope stability analysis we can conclude that:
- ○
- The hydro-geotechnical coupled analysis was conducted to confirm the effects of rainfall, vegetation and soil properties on landslide. The infiltration analysis was performed by applying unsaturated soil characteristics and actual recorded rainfall. In order to apply the infiltration analysis method used for simple slopes to the watershed-scale interpretation, special attention was given to the infiltration of rainfall into the underground using a one-dimensional infiltration model. It was used to determine the wetting band depth in shallow depth failure analysis and to determine the initial matric suction of the ground effect of the anticipated rainfall in deep-seated failure analysis. From the results of the numerical limiting equilibrium analysis, conducted in an elongated and narrow watershed, we can conclude that rainfall infiltration due to the considered rainfall event was responsible for triggering the observed landslide.
- ○
- The simulated results agree closely with the investigation results, which indicate that the applied method is appropriate for use in the simulation of the landslide in unsaturated soils. The simulated results has illustrated that the methodology applied in this work are consistent with both shallow and deep-seated rainfall-induced landslides. In fact, the simulated critical slip surfaces corresponding to the minimum factor of safety are in reasonable agreement with the observed ones.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 2.**Satellite image showing 33 debris flow gullies (marked in blue), watersheds (outlined by white lines), and 151 landslides (modified from Korean Geotechnical Society, 2012).

**Figure 3.**Photographs of geotechnical and geological characteristics of landslide damage sites. (

**a**) main scarp of landslide at a ridge crest; (

**b**) soil profile at the center of a channel; (

**c**) expose of the gneiss bedrock; (

**d**) transported material at bottom of watershed.

**Figure 6.**Hourly and cumulative rainfall for 26–27 July 2011: (

**a**) Seocho station; (

**b**) Namhyun station.

**Figure 7.**Comparison of measured rainfall intensity–duration data and existing intensity–duration threshold curves.

**Figure 12.**Soil profile of the studied area: (

**a**) Typical soil profile of Umyeonsan; (

**b**) representative soil profiles of two watersheds.

**Figure 14.**Soil slope mesh used for two-dimensional seepage analysis, boundary conditions, and results of pore-water pressure distribution.

**Figure 15.**Critical slip surface from coupled analysis: (

**a**) Raemian watershed; (

**b**) Dukwooam watershed.

ID | Basin Area (×103 m^{2}) | Runout Distance (m) | Landslides Volume (m^{3}) | Average Slope (°) |
---|---|---|---|---|

W1 | 75.6 | 606.7 | 1827.0 | 44 |

W2 | 54.1 | 267.3 | 45.7 | 29 |

W3 | 214.4 | 663.6 | 105.0 | 26 |

W4 | 421.4 | 900.0 | 153.6 | 36 |

W5 | 233.4 | 307.1 | 19.8 | 16 |

W6 | 271.8 | 454.4 | 62.9 | 27 |

W7 | 786.4 | 1365.1 | 34.2 | 35 |

W8 | 178.2 | 632.0 | 125.2 | 26 |

W9 | 678.9 | 941.3 | 182.0 | 36 |

W10 | 64.2 | 229.7 | 129.3 | 29 |

W11 | 444.8 | 960.3 | 75.4 | 34 |

W12 | 41.4 | 201.5 | 70.2 | 29 |

W13 | 324.0 | 385.4 | 73.2 | 30 |

W14 | 17.7 | 176.3 | 108.6 | 33 |

W15 | 48.9 | 246.8 | 55.4 | 34 |

W16 | 90.9 | 435.7 | 134.3 | 40 |

W17 | 57.0 | 130.6 | 8.1 | 37 |

W18 | 183.6 | 562.9 | 112.3 | 40 |

W19 | 76.2 | 495.2 | 86.9 | 35 |

W20 | 90.8 | 625.3 | 98.7 | 35 |

Total | 4353.7 | 10,450.2 | 3507.8 | - |

**Table 2.**In situ soil properties from constant head permeability and shear tests performed in boreholes.

Borehole | Depth (m) | Soil Type | k (m/s) | c (kPa) | φ (°) |
---|---|---|---|---|---|

B-1 | 1–2 | Colluvium | 4.67 × 10^{−6} | 7.5 | 22.3 |

B-2 | 3–4 | Colluvium | 8.08 × 10^{−6} | 6.9 | 25.1 |

5–6 | Weathered rock | 1.99 × 10^{−6} | 18.1 | 27.3 | |

B-3 | 1–2 | Colluvium | 8.08 × 10^{−4} | 8.36 | 24.78 |

3–4 | Weathered soil | 1.02 × 10^{−4} | 18.55 | 28.22 | |

B-4 | 2–3 | Colluvium | 7.92 × 10^{−4} | 11.89 | 27.01 |

8–9 | Colluvium | 9.55 × 10^{−5} | 14.96 | 32.13 |

Test Pit | Depth (m) | w (%) | PL (%) | LL (%) | % Fines | USCS | c (kPa) | φ (°) |
---|---|---|---|---|---|---|---|---|

TP-1 | 0.5 | 18.2 | 21.2 | 36.6 | 51.9 | CL | 9.2 | 21.7 |

TP-2 | 0.5 | 14.1 | 22.3 | 31.6 | 28.9 | SC | 10.9 | 23.7 |

TP-3 | 0.5 | 32.1 | 23.7 | 40.6 | 55.7 | CL | 11.3 | 23.1 |

TP-4 | 0.5 | 15.8 | 20.9 | 35.9 | 44.4 | SC | 11.8 | 22.7 |

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

Hydraulic conductivity, ks | 8 × 10^{−6} m/sec (28.8 mm/h) | In situ permeability test |

Initial water contents, ${\theta}_{i}$ | 28.0~32.0 (30.0)% | SWCC test |

Water-content deficit, $\Delta \theta $ | 0.20 | SWCC test |

Wetting front suction head, ${\psi}_{f}$ | 830 mm | SWCC test |

Soil cohesion, ${c}_{s}^{\prime}$ | 6.9~18.5 (11.7) kPa | Direct shear test, borehole shear test |

Soil friction angle, ${\phi}_{}^{\prime}$ | 21.7~32.1 (25.3) deg. | Direct shear test, borehole shear test |

Total unit weight of soil, ${\gamma}_{t}^{}$ | 17.0~18.5 (18.0) kN/m^{3} | laboratory density test |

Additional shear strength by roots of tree, ${c}_{r}^{\prime}$ | 1.0 kPa | suggested by Norris et al. [49] |

Uniform load by tree, ${q}_{0}^{}$ | 0.253 kPa | suggested by KFRI, [50] |

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## Share and Cite

**MDPI and ACS Style**

Jeong, S.; Lee, K.; Kim, J.; Kim, Y. Analysis of Rainfall-Induced Landslide on Unsaturated Soil Slopes. *Sustainability* **2017**, *9*, 1280.
https://doi.org/10.3390/su9071280

**AMA Style**

Jeong S, Lee K, Kim J, Kim Y. Analysis of Rainfall-Induced Landslide on Unsaturated Soil Slopes. *Sustainability*. 2017; 9(7):1280.
https://doi.org/10.3390/su9071280

**Chicago/Turabian Style**

Jeong, Sangseom, Kwangwoo Lee, Junghwan Kim, and Yongmin Kim. 2017. "Analysis of Rainfall-Induced Landslide on Unsaturated Soil Slopes" *Sustainability* 9, no. 7: 1280.
https://doi.org/10.3390/su9071280