# Computer-Aided Slope Stability Analysis of a Landslide—A Case Study of Jhika Gali Landslide in Pakistan

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## Abstract

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^{3}. Unconfined compression tests and unconsolidated-undrained (UU) triaxial tests were performed on samples reconstituted at in-situ dry unit weight, standard Proctor and modified Proctor maximum dry unit weights. The test results show that the shear strength and deformation parameters, i.e., undrained shear strength, angle of internal friction and deformation modulus decreased from 200 kPa to 90 kPa, 23° to 12° and 51 MPa to 32 MPa, respectively, with an increase in the percentage of saturation from 35% to 95% at a specific dry unit weight. The slope was also modeled in GeoStudio for limit equilibrium analysis, and slope stability analysis was performed using the values of undrained shear strength and the angle of internal friction as determined in the laboratory at varying degrees of saturation. The limit equilibrium analysis showed that the factor of safety reduces from 1.854 to 0.866 as the saturation of material increases from 35% to 95%. The results also suggest that, as the percentage of saturation increases above 85%, the soil loses its shear strength significantly and gains in bulk unit weight, so at this stage the material starts sliding. Additionally, slope stability analysis was carried out by changing the slope geometry in three different ways, i.e., by reducing the height of the slope, adding a counterweight at the toe of the slope and by making benches on the slope. The results of GeoStudio analysis showed that the slope will be stable even above 85% degree of saturation.

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Soil Sample Collection

#### 2.2. Soil Sample Properties

^{3}and 20.31 kN/m

^{3}for the sample, respectively. The in-situ dry unit weight of soil as measured by the block sample is 18.63 kN/m

^{3}[18,19].

#### 2.3. Shear Strength of Soil Matrix

_{50}). The deformation modulus was calculated by using the axial stress versus axial strain relationship.

#### 2.4. Slope Stability Analysis

## 3. Results

#### 3.1. Triaxial Compression Test

^{3}) and degrees of saturation varying from 35~95%. Samples remolded at a specific dry unit weight and degree of saturation were tested at confining pressures of 50 kPa, 100 kPa and 150 kPa. Through these Triaxial tests shear strength parameters, i.e., the undrained shear strength and friction angle were calculated and used for slope stability analysis. In a series of triaxial tests performed at standard Proctor dry unit weight of 18.14 kN/m

^{3}, modified Proctor dry unit weight of 20.31 kN/m

^{3}and in-situ dry unit weight of 18.63 kN/m

^{3}the value of undrained shear strength (c

_{u}) decreased from 186.03 kPa to 89.57 kPa, 199.81 kPa to 96.46 kPa and 189 kPa to 91 kPa respectively, as the saturation increased from 35% to 95%. In the same way, in a series of triaxial tests at standard Proctor dry unit weight of 18.14 kN/m

^{3}, modified Proctor dry unit weight of 20.31 kN/m

^{3}and in-situ dry unit weight of 18.63 kN/m

^{3}the value of angle of internal friction (ϕ) decreased from 22.54° to 12.3°, 23.27° to 13.69° and 22.7° to 12.6° respectively, as the saturation increased from 35% to 95%.

_{u}) as shown in Figure 5 was used to investigate the decreasing trend with an increase in the degree of saturation at different dry unit weights. It was observed that the undrained shear strength decreased by about 51% as the saturation increased from 35% to 95%.

#### 3.2. Unconfined Compression Test

_{50}) for each sample was calculated using the axial stress versus axial strain relationship.

^{3}, at standard Proctor dry unit weight, i.e., 18.14 kN/m

^{3}and at in-situ dry unit weight 18.63 kN/m

^{3}with the increasing degree of saturation is shown in Figure 7. In a number of unconfined compression tests carried out on samples remolded at standard Proctor dry unit weight of 18.14 kN/m

^{3}, modified Proctor dry unit weight of 20.31 kN/m

^{3}and in-situ dry unit weight of 18.63 kN/m

^{3}the value of unconfined compression strength decreased from 707 kPa to 344 kPa, 755 kPa to 383 kPa and 718 kPa to 353 kPa, respectively, as the saturation increased from 35% to 95%. This is due to increased saturation as a result of rainfall infiltration, which leads to slope failure.

_{50}varies from 44.7 MPa to 32.2 MPa in the case of standard Proctor dry unit weight as the percentage of saturation increases from 35% to 95%. Similarly, the E

_{50}value decreases from 50.8 MPa to 36.8 MPa in the case of modified Proctor dry unit weight as the percentage of saturation increases from 35% to 95%. The E

_{50}value decreases from 46.1 MPa to 33.2 MPa in the case of in-situ dry unit weight as the percentage of saturation increases from 35% to 95%. Figure 9 shows the decreasing trend of deformation modulus (E

_{50}) with the increase in the saturation at in-situ dry unit weight i.e., 18.61 kN/m

^{3}, standard Proctor dry unit weight i.e., 18.14 kN/m

^{3}and modified Proctor dry unit weight 20.31 kN/m

^{3}.

#### 3.3. Jhika Gali Slope Stability Analysis

_{u}and ϕ) and saturations. The results of these analysis in Figure 11 shows that as the saturation increases, the factor of safety decreases dramatically and the slope becomes critical; furthermore, the factor of safety is reduced to below 1, somewhere between 85% to 95% degrees of saturation for various dry unit weights.

#### Alteration in Slope Geometry for Stability

## 4. Conclusions

- Based on the triaxial compression test results, it can be concluded that the undrained shear strength (c
_{u}) of the remolded samples at a particular degree of saturation increases by about 7~10% with the increase in dry unit weight. However, the cohesion decreases by about 51% as the degree of saturation increases from 35% to 95%. The friction angle (ϕ) improved approximately 6~11% with the increase in the dry unit weight at a specific degree of saturation, although the friction angle is reduced by about 41% as the degree of saturation is increased from 35% to 95%. - According to a slope stability study conducted on GeoStudio, the landslides along the Jhika Gali road were caused by a reduction in the shear strength of the slide material due to a rise in saturation level. Consequently, with the increase in saturation levels the effective normal stress decreased along the slip surface. Stability analysis of the landslide shows that the factor of safety reduced from 1.854 to 0.886 as the saturation increased from 35% to 95%. When the saturation was about 85% to 95%, the slope was in the critical condition because of the low factor of safety.
- Landslide occurrence can be mitigated by flattening the slope geometry even when the degree of saturation is more than 85%. By reducing the height of the slope by about 10m the factor of safety is increased from 0.893 to 1.042. Similarly, by adding the counterweight at the toe of the slope by 10m the factor of safety increased from 0.893 to 1.12. In the last case by making the benches of the slope with angle variation the factor of safety increased from 0.893 to 1.307.
- The results of this study could be useful for a precise evaluation of possible landslides in other areas of Pakistan as well as in other parts of the world.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 5.**Degree of saturation vs. undrained shear strength of samples remolded to in-situ, standard and modified Proctor compaction energy.

**Figure 6.**Degree of saturation vs. friction angle of samples remolded to in-situ, standard and modified Proctor compaction energy.

**Figure 7.**Stress versus axial strain of samples with varying degrees of saturation (35~95%) in an unconfined compression test remolded at (

**a**) Modified Proctor dry unit weight, (

**b**) Standard Proctor dry unit weight, and (

**c**) in-situ dry unit weight.

**Figure 8.**Degree of saturation vs. unconfined compression strength of samples remolded to in-situ, standard and modified Proctor dry unit weights.

**Figure 9.**Degree of saturation versus deformation modulus of samples remolded at in-situ, standard and modified Proctor dry unit weight.

**Figure 10.**Jhika Gali slope modeled in GeoStudio for slope stability analysis for varying degrees of saturation.

Sr. No. | Description | Result |
---|---|---|

1 | Gravel (%) | 1 |

2 | Sand (%) | 21 |

3 | Silt (%) | 43 |

4 | Clay (%) | 35 |

5 | Liquid Limit, LL (%) | 38 |

6 | Plastic Limit, PL (%) | 23 |

7 | Plasticity Index, PI | 15 |

8 | Specific Gravity, Gs | 2.72 |

9 | USCS Classification | CL |

10 | AASHTO Classification | A-6(6) |

11 | Standard Proctor dry unit weight at OMC 10.94% | 18.14 kN/m^{3} |

12 | Modified Proctor dry unit weight at OMC 9.5% | 20.31 kN/m^{3} |

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**MDPI and ACS Style**

Amin, M.N.; Umair Ashfaq, M.; Mujtaba, H.; Ehsan, S.; Khan, K.; Faraz, M.I.
Computer-Aided Slope Stability Analysis of a Landslide—A Case Study of Jhika Gali Landslide in Pakistan. *Sustainability* **2022**, *14*, 12954.
https://doi.org/10.3390/su142012954

**AMA Style**

Amin MN, Umair Ashfaq M, Mujtaba H, Ehsan S, Khan K, Faraz MI.
Computer-Aided Slope Stability Analysis of a Landslide—A Case Study of Jhika Gali Landslide in Pakistan. *Sustainability*. 2022; 14(20):12954.
https://doi.org/10.3390/su142012954

**Chicago/Turabian Style**

Amin, Muhammad Nasir, Muhammad Umair Ashfaq, Hassan Mujtaba, Saqib Ehsan, Kaffayatullah Khan, and Muhammad Iftikhar Faraz.
2022. "Computer-Aided Slope Stability Analysis of a Landslide—A Case Study of Jhika Gali Landslide in Pakistan" *Sustainability* 14, no. 20: 12954.
https://doi.org/10.3390/su142012954