Analysis on Failure of Slope and Landslide Dam

Ram Krishna Regmi,이기하,정관수 대한토목학회 2013 KSCE JOURNAL OF CIVIL ENGINEERING Vol.17 No.5

A three-dimensional (3D) seepage-flow numerical simulation model was developed for seepage analysis of a landslide dam. A 3D seepage-flow numerical simulation model coupled with a two-dimensional (2D) surface flow and erosion/deposition model was developed for seepage analysis of a slope due to a rainfall event. The conventional water-phase (one-phase) seepage-flow model assumed only water-phase flow in seepage analysis, which was inadequate for unsaturated soil domains. A water-air two-phase seepage-flow model that considers both the water and air phases in the seepage-flow process was also used for the seepage analysis. The pore-water pressure and moisture-content data obtained from the seepage-flow model were used to analyze the stability. Janbu’s simplified method and the extended Spencer method were used for the stability analysis. The numerical simulation results almost compared well with laboratory experimental measurements.

Analysis on Failure of Slope and Landslide Dam

Regmi, Ram Krishna,Lee, Gi Ha,Jung, Kwan Sue 한국방재학회 2011 한국방재학회 학술발표대회논문집 Vol.10 No.-

A 3D seepage flow numerical simulation model was developed for seepage analysis of a landslide dam. A 3D seepage flow numerical simulation model coupled with a 2D surface flow and erosion/deposition model was developed for seepage analysis of a slope due to a rainfall event. The conventional water-phase (one-phase) seepage flow model assumes only water phase flow in seepage analysis, which is inadequate for unsaturated soil domains. Hence, a water-air two-phase seepage flow model that considers both water and air phase in the seepage flow process is also used for seepage analysis. Pore water pressure and moisture content data obtained by the seepage flow model were then used to analyze the stability of the slope. Janbu’s simplified method as well as the extended Spencer method was used for the stability analysis. Numerical simulation results and experimental measurements are satisfactorily in agreement.

Application of Dynamic Programming to Locate the Critical Failure Surface in a Rainfall Induced Slope Failure Problem

Ram Krishna Regmi,정관수 대한토목학회 2016 KSCE JOURNAL OF CIVIL ENGINEERING Vol.20 No.1

In this paper, a dynamic programming method is employed in conjunction with limit-equilibrium techniques to determine the location of the non-circular critical slip surface in the analysis of a slope failure due to a rainfall event. The Spencer method of slope stability analysis was incorporated into dynamic programming to predict the time of a slope failure and shape of the failure surface. A one-Dimensional (1D) sliding block model was used to analyze the motion of the failure mass. Furthermore, during the movement of the sliding mass, the stability of the model slope was analyzed by updating the shape of the model slope according to the new position of the sliding mass. The suction head in the soil pore provides soil shear strength to maintain the stability of the slope. The positive pore water pressure reduces the soil shear strength so that the slip surface beneath the water table will be more unstable. Numerous studies (using non-cohesive soil) have been conducted to locate the critical slip surface in slope stability problems without considering the increase in shear strength due to suction and apparent cohesion. The studies do not clearly illustrate the reasonable boundary conditions to avoid the slip surface alignment totally outside the actual slope. This paper clearly discusses the boundary conditions for such a slope stability problem reasonably. The step-by-step calculation procedure is presented through the flow diagram. Three cases of slope failure obtained from a laboratory flume experiment have been analyzed using the proposed method. The numerical simulation results and the results obtained from experiments are comparable.

Sediment Erosion and Transport Experiments in Laboratory using Artificial Rainfall Simulator

Ram Krishna Regmi,Kwansue Jung,Hajime Nakagawa,Jaewon Kang,Giha Lee 한국지반환경공학회 2014 한국지반환경공학회논문집 Vol.15 No.4

Catchments soil erosion, one of the most serious problems in the mountainous environment of the world, consists of a complex phenomenon involving the detachment of individual soil particles from the soil mass and their transport, storage and overland flow of rainfall, and infiltration. Sediment size distribution during erosion processes appear to depend on many factors such as rainfall characteristics, vegetation cover, hydraulic flow, soil properties and slope. This study involved laboratory flume experiments carried out under simulated rainfall in a 3.0 m long × 0.8 m wide × 0.7 m deep flume, set at 17° slope. Five experimental cases, consisting of twelve experiments using three different sediments with two different rainfall conditions, are reported. The experiments consisted of detailed observations of particle size distribution of the out-flow sediment. Sediment water mixture out-flow hydrograph and sediment mass out-flow rate over time, moisture profiles at different points within the soil domain, and seepage outflow were also reported. Moisture profiles, seepage outflow, and movement of overland flow were clearly found to be controlled by water retention function and hydraulic function of the soil. The difference of grain size distribution of original soil bed and the out-flow sediment was found to be insignificant in the cases of uniform sediment used experiments. However, in the cases of non-uniform sediment used experiments the outflow sediment was found to be coarser than the original soil domain. The results indicated that the sediment transport mechanism is the combination of particle segregation, suspension/saltation and rolling along the travel distance.

Sediment Erosion and Transport Experiments in Laboratory using Artificial Rainfall Simulator

Regmi, Ram Krishna,Jung, Kwansue,Nakagawa, Hajime,Kang, Jaewon,Lee, Giha Korean Geo-Environmental Society 2014 한국지반환경공학회논문집 Vol.15 No.4

Catchments soil erosion, one of the most serious problems in the mountainous environment of the world, consists of a complex phenomenon involving the detachment of individual soil particles from the soil mass and their transport, storage and overland flow of rainfall, and infiltration. Sediment size distribution during erosion processes appear to depend on many factors such as rainfall characteristics, vegetation cover, hydraulic flow, soil properties and slope. This study involved laboratory flume experiments carried out under simulated rainfall in a 3.0 m long ${\times}$ 0.8 m wide ${\times}$ 0.7 m deep flume, set at $17^{\circ}$ slope. Five experimental cases, consisting of twelve experiments using three different sediments with two different rainfall conditions, are reported. The experiments consisted of detailed observations of particle size distribution of the out-flow sediment. Sediment water mixture out-flow hydrograph and sediment mass out-flow rate over time, moisture profiles at different points within the soil domain, and seepage outflow were also reported. Moisture profiles, seepage outflow, and movement of overland flow were clearly found to be controlled by water retention function and hydraulic function of the soil. The difference of grain size distribution of original soil bed and the out-flow sediment was found to be insignificant in the cases of uniform sediment used experiments. However, in the cases of non-uniform sediment used experiments the outflow sediment was found to be coarser than the original soil domain. The results indicated that the sediment transport mechanism is the combination of particle segregation, suspension/saltation and rolling along the travel distance.

Study on the Formation and Geometries of Rainfall-Induced Landslide Dams

Xuan Khanh Do,Ram Krishna Regmi,Ho Phuong Thao Nguyen,정관수 대한토목학회 2017 KSCE JOURNAL OF CIVIL ENGINEERING Vol.21 No.5

Recently, the occurrence of landslide dam problems has been aggravated due to the effects caused by climate change and the further expansion of land use in mountainous areas. Knowledge of landslide dam formation and its geometry is very necessary to accurately evaluate dam stability and hence provide good predictions for disaster preparedness. In previous studies, landslide dam formation was considered as a separate issue; its link with slope failure was ignored, and thus their description and interpretation are still inadequate. Previous models also were limited to evaluate landslide dam shape in 2 dimensions (2D). Through a series of experiments, this study aims to analyze the effect of different slope failure mechanism on the formation of landslide dam. This study also integrated 2D seepage flow model, 2D slope stability model, landslide dam-geometry evaluation model in a single unit and proposed a new method to estimate the landslide dam shape in lateral direction (3D). The experimental results indicated that in the sudden failure, the failed mass can quickly block the river width, making a high impact in regard to dam construction. In retrogressive failure, the shape of dam was formed layers by layers which slowly span to the other side of river and to both up and downstream direction. The results also showed the failed volume and river bottom slope are the two most essential factors that dominate the shape of landslide dam. The simulation results including the failure surface, landslide volume and shape of landslide dam in lateral direction were comparable with those observed from the experiments.

A Study on Rainfall Induced Slope Failures

Xuan Khanh Do,Kwansue Jung,Giha Lee,Ram Krishna Regmi 한국지반환경공학회 2016 한국지반환경공학회논문집 Vol.17 No.5

A rainfall induced slope failure is a common natural hazard in mountainous areas worldwide. Sudden and rapid failures which have a high possibility of occurrence in a steep slope are always the most dangerous due to their suddenness and high velocities. Based on a series of experiments this study aimed to determine a critical angle which could be considered as an approximate threshold for a sudden failure. The experiments were performed using 0.42 ㎜ mean grain size sand in a 200 ㎝ long, 60 ㎝ wide and 50 ㎝ deep rectangular flume. A numerical model was created by integrating a 2D seepage flow model and a 2D slope stability analysis model to predict the failure surface and the time of occurrence. The results showed that, the failure mode for the entire material will be sudden for slopes greater than 67°; in contrast the failure mode becomes retrogressive. There is no clear link between the degree of saturation and the mode of failure. The simulation results in considering matric suction showed good matching with the results obtained from experiment. A subsequent discarding of the matric suction effect in calculating safety factors will result in a deeper predicted failure surface and an incorrect predicted time of occurrence.

A Study on Rainfall Induced Slope Failures: Implications for Various Steep Slope Inclinations

Do, Xuan Khanh,Jung, Kwansue,Lee, Giha,Regmi, Ram Krishna Korean Geo-Environmental Society 2016 한국지반환경공학회논문집 Vol.17 No.5

A rainfall induced slope failure is a common natural hazard in mountainous areas worldwide. Sudden and rapid failures which have a high possibility of occurrence in a steep slope are always the most dangerous due to their suddenness and high velocities. Based on a series of experiments this study aimed to determine a critical angle which could be considered as an approximate threshold for a sudden failure. The experiments were performed using 0.42 mm mean grain size sand in a 200 cm long, 60 cm wide and 50 cm deep rectangular flume. A numerical model was created by integrating a 2D seepage flow model and a 2D slope stability analysis model to predict the failure surface and the time of occurrence. The results showed that, the failure mode for the entire material will be sudden for slopes greater than $67^{\circ}$; in contrast the failure mode becomes retrogressive. There is no clear link between the degree of saturation and the mode of failure. The simulation results in considering matric suction showed good matching with the results obtained from experiment. A subsequent discarding of the matric suction effect in calculating safety factors will result in a deeper predicted failure surface and an incorrect predicted time of occurrence.