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

Do, Xuan Khanh; Jung, Kwansue; Lee, Giha; Regmi, Ram Krishna

doi:10.14481/jkges.2016.17.5.5

[국내논문] A Study on Rainfall Induced Slope Failures: Implications for Various Steep Slope Inclinations 원문보기

한국지반환경공학회논문집 = Journal of the Korean Geoenvironmental Society, v.17 no.5, 2016년, pp.5 - 16

Do, Xuan Khanh (Department of Civil Engineering Chungnam National University) , Jung, Kwansue (Department of Civil Engineering Chungnam National University) , Lee, Giha (Department of Construction and Disaster Prevention Engineering, Kyungpook National University) , Regmi, Ram Krishna (Institute for the Advanced Study of Sustainability, United Nations University)

Abstract ▼ AI-Helper

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.

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문제 정의

Based on a series of experiments, this study aims to determine a critical slope which could be considered as a slope threshold for distinguishing between sudden and gradual retrogressive failures. The study also attempts to discover the relationship between the mode of failure and the degree of saturation inside the soil model. For prediction and simulation purposes, the Spencer method is integrated with an effective minimization procedure based on dynamic programming by which the minimum of safety coefficient and their corresponding critical non-circular slip surfaces are defined simultaneously.

가설 설정

2. Retrogressive failures: After the first sign of failure, there were a number of small retrogressive slides that occurred in experiment 1, whereas only 2 localized slides could be observed in experiments 2 and 3. These two slides might be due to the local instability caused by the first slide. Interestingly, in experiment 4, no retrogressive failures were recorded.
b) The slip surface within one or two stages after entrance to the soil model had to pass at least a small portion through the water table in which only the pores below it will exert positive pressure resulting instability.

제안 방법

, 2006). In their studies, pore water pressure, moisture content at some critical positions within the slope, and rainfall were identified as the fundamental information for the prediction of slope failures. Orense et al.
Based on a series of experiments, this study aims to determine a critical slope which could be considered as a slope threshold for distinguishing between sudden and gradual retrogressive failures. The study also attempts to discover the relationship between the mode of failure and the degree of saturation inside the soil model.
To determine whether sudden or slow retrogressive failures mode will occur, the flume experiments were set at various angles with the same model geometry, rainfall intensity, and initial conditions. During the time of research, a series of experiments were conducted; however, only 4 representative experiments in which the differences between the modes of failure could clearly be distinguished were chosen to be presented in this paper (Table 1).
The sudden failure in experiment 4 was chosen for prediction due to the danger such failures present for people and properties. In the seepage flow model, the simulation was performed at 0.02 second time steps, 2.0 cm space steps in the horizontal direction and 1.0 cm space steps in the vertical direction. The time t=0 seconds was considered as the starting time for experiment.
Moreover, the observation only was implemented from the flume side. The aim of this study were simulation of failure in 2D, but the real failure shapes was 3D in geometry, in which their length and depth on either side is shorter and shallower than in the inner part. Therefore, to perfectly match the results obtained from experiments with those computed through calculations, a 3D numerical analysis is necessary.
In this study, a series of laboratory experiments were conducted to investigate the failure process of rainfall induced slope failures at various steep slope angles. The results showed that the mode of failure are highly dependent on the effect of a slope’s inclination: starting from slow retrogressive in a 55° slope, changing to gradual retrogressive in a steeper slope (59° and 63°) and becoming a sudden failure in 67° slope for the entire material d₅₀=0.
The numerical model was created by integrating a 2D seepage flow model and a 2D slope stability model to predict the failure surfaces and their corresponding times of occurrence. The results showed that besides the effect of pore water pressure, matric suction is also a very important factor in slope stability analysis.

대상 데이터

1. The experiments were carried out in a 200 cm long, 60 cm wide and 50 cm high rectangular glass-side flume, to generate slope failure. This flume could be adjusted with different angles by a chain pulley arrangement to create varied scenarios which were in turn at 15°, 19°, 23°, and 27°.
A rainfall simulator was installed above the structure to provide artificial rainfall over the flume. The complete processes, including the initiation of slope failure and movement of the failure mass, were captured by two digital cameras. One was placed in front of the flume; the other was placed on the right side (side A) of flume.

이론/모형

For prediction and simulation purposes, the Spencer method is integrated with an effective minimization procedure based on dynamic programming by which the minimum of safety coefficient and their corresponding critical non-circular slip surfaces are defined simultaneously. The additional shear strength of unsaturated soil caused by matric suction is estimated using an empirical procedure proposed by Vanapalli et al. (1996) which is compatible for using a limit equilibrium method. The simulation results will then be compared to those obtained by experiments.
Thereafter the moisture retention curve was constructed and van Genuchten parameters i.e., alpha α and eta η were calculated by non-linear regression method (Fig. 2).
Dynamic programming is a generalized method to achieve the best result (Yamagami & Ueta, 1986). In this study, in each time step, one critical slip surface (have the minimum value) was chosen among a list of potential slip surfaces by a numerical algorithm in mathematical programming for optimization of sequential multi-stage decision problems. Such problems are characterized by the following two properties (Baker, 1980):

성능/효과

The results showed that the mode of failure are highly dependent on the effect of a slope’s inclination: starting from slow retrogressive in a 55° slope, changing to gradual retrogressive in a steeper slope (59° and 63°) and becoming a sudden failure in 67° slope for the entire material d50=0.420 mm.

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