• Journal of the Korean Geotechnical Society
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• 제16권1호
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• pp.51-64
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• 2000

This paper presents a procedure of calculating a safety factor of the unsaturated slope suffering from the rainfall infiltration. The process of infiltration into a slope due to rainfall and its effect on the behavior of the soil slope are examined by using a two dimensional finite element flow-deformation coupled analysis. A factor of safety is calculated at various elapsed times after the commencement of rainfall as in the following procedure. First, stresses are estimated at each Gaussian point from the coupled finite element analysis. Then, the global stress smoothing method is applied to get a continuous stress field. Based on this stress field, a factor of safety is calculated for a specified slip surface by a stress integration scheme. Then, a search strategy is used to find out a critical slip surface which is associated with the minimum factor of safety. Some numerical examples are analyzed in order to study the effect of hydraulic conductivity on the slope stability during rain-induced infiltration. According to the results, local failure zone can be formed near the slope surface due to inhomogeneous distribution of hydraulic conductivity If the failure zone is once formed, then the region extends until a large amount of slide activates. Therefore the local failure can be neglected no longer in the stability analysis.

• Journal of the Korean Geotechnical Society
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• 제17권2호
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• pp.51-57
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• 2001

지하수위의 상승에 따른 간극수압의 증가는 사면의 불안정을 야기할 수 있다. 그러나 모델링 오차, 계측오차, 모델변수의 불확실성 등과 같은 오차로 인하여 사면에서의 지하수위 변동을 예측하는 것은 매우 어렵다. 이러한 불확실성을 극복하고 지하수위 변동을 평가하기 위한 최적의 모델변수를 구하기 위하여 역해석 기법이 사용되고 있다. 본 논문에서는 사면에서의 지하수위 변동을 예측하기 위하여 포화대에서의 지하수 흐름과 불포화대에서의 지하수 흐름을 동시에 고려할 수 있는 수치해석 모델과 변수예측기법을 적용하였다. 따라서, 본 논문에서는 포화투수계수($K_{s}$ ), 포화흡인력($\psi$$_{e}$) 및 불포화 투수계수의 함수에 사용되는 경험적인 상수(b)를 주요 매개변수로 선정하여 역해석을 실시하였다. 그리고, 역해석 기법 가운데 Maximum Likelihood(MK), Maximum-A-Posterior(MAP) 및 Extended Bayesian Method(EBM)에 대하여 비교연구를 실시하였다. 위의 세가지 방법 가운데 EBM은 가상의 변수(Hyperparameter) $\beta$를 도입함으로써 현장계측치와 사전정보를 가장 잘 조화시키는 방법으로 다른 ML, MAP 보다 탁월한 방법인 것을 알 수 있었다.

• KSCE Journal of Civil and Environmental Engineering Research
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• 제35권5호
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• pp.1073-1082
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• 2015

The behavior of the unsaturated soil slope can be influenced by the various factors such as the hydraulic characteristics, the mechanical characteristics, the coefficient of conductivity, the stratifications, the rainfall conditions i.e. the rainfall intensity, the rainfall pattern, the duration time of the rainfall and the antecedent rainfall etc. It is known that the slope failure is influenced greatly by the antecedent rainfall rather than the rainfall condition at the failure time, so the antecedent rainfall is supposed to be a very important factor in slope stability analysis among these factors. To predict and to prevent the slope failure by the rainfall, the distribution of the matric suction by the antecedent rainfall must be considered first of all and the slope stability analysis should be carried out by considering the successive rainfall characteristics. In this research, 3 samples with different quantity (5%, 10%, 20%) of silts were prepared and the SWCC (Soil-water characteristic curve) tests were carried out and the associated parameters were analyzed. After analyzing the distribution of the matric suction and the change of the mechanical characteristics such as the stress and the strength when applying the antecedent rainfall for one month and the successive intensive rainfall for 12 hours, the slope stability analyses were carried out numerically. And the influence of the antecedent rainfall for one month and the SWCC on the stability of a slope were compared and analyzed.

• Journal of the Korean Geotechnical Society
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• 제18권6호
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• pp.5-16
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• 2002

A hydrogeomechanical numerical model is presented to evaluate rainfall impacts on groundwater flow in slopes and slope stability. This numerical model is developed based on the fully coupled poroelastic governing equations for groundwater flow in deforming variably saturated geologic media and the Galerkin finite element method. A series of numerical experiments using the model developed are then applied to an unsaturated slope under various rainfall rates. The numerical simulation results show that the overall hydromechanical slope stability deteriorates, and the potential failure nay initiate from the slope toe and propagate toward the slope crest as the rainfall rate increases. From the viewpoint of hydrogeology, the pressure head and hence the total hydraulic head increase as the rainfall rate increases. As a result, the groundwater table rises, the unsaturated zone reduces, the seepage face expands from the slope toe toward the slope crest, and the groundwater flow velocity increases along the seepage face. From the viewpoint of geomechanics, the horizontal displacement increases, and the vertical displacement decreases toward the slope toe as the rainfall rate increases. This may result from the buoyancy effect associated with the groundwater table rise as the rainfall rate increases. As a result, the overall deformation intensifies toward the slope toe, and the unstable zone, in which the factor of safety against shear failure is less than 1, becomes thicker near the slope toe and propagates from the slope toe toward the slope crest. The numerical simulation results also suggest that the potential tension failure is likely to occur within the slope between the potential shear failure surface and the ground surface.

• Journal of the Korean GEO-environmental Society
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• 제11권5호
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• pp.35-43
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• 2010

Partially saturated permeability should be defined by the function of suction (or degree of saturation) and porosity. However, commercial software and most researchers' model often describe as the function of suction. The stability of a soil slope can be affected by both hydraulic and shear strength properties of partially saturated soils. For both studies, we generally use an uncoupled seepage analysis program Seep/W(Geo-Slope, 2007) and a series stress-deformation analysis program Sigma/W, or slope stability analysis program Slope/W. Seep/W is performed for simulations of partially saturated flow problems in non-deformable soil media. However, under real situations, the water flow processes in a deformable soil are influenced by soil skeleton movement and the pore water pressure changed due to seepage will lead to changes in stresses and to deformation of a soil. Many researchers are currently developing their models for solving coupled hydro-mechanical problems to simulate slope stability during a rainstorm. For a proper implementation in the field, the developed model should be still needed in order to achieve appropriate accuracy of the solution for coupled hydro-mechanical problems in soil slope stability. Thus, the paper presents the comparison of slope stability between uncoupled and coupled analyses of seepage and stress deformation problems.

• Journal of the Korean Geotechnical Society
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• 제22권7호
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• pp.55-64
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• 2006

Shallow failures of slopes in weathered soils are caused by infiltration caused by prolonged rainfall. These failures are mainly triggered by the deepening of the wetting band accompanied by a decrease in suction induced by the water infiltration. In this paper, hysteresis on soil-water characteristic curve (SWCC) of granite and gneiss weathered soils is investigated using transient flow analysis respectively. Each case was subjected to artificial rainfall intensities and time duration depending on the laboratory-based drying and wetting processes. The results show that the unsaturated seepage on weathered slopes are very much affected by the initial suction of soils and unsaturated permeability of the soils. In addition, a granite weathered soil has a lower air-entry value, residual matric suction, and wetting front suction and less hysteresis loop than a gneiss weathered soil.

• Journal of the Korean Geotechnical Society
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• 제33권9호
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• pp.5-22
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• 2017

The procedure that combines the result of infiltration analysis into stability analysis based on the limit equilibrium method is widely used to evaluate the impact of rainfall infiltration on slope stability. Accurate prediction of rainfall infiltration is essential to the prediction of landslides caused by rainfall, requires to obtain accurate unsaturated hydraulic properties of the soil. Among the unsaturated hydraulic characteristics of the soil, the importance of the soil-water characteristic curve describing the retained water characteristics of the soil is relatively well known and the measurement by test method to obtain the SWCC is gradually increasing. However, it takes a lot of time and expenses to experimentally measure the unsaturated conductivity characteristics of the soil. Therefore, it is common practice to estimate the hydraulic conductivity function from the SWCC. Although it is widely known that the SWCC has a great influence on rainfall infiltration, studies on the effect of the hydraulic conductivity function estimated from the SWCC on rainfall infiltration are very limited. In this study, we explained how the estimation model of the hydraulic conductivity function affects rainfall infiltration and slope stability analysis. To this end, one-dimensional infiltration analysis and slope stability analysis were conducted by using the data on the SWCC of weathered granite soil widely distributed in Korea. The applicability of each estimation model is discussed through review of the analysis results.

• Journal of the Korean Geotechnical Society
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• 제31권2호
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• pp.5-11
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• 2015

Slope stability is affected by duration of precipitation, probable rainfall intensity, unsaturated soil property, and soil strength. The recent analyses of slope stability tend to include unsaturated analysis based on infiltration properties of soil, while researches of unsaturated soil slope tend to include the analysis of deformation and stress distribution of soil over time. However, infiltration property of unsaturated soil slope depends not only on intensity or duration of precipitation, but also on relief and surface condition, which is not considered in status quo. This research uses hydrologic model parameters of soil in order to consider effects of inclination on filtration, and carries out analysis of unsaturated soil slope to confirm the effects according to slope inclination and surface condition. In conclusion, using slope stability analysis, the need to consider infiltration rate according to inclination and surface condition was confirmed even under the same precipitation conditions.

• KSCE Journal of Civil and Environmental Engineering Research
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• 제29권5C호
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• pp.191-198
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• 2009

It has been reported in Korea that surface slope failures in weathered soil are mainly caused by downward infiltration due to rainfall. These failures are triggered by the deepening of the wetting band in soils accompanied by a decrease in matric suction induced by the water infiltration. So, a need exists that these trends of wetting path in gneiss weathered soils, which is commonly found in Korea, are assessed by phenomenological approach. In this paper, numerical analyses of unsaturated soil slope under rainfall conditions are presented based on the wetting path soil-water characteristic curve in the laboratory. As the field SWCC matches well with the wetting path of the laboratory SWCC from the literatures, it seems reasonable to adopt the laboratory wetting SWCC as an upper boundary condition in the assessment of unsaturated slope instability.

• Journal of the Korean GEO-environmental Society
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• 제7권2호
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• pp.5-14
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• 2006

It has been reported that the failure of a slope in weathered soils or rocks induced by intensive rainfall occurs mainly within 2.0m below the ground surface, and that the effect of rainfall on the rise of the ground water level is not significant if the ground water level inside the slope is not so high before rainfall. Most slopes in Korea, however, have been examined to rather fail at the deeper part since they are usually designed on the basis of assumption that the ground water level rises up to the surface when raining. In this study, the rise of ground water level and slope stability were examined in order to verify the appropriateness of the current ground water level condition that has a significant effect on slope stability using the average daily rainfall in Seoul for the last 30 years. The result showed that the ground water level appears to rise up to 6.0~41.0% of the slope height respectively, and therefore the currently applied condition of ground water level may be somewhat overestimated.