• Journal of the Korean Geotechnical Society
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• v.25 no.1
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• pp.65-76
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• 2009

The unsaturated characteristics of Korean weathered granite soils have been studied to investigate the influence of saturated permeability, rainfall intensity and soil-water characteristic curve (SWCC) on the slope stability. The upper, average and lower SWCCs were estimated from the publication and experimental results using the statistical concept. The roughly estimated SWCC can be used for the soils without experimental results by relating SWCC with the particle size distribution curve. An appropriate ratio between the saturated permeability and the rainfall intensity ($k_s$/i) was also suggested for practical use in designing the slopes by investigating the time-dependent variation of slope instability during the rainfall. The slope stability was deteriorated from the initiation of rainfall and recovered again after the factor of safety reached the critical value. The FS of the slope decreased at first and then increased after reaching the critical value during the rainfall. As a result, the slope instability was not related with an absolute rainfall intensity but with the ratio between the saturated permeability and the rainfall intensity. In case of the upper SWCC, the critical condition occurred when the ratio between the saturated permeability and the rainfall intensity was in the range of $1.0{\sim}2.0$.

• Journal of the Korean Geotechnical Society
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• v.21 no.6
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• pp.147-154
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• 2005

This study deals with the calibration of low-frequency water content reflectometer for measuring the volumetric water content of soils in landfill final cover layer, and the validity of calibrations was evaluated by electrical conductivity and index properties of the soils. Linear calibrations concerning volumetric water content to WCR period provided good agreement with the data. Analysis of the calibration data indicates that the slope of the calibration decreases as the electrical conductivity of the soil increases. Lower slopes correspond to soils with greater clay content, organic content, liquid limit, and plasticity index, which typically have higher electrical conductivity. It could be well explained that WCR can operate in a lower frequency range.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.11
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• pp.3287-3295
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• 2009

This present study implemented a rainfall simulation system, and performed simulation and numerical analysis according to rainfall and slope conditions using a model slope built with weathered granite soil. Extensive analysis were conducted on the characteristics of changes in volumetric water content and pore water pressure measured in the simulation, and compared them with the results of numerical analysis. It took longer for the volumetric water content to reach the limit when rainfall intensity was high and the slope was steep and shorter when rainfall intensity was low and the slope was gentle. When rainfall intensity was low and the duration of rainfall was short, negative pore water pressure was higher and the time for restoration was shorter. On the contrary, when rainfall intensity was high and the duration of rainfall was long, it took a longer time to restore negative pore water pressure. In the results of rainfall simulation and numerical analysis, the distribution of volumetric water content and pore water pressure was similar between the two. However, the volumetric water content was different by up to 5%, and pore water pressure by up to 3kPa.

• Journal of the Korean GEO-environmental Society
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• v.11 no.2
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• pp.13-21
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• 2010

The unsaturated slope usually is stable for a long time, but fails during heavy rainfall. And the factors of the rainfall intensity exhibit significant roles because the water content and the shear stress developed along the potential failure surface will be changed by the rainfall intensity. The objective of the study presented in this paper is to analyze the relationship between rainfall intensity and shear stress of the soil slopes by applying the laboratory slope model apparatus and undrained direct shear test with rainfall intensity controlled. The soil sample was taken from the field slope of Youngdong, and particle size analysis was done. To look over the relationship between rainfall intensity and shear strength of slope, the three-dimensional relationships among shear strength, normal stress and water content of the slope soil samples are examined; those are based on the data from the TDR sensor and undrained direct shear test.

• Journal of the Korean Geosynthetics Society
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• v.9 no.3
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• pp.77-85
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• 2010

The change of water level in reservoirs is an important factor causing failure of bank slopes, i.e. landslide. The water level of Three Gorges reservoir in China fluctuate between 145 m and 175 m, as a matter of flood control. During its normal operational state, the rate of water level fluctuation is supposed to range from 0.67 m/d to 3.0 m/d. Majiagou slope is located on the left bank of Zhaxi River, 2.1 km up from the outlet. Zhaxi River is a tributary of the Yangtze River within the Three Gorges area, of which the water level changes with the reservoir. At the back of Majiagou slope, a 20 m long and 3~10 cm wide fissure developed just after the reservoir water level rose from 95 m to 135 m in 2003. This big fissure was a full suggestion of potential failure of this slope. In this study, unsaturated-saturated seepage analyses were carried out to simulate the change of pore-water pressures in the bank slope subjected to the reservoir water level change. The obtained pore-water pressures were then used to evaluate the change in factor of safety (FS) with reservoir water level. It was found that the phreatic line showed a delayed response with respect to the change of the reservoir water level, because the seepage through soil layer was generally slower than water flows itself. During the rising and drawdown process, the phreatic lines take the shapes of concave and convex, respectively. And the fluctuation of reservoir water level just affected the front part of the bank slope, but had little influence on the back of the slope.

• Journal of the Korean Geotechnical Society
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• v.29 no.7
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• pp.75-89
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• 2013

This paper describes a GIS-based geohydrologic methodology, called YSGWF (YonSei GroundWater Flow) for predicting the rainfall infiltration-groundwater flow of slopes. This physical-based model was developed by the combination of modified Green-Ampt model that considers the unsaturated soil parameters and GIS-based raster model using Darcy's law that reflects the groundwater flow. In the model, raster data are used to simulate the three dimensional inclination of bedrock surface as actual topographic data, and the groundwater flow is governed by the slope. Also, soil profile is ideally subdivided into three zones, i.e., the wetting band zone, partially saturated zone, and fully saturated zone. In the wetting band and partially saturated zones the vertical infiltration of water (rainfall) from surface into ground is modeled. When the infiltrated water recharges into the fully saturated zone, the horizontal flow of groundwater is introduced. A comparison between the numerical calculation and real landslide data shows a reasonable agreement, which indicate that the model can be used to simulate real rainfall infiltration-groundwater flow.