• Title/Summary/Keyword: Unsaturated weathered soils

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A Study on Analytical Solution of Unsaturated Infinite Slope Stability (불포화 무한사면 안전율의 수정방정식에 대한 연구)

  • Chae, Yu-Mi;Kim, Jae-Hong;Jeong, Young-Hun;Kim, Tae-Heon
    • Journal of the Korean Geotechnical Society
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    • v.34 no.4
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    • pp.5-11
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    • 2018
  • In conventional analytical solutions for rainfall-induced soil slope stability, the Green-Ampt (1911) equation for estimating the saturation depth and the Skempton & DeLory (1957) equation for calculating the infinite slope shallow failure were compared with the numerical analysis to confirm the error. In the simple evaluation of the reason of soil slope instability due to rainfall using the conventional equations, there are many errors and, overestimation or underestimation of the calculation results. In this study, the equation consisting of the results obtained from infiltration analysis on unsaturated soil slope is proposed by applying the average range of the strength parameters of the granite weathered soils, and its reliability is verified by comparing with the numerical analysis results. The developed equation can be used easily in various fields for the estimation of slope safety factor by checking the rainfall duration and saturation depth.

Ground Characterization of the Cheongju Granite Area Using the Geophysical Methods (물리탐사를 이용한 청주 화강암 지역의 지반특성 파악)

  • Kim Ji-Soo;Han Soo-Hyung;Seo Yong-Seok;Lee Yong-Jae
    • The Journal of Engineering Geology
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    • v.15 no.1
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    • pp.41-55
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    • 2005
  • This research is aimed at investigating the ground characterization of the Cheongju granite area using the geophysical methods. Test site was chosen from the building site in Chungbuk University, Chongju, Chungbuk province. Furthermore, geophysical methods are employed on the outcrops in the east to map the distribution of fault and intrusion and reveal the degree of weathering. The subsurface structure mapped from seismic re-fraction survey mainly consists of two units of weathered soil and rock. Threshold of the units were determined on the basis of seismic velocity of 800 m/s, supported from the standard classification table. From the results of standard penetrating test(SPT), these units are found to show medium-high and high density, respectively. Weathering soil is subdivided in unsaturated layer and saturated layer with thresholds of seismic velocity (500 m/s) and resistivity (200 ohm-m). In particular, unsaturated layer is again classified into dry and wet portions using the GPR section. The boundary between unsaturated and saturated weathering soils corresponds to the groundwater table at depth of approximately 5~6.2 m, which is well correlated with the one from drill-core data. However, bedrock is not delineated by geophysical methods. In the GPR section, fault and intrusion observed on the outcrop are revealed not to extend to the building site. With respect to weathering degree, the outcrop characterized by low resistivity and velocity corresponds to the grade of 'completely weathered' from the geotechnical investigations.

Influence of Rainfall Intensity and Saturated Permeability on Slope Stability during Rainfall Infiltration (강우침투시 강우강도와 포화투수계수가 안전율에 미치는 영향)

  • Lee, Seung-Rae;Oh, Tae-Kyu;Kim, Yun-Ki;Kim, Hee-Chul
    • 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 $1.0{\sim}2.0$.

Effect of Rainfall-Patterns on Slope Stability in Unsaturated Weathered Soils (강우사상의 영향을 고려한 불포화 풍화사면의 안정성)

  • Kim, Byeong-Su;Park, Seong-Wann
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.33 no.3
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    • pp.1027-1035
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    • 2013
  • In this study, two rainfall patterns are utilized for practical consideration of rainfall phenomena in unsaturated soil slope design. One is the I.D.F (Intensity-Duration-Frequency) method which is an existing design rainfall method and ignores the effect of the variation of the rainfall according to the time. The other is the Huff method which considers this effect oppositely. First, the safety of factor of the slope according to the variation of an initial suction which means the precedent rainfall effect was examined by means of the application of the I.D.F method. Through the application of two rainfall patterns, it was discussed how the rainfall pattern affects the factor of safety of the slope. As a result, it is found that the Huff method is more practical on the evaluation of the slope stability than the I.D.F method.

Reliable Assessment of Rainfall-Induced Slope Instability (강우로 인한 사면의 불안정성에 대한 신뢰성 있는 평가)

  • Kim, Yun-Ki;Choi, Jung-Chan;Lee, Seung-Rae;Seong, Joo-Hyun
    • Journal of the Korean Geotechnical Society
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    • v.25 no.5
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    • pp.53-64
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    • 2009
  • Many slope failures are induced by rainfall infiltration. A lot of recent researches are therefore focused on rainfall-induced slope instability and the rainfall infiltration is recognized as the important triggering factor. The rainfall infiltrates into the soil slope and makes the matric suction lost in the slope and even the positive pore water pressure develops near the surface of the slope. They decrease the resisting shear strength. In Korea, a few public institutions suggested conservative slope design guidelines that assume a fully saturated soil condition. However, this assumption is irrelevant and sometimes soil properties are misused in the slope design method to fulfill the requirement. In this study, a more relevant slope stability evaluation method is suggested to take into account the real rainfall infiltration phenomenon. Unsaturated soil properties such as shear strength, soil-water characteristic curve and permeability for Korean weathered soils were obtained by laboratory tests and also estimated by artificial neural network models. For real-time assessment of slope instability, failure warning criteria of slope based on deterministic and probabilistic analyses were introduced to complement uncertainties of field measurement data. The slope stability evaluation technique can be combined with field measurement data of important factors, such as matric suction and water content, to develop an early warning system for probably unstable slopes due to the rainfall.