• Title/Summary/Keyword: Unsaturated soil

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Effect of suction on volume change and shear behaviour of an overconsolidated unsaturated silty soil

  • Estabragh, A.R.;Javadi, A.A.
    • Geomechanics and Engineering
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    • v.4 no.1
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    • pp.55-65
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    • 2012
  • This paper presents the results of an experimental study on the effect of suction on compressibility and shear behaviour of unsaturated silty soil under various types of loading. A series of laboratory experiments were conducted in a double-walled triaxial cell on samples of a compacted silty soil. In the experiments the soil samples were subjected to isotropic consolidation followed by unloading and subsequent reloading under constant suction and prescribed overconsolidated ratio. The experimental results are presented in the context of an elasto-plastic model for unsaturated soil. The effects of suction on mechanical behaviour of unsaturated silty soil are presented and discussed. It is shown that increasing suction affects the shear behaviour of unsaturated soils, but there is a limit beyond which, further increase in suction will not result in any significant change in the behaviour.

Triaxial Compressive Behaviour of Unsaturated Silt under Different Drainage Conditions (다양한 경계조건에서의 불포화 실트의 삼축압축 거동)

  • Kim, Young-Seok;Oka, Fusao
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.998-1003
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    • 2008
  • It has been recognized unsaturated soil behaviour playing an important role in geomechanics. Up to now, only a few experimental data are available for the technical difficulties related to both volume changes and suction measurements. In this study, the volume changes of unsaturated compacted silty soil were monitored with proximeter (i.e. non-contactable transducer) during various triaxial compression tests, which gave a realistic estimation in the volume changes of unsaturated soil sample. Various triaxial compression tests for unsaturated soil under different drainage conditions are carried out. The behaviour of the pore pressure, namely, the pore-air pressure and the pore-water pressure, and matric suction during the shearing tests are investigated. The experimental results have revealed that the mechanical behaviour of unsaturated soil can be significantly affected by the matric suction.

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Experimental Study for Soil-Water Characteristic Curves of Unsaturated Soil (불포화토의 흙-수분 특성곡선에 관한 실험적 고찰)

  • 송창섭;김명환
    • Magazine of the Korean Society of Agricultural Engineers
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    • v.45 no.6
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    • pp.153-161
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    • 2003
  • The purpose of this paper is to investigate the soil-water characteristic curves for an unsaturated soil. To this ends, a series of suction measured test was conducted on the selected 4 kinds of soils taken from different sites of Korea, using modified pressure plate apparatus. Form the test results, the water contents, degree of saturation and volumetric water contents were analyzed with the suction. And the soil-water characteristic curves of unsaturated soil were drawn from the test results for various factors. The characteristic curves drawn with water content vs matric suction were classified certainly the difference in wet side but were not classified in dry side. The characteristic curves drawn with degree of saturation vs matric suction for unsaturated soil were shown the opposite inclination as compared with the former curve. But the characteristic curve with volumetric water content vs matric suction was described suitably not only in wet side condition but also in dry side. And it was found that the volumetric water contents of loose soil was high at the initial condition but that of dense soil was high at final condition (dry side).

Estimation of Unsaturated Permeability Function from Water Retention Characteristics for Korean Weathered Soils (함수특성에 근거한 국내 풍화토의 불포화 투수곡선 추정)

  • Kim, Yun-Ki;Choi, Kyung-Lim;Lee, Sung-Jin;Lee, Seung-Rae;Kwon, Hyoung-Seok
    • Journal of the Korean Geotechnical Society
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    • v.26 no.10
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    • pp.49-60
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    • 2010
  • Unsaturated permeability function is an important factor in the design and analysis of various unsaturated soil structures. Generally the permeability characteristics decrease as the mat ric suction increases and the trend is similar to water retention characteristics of a soil. The permeability of unsaturated soils can be obtained directly by laboratory tests or indirectly by estimation methods from other soil properties. For unsaturated soils sampled from 7 areas in KOREA, SWCCs and unsaturated permeability functions were obtained by experimental tests. The unsaturated permeability results were also compared with the unsaturated permeability functions derived from the SWCCs theoretically. However, the current estimation models of unsaturated permeability function did not express the unsaturated permeability characteristics. Therefore, the FXK-M permeability function was modified to predict more accurate permeability functions for Korean weathered soils using a correction factor that can be calculated from the air-entry value of SWCC. The new estimation model resulted in good agreements for all tested soils.

Evaluation of Overburden Pressure on Soil-Water Retention Characteristics of Unsaturated Weathered Soils (상재하중의 영향을 고려한 불포화 풍화토의 함수특성 평가)

  • Park, Jai-Young;Park, Seong-Wan
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.111-118
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    • 2008
  • The purpose of this paper was to investigate the overburden effect on soil-water characteristic curve and unsaturated permeability of unsaturated weathered soils. For this, unsaturated suction and permeability tests under various overburden stress were conducted respectively. Then, the coefficient of unsaturated permeability and moisture capacity of weathered soils were estimated and compared. All these results are presented in the paper.

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Unsaturated Soil Mechanics for Slope Stability

  • Rahardjo, Harianto;Satyanaga, Alfrendo;Leong, Eng-Choon
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.481-501
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    • 2007
  • Excessive rainfalls due to climatic changes can trigger an increase in rainfall-induced slope failures that pose real threats to both lives and properties. Many high slopes in residual soils could stand at a steep angle, but failed during or after rainfall. Commonly, these slopes have a deep groundwater table and negative pore-water pressures in the unsaturated zone above the groundwater table contribute to the shear strength of soil and consequently to factor of safety of the slope. Stability assessment of slope under rainfall requires information on rate of rainwater infiltration in the unsaturated zone and the resulting changes in pore-water pressure and shear strength of soil. This paper describes the application of unsaturated soil mechanics principles and theories in the assessment of rainfall effect on stability of slope through proper characterization of soil properties, measurement of negative pore-water pressures, seepage and slope stability analyses involving unsaturated and saturated soils. Factors controlling the rate of changes in factor of safety during rainfall and a preventive method to minimize infiltration are highlighted in this paper.

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Calculation model for the shear strength of unsaturated soil under nonlinear strength theory

  • Deng, Dongping;Wen, Shasha;Lu, Kuan;Li, Liang
    • Geomechanics and Engineering
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    • v.21 no.3
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    • pp.247-258
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    • 2020
  • The shear strength of unsaturated soils, a research hotspot in geotechnical engineering, has great guiding significance for geotechnical engineering design. Although kinds of calculation models for the shear strength of unsaturated soil have been put forward by predecessors, there is still need for new models to extensively consider the nonlinear variation of shear strength, particularly for the nonlinear effect of the net normal stress on the shear strength of unsaturated soil. Here, the shear strength of unsaturated soils is explored to study the nonlinear effects of net normal stress with the introduction of a general nonlinear Mohr-Coulomb (M-C) strength criterion, and the relationship between the matric suction (or suction stress) and degree of saturation (DOS) constructed by the soil-water characteristics curve (SWCC) of van Genuchten is also applied for unsaturated soil. Then, two calculation models (i.e., an envelope shell model and an effective stress model) are established for the shear strength of unsaturated soils under the nonlinear strength theory. In these two models, the curve of the shear strength of unsaturated soils versus the net normal stress exhibits a tendency to gently. Moreover, the proposed formulas have flexibility and convenience with five parameters (for the effective stress model) or six parameters (for the envelope shell model), which are from the M-C strength parameters of the saturated soil and fitting parameters of SWCC of van Genuchten. Thereafter, by comparison with the classical theory of the shear strength of unsaturated soils from some actual cases, the rationality and accuracy of the present models were verified.

LE analysis on unsaturated slope stability with introduction of nonlinearity of soil strength

  • Deng, Dong-ping;Lu, Kuan;Li, Liang
    • Geomechanics and Engineering
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    • v.19 no.2
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    • pp.179-191
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    • 2019
  • Based on the effective stress principle, a new formula for shear strength of unsaturated soil is derived under the general nonlinear Mohr-Coulomb (M-C) strength criterion to improve the classical strength criterion of unsaturated soil. Meanwhile, the simple irrigation model under steady seepage is adopted to obtain the distribution of the matrix suction or the degree of saturation (DOS) above the groundwater table in the slope. Then, combined with the improved strength criterion of unsaturated soil and the simple irrigation model under steady seepage, the limit equilibrium (LE) solutions for the unsaturated slope stability are established according to the global LE conditions of the entire sliding body with assumption of the stresses on the slip surface. Compared to the classical strength criterion of unsaturated soil, not only the cohesion soil but also the internal friction angle is affected by the matric suction or the DOS in the improved strength criterion. Moreover, the internal friction angle related to the matric suction has the nonlinear characteristics, particularly for a small of the matric suction. Thereafter, the feasibility of the present method is verified by comparison and analysis on some slope examples. Furthermore, stability charts are also drawn to quickly analyze the unsaturated slope stability.

Modelling the hydraulic/mechanical behaviour of an unsaturated completely decomposed granite under various conditions

  • Xiong, Xi;Xiong, Yonglin;Zhang, Feng
    • Geomechanics and Engineering
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    • v.25 no.2
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    • pp.75-87
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    • 2021
  • Because the hydraulic/mechanical behaviour of unsaturated soil is more complicated than that of saturated soil, one of the most important issues in modelling unsaturated soil is to properly couple its stress-strain relationship with its water retention characteristics. Based on the results of a series of tests, the stress-strain relationship and the changes in suction and saturation of unsaturated completely decomposed granite (CDG, also called Masado) vary substantially under different loading/hydraulic conditions. To precisely model the hydraulic/mechanical behaviour of unsaturated Masado, in this study, the superloading concept was firstly introduced into an existing saturated/unsaturated constitutive model to consider the structural influences. Then a water retention curve (WRC) model considering the volumetric change in the soil, in which the skeleton and scanning curves of the water retention characteristics were assumed to shift in parallel in accordance with the change in the void ratio, was proposed. The proposed WRC model was incorporated into the constitutive model, and the validity of the newly proposed model was verified using the results of tests conducted on unsaturated Masado, including water retention, oedometer and triaxial tests. The accuracy of the proposed model in describing the stress-strain relationship and the variations in suction and saturation of unsaturated Masado is satisfactory.

Infiltration Characteristics for Unsaturated Residual Soil (화강풍화토의 불포화 침투특성에 관한 연구)

  • 김영욱;김도형;성상규;이인모
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.147-152
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    • 2001
  • This study investigated one-dimensional vertical infiltration to an unsaturated residual soil by numerical solutions, FDM. In order to estimate the parameters needed for numerical analysis, tire soil-water characteristic curve(SWCC) of Shinnae-dong soil, one of the most typical residual soils in Korea, were experimentally obtained. Then, the statistical analysis for obtaining the SWCC was performed. The numerical solution to the linearized governing equation for unsaturated groundwater flow provides the infiltration characteristics for the unsaturated residual soil represented by transient pressure profiles and water contents profiles.

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