• Geomechanics and Engineering
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• 제23권5호
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• pp.419-429
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• 2020

To investigate the effect of spatial variability on hydraulic properties of unsaturated soils, a numerical model is set up which can simulate seepage process in an unsaturated heterogeneous soil. The unsaturated heterogeneous soil is composed of matrix sand embedded with a small proportion of clay for simulating the heterogeneity. Soil-water characteristic curve and unsaturated hydraulic conductivity curve of the unsaturated soil are expressed by Van Genuchten model. Hydraulic parameters of the matrix sand are considered as random fields. Different autocorrelation lengths (ACLs) of hydraulic parameter of the matrix sand and different proportions of clay are assumed to investigate the influence of spatial variability on the equivalent hydraulic properties of the heterogeneous soil. Four model sizes are used in the numerical experiments to investigate the influence of scale effects and to determine the sizes of representative volume element (RVE) in the numerical simulations. Through a number of Monte Carlo simulations of unsaturated seepage analysis, the means and the coefficients of variations (COVs) of the equivalent hydraulic parameters of the heterogeneous soil are calculated. Simulations show that the ACL and model size has little influence on the means of the equivalent hydraulic parameters, but they have a large influence on the COVs of the equivalent hydraulic parameters. The size of an RVE is mainly affected by the ACL and the proportion of heterogeneity. The influence of spatial variability on the hydraulic parameters of the heterogeneous unsaturated soil can be used as a guidance for geotechnical reliability analysis and design related to unsaturated soils.

• 한국지반공학회논문집
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• 제38권3호
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• pp.27-34
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• 2022

지반내 물의 흐름은 입자 사이의 공극 분포에 의존하므로 입자의 크기를 이용한 수리학적 물성치의 예측은 정확도가 낮다. 본 논문은 Silveria의 방법을 이용하여 입도분포곡선으로부터 수축 공극크기분포를 산정하고, 포화-불포화 수리학적 물성치를 산정하는 방법을 제시하였다. 입도분포가 양호한 흙은 단봉의 공극크기분포를 보이고, 입도분포가 불량한 흙은 쌍봉의 공극크기분포를 보였다. 공극크기분포를 이용한 이론적 포화투수계수 모델식 중에서 Marshall 모델이 실내실험결과와 가장 부합되었다. 불포화토 수리해석에 필요한 함수특성곡선과 불포화투수계수에 대한 모델식을 공극크기분포를 이용하여 제안하였다. 개발된 모델식을 다양한 흙에 적용하여 수리학적 물성치의 예측에 적합한 모델을 선정하는 지속적인 연구가 필요하다.

• 한국지반공학회논문집
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• 제21권3호
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• pp.119-131
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• 2005

댐 안정성 평가를 위하여 불포화 이론을 이용한 침투해석 기법이 널리 사용되고 있으나 지반의 불포화 거동에 대한 인식 부족과 실험절차의 번거로움으로 인하여 입력 물성치에 대한 체계적인 실험과 평가가 일반화되지 못하고 있는 실정이다. 불포화 수리특성에 따라 침투수량 및 간극수압 분포 등 댐체 및 제방의 침투거동이 달라지나 국내 지반에 대한 실험자료 부족으로 대부분 단순 추정에 의한 값을 사용하고 있다 본 연구에서는 불포화 수리특성이 댐체의 침투 거동에 미치는 영향을 살펴보기 위하여 함수특성곡선과 투수계수곡선에 대하여 고찰하고 수치해석을 수행하였다. 해석결과 변수 a와 n은 값이 클수록, m은 값이 작을수록 중력에 의한 침윤선의 하강이 빨리 발생하였다. 유한차분 형식에 의해 개략적으로 계산한 민감도는 침윤선 부근에서 가장 크게 나타났고 시간에 따른 민감도의 변화에는 함수특성곡선의 기울기와 관련된 n값이 가장 큰 영향을 주었다.

• 한국농공학회논문집
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• 제54권5호
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• pp.129-139
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• 2012

Accurate modeling rainfall induced landslide and slope stability requires a detailed knowledge of the distribution of material strength characteristics and suction distribution. However, material properties obtained from the drying cycle are still used for infiltration analysis in many cases, even though material properties of wetting cycle are quite different from those of drying cycle due to hydraulic hysteresis and air occlusion. Therefore, the selection of proper material properties such as soil-water retention curve (SWRC) and the hydraulic conductivity function (HCF) reflecting characteristics of wetting cycle and air occlusion is an essential prerequisite in order to simulate the infiltration phenomena and to predict the suction and water content distribution in unsaturated soils. It is concluded that the simulation of infiltration with material properties from the drying cycle did not reasonably match with experimental outputs. Further discussion is made on how to describe the material properties considering air occlusion during wetting cycle over the entire suction range in order to simulate infiltration phenomena.

• 한국지반공학회논문집
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• 제22권4호
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• pp.73-83
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• 2006

댐 안정성 평가를 위하여 불포화 이론을 이용한 침투해석 기법이 널리 사용되고 있으나 지반의 불포화 거동에 대한 인식 부족과 실험절차의 번거로움으로 인하여 입력 물성치에 대한 체계적인 실험과 평가가 일반화되지 못하고 있는 실정이다. 지반의 불포화 수리특성은 변동성과 불확실성을 내포하고 있으며, 이는 침투수량 및 간극수압 분포 등 댐체 및 제방의 침투거동에 영향을 미친다. 본 연구에서는 댐의 심벽재료에 대한 함수특성곡선과 투수계수를 구하는 실내실험을 수행하였다. 그리고, 불포화수리특성이 댐체의 침투 거동에 미치는 영향을 살펴보기 위하여 입력물성의 확률론적 분포를 고려한 수치해석을 수행하였다. 해석결과 Fredlund와 Xing 함수특성곡선식과 Campbell의 투수계 수식에서 변수 a와 n은 값이 클수록, n과 p는 값이 작을수록 정상상태에서의 침투수량이 많았다.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2005년도 지반공학 공동 학술발표회
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• pp.649-654
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• 2005

Shallow slope failures in residual soil during periods of prolonged infiltration are common over the world. One of the key factors that dominate slope stability is hydrological response associated with infiltration. Hence, the soil-water profile during rainfall infiltration into unsaturated soil must me examined to evaluate slope stability. However, the hydraulic response of unsaturated soil is complicated by inherent uncertainties of the soil hydraulic properties. This study presents a methodology for assessing the effects of parameter uncertainty of hydraulic properties on the response of a analytical infiltration model using first-order reliability method. The unsaturated soil properties are considered as uncertain variables with means, standard deviations, and marginal probability distributions. Sensitivities of the probabilistic outcome to the basic uncertainties in the input random variables are provided through importance factors.

• Geomechanics and Engineering
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• 제33권2호
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• pp.141-154
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• 2023

This study investigated the effect of water retention characteristics between aged and fresh Municipal Solid Waste (MSW) on the stability of the landfill. A series of transient numerical modeling for the slope of an MSW landfill was performed considering the variation of water retention characteristics due to leachate circulation. Four different scenarios were considered in this analysis depending on how to obtain hydraulic conductivity and the aging degree of materials. Unsaturated hydraulic properties of the MSW used for the modeling were evaluated through modified hanging column tests. Different water retention properties and various landfill conditions, such as subgrade stiffness, leachate injection frequency, and gas and leachate collection system, were considered to investigate the pore water distribution and slope stability. The stability analyses related to the factor of safety showed that unsaturated properties under those varied conditions significantly impacted the slope stability, where the factor of safety decreased, ranging between 9.4 and 22%. The aged materials resulted in a higher factor of safety than fresh materials; however, after 1000 days, the factor of safety decreased by around 10.6% due to pore pressure buildup. The analysis results indicated that using fresh materials yielded higher factor of safety values. The landfill subgrade was found to have a significant impact on the factor of safety, which resulted in an average of 34% lower factor of safety in soft subgrades. The results also revealed that a failed leachate collection system (e.g., clogging) could result in landfill failure (factor of safety < 1) after around 298 days, while the leachate recirculation frequency has no critical impact on stability. In addition, the accumulation of gas pressure within the waste body resulted in factor of safety reductions as high as 24%. It is essential to consider factors related to the unsaturated hydraulic properties in designing a landfill to prevent landfill instability.

• 한국농공학회논문집
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• 제54권4호
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• pp.93-103
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• 2012

A suction-saturation control technique based on flow pump system was developed to investigate hydraulic properties in unsaturated soils. The flow pump system is designed based on the principle of the axis-translation technique and triaxial equipment, and gives the suction-time and suction-saturation curves, the primary relationship needed for interpreting the response of unsaturated soils and link between theory and the material properties in unsaturated soil mechanics. Using the suction-saturation control technique, suction-time relationship and soil-water retention curve (SWRC) during hydraulic hysteresis were investigated with different net confining pressures and porosities. Three types of soils-two sands and a silt were used in this paper. This paper showed the effect of the hysteresis on the SWRC due to different net confining pressures and porosities. This means that a careful decision must be made as to which condition is to be modeled, since the delicate difference of the conditions in physical modeling can cause the different experimental output.

• 대한토목학회논문집
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• 제31권1D호
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• pp.73-82
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• 2011

일반적으로 다져진 도로기초 지반재료의 장기적인 강도나 배수 거동 평가시 불포화상태를 고려하지 않고 있다. 그래서 보조기층과 노상토와 같은 도로하부 기초에서의 불포화상태와 이력현상을 고려하는 것은 현실적이다. 따라서 보다 정량적인 불포화 지반의 평가를 위하여 흡수력과 함수특성을 평가하기 위한 적절한 장비, 재료모델, 그리고 투수계수에 대한 평가가 필요하다. 본 논문에서는 이러한 거동을 알아보기 위해서는 흙-수분 관계인 재료의 함수특성곡선 측정 자료를 통하여 불포화 투수계수를 추정하고 이를 통하여 국내 대표적인 입상재료에 대한 불포화상태에서의 수분의 이동에 대한 관찰 및 분석을 수행하였다.

• Geomechanics and Engineering
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• 제36권4호
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• pp.343-353
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• 2024

The hydraulic anisotropic behavior of unsaturated soil has not been fully explored in relation to the grain-size distribution. The present study conducted laboratory assessments to examine the hydraulic anisotropy condition of statically compacted specimens in various initial states. The investigation incorporated the concept of hydraulic anisotropy by employing two discrete forms of soil stratification: horizontal-layering (HL) and vertical-layering (VL). The examined soils comprised sandy silt and silty sand, exhibiting either unimodal or bimodal soil-water characteristic curve (SWCC). This study aimed to investigate the potential correlation between the hydraulic anisotropy ratio and soil properties. The present study established a correlation between the hydraulic anisotropy ratio and several soil parameters, including fine content, dry density, plastic limit, and liquid limit. The study results indicate a non-linear relationship between the percentage of fine and dry density in soils with unimodal and bimodal soil-water characteristic curve (SWCC) and hydraulic anisotropy ratio.