• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
• /
• 2002.09a
• /
• pp.28-31
• /
• 2002

The water level fluctuation (WLF) method is a conventional method for quantifying groundwater recharge by multiplying the specific yield to the water level rise. A 2-D unconfined flow model with a time series of the recharge rate is developed. It is used for elucidating the errors of the WLF method which is implicitly based on the tank model where the horizontal flow in the saturated zone is ignored. Simulations show that the recharge estimated by the WLF method is underestimated for the observation well near the discharge boundary. This is due to the fact that the hydraulic stress resulting from the recharge is rapidly dissipating by the horizontal flow near the discharge boundary Simulations also reveal that the recharge was significantly underestimated with increase in the hydraulic conductivity and the recharge duration, and decrease in the specific yield.

• Journal of the Korean Geotechnical Society
• /
• v.22 no.4
• /
• pp.73-83
• /
• 2006

Seepage analysis through unsaturated zone based on the theory of unsaturated flow is commonly performed to evaluate dam safety. However, the concepts of unsaturated soil behavior have not been transferred into the hands of practicing geotechnical engineers since the problems involving unsaturated soils often have the appearances of being extremely complex. There is variability and uncertainty associated with the unsaturated hydraulic properties that in turn will lead to variability in predicting unsaturated soil behavior such as seepage rate and the pore water pressure distribution. In this paper, measurements of the soil-water characteristic curve and saturated hydraulic conductivity for the core material of dam were conducted. Then, finite element stochastic analysis was used to capture the effect of unsaturated hydraulic properties on the seepage behavior of dam. It is observed that the amount of seepage increases, as the values of unsaturated soil parameters a and n increase. The values of m and p showed opposite trend.

• Journal of the Korean Geotechnical Society
• /
• v.30 no.1
• /
• pp.37-47
• /
• 2014

Unsaturated hydraulic conductivity (HC) is integrated theoretically from soil water retention curves (SWRC) by Mualem capillary model, but the prediction of HC is extremely sensitive to small variation of matric suction near saturation. Near saturation, the Mualem HC based on smooth SWRC decreases abruptly and has problems in the reliability of hydraulic behavior and the stability of numerical solutions. To improve van Genuchten-Mualem (VGM) HC, the van Genuchten SWRC model is modified within range of low matric suction (arbitrary air entry pressure). At an arbitrary air entry pressure, the VG SWRC is linearized in log scale until full saturation. The modified VG SWRC does not affect the fit of actual retention behavior and either the parameters of original VG SWRC fit. Using the modified VG SWRC, the VGM HC is modified to integrate for each interval decomposed by arbitrary air entry pressure. An analytical solution on modified VGM HC is proposed each interval, to protect the rapid change in HC near saturation. For silty soils, VGM models of HC function underestimate the unsaturated permeability characteristics and especially show rapid reduction near saturation. The modified VGM model predicts more accurate HC functions for Korean weathered soils. Furthermore, near saturation, the saturated HC is conserved by the modified VGM model. After 2-D infiltration analysis of an actual slope, the hydraulic behaviors are compared for VGM and the modified models. The prediction by the proposed model conserved the convergence of solutions on various rainfall conditions. However, the solution by VGM model did not converge since the conductivity near saturation reduced abruptly for heavy rainfall condition. Using VGM model, the factor of safety is overestimated in both initial and final stage during heavy rainfall. Stability analysis based on infiltration analysis could simulate the actual slope failure by the proposed model on HC.

• Journal of the Korean Geotechnical Society
• /
• v.30 no.3
• /
• pp.47-58
• /
• 2014

To investigate the unsaturated characteristics of the tailings obtained from the waste dump at Imgi mine, matric suction and volumetric water content were measured in both drying and wetting processes using Automated Soil-Water Characteristics Curve Apparatus. Based on the measured result, Soil Water Characteristic Curves (SWCC) were estimated by van Genuchten model. According to the unsaturated soil classification method, the tailings of the waste dump correspond to clayey sand. As a result of Suction Stress Characteristic Curve (SSCC) by Lu and Likos model, SSCC has a shape of S which is similar to SWCC. The hysteresis phenomenon occurred in SSCCs, which means the suction stress of drying path is larger than that of wetting path in the same effective degree of saturation. The effective stress of unsaturated soil is equal to that of saturated soil when matric suction is less than Air Entry Value (AEV). However, the effective stress of unsaturated soil is larger than that of saturated soil when matic suction is more than AEV. Meanwhile, unsaturated hydraulic conductivity by van Genuchten model decreased with increasing matric suction, and the hydraulic conductivity of drying path is larger than that of wetting path.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2006.05a
• /
• pp.180-188
• /
• 2006

수리 전도도는 hydraulic gradient에 대한 flux의 비율 또는, flux-hydraulic gradient 직선의 기울기를 나타내며, 포화된 토양에서의 물의 이동이 포화수리 전도도이고 불포화된 토양에서의 이동이 불포화수리전도도이다. 일반적인 밭 상태에서의 토양수분 조건은 불포화수리 전도도로 표시하는 것이 적절하나 그 상태를 표현하기가 쉽지 않다. 토양의 불포화 상태를 나타내는데 가장 많이 쓰이고 있는 VGM(van Genuchten Mualem) 모형은 토양수분 포텐셜과 수분함량의 함수로 구성된 모형이며 몇 가지 매개변수가 필요하다. VGM 모형의 매개변수를 얻기 위해 본 연구에서는 VGM 모형의 매개변수를 계산해주는 프로그램인 Rosetta를 사용하였다. Rosetta 모형은 신경그물 얼개(neural network)를 이용하여 토양의 물리적 자료들인 토성이나 모래, 미사, 점토 함량 또는 용적밀도나 33kPa, 1500kPa에서의 토양수분 함량 자료를 가지고 VGM의 매개변수인 Ko(effective saturated hydraulic conductivity), ${\theta}r$(residual soil water content), ${\theta}s$(saturated soil water content), L, n, m(=1-1/n)을 예측하는 모형으로 미국 농무성(USDA-ARS)에서 개발한 프로그램이다. Rosetta를 이용하여 10kPa에서의 불포화수리 전도도를 예측하였다. 또한 Gardner(1958)와 Wooding (1968)의 모형을 기반으로 하여 만들어진 tension infiltrometer의 포화수리 전도도 값을 Gardner 식에 적용하여 1, 3, 5, 7kPa에서의 불포화수리 전도도 값을 17개 토양통을 대상으로 하여 구했다. 토양수분 potential이 3kPa에서는 물의 이동이 거의 없는 토양들이 있었는데 반해 남계통을 비롯한 학곡통, 회곡통, 백산통, 상주통, 석천통, 예산통 등 7개의 토양은 3kPa에서도 약간의 물의 이동이 있었다. 이는 모암이 화강 편마암인 관계로 토양 내에 물의 이동에 영향을 미치는 자갈의 함량이 높았기 때문일 것으로 생각되고 추후의 연구에서는 이 부분에 대한 내용도 검토되어야 할 것이다. 또한, 1kPa에서 물의 이동은 삼각통에서 35.21 cm/day로 이동 속도가 가장 컸으며 그 뒤로 예산통, 화봉통, 학곡통, 백산통 등이 토양에서 빠른 속도로 이동하였다. 가천통이나 석천통 및 우곡통은 1kPa에서의 이동 속도가 아주 느린 토양으로 판단되었다. 또한, 포화되지 않은 상태인 1kPa에서 물의 이동 속도를 VGM 모형에 의해 예측된 값과 측정된 값으로 비교하였을 때 불포화 수리 전도도가 예측되지 않은 토양(석천통, 지곡통, 풍천통)이 존재하여 불포화 수리 전도도 특성평가에 대한 VGM 모형의 적용성에 문제를 보였다. 이는 결과적으로 논이라는 영농형태가 존재하는 우리나라에서 토양의 수리적 특성해석을 위한 VGM 모형의 적용성에 한계가 있을 것으로 판단되었다.

• Korean Journal of Soil Science and Fertilizer
• /
• v.43 no.1
• /
• pp.1-7
• /
• 2010

This study was carried out to investigate plow pan characteristics and to grasp the relationship between its particle size distribution fractal dimension ($D_m$) and water transport in paddy plow pan. Twenty four soil sampling sites with different management groups, ordinary and sandy-textured, were selected and investigated for physical properties of soils such as Yamanaka hardness in April, non-submerged condition, before rice seedling transplanting. The plow pan appearing depth and thickness was determined by penetration resistance profile. Undisturbed core samples with five replicates were sampled at plow pan layerwith 2 inch cores for measuring soil bulk density and saturated hydraulic conductivity. The particle size distribution fractal dimension ($D_m$) was calculated by the method following the procedure Tylerand Wheacraft (1992), using the USDA-based particle size analysis datawith fractions of 0-0.002, 0.002-0.053, 0.053-0.1, 0.1-0.25, 0.25-0.5, 0.5-1.0, and 1.0-2.0 mm. The plow pan of investigated fields appeared at a range from 5 to 30 cm depth, showing minimum value in sandy-textured management group and maximum value in ordinary management group. The thickness of plow pan were distributed from 5 to 17 cm, showing both minimum and maximum values in sandy-textured management group. Averagely, the plow appearing depth were deeper in ordinary management group than in sandy-textured management group, whereas the reverse in the thickness of plow pan. The particle size distribution fractal dimension ($D_m$) had higher value with finer textures, with higher fractality in coarser texture. Saturated hydraulic conductivities, $K_s$, of plow pan soils distributed from 0.5 to 1420 mm $day^{-1}$, having the highest value in sandy skeletal soils. The $K_s$ decreased with decreasing clay content and $D_m$, showing power function relationships. The coefficient of determination, $R^2$, of the fitted power functions were higher in $D_m$ as x-axis than in clay content. This means that $D_m$ could give us more effective estimation than clay content. Especially, sandy-textured paddy soils had higher $R^2$, compared to ordinary paddy soils. $K_s$ of relatively coarse-textured soils with less than 18%of clay content, therefore, was more dependent on particle size distribution than that of relatively fine-textured soils. From these results, it could be concluded that the fractal scaling gives us a unique quantity describing particle size distribution and then can be applied to estimate saturated hydraulic conductivity, especially more effective in coarse-textured soils.

• Journal of Soil and Groundwater Environment
• /
• v.12 no.3
• /
• pp.10-16
• /
• 2007

The slug test by adding water to well and measuring falling head was conducted to investigate the hydrogeological property of unsaturated or partially saturated mine tailings in the Imgi abandoned mine in Busan. In case that wells were installed with a full screen through two layers with different hydraulic properties, Bouwer and Rice method was useful to estimate the hydraulic conductivity and the depth of mine tailings. In particular, when groundwater dried out in the dry season, the slug test performed by adding water into well to form artificial water table and then conducting falling head test produced the reasonable hydraulic conductivity values. The slug test using falling head test can be an alternative to investigate the hydrogeological property of abandoned mine tailings.

• Journal of the Korean Society for Industrial and Applied Mathematics
• /
• v.19 no.2
• /
• pp.197-215
• /
• 2015

Modeling water flow in variably saturated, porous media is important in many branches of science and engineering. Highly nonlinear relationships between water content and hydraulic conductivity and soil-water pressure result in very steep wetting fronts causing numerical problems. These include poor efficiency when modeling water infiltration into very dry porous media, and numerical oscillation near a steep wetting front. A one-dimensional finite element formulation is developed for the numerical simulation of variably saturated flow systems. First order backward Euler implicit and second order Crank-Nicolson time discretization schemes are adopted as a solution strategy in this formulation based on Picard and Newton iterative techniques. Five examples are used to investigate the numerical performance of two approaches and the different factors are highlighted that can affect their convergence and efficiency. The first test case deals with sharp moisture front that infiltrates into the soil column. It shows the capability of providing a mass-conservative behavior. Saturated conditions are not developed in the second test case. Involving of dry initial condition and steep wetting front are the main numerical complexity of the third test example. Fourth test case is a rapid infiltration of water from the surface, followed by a period of redistribution of the water due to the dynamic boundary condition. The last one-dimensional test case involves flow into a layered soil with variable initial conditions. The numerical results indicate that the Crank-Nicolson scheme is inefficient compared to fully implicit backward Euler scheme for the layered soil problem but offers same accuracy for the other homogeneous soil cases.

• Journal of the Korean Geotechnical Society
• /
• v.28 no.4
• /
• pp.91-100
• /
• 2012

Soil-water characteristic curves (SWCCs), which represent a physical property in partially saturated soils, show the relation between volumetric water content and matric suction. The SWCCs exhibit hysteresis during wetting and drying, however experimental expressions used to describe SWCCs have generally ignored the hysteresis. In addition, the shape of SWCC may depend on the void ratio which is changed by soil skeleton deformations or hysteretic behavior under various loading conditions. Hence, it is necessary to understand, both empirically and analytically, the relationship between soil skeleton deformations and the SWCCs of various soils. The typical SWCCs experimentally have drying, wetting, and the second drying curve. The measurement of a complete set of hysteretic curves is severely time-consuming and difficult works, then the first drying curve of SWCC is generally determined to estimate the hydraulic conductivity and shear strength function of partially saturated soils. This paper presents the hydraulic-mechanical behavior of partially saturated soils (weathered soil and silty soil) for volume changes and hysteresis in SWCCs regarding the difference between the first drying and wetting curve.

• Korean Journal of Soil Science and Fertilizer
• /
• v.43 no.4
• /
• pp.407-414
• /
• 2010

Until now, the pore size distribution, PSD, of soil profile has been calculated from soil moisture characteristic data by water release method or mercury porosimetry using the capillary rise equation. But the current methods are often difficult to use and time consuming. Thus, in this work, theoretical framework for an easy and fast technique was suggested to estimate the PSD from unsaturated hydraulic conductivity data in an undisturbed field soil profile. In this study, unsaturated hydraulic conductivity data were collected and simulated by the variation of soil parameters in the given boundary conditions (Brooks and Corey soil parameters, ${\alpha}_{BC}=1-5L^{-1}$, b = 1 - 10; van Genuchten soil parameters, ${\alpha}_{VG}=0.001-1.0L^{-1}$, m = 0.1 - 0.9). Then, $K_s$ (1.0 cm $h^{-1})$ was used as the fixed input parameter for the simulation of each models. The PSDs were estimated from the collected K(h) data by model simulation. In the simulation of Brooks-Corey parameter, the saturated hydraulic conductivity, $K_s$, played a role of scaling factor for unsaturated hydraulic conductivity, K(h) Changes of parameter b explained the shape of PSD curve of soil intimately, and a ${\alpha}_{BC}$ affected on the sensitivity of PSD curve. In the case of van Genuchten model, $K_s$ and ${\alpha}_{VG}$ played the role of scaling factor for a vertical axis and a horizontal axis, respectively. Parameter m described the shape of PSD curve and K(h) systematically. This study suggests that the new theoretical technique can be applied to the in situ prediction of PSD in undisturbed field soil.