Soil-water Characteristic Curve Model Research Articles

Slope stability analysis based on rate-dependent soil-water characteristic curve

Rainfall-induced landslides are a common geological hazard. Analyzing this problem necessitates the use of the unsaturated soil theory, with the soil-water characteristic curve (SWCC) being a crucial component. Most existing unsaturated soil seepage models are established based on the static soil-water characteristic curve model. However, the dynamic capillary pressure has been observed in experiments conducted in previous studies. In this study, experiments were performed to explore the dynamic impact of the SWCC. The influence of different infiltration rates was studied, and a rate-dependent SWCC model under dynamic conditions was established. The model was validated through experiments. The behavior of an unsaturated soil slope under different rainfall intensities was analyzed based on the equivalent model. The equation for the slope safety factor was then derived using the Bishop method. The slope safety factors based on both the rate-independent and rate-dependent SWCC models were compared. The results indicated that the safety factor continuously decreased with increasing rainfall intensity. The dynamic effect reduced the safety factor, making the slope more susceptible to instability.

Assessment of Two SWCC models for predicting the curve fitting parameters of lateritic soil: bentonite mixtures

This study compares the capabilities of soil water characteristic curve (SWCC) models by Brooks-Corey (BC) and van Genuchten (vG) in estimating the curve fitting parameters for lateritic soil—bentonite mixtures. The SWCCs of soil treated with 0–10% bentonite and compacted with British standard light (BSL) energy at compaction states representing dry of optimum, optimum, and wet of optimum conditions were measured by sequential desorption using pressure plate extractor. The fitting parameters of the two equations were determined by a non-linear fitting program. The fitting capabilities of the models on the measured data were compared by statistical indices namely the root mean square error (RMSE), linearity (R2) and index of agreement (d). Results revealed that volumetric water content increased as bentonite content increased with specimens containing 10% bentonite recording the highest and therefore has greater capacity to retain water/contaminants, while the air entry value (AEV) for the various soil mixtures also increased with higher bentonite content. The study also found that the estimated volumetric water contents approximated the measured values at all suctions with a high degree of accuracy with RMSE values that ranged from 0.0035 to 0.0150 for vG model which are somewhat lower than the values for BC equation. Similarly, R2 for the vG equation (≥ 0.99) are, on average, slightly higher than those of the BC fits. However, the d values associated with the BC model which varied between 0.788 and 0.971 are higher than those of the vG (0.784–0.968). Overall, the study established that the vG model proved marginally superior in respect of goodness of fit.

A Novel Three-Segment Model to Describe the Entire Soil–Water Characteristic Curve

This study aims to accurately describe the soil–water characteristic curve (SWCC) across the full range from saturation to oven dryness. We propose a smooth, continuous three-segmented SWCC model that divides the saturation range into wet, air-dried, and oven-dried segments. The two model junction points are anchored at matric suctions of 104.5 and 106.5 cm, respectively. The soil water content at 104.5 cm represents the maximum soil hygroscopy, reflecting the maximum water content in air-dried soil, while the soil water content at 106.5 cm characterizes the minimum soil water content. This imbues the junction points with specific physical significance regarding soil moisture content and matric potential. The model was tested with the water retention data of nine soils across the SWCC and compared with three existing SWCC models based on the adjusted coefficient of determination (adjR2) and root mean square error (RMSE). The results indicated that the proposed model accurately described the entire SWCC. The three-segmented model yielded an adjR2 of >0.99 and an RMSE of ≤0.022 cm3 cm−3, outperforming other models. We also introduce a new method for predicting soil water data in air-dried and oven-dried segments. The results showed that the predicted soil water content values were accurate.

Models for Considering the Thermo-Hydro-Mechanical-Chemo Effects on Soil–Water Characteristic Curves

The soil–water characteristic curve (SWCC) is widely used as a tool in geotechnical, geo-environmental, hydrology, and soil science fields for predicting and interpreting hydro-mechanical behaviors of unsaturated soils. Several previous studies focused on investigating the influence of initial water content, stress history, temperature, and salt content on the SWCC behavior. However, there is still limited understanding to be gained from the literature on how we can systematically incorporate the influence of complex thermo-hydro-mechanical-chemo (THMC) effects into interpreting and predicting the behavior of unsaturated soils. To address that knowledge gap, in this study, the coupled influence of temperature, initial stress state, initial density, soil structure, and chemical solution effects was modeled using established SWCC equations from the literature. The methodology for incorporating the coupled effects of these influential factors is presented herein. Furthermore, we evaluated the SWCC models proposed in this study, enabling us to provide a comprehensive discussion of their strengths and limitations, using the published SWCC data from the literature. The developments outlined in this paper contribute toward facilitating a rigorous approach for analyzing the THMC behaviors of unsaturated soils.

Engineering properties on salt rock as subgrade filler in dry salt lake: Water retention characteristics and water migration patterns

As a special road material, salt rock has high strength and stability in arid areas, and has broad application prospects in highway engineering construction in dry salt lake areas. To investigate the water retention characteristics of salt rock and its water migration patterns under the covering effect, and to ensure the long-term durability of salt rock subgrade, this study presents a comprehensive analysis of the effect of initial dry density on the water retention characteristics of fine-grained salt rock fillers. The soil–water characteristic curve model of salt rock was recommended. The capillary water migration patterns of salt rock were proved. The water migration patterns of salt rock under freeze–thaw cycles were clarified. The hydrothermal state of salt rock subgrade under the influence of pavement overburden was evaluated based on the field monitoring. The results indicated that the water retention capacity of fine-grained salt rock filler increases with the decrease of initial dry density during the high suction stage. The Fredlund & Xing model is recommended to address the soil–water characteristic curve of fine-grained salt rock. The capillary action of salt rock filler decreases with increasing compaction. The temperature gradient from the upper boundary to the bottom of the soil column under the effect of freeze–thaw cycles is significant. After five freeze–thaw cycles, the water in the soil column continued to migrate upwards under the covering effect and temperature gradient. The relative humidity and temperature of the salt rock subgrade both show a sinusoidal cyclical variation. When the temperature range of salt rock subgrade is −6°C–30 °C, the relative humidity of subgrade is significantly correlated with temperature. This study can provide theoretical support for the disaster prevention and control of salt rock subgrade covering effect in the Lop Nor area.

Environmental Temperature Effect on Hydraulic Behavior and Stability of Shallow Slopes

This study established a study framework to quantify the safety factors of unsaturated shallow slopes at different temperatures. This study is based on a non-isothermal soil water characteristic curve model quantifying the temperature-dependent hydraulic properties of soils. The hydraulic coupling analysis models HYDRUS 2D and The Slope Cube Module were used for finite element modeling. A slope stability analysis was performed based on the local factor of safety (LFS) theory. An increased temperature decreased the soil matric suction, suction stress, effective stress, and LFS, weakening the soil strength. Slope modeling analysis showed that soils were dominated by different water retention mechanisms before and after rainfall infiltration, and the trends caused by temperature changes also changed accordingly. This study provides insights into the relationship between soil mechanical properties and temperature, which is valuable for maintaining soil stability and preventing geological hazards.

Hydraulic properties of bentonite buffer material beyond 100 °C

Bentonite buffer materials are important components of engineered barrier systems for the disposal of high-level radioactive waste produced during nuclear power generation. The design temperature of the buffer material is < 100 °C, and increasing the design temperature can reduce the required disposal area. This characteristic necessitates the evaluation of the thermal–hydraulic–mechanical properties of the buffer at temperatures above 100 °C to increase its target temperature. Therefore, the hydraulic properties of Gyeongju (KJ) bentonite buffer material were evaluated in this study, including the soil–water characteristic curve (SWCC) and hydraulic conductivity. An experimental system was manufactured to measure the suction and saturated hydraulic conductivity of KJ bentonite buffer material above 100 °C; the relative humidity of KJ bentonite buffer material was measured at 25–149 °C with an initial water content of 0, 0.06, and 0.12 under constant saturation conditions. The suction decreased as the temperature increased (10%–25% reduction at 99 °C-149 °C). The Van-Genuchten SWCC fitting parameters were also derived at 25 °C-149 °C using previously reported and newly generated experimental results, and the applicability of the modified Van-Genuchten SWCC model in this temperature range was verified. The hydraulic conductivity was proportional to temperature up to 100 °C, in agreement with the theoretical model results. Between 100 °C and 150 °C, the hydraulic conductivity increased nonlinearly because of molecular motion and structural changes inside the sample.

Research on the saturated/unsaturated seepage laws of ionic rare earth ore under different leaching conditions

The seepage law of the ionic rare earth leaching process plays an important role in the efficient development and utilization. The saturation permeation test of ionic rare earth under different leaching conditions was carried out using the variable head method, and the influence of type, concentration, and leaching path on the saturation permeability coefficient was revealed. The relationship between the water content and the matric suction of ionic rare earths under different leaching conditions was measured with the Geo-Experts pressure plate apparatus, and the soil-water characteristic curves under different leaching conditions were obtained. Based on the soil-water characteristic curve model, the unsaturated permeability coefficient function of ionic rare earths under different conditions was studied. The results show that the saturated-unsaturated permeability coefficients of ionic rare earths are pure water, 3% (NH4)2SO4, and 3% MgSO4, in descending order, when the type of leaching solution is different. For different concentrations of the leaching solution, when the concentration of (NH4)2SO4 increases from 2% to 5%, the saturated permeability coefficient first increases and then decreases. The matrix suction is an important factor affecting the unsaturated permeability coefficient when the ore body is unsaturated, and the unsaturated permeability coefficient decreases with the increase of the leaching solution concentration under the same matric suction. The seepage law is related to the leaching path, and the permeability coefficient increases when leaching at high concentrations followed by low concentrations, in reverse order, the permeability coefficient decreases. The research results can provide theoretical guidance for the design of injection parameters, and improve the theory of in-situ leaching.

Modeling the combined effect of initial density and temperature on the soil–water characteristic curve of unsaturated soils

The soil–water characteristic curve (SWCC) plays an important role in solving the stability and deformation problems of unsaturated soils. In many practical situations, soils are usually experienced by both deformations and thermal conditions. In this interest, the paper proposes a simple and effective model to predict the combined effect of initial density and temperature on the SWCC and to be able to quantify the changes in thermal-hydro-mechanical behavior of unsaturated soils. In the first step, an initial density-dependent SWCC model is presented using the translation principle between particle-size distribution curve and soil–water characteristic curve. In the second part, a non-isothermal model is proposed to predict the effect of temperature on the SWCC. The key to the non-isothermal model is considering five different temperature-dependent functions, which are surface tension, contact angle, particle-size expansion, void ratio, and water density. On the basis of 22 data sets of thermal volume change, this study also developed further a theoretical correlation between void ratio and temperature that is directly related to soil plasticity. It was observed that the value of the thermal void ratio increases as soil plasticity increases, and there is a nonlinear relationship between the plasticity index and the void ratio. Because of this, soils with high plasticity are more susceptible to volume changes caused by temperature fluctuations than soils with low plasticity. A coupled mechanical–thermal model is then produced which is capable to predict separately or simultaneously the effect of temperature and initial density on SWCC. The proposed model is validated against several test data sets available in the literature. The results show that the proposed model has a good performance in predicting the variation in SWCC with arbitrary temperature and initial density.

Hydraulic behavior of sandy subgrade under extreme rainfall in Alashan, China

Unsaturated aeolian sand is the main railway/highway sandy subgrade filling material in the desert area of Alashan, Inner Mongolia. In recent years, the extreme climate events frequently happen in Inner Mongolia. Under the extreme rainfall conditions in the desert area, the rainfall infiltration process and the unsaturated state evolution of aeolian sand subgrade is significant to the hydraulic behavior distribution and the deformation development under traffic loading.In this study, the SWCC (soil-water characteristic curve) parameters of aeolian sand was firstly calculated and discussed through van Genuchten model and the linear approach. Then a unified framework was proposed to describe the hydraulic behavior evolution of aeolian sand subgrade under extreme rainfall. Considering the effect of soil properties and rainfall conditions on infiltration process of aeolian sand subgrade, a normalized SWCC model was established. An analytical model of maximum infiltration depth for aeolian sand subgrade affected by extreme rainfall was further developed. Lastly, the numerical simulation results of the maximum infiltration depth for aeolian sand subgrade under an extreme rainfall event verified the correctness of the analytical model. This research can lay a foundation for studying the hydro-mechanical coupling behavior of sandy subgrade in desert areas.

Reliability assessment of rainfall-induced slope stability using Chebyshev–Galerkin–KL expansion and Bayesian approach

Soil spatial variability has essential influence on the reliability of geotechnical structures. Karhunen–Loève (KL) series expansion is an effective approach to characterize such features of soil properties. Acquiring solution for Fredholm integral equation of the second type is a necessary prerequisite; however, the corresponding analytical expressions are only available for limited circumstances. To overcome this challenge, a newly proposed method called Chebyshev–Galerkin–KL expansion was developed to discretize the random fields of soil parameters, from which the approximated eigenvalues and eigenfunctions can be obtained using the Chebyshev orthogonal polynomials of the second kind combined with Galerkin technique. Application of the proposed approach is illustrated through reliability analysis of an unsaturated slope example under different rainfall patterns, where the uncertainty in selection of a “best” soil-water characteristic curve (SWCC) model and statistical uncertainties in SWCC model parameters are taken into account. Results show that the developed approach is feasible to generate random fields with sufficient accuracy. Under a constant rainfall duration, the Advanced pattern may lead to shallow landslide with the highest probability, followed by Intermediate and Delayed. It should be noted that Bayesian inference and determination of optimal SWCC model should be carried out prior to reliability analysis. Otherwise, the landslide risk level would be exaggerated.

A universal soil–water characteristic curve model based on the particle size distribution and fractal theory

A soil–water characteristic curve (SWCC) is a prerequisite relationship to study water transport in unsaturated soils and has significant applications in agriculture and engineering sciences. Therefore, many SWCC models were proposed and improved the understanding of water transport. Many of these models were obtained depending on the particle size distribution (PSD) because the water content is related to the sizes and distributions of particles and pores in soils. However, the continuous PSD was divided into many fractions to calculate the water content in these models. Moreover, the particles and pores were simplified as spheres and cylinders, respectively. These simplifications are idealistic and contrary to actual complex structures in soils. To solve the above limitations of these models, we considered the Weibull distribution and fractal theory and established a universal SWCC model called PF (PSD and fractal theory) based on the continuous PSD and irregularities of the pore structures. The new proposed PF model was verified by measured data in the UNSODA database. Furthermore, its accuracy was proven by comparison with two classical models widely used. The verification and comparison results show that the PF model can well describe the measured data of various types of soils and has fewer errors than the other two models. Meanwhile, the continuous PSD in this study overcomes the limitation of artificial discretization and is convenient for subsequent calculation. In addition, the actual irregular soil geometry is described by the fractal theory, which well matches complex soil structures. Therefore, the PF model based on the PSD and the fractal theory is reasonable and reliable in predicting SWCC. In addition, this study can provide a foundation and reference for subsequent studies on hydraulic behaviors.

Introduction of density distribution in soil-water characteristic curve model with regular packing structure

土粒子構造を規則充填構造でモデル化し，間隙スケールの保水メカニズムに基づいて水分特性曲線（以下SWCC）を推定する理論的モデルを構築した。規則充填構造の単位構造には構造や密度が変化可能な菱面体構造を採用し，単位構造の構造を決定するパラメータに存在割合分布を仮定することで，規則充填構造を仮定した SWCC モデルに密度分布を導入した。具体的な密度分布には等球径ランダム充填における密度分布を採用し，その適用性を検討した。本モデルと従来の均一構造を仮定したモデルを比較したところ，密度分布を考慮することにより間隙への空気の通り道である throat の径の分散が大きくなり，緩やかに排水することで比水分容量が小さくなる現象を再現可能であることがわかった。throat 径の分散は粒度分布と密度分布の両者に影響されるため密度分布の影響は粒径幅が狭い供試体ほど顕著であることを明らかにした。

MODELS OF SOIL WATER CHARACTERISTICS CURVES: A REVIEW

Soil water characteristic curve (SWCC) is very important in the study of unsaturated soil because it represents a soil’s ability to store and release water as it is subjected to various soil suctions. This storage of moisture in soil is paramount in irrigation engineering, as it determines the irrigation scheduling in dryland. This study reviews some SWCC model’s performance to predict the soil from some selected soils in Nigeria from previous work. This previous work contains 7 points between 0 – 1,500 kPa. Seki, which is bimodal, tends to perform better than the rest with R2 value of 0.99. Generally, the bimodal models performed better than the unimodal, due to the flexibility of the curve. However, with the removal of the reference point of 1,000,000kPa (leaving 7 points), all the unimodal models had good performance in the topsoil. Fredlund and Xing performed best and van Genuchten performed poorly in the subsoil. Also, it was observed with the increase in both the organic matter contents and electrical conductivity, the performance of the models decreased. From this review, it is observable that in analysis of 8 or more points and 3 to 7 points Seki’s model and Fredlund and Xing model can be used respectively for further research in the study of dryland soils of Nigeria

Effect of Shelterbelt Construction on Soil Water Characteristic Curves in an Extreme Arid Shifting Desert

To explore the impact of artificial shelterbelt construction with saline irrigation on the soil water characteristic curve (SWCC) of shifting sandy soil in extreme arid desert areas, three treatments including under the shelterbelt (US), bare land in the shelterbelt (BL) and shifting sandy land (CK) in the hinterland of the Taklimakan Desert were selected. The age of the shelterbelt is 16, and the vegetation cover is mainly Calligonum mongolicum. The soils from different depths of 0–30 cm were taken keeping in view the objective of the study. The SWCCs were determined by the centrifugal method and fitting was performed using various models such as the Gardner (G) model, Brooks–Corey (BC) model and Van Genuchten (VG) model. Then, the most suitable SWCC model was selected. The results showed that electrical conductivity (EC) and organic matter content of BL and US decreased with the increasing soil depth, while the EC and organic matter content of CK increased with the soil depth. The changes in soil bulk density, EC and organic matter of 0–5 cm soil were mostly significant (p &lt; 0.05) for different treatments, and the differences in SWCCs were also significant among different treatments. Moreover, the construction of an artificial shelterbelt improved soil water-holding capacity and had the most significant impacts on the surface soil. The increase in soil water-holding capacity decreased with increasing soil depth, and the available soil water existed in the form of readily available water. The BC model and VG model were found to be better than the G model in fitting results, and the BC model had the best fitting result on CK, while the VG Model had the best fitting result on BL with higher organic matter and salt contents. Comparing the fitting results of the three models, we concluded that although the fitting accuracy of the VG model tended to decrease with increasing organic matter and salinity, the VG model had the highest fitting accuracy when comparing with BC and G models for the BL treatment with high organic matter and salinity. Therefore, the influence of organic matter and salinity should be considered when establishing soil water transfer function.

A flexible soil-water characteristic curve model considering physical constraints of parameters

Most soil-water characteristic curve (SWCC) models contain more than three parameters, but these parameters typically do not have a distinct geometric or physical meaning, or the parameters used are inaccurate because of their high reliance on inverse parameter estimation methods. This phenomenon greatly limits the application of SWCC in the study of mechanical and hydraulic properties of unsaturated soils. To explore the nature of SWCC, this paper reexamines the role of the parameters in the two most commonly used SWCC models from the perspective of the relationship between capillary and adsorbed water. A new and flexible SWCC model (named the MVF model) with four parameters is proposed, which can separate the two different regions by physical constraints on the residual saturation Sr and residual suction ψr. Ten published experimental data sets are compiled to evaluate the performance of the MVF model by comparing it with both the existing models and the experimental data. The MVF model shows excellent performance in terms of goodness of fit, and the fitting parameters are provided with a clear geometric meaning, which helps to establish the connection between the basic properties of the soil and the SWCC. This study consolidates the application and reliable determination of SWCC in estimating the mechanical and hydraulic properties of unsaturated soils.

Numerical scheme for transient seepage analysis under unsaturated conditions

Unsaturated soil behaviors characterize the failure mechanisms of geotechnical infrastructures with transient seepage conditions. Therefore, an accurate estimate of the unsaturated groundwater flow is vital in improving hazard management and assessment. This study attempts to develop a numerical scheme for 2-D transient analysis under unsaturated conditions. First, the unsaturated groundwater flow was described using the mass conservation law. Then, the Finite Difference Method and Backward Euler approximation were applied for space and time discretization, respectively. Furthermore, the simple Picard iteration was applied to linearize the governing equation. The reliability of the presented method was verified with the analytical solution. The evaluation results demonstrated the sufficiency of the proposed method, quantitatively expressed by the maximum error of 0.04% for opened boundary conditions and 0.15% for closed boundary conditions. The significant advantage of the proposed method is the flexibility with various soil-water characteristic curve models and associated hydraulic conductivity functions, which helps to improve the applicability in practice.

A Flexible Soil-Water Characteristic Curve Model Considering Physical Constraints of Parameters

A Flexible Soil-Water Characteristic Curve Model Considering Physical Constraints of Parameters

Probabilistic Estimation of Soil-Water Characteristic Curves of Fine-grained Soils

The soil-water characteristic curve (SWCC), which describes the relationship between water content in soil and its matric suction, is important for studying the mechanical properties of unsaturated soil. Because measuring SWCC is time-consuming and labor-intensive, many empirical methods have been proposed for assessing SWCC. However, the uncertainty associated with empirical methods is rarely quantified. The purpose of this paper is to propose a probabilistic method for assessing the SWCC of fine-grained soils based on the particle size distribution and index properties. First, the van Genuchten model, which is a widely used SWCC model, is briefly presented. A comprehensive soil database was then compiled and the parameters of the van Genuchten model were calibrated for each soil sample. After that, a regression analysis was performed to estimate the mean value of the parameters of the van Genuchten model, and a correlation analysis was performed to estimate the covariance matrix of the parameters of the van Genuchten model. Finally, a joint probability density function of the parameters of the van Genuchten model is constructed. In this paper, Johnson distributions are used to construct the joint probability distribution of the parameters in the van Genuchten model that do not obey the multivariate normal distribution. The proposed method has been rigorously tested to assess its accuracy and applicability. The method proposed in this paper can provide a practical tool for estimating SWCC with limited data and accounting for the associated uncertainty.

Spatiotemporal variability of soil-water characteristic curve model parameters of Lanzhou collapsible loess

Abstract The spatiotemporal variation of the model parameters of the soil-water characteristic curve (SWCC) reflect the soil water holding capacity and soil pore distribution state. It is an integral part of interdisciplinary disciplines such as soil hydrodynamics and ecohydrology. The authors selected the optimal SWCC model for the Lanzhou collapsible loess, used classical statistics and geostatistics methods to study the spatiotemporal variability of the SWCC model parameters, and using the comprehensive comparison of the mean relative differences (MRD), standard deviations (SDRD) and an index of temporal stability (ITS) determined the representativeness measuring point. The results showed that the SWCC parameter α was medium variability in the 0–30 cm soil layer, n and θs were of low variability, and the spatial distribution of the parameters of different soil layers was consistent. Migration direction prediction of θs was very similar in each layer, α, n and θs were all strongly significantly correlated positively. Moreover, the determination coefficient of representative measuring point 16 had the highest prediction accuracy for the measured values of SWCC. The results of this paper can be used as a simple method to predict SWCC and provide theoretical guidance for soil water management and soil collapse erosion monitoring in the collapsible loess area.