Predicting the entire soil-water characteristic curve using measurements within low suction range

The soil-water characteristic curve(SWCC) is widely used in the design and evaluation in the practice of geotechnical and geoenvironmental engineering such as the slope stability under the influence of environmental factors. The SWCC has distinct features in the capillary and adsorption zones due to different physical mechanisms. Measurements of the SWCC are typically limited within the capillary zone(i.e., low suction range). It is cumbersome and time-consuming to measure the SWCC in the adsorption zone(i.e., high suction range). This study presents a simple method to predict the entire SWCC within both the capillary and adsorption zones, using measured data only from low suction range(e.g., from 0 to 500 kPa). Experimental studies were performed on a completely weathered granite residual soil to determine its entire SWCC from saturated to dry conditions. The resultant SWCC, along with the SWCC measurements of 14 soils reported in the literature, were used to validate the proposed method. The results indicate that the proposed method has good consistency with a wide array of measured data used in this study. The proposed method is easy to use as it only requires a simple parameter calibration for a commonly used SWCC model. It can be used to improve the reliability in the prediction of the SWCC over the entire suction range when measurements are limited within the low suction range.

Effects of sample dimensions and shapes on measuring soil-water characteristic curves using pressure plate

It is well known that soilewater characteristic curve （SWCC） plays an important role in unsaturated soil mechanics, but the measurement of SWCC is inconvenient. In laboratory it requires days of testing time. For fine-grained clays, it may last for a couple of months using pressure plate tests. In this study, the effects of sample dimensions and shapes on the balance time of measuring SWCCs using pressure plate tests and the shape of SWCCs are investigated. It can be found that the sample dimensions and shapes have apparent influence on the balance time. The testing durations for circular samples with smaller diameters and annular samples with larger contact area are significantly shortened. However, there is little effect of sample dimensions and shapes on the shape of SWCCs. Its mechanism is explored and discussed in details through analysing the principle of pressure plate tests and microstructure of the sample. Based on the above findings, it is found that the circular samples with smaller dimensions can accelerate the testing duration of SWCC using the pressure plate.

Characteristics of In-Situ Soil Water Hysteresis Observed through Multiple-Years Monitoring

A soil water retention curve (SWRC) is an essential soil physical property for analyzing transport and retention of water in a soil layer. A SWRC is often described as a single-valued function that relates the soil water potential ψ to volumetric water content θ of the soil. However, an in-situ ψ − θ relation should show soil water hysteresis, though this fact is often neglected in analyses of field soil water regimes while long-term in-situ soil water hysteresis is not well characterized. This study aimed at probing and characterizing in-situ ψ − θ relations. The developments of large hysteresis in the in-situ ψ − θ relations were observed only a few times during the study period of 82 months. Any of the large hysteretic behaviors in the ψ − θ relations began with an unusually strong continual reduction in ψ. The completion of a hysteresis loop required a recorded maximum rainfall. Because the study field had very small chances to meet such strong rainfall events, it took multiple years to restore the fraction of soil water depleted by the unusually strong continual reduction in ψ. While wetting-drying cycles had occurred within a certain domain of ψ, hysteretic behaviors tended to be so small that the in-situ ψ − θ relation can be approximated as a single-valued function of θ(ψ). These observed patterns of the in-situ ψ − θ relations were characterized by kinds of difference in dθ/dψ between a drying process and a wetting process at a given ψ. Thus, more amounts of experimental facts about wetting SWRCs in parallel with drying SWRCs should be needed for correct modelling, analyzing, and predicting soil water regimes in fields. It is also necessary to increase our understandings about the long-term trends of occurrences of extreme weather conditions associated with possible change in climate.

Design and Application of Automatic Test System of Soil Water Characteristic Curve

[Objective] In order to better meet the requirement of crops on a more and more accurate water content under various planting environment of modern agri-culture, an automatic test system of soil water characteristic curve was designed by combining the conceptions of soil moisture content and soil water potential. [Method] Electronic soil moisture tension meter was used to determine the real-time tension value of soil moisture in the tested container, and the electronic Weigh sensor was used to determine soil Weigh. Minusing method was used to calculate soil moisture content, based on which the soil water characteristic curve was plotted. [Result] Through the filed survey of 2 different kinds of soil in Jiangsu Province, the results were as fol ows： soil of different composition showed different trend in soil water characteristic curve that the soil water characteristic relation of the sandy soil in the old course of the Yel ow River in Xuzhou was Y=-0.000 2X3＋0.027 7X2-1.644 5X＋38.161, R2=0.991 9; while the soil water characteristic relation of the saline-alkali soil in Jinhai Farm of Dafeng was Y=-0.00 2X2-0.426X＋39.905, R2=0.991 3. [Con-clusion] The automatic test system of soil water characteristic curve soil water char-acteristics curve could reflect soil moisture content and soil water potential, as wel as reflect the effectiveness of soil water to plant growth, providing basis for the sci-entific irrigation.

Soil freezing process and different expressions for the soil-freezing characteristic curve

The soil-freezing characteristic curve(SFCC),which represents the relationship between unfrozen water content and subfreezing temperature(or suction at ice-water interface)in a freezing soil,can be used for understanding the transportation of heat,water,and solute in frozen soils.In this paper,the soil freezing process and the similarity between the SFCC of saturated frozen soil and soil-water characteristic curve(SWCC)of unfrozen unsaturated soil are reviewed.Based on similar characteristics between SWCC and SFCC,a conceptual SFCC is drawn for illustrating the main features of soil freezing and thawing processes.Various SFCC expressions from the literature are summarized.Four widely used expressions(i.e.,power relationship,exponential relationship,van Genuchten 1980 equation and Fredlund and Xing 1994 equation)are evaluated using published experimental data on four different soils(i.e.,sandy loam,silt,clay,and saline silt).Results show that the exponential relationship and van Genuchten(1980)equation are more suitable for sandy soils.The simple power relationship can be used to reasonably best-fit the SFCC for soils with different particle sizes;however,it exhibits limitations when fitting the saline silt data.The Fredlund and Xing(1994)equation is suitable for fitting the SFCCs for all soils studied in this paper.

Prediction of soil–water characteristic curve for Malan loess in Loess Plateau of China

To predict the soil-water characteristic curve(i.e.SWCC)of natural and remoulded Malan loess from soil physical properties,one-point methods for determining the SWCC that are much simpler than experimental methods are proposed.The predicted SWCC is presented in the form of the BRUTSAERT equation,in which the four model parameters can be estimated from soil physical properties using the best correlations obtained in the present study along with one measured data point.The proposed one-point methods are validated using the measured SWCC data reported in the literature.The results of validation studies suggest that the proposed one-point methods can provide reasonable prediction of the SWCC for natural and remoulded Malan loess.The measured data point should be within the transition zone;the measured suction is suggested between25to100kPa for natural loess,while between100to500kPa for remoulded loess.

Rapid testing and prediction of soil–water characteristic curve of subgrade soils considering stress state and degree of compaction

The subgrade soil is generally in saturated or unsaturated condition. To analyze complex thermo-hydro-mechanical-chemical (THMC) behaviors of subgrade, it is essential to determine the soil–water characteristic curve (SWCC) that represents the relationship between matric suction and moisture content. In this study, a full-automatic rapid stress-dependent SWCC pressure-plate extractor was developed. Then, the influences of overburden stress and degree of compaction on the SWCC of subgrade soil such as high liquid limit silt (MH) and low liquid limit clay (CL) were analyzed. Accordingly, a new model taking into account the influences of overburden stress and degree of compaction based on the well-known Van Genuchten (VG) SWCC fitting model was presented and validated. The results show that with the increase of the degree of compaction and overburden stress, the saturated moisture content of subgrade soil decreases, while the air-entry value increases and the transition section curve becomes flat. The influences of the degree of compaction and overburden stress on the SWCC of MH is greater than that of CL. Meanwhile, there was a satisfactory agreement between the prediction and measurement, indicating a good performance of the new model for predicting the SWCC.

Soil Water Characteristic Curve of an Unsaturated Soil under Low Matric Suction Ranges and Different Stress Conditions

Accurate evaluation of unsaturated soil properties is critical for the design of geotechnical and geo-environmental structures such as road pavements, foundations, and earth dams. Water retention activity in soils which is used to predict the stability or seepage problems in the ground is one of the key features in unsaturated soil mechanics. Thus, many experimental works have reported on the unsaturated soil properties, and the soil-water characteristic curve (SWCC) test has contributed significantly to the interpretation of matric suction. Since traditional instruments cannot apply stress in SWCC tests, some researchers have developed suction controlled triaxial apparatus, by which SWCC tests are performed under different stress states. Determination of SWCCs under stress conditions similar to those in the field is key for interpretation of the hydro-mechanical behavior of unsaturated soils. This study conducted SWCC tests of unsaturated silt soil in low matric suction ranges under both drying and wetting conditions. The SWCCs were measured under one-dimensional and isotropic confining stresses ranging from 50 to 450 kPa. The micro porous membrane method was used instead of high air entry ceramic disk for controlling relatively low matric suction. The range of matric suction controlled was from 0 to 20 kPa. The study revealed that the measured SWCC in low matric suction ranges seems to be affected by the influence of stress conditions. Isotropic confining stress caused the void structure of the specimen to become dense and consequently, soil moisture flow movement also decreased. The water retention activity was obviously high, and the point regard to air entry value was larger. The study further suggests that the current methods adopted for estimating unsaturated soil properties require further development to take into account the effect of different stress conditions.

Factors Influencing the Soil-Water Characteristics of Unsaturated Tropical Silty Sand

Fine grain soils have a complex engineering behaviour which depends but not limited to moisture content, changes in external pressure and characteristics of the pore medium. Sand often contains a considerable percent of silt which is expected to alter its natural behaviour. This composite matrix is referred to as silty-sand. To understand the behaviour of this matrix under varying moisture conditions, some of the factors influencing the soil-water characteristics of unsaturated silty sands were investigated. Representative samples were collected from a river bank after its index properties were predetermined in the laboratory. The samples were compacted at different moisture conditions and compactive efforts. With the pressure plate extractor device, the Soil-Water Characteristic (SWC) was obtained and SWC Curves plotted. Compaction at greater compactive effort (modified proctor) and optimum moisture content produced the largest air entry value and reduced air voids. The air entry values of the soils obtained ranged from 21 kPa to 57 kPa. Also changes in the shape of the SWCC were consistent with changes in pore size which occur by varying compaction conditions. Result shows that soil structure, compaction water content, compactive effort and percentage of fine particles are factors affecting the Soil-Water Characteristics.

Towards an improved prediction of soil-freezing characteristic curve based on extreme gradient boosting model

As an essential property of frozen soils,change of unfrozen water content(UWC)with temperature,namely soil-freezing characteristic curve(SFCC),plays significant roles in numerous physical,hydraulic and mechanical processes in cold regions,including the heat and water transfer within soils and at the land–atmosphere interface,frost heave and thaw settlement,as well as the simulation of coupled thermo-hydro-mechanical interactions.Although various models have been proposed to estimate SFCC,their applicability remains limited due to their derivation from specific soil types,soil treatments,and test devices.Accordingly,this study proposes a novel data-driven model to predict the SFCC using an extreme Gradient Boosting(XGBoost)model.A systematic database for SFCC of frozen soils compiled from extensive experimental investigations via various testing methods was utilized to train the XGBoost model.The predicted soil freezing characteristic curves(SFCC,UWC as a function of temperature)from the well-trained XGBoost model were compared with original experimental data and three conventional models.The results demonstrate the superior performance of the proposed XGBoost model over the traditional models in predicting SFCC.This study provides valuable insights for future investigations regarding the SFCC of frozen soils.

Soil-Water Charateristics of Tropical Clay Soil under High and Low Suction Conditions

Tropical clay soil was compacted at different moisture conditions (dry, wet and optimum) and compactive efforts (Reduced proctor, Standard proctor, West African standard and Modified proctor). Experimental Soil-Water Characteristics (SWC) of the soil was derived using the pressure plate extractor equipment and SWC Curves (SWCC) plotted as gravimetric water content versus logarithm of matric suction. The Air Entry Values (A.E.V) obtained from experimental work ranged from 21 kPa to 59 kPa and compared favourably well with those estimated from predictive models with values of 23 kPa to 52 kPa. Specimens compacted with greater compactive effort (Modified proctor) and at optimum moisture content produced the largest air entry value of 59 kPa and reduced air voids. Changes observed in the shape of the SWCC were consistent with changes in pore size which occurred by varying compaction conditions. The shape of the soil-water characteristics curve was found to depend on the soil structure, compactive water content and compactive effort and not solely on the percentage of fine particles.

长春地区非饱和粉质黏土的土-水特征试验研究

土-水特征曲线(SWCC)是反映非饱和土工程特性的重要表现形式。为研究长春地区非饱和土的土-水特征曲线变化,以长春地区广泛分布的粉质黏土作为研究对象,采用GEO-Experts压力板仪和GDS非饱和土三轴仪对不同初始含水率的土体进行土-水特征曲线的测量分析。结果表明:不同设备所测得的结果基本一致,土体的进气值均在基质吸力25 kPa左右时达到。土体的体积含水率均随基质吸力的增加而减小,土-水特征曲线均在基质吸力为125 kPa时出现明显拐点。同一含水率土样,GEO-Experts压力板仪的排水体积比GDS非饱和土三轴仪多,压力板仪所对应的体积含水率相对较低。

A Flexible Model for Moisture-Suction Relationship for Unsaturated Soils and Its Application

The mathematical equation for the moisture-suction relationship also known as soil water characteristic curve (SWCC) is one of the constitutive relations necessary for the computational modeling of deformation and flow problems of unsaturated soil using the finite element method. In this paper, a new empirical equa-tion for the SWCC is developed that incorporates the actual airentry suction and the maximum possible suction of the soil as input parameters. The capability of the new model is investigated by fitting the experimental data for twelve different soils that includes sands, silts, and clays. The model fits the experimental data well including in high suction range which is one of the difficulties observed in other commonly used models such as the Brooks and Corey, van Genuchten, and Fredlund and Xing models. The numerical stability and the performance of the new model at low and high degrees of saturations in finite element simulation are investigated by simulating the dynamic response of a compacted embankment and the results are compared with similar predictions made using widely used SWCC models.

原状黄土土-水特征曲线与湿陷性的相关性

为了研究原状黄土土-水特征曲线与黄土湿陷性之间的联系,在陕西西安长安区取地表以下30 m范围内的原状黄土土样,进行基本物理指标试验和湿陷性试验。对不同典型地层的黄土-古土壤试样进行土水特征曲线试验,通过电镜扫描从微观角度分析。研究结果表明:大孔隙的数量与饱和体积含水率呈正相关;中孔隙的数目与过渡区斜率的大小呈正相关,孔隙数目越多土体失水速度越快;微小孔隙的数目和土的塑性指数影响残余含水率的大小。对于不同深度土层,饱和体积含水率和过渡区斜率与土层的湿陷系数呈正相关;塑性指数接近土层的湿陷系数对残余体积含水率的影响不明显;古土壤层的SWCC与湿陷系数之间存在与黄土层相同的正相关性。文章从非饱和土力学的方向去研究黄土的湿陷性,为湿陷性的研究提供一种新的研究角度。

干湿循环作用下原状黄土渗透性及其对土-水特征曲线的影响

为评估干湿循环作用对原状黄土水力特性的劣化效应,对甘肃Q 3黄土进行了4组不同干湿循环路径下的饱和渗透试验与土-水特征测试,分析土体饱和渗透系数和土-水特征曲线随干湿循环次数、循环幅度和下限含水率的变化规律。结果表明:原状黄土饱和渗透系数劣化度与干湿循环次数间关系可采用双曲线函数进行描述,6次循环后劣化度变化趋于稳定;土体饱和渗透系数劣化度随循环幅度增大而线性增大,随下限含水率增大而线性减小。V-G模型对干湿循环下黄土的土-水特征曲线拟合效果良好,模型参数θs、α、n变化幅度较小,而参数θr随循环次数增加呈指数下降趋势。依据试验结果建立了考虑干湿循环3参数的饱和渗透系数劣化模型与土-水特征曲线模型,并对土体非饱和渗透系数进行了预测分析。

脱湿路径下土体水分变化路径及体积变形对滤纸法测定土-水特征曲线的影响

尽管滤纸法已被广泛应用于非饱和土-土水特性研究,但滤纸法试验数据与压力板试验数据始终存在无法连续、光滑连接的问题。为此,以收缩变形不同的两类土为研究对象,从试样水分变化路径和体积变形两个方面着手,分别开展了一系列室内压力板试验、滤纸法试验和土的收缩试验,并基于土-水特征曲线(soil-water characteristic curve,SWCC)试验结果,进行统计指标的量化分析。研究结果表明,饱和脱湿路径下的滤纸法与压力板法联合测定的SWCC试验数据能够有效地连接,而无水分变化路径下的滤纸法与压力板法各自测定的SWCC试验数据在连接时可能会错开或交叉,尤其是变形显著的土体;为了较准确地估算基于饱和度的SWCC,提出一个简单的改进法,仅需补充土的收缩试验;针对不同滤纸法试验结果呈现的差异性,给出了合理的解释。该研究结果不仅能为试验人员在同时处理滤纸法和压力板法试验数据时提供重要的理论指导,还能拓展滤纸法的适用范围。

蔗渣生物炭对岩溶峰丛洼地石灰土土-水特征曲线的影响

岩溶地区岩石裸露、土壤浅薄且持水能力较低,其土壤水分保持和运动状况是其水土漏失治理的重要因素。生物炭拥有多孔结构和高比表面积,能改变土壤持水能力,但其种类、粒径、添加量和土壤类型等因素对土壤持水能力产生正面或负面作用,为了研究添加不同比例蔗渣生物炭对岩溶峰丛洼地石灰土持水特性的影响,选取位于广西河池地区岩溶峰丛洼地的石灰土壤作为试验样本,分别添加不同比例生物炭(0%,2%,4%,6%),利用滤纸法测定生物炭-石灰土混合土在0~40MPa吸附能力下土水特性曲线,并使用扫描电子显微镜(SEM)和核磁共振波谱法(NMR),观察生物炭-石灰土混合土的微观孔隙结构。研究结果表明:当土壤含水量超过30%时,不同比例生物炭掺入量的持水能力相似,土壤的持水性并不会随着生物炭掺入量的增加而持续提升。在添加生物炭0%,2%,4%时混合土的持水能力是在逐渐提高的,添加4%和6%时混合土的持水能力基本一致,这表明添加生物炭存在一个最优值和适用范围。建议岩溶峰丛地区石灰土含水率在30%以下时,施加4%~6%的蔗渣生物炭,可提高土壤的持水能力。

基于DE-MCMC方法的滑坡堆积体土壤水力参数反演研究

极端降雨条件下,入渗可急剧增大滑坡体中软弱结构面上的土壤含水量和孔隙水压力,是触发滑坡的重要因素。因此,土壤水动力模拟是滑坡预警系统的核心,其模拟结果的准确与否取决于土壤渗透特性的参数化。该文以汶川地震扰动区的银洞子滑坡堆积体为例,以土壤含水量野外观测数据为基础,基于Richards方程构建入渗模型,利用自适应差分演化算法的马尔科夫链蒙特卡罗方法(Differential Evolution Markov chain Monte Carlo,简称DE-MCMC)反演van Genuchten土壤水力参数。通过比较土壤含水量的模拟值与实测值的拟合程度,评估DEMCMC算法在土壤水动力模拟中的适用性。结果表明,基于DE-MCMC算法反演的后验参数极大降低了先验参数分布的不确定性,并显著提升土壤含水量的模拟精度。将DE-MCMC方法应用于滑坡水文,可有效率定土壤渗透特性参数,为降雨型滑坡预警提供更为可靠的土壤水动力模拟结果。

A Statistical Model for the Relative Hydraulic Conductivity of Water Phase in Unsaturated Soils

Permeability coefficients of fluids occupying the pore space of a porous medium have significant influence on the flow of these fluids through the porous medium. In the case of unsaturated soils, in addition to other parameters such as void ratio, void distribution, particle size distribution and initial density the degree of saturation also affects the permeability coefficient of water. The degree of saturation, in unsaturated soil, is directly related to the matric suction of the soil through soil water characteristic curve. Matric suction is one of the two stress state variables widely used to characterize the deformation behavior of unsaturated soils. Therefore, it can be stated that both flow and deformation behaviors of unsaturated soil are affected by the permeability coefficient of water. Numerical modeling of coupled deformation-flow behavior of unsaturated soil requires a mathematical equation that relates the permeability coefficient to the degree of saturation. Since the parameters that affect the permeability coefficient of water in unsaturated soil have similar direct or indirect effects on the soil water characteristic curve, permeability can be effectively predicted using the soil water characteristic curve as done in statistical models. In this paper, a statistical model is proposed for the permeability of water in unsaturated soil using soil water characteristic curve of the soil. The calibrated parameters of the soil water characteristic curve are directly used in the prediction of permeability with- out additional calibration using measured permeability data. The predictive capability of the new equation is verified by matching the measured data of eight different soils found in the literature.

考虑初始孔隙率的非饱和土土 -水特征曲线模型

为了构建考虑初始孔隙率的非饱和土土-水特征曲线模型,从土体的孔隙分布规律出发,通过对参考状态下孔隙尺寸分布曲线的平移缩放得到任意荷载状态下的土体孔隙尺寸分布曲线,并结合Yong-Laplace方程与VG模型推导得到了可考虑初始孔隙率的土水特征曲线模型。该模型仅包含5个参数且其物理意义明确,且仅需通过参考状态的土水特征曲线确定模型参数,即可用得到的参数预测任意初始孔隙比的土水特征曲线。通过5组试验数据对所提出的模型进行验证,结果表明该模型可较好地模拟不同初始孔隙率下的非饱和土持水特征。