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.

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.

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.

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.

Influences affecting the soil-water characteristic curve

The soil-water characteristic curve (SWCC) is the primary partially saturated soil information as its behavior and properties can be derived from it. Although there have been many studies of unsaturated soils and the SWCC, there is still no combined constitutive model that can simulate soil characteristics accurately. In cases when hydraulic hysteresis is dominant (e.g. under cyclic loading) it is particularly important to use the SWCC. In the past decades, several mathematical expressions have been proposed to model the curve. There are various influences on the SWCC as a source of information, so the curves obtained from conventional tests often cannot be directly applied; and the mathematical expressions from one scenario cannot be used to simulate another situation. The effects of void ratio, initial water content, stress state and high suction were studied in this work revealing that water content and stress state are more important than the other effects; but that the influences tend to decrease when suction increases. The van Genuchten model was modified to simulate better the changes in the degree of saturation at low values of suction. Predictions were compared with experimental results to determine the simulation capability of the model.

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.

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.

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.

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.

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.

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.

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.

植被根系含量对膨胀土持水和渗透特性的影响

为探究植被根系含量对膨胀土持水和渗透特性的影响规律,选取南宁市膨胀土地区的膨胀土,掺入狗牙根根系制备不同含根率的试样,进行室内压力板仪试验和变水头渗透试验,采用Van Genuchten-Mualem模型预测不同含根率的膨胀土非饱和渗透系数。结果表明:掺入根系后,膨胀土持水能力减弱,根系掺入增加膨胀土内部大孔隙的比例,含根率越高,大孔隙体积占总孔隙体积比例越大,持水能力相较于纯土下降的幅度越大;饱和状态下,膨胀土渗透系数随含根率增大而增大;非饱和状态下,低吸力阶段(0~25 kPa)的膨胀土渗透性随含根率的增大而增大,随着基质吸力增加到高吸力阶段(200~1 000 kPa),根系优势流效果减弱,含根膨胀土渗透性逐渐趋近低于纯土;细观结果表明根系的掺入使膨胀土内部产生贯通裂缝,是影响膨胀土持水和渗透能力的关键因素。研究进一步揭示了植被根系对膨胀土增渗作用的机理和规律,为综合评价植被防护膨胀土边坡效果提供参考。

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。

基于土−水特征曲线的非饱和冻土未冻水含量预测

对非饱和土体在冻结温度以下土中未冻水含量进行了理论分析,给出了一种简便的预测方法及其数学模型。基于非饱和土孔隙中气液两相的化学平衡和力学平衡关系,结合土−水特征曲线,给出了以Van Genuchten土−水特征曲线模型参数表示的土体孔隙体积分布密度函数,且水分冻结与土体孔径分布密切相关。根据初始有效饱和度对应的液态水填充的最大孔径和一定冻结温度对应的起始结晶孔径之间的关系,分析了土体孔隙中冰水相变的特征及其临界有效饱和度。当初始有效饱和度超过临界结晶饱和度后,冻结现象才会发生。给出了非饱和土体在冻结温度以下土中未冻水饱和度的预测公式,并通过已有的试验结果对其有效性进行了验证。

干密度和增减湿对压实黄土水力特性的影响

研究非饱和土的水力特性对分析工程建设活动中黄土的渗流和变形问题具有重要意义。以西安市南郊黄土为研究对象,采用滤纸法和饱和盐溶液法测得3种干密度下的黄土增减湿土-水特征曲线,并通过Childs&Collis-George模型预测试验黄土的非饱和渗透系数,探究干密度和增减湿对非饱和渗透特性的影响。结果表明:经历增减湿的土样的非饱和渗透系数曲线应结合土样的含水状态和对应的土-水特征曲线来预测;土体非饱和渗透系数随体积含水率的增大而增大,随基质吸力的增大而减小,在0~5 MPa范围快速衰减4~5个数量级;受瓶颈效应和减湿收缩的影响,土体增湿阶段渗透系数变化幅度小于减湿阶段的变化幅度;渗透系数随干密度增大而减小,干密度从1.719 g/cm^(3)增大到1.834 g/cm^(3),渗透系数减小2个数量级;压实黄土在增减湿阶段的非饱和渗透系数最大可相差3~4倍。

广州原状花岗岩残积土非饱和力学特性的试验研究

【目的】为了研究原状花岗岩残积土的表观黏聚力和有效内摩擦角随基质吸力的变化趋势,【方法】以广州市白云区某医院在挖基坑的原状花岗岩残积土为研究对象,通过室内试验研究原状花岗岩残积土的矿物成分、颗粒级配曲线、基本物理性质指标和微观结构等,借助变水头试验研究原状饱和花岗岩残积土的渗透性,结合非饱和三轴固结排水试验与原状土的微观试验,解释微观结构在宏观层面上对残积土的渗透系数、崩解及基质吸力等的影响,探讨了不同类型土体土水特征曲线差异的原因。【结果】结果显示,研究对象的花岗岩残积土为残积黏性土,主要由高岭土、石英和云母等组成,土体内部孔隙大小不均匀且相互贯通。饱和状态下原状花岗岩残积土的有效黏聚力和有效内摩擦角分别为29.17 kPa和20.99°。【结论】结果表明,非饱和状态下,随饱和度的减小,基质吸力对花岗岩残积土的表观黏聚力和有效内摩擦角均有着明显的影响;Khalili给出的非饱和抗剪强度方程式对花岗岩残积土较为合适。

高速铁路微膨胀泥岩破碎土非饱和渗透特性研究

非饱和渗透系数是非饱和膨胀泥岩土体渗流分析及水-力耦合研究的基础,对工程建设和工程病害预防具有重要意义。以新疆哈密地区微膨胀泥岩破碎土为例,制备4种不同初始干密度重塑土样,采用压力板法和滤纸法试验测量其土-水特征曲线,采用变水头试验测量土样饱和渗透系数;通过自主研制的土柱渗流试验装置进行恒定体积条件下一维土柱入渗试验,探究湿润锋前进法和瞬态剖面法的适用性,以获得不同初始干密度土体的非饱和渗透性曲线,并结合试验值对Childs和Collis-Geroge (CCG)渗透系数预测模型进行修正。结果表明:新疆哈密微膨胀泥岩破碎土的基质吸力范围为1~10^(5)kPa,渗透系数范围为10^(-9)~10^(-4)cm·s^(-1);试验土样初始干密度越大,大孔隙占比越小,阻渗作用越明显;CCG渗透系数预测模型可较好地反映土体渗透性曲线发展趋势,但在量值上随吸力的增加逐渐“远离”土体实测渗透性曲线;修正后的CCG渗透系数预测模型可反映不同初始干密度下土体渗透性曲线的发展规律。

施肥对热带砖红壤胶林土壤水分特征曲线的影响及模型优选

[目的]探明热带橡胶林施肥措施对土壤持水能力及孔隙特征的影响,并优选描述砖红壤胶林土壤水分特征曲线的模拟模型。[方法]采用沙箱排水法结合压力膜仪法测定砖红壤橡胶林5种施肥处理(有机肥混施化肥(H)、施化肥(C)、种植绿肥(G)、施专用肥(S)及不施肥(CK))的全吸力段土壤水分特征曲线,应用Brooks-Corey(BC)、van Genuchten(VG)、Gardner和Log-Normal Distribution(LND)4种经验模型拟合,分析了不同模型的适用性及拟合精度。[结果]0—5 cm土层,在低吸力段(pF≤3.01)C处理降幅最大(0.44 cm^(3)/cm^(3));在高吸力阶段(3.01<pF≤4.18)的降幅整体偏小,H处理的降幅最大(0.05 cm^(3)/cm^(3))。5—10 cm土层,在低吸力段C处理的下降幅度最大(0.52 cm^(3)/cm^(3)),高吸力段C处理的降幅最大(0.03 cm^(3)/cm^(3))。在0—5 cm土层中,CK处理的土壤有效含水率最大(0.19 cm^(3)/cm^(3))。在5—10 cm土层中,C处理的土壤有效含水率最大(0.11 cm^(3)/cm^(3))。0—5 cm土层,CK处理的土壤空隙占比(5.2%)和大孔隙占比(0.8%)最小,中等孔隙占比(48.7%)最高。5—10 cm土层中,C处理的土壤空隙和大孔隙的占比均最小。BC,VG,Gardner和LND 4种模型均可用来拟合砖红壤胶林不同施肥处理的土壤水分特征曲线(R 2≥0.84),不同模型的拟合精度由大到小依次为:BC,VG,LND和Gardner模型。[结论]0—5 cm土层不同施肥处理中CK处理是最优施肥管理方式,5—10 cm土层所有施肥处理中C处理是最优施肥管理方式。同时,BC模型是拟合研究区内不同施肥处理土壤水分特征曲线是最优模型。

Study on the Soil Moisture Stress Level in Regulated Deficit Irrigation Experiment

On the basis of discussing the influencing mode of plant moisture stress on plant physiological process and the division of soil moisture availability range, the water suction values partitioning soil moisture were put forward, and then the corresponding water moistures under water stress were obtained by conversing together with characteristic curve of water moisture.