Modeling of the Soil-Water Characteristic Curve-Case Study in Bom Brinquedo Hill’s, Antonina, Brazil

In this paper,we present a modeling of the soil-water characteristic curve for residual and sedimentary soils of Bom Brinquedo Hill’s,located in Antonina,Brazil.This mountain range region is characterized as a natural disaster risk area,requiring continuous research related to the stability of the area.To obtain the soil-water characteristic curve,undisturbed samples of residual and sedimentary soil were collected,followed by suction testing using the filter paper method.Considering the bimodal characteristic presented by the soil,LABFIT software was employed for curve fitting using the generic formulation“Harris+C”.The results of the tests indicated that the phenomenon of hysteresis had a greater influence in situations with higher suction levels.When comparing the residual moisture values of the macropores between residual soil and sedimentary soil,the former exhibited the lower value.This suggests that the residual soil has a coarser grain size and larger pores,which facilitates the release of water retained in the soil’s macropores.

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.

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.

Prediction of loess soil-water characteristic curve by mercury intrusion porosimetry

Mercury intrusion porosimetry(MIP)is a simple and fast way to obtain the pore distribution of soil and can be used to estimate the soil-water characteristic curve(SWCC).In previous studies,soil was assumed to be a perfect wettability material,and the contact angle(CA)of the soil-water interface was taken as zero in the SWCC prediction method.However,the CA has proved to be much greater than zero even for hydrophilic soils according to some soil wettability experiments,and it has a significant effect on predicting the SWCC.In this research,a method for predicting the SWCC by MIP,which takes the CA as a fitting coefficient,is proposed.The pore size distribution curves are measured by MIP,and the SWCCs of two loess soils are measured by pressure plate and filter paper tests.When the CA is taken as70°and 50°for the wetting and drying process,respectively,the SWCCs predicted by the pore size distribution curves agree well with the measured SWCCs.The predicted suction range of the proposed method is 0-105 k Pa.The consistency of the results suggests that utilizing the MIP test to predict the SWCC with a proper CA is effective for loess.

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.

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.

Soil-water characteristics and shear strength in constant water content triaxial tests on Yunnan red clay

The shear strength parameters for geotechnical designs are obtained mainly from consolidated drained （CD） or consolidated undrained （CU） triaxial tests. However, during construction, the excess pore-air pressure generally dissipates instantaneously while the excess pore-water pressure dissipates with time. This condition needs to be simulated in a constant water content （CW） triaxial test. The study on Yunnan red clay is carried out to investigate the soil-water characteristics and the shear strength characteristics under the constant water content condition. Osmotic technique is used to obtain the soil-water characteristic curve. A series of CW triaxial tests are conducted on statically compacted specimens. The experimental results show that the soil-water characteristic curve has a low air entry value of 7 kPa due to large pores in non-uniform pore size distribution, and a high residual value exceeding 10 MPa. In addition, the initial degree of saturation and net confining stress play an important role in affecting the shear characteristics under the constant water content condition. Finally, a new semi-empirical shear strength model in terms of degree of saturation is proposed and then applied to Yuunan red clay. Simulation result shows that the model is capable of capturing some key features of soils. The model can be used in whole engineering practice range, covering both unsaturmed and saturated soils.

Water infiltration and soil-water characteristics of compacted loess under applied vertical stress

Additional stress formed by postconstruction buildings in loess-filling areas affects water infiltration in soil and causes soil deformation.To investigate this effect,under constant water head,vertical infiltration tests on compacted loess with two initial dry densities for different applied vertical stresses were developed using vertical stresscontrollable one-dimensional soil columns.The timehistory curves of vertical deformation,wetting front depth,cumulative infiltration depth,volumetric water content(VWC)and suction were measured,and the soil-water characteristic curves(SWCCs)were determined.The results showed that:(1)the infiltration ability of the soil column weakens with increasing applied vertical stress and initial dry density;(2)vertical deformation increases rapidly at first and then tends to be stable slowly at the consolidation and wetting-induced deformation stage,and is positively correlated with applied vertical stress and is negatively correlated with initial dry density.The stability time of wetting-induced deformation and the corresponding wetting front depth increase with the increase of applied vertical stress,while they decrease obviously when initial dry density increases;(3)the influence of applied vertical stress on soilwater characteristics in soil columns with various initial dry densities is related to the deformation depth of soil column.The VG(Van Genuchten)model is suitable for fitting the SWCCs at different monitoring positions.A normalized SWCC model introducing the applied vertical stress was proposed for each initial dry density using the mathematical relationship between the fitting parameters and the applied vertical stress.

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 characteristics of weathered crust elution-deposited rare earth ores

The permeability of the weathered crust elution-deposited rare earth ores directly affects the efficiency of in-situ leaching.The soil−water characteristic curve(SWCC)is an important constitutive relation for calculating the permeability of ore body,which is related to many factors.Soil−water characteristic tests of rare earth ore samples considering different factors were carried out by using the pressure plate instrument.Effects of dry density,particle size and solution leaching on water holding behavior and the mechanism were investigated.The experimental observations indicate that with the decrease of dry density,the pore ratio increases gradually,and the saturated water content increases.Under the same matric suction,the water content decreases gradually with the increase of particle size,thus decreasing water holding capacity of ore accordingly.In the same water content,matric suction is inversely proportional to particle size.Under the same matric suction,the water content of ore samples after leaching is less than that of the ore samples before leaching,indicating that solution leaching can decrease water holding capacity of ore.

Effect of the uncertainty in soil-water characteristic curve on the estimated shear strength of unsaturated soil

Most failures or instabilities of geotechnical structures commonly result from shear failure in soil. In addition, many infrastructures are constructed within the unsaturated zone. Therefore, the determination of shear strength of unsaturated soil is crucial in geotechnical design. The soil-water characteristic curve(SWCC) is commonly used to estimate the shear strength of unsaturated soil because the direct measurement is time-consuming and costly. However, the uncertainty associated with the determined SWCC is rarely considered in the estimation of the shear strength. In this paper, the uncertainties of SWCC resulted from different factors are reviewed and discussed. The variability of the estimated shear strength for the unsaturated soil due to the uncertainty of SWCC associated with the best fit process is quantified by using the upper and lower bounds of the determined SWCC. On the other hand, the uncertainties of the estimated shear strength due to different initial void ratios or different confining pressures are quantified by adopting different SWCCs. As a result, it is recommended that the measured SWCC from the conventional Tempe cell or pressure plate needs to be corrected by considering different stress levels in the estimation of the shear strength of unsaturated soil.

Effect of initial gravimetric water content and cyclic wetting-drying on soil-water characteristic curves of disintegrated carbonaceous mudstone

The soil-water characteristic curve(SWCC)is often used to estimate unsaturated soil properties(e.g.strength,permeability,volume change,solute and thermal diffusivity).The SWCC of soil samples is significantly affected by cyclic wetting-drying.To examine how water content and cyclic wetting-drying affect the SWCC of disintegrated carbonaceous mudstone(DCM),SWCC tests were implemented using a pressure-plate apparatus.In addition,SWCC models for DCM considering the initial gravimetric water content and cyclic wetting-drying were developed.The test results showed that the volumetric water content(θ)of the DCM first decreased rapidly and then became stable as matric suction(s)increased.The initial water content affected the SWCC by altering the pore structure of the DCM.For a given number of wetting-drying cycles,the higher the initial water content,the higher the stabilizedθ.At a given s value,θdecreased as the number of wetting-drying cycles increased,which suggests that cyclic wetting-drying reduces the water-holding capacity of DCM.The Gardner model for DCM was constructed considering initial water content and cyclic wetting-drying,and was effective at describing and predicting the SWCC model for DCM.

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.

非饱和土双应力变量广义土水特征曲线理论模型构建

土水特征曲线(soil-water characteristic curve,SWCC)方程是非饱和土力学中最重要的土性表征手段之一。该文评价当前经典的SWCC方程,指出其未具有包容复杂因素的能力,具有灵活性的优点但却同时具有对试验数据量依赖性高的缺点,不能处理多孔隙尺度集群土体固-液-气共同运动及作用的水力-力学耦合效应问题。建立双应力变量广义SWCC概念图示并定义相对体积含水比,基于Fredlund双应力变量理论及van Genuchten土-水表征方程,构建考虑土体变形及多孔隙分布形态的双应力状态变量的广义SWCC方程。相较于2个参数的Brooks等的方程、3个参数的van Genuchten方程以及4个参数的Fredlund等的方程,广义SWCC方程仅3个参数,其中2个参数在双对数坐标系的"相对体积含水比-吸力"平面中进行最小二乘法线性拟合得到,仅1个参数需非线性最小二乘法拟合得到。该模型可利用不同应力状态下的至少3个土水试验数据点,绘制出1条具有适宜精度的单峰SWCC;方程考虑了多峰孔隙概率密度函数分布及土体变形因素,实现了从应力历史推广到应力状态的广义情况,为定量描述不同孔隙结构土体双应力状态下的持水特性、渗透特性和强度特性提供了一条途径。

膨胀土干湿循环裂隙演化及其土-水特征曲线研究

周期性的蒸发和降雨作用导致膨胀土裂隙发育,而裂隙对土体的水力及强度特性有很大影响。为研究膨胀土在干湿循环作用下的裂隙发展规律及其土水特征,采用CT扫描技术和图像处理程序定量分析裂隙的发育演化过程;利用压力板仪进行合肥膨胀土土水特征曲线的干湿循环效应试验。研究结果表明:基于CT图像可以得到试样内部裂隙的二维、三维结构及其定量化分析指标;各个裂隙指标的裂隙发育过程可以用logistic函数的生长曲线描述。裂隙对膨胀土的土水特征曲线影响较大,裂隙膨胀土的土水特征曲线呈"双峰"形态;随干湿循环次数增加,膨胀土持水能力降低,不同循环次数的土水特征曲线差异性明显,吸力200~450 k Pa阶段,不同循环次数下膨胀土土水特征曲线逐渐趋于一致;干湿循环对SWCC1和SWCC2特征点(进气值,残余值)的影响程度不同;通过灰色关联法分析发现,平均裂隙宽度、内表面积、平均裂隙长度、体积裂隙度和面积裂隙度均与裂隙膨胀土的持水能力有显著的相关联性;以干湿循环次数和基质吸力为变量建立双峰土水特征曲线模型,采用合肥膨胀土的土水特征曲线试验数据验证了模型的精确性及可靠性,饱和含水率、拟合参数都与干湿循环次数呈良好的相关性。

单峰和双峰土水特征曲线基本参数的确定

土水特征曲线基本参数(如进气值、残余吸力值和过渡区脱湿曲线的斜率等)的确定是预测非饱和土强度、渗透性以及本构关系的基础。基于修正Fredlund和Xing拟合方程,提出单峰和双峰土水特征曲线基本参数的确定方法。首先,对试验数据点进行最优化拟合,获取拟合方程的参数,对于双峰土水特征曲线,根据试样的孔径分布特征将其分段进行拟合;其次,根据拟合方程的几何关系求出过渡区和残余区土水特征曲线的斜率及其切线方程;再根据切线方程可以准确地确定单峰和双峰土水特征曲线的进气值、残余吸力值等。最后,利用单峰和双峰土水特征曲线的试验数据,验证了此方法的可行性。

考虑初始孔隙比影响的单/双峰土-水特征曲线模型研究

土样内部的孔隙结构可以分为单峰、双峰甚至多峰孔径分布。双峰孔隙结构土体的土-水特征曲线(SWCC)通常为双峰形态。现有的双峰SWCC模型大部分为单峰SWCC模型经过分段或叠加的方式所得,两段SWCC交点处的相关参数往往难以准确确定,而且很少有模型考虑孔隙比对双峰土水特性的影响。首先,详细分析和论述了广吸力范围内不同初始孔隙比土试样单/双峰SWCC的特点。其次,基于上述土水特性的分析,以及吸附水和毛细水两种不同的作用机制,结合开尔文方程推导出指数函数形式的吸附项方程,毛细项方程则以修正的Fredlund和Xing SWCC模型为基础。最后,提出一个考虑初始孔隙比影响的单/双峰SWCC模型,并利用不同类型土单/双峰SWCC的实测数据验证了模型的有效性。

双峰土-水特征曲线分形模型及其在孔隙分类中的应用

现有的双峰土-水特征曲线(SWCC)模型大多由单峰土-水特征曲线的经验模型衍生而来,这些模型中的参数一般没有明确的物理意义和取值范围,导致通过拟合方法确定参数大小时出现多解情况,因而从很大程度上限制了双峰SWCC的理论研究。根据毛细管理论,双峰SWCC对应于双峰孔径分布(PSD),基于此,利用Menger海绵模型对双峰PSD进行了分形描述,并以此建立了双峰SWCC的物理模型。利用该模型对20组已发表的双峰SWCC试验数据进行拟合,并与其他学者提出的模型进行对比。结果表明本文模型具有良好的拟合效果。此外,基于建立的双峰SWCC模型提出了一种孔隙分类方法,经验证,该方法所得结果较为准确,且具有应用简便、理论基础明确的优势。

Variations of suction and suction stress of unsaturated loess due to changes in lignin content and sample preparation method

Unsaturated loess in natural sites loses stability as the overburden load continuously increases.Traditional soil modifiers such as cement and fly ash affect the surrounding environment.A new type of material,i.e.,lignin,is environmentally friendly and able to increase the strength of loess.However,the engineering characteristics of the improved loess under unsaturated conditions are not yet clear.In this study,the soil-water characteristic curves(SWCCs)of lignin-improved loess samples were determined from 0 kPa to 700 kPa using a pressure plate instrument,and then,they were fitted using the van Genuchten(VG)model and the Fredlund and Xing(FX)model.In addition,the effects of the lignin content and sample preparation methods on the SWCCs were investigated to determine the optimal lignin content and a suitable sample preparation method for loess foundations.As the lignin content increases,the matric suction and residual water content of the improved loess increase.The suction stress increases with the increasing lignin contents of 1%–2%.At lignin contents of 3%–4%,the suction stress begins to decrease and the samples prepared using the slurry method has a lower suction stress than that prepared using the wet mixing method.The air entry value(AEV)increases with increasing lignin content.In addition,scanning electron microscopy(SEM)was used to investigate the microstructural variations.It was found that after the addition of lignin,the entrapment of the loess particles by the lignin fibers created some larger particles and smaller pore diameters,which in turn led to poor connectivity of the loess pores.These changes cause the matric suction of the modified loess to increase.

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.