Measurement Of Soil-water Characteristic Curve Research Articles

Variability in the measured soil‒water characteristic curve with respect to the numbers of specimens

The soil‒water characteristic curve (SWCC) defines the relationship between the amount of water in soil and soil suction. The SWCC is commonly used to estimate the hydraulic conductivity function (HCF) and the shear strength function (SSF). Therefore, an accurate determination of the SWCC is crucial for implementing the principles of unsaturated soil mechanics. The SWCC is commonly determined from a limited number of experimental data because SWCC measurements are time-consuming and costly. As a result, the minimum number of required soil specimens is crucial for a SWCC test when considering the accuracy of the determined SWCC and the experimental expenses. In this study, both engineered from sand and kaolin mixtures and residual soils from Bukit Timah Formation in Singapore are selected to prepare soil specimens for SWCC measurements. The SWCCs obtained from the specimens with engineered soil are consistent, while those from specimens with residual soil are slightly scattered. This indicates that one specimen is sufficient to determine the SWCC for engineered soil samples, while a minimum of two specimens should be prepared for the determination of SWCC for residual soil samples from Bukit Timah Formation.

Soil Water Retention and Pore Characteristics of Intact Loess Buried at Different Depths

Surface water infiltration is a primary factor responsible for engineering challenges and geological disasters on the Loess Plateau of China (LPC). Due to the extensive groundwater in this region, surface water must pass through thick unsaturated zones to recharge the groundwater reservoirs. Exploring the unsaturated hydraulic characteristics of loess, especially at varying depths, may significantly contribute to disaster prevention and mitigation and the pursuit of sustainable development in the Loess Plateau. The soil-water characteristic curve (SWCC), intricately linked to the soil’s pore structure, is a critical hydraulic parameter of loess. An exploration well with a depth of 30 m was excavated in the LPC to obtain intact specimens at depths of 5 m, 15 m, and 25 m. Basic physical property tests, SWCC measurements, and particle size distribution (PSD) analyses were conducted. Additionally, the relationship between PSD and SWCC is discussed in this paper. The findings highlight the influence of depth on the dominant pore size and distribution density, both of which decreased with increasing depth. The air occlusion value of the SWCC experienced an increase, and the slope of the SWCC in the transition zone exhibited consistency. These observations underscore the pivotal role played by pore structure in shaping the soil’s water-retention behavior. Furthermore, predictions based on PSD data demonstrated excellent accuracy in replicating the wetting SWCC of loess over a wide suction range (e.g., 10–104 kPa).

Uncertainty in Determination of the Soil-Water Characteristic Curve Based on the van Genuchten Model

The van Genuchten (1980) (VG) model is one of the most popular and widely used equations to achieve the best fitting of experimental data of the soil-water characteristic curve (SWCC) test. As the best-fitted SWCC is determined from regression analysis, there are various possible solutions for the best-fitted curve (or the determined fitting parameters). The residual error, which is obtained from regression analyses, can be considered as an index indicating the incapability of the SWCC equation in representing the measured SWCC experimental data. As a result, there is always uncertainty associated with determined SWCC from the regression analysis. The residual error from the regression analysis can be used for the quantification of the uncertainty of determined SWCC. In this paper, Taylor’s expansion is adopted, and only the first-order term is considered for the VG model. Subsequently, the variances of the fitting parameters in the VG model are estimated from the residual error. Consequently, the uncertainty of SWCC using the VG model is quantified by using the upper and lower bounds with a 95% confidence level. The artificial soil, composed of mixed sand and kaolin, was prepared for the SWCC measurement. This paper shows that the proposed method can be used to quantify the uncertainty of SWCC from different specimens (a total of 12 specimens).

Use of osmotic tensiometers in the determination of soil-water characteristic curves

Soil-water characteristic curve (SWCC) correlates the water content of a soil to its soil suction, which is a very important property of unsaturated soils. Osmotic tensiometers (OTs) have shown their capability in high soil suction measurement. This study tried to use OT combined with a pressure plate and WP4C dewpoint potentiometer to determine the SWCC of a residual soil. Three OTs with different measuring ranges (900 kPa, 1200 kPa, 2300 kPa) were prepared for soil suction measurement. The temperature effect on the pressure variations of OTs was illustrated based on Flory-Huggins polymer theories and an appropriate calibration equation was proposed to eliminate the temperature effect on the accuracy of soil suction measurement using OT. The OT showed a fast response in soil suction measurement and the equilibrium can be established in 10–15 min during SWCC measurement. Comparison of the SWCC data obtained from the pressure plate, OT, and WP4C dewpoint potentiometer proves that the OT had good performance in the determination of SWCC, especially for the transition zone (i.e., 10–1500 kPa suction range), by providing more data points to define the SWCC in a shorter period. The results show that the usage of OT will shorten the time required for the determination of SWCC and could be considered as a new and reliable technique in SWCC measurement.

Global Mapping of Soil Water Characteristics Parameters— Fusing Curated Data with Machine Learning and Environmental Covariates

Hydrological and climatic modeling of near-surface water and energy fluxes is critically dependent on the availability of soil hydraulic parameters. Key among these parameters is the soil water characteristic curve (SWCC), a function relating soil water content (θ) to matric potential (ψ). The direct measurement of SWCC is laborious, hence, reported values of SWCC are spatially sparse and usually have only a small number of data pairs (θ, ψ) per sample. Pedotransfer function (PTF) models have been used to correlate SWCC with basic soil properties, but evidence suggests that SWCC is also shaped by vegetation-promoted soil structure and climate-modified clay minerals. To capture these effects in their spatial context, a machine learning framework (denoted as Covariate-based GeoTransfer Functions, CoGTFs) was trained using (a) a novel and comprehensive global dataset of SWCC parameters and (b) global maps of environmental covariates and soil properties at 1 km spatial resolution. Two CoGTF models were developed: one model (CoGTF-1) was based on predicted soil covariates because measured soil data are not generally available, and the other (CoGTF-2) used measured soil properties to model SWCC parameters. The spatial cross-validation of CoGTF-1 resulted, for the predicted van Genuchten SWCC parameters, in concordance correlation coefficients (CCC) of 0.321–0.565. To validate the resulting global maps of SWCC parameters and to compare the CoGTF framework to two pedotransfer functions from the literature, the predicted water contents at 0.1 m, 3.3 m, and 150 m matric potential were evaluated. The accuracy metrics for CoGTF were considerably better than PTF-based maps.

Field instrumentation for real-time measurement of soil-water characteristic curve

Suction measurement has a vital role in unsaturated soil analysis. However, measuring soil suction remains a challenging task due to a number of issues such as the limited range of suction measurement, cavitation, or long equilibrium time. It is even more challenging when the suction measurement is to be carried on the field. Hence, the development of a new suction measurement device which is able to measure high suction range for a long duration without tedious maintenance and yet portable enough for site measurement is required. In this study, a new sensor which is referred to as NTU Osmotic Tensiometer was developed along with the method to correct for decay and temperature. The NTU osmotic tensiometer is based on polymer swelling capacity in order to measure in-situ soil suction in real time for a very long duration. It requires minimum maintenance as the polymer-based sensor is not affected by the cavitation phenomenon. However, correction for decay and temperature is of paramount importance and therefore explained in this paper. Verification of the NTU osmotic tensiometer was carried out by comparing the field measurement results from the NTU osmotic tensiometer and the small tip tensiometer. The performance of the NTU osmotic tensiometer was found to be comparable with that of the small tip tensiometer, but the NTU osmotic tensiometer is able to measure more than 100 kPa soil suction. Therefore, it is possible to obtain the field soil-water characteristic curve by combining the measured in-situ soil suction from the NTU osmotic tensiometer with the measured in-situ water content from the moisture sensor as illustrated in the paper.

A new framework to determine general multimodal soil water characteristic curves

A soil water characteristic curve (SWCC) model named as discrete-continuous multimodal van Genuchten model with a convenient parameter calibration method is developed to describe the relationship between soil suction and the water content of a soil with complex pore structure. The modality number N of the SWCC in the proposed model can be any positive integer (the so-called multimodal or N-modal SWCC). A unique set of parameters is determined by combining curve fitting and a graphical method based on the shape features of the SWCC in the log s–log Se plane. In addition, a modality number reduction method is proposed to minimize the number of parameters and simplify the form of SWCC function. The proposed model is validated using a set of bimodal and trimodal SWCC measurements from different soils, which yield a strong consistency between the fitted curves and the measured SWCC data. The uniqueness in the set of parameters provides the possibility to further improve the proposed model by correlating the parameters to soil properties and state parameters.

Predicting hysteresis loops of the soil water characteristic curve from initial drying

AbstractThe soil water characteristic curve (SWCC) is an important tool in soil science and geotechnical applications. Most previous studies on SWCCs are limited to one to three drying‐wetting cycles, mainly because SWCC measurement is complex and time‐consuming. The numerous models that have been proposed to predict hysteresis in SWCCs. In this paper, a novel simplified equation is proposed based on the van Genuchten model, whereby SWCC hysteresis can be easily predicted from the initial drying SWCC. The theory for describing the main wetting curve and main drying curve considers the ink‐bottle effect and entrapped air. Laboratory experiments on silicon micro‐powder (SMP) and Guangxi Guiping clay (GGC) were conducted to determine SWCCs for five drying‐wetting cycles. The proposed equation was verified using the experimental SMP data and data from the literature. The predicted hysteretic SWCCs are in good agreement with the experimental data.

Probabilistic analysis of soil-water characteristic curve using limited data

Soil-water characteristic curve is an essential constitutive relationship required for modelling unsaturated soil behaviour. Earlier supposed to be characteristic or unique for a particular soil, it is now well established that there are various sources of uncertainty which lead to different curves for the same soil. Hence it becomes necessary to probabilistically characterize soil-water characteristic curve for the purpose of reliability based design in unsaturated geotechnical engineering projects. Popular approach is to parametrize the soil-water characteristic curves into a set of curve fitting parameters and evolve the joint distribution of these parameters. However, a satisfactory probabilistic characterization requires large number of realizations and is practically not feasible since experimental measurement of soil-water characteristic curve is an expensive, time intensive and difficult task. For all practical purposes, it is impossible to obtain a large number of curves to arrive at the site-specific probability distribution. Therefore, this study proposes a Bayesian approach integrated with copula theory to obtain the joint probability distribution of soil-water characteristic curve parameters from limited number of curves. Limited number of curves are incorporated with prior knowledge to obtain the updated probability distribution of soil-water characteristic curve parameters. Efficacy of the proposed approach is demonstrated on three databases, one each for loam, fly ash and bentonite. Finally for demonstrating a practical application of the proposed approach, a site specific reliability based design of unsaturated slope is conducted.

The effects of soil shrinkage during centrifuge tests on SWCC and soil microstructure measurements

Significant soil shrinkage occurred during SWCC (soil-water characteristic curve) measurement using centrifuge methods and thus affects the soil microstructure. This paper deals with an experimental investigation aimed at correcting for SWCC measurements with soil shrinkage considered and at studying the evolution of the microstructure induced by soil consolidation along drying path. Microstructure evolution of samples subjected to different pressure heads is determined using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) techniques. The relationship between the soil microstructural features and the soil mechanical and hydraulic properties is explained from the microstructure point of view. The results show that after the centrifugation ended, the height shrinkages of the soil samples were 10.36, 8.86, 6.39, 4.16, and 2.37 mm, while the corresponding initial bulk densities were 1.30, 1.36, 1.46, 1.55, and 1.68 g/cm3. The corrected volumetric water contents were higher than the uncorrected values at a specific pressure head. The soil shrinkage, void ratio, and degree of saturation changed at a decreasing rate as the pressure head gradually increased. These changes occurred because of the greater compressibility of the macropores, which allowed for consolidation, and because the water in the macropores was removed at a relatively low pressure head. The drastic decrease in the pore volume and the dominant diameter of such macropores inhibited further and obvious soil consolidation at a higher pressure head and inhibited the flow of water out of the soil matrix.

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.

Influence of Stress on Soil-water Characteristic Curve of Intact Loess

The soil-water characteristic curve plays a significant role in describing the unsaturated soil characteristics, and is greatly affected by stress state, wetting and drying cycles and other factors. Accurate measurement of soil-water characteristic curve is the key to the unsaturated soil mechanics studies. In this paper, based on the GCTS soil-water characteristic curve (SWCC) instrument, the undisturbed loess was tested to study the effects of one-dimensional consolidation stress, stress history and wetting and drying cycles on the soil-water characteristic curve of undisturbed loess. The experimental results show that: (1) As the one-dimensional axial stress increases, the air intake of the soil sample gradually increases, and the decrease of the water content slows down as the matrix suction increases; (2) Due to the increase of the pre-consolidation stress, soil compactness increases, and pore size reduces, which makes the water migration more difficult. Therefore, as the sampling depth of soil increases, the air intake increases and the residual water content decreases. (3) Due to the pore size, bottleneck effect and contact angle, the water content of the humidification curve is higher under the same matrix suction, and the soil-water characteristic curve in the humidification test shows obvious hysteresis effect.

Comparison of Soil–Water Characteristic Curves from Conventional Testing and Combination of Small-Scale Centrifuge and Dew Point Methods

Soil–water characteristic curve (SWCC) is an important unsaturated soil property relating the water content of a soil to soil suction and it is conventionally measured using Tempe cell, pressure plate and salt solution methods. However, these tests are tedious and time consuming. The SWCC measurements using fast and efficient methods are required for engineering designs such as excavation, slope protection, retaining wall and landfill cover designs. This paper describes the testing procedures and apparatuses associated with rapid measurements of a complete SWCC of a residual soil as obtained from combined measurements using a small-scale centrifuge and dew point methods. The SWCC test results obtained using these alternative methods were compared with the SWCC data from Tempe cell, pressure plate and salt solution methods. Shrinkage tests were carried out in this study to incorporate the volume change of soil into SWCC. The experimental data from all SWCC tests were evaluated using first order analysis with 95% confidence interval for determination of upper and lower bounds of SWCC. The analysis results showed that the SWCC data obtained from tests using small-scale centrifuge and dew point methods were in good agreement with those obtained from Tempe cell, pressure plate and salt solution methods. This indicates that the combination of small-scale centrifuge and dew point methods can be used to generate a complete curve of SWCC for the residual soil. In addition, the time required to perform SWCC tests using the alternative methods is shorter than the SWCC tests using the conventional methods.

Uncertainty of Soil Water Characteristic Curve Measurements Using an Automated Evaporation Technique

Core Ideas Replicate‐specimen measurement of SWCCs was performed with an automated technique. Inter‐specimen uncertainty for SWCC parameters was quantified. A comparison of automated evaporation technique vs the standard hanging column is presented. Soil water characteristic curve (SWCC) measurements are used to estimate hydrologic parameters for a variety of engineering and agricultural applications. The SWCC parameterization is often based on a single measurement or with little or no consideration and/or quantification of the uncertainty in collection of SWCC data. Uncertainty in the SWCC data stems from random variability in pore geometry associated with irregular granular media. Additionally, a substantial opportunity exists in SWCC measurements for systematic or experimental errors that can increase uncertainty. This study utilized an evaporation method for measuring 10 SWCCs for each of four different test soils. One low‐plasticity and three high‐plasticity silty sand soils were sampled from the Sagehen Creek Experimental Watershed in Sierra County, California. The uncertainty in the SWCC measurements was quantified by the distribution and variability of van Genuchten parameters evaluated via nonlinear regressions on each set of SWCC data. Despite a careful, replicated specimen preparation, variability in the measured SWCC data was indicated by variability of estimates of van Genuchten's fitting parameters α and n and residual volumetric water content θr, suggesting that packing specimens to identical dry densities does not produce identical SWCC measurements for a single soil type. For the soils tested in this study, replicate specimen testing resulted in uncertainty in α between ×/1.2 and ×/1.5 the geometric mean estimate for α, between ±2 and ±6% of the mean estimate for n, and between ±2 and ±6% for θr. Additional testing is required before applying these uncertainties generally to soils of different textural classifications.

New approach to improve soil-water characteristic curve to reduce variation in estimation of unsaturated permeability function

The unsaturated permeability function is often estimated from the soil-water characteristic curve (SWCC) of a soil. A complete SWCC measurement can improve the estimation of the unsaturated permeability function. In most laboratories, the SWCC can be measured up to a suction of 100 kPa using a Tempe cell. However, complete measurement of the SWCC is an expensive and time-consuming task. Therefore, this paper presents a new approach to estimate SWCC data points beyond 100 kPa suction to complement the SWCC measured up to a suction of 100 kPa. The new SWCC is then used to estimate the unsaturated permeability function. The proposed approach uses knowledge of the grain-size distribution curve and measured SWCC data at 100 kPa suction to estimate the SWCC data points beyond 100 kPa suction. To verify the proposed procedure, SWCC tests were conducted over a wide range of suctions for coarse kaolin and a triaxial permeameter system was used to directly measure unsaturated permeability of the coarse kaolin. The proposed procedure is found to reduce the variation between unsaturated permeability functions estimated by various estimation models.

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

It is well known that soil–water 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.

Study on soil shrinkage characteristics during soil water characteristic curve measurement

Clay loam soil from Yunnan(KM),sandy loam soil from Liaoning(SY) and Xinjiang(KL),and loam soil from Shandong(LY) were selected for analyzing the characteristics of soil shrinkage duringsoil water characteristic curve measurement in order to explore the suitability of fitting models for soil watercharacteristic curve,and the changes of soil shrinkage. In this paper,soil water characteristic curves ofthe four soil types were measured by centrifuge method and were fitted by RETC software. The resultsshowed that as for KM,SY,KL and LY soils,firstly,soil moisture content gradually decreased with soilsuction increasing from 0 to 7000 cm. And the rate of soil water reduction decreased,and then increased,and then decreased again. Secondly,all fitting models presented in RETC software were of high accuracyfor the simulation of soil water characteristic curves,and the appropriate fitting models for KM,SY,KLand LY soils were VG-M(m,n),LND-M,VG-M(1-1/n,n) and VG-B(m,n),respectively. And VGmodel could be considered as the optimal model for simulation of soil water characteristic curves of thefour soils. Thirdly,a logarithmic function was found between degree of effective soil shrinkage and soil suc-tion during soil water characteristic curve measurement. The soil shrinkage was ranked in the order of SY KM LY KL,and the bulk density of the four soils increased by 0.19 ~ 0.46 g/cm3. Fourthly,soil shrink-age process could be roughly divided into pseudo-saturated stage,structural stage and super-normal stage.And as for KM,SY,KL and LY soils,the three stages could be described by linear function,but theranges of soil suction corresponding to three soil shrinkage stages were different.

Effect of range of soil–water characteristic curve measurements on estimation of permeability function

The most commonly used indirect method to determine unsaturated permeability of soil is to estimate the unsaturated permeability function from soil–water characteristic curve and saturated permeability. The suction range that soil–water characteristic curve (SWCC) can be measured in the laboratory depends on the type of equipment used for measurement. As the estimation models use the available measured SWCC data to obtain the unsaturated permeability function, the estimated permeability function can be affected by the number of measured data of SWCC and the suction range over which the SWCC data are measured. Therefore, there is a need to investigate the effect of the range of SWCC measurements on the estimation of unsaturated permeability function. In this study, the effect of the range of SWCC measurements is investigated through different estimation models that are based on four different best-fit soil–water characteristic curve equations and three different relative permeability equations. It was found that the range of SWCC measurements greatly affect the estimated permeability functions. It was also found that the effect of the range of SWCC measurements is more significant than the selected best-fit SWCC equation used.

Microporous Membrane Technology for Measurement of Soil-Water Characteristic Curve

Abstract High air entry ceramic disks are commonly used for the control of matric suction in triaxial and direct shear tests and in tests for obtaining the soil-water characteristic curve. Geotechnical engineers have made increasing use of high air entry ceramic disks in order to measure or control matric suctions during unsaturated soil testing. One of the limitations associated with the use of high air entry ceramic disks has been the time required in order for equilibrium conditions to be established across the disk. Improving the efficiency associated with establishing suction equilibrium is of considerable interest to geotechnical engineers. Pressure membrane technology involving the use of microporous membranes is one possible technology that could result in improved performance for the measurement or control of matric suctions. If microporous membrane technology can be used, the end result could provide considerable time and cost savings, particularly in the measurement of soil-water characteristic curves (SWCCs). In this study, a new apparatus was developed to make use of a microporous membrane for the measurement of the SWCC with matric suction of up to 25 kPa. The maximum AEV (i.e., air-entry value) of the membrane is 250 kPa. This paper presents the results of laboratory SWCC measurements on several soil types using pressure membrane technology. Comparisons are made between the soil-water characteristic curves measured using a conventional pressure plate apparatus and those obtained from the new micro membrane apparatus.

A New Method for Soil Water Characteristic Curve Measurement Based on Similarities Between Soil Freezing and Drying

Abstract The soil water characteristic curve (SWCC) is the basis to explain a variety of processes in unsaturated soils, ranging from transport phenomena to mechanical behaviors. In this paper, a new method is developed for SWCC estimation based on the similarity between the freezing/thawing process and drying/wetting process in soils. The theoretical basis for this method is first reviewed. The concept of the soil freezing characteristic curve (SFCC) is introduced to describe the relationship between the unfrozen water content and matric suction in frozen soils. SFCC is analogous to SWCC in that both of them describe the energy status of liquid water associated with liquid water content. Relationships between SWCC and SFCC are discussed. To measure the SFCC, a thermo-time domain reflectometry (TDR) sensor was developed which combines both temperature sensors and conventional TDR sensor. The TDR module and algorithm measured the bulk free water content of soils during the freezing/thawing processes, while the built-in thermocouples measured the internal temperature distribution. SFCCs were obtained from the simultaneously measured TDR and temperature data. Experiments were conducted on a few types of soils to validate this new procedure. The SFCC was obtained from thermo-TDR data collected in specimens subjected to a controlled thawing process, while the SWCC was directly measured by ASTM D5298, the filter paper method. Reasonable agreements were found between SWCC and SFCC. The experimental results implied that the SWCC could be estimated from SFCC, which also provided more evidence of the similarity of freezing/thawing processes and desorption/sorption processes.