Determination Of Soil Water Characteristic Curve Research Articles

Evaluation of Soil Water Characteristic Curves of Boron added Sand-bentonite Mixtures using the Evaporation Technique

Compacted bentonite or sand-bentonite mixtures are considered buffer/backfill materials in the engineering barriers of deep geological repositories for high-level nuclear waste (HLW) disposal in many countries. The design and long-term functionality of nuclear repositories have critical importance for environmental safety and public health. The initially unsaturated buffer material could become re-saturated long after following the sealing of the repository. Although the saturation degree of the buffer might decrease due to high temperatures and evaporation, it tends to increase with groundwater intrusion. Therefore, the soil water characteristic curves (SWCCs) for these unsaturated soils are a key factor in geotechnical engineering. Yet, the determination of SWCCs can be time-consuming and prone to inaccuracies. The HYPROP (Hydraulic Property Analyzer) evaporation technique is a preferred method for accurately determining water retention curves of soils. This reliable method was applied to estimate the water retention curves for sand-bentonite mixtures in the presence of boron minerals. Known for their minimal thermal expansion and commonly used in various industries, boron minerals may improve the thermal stability of sand-bentonite mixtures. The findings revealed that the boron addition increased the water retention capacity of the 10% bentonite mixtures but had a negligible impact on the 20% bentonite mixtures.

Determining the Bimodal Soil–Water Characteristic Curve of Fine-Grained Subgrade Soil Derived from the Compaction Condition by Incorporating Pore Size Distribution

The soil–water characteristic curve (SWCC) is a key constitutive relationship for unsaturated soil which can be unimodal or bimodal. For the fine-grained compacted subgrade soil with a bimodal pattern, the determination of SWCC is complicated and needs a wide-range suction measurement. In this paper, the bimodal SWCC of a subgrade soil derived from the compaction condition was measured and determined by incorporating pore size distribution. For this purpose, a series of laboratory tests were conducted, including the pressure plate method, filter paper method, and vapor equilibrium method, which were used to measure SWCC at the low, medium, and high suction range, respectively. The pore size distribution (PSD) data were obtained by mercury intrusion porosimetry (MIP) tests and used to predict SWCC. Based on the analysis of hydraulic paths and SWCC-PSD correlations, the SWCC of the subgrade soil should be determined to follow the actual hydraulic path. SWCC within a low suction range can be filled by PSD-based data to improve the fitting accuracy. Then, a graphical method is applied to predict the bimodal SWCC by combining the filter paper method, vapor equilibrium method, and PSD-based data. The prediction curves fit well with the test data for all selected compaction conditions. Furthermore, the prediction method can still provide good prediction performance in the absence of high suction section data, which is beneficial for the application of bimodal SWCC.

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.

The effects of fitting parameters in best fit equations in determination of soil-water characteristic curve and estimation of hydraulic conductivity function

It is known that the soil-water characteristic curve (SWCC) contains important information for understanding the engineering behaviors of unsaturated soils. Best fit equations (or SWCC equations) are commonly used for the representation of the SWCC. As the infiltration into the unsaturated soil is mainly governed by the hydraulic properties of soil, the SWCC equations play important roles in the assessment of the rainwater infiltration. In this paper, four types of SWCC equations were adopted and different applicable suction ranges for four best fit equations are investigated and discussed. The fitting parameters and input values in these best fit equations are also demonstrated and discussed. The residual volumetric water content (or residual saturation) as defined in the original equations was recommended to be considered as an input value for a rough estimation of the residual condition. Parametric studies were also carried out to investigate the effects of different fitting parameters on the estimation of SWCC variables and hydraulic conductivities.

Determining Soil-Water Characteristic Curves from Mercury Intrusion Porosimeter Test Data Using Fractal Theory

Accurate determination of soil-water characteristic curve (SWCC) is of immense importance for understanding the mechanical behavior of unsaturated soils. Due to the difficulty and long duration of experimental procedures, it is of great significance to estimate the SWCC by indirect methods. To address this issue, in this article an effective fractal method is proposed for predicting the SWCC based on mercury intrusion porosimeter (MIP) data. Only two characteristic parameters, namely the fractal dimension and air-entry value, are needed in the presented approach. Detailed procedures for determining the parameters are clearly elaborated. Due to the influence of sample size difference on the equivalent connected pore size, a sample scale effect coefficient is proposed to predict air-entry values. The concept of “critical pore size” is introduced to obtain the optimal fractal dimension, which can accurately reflect the fractal behaviour of SWCC samples. By comparisons between predicted and experimental SWCCs, the validation of the proposed method is verified. The comparisons reveal the good agreement between the proposed approach and laboratory experiments.

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.

Comparison of Soil–Water Characteristic Curves in One-Dimensional and Isotropic Stress Conditions

Understanding unsaturated soil behavior is key to the design of foundations and embankment structures. Geotechnical engineers have applied net normal stress and matric suction to these engineering problems. Water retention activity in soils is used to predict seepage problems and stability of slope failures. Soil–Water Characteristic Curve (SWCC) tests contribute largely to matric suction interpretation. Determination of SWCCs in the laboratory is usually done using a pressure plate apparatus where vertical or confining stress cannot be applied. Mathematical models of SWCC though commonly accepted in geotechnical engineering practices, do not take into consideration stress conditions such as the difference between a one-dimensional condition and isotropic confining conditions. This study conducted SWCC tests of a silt soil under one-dimensional and isotropic confining stress conditions and focused on the differences between these types of SWCC data sets. Vertical and isotropic confining stresses ranging from 100 to 600 kPa were applied under both stress conditions. SWCCs appears to be affected by the influence of different stress conditions. Lateral pressure and confinement on an isotropic compression condition caused the soil specimen to become dense in void structure and consequently, soil moisture flow movement decreased. This probably induced high retention activities in the silt soil specimen. The study further suggests that the current SWCC models require further development to take into consideration the effect of different stress conditions.

Modified Pressure Plate Apparatus and Column Testing Device for Measuring SWCC of Sand

Abstract The determination of soil-water characteristic curve (SWCC) is of major concern in unsaturated soil mechanics. For decades experimental and theoretical studies are performed to investigate the constitutive relationship between soil suction and volumetric water content. The major objective of our study is to generate an extensive experimental database for sand with a relevant suction range of just a few kPa. This database enables to derive conclusions on the sensitivity of hydraulic properties regarding different experimental procedures. Further, one objective is the comparison of results for SWCC derived from steady state and transient state tests. While the first type of tests considers equilibrium states, the subsequent test is related to non-equilibrium states. Experimental results are generated from a so called homogenous element test (modified pressure plate apparatus) and an initial boundary value experiment (column testing device) considering different hydraulic loading path directions. The experiments are analysed for sand with different initial states. Finally results are presented for SWCC including initial curves, main curves, and scanning paths. Discussion is focused on transient state versus steady state flow tests. No significant dynamic effects are observed for the sand studied. Results of well controlled element tests compare very well to initial boundary value experiments implying higher experimental efforts.

Predicting Bimodal Soil–Water Characteristic Curves

Failure of colluvial soil deposits induced by rainfall is a common geohazard in the natural terrain. Many colluvial soils are widely and gap graded with a minimal sand fraction. In order to study the pore water pressures in such gap-graded, unsaturated soil deposits through a seepage analysis and to evaluate the stability of these soil deposits, the soil-water characteristic curves for gap-graded soils must be known. Usually, gap-graded soils exhibit bimodal grain-size and pore-size distributions. This technical note presents a theoretical continuum method for the determination of soil-water characteristic curves for soils with a bimodal or multimodal pore-size distribution. Based on the capillary law, the water content in a multimodal soil is equal to the sum of water stored in each pore series in the soil. Therefore, the bimodal or multimodal soil-water characteristic curves can be obtained by combining the unimodal soil-water characteristic curves for all components of the soil corresponding to the pore series weighted by the respective volumetric percentages. The proposed method is verified using experimental soil-water characteristics data of sand-diatomaceous earth mixtures with dual porosity.