Unsaturated Compacted Bentonite Research Articles

Modeling unsaturated hydraulic conductivity of compacted bentonite using a constrained CatBoost with bootstrap analysis

Accurately determining the hydraulic conductivity of unsaturated bentonite is important for modeling subsurface thermo-hydro-mechanical and chemical processes. This study introduced a new hybrid model that employs a constrained categorial boosting (CatBoost) algorithm, combined with a genetic algorithm for hyperparameter tuning, to estimate the hydraulic conductivity of unsaturated compacted bentonite The performance of the constrained CatBoost model was benchmarked against a diverse set of data-driven baseline regression models, including lasso, elastic net, polynomial, k-nearest neighbors, decision tree, bagging tree, random forest, and CatBoost. The results indicated that the constrained CatBoost model offers a superior balance between model robustness and predictive accuracy in estimating the hydraulic conductivity of compacted bentonite-based materials during the wetting phase. The model effectively captured the U-shape relationship between hydraulic conductivity and suction, a key characteristic of bentonite behavior. Additionally, bootstrapping analyses confirmed the model's reliability under data variability, further validating its applicability in environmental and geotechnical applications.

NMR-based analysis of pore water state of unsaturated compacted bentonite considering the saline effect

Understanding the saline effect on the pore water state of unsaturated compacted bentonite at the microscale is critical for evaluating the performance of barrier materials in nuclear waste geological repositories. In this paper, the pore water redistribution and pore water state of unsaturated compacted bentonite considering the saline effect is investigated via nuclear magnetic resonance (NMR) technique. It is found that, with increasing pore water salinity, the pore water content decreases and pore water is distributed in smaller pores, which is consistent with the swelling deformation behavior of bentonite at the macroscale and the diffuse double layers (DDL) theory at the microscale. In addition, the saline effect on pore water distribution is weakened with the increase in matric suction due to the shrinkage deformation of bentonite during drying. Then, the saline effect on the pore water state of unsaturated compacted bentonite is discussed via the concept of effective activation energy. It is shown that the drainage of pore water and increased pore water salt concentration (or the increase of matric suction and osmotic suction) both elevate the effective activation energy of bentonite, decreasing pore water mobility. The saline effect on the permeability coefficient of bentonite is also discussed. For bentonite under larger matric suction and osmotic suction conditions, the permeability coefficient is smaller.

A coupled THM finite element formulation for unsaturated soils and a strategy for its nonlinear solution

This paper presents a coupled thermo-hydro-mechanical (THM) finite element (FE) formulation which is capable of accounting for the effects of temperature change on the behaviour of unsaturated soils. Both vapour flow and density variation are taken into account in the development of this formulation. The full derivation procedure is provided and the adopted assumptions are stated and explained. To improve the efficiency of the nonlinear solution process while maintaining the accuracy of the prediction, a novel approach for determining iterative corrections when modelling coupled transient problems with the Newton-Raphson algorithm is established and presented here. The performance of the proposed FE formulation and of the new strategy for iterative corrections in a nonlinear solver is subsequently demonstrated and verified by simulations of laboratory experiments on unsaturated compacted bentonite, showing good agreement between the numerical and experimental results.

A bounding surface model for unsaturated compacted bentonite

This paper presents a bounding surface model for unsaturated compacted bentonite based on the critical state theory and bounding surface plasticity. In the proposed model, a shear bounding surface was proposed for describing shear behaviour. A new loading collapse curve was defined to describe suction effects on the yield behaviour. Meanwhile, wetting and drying bounding surfaces were proposed to represent the swelling and shrinkage volume change induced by suction changes. Furthermore, isotropic hardening rule was incorporated into the bounding surface formulation with an associated flow rule. Finally, a simplified form of plastic modulus was introduced to describe elastoplastic behaviour within the bounding surface and the Hvorslev surface was employed to describe the peak strength of heavily compacted bentonite. The proposed model has great potential to realistically describe some features of compacted bentonite including strain hardening or strain softening; monotonic loading under isotropic or one-dimensional compression; yield behaviour due to suction; swelling behaviour induced by changes of suction, etc. With the proposed model, simulations were performed on results of oedometer tests conducted under different suctions, triaxial compression tests and swelling potential tests. Validation showed that good agreements were reached between simulations and the experimental results. Highlights A loading collapse bounding surface was proposed to describe the influence of initial dry density on yield stress A shear bounding surface was proposed for describing hardening or softening behaviour The Hvorslev surface was introduced to reflect the peak strength and the water content effect on the peak strength Wetting–drying bounding surface was proposed for describing the volume change induced by suction changes

Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite

Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.

Changing of Properties of Unsaturated Compacted Bentonite due to Hydration Effort

Changing of Properties of Unsaturated Compacted Bentonite due to Hydration Effort

Longitudinal and Shear Wave Velocities Measurement in Compacted Bentonite for Water Content

Bentonite is a good candidate of buffer material for disposal repository of high-level radioactive waste. Understanding groundwater behavior in bentonite buffer material is important in order to evaluate the bentonite buffer performance and guarantee long-term safety. Elastic constants of the bentonite buffer material are important parameters for the long-term safety. Water content in buffer material may have an influence on its elastic properties. For this reason, the monitoring system of the water saturation level in compacted bentonite is required. In this study, the ultrasonic velocity measurement method for evaluation of water content in compacted bentonite was proposed. At first, the effect of a degree of saturation in compacted bentonite on the longitudinal and shear wave velocities was investigated experimentally. In addition, the elastic property, bulk modulus, in unsaturated compacted bentonite were evaluated by ultrasonic velocities. As a result, it can be confirmed that ultrasonic velocities can evaluate a degree of saturation and bulk modulus of compacted bentonite.

The gas permeability, breakthrough behaviour and re-sealing ability of Czech Ca–Mg bentonite

Abstract The study's objectives were to test the gas permeability of unsaturated compacted bentonite, the gas-breakthrough behaviour of saturated bentonite, and the re-sealing ability compared with the material's dry density, hydraulic conductivity and swelling pressure. During the project's pilot phase, a series of laboratory experiments were conducted on samples of two Czech Ca–Mg bentonites (Bentonite 75 and Cerny Vrch). The testing procedure, employing repeated hydration and gas-breakthrough test phases, simulated conditions within deep geological repositories for radioactive waste, wherein the buffer will be progressively hydrated from the surrounding host rock and loaded with pressure exerted by gases created within the repository. It was found that the gas permeability of the bentonites tested was affected by dry density and, as with the two materials (FEBEX and MX-80) used for comparison, it correlated well with the accessible void ratio. Gas-breakthrough tests revealed correlations between time to breakthrough and both dry density and sample height. The recorded repeated test breakthrough times on the same samples varied, which implies that the pathways created are complex and differ for repeated gas-injection cycles. However, no systematic deterioration in sample properties was detected, thus indicating the extensive re-sealing ability of the materials tested.

A homogenization-based model for the effective thermal conductivity of bentonite–sand-based buffer material

A homogenization-based effective thermal conductivity model was proposed for unsaturated compacted bentonite–sand-based buffer materials. The microstructure of the mixture was approximated with pores and sand particles of spheroidal shape and random orientation embedded in the homogeneous bentonite matrix. By virtue of the analytical solution to the inhomogeneous inclusion problem in heat conduction, the model was developed using homogenization techniques such as the Mori–Tanaka (MT) and interaction direct derivative (IDD) schemes for different consideration of the interactions between pores, sand particles, and the bentonite matrix. The proposed estimates are dependent on the thermal conductivities of the bentonite matrix, the liquid and gas phases and sand, porosity, the degree of saturation, sand content, and the aspect ratios of pores and sand particles. The proposed model was validated against four sets of laboratory measurement data on the Kunigel-V1, MX-80, Kyungju, and GMZM bentonite–sand mixtures. It is demonstrated that although with simplified considerations of the microstructural features and mechanisms, the model predicts the effective thermal conductivity of the bentonite–sand mixtures with a fairly good agreement.

Vapour transfer in unsaturated compacted bentonite

Results of an experimental and theoretical investigation of heat and moisture movement in unsaturated MX-80 bentonite are presented. A thermo-hydraulic cell that allows measurement of transient temperatures and facilitates the determination of pseudo-transients of moisture content, dry density and chemical composition has been used to perform thermal gradient tests. Results of a number of tests are presented, and observation of the accumulation of chloride ions near the hot end clearly indicates that there is a cycle of vapour and liquid moisture movement, with vapour moving from hotter to cooler regions, condensing, and then moving as liquid towards the hotter regions. An empirical method is applied to calculate approximate vapour fluxes using measured variations in chloride ion concentration and moisture content with time. The vapour fluxes calculated empirically are found to be lower than those determined by some existing vapour flow theories. Subsequently, an existing vapour flow model is modified to represent the observed vapour fluxes more closely.

Hydro-mechanical constitutive model for unsaturated compacted bentonite–sand mixture (BSM): Laboratory tests, parameter calibrations, modifications, and applications

A bentonite–sand mixture (BSM) is one of the clay-based sealing components proposed for use in a Canadian deep geological repository (DGR) for used nuclear fuel. Numerical modelling to assess the overall design of the proposed DGR requires characterisation of the hydraulic–mechanical (H–M) of each of the components of the sealing system, including the BSM. The BSM currently under consideration is a 50/50 mixture (by dry mass) of bentonite and well-graded silica sand, compacted to a dry density of at least 1.67 Mg/m 3. This paper presents the H–M constitutive model parameters, calibrated for BSM specimens under saturated and unsaturated conditions, based on various laboratory tests. A set of parameters for an elastoplastic model for unsaturated soil, Basic Barcelona Model (BBM), have been determined to simulate the mechanical behaviour of the BSM specimen. A set of parameters for van Genuchten’s Soil–Water Characteristic Curve (SWCC) and Kozeny’s hydraulic permeability model have been determined to simulate the hydraulic behaviour of the BSM specimen. Using a finite element computer code, CODE_BRIGHT, these sets of parameters have been used to simulate H–M processes in BSM specimens during water infiltration under constant volume (CV) and constant mean stress (CMS) boundary conditions. The key features of the selected constitutive models that are different from the laboratory tests of the BSM specimen have been summarised. The functions to improve the capability of the selected constitutive models to match the laboratory test results of the BSM specimen have been proposed.

Suction and water uptake in unsaturated compacted bentonite

Suction is an important process dominating water movement in unsaturated compacted bentonite. Suction measurements were carried out to investigate the characteristics of suction in unsaturated compacted bentonite for the buffer of a HLW repository. The suction values decreased with increasing the water content at a given dry density, and it revealed higher value at the higher dry density of compacted bentonite. The suction variation with an increase of temperature was negligible for the water content up to 17%. For the dry density of 1600 kg/m 3, the suction value in the vertical measurement was a little lower than that in the horizontal one due to the microstructural anisotropy of compacted bentonite. The modeling of water uptake in unsaturated compacted bentonite is allowed to predict the re-saturation for the buffer of a HLW repository. To this end, water uptake tests were conducted to compare their results with those calculated using the coupled hydro-mechanical model of a computer code ABAQUS. Both results were in reasonably good agreement with each other, which suggests that the model can be used to predict the re-saturation of the bentonitic buffer in a HLW repository.

Swelling pressures and one-dimensional compressibility behaviour of bentonite at large pressures

In this study, the swelling pressures and one-dimensional compressibility behaviour of several compacted saturated bentonite specimens were determined. Laboratory oedometer tests were carried out on a bentonite from Germany, using distilled water as the bulk fluid. A newly developed high pressure oedometer device was used that enabled measurement of swelling pressures of initially unsaturated compacted bentonite specimens (strain-controlled tests) and then facilitated subsequent loading the specimens up to 25 MPa. Several initial compaction conditions (i.e., dry density and water content) of the bentonite were considered. The test results showed that both initial water content and compaction dry density influenced the swelling pressure of the bentonite. For the range of void ratio considered in this study, the swelling pressures of compacted saturated specimens were found to be significantly smaller than the applied vertical pressures during the consolidation tests. Also, the compression paths of compacted saturated specimens remained distinctly below that of the compression path for the initially saturated bentonite specimen even at very large pressures. The initial compaction conditions affected the compression index of the bentonite, whereas the effects were found to be less significant on the decompression index. The effect of initial compaction conditions on the coefficient of consolidation was found to be significant at void ratios greater than 0.5. The variation of the coefficient of permeability with the void ratio was found to be distinctly bilinear for an initially saturated bentonite specimen, whereas the permeability varied linearly with the void ratio for compacted saturated specimens. In general, the compacted saturated bentonite specimens were found to be more permeable than the initially saturated bentonite.

Simulation of Heat and Water Counterflow in Unsaturated Compacted Bentonite

Abstract An engineer-scaled experiment, KENTEX is being performed at the Korea Atomic Energy Research Institute to study the thermal, hydraulic, and mechanical (THM) processes occurring in the engi...

Identification of relative conductivity models for water flow and solute transport in unsaturated bentonite

Unsaturated compacted bentonite is the buffer material foreseen for deep geological high-level radioactive waste repositories because it provides mechanical and chemical stability, hydraulic isolation, and radionuclide retardation. FEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project for the engineered barrier of a high-level radioactive waste repository in crystalline rock. An inverse methodology has been applied to estimate FEBEX bentonite hydraulic parameters and identify its relative conductivity function from infiltration tests using transient cumulative water inflow and final water content data. Model identification criteria developed within the context of information theory have been used to select the best relative conductivity function among four candidates, including that of Irmay and three proposed by van Genuchten. The Irmay model provides the best fit in almost all tests and is rated the best by model identification criteria for the test having the largest number of data values. The general van Genuchten model provides the second best fit to all tests and is rated either first or second best model by identification criteria. The relevance of different relative conductivity functions for solute transport has been evaluated by comparing concentrations of two conservative tracers computed with Irmay and van Genuchten models. Concentrations computed with van Genuchten model are steeper than those computed with Irmay model at the solute front.

Diffusion of soluble salts under a temperature gradient after the hydration of compacted bentonite

The transport of the soluble salts contained in a compacted bentonite has been studied in a hydration and heating experiment. The objective is to follow the processes of transport produced during and after the hydration of compacted bentonite in order to detect the generation of localized saline environments under the presence of a temperature gradient. Permanent saline environments may affect the surfacial and swelling properties of a bentonite when used as a clay liner for the isolation of high-level nuclear waste (HLNW). Several hermetic cells have been designed to constrain an unsaturated compacted bentonite cylinder (dry density of 1.65 g/cm 3). The bentonite is moisturized by injecting an aqueous solution in an opposite direction to a heat source. The tests are conducted during pre-saturation to post-saturation stages. The hydration is carried out with two different solutions, synthetic granitic water of low salinity and saline water. Two gradients with different average temperatures are generated: 100–60°C (16°C/cm) and 60–35°C (10°C/cm). Convection and diffusion processes govern the transport of salts in the compacted bentonite. The temperature is the variable that mostly influences the processes of transport, while the concentration of the injected solution had smaller importance. The convective transport of salts, driven by the hydration, occurs in the presence of the thermal gradient. By means of this mechanism, the dissolved ions concentrate on the heated area of the cell. The process permits the measurement of apparent diffusion coefficients ( D a) for ions and salts during the homogenization of concentration profiles in the post-saturation stages. D a for Cl −, SO 4 2−, Br −, Ca 2+, Mg 2+, Na +, K +, Sr 2+ and Zn 2+ have been calculated. The values vary in the range of 1.5×10 −10–4×10 −12 m 2/s; the ions Cl −, Na + and Ca 2+ being of higher mobility. Codiffusion of Ca 2+ and Na + with Cl − is the main mechanism found for salt transport in the compacted bentonite.