Bentonite Specimens Research Articles

Migration Behaviour of Lanthanides in Compacted Bentonite with Iron Corrosion Product Using Electrochemical Method

ABSTRACTAfter the closure of a high-level waste repository, corrosion of the carbon steel overpack will occur. The corrosion products can then migrate into bentonite and affect the migration behavior of radionuclides in bentonite. Therefore, electrochemical experiments, with Fe2+ supplied by anodic corrosion of carbon steel, were carried out to study trivalent lanthanides in compacted bentonite. The interface between a carbon steel coupon and bentonite (dry density, 1.5 Mg/m3) was spiked with a tracer solution containing Nd(NO3)3, Eu(NO3)3, Dy(NO3)3, and Er(NO3)3. The carbon steel coupon was connected as the working electrode to a potentiostat and held at a constant potential between -550 and 0 mV (vs. Ag/AgCl reference electrode) for 7 days. A model using dispersion and electromigration could explain the measured profiles in the bentonite specimens. The best-fit electromigration velocity was related to the applied electric potential and was 1.0–3.8 nm/s for Nd, Eu, Dy, and Er ions. For these lanthanides, the best-fit dispersion coefficient was also related to the applied potential and was 0.8–1.6 μm2/s, and the dispersion length was calculated as 0.2 mm from the linear relationship between the dispersion coefficient and electromigration velocity. Finally, the apparent diffusion coefficient for these lanthanides was estimated as 0.6–0.9 μm2/s.

물의 염도가 압축벤토나이트의 수리전도도에 미치는 영향

The influence of water salinity on the hydraulic conductivities of compacted bentonite with several dry densities were studied. The hydraulic conductivity increases with increasing salinity only when the dry density of bentonite is relatively low. The degree of increase becomes more remarkable at a lower dry density of bentonite. For bentonite with the density of 1.0 Mg/m{sup 3} and 1.2 Mg/m{sup 3}, the hydraulic conductivity of the 0.4 M NaCl solution increases up to about 7 times and 3 times, respectively higher than that of freshwater. However, for the bentonite with a dry density higher than 1.4 Mg/m{sup 3}, the salinity has an insignificant effect on the hydraulic conductivity, and the hydraulic conductivity is nearly constant within the salinity range of 0.04 to 0.4 M NaCl. The pre-saturation of the bentonite specimen with freshwater has no significant influence on the hydraulic conductivity.

Swelling behaviors of GMZ bentonite–sand mixtures inundated in NaCl–Na2SO4 solutions

a b s t r a c t In this study, the swelling behaviors of compacted GMZ bentonite–sand mixtures inundated in NaCl–Na2SO4 solutions are investigated and the influence of chemical solutions on the swelling behaviors of GMZ bentonite–sand mixtures as backfill/buffer material in China for high level radioactive waste (HLW) is investigated. The sand addition ratios of the bentonite–sand mixtures are 0%, 20%, 30% and 50%, and the total dissolved solids (TDS) of the NaCl–Na2SO4 (NaCl:Na2SO4 = 2:1 by mass) solution are 0.5, 1.0, 3.0, 6.0 and 12.0 g/L (pH 7.1). The specimens of bentonite–sand mixtures for swelling tests are prepared by static-compaction to various dry densities, ranging from 1.50 to 1.90 g/cm3. Test results indicate that liquid limit (LL) and plasticity limit (PL), swell time, maximum swelling pressure and maximum swelling strain decrease with the increase of TDS for GMZ bentonite–sand mixtures. All of the LL, PI and maximum swelling strain are decreased exponentially with TDS increase: very quickly as TDS 6.0 g/L. The maximum swelling pressure shows a linear reduction with the TDS increasing, but the pure bentonite indicates a high sensitivity than the bentonite–sand mixtures with 30% sand addition ratio. As NaCl–Na2SO4 (TDS = 0.5 g/L) solution was used according to the ground water, with initial dry density of 1.70 g/cm3, the maximum swelling pressure of specimens decrease exponentially while the maximum strain decrease linearly with the increase of sand addition. With 30% sand addition in 0.5 g/L NaCl–Na2SO4 solution, the maximum swelling pressure increase exponentially while the maximum strain increase linearly with the increase of initial dry density. Compared with the pure bentonite, bentonite–sand mixtures show a better tolerance withstanding the chemical attack to ground water chemistry because of the replacement of some quantity of expansive clay by quartz sand in the mixtures.

Nonlinear Creep and Swelling Behavior of Bentonite Mixed with Different Sand Contents Under Oedometric Condition

Due to the viscous nature, especially the swelling property, bentonite is selected as buffer and backfill material for marine radioactive waste disposal. This article presents experimental results from one-dimensional oedometric compression tests with multi-stage loading and unloading/reloading on specimens of bentonite mixed with different sand contents. The sand contents are 50%, 60%, 70%, 80%, and 90%, respectively, in terms of the dry mass of sand over the dry mass of the mixture. Influences of sand contents on time-dependent stress-strain behavior of the bentonite-sand mixture under 1-D straining are examined with special attention to the nonlinear creep and swelling behavior. It is found that the bentonite-sand mixtures exhibit distinct nonlinear creep and swelling behavior that cannot be neglected. Moreover, a nonlinear creep function proposed by Yin (1999) for Hong Kong Marine Deposits has been modified to describe the nonlinear creep behavior of the bentonite-sand mixture.

On the intrusion of polyethylene glycol during osmotic tests

In the laboratory, semi-permeable membranes and polyethylene glycol (PEG) solutions are used for applying suction in soils using the osmotic technique. In this study, the pore structures of cellulose semi-permeable membranes with molecular weight cut-off values of 3500 and 14 000 were examined via an atomic force microscope (AFM). Fourier transform infrared (FTIR) spectroscopy was used to identify any degradation of PEG molecules with elapsed time. Freshly prepared PEG solutions and solutions aged for 15 days were considered for this purpose. Osmotic tests were carried out on initially saturated MX80 bentonite specimens at two suction levels (0·44 and 7·04 MPa). Comparison of the AFM images of the semi-permeable membranes before and after the osmotic tests clearly revealed that the pore size of the semi-permeable membranes increased significantly, particularly at the higher applied suction that enabled passing of PEG molecules into the clay specimens. FTIR spectrums of PEG 20 000 and PEG 6000 did not provide any evidence of degradation of the PEG molecules with elapsed time.

Effect of GCL Properties on Shrinkage When Subjected to Wet-Dry Cycles

The potential shrinkage of eight different geosynthetic clay liners (GCLs) subjected to wetting and drying cycles is examined. It is shown that the initial (e.g, off-the-roll) moisture content may affect the initial shrinkage but did not notably affect the final equilibrium shrinkage. For GCLs with granular bentonite and wetted to a moisture content of about 60% (or greater) in the hydration phase, the actual moisture content did not appear to affect the magnitude of the final equilibrium shrinkage. However, it did affect the rate of shrinkage. Specimens brought to about 100% moisture content in each cycle reached a constant shrinkage value much faster than those brought to about 60% in each wetting cycle. GCLs containing powdered bentonite generally shrank more than those containing granular bentonite. All of the powdered bentonite specimens continued a slow accumulation of strain with increasing cycles, even up to 75 cycles. The shrinkage of a needle-punched GCL with a thermally treated scrim-reinforced...

Advances on the knowledge of the buffer/backfill properties of heavily-compacted GMZ bentonite

GMZ bentonite has been selected as a potential material for the construction of engineered barrier in the Chinese program of geological nuclear waste disposal, for its high montmorillonite content, high cation exchange capacity (CEC) and large specific surface etc. Studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior of GMZ bentonite were performed from 1980s. Based on a review of the former studies, achievements on experimental and theoretic results obtained on compacted GMZ bentonite specimens including basic properties, thermal, hydraulic and mechanical behaviors are presented in this paper. Results show the thermal conductivity of GMZ bentonite and the bentonite-based mixtures influenced by its dry density, water content, mixture of other materials and degree of saturation etc. Water retention capacity of highly-compacted GMZ bentonite decreases as the temperature increases under confined and unconfined conditions. The hysteretic behavior in the water retention curves of the compacted GMZ bentonite is not so significant at 20 or 40 °C. The unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions. This is possibly induced by the difference in the mechanism of microstructural changes during hydration under different confining conditions. The compaction curves for GMZ bentonite with different dry densities are clearly step-phased. And the optimum water content for GMZ bentonite is about 15%. An exponential relationship between swelling pressure and dry density of highly-compacted GMZ bentonite was determined for the prediction of swelling pressure. Furthermore, the void ratio after swelling for unconfined sample also can be predicted using diffuse double layer (DDL) theory.

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.

Migration Behavior of Alkali Earth Ions in Compacted Bentonite With Iron Corrosion Product Using Electrochemical Method

AbstractCarbon steel overpack will corrode by consuming oxygen introduced during repository construction after closure of repository, that will keep the environment in the vicinity of repository reducing. The iron corrosion products can migrate in bentonite as ferrous cations (Fe2+) through the interlayer of montmorillonite replacing the exchangeable sodium ions in the interlayer. This replacement of sodium may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore we have carried out electrochemical analysis, of calcium, strontium or barium with the ferrous ion supplied by anodic corrosion of iron coupons in compacted bentonite. Fifteen micro liters of tracer solution containing 8.6 M of CaCl2 or 3.0 M of SrCl2 or 1.5 M BaCl2 were sspiked on the interface between the iron coupon and bentonite, for which the dry density was in the range of 1.4 to 1.5 Mg/m3, before assembling. The iron coupons were connected as working electrodes to the potentiostat and held at a constant supplied potential between - 500 to +300 mV (vs. Ag/AgCl reference electrode) for up to 7 days. Calcium and strontium could migrate faster and deeper into the bentonite than iron in each condition, while barium could migrate slower than iron. A model using dispersion and electromigration can explain the measured profiles in the bentonite specimens. The fitted value of electromigration velocity was a function of applied electrical potential and 10 to 23 nm/s for calcium, 11 to 19 for strontium, around 4 nm/s for barium and 5 to 15 nm/s for iron, respectively. Alternatively, the fitted value of the dispersion coefficient was not a function of applied potential, and the values were 3 - 8 × 10-12 m2/s for calcium, 2 - 4 × 10-12 m2/s for strontium, 5 - 10 × 10-12m2/s for barium and 3 - 9 × 10-12 m2/s for iron, respectively.

An Accelerated Method to Measure the Swelling Behaviour of Caprock (Shale) in Steam Stimulation Process

Abstract Shale formations are used to serve as hydraulic and thermal barriers in steam stimulated processes such as steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). They are potentially subjected to swelling and softening when they are exposed to fresh water. A conventional swell test on clay shale is very slow and it can take months or years. In this study, an accelerated swell test is proposed, which applies an electrical potential gradient through the specimen. The applied external electrical field accelerates the ionic flows and subsequently speeds up the swelling process. Experimental results of this study on reconstituted bentonite specimens saturated with sodium chloride solution have proved the proposed idea. Possible electrochemical reactions and their influence on the experimental set up have been discussed. A mathematical model based on coupling the flow equations in micro and macro levels has been developed to quantify this process. Introduction Multi-component mass transport under electrical, chemical and hydraulic gradients is gaining increasing attention and interest in different fields of engineering. Acar and Alshawabkeh(1–3) applied an electrical field through soils to extract contaminant ions from the soil. Roy and Cooper(4) and Cooper and Roy(5) proposed the application of an electrical field to prevent the bit balling during the drilling process. In this study, the electrical field is applied to accelerate the swell test in shale specimens. Shale formations are used to serve as an hydraulic and/or thermal barrier in oil production. The high tendency of shale to absorb fresh water and its high swelling potential may cause problems, such as drilling bit balling(6) or wellbore instability(7). Fresh water can access shale formations in different ways from different sources. Practical examples include: the use of steam and water in the oil recovery processes and the use of water-based drilling mud in borehole drilling. Control of swelling is critical in these practical applications, and thus, characterizing the swelling behaviour in shale formations is of great importance in these engineering activities. However, a conventional swell test on shale is very slow and it can take months or years(8). Thus, an accelerated swell test could be very helpful to overcome this problem in cost and time savings. Ion extraction from macro voids in shale and the subsequent induced concentration gradients along the macro voids and inter-particle micro channels are the main sources of swelling in shale. However, due to the low permeability of shale, the ion transport through the bulk fluid of this material is a time consuming process(8). Consequently, the fluid flow towards the inter-particle micro channels has a very low rate. If the ion extraction from the bulk fluid is accelerated, the fluid flow towards the micro channels will occur faster and the specimen swells more quickly. In this study, an electrical potential gradient is applied to accelerate the ion removal from shale which finally leads to a quick swell test. Experimental results on reconstituted pure bentonite specimens saturated with NaCl solution proved this idea.

Swelling pressure of a divalent‐rich bentonite: Diffuse double‐layer theory revisited

Physicochemical forces are responsible for the swelling pressure development in saturated bentonites. In this paper, the swelling pressures of several compacted bentonite specimens for a range of dry density of 1.10–1.73 Mg/m3were measured experimentally. The clay used was a divalent‐rich Ca‐Mg‐bentonite with 12% exchangeable Na+ions. The theoretical swelling pressure–dry density relationship for the bentonite was determined from the Gouy‐Chapman diffuse double‐layer theory. A comparison of experimental and theoretical results showed that the experimental swelling pressures are either smaller or greater than their theoretical counterparts within different dry density ranges. It is shown that for dry density of the clay less than about 1.55 Mg/m3, a possible dissociation of ions from the surface of the clay platelets contributed to the diffuse double‐layer repulsion. At higher dry densities, the adsorptive forces due to surface and ion hydration dominated the swelling pressures of the clay. A comparison of the modified diffuse double‐layer theory equations proposed in the literature to determine the swelling pressures of compacted bentonites and the experimental results for the clay in this study showed that the agreement between the calculated and experimental swelling pressure results is very good for dry densities less than 1.55 Mg/m3, whereas at higher dry densities the use of the equations was found to be limited.

Migration Behavior of Potassium and Rubidium in Compacted Bentonite Under Reducing Condition With Iron Corrosion Product

AbstractCarbon steel overpack will corrode by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The iron corrosion products can migrate in bentonite as ferrous ion through the interlayer of montmorillonite replacing exchangeable sodium ions in the interlayer. This replacement of sodium with ferrous ion may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore the authors have carried out electromigration experiments of potassium or rubidium with source of iron ions supplied by anode corrosion of iron coupon in compacted bentonite. Five to fifteen micro liter of tracer solution containing 3.3 M of KCl or 2.2 M of RbCl was spiked on the interface between an iron coupon and bentonite, which dry density was around 1.4 Mg/m3, before assembling. The iron coupon was connected as the working electrode to the potentiostat and was held at a constant supplied potential between - 600 and 300 mV vs. Ag/AgCl reference electrode for up to 8 days. Potassium could migrate faster and deeper in bentonite specimen than iron in each condition. On the other hand rubidium could migrate slower than iron. Migration velocity was a function of applied electrical potential and 8 to 14 nm/s for potassium, 5 to 10 nm/s for iron and 3 to 5 for rubidium, respectively. Dispersion coefficient was also a function of applied potential and 10 to 14 × 10−12 m2/s for potassium, 4 to 8 overv 10−12 m2/s for rubidium and 2 to 4 overv 10−12 m2/s for iron, respectively. Diffusion experiments were also carried out for comparison. Potassium and rubidium might migrate slightly slower in the altered bentonite by iron corrosion than in ordinary compacted bentonite.

Migration Behaviour of Ferrous Ion in Compacted Bentonite Under Reducing Conditions Controlled With Potentiostat

AbstractCarbon steel overpack is corroded by consuming oxygen introduced by repository construction after closure of the repository and then maintains the reducing environment in the vicinity of the repository. The migration of iron corrosion products through the buffer material will affect the migration of redox-sensitive radionuclides. Therefore, it is important to study the migration of iron corrosion products through the buffer material because it may affect the corrosion rate of overpack, and migration of redox-sensitive radionuclides. Electromigration experiments have been conducted with the source of iron ions supplied by anode corrosion of the iron coupon in compacted bentonite. The carbon steel coupon was connected as the working electrode to the potentiostat and was held at a constant supplied potential between - 650 to +300 mV vs. Ag/AgCl electrode for up to 168 hours. The amount of iron penetrated into a bentonite specimen was in good agreement with the calculated value from the corrosion current under the assumption that iron is dissolved as ferrous ions. A model using dispersion and electromigration could explain the measured iron profiles in the bentonite specimens. The fitted value of electromigration velocity depended on the potential supplied. On the other hand the fitted value of the dispersion coefficient did not depend on the potential supplied but a constant. This constant dispersion coefficient could be due to the much larger diffusion coefficient of ferrous ion in bentonite compared with the effect of mechanical dispersion. The experimental configurations used in this study are applicable to the examination of the migration behaviour of cations with the source of iron ions under a reducing condition controlled with a potentiostat.

A study on the thermal conductivity of compacted bentonites

Thermal conductivity of compacted bentonite is one of the most important properties in the design of high-level radioactive waste repositories where this material is proposed for use as a buffer. In the work described here, a thermal probe based on the hot wire method was used to measure the thermal conductivity of compacted bentonite specimens. The experimental results were analyzed to observe the effects of various factors (i.e. dry density, water content, hysteresis, degree of saturation and volumetric fraction of soil constituents) on the thermal conductivity. A linear correlation was proposed to predict the thermal conductivity of compacted bentonite based on experimentally observed relationship between the volumetric fraction of air and the thermal conductivity. The relevance of this correlation was finally analyzed together with other existing methods using experimental data on several compacted bentonites.

Artificial neural networks approach for swell pressure versus soil suction behaviour

In this study, the swell pressure versus soil suction behaviour was investigated using artificial neural networks (ANNs). To achieve this, the results of the total suction measurements using thermocouple psychrometer technique and constant-volume swell tests in oedometers performed on statically compacted specimens of Bentonite–Kaolinite clay mixtures with varying soil properties were used. Two different ANN models have been developed to predict the total suction and swell pressure. The ANNs results were compared with the experimental values and found close to the experimental results. Moreover, several performance indices such as correlation coefficient, variance account for (VAF), and root mean square error (RMSE) were calculated to check the prediction capacity of the ANN models developed. Both ANN models have shown a high prediction performance based on the performance indices. Therefore, it can be concluded that the initial soil suction is the most relevant state of suction that characterizes the potential swell pressures.

A Study on Gas Migration Behavior in Buffer Material using X-ray CT Method

ABSTRACTThis paper presents a study on gas migration behavior in a bentonite specimen with the aid of X-ray computer tomography (CT) scan data. The laboratory experiment was carried out to clarify gas migration behavior through saturated, compacted bentonite. X-ray CT was used to estimate the spatial distribution of gas and water saturation during gas migration test in the bentonite. For the gas migration test, the controlled flow rate of gas injection was adopted for pre-compacted samples of Kunigel V1 bentonite using helium gas, which is safer than hydrogen gas.A specimen was isotropically consolidated and saturated by synthetic seawater, simultaneously, by applying a backpressure. This was followed by injecting the gas using a syringe pump. Inlet and outlet gas fluxes were monitored. This test exhibited a significant threshold pressure for breakthrough, somewhat larger than the sum of the swelling pressure and the backpressure.The procedure of the X-ray CT measurement is as follows; i) measurement of the initial condition (saturated condition) of the compacted bentonite, ii) measurement of the gas injection condition as a function of time. The digital data obtained from the X-ray CT usually includes some noise. The stacking method can reduce the noise in CT values and enables to identify the gas migration area. The results indicate that gas is transported through preferential pathways in compacted bentonite, and is not homogenous.

Corrosion of carbon steel in compacted bentonite and its effect on neptunium diffusion under reducing condition

The corrosion of carbon steel and its effect on neptunium diffusion were studied by corrosion tests of carbon steel and neptunium diffusion tests under conditions designed with consideration of constituents of a repository for High-level Radioactive Waste disposal. The major constituents of the conditions were carbon steel, corrosion products pre-formed under aerobic conditions, compacted bentonite, reductant and a low-oxygen glove box. The corrosion tests were performed in the presence of water-saturated compacted bentonite. The neptunium diffusion tests were performed with and without carbon steel. The carbon steel and the bentonite were observed by taking photographs and analyzed by X-ray diffraction. The color of the corroded surface of the carbon steel changed from red/orange/black under aerobic conditions to a dark-green under reducing conditions. Portions of the bentonite changed their original color, pale-gray, to a brownish color after the test. Gas-induced cracks were also observed in some bentonite specimens. These changes were discussed combining thermodynamic predictions under the present experimental conditions. The results revealed that anaerobic corrosion of carbon steel occurred in the bentonite. The corrosion processes were also discussed. The corrosion rate of carbon steel was estimated to be ∼0.1 μm/year from iron profiles in the bentonite. This value was much lower than the conservative setting value 20 μm/year in the current performance assessment. The significant restriction in corrosion could be attributed to the presence of bentonite and corrosion products. Oxidation states of neptunium in the bentonite were investigated by thenoyltrifluoroacetone in xylene extraction technique. It was found that tetravalent neptunium accounted for 99% of the total neptunium in the presence of carbon steel and only 63% without. The results revealed that anaerobic corrosion of carbon steel could maintain strong reducing conditions to keep most tetravalent neptunium from being oxidized to pentavalent neptunium, thus effectively restraining migration of neptunium in the bentonite. The apparent diffusions were estimated to be in the order of 10 −15–10 −14 m 2/s for tetravalent neptunium and 10 −13–10 −12 m 2/s for the pentavalent. No significant effect of the corrosion products on neptunium sorption was found through comparing the apparent diffusion coefficients obtained in the diffusion with and without carbon steel. Corrosion of carbon steel under conditions relevant to the repository is essential for further studies, providing a confident and defensible value of corrosion rate for repository design. The effects of corrosion on radionuclide migration, especially redox-sensitive radionuclides in the buffer, should be considered for a reliable safety assessment of geological disposal.

A capillarity-advective model for gas break-through in clays

Laboratory testing has investigated how gases can break through compacted specimens of illite, bentonite, and sand–illite or sand–bentonite mixtures. Specimens were formed with a wide range of initial clay densities, water contents and degrees of saturation. Tests were done using two different test procedures. In one, equal increments of gas pressure were applied at constant time intervals until break-through was observed. In the second, the pressure was held constant, and the time required for break-through recorded. Results show that the pressure at break-through increases with clay density and decreases with degree of saturation. When the degree of saturation is below about 85% in illite and clay–illite, and 93% in bentonite and sand–bentonite, there is only a small resistance to gas migration. Above these degrees of saturation, break-through pressures rise sharply. In an approach that differs from some others that have been reported, it is postulated that gas migration is only possible when its pressure is higher than a Gas Entry Value (GEV) that is related to capillarity effects in the largest pores of the material. Thereafter, the rate of advance of the gas–water interface depends on advective flow, that is, on the pressure (hydraulic) gradient across the specimen. Analysis shows that times to break-through should decrease inversely with pressure increase and this was observed in the experiments. Tests were also done on specimens made with non-polar paraffin instead of water. This inhibited the development of bound water in diffuse double layers (DDLs) and led to break-through at much lower pressures.

Backanalysis of thermohydraulic bentonite properties from laboratory tests

The paper presents a general methodology to perform backanalysis of laboratory tests involving thermohydraulic behaviour of bentonite in a systematic manner. The procedure is based on a maximum likelihood approach that defines a probabilistic framework in which error measurements and the reliability of the parameters identified can be estimated. The method is applied to the identification of some thermal and hydraulic properties of a bentonite specimen, using temperature and water content measurements as input data. Three basic cases have been considered: (a) thermal case, identifying the global thermal conductivity, λ e, and the global specific heat, c e; (b) hydraulic case, identifying the tortuosity factor, τ, and the exponent of the unsaturated permeability law, n; and (c) thermohydraulic case, in which τ, n, and the saturated thermal conductivity, λ sat, have been estimated. A numerical code that solves the coupled thermohydraulic equations has been used as main computational tool. A detailed description of the experimental device and a discussion on the tests results and on the parameters identified are also included. The values obtained are within the normal range of these parameters, but the method provides a systematic and consistent procedure to find the best parameters that reproduce the measurements for the selected models. Also the method gives an insight into the model structure, and allows detecting dependence and coupling between parameters.

Apparatus and Procedures for Assessing Inorganic Diffusion Coefficients for Geosynthetic Clay Liners

Abstract The apparatus and procedure for performing tests to obtain inorganic diffusion coefficients for Geosynthetic Clay Liners (GCLs) are described, and the processes (diffusion, anion exclusion and osmosis) that can affect the interpretation of these tests are discussed. Results from several inorganic diffusion tests on GCL and bentonite specimens show that the diffusion coefficients deduced for sodium (Na+) and chloride (Cl−) are directly related to the final bentonite void ratio. It is shown that diffusion through GCLs (or thin bentonite layers) can be modeled using a bulk porous media diffusion coefficient (Dp) consisting of the total porosity, nt, and a deduced diffusion coefficient, Dt, (Dp = ntDt) without the need to establish the true effective diffusion coefficient () without the need to establish the true effective diffusion coefficient (De) and effective porosity (ne). However, for a longer bentonite plug sample it is shown that the true effective diffusion coefficient (De) and effective porosity (ne) must be established to predict the contaminant transport through the sample.