Hydromechanical behaviour of bentonite pellet mixtures (original) (raw)
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Pellet mixtures in isolation barriers
2009
Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydro-mechanical (HM) behaviour of compacted pellet mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen's dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range from 1.3 to 1.5 Mg/m 3. Pellets had a very high dry density, close to 2 Mg/m 3. The outstanding characteristic of these mixtures is their discontinuous porosity. Pore sizes of the compacted pellets varied around 10 nm. However, the inter-pellet size of the pores was four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Performed tests include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction-controlled oedometer tests and swelling pressure tests. The interpretation of some performed tests required back analysis procedures using a hydro-mechanical (HM) computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al. (1990) (Barcelona basic model, BBM). Parameters for the model were identified and also a set of hydraulic laws are necessary to perform coupled HM analysis. A large scale in-situ test (the "EB" test in Mont Terri, Switzerland) was described and analyzed. Rock barrier parameters were adjusted on the basis of available tests. The test excavation, barrier emplacement and forced hydration were simulated by means of the CODE_BRIGHT program. The comparison between measurements and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacements.
Engineered barrier of bentonite pellets and compacted blocks: State after reaching saturation
Engineering Geology, 2015
The EB experiment was a large-scale test performed in the Underground Research Laboratory of Mont Terri (Switzerland) for the demonstration of an engineered barrier concept for nuclear waste disposal consisting of the simultaneous use of high-density bentonite blocks and a lower-density bentonite pellets mixture (the granular buffer material, GBM). For that purpose, a gallery was excavated in the Opalinus clay and a dummy waste canister was placed on a bed made of bentonite blocks and surrounded by the GBM material. The bentonite barrier was artificially hydrated with Pearson water and after 10.5 years of operation at isothermal conditions it was considered that the bentonite was completely saturated and the dismantling of the barrier was undertaken. A sampling campaign was done to assess the final state of the bentonite barrier with regards to dry density and water content. Upon dismantling, the GBM looked perfectly homogeneous, with every void and gap between the different elements (blocks/GBM, GBM/host rock, GBM/canister, etc.) having been sealed. Full saturation had been reached all through the barrier. Moreover, the dry density of the blocks had decreased to values similar to those of the GBM, and the average water contents for both kinds of materials were similar. Nevertheless, the initial conditions of the system did have a certain impact on the final distribution of dry density and water content: the bottom of the barrier had a chance to quicker and higher water uptake (due to the heterogeneities in the initial porosity and characteristics of the artificial hydration system), which gave place to immediate swelling that resulted irreversible, with permanent higher water contents and lower dry densities towards the floor and back of the gallery, particularly in the GBM. Despite these heterogeneities, the water contents and dry densities of the whole barrier (GBM and blocks) were much more homogeneous than at the beginning of the test and remained within a relatively narrow range. The bentonite degree of saturation was homogeneous and very close to 100% all through the barrier. The feasibility and performance of this kind of initially heterogeneous barrier was proved in that it had an optimal sealing capacity and developed acceptable swelling pressures between 1.3 and 2.2 MPa.
Physics and Chemistry of the Earth, Parts A/B/C, 2008
The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests performed in cylindrical cells (length 60 cm, diameter 7 cm). Inside the cells, six blocks of FEBEX bentonite compacted to dry density 1.65 g/cm 3 were piled up, giving rise to a total length similar to the thickness of the clay barrier in a repository according to the Spanish concept. The bottom surface of the material was heated at 100°C and the top surface was injected with granitic water. The duration of the tests was 6, 12, 24 and 92 months. The temperatures inside the clay and the water intake were measured during the tests and, at the end, the cells were dismounted and the dry density, water content and hydro-mechanical properties were measured at different positions. The injection of water provokes, near the hydration surface, a decrease of the dry density due to the increase of the water content and the clay swelling, while heating gives rise to an increase of the dry density and a reduction of the water content in the hottest areas.
Soils and Foundations, 2013
Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure the disposal safety. In this study, a mixture of MX80 bentonite and sand was characterized in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil is referred to as the common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suctions, the amount of water absorbed in the soil depends solely on suction, whereas at low suctions it depends on both suction and bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite-sand mixture yielded at a higher pre-consolidation stress.
E3S Web of Conferences, 2020
In most of the concept designs for nuclear waste disposals, bentonite-based materials are used to seal underground galleries and shafts. In order to assess the safety of such engineered barriers and to provide a good prediction for the material behaviour, a number of experimental campaigns and constitutive models have been developed mainly focusing on bentonite compacted blocks. On the other hand, in recent years, the use of high density bentonite pellets combined with powdered bentonite has also been considered as an interesting alternative. The bentonite pellets mixtures present a prominent initial heterogeneity level as a result of the inter-pellets porosity, which evolves strongly during hydration. This paper aims to present the numerical modelling strategy adopted for a swelling pressure test on pellets mixture carried by CEA (France). Taking advantage of the finite element code LAGAMINE, the Barcelona Basic model is considered for the bentonite mechanical behaviour and the dou...
Environmental Earth Sciences, 2014
This study assesses the geotechnical performance of a compacted bentonite-sand mixture with a bentonite content ranging from 15 to 30 % by weight to be used as a material component in a waste sealing system. Geotechnical laboratory tests such as compaction, falling head permeability, swelling, unconfined compression and shear strength tests were conducted to select an optimum mixture which eventually led to a recommendation to select an optimum bentonite-sand mixture possessing a bentonite content of 30 % for the isolation of underground waste disposal facilities. Bentonite-sand seal design was performed for the optimum seal selected as a function of the axial stress applied to the seal and seal length-to-radius ratio.
Permeability and swelling characteristics of bentonite
International Journal of the Physical Sciences, 2010
Permeability and swelling characteristics of bentonite and bentonite-sand are the essential parameters for designing any type of waste disposal or in geo-environmental engineering applications. To design and construct these facilities accurate values of permeability for these bentonite-sand mixtures must be evaluated. For this purpose, a series of various laboratory tests were performed to investigate the coefficient of permeability using direct and indirect test methods derived from consolidation theory using liquid limit 1.5 of bentonite. Permeability tests were also carried out for dynamically compacted bentonite. Void ratio of bentonite is a key parameter of permeability for bentonite and bentonite-sand mixtures. The specimen manufacture method had no effect on permeability. A series of swelling pressure and deformation tests are performed using variable content (30 to 90%) of bentonite at initial dry density of 2 g/cm(3) to investigate the characteristics of buffer material for radioactive waste disposal. Content of bentonite in bentonite-sand mixture is the prime criteria of buffer material and must be taken into consideration in designing any types of waste disposal facilities. Content of bentonite and loading pressure on the specimens is noticeably influenced on maximum swelling rate.
Chemo Mechanical Behavior of Bentonite Sand Mixtures Inundated with Organic Pore Fluid 1
Bentonite-sand mixture is considered to be a potential liner material for landfill facilities owing to its excellent sorption potential and swelling characteristics. Such liner materials are expected to perform satisfactorily if the hydraulic conductivity is maintained less than 10 −7 cm/s and thereby inhibiting the migration of the generated leachate. However, the performance assessment of these liners in the presence of highly concentrated organic fluids and under the overburden stresses that is usually expected in landfill conditions is scarce. Therefore, in the present work, the hydraulic performance of the bentonite-sand mixtures in the presence of the organic compounds commonly encountered in the landfill leachate is studied, which is a need of the hour. Flow tests were performed to evaluate the hydraulic conductivity of different bentonite-sand mixtures in unhydrated conditions and permeated with varied proportions of methanol-water and ethanol-water mixtures. The study reveals that the proportions of the organic pore fluids significantly influence the hydraulic conductivity of the bentonite-sand mixtures. A very good linear relationship was observed between the pore fluids' dielectric constants and the hydraulic conductivity of the bentonite-sand mixtures. The chemomechanical behavior suggests that the conventional bentonite-sand mixtures perform poorly in terms of the hydraulic and swelling characteristics with the change in the dielectric constant of the organic fluids. The microstructural analysis also supports the experimental findings that as the dielectric constant of the fluid are reduced, the effective flow path through the soil increases considerably resulting in higher hydraulic conductivity.
Membrane behavior of compacted sand–bentonite mixture
Canadian Geotechnical Journal, 2017
Semi-permeable membrane behavior contributes to the containment function of engineered barriers used for waste containment by restricting the migration of dissolved chemical species (solutes) such as aqueous-phase contaminants. The existence of membrane behavior has been demonstrated extensively for virtually all categories of bentonite-based containment barriers except compacted sand–bentonite mixtures. Accordingly, membrane tests were conducted on two specimens of a compacted sand–bentonite mixture comprising 15% bentonite (dry weight) with sufficiently low hydraulic conductivity (i.e., ≤1.0 × 10−9 m/s) to be suitable for use as a waste containment barrier. Despite the imposition of relatively complex chemical conditions, including the use of tap water versus de-ionized water as a circulating liquid and incomplete flushing (leaching) of soluble salts from the specimens prior to membrane testing, the results were in good agreement with those previously reported for other bentonite-...
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Environmental Earth Sciences
For the deep geological disposal of high-level radioactive waste in argillaceous rocks, the heat production of the waste is an important driver for thermal-hydraulic-mechanical (THM)-coupled processes. These THM processes influence the properties and conditions of the near field that in many repository designs contains bentonite as a clay buffer. One task in the DECOVALEX-2015 (DEvelopment of COupled models and their VALidation against Experiments) project was the modelling of a heated bentonite column (Villar et al. in Long-term THM tests reports: THM cells for the HE-E test: update of results until February 2014. Deliverable-no: D2.2-7.3. CIEMAT Technical Report IEMAT/DMA/2G210/03/2014, 2014) in preparation for the in situ heater experiment HE-E at the underground rock laboratory Mont Terri. DECOVALEX is an international cooperative project that focuses on the development and validation of mathematical models for simulating such coupled processes associated with disposal in deep geological repositories. Eight modelling teams developed their own THM-coupled models for the bentonite column experiment, using six different simulation codes. Each of the teams individually calibrated the THM parameters for the bentonite material. The eight resulting parameter sets agree well and allow a satisfactory reproduction of the TH measurements by all models. The modelling results for the evolution of temperature and relative humidity over time at three sensors in the bentonite column are in good agreement between the teams and with the measured data. Also, changes of the temperature due to modifications of the insulation and the adjustment of the heating power during the course of the experiment are well reproduced. The models were thus able to reproduce the main physical processes of the experiment, both for vapour-dominated diffusion during the heating phase and combined liquid and vapour transport during a subsequent heating and hydration phase. Based on the parameter sets, the teams predict a penetration of the water infiltration front in the 48-cm column filled with bentonite pellets to a depth between 25 and 35 cm over the 15,000 h (i.e. over 20 months) of the hydration phase of the experiment.
EPJ Nuclear Sciences & Technologies
Pellet-based expansive clay materials are considered as a sealing material for closing the galleries in radioactive waste disposal concepts. In repository conditions, the granular mixture progressively homogenises upon hydration by the host rock pore water. The present study focuses on the material behaviour before homogenisation. A grain-scale experimental characterisation is first performed in the laboratory. A model describing the hydromechanical behaviour of a pellet is proposed based on the experimental results. Then, suction-controlled swelling pressure tests are performed in the laboratory. Using Discrete Element Method (DEM) and the model proposed for a single pellet, the tests are successfully simulated. It is highlighted that (i) the swelling pressure evolves in two phases in the investigated suction range, controlled by the granular structure of the mixture; (ii) wall effects at the laboratory scale affects the material response; (iii) measurement variability associated t...
Modelling the hydromechanical behaviour of expansive granular mixtures upon hydration
E3S Web of Conferences
Bentonite pellet-powder mixtures are candidate sealing materials in radioactive waste disposal concepts. The mixture is installed in galleries in dry state as a granular material. The material is progressively hydrated by the pore water of the host rock and becomes homogeneous. Before homogenisation, the granular structure controls the material behaviour. In the present work, a modelling approach able to address particular features of pellet-powder mixtures is introduced. Two domains are considered: i) granular, and ii) homogeneous. The material behaviour before homogenisation is studied through Discrete Element Method (DEM) simulations. Constitutive laws for the granular state are proposed from DEM results. The behaviour of the homogenised material is described by a modified Barcelona Basic Model (BBM). Transition from granular to homogeneous states depends on suction and relative volume fractions of pellets and powder. Swelling pressure tests performed in the laboratory are satisf...
Acta Geotechnica, 2022
Despite the increasing understanding of bentonite behaviour, there is still missing evidence on how different hydro-mechanical loadings, including sequences of hydration and compression, affect the fabric and the volume change behaviour of the material. It is generally assumed that the interplay between the behaviour of clay assemblages and the overall fabric of the material is the reason of having final states that are dependent on the stress path followed. Here the results of an experimental campaign aiming to study these factors are reported and discussed. Free swelling and swelling pressure tests were performed, both followed by compression to a relatively high stress. The experimental program involved various samples that were dismantled at intermediate states in order to perform microstructural observations by means of mercury intrusion porosimetry and electronic scanning microscopy. It was observed that while the void ratio at a given stress level depends on the stress path, ...
Engineering Geology, 2020
Bentonite pellet-powder mixtures are candidate materials for sealing the galleries in deep geological repositories for radioactive waste. In the present work, swelling pressure tests are performed on pellet-powder mixtures with different powder contents. Results highlight the influence of the initial granular structure on the mechanical behaviour of pellet-powder mixtures. In mixtures with a low density powder in the inter-pellet porosity, the macroscopic response of pellet-powder mixtures is identical to that of a pellet assembly with no powder. A model is proposed to describe the hydromechanical behaviour of pellet-powder mixtures. The formulation considers two distinct states of the material, Granular and Continuous. In the Granular state, the pellets control the mechanical behaviour of the mixture. In the Continuous state, both pellets and powder contribute to the mechanical behaviour of the mixture. In the Granular domain, the material behaviour is described by constitutive laws proposed after a numerical study using Discrete Element Method. In the Continuous domain, the material behaviour is described by a modified Barcelona Basic Model. Transition between the two domains depends on the density of the powder phase and suction. The model is implemented in a Finite Element Method code, and the swelling pressure tests performed in the laboratory are simulated with a single set of parameters. These results improve the knowledge on the behaviour of bentonite pellet-powder mixtures during hydration in repository conditions.
Geotechnical and Geological Engineering, 2008
With the advent of modern microstructural testing techniques and microstructure based constitutive models the microstructural characterisation of soils is gaining prominence. This paper reviews the history of microstructure investigation in unsaturated soils and discusses the engineering significance of this research to date. After a brief overview of the main microstructural techniques, the paper focuses on the evaluation of the current state of use and the development of two widely used techniques to study the microstructure of partially saturated soils, namely mercury intrusion porosimetry and the environmental scanning electron microscopy. The details of these techniques, their advantages and limitations, are first covered, followed by the presentation of selected test results. These results highlight the use of these techniques for understanding different hydro-mechanical behavioural features observed at macroscopic scale. Specifically, the paper shows the use of these techniques to explore the fundamental properties of water retention characteristics, water permeability, and micro and macrostructural interactions along different hydro-mechanical paths.
Microstructure and hydro-mechanical behaviour of compacted granular bentonite
E3S Web of Conferences
In recent years, granular bentonite GB has become a reference material since it facilitates the backfilling operation due to high-density compacted granules with extended grain size distribution(maximum sizes around 10 mm). This investigation explores the initial microstructure of MX80-type GB and the hydro-mechanical HM response observed at the phenomenological scale. Samples were statically compacted at a fixed dry density (1.55 Mg/m3 ), encompassing dry and wet sides of the optimum. Themicrostructural study was performed using a combination of X-ray micro-computed tomography and mercury intrusion porosimetry. The results indicate that the pore size density function of compacted GB can be simplified as a double-porosity network with micropores (intra-granular/aggregate pores) and macropores(inter-granular/aggregate pores). Compacting at the dry side increases macropores, making the sample more compressible on loading under as-compacted states despite the higher matric suction. The...
Applied Sciences, 2018
The study discusses the issue of introducing drilling fluid (bentonite) into the environment during the construction of linear underground investments, considering the example of the construction of the high-pressure gas pipeline, Czeszów—Kiełczów DN1000, which involved the use of the Horizontal Directional Drilling (HDD) method. The analyses concern the drilling stage as a low-waste technology, indicating the use and management of bentonite drilling fluid in a closed circulation cycle in the area of Pęciszów, poviat Trzebnica (Poland). The loss of drilling fluid in valuable natural areas during the construction stage of the gas pipeline has been analyzed. Drilling fluid is an element of the horizontal directional drilling technology (HDD). The analyzed area included a section of the route of the strategic gas pipeline, realized in June–July 2017 in an area of lowland ash and alder forest 91E0-3, a probable breeding site of the Bluethroat for a detailed description and common frog. ...
Environmental Earth Sciences, 2017
Process understanding and parameter identification using numerical methods based on experimental findings are a key aspect of the international cooperative project DECOVALEX. Comparing the predictions from numerical models against experimental results increases confidence in the site selection and site evaluation process for a radioactive waste repository in deep geological formations. In the present phase of the project, DECOVALEX-2015, eight research teams have developed and applied models for simulating an in-situ heater experiment HE-E in the Opalinus Clay in the Mont Terri Rock Laboratory in Switzerland. The modelling task was divided into two study stages, related to prediction and interpretation of the experiment. A blind prediction of the HE-E experiment was performed based on calibrated parameter values for both the Opalinus Clay, that were based on the modelling of another in-situ experiment (HE-D), and modelling of laboratory column experiments on MX80 granular bentonite and a sand/bentonite mixture .. After publication of the experimental data, additional coupling functions were analysed and considered in the different models. Moreover, parameter values were varied to interpret the measured temperature, relative humidity and pore pressure evolution. The analysis of the predictive and interpretative results reveals the current state of understanding and predictability of coupled THM behaviours associated with geologic nuclear waste disposal in clay formations..
Homogenization of unsaturated bentonite during hydration
E3S web of conferences, 2023
Bentonite barriers are a key component in may designs of deep geological repositories for high level nuclear waste. During the hydration stage, the bentonite undergoes non-uniform changes in dry density that may persist even after reaching a fully saturated state. Since dry density controls the properties of the bentonite that ensure the functions of safety of the barrier, the potential of bentonite for homogenization or otherwise is a matter of high relevance. The paper presents the results and the analyses of two hydration tests on initially heterogenous samples, especially designed to explore the phenomena and processes underlying the homogenisation of bentonite materials. The formulation and constitutive model used in the analyses are briefly presented followed by a description of the tests. Subsequently, the experimental data obtained from the tests are discussed together with the results of the hydromechanical analyses performed. Particular attention is given to the degree of homogenization of the specimens achieved at the end of the tests. The simulations have resulted in a very satisfactory agreement with test observations, especially regarding the final state of the samples, thus enhancing the confidence in the numerical tool employed.
Acta Geotechnica, 2021
In the context of the French Cigéo-project, a mixture composed of 70% processed Callovo-Oxfordian claystone spoil and 30% MX80-bentonite could be a potential backfill material, whose installation aims to stabilize the surrounding rock formation and to limit the propagation of the excavation damaged zone. The backfill material must sustain the overburden pressure, despite it might be exposed to different hydraulic and mechanical paths. The reference concept considers employing conventional compaction techniques, although their employment involves spatial variations in the dry density after compaction. In general, as the initial dry density has a significant impact on the hydro-mechanical behavior of backfill materials, it is of major importance to relate the variations in the initial dry density to differences in the behavior. This experimental laboratory study aimed to analyze how variations in the initial dry density affects the swelling and compression behavior of the claystone/ bentonite-mixture, in particular in unsaturated state. Further, it evaluated whether those variations affected possible hydromechanical path dependences. The experimental program comprised suction-controlled oedometer and constant-volume swelling pressure experiments, in which samples characterized by different initial dry densities were exposed to different hydro-mechanical paths. The analysis of microstructural and water retention characteristics complemented the program. Major results indicated that the magnitude of swelling pressure at a given suction depends considerably on the initial dry density, but it is independent of the imposed hydro-mechanical path. Interestingly, the dependency of the yield behavior on the hydromechanical path appears to be more pronounced as the initial dry density increases.
Applied Clay Science, 2023
Bentonite-based materials are employed in some nuclear waste disposal concept designs to seal underground tunnels and shafts. In some cases, these barriers consist of two parts: highly compacted blocks and granular buffer material consisting of pellets. These two components have highly distinct initial properties in terms of dry density, water content, and microstructure, but, at full water saturation, those tend to homogenise. However, the simultaneous application of pellets and blocks in the same barrier section creates challenges for understanding and modelling system performance that must be tackled. Therefore, this work merges experimental and computational approaches to better understand the hydro-mechanical processes that take place throughout the interaction of the various assemblies with one another and with the environment during hydration. Hence, the role of a variety of material configurations, considering simultaneously compacted blocks and pellets, and hydration boundary conditions is analysed giving great insight about intermediate saturation states. Discrepancies between experimental measurements and model results were explained by the effect of friction after a sensitivity analysis performed with the finite element code LAGAMINE. Results from experiments and calculations were in good agreement and offer supplementary knowledge about a relevant amount of the numerous phenomena (for instance related to dry density evolution and water distribution inside the sample) taking place during initial heterogeneous bentonite samples resaturation in isochoric conditions.
Environmental Earth Sciences, 2017
Modeling the mechanical behavior of expansive clays is of interest in understanding the performance of nuclear waste disposal designs that include clay materials as buffers around waste containers, backfill for underground openings, seals between adjacent openings, or as host-rock constituents. The buffers, backfill, and seals will be unsaturated during construction and will re-saturate at varying rates after cessation of disposal operations. The mechanical behavior of the clay materials during re-wetting could affect the pressure on waste containers, other engineered components, or the host rock and could influence fluid flow and radionuclide transport. The authors describe an approach to mechanical modeling of unsaturated soils using the moisture retention characteristic curve and the Bishop principle of effective stress to evaluate suction effects on stress. The approach incorporates swelling, thermal expansion, and soil hardening and stiffening due to suction or compaction and uses stress-strain relationships based on elastoplasticity. Suction contributions to soil stress, strength, and stiffness are evaluated using the same moisture retention characteristic curve that is a standard input for hydrological modeling. The model is illustrated through several numerical simulations of oedometer free-swell and confined-swell testing of a bentonite-sand mixture and a granular bentonite. Results are presented with and without the effect of physicochemical swelling to illustrate magnitude of the swelling influence on independent variables and different mechanical responses. The model prediction is highly dependent on the swelling/shrinkage behavior of bentonite, which could be a source of uncertainty in estimating potential pressure due to a bentonite buffer in a nuclear waste repository design.
Related papers
Hydraulic and mechanical properties of compacted bentonite after 18 years in barrier conditions
Applied Clay Science, 2018
The FEBEX "in situ" test was performed at an underground laboratory in Grimsel (Switzerland) with the aim of studying the behaviour of components in the near-field of a nuclear waste repository. A gallery of 2.3 m in diameter was excavated through the granite and two heaters, simulating the thermal effect of the wastes, were placed inside, surrounded by a barrier of highly-compacted bentonite blocks. In 2015, after 18 years of operation, the experiment was dismantled. Some of the bentonite samples taken were tested in the laboratory to characterize, among others, their physical state and determine their permeability and swelling capacity. There were significant changes in water content and dry density across the bentonite barrier: their distribution was radial around the axis of the gallery, with the water content decreasing from the granite towards the axis of the gallery and the dry density following the inverse pattern. The swelling capacity of the samples was related to their position in the barrier. In the internal, drier part of the barrier an increase of the swelling capacity with respect to the reference bentonite was detected, whereas the samples from the external part swelled less than expected. This was attributed to the different salinity of the samples. The hydraulic conductivity was mainly related to the dry density of the samples and decreased with respect to the reference bentonite. This decrease was not related to the position of the samples and could be related to the microstructural reorganization of the bentonite during the 18-year operation-which brought about an average decrease in the pore size-and to the low hydraulic gradients applied to determine the permeability of the samples retrieved.
Observed heterogeneities after hydration of MX-80 bentonite under pellet/powder form
Applied Clay Science, 2020
Bentonite is considered for engineered barriers in radioactive waste geological repositories due to its low permeability, swelling pressure and radionuclide retention properties. The French concept favors the use of bentonite under granular form-pellets and crushed pellets mix-for an easier building of plugs and seals relatively to bricks. However, this introduces a significant degree of macroscopic initial heterogeneity (at several scales) that is expected to disappear via mass redistribution during resaturation in order to obtain homogeneous hydraulic and mechanical performance. This also may have practical effects on the reliability of results from tests with small volumes of bentonite compatible with the time scales manageable at the laboratory. In this study, isochoric swelling pressure tests at various densities and sample sizes were carried out and evidenced bias mainly due to the size of the cell compared to the size of the pellets, although the average density is equivalent, pointing towards incomplete homogenization. Large-scale swelling pressure tests, however, converged towards values previously measured for more finely grained materials. Local pressure measurements also evidenced some stress heterogeneities that do not seem to disappear at medium timescales. These stress heterogeneities could be correlated to local density variations of the final state assessed using destructive measurements and X-ray computed tomography.
A parametric study on the hydromechanical properties of claystone/bentonite mixtures
E3S web of conferences, 2023
Mixtures of crushed excavated COx claystone and bentonite are considered as a potential backfill solution for the French disposal concept of radioactive waste. In the present work, the hydro-mechanical behaviour of claystone/sodium bentonite mixtures was experimentally investigated for various bentonite contents (≤40% in total mass) and densities. Demineralized, site water and cementitious alkaline solution were used. Test on mixtures physicochemical properties showed that Plasticity Index (PI) and Free Swelling Index (FSI) values increase with bentonite content. Introducing bentonite in the mixture induces to lower density for the same compaction energy. No clear impact of the cementitious solution was observed compared to the site water. The expected swelling pressure increase upon bentonite increment was obtained. The effect was more important at higher densities. Mixtures primary wetting, resulted in lower swelling pressure.
Minerals Engineering, 2009
This study investigates the feasibility of using bentonite-paste tailings (BPT) as a barrier (liner, cover) material for mine waste containment facilities. Improvements of the hydraulic properties are realized by using compaction to densify the paste tailings in the first stage and mixing an additive-like natural bentonite to paste tailings to further reduce the voids that control hydraulic conductivity in the second stage. A significant decrease in hydraulic conductivity is observed with these transformations. Values that are as low as 1 Â 10 À9 and 4 Â 10 À9 cm/s are obtained in 8% and 4% bentonite and BPT, respectively. Based on these satisfactory values, additional investigations are conducted to evaluate the freeze-thaw and wet-drying performance of BPT up to 5 and 6 cycles, respectively. The results show that negligible to acceptable changes in hydraulic conductivity occur. None of the changes reach one order of magnitude. As a final step, a cost analysis is undertaken to evaluate the economical benefits that can be obtained from such new material. When compared to conventional compacted clay-bentonite barrier or sand-bentonite with 12% bentonite concentration, it is found that the 4% BPT is less expensive by 66%. The results place this recycled BPT material as a promising candidate for barrier design while reducing the amount of waste to be managed and the cost of surface tailings management.
Thermo-hydro-mechanical behavior of compacted bentonite-sand mixtures: an experimental study
2008
Heat, gas, and leachate are primary by-products of landfill processes in municipal solid waste landfills. In nuclear waste repository, temperature of the waste also raises due to radioactivity processes. Temperature increase in the repository induces hydro-mechanical processes of its sealing material. Moderate to high temperature is expected to be encountered in the field situation. In this thesis, a study on the thermo-hydro-mechanical behavior of compacted bentonite-sand mixtures which are among the materials proposed to be used as sealing material for landfills and hazardous waste repository is presented. Mixtures of a calcium-type bentonite, Calcigel, and quartz sand were used in this study. Series of tests including suction and swelling pressure measurement, drying-wetting under unconfined and confined conditions were conducted at a moderately high temperature. Tests at room temperature including basic and physico-chemical characterization, microstructure and fabric studies, an...
Compressibility of Compacted Clays Mixed with a Wide Range of Bentonite for Engineered Barriers
Arabian Journal for Science and Engineering, 2019
Compacted clay-bentonite mixtures are often used as compacted blocks and landfills in high-level radioactive waste repositories and clay liners, respectively. A better understanding of their compressibility behaviour is essential to guarantee the disposal safety. In this study, mixtures of clays and bentonite are used to investigate the compressibility and compaction behaviour. Three natural clays were selected with different physical characteristics, and clay-bentonite mixtures were prepared by mixing the bentonite content up to 50%. A series of standard compaction tests and consolidation tests were performed in the laboratory; the samples for the consolidation test were prepared at the optimum moisture contents and maximum dry unit weights. With increasing bentonite content, the liquid limit, plasticity index, optimum moisture content, initial void ratio and compression index of natural clays increase and maximum dry unit weight and yield stress decrease. It was observed that the effect of bentonite on geotechnical characteristics pronounced when bentonite content exceeded 10%. Correlations were also proposed to estimate the compression characteristics and compression curves of compacted clays using three physical parameters such as the initial void ratio, optimum moisture content and maximum dry unit weight.
Modelling of oedometer tests on pellet-powder bentonite mixtures to support mock-up test analysis
E3S Web of Conferences, 2020
Bentonite mixtures of MX-80 (80% of high-density pellets and 20% of bentonite powder on a mass basis) have been recently proposed as a candidate material for sealing deep geological disposals of high-level radioactive waste. A loading/unloading oedometer test at constant water content has been performed on this mixture, which has been modelled using the finite element Code_Bright. The constitutive model used to represent the mechanical response is the Barcelona Expansive Model (BExM), since a multi-modal pore size distribution characterises the pore network of the mixture. During compression at constant water content, an increase in the degree of saturation and a consequent reduction of suction is induced. Consequently, two competing effects occur at different pore-size scales: (a) compression due to mean net stress increase; and (b) expansion on induced suction reduction that mainly affects the micro-porosity level inside aggregates. A sensitivity analysis has been performed to exp...
Compacted sand–bentonite mixtures for the confinement of waste landfills
Acta geotechnica, 2024
This paper illustrates the results of an experimental study on sand-bentonite mixtures for their use as confinement barriers for solid waste landfills. The mixtures have been prepared parametrically varying the percentage of bentonite. The sample preparation method was established willing to simulate the compaction processes on site. In fact, the compacted samples were tested following two different stress-wetting paths representative of the possible stress and imbibition sequences occurring on a landfill confinement barrier. In the first case, the barrier comes into contact with rainwater before being subjected to the overloading stress induced by waste disposal, while, in the second case, the barrier is overloaded by the waste before being wetted by the leachate. The compressibility and permeability of the sand-bentonite mixtures were determined, in both cases, by oedometric compression tests. The experimental results are analysed and compared in order to evaluate the influence of the bentonite content on the mechanical and hydraulic behaviour of the mixture. Interpretation of the results is also accomplished with a micro-mechanical investigation of the mixtures fabric. Suitable compositions of sand and bentonite are finally proposed for the design of effective confinement barriers. Keywords Bentonite compaction Á Bentonite swelling Á Bentonite hydraulic conductivity Á Compaction of sand-bentonite mixtures Á Hydraulic conductivity of sand-bentonite mixtures Á Oedometer test on bentonite Á Oedometer test on sandbentonite mixtures Á Sand-bentonite mixtures Á Swelling of sand-bentonite mixtures Á Waste landfills confinement & Erminio Salvatore
2016
In this study, natural bentonite was used as natural<br> clay material and samples were taken from the Kalecik district in<br> Ankara. In this research, bentonite is the subject of an analysis from<br> standpoint of assessing the basic properties of engineered barriers<br> with respect to the buffer material. Bentonite and sand mixtures were<br> prepared for tests. Some of clay minerals give relatively higher<br> hydraulic conductivity and lower swelling pressure. Generally,<br> hydraulic conductivity of these type clays is lower than <10-12 m/s.<br> The hydraulic properties of clay-sand mixtures are evaluated to<br> design engineered barrier specifications. Hydraulic conductivities of<br> bentonite-sand mixture were found in the range of 1.2x10-10 to<br> 9.3x10-10 m/s. Optimum B/S mixture ratio was determined as 35% in<br> terms of hydraulic conductivity and mechanical stability. At the<br> second stag...
A Study on Compressibility, Swelling and Permeability Behaviour of Bentonite–Sand Mixture
2019
In geotechnical engineering field, bentonite–sand mixtures have been proposed and used as engineered barriers for containing the waste. This paper presents the laboratory evaluation of compressibility by performing one-dimensional consolidation tests on six different mixtures of bentonite with sand. The bentonite–sand mixtures were formed by varying sand content in bentonite in increments of 5% from 5 to 25% by dry weight. Dry bentonite–sand mixtures were placed initially in the consolidation cell at their loosest dry state and then allowed to saturate. Swelling characteristics and swelling pressures of the bentonite–sand mixtures were also evaluated. This paper also presents the laboratory evaluation of permeability of the bentonite–sand mixtures by performing falling head test after every load increment during the consolidation test. This study arrived at the conclusion that amount of swelling (expressed in percentage) and swelling pressure decreased with addition of sand. Moreove...
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