Hydromechanical behaviour of a heterogeneous compacted soil: experimental observations and modelling (original) (raw)
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Hydromechanical behaviour of bentonite pellet mixtures
Physics and Chemistry of The Earth, 2007
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 described the experimental program performed to characterize the hydromechanical behaviour of compacted pellet's mixtures used in the engineered barrier (EB) experiment.
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...
Influence of dry density and water content on the swelling of a compacted bentonite
Applied Clay Science, 2008
In the context of a project for the study of the behaviour of the clay barrier in a nuclear waste repository, the swelling properties of compacted bentonite have been investigated. Results on the influence of initial dry density and water content on the swelling pressure and swelling deformation of a compacted bentonite are presented. Swelling pressure is exponentially related to dry density but is rather independent of the initial water content of the clay. The swelling capacity is mainly affected by the vertical load under which saturation takes place. It increases with initial dry density but decreases as the initial water content is higher. The effect of the initial water content on the final swelling strain is less important for the low dry densities and the high vertical loads, becoming negligible for vertical loads close to the swelling pressure. These features of behaviour agree with the predictions of conceptual models that consider the interaction between the responses of the microstructure and the macrostructure of the material to suction and stress changes.
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.
Experimental study on the swelling behaviour of bentonite/claystone mixture
Engineering Geology, 2012
The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, as a possible sealing and backfill material in the French geological high-level radioactive waste disposal. In this study, the effects of the grain size distribution and the microstructure on the hydro-mechanical behaviour of the compacted crushed argillite have been investigated. The volume change properties were investigated by running one-dimensional compression tests under constant water content (2.4-2.8%) with loadingunloading cycles. Under various vertical stresses, water flooding tests were carried out under constantvolume condition. Depending on the vertical stress level, either swelling or collapse behaviour was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identified: finer samples exhibit stiffer compression behaviour and higher swelling potential. To provide a microstructure insight into the macroscopic behaviour feature observed, both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) observations were performed, evidencing that: (i) at the same dry density, the size of inter-aggregate pores is larger for the coarser crushed material; (ii) mechanical compression only reduces the inter-aggregate porosity in the stress range considered; (iii) the micro-mechanisms governing the flooding under constant-volume condition include the swelling of the clay particles, the increase of the intra-aggregate pores and the collapse of the inter-aggregate pores. The results show a strong effect of the grain size distribution on the hydro-mechanical behaviour and thus the close link between the microstructure and the hydro-mechanical behaviour.
Applied Clay Science, 2020
Mixtures composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80-bentonite are considered as materials, that could be used for backfilling a future radioactive waste repository in deep sedimentary rock formations. Their characterization is of interest, as the replacement of fractions of crushed claystone by bentonite enhances the chemo-hydro-mechanical performance of backfill. The materials are envisaged to be installed directly in the drifts and shafts by means of conventional compaction techniques. The hydro-mechanical behavior of materials containing expansive mineral phases, and especially their swelling behavior, is known to be significantly affected by the initial material properties and environmental and stress conditions. The present study aimed to assess the combined impact of variations in the material properties and environmental conditions, particularly the grain size distribution, dry density and saturating solution chemistry, on the swelling pressure of the mixtures, by conducting a comprehensive laboratory experimental program. The results revealed that the adjustment of the grain size distribution of employed bentonite enhanced the compaction behavior and, in turn, the swelling behavior of the mixtures. Generally, swelling pressures of mixtures were less affected by the employed saline and alkaline solutions than those of crushed claystone. The measured swelling pressures were exponentially related to the initial dry density of the expansive mineral phase, regardless of the grain size distribution. Based upon the finding that the expansive mineral phase being present in crushed claystone contributed to measured swelling pressures, a new approach was introduced to calculate the dry density of the expansive mineral phase in bentonites and their mixtures with non-expansive or less-expansive materials.
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.
Hydro mechanical behaviour of a heterogeneous swelling clay material
CRC Press eBooks, 2020
This paper presents the main results of a research program devoted to the experimental study and the modelling of the hydro mechanical behaviour of an heterogeneous swelling clay material. This material is constituted of a mixture of bentonite powder and high dry density bentonite pellets. It may be used to seal shafts and galleries thanks to its low permeability and large swelling capacity. The behaviour of the material during the hydration phase, observed experimentally on small scale samples, is compared to numerical simulations.
Investigation of the swelling behaviour of compacted bentonite/sand mixture by mock-up tests
Canadian Geotechnical Journal, 2014
Two small-scale mock-up tests were carried out on a compacted mixture of bentonite and sand, one with saturation from the bottom only and the other with saturation from the top and bottom. Swelling pressure was monitored at different positions of the soil sample in both axial and radial directions. Results showed that the two-side saturation accelerates the swelling pressure kinetics by a factor of 4. The radial swelling pressure kinetics depends on the soil swelling, soil collapse, and interaction between the soil layers. Further analysis based on the relative humidity measurements in an infiltration test performed previously showed that the variations of swelling pressure with suction are similar to those found in literature, and that threshold points exist indicating the initiation of soil collapse. These threshold points appeared at higher suction when the soil was farther from the wetting face. The failure of the axial confining elements in the storage galleries was also simulated by releasing the piston and allowing a limited axial swelling of 20%. The radial swelling pressures decreased sharply upon the piston release and during the free axial swelling. A swelling pressure gradient was found from the bottom to the top with a higher value at the bottom. The density gradient was estimated based on the swelling pressure profile.
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.