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Nathalie Conil - One of the best experts on this subject based on the ideXlab platform.
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Thermo-Poro-Elastic Behaviour of a Transversely Isotropic Shale: Thermal Expansion and Pressurization
Rock Mechanics and Rock Engineering, 2020Co-Authors: Philipp Braun, Siavash Ghabezloo, Pierre Delage, Jean Sulem, Nathalie ConilAbstract:The Callovo-Oxfordian (COx) Claystone is considered as a candidate host rock for a deep geological radioactive waste repository in France. Due to the exothermic waste packages, the rock is expected to be submitted to temperatures up to 90 °C. The temperature rise induces deformations of the host rock, together with an increase in pore pressures, involving complex thermo-hydro-mechanical (THM) couplings. This study aims to better characterize the THM response of the COx Claystone to temperature changes in the laboratory. To this end, Claystone specimens were tested in a temperature controlled, high pressure isotropic compression cell, under stress conditions close to the in-situ ones. Thermal loads were applied on the specimens along different heating and cooling paths. A temperature corrected strain gage system provided precise measurements of the anisotropic strain response of the specimens. Drained and undrained thermal expansion coefficients in both transversely isotropic directions were determined. The measurement of pore pressure changes in undrained condition yielded the thermal pressurization coefficient. All parameters were analysed for their compatibility within the thermo-poro-elastic framework, and their stress and temperature dependency was identified.
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Assessment of Swelling Pressure Determination Methods with Intact Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2020Co-Authors: Feng Zhang, Yu-jun Cui, Nathalie Conil, Jean TalandierAbstract:There are several methods for the determination of soil swelling pressure. In this study, such common methods were assessed using intact Callovo-Oxfordian (COx) Claystone samples extracted in the direction perpendicular to the bedding plane, in the French Underground Research Laboratory in Bure. Higher swelling pressure was found with the swell-consolidation method, the swelling pressures determined by other methods being comparable. The swelling line that defined the swelling limit was lower than the consolidation curve, confirming the limitation of the swell-consolidation method in determining soil swelling pressure. Moreover, the swelling pressure corresponding to the in situ void ratio was expected to be 12 MPa–14 MPa, while the value corresponding to the sample void ratio after sampling was estimated between 0.3 and 1.9 MPa, much lower than the expected one. This large difference might be attributed to the Claystone rebound due to unloading and Claystone damage due to sampling. It could be also due to the natural material variability in terms of mineralogy and dry density. Further studies are, thus, needed for clarifying this point.
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In Situ Investigation of the THM Behavior of the Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2020Co-Authors: Nathalie Conil, Manon Vitel, Carlos Plua, Minh Ngoc Vu, Darius Seyedi, Gilles ArmandAbstract:The thermo-hydro-mechanical behavior of the host rock is essential while designing an underground radioactive waste disposal repository, and in particular, while considering the long-term safety of the facility. In 2000, the French National Radioactive Waste Management Agency (Andra) started constructing an underground research laboratory located in the Meuse Haute Marne to carry out a research program aiming to demonstrate the feasibility of constructing and operating a radioactive waste disposal facility in the Callovo-Oxfordian Claystone, and to optimize its implementation. To study the thermo-hydro-mechanical effects of the early thermal phase on the clay host rock of a deep repository, Andra has performed various in situ heating tests; one of them is called the TED experiment. The aim of the TED experiment was to measure the evolution of the temperature and pore pressure fields around several heaters and to back-analyze the thermo-hydro-mechanical properties of the Callovo-Oxfordian Claystone. Thermal conductivity and heat capacity values were determined based on the back-analysis of the in situ measurements and compared to those measured on samples. The in situ experimental data and numerical model confirm the anisotropic behavior of the Claystone. The TED experiment results demonstrate the ability of current models to predict the evolution of temperature and pore pressure in the far field of disposal cells.
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Investigating the contribution of Claystone to the swelling pressure of its mixture with bentonite
'EDP Sciences', 2020Co-Authors: Zhixiong Zeng, Yu-jun Cui, Nathalie Conil, Jean TalandierAbstract:Compacted MX80 bentonite/Callovo-Oxfordian (COx) Claystone mixture has been considered as a possible sealing/backfilling material in the French deep geological radioactive waste disposal. The swelling pressure of such mixture is an important factor in the design and long-term safety assessment of deep geological repositories. In this study, constant-volume swelling pressure tests were performed on the mixtures with different Claystone fractions and dry densities. The test results show that the swelling pressure of the mixtures decreased with the increasing Claystone fraction and decreasing dry density. According to the experimental results, the contribution of Claystone to the global swelling pressure was further investigated. It was found that the deformation of Claystone and its contribution to swelling pressure was highly dependent on the Claystone fraction. As the Claystone fraction was larger than 30%, the Claystone in the mixture swelled, contributing to the global swelling pressure; On the contrary, as the Claystone fraction was less than 30%, the swelling of Claystone was inhibited by the bentonite and it worked an inert material without any contribution to the swelling pressure
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Swelling behavior of unsaturated Claystone/ bentonite mixtures
'EDP Sciences', 2020Co-Authors: Marvin Middelhoff, Jean Talandier, Olivier Cuisinier, Farimah Masrouri, Nathalie ConilAbstract:This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian Claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of Claystone/ bentonite mixtures remained questionable
Jean Talandier - One of the best experts on this subject based on the ideXlab platform.
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Assessment of Swelling Pressure Determination Methods with Intact Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2020Co-Authors: Feng Zhang, Yu-jun Cui, Nathalie Conil, Jean TalandierAbstract:There are several methods for the determination of soil swelling pressure. In this study, such common methods were assessed using intact Callovo-Oxfordian (COx) Claystone samples extracted in the direction perpendicular to the bedding plane, in the French Underground Research Laboratory in Bure. Higher swelling pressure was found with the swell-consolidation method, the swelling pressures determined by other methods being comparable. The swelling line that defined the swelling limit was lower than the consolidation curve, confirming the limitation of the swell-consolidation method in determining soil swelling pressure. Moreover, the swelling pressure corresponding to the in situ void ratio was expected to be 12 MPa–14 MPa, while the value corresponding to the sample void ratio after sampling was estimated between 0.3 and 1.9 MPa, much lower than the expected one. This large difference might be attributed to the Claystone rebound due to unloading and Claystone damage due to sampling. It could be also due to the natural material variability in terms of mineralogy and dry density. Further studies are, thus, needed for clarifying this point.
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Investigating the contribution of Claystone to the swelling pressure of its mixture with bentonite
'EDP Sciences', 2020Co-Authors: Zhixiong Zeng, Yu-jun Cui, Nathalie Conil, Jean TalandierAbstract:Compacted MX80 bentonite/Callovo-Oxfordian (COx) Claystone mixture has been considered as a possible sealing/backfilling material in the French deep geological radioactive waste disposal. The swelling pressure of such mixture is an important factor in the design and long-term safety assessment of deep geological repositories. In this study, constant-volume swelling pressure tests were performed on the mixtures with different Claystone fractions and dry densities. The test results show that the swelling pressure of the mixtures decreased with the increasing Claystone fraction and decreasing dry density. According to the experimental results, the contribution of Claystone to the global swelling pressure was further investigated. It was found that the deformation of Claystone and its contribution to swelling pressure was highly dependent on the Claystone fraction. As the Claystone fraction was larger than 30%, the Claystone in the mixture swelled, contributing to the global swelling pressure; On the contrary, as the Claystone fraction was less than 30%, the swelling of Claystone was inhibited by the bentonite and it worked an inert material without any contribution to the swelling pressure
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Swelling behavior of unsaturated Claystone/ bentonite mixtures
'EDP Sciences', 2020Co-Authors: Marvin Middelhoff, Jean Talandier, Olivier Cuisinier, Farimah Masrouri, Nathalie ConilAbstract:This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian Claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of Claystone/ bentonite mixtures remained questionable
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Investigation of swelling pressure of bentonite/Claystone mixture in the full range of bentonite fraction
Applied Clay Science, 2019Co-Authors: Zhixiong Zeng, Yu-jun Cui, Feng Zhang, Nathalie Conil, Jean TalandierAbstract:Abstract MX80 bentonite/Callovo-Oxfordian (COx) Claystone mixture has been proposed as a sealing/backfilling material in a deep geological repository of radioactive waste in France. A good understanding of the swelling behaviour of this mixture is essential when evaluating the long-term performance of the repository. In this work, the swelling pressure of MX80 bentonite/COx Claystone mixture was investigated by the constant-volume method for a full range of bentonite fraction. Results show that the swelling of Claystone in the mixture can be inhibited by bentonite and its contribution to the global swelling pressure depends on the bentonite fraction. For the mixture with >70% bentonite, Claystone behaves as an inert material, and its contribution to the global swelling pressure can be ignored. However, for the mixture with less bentonite, the swelling of Claystone will significantly contribute to the global swelling pressure. A method was proposed allowing the swelling pressure of bentonite/Claystone mixture to be predicted in the full range of bentonite fraction.
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fundamental aspects of the hydromechanical behaviour of callovo oxfordian Claystone from experimental studies to model calibration and validation
Computers and Geotechnics, 2017Co-Authors: Gilles Armand, Nathalie Conil, Jean Talandier, Darius SeyediAbstract:Abstract Within the framework of feasibility studies for a reversible, deep geological repository of high- and intermediate-level long-lived radioactive waste (HLW, IL-LLW), the French National Radioactive Waste Management Agency (Andra) is investigating the Callovo-Oxfordian (COx) Claystone formation near Bure (northeast France) as a potential host formation for the repository. In 2000, Andra initiated construction of the Meuse Haute-Marne Underground Research Laboratory (MHM URL). Concurrently with on-site construction work, an extensive research program has been conducted, including theoretical and experimental studies and constitutive modelling. In-situ experiments provide an extensive data set for performance evaluation of models describing the thermo-hydro-mechanical behaviour of COx Claystone under different conditions (excavation, heat, etc.). In 2012, a benchmark exercise was initiated to evaluate proposed models, including basic assumptions, mathematical descriptions, input variables and parameters, and to determine how these assumptions influence model outcomes with respect to in-situ experimental observations. The present paper outlines the main features of the hydromechanical behaviour of COx Claystone and presents a series of laboratory test results (triaxial compression and creep tests) providing model development teams with a coherent database for model calibration and validation.
Pierre Delage - One of the best experts on this subject based on the ideXlab platform.
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Thermo-Poro-Elastic Behaviour of a Transversely Isotropic Shale: Thermal Expansion and Pressurization
Rock Mechanics and Rock Engineering, 2020Co-Authors: Philipp Braun, Siavash Ghabezloo, Pierre Delage, Jean Sulem, Nathalie ConilAbstract:The Callovo-Oxfordian (COx) Claystone is considered as a candidate host rock for a deep geological radioactive waste repository in France. Due to the exothermic waste packages, the rock is expected to be submitted to temperatures up to 90 °C. The temperature rise induces deformations of the host rock, together with an increase in pore pressures, involving complex thermo-hydro-mechanical (THM) couplings. This study aims to better characterize the THM response of the COx Claystone to temperature changes in the laboratory. To this end, Claystone specimens were tested in a temperature controlled, high pressure isotropic compression cell, under stress conditions close to the in-situ ones. Thermal loads were applied on the specimens along different heating and cooling paths. A temperature corrected strain gage system provided precise measurements of the anisotropic strain response of the specimens. Drained and undrained thermal expansion coefficients in both transversely isotropic directions were determined. The measurement of pore pressure changes in undrained condition yielded the thermal pressurization coefficient. All parameters were analysed for their compatibility within the thermo-poro-elastic framework, and their stress and temperature dependency was identified.
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Drained Triaxial Tests in Low-Permeability Shales: Application to the Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2018Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Nathalie ConilAbstract:Drained triaxial testing is challenging in low-permeability Claystones (10−20 m2). This paper presents a method of testing low-permeability clay rocks in a standard triaxial cell. In this system, the resaturation of the specimen and the drainage conditions were enhanced by reducing the drainage length to 19 mm, the specimen radius. To do so, two geotextiles were placed around the top and bottom ends of the specimen, with no connection between them. Resaturation was hence performed by forcing water infiltration into the specimen from the upper and lower geotextiles, with a maximum infiltration length of around 19 mm, resulting in reasonable saturation durations. High-precision local measurements of radial strains were also achieved by ensuring direct contact between the LVDT rod and the specimen through the membrane. A poroelastic numerical calculation was carried out, and it was shown that, with these drainage conditions, a strain rate of 6.6 × 10−8 s−1 was satisfactory to ensure good drainage when shearing Claystone specimens. After a check test made on a low-permeability sandstone with well-known mechanical characteristics, two tests were carried out to investigate specimens of the Callovo-Oxfordian Claystone, a possible host rock for deep geological disposal in France. The results compare well with other published data from drained triaxial tests.
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thermal volume changes and creep in the callovo oxfordian Claystone
Rock Mechanics and Rock Engineering, 2017Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Nathalie ConilAbstract:The Callovo-Oxfordian (COx) Claystone is considered as a potential host rock for high-level radioactive waste disposal at great depth in France. Given the exothermic nature of radioactive wastes, a temperature elevation planned to be smaller than 100 °C will affect the host rock around the disposal cells. To gain better understanding of the thermal volumetric response of the COx Claystone, a new thermal isotropic compression cell was developed with particular attention devoted to monitoring axial and radial strains. To do so, a high-precision LVDTs system ensuring direct contact between the LVDT stem and the Claystone sample through the membrane was developed. A short drainage length (10 mm) was also ensured so as to allow full saturation of the sample under stress conditions close to in situ, and fully drained conditions during compression. High-precision strain monitoring allowed to observe a volumetric creep under stress conditions close to in situ. A drained heating test under constant stress carried out afterwards up to 80 °C exhibited a thermoelastic expansion up to a temperature of 48 °C, followed by thermoplastic contraction at higher temperature. Creep volume changes, that appeared to be enhanced by temperature, were modelled by using a simple Kelvin–Voigt model, so as to estimate the instantaneous response of the COx Claystone and to determine its thermal expansion coefficient. The temperature at which the transition between thermal expansion and contraction appeared is close to the maximum burial temperature of the Callovo-Oxfordian Claystone, estimated at 50 °C. This is in agreement with what has been already observed on the Opalinus Clay by Monfared et al. (2012) that was interpreted as a thermal hardening phenomenon, showing that the material kept the memory of the highest temperature supported during its geological history.
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poroelasticity of the callovo oxfordian Claystone
Rock Mechanics and Rock Engineering, 2017Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Anh Minh Tang, Hamza Menaceur, Nathalie ConilAbstract:This work is devoted to an experimental investigation of the poroelastic behavior of the Callovo–Oxfordian Claystone, a potential host rock for the deep underground repository of high-level radioactive waste in France. Drained, undrained, pore pressure loading and unjacketed tests were carried out in a specially designed isotropic compression cell to determine the poroelastic parameters of fully saturated specimens. Great care was devoted to the saturation procedure, and small loading rates were used to ensure full drainage conditions in drained and pore pressure tests (0.5 kPa/min) and in the unjacketed test (2 kPa/min). High-precision strain measurements were performed by ensuring direct contact between the LVDT stems and the specimen. An analysis in the framework of transverse isotropic poroelasticity provided the Biot effective stress coefficients b1 (perpendicular to bedding) between 0.85 and 0.87 and b2 (parallel to bedding) between 0.90 and 0.98 under different stress conditions (pore pressure 4 MPa, total isotropic stresses of 14 and 12 MPa, respectively). A set of equivalent isotropic poroelastic parameters was also determined and a very good compatibility between the results of different tests was found, giving confidence in the parameters determined. The unjacketed test provided a directly reliable measurement of the unjacketed modulus (Ks = 21.7 GPa) that was afterward confirmed by an indirect evaluation that showed the non-dependency of Ks with respect to the stress level. These parameters were obtained for specimens cored and trimmed in the laboratory. A parametric study was then conducted so as to provide an estimation of the parameters in situ, i.e., not submitted to the damage supported by laboratory specimens. A minimal value b = 0.77 seems to be a reasonable lower bound for the equivalent isotropic Biot parameter.
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The Status of Water in Swelling Shales: An Insight from the Water Retention Properties of the Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2016Co-Authors: Hamza Menaceur, Anh Minh Tang, Pierre Delage, Jean TalandierAbstract:The Callovo-Oxfordian (COx) Claystone is considered in France as a possible host rock for the disposal of high-level long-lived radioactive waste at great depth. During the operational phase, the walls of the galleries and of the disposal cells will be successively subjected to desaturation induced by ventilation followed by resaturation once the galleries are closed. To better understand this phenomenon, a sound understanding of the water retention properties of the COx Claystone is necessary. Following a previous study by the same group, this paper presents an investigation of microstructure changes in COx Claystone under suction changes. Microstructure was investigated by means of mercury intrusion porosimetry tests on freeze-dried specimens previously submitted to various suctions. Along the drying path, the initial microstructure, characterised by a well-classified unimodal pore population around a mean diameter value of 32 nm, slightly changed with the same shape of the PSD curve and slightly moved towards smaller diameters (27–28 nm) at suctions of 150 and 331 MPa, respectively. The infra-porosity too small to be intruded by mercury (diameter smaller than 5.5 nm) reduced from 4.3 to 3.3 %. Oven drying reduced the mean diameter to 20 nm and the infra-porosity to 1 %. Wetting up to 9 MPa suction leads to saturation with no significant change in the PSD curve, whereas wetting at zero suction gave rise to the appearance of a large pore population resulting from the development of cracks with width of several micrometres, together with an enlargement of the initial pore population above the mean diameter. The concepts describing the step hydration of smectites (by the successive placement within the clay platelets along the smectite faces of 1, 2, 3 and 4 layers of water molecules with respect to the suction applied) appeared relevant to better understand the changes in microstructure of the COx Claystone under suction changes. This also allowed to better define the status of water in Claystones and shales containing smectite, with a distinction made between the water adsorbed within the clay platelets, and the free inter-platelet water involved in hydromechanical couplings through changes in pore pressure and water transfers.
K. A. Bogus - One of the best experts on this subject based on the ideXlab platform.
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Late Cretaceous stratigraphy and paleoceanographic evolution in the Great Australian Bight Basin based on results from IODP Site U1512
Gondwana Research, 2020Co-Authors: K. G. Macleod, L. T. White, C. C. Wainman, Mathieu Martinez, M. M. Jones, Sietske Batenburg, L. Riquier, S. J. Haynes, D. K. Watkins, K. A. BogusAbstract:The Upper Cretaceous sedimentary sequence at International Ocean Discovery Program Site U1512 in the Ceduna Sub-basin of the Great Australian Bight represents a continuous, > 690 m thick interval of black silty clay and Claystone spanning the lower Turonian through Lower Campanian (~10 million years). Sediments were deposited in an elongate, ~E-W oriented, ~2500 km long rift system that developed between Australia and Antarctica with an open-ocean connection to the west and a continental bridge to the east. Site U1512 cores provide a unique, continuous record of Late Cretaceous deposition in the Ceduna Sub-basin on the hanging wall of the Wallaroo Fault Zone. Study of U1512 samples could provide both an important high-latitude, southern hemisphere perspective on climatic evolution during the peak and demise of the Cretaceous hothouse and an offshore record of the sedimentation history in the basin during the Late Cretaceous portion of the Gondwanan breakup. The Upper Cretaceous sequence at Site U1512 is notable for its lithologic uniformity. Burrow-mottled to massive Claystone and silty Claystone make up the majority of the almost 700 m section, while rare (n = 28) isolated, 2 to 21 cm thick medium to fine sandstone beds are a minor lithology. Macrofossils present include common inoceramids and rare occurrences of other bivalves and ammonites. Microfossils include common occurrences of calcareous nannofossils, agglutinated and calcareous benthic foraminifera, radiolaria and organic-walled dinoflagellate cysts as well as rare small, surface dwelling planktonic foraminifera. Carbonate (
Siavash Ghabezloo - One of the best experts on this subject based on the ideXlab platform.
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Thermo-Poro-Elastic Behaviour of a Transversely Isotropic Shale: Thermal Expansion and Pressurization
Rock Mechanics and Rock Engineering, 2020Co-Authors: Philipp Braun, Siavash Ghabezloo, Pierre Delage, Jean Sulem, Nathalie ConilAbstract:The Callovo-Oxfordian (COx) Claystone is considered as a candidate host rock for a deep geological radioactive waste repository in France. Due to the exothermic waste packages, the rock is expected to be submitted to temperatures up to 90 °C. The temperature rise induces deformations of the host rock, together with an increase in pore pressures, involving complex thermo-hydro-mechanical (THM) couplings. This study aims to better characterize the THM response of the COx Claystone to temperature changes in the laboratory. To this end, Claystone specimens were tested in a temperature controlled, high pressure isotropic compression cell, under stress conditions close to the in-situ ones. Thermal loads were applied on the specimens along different heating and cooling paths. A temperature corrected strain gage system provided precise measurements of the anisotropic strain response of the specimens. Drained and undrained thermal expansion coefficients in both transversely isotropic directions were determined. The measurement of pore pressure changes in undrained condition yielded the thermal pressurization coefficient. All parameters were analysed for their compatibility within the thermo-poro-elastic framework, and their stress and temperature dependency was identified.
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Drained Triaxial Tests in Low-Permeability Shales: Application to the Callovo-Oxfordian Claystone
Rock Mechanics and Rock Engineering, 2018Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Nathalie ConilAbstract:Drained triaxial testing is challenging in low-permeability Claystones (10−20 m2). This paper presents a method of testing low-permeability clay rocks in a standard triaxial cell. In this system, the resaturation of the specimen and the drainage conditions were enhanced by reducing the drainage length to 19 mm, the specimen radius. To do so, two geotextiles were placed around the top and bottom ends of the specimen, with no connection between them. Resaturation was hence performed by forcing water infiltration into the specimen from the upper and lower geotextiles, with a maximum infiltration length of around 19 mm, resulting in reasonable saturation durations. High-precision local measurements of radial strains were also achieved by ensuring direct contact between the LVDT rod and the specimen through the membrane. A poroelastic numerical calculation was carried out, and it was shown that, with these drainage conditions, a strain rate of 6.6 × 10−8 s−1 was satisfactory to ensure good drainage when shearing Claystone specimens. After a check test made on a low-permeability sandstone with well-known mechanical characteristics, two tests were carried out to investigate specimens of the Callovo-Oxfordian Claystone, a possible host rock for deep geological disposal in France. The results compare well with other published data from drained triaxial tests.
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thermal volume changes and creep in the callovo oxfordian Claystone
Rock Mechanics and Rock Engineering, 2017Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Nathalie ConilAbstract:The Callovo-Oxfordian (COx) Claystone is considered as a potential host rock for high-level radioactive waste disposal at great depth in France. Given the exothermic nature of radioactive wastes, a temperature elevation planned to be smaller than 100 °C will affect the host rock around the disposal cells. To gain better understanding of the thermal volumetric response of the COx Claystone, a new thermal isotropic compression cell was developed with particular attention devoted to monitoring axial and radial strains. To do so, a high-precision LVDTs system ensuring direct contact between the LVDT stem and the Claystone sample through the membrane was developed. A short drainage length (10 mm) was also ensured so as to allow full saturation of the sample under stress conditions close to in situ, and fully drained conditions during compression. High-precision strain monitoring allowed to observe a volumetric creep under stress conditions close to in situ. A drained heating test under constant stress carried out afterwards up to 80 °C exhibited a thermoelastic expansion up to a temperature of 48 °C, followed by thermoplastic contraction at higher temperature. Creep volume changes, that appeared to be enhanced by temperature, were modelled by using a simple Kelvin–Voigt model, so as to estimate the instantaneous response of the COx Claystone and to determine its thermal expansion coefficient. The temperature at which the transition between thermal expansion and contraction appeared is close to the maximum burial temperature of the Callovo-Oxfordian Claystone, estimated at 50 °C. This is in agreement with what has been already observed on the Opalinus Clay by Monfared et al. (2012) that was interpreted as a thermal hardening phenomenon, showing that the material kept the memory of the highest temperature supported during its geological history.
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poroelasticity of the callovo oxfordian Claystone
Rock Mechanics and Rock Engineering, 2017Co-Authors: Malik Belmokhtar, Pierre Delage, Siavash Ghabezloo, Anh Minh Tang, Hamza Menaceur, Nathalie ConilAbstract:This work is devoted to an experimental investigation of the poroelastic behavior of the Callovo–Oxfordian Claystone, a potential host rock for the deep underground repository of high-level radioactive waste in France. Drained, undrained, pore pressure loading and unjacketed tests were carried out in a specially designed isotropic compression cell to determine the poroelastic parameters of fully saturated specimens. Great care was devoted to the saturation procedure, and small loading rates were used to ensure full drainage conditions in drained and pore pressure tests (0.5 kPa/min) and in the unjacketed test (2 kPa/min). High-precision strain measurements were performed by ensuring direct contact between the LVDT stems and the specimen. An analysis in the framework of transverse isotropic poroelasticity provided the Biot effective stress coefficients b1 (perpendicular to bedding) between 0.85 and 0.87 and b2 (parallel to bedding) between 0.90 and 0.98 under different stress conditions (pore pressure 4 MPa, total isotropic stresses of 14 and 12 MPa, respectively). A set of equivalent isotropic poroelastic parameters was also determined and a very good compatibility between the results of different tests was found, giving confidence in the parameters determined. The unjacketed test provided a directly reliable measurement of the unjacketed modulus (Ks = 21.7 GPa) that was afterward confirmed by an indirect evaluation that showed the non-dependency of Ks with respect to the stress level. These parameters were obtained for specimens cored and trimmed in the laboratory. A parametric study was then conducted so as to provide an estimation of the parameters in situ, i.e., not submitted to the damage supported by laboratory specimens. A minimal value b = 0.77 seems to be a reasonable lower bound for the equivalent isotropic Biot parameter.