In Switzerland,the Opalinus Clay has been selected as a potential host rock for the deep geological disposal of radioactive waste due to its low hydraulic conductivity and favorable swelling properties.During the oper...In Switzerland,the Opalinus Clay has been selected as a potential host rock for the deep geological disposal of radioactive waste due to its low hydraulic conductivity and favorable swelling properties.During the operational phase of the repository,the host rock will be exposed to pH values as high as 13.5 due to concrete degradation,which will certainly affect its hydraulic properties.This study investigates the effect of pH increase on the water retention properties of Opalinus Clay.A series of samples from the lower sandy facies of the Mont Terri site in Switzerland,at initial dry state,were exposed to a hyperalkaline solution of pH=13.5 and to the synthetic water of pH=7.5 at different water contents.After equilibrium,the total suction was measured with a dew point potentiometer and microstructural analyses were conducted via mercury intrusion porosimetry(MIP)and nitrogen adsorption-desorption technique.It was found that the total suction decreased with hydration and pH increase.Since the two investigated solutions have the same osmotic suction,the decrease in total suction was attributed to the decrease in matric suction.Indeed,the total porosity increased with saturation and pH increase.This was confirmed by MIP data that evidenced an increase in the proportion of macropores,and by Barret eJoynereHalenda(BJH)data that showed mesopore generation.The specific surface area(SSA)also increased.The previous results were due to mineral hydration and,exclusively in the case of alkaline solution,to(1)the dissolution of quartz and calcite and(2)the acid-base reactions,which were concentrated at the edges of the clay particles,leading to an increase in negatively charged groups and thus to a face-to-face association of the clay particles(dispersion),causing an increase in the repulsive forces between the clay particles.In addition,the weakening of covalent bonds led to the primary dissolution of clay minerals,i.e.silicon and aluminum detachment.展开更多
In many engineering applications,it is important to determine both effective rock properties and the rock behavior which are representative for the problem’s in situ conditions.For this purpose,rock samples are usual...In many engineering applications,it is important to determine both effective rock properties and the rock behavior which are representative for the problem’s in situ conditions.For this purpose,rock samples are usually extracted from the ground and brought to the laboratory to perform laboratory experiments such as consolidated undrained(CU)triaxial tests.For low permeable geomaterials such as clay shales,core extraction,handling,storage,and specimen preparation can lead to a reduction in the degree of saturation and the effective stress state in the specimen prior to testing remains uncertain.Related changes in structure and the effect of capillary pressure can alter the properties of the specimen and affect the reliability of the test results.A careful testing procedure including back-saturation,consolidation and adequate shearing of the specimen,however,can overcome these issues.Although substantial effort has been devoted during the past decades to the establishment of a testing procedure for low permeable geomaterials,no consistent protocol can be found.With a special focus on CU tests on Opalinus Clay,this study gives a review of the theoretical concepts necessary for planning and validating the results during the individual testing stages(saturation,consolidation,and shearing).The discussed tests protocol is further applied to a series of specimens of Opalinus Clay to illustrate its applicability and highlight the key aspects.展开更多
Deep repository in geological formations is the preferential solution considered in many countries to manage high-level nuclear wastes. In Switzerland, the Opalinus Clay is a candidate host rock. In this context, in s...Deep repository in geological formations is the preferential solution considered in many countries to manage high-level nuclear wastes. In Switzerland, the Opalinus Clay is a candidate host rock. In this context, in situ and laboratory tests are conducted on Opalinus Clay to demonstrate the feasibility of deep disposal in this argillaceous formation. This paper presents a constitutive model able to fit the experimental data obtained from some triaxial tests conducted by Jahns(2013) on cores from borehole Schlattingen SLA-1. The elasto-plastic behaviour of Opalinus Clay is reproduced thanks to a DruckerPrager model, taking into account the anisotropy behaviour of this sedimentary rock. The objective is to employ a single set of parameters representative of the material. In a second version of the model, the stress-dependence of the elastic properties and damage are taken into account. Finally, the parameters calibrated with experimental tests are used to simulate the excavation of a gallery with a second gradient approach.展开更多
Shales are considered to be sealing units for geological carbon storage and suitable host rocks for nuclear waste repositories due to their low permeability.However,the presence of fractures within these formations ca...Shales are considered to be sealing units for geological carbon storage and suitable host rocks for nuclear waste repositories due to their low permeability.However,the presence of fractures within these formations can significantly alter their flow,transport,and deformation behavior,which is central to the safe implementation of underground storage projects.Fractures not only increase the overall permeability of the rock but also contribute to its anisotropic behavior.This study focuses on characterizing the hydro-mechanical response of a natural fracture to aqueous fluid injection within a shaly specimen of Opalinus Clay.Laboratory experiments were performed to measure the mechanical and flow properties of intact and fractured rock specimens.Subsequently,a three-dimensional(3D)numerical model of water injection into the fractured specimen was developed.This model explicitly accounts for fracture geometry with strain-dependent aperture changes based on the cubic law assumption.Experimental measurements indicate that the fractured shale exhibits permeability up to two orders of magnitude higher than that of the intact counterpart.However,the simulations reveal that fracture permeability locally spans up to eight orders of magnitude.This significant change in permeability affects fluid flow within the rock specimen.The numerical model best reproduces the experimental results for a normal stiffness of the natural fracture of 18.7 MPa/mm at effective mean stresses below 15 MPa,and of 187.2 MPa/mm at higher confinements.This outcome highlights the critical importance of defining the hydro-mechanical parameters of fractures under realistic effective stress conditions with far-reaching implications for secure underground storage.展开更多
The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay.The emplacement drifts for high-level waste(approximately 3.5 m diameter)are ...The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay.The emplacement drifts for high-level waste(approximately 3.5 m diameter)are planned to be excavated with a shielded tunnel boring machine(TBM)and supported by a segmental lining.At the repository depth of 900 m in the designated siting region Nordlich Lagern,squeezing conditions may be encountered due to the rock strength and the high hydrostatic pressure(90 bar).This paper presents a detailed assessment of the shield jamming and lining overstressing hazards,considering a stiff lining(resistance principle)and a deformable lining(yielding principle),and proposes conceptual design solutions.The assessment is based on three-dimensional transient hydromechanical simulations,which additionally consider the effects of ground anisotropy and the desaturation that may occur under negative pore pressures generated during the drift excavation.By addressing these design issues,the paper takes the opportunity to analyse some more fundamental aspects related to the influences of anisotropy and desaturation on the development of rock convergences and pressures over time,and their markedly different effects on the two lining systems.The results demonstrate that,regardless of these effects,shield jamming can be avoided with a moderate TBM overcut,however overstressing of a stiff lining may be critical depending on whether the ground desaturates.This uncertainty is eliminated using a deformable system with reasonable dimensions of yielding elements,which can also accommodate thermal strains generated due to the high temperature of the disposal canisters.展开更多
文摘In Switzerland,the Opalinus Clay has been selected as a potential host rock for the deep geological disposal of radioactive waste due to its low hydraulic conductivity and favorable swelling properties.During the operational phase of the repository,the host rock will be exposed to pH values as high as 13.5 due to concrete degradation,which will certainly affect its hydraulic properties.This study investigates the effect of pH increase on the water retention properties of Opalinus Clay.A series of samples from the lower sandy facies of the Mont Terri site in Switzerland,at initial dry state,were exposed to a hyperalkaline solution of pH=13.5 and to the synthetic water of pH=7.5 at different water contents.After equilibrium,the total suction was measured with a dew point potentiometer and microstructural analyses were conducted via mercury intrusion porosimetry(MIP)and nitrogen adsorption-desorption technique.It was found that the total suction decreased with hydration and pH increase.Since the two investigated solutions have the same osmotic suction,the decrease in total suction was attributed to the decrease in matric suction.Indeed,the total porosity increased with saturation and pH increase.This was confirmed by MIP data that evidenced an increase in the proportion of macropores,and by Barret eJoynereHalenda(BJH)data that showed mesopore generation.The specific surface area(SSA)also increased.The previous results were due to mineral hydration and,exclusively in the case of alkaline solution,to(1)the dissolution of quartz and calcite and(2)the acid-base reactions,which were concentrated at the edges of the clay particles,leading to an increase in negatively charged groups and thus to a face-to-face association of the clay particles(dispersion),causing an increase in the repulsive forces between the clay particles.In addition,the weakening of covalent bonds led to the primary dissolution of clay minerals,i.e.silicon and aluminum detachment.
基金funded by the Swiss Federal Nuclear Safety Inspectorate ENSI (Grant no. H-100897)
文摘In many engineering applications,it is important to determine both effective rock properties and the rock behavior which are representative for the problem’s in situ conditions.For this purpose,rock samples are usually extracted from the ground and brought to the laboratory to perform laboratory experiments such as consolidated undrained(CU)triaxial tests.For low permeable geomaterials such as clay shales,core extraction,handling,storage,and specimen preparation can lead to a reduction in the degree of saturation and the effective stress state in the specimen prior to testing remains uncertain.Related changes in structure and the effect of capillary pressure can alter the properties of the specimen and affect the reliability of the test results.A careful testing procedure including back-saturation,consolidation and adequate shearing of the specimen,however,can overcome these issues.Although substantial effort has been devoted during the past decades to the establishment of a testing procedure for low permeable geomaterials,no consistent protocol can be found.With a special focus on CU tests on Opalinus Clay,this study gives a review of the theoretical concepts necessary for planning and validating the results during the individual testing stages(saturation,consolidation,and shearing).The discussed tests protocol is further applied to a series of specimens of Opalinus Clay to illustrate its applicability and highlight the key aspects.
文摘Deep repository in geological formations is the preferential solution considered in many countries to manage high-level nuclear wastes. In Switzerland, the Opalinus Clay is a candidate host rock. In this context, in situ and laboratory tests are conducted on Opalinus Clay to demonstrate the feasibility of deep disposal in this argillaceous formation. This paper presents a constitutive model able to fit the experimental data obtained from some triaxial tests conducted by Jahns(2013) on cores from borehole Schlattingen SLA-1. The elasto-plastic behaviour of Opalinus Clay is reproduced thanks to a DruckerPrager model, taking into account the anisotropy behaviour of this sedimentary rock. The objective is to employ a single set of parameters representative of the material. In a second version of the model, the stress-dependence of the elastic properties and damage are taken into account. Finally, the parameters calibrated with experimental tests are used to simulate the excavation of a gallery with a second gradient approach.
基金the funding from the European Research Council(ERC)under the European Union's Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST(www.georest.eu)under Grant agreement No.801809IMEDEA is an accredited“Maria de Maeztu Excellence Unit”(Grant CEX2021-001198+2 种基金funded by MICIU/AEI/10.13039/501100011033)Iman R.Kivi also acknowledges the funding from the Engineering and Physical Sciences Research Council through the UKRI Postdoc Guarantee Award THMC4CCS(Grant No.EP/X026019/1)Hyunbin Kim and Roman Y.Makhnenko are supported by the US National Science Foundation(Grant No.CMMI-2239630).
文摘Shales are considered to be sealing units for geological carbon storage and suitable host rocks for nuclear waste repositories due to their low permeability.However,the presence of fractures within these formations can significantly alter their flow,transport,and deformation behavior,which is central to the safe implementation of underground storage projects.Fractures not only increase the overall permeability of the rock but also contribute to its anisotropic behavior.This study focuses on characterizing the hydro-mechanical response of a natural fracture to aqueous fluid injection within a shaly specimen of Opalinus Clay.Laboratory experiments were performed to measure the mechanical and flow properties of intact and fractured rock specimens.Subsequently,a three-dimensional(3D)numerical model of water injection into the fractured specimen was developed.This model explicitly accounts for fracture geometry with strain-dependent aperture changes based on the cubic law assumption.Experimental measurements indicate that the fractured shale exhibits permeability up to two orders of magnitude higher than that of the intact counterpart.However,the simulations reveal that fracture permeability locally spans up to eight orders of magnitude.This significant change in permeability affects fluid flow within the rock specimen.The numerical model best reproduces the experimental results for a normal stiffness of the natural fracture of 18.7 MPa/mm at effective mean stresses below 15 MPa,and of 187.2 MPa/mm at higher confinements.This outcome highlights the critical importance of defining the hydro-mechanical parameters of fractures under realistic effective stress conditions with far-reaching implications for secure underground storage.
文摘The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay.The emplacement drifts for high-level waste(approximately 3.5 m diameter)are planned to be excavated with a shielded tunnel boring machine(TBM)and supported by a segmental lining.At the repository depth of 900 m in the designated siting region Nordlich Lagern,squeezing conditions may be encountered due to the rock strength and the high hydrostatic pressure(90 bar).This paper presents a detailed assessment of the shield jamming and lining overstressing hazards,considering a stiff lining(resistance principle)and a deformable lining(yielding principle),and proposes conceptual design solutions.The assessment is based on three-dimensional transient hydromechanical simulations,which additionally consider the effects of ground anisotropy and the desaturation that may occur under negative pore pressures generated during the drift excavation.By addressing these design issues,the paper takes the opportunity to analyse some more fundamental aspects related to the influences of anisotropy and desaturation on the development of rock convergences and pressures over time,and their markedly different effects on the two lining systems.The results demonstrate that,regardless of these effects,shield jamming can be avoided with a moderate TBM overcut,however overstressing of a stiff lining may be critical depending on whether the ground desaturates.This uncertainty is eliminated using a deformable system with reasonable dimensions of yielding elements,which can also accommodate thermal strains generated due to the high temperature of the disposal canisters.
