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
Pre-compacted elements （disks, tutus） of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal. Choice of this material is mainly based on its swelling capacity allowing all ...Pre-compacted elements （disks, tutus） of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal. Choice of this material is mainly based on its swelling capacity allowing all gaps in the system to be sealed, and on its low permeability. When emplaced in the gallery, these elements will start to absorb water from the host rock and swell. Thereby, a swelling pressure will develop in the radial direction against the host rock and in the axial direction against the support structure. In this work, the swelling pressure of a small scale compacted disk of bentonite and sand was experimentally studied in both radial and axial directions. Different swelling kinetics were identified for different dry densities and along different directions. As a rule, the swelling pressure starts increasing quickly, reaches a peak value, decreases a little and finally stabilises. For some dry densities, higher peaks were observed in the radial direction than in the axial direction. The presence of peaks is related to the microstructure change and to the collapse of macro- pores. In parallel to the mechanical tests, microstructure investigation at the sample scale was conducted using microfocus X-ray computed tomography （BCT）. Image observation showed a denser structure in the centre and a looser one in the border, which was also confirmed by image analysis. This structure heterogeneity in the radial direction and the occurrence of macro-pores close to the radial boundary of the sample can explain the large peaks observed in the radial swelling pressure evolution. Another interesting result is the higher anisotropy found at lower bentonite dry densities, which was also analysed by means ofμCT observation of a sample at low bentonite dry density after the end of test. It was found that the macro-pores, especially those between sand grains, were not filled by swelled bentonite, which preserved the anisotropic microstructure caused by uniaxial compression due to the absence of microstructure collapse.展开更多
In the context of research into deep nuclear waste disposal,various works have concerned the hydromechanical behavior of Boom clay,a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the...In the context of research into deep nuclear waste disposal,various works have concerned the hydromechanical behavior of Boom clay,a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the Mol City(Belgium),at a depth of 223 m.Due to some amount of smectite minerals in the clay fraction,Boom clay exhibits swelling properties when hydrated under low stresses.To investigate some aspects of the hydromechanical behavior of Boom clay,oedometer compression tests were carried out on samples of Boom clay close to saturation and submitted to an initial suction.During oedometer compression,the changes in suction with increased vertical stress are monitored by means of a high capacity tensiometer installed at the bottom of the sample.Some aspects related to hydromechanical couplings are examined through the investigation of the changes in suction during oedometer compression,a somewhat delicate and poorly documented experimental approach.A comparison is also made with a completely different soil sample under suction,i.e.a statically compacted unsaturated low plasticity silt.Some technical difficulties typical of this new experimental approach are first described in detail so as to optimize the interpretation of the data obtained.The experiment allows the determination of the point at which suction is changed to positive pressure during compression.Below this point,the ratio between the vertical stress and the change in suction are determined.Above this point,the data show that positive pore pressures are dissipated in a common way.The suction/stress behavior during unloading is also described and discussed.Finally,an interpretation in terms of microstructure effects is provided for both samples.The experimental approach initiated here seems to provide interesting further application to better understand hydromechanical couplings in natural soils in relation with suction increase during stress release.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.
文摘Pre-compacted elements （disks, tutus） of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal. Choice of this material is mainly based on its swelling capacity allowing all gaps in the system to be sealed, and on its low permeability. When emplaced in the gallery, these elements will start to absorb water from the host rock and swell. Thereby, a swelling pressure will develop in the radial direction against the host rock and in the axial direction against the support structure. In this work, the swelling pressure of a small scale compacted disk of bentonite and sand was experimentally studied in both radial and axial directions. Different swelling kinetics were identified for different dry densities and along different directions. As a rule, the swelling pressure starts increasing quickly, reaches a peak value, decreases a little and finally stabilises. For some dry densities, higher peaks were observed in the radial direction than in the axial direction. The presence of peaks is related to the microstructure change and to the collapse of macro- pores. In parallel to the mechanical tests, microstructure investigation at the sample scale was conducted using microfocus X-ray computed tomography （BCT）. Image observation showed a denser structure in the centre and a looser one in the border, which was also confirmed by image analysis. This structure heterogeneity in the radial direction and the occurrence of macro-pores close to the radial boundary of the sample can explain the large peaks observed in the radial swelling pressure evolution. Another interesting result is the higher anisotropy found at lower bentonite dry densities, which was also analysed by means ofμCT observation of a sample at low bentonite dry density after the end of test. It was found that the macro-pores, especially those between sand grains, were not filled by swelled bentonite, which preserved the anisotropic microstructure caused by uniaxial compression due to the absence of microstructure collapse.
基金EURIDICE (European Undenground Research Infn-structure for Disposal of nuclear waste In Clay Environment, Mol, Belgium)is gatefully acknowledged for the financial support provided for this workthat is part of the fist author's PhD thesis.
文摘In the context of research into deep nuclear waste disposal,various works have concerned the hydromechanical behavior of Boom clay,a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the Mol City(Belgium),at a depth of 223 m.Due to some amount of smectite minerals in the clay fraction,Boom clay exhibits swelling properties when hydrated under low stresses.To investigate some aspects of the hydromechanical behavior of Boom clay,oedometer compression tests were carried out on samples of Boom clay close to saturation and submitted to an initial suction.During oedometer compression,the changes in suction with increased vertical stress are monitored by means of a high capacity tensiometer installed at the bottom of the sample.Some aspects related to hydromechanical couplings are examined through the investigation of the changes in suction during oedometer compression,a somewhat delicate and poorly documented experimental approach.A comparison is also made with a completely different soil sample under suction,i.e.a statically compacted unsaturated low plasticity silt.Some technical difficulties typical of this new experimental approach are first described in detail so as to optimize the interpretation of the data obtained.The experiment allows the determination of the point at which suction is changed to positive pressure during compression.Below this point,the ratio between the vertical stress and the change in suction are determined.Above this point,the data show that positive pore pressures are dissipated in a common way.The suction/stress behavior during unloading is also described and discussed.Finally,an interpretation in terms of microstructure effects is provided for both samples.The experimental approach initiated here seems to provide interesting further application to better understand hydromechanical couplings in natural soils in relation with suction increase during stress release.
