Investigation of thermal effects on the strain rate-dependent properties of compacted bentonite is crucial for the long-term safety assessment of deep geological repository for disposal of high-level radioactive waste...Investigation of thermal effects on the strain rate-dependent properties of compacted bentonite is crucial for the long-term safety assessment of deep geological repository for disposal of high-level radioactive waste.In the present work,cylindrical GMZ01 bentonite specimens were compacted with suction-controlled by the vapor equilibrium technique.Then,a series of temperature-and suction-controlled stepwise constant rate of strain(CRS)tests was performed and the rate-dependent compressibility behavior of the highly compacted GMZ01 bentonite was investigated.The plastic compressibility parameterλ,the elastic compressibility parameterκ,the yield stress p0,as well as the viscous parameterαwere determined.Results indicate thatλ,κandαdecrease and p0 increases as suction increases.Upon heating,parametersλ,αand p0 decrease.It is also found that p0 increases linearly with increasing CRS in a double-logarithm coordinate.Based on the experimental results,a viscosity parameterα(s,T)was fitted to capture the effects of suction s and temperature T on the relationship between yield stress and strain rate.Then,an elastic-thermo-viscoplastic model for unsaturated soils was developed to describe the thermal effects on the rate-dependent behavior of highly compacted GMZ01 bentonite.Validation showed that the calculated results agreed well to the measured ones.展开更多
In the high-level radioactive waste(HLW)deep geological repository,bentonite is compacted uniaxially,and then arranged vertically in engineered barriers.The assembly scheme induces the initial anisotropy,and with hydr...In the high-level radioactive waste(HLW)deep geological repository,bentonite is compacted uniaxially,and then arranged vertically in engineered barriers.The assembly scheme induces the initial anisotropy,and with hydration,it develops more evidently under chemical conditions.To investigate the anisotropic swelling of compacted Gaomiaozi(GMZ)bentonite and the further response to saline effects,a series of constant-volume swelling pressure tests were performed.Results showed that dry density enhanced the bentonite swelling and raised the final anisotropy,whereas saline inhibited the bentonite swelling but still promoted the final anisotropy.The final anisotropy coefficient(ratio of radial to axial pressure)obeyed the Boltzmann sigmoid attenuation function,decreasing with concentration and dry density,converging to a minimum value of 0.76.The staged evolution of anisotropy coefficient was discovered,that saline inhibited the rise of the anisotropy coefficient(Dd)in the isotropic process greater than the valley(d1)in the anisotropic process,leading to the final anisotropy increasing.The isotropic stage amplified the impact of soil structure rearrangement on the macro-swelling pressure values.Thus,a new method for predicting swelling pressures of compacted bentonite was proposed,by expanding the equations of Gouy-Chapman theory with a dissipative wedge term.An evolutionary function was constructed,revealing the correlation between the occurrence time and the pressure value due to the structure rearrangement and the former crystalline swelling.Accordingly,a design reference for dry density was given,based on the chemical conditions around the pre-site in Beishan,China.The anisotropy promoted by saline would cause a greater drop of radial pressure,making the previous threshold on axial swelling fail.展开更多
An effective thermal conductivity model was proposed for unsaturated compacted bentonites with consideration of the bimodal shape of pore size distribution curves. The pores of soils were grouped into two dominant por...An effective thermal conductivity model was proposed for unsaturated compacted bentonites with consideration of the bimodal shape of pore size distribution curves. The pores of soils were grouped into two dominant pore size modes corresponding to the intra- and inter-particle pores, and were simulated with randomly distributed spheroidal inclusions of different aspect ratios. With the assumption of preferential invasion of the wetting fluid （water） into pores of smaller sizes and by virtue of the analyt- ical solution to the inhomogeneous inclusion problem in heat conduction, the model was developed using the Mori-Tanaka （MT）, Ponte Castafieda-Willis （PCW） and self-consistent （SC） homogenization approaches for different considerations of the interactions between pores and the solid phase. The proposed model is functions of the thermal conductivities of the solid, liq- uid and gas phases, porosity, the degree of saturation, the aspect ratios of pores and/or soil particles, and the fraction of the smaller group of pores. The proposed model was validated against five sets of laboratory measurement data on the MX-80, FEBEX, KunigeI-V1 and GMZ01 bentonites, showing a good agreement between the model predictions and the laboratory measurements. The responses of the model with respect to the geometries of pores and solid particles were examined. Com- pared to series-parallel structural models, the proposed model may overall exhibit better performance if proper homogenization schemes are adopted, but as an advantage, the model has clearer physical mechanisms and a smaller number of parameters.展开更多
The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces,with particular emphasis on the self-healing(automatic healing upon wetting)of assemb...The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces,with particular emphasis on the self-healing(automatic healing upon wetting)of assembled bentonite-bentonite interfaces.This study determined the shear resistance(including the peak shear strength and secant modulus)of densely compacted Gaomiaozi(GMZ)bentonite and its assembled interface after confined water saturation.The effect of bentonite dry density and saturation time on the shear resistance of saturated healed interfaces was elucidated,and the interfacial self-healing capacity was assessed.The results indicate that the shear resistance of the saturated healed interfaces increased with the bentonite dry density but had a non-monotonic correlation with the saturation time.For a given dry density of the bentonite,the saturated healed interface exhibits a lower peak shear strength than the saturated intact bentonite but a higher peak shear strength than the saturated separated interface.The saturated healed and separated interfaces have comparable shear moduli(secant moduli),which are lower than that of the saturated intact bentonite.The saturated healed interfaces display smooth shear failure planes,while the saturated assembled interfaces and intact bentonite exhibit comparable frictional angles.This indicates that interfacial self-healing plays a pivotal role in enhancing interfacial peak shear strength by facilitating microstructural bonding at the assembled interface.Finally,it can be stated that densely compacted GMZ bentonite has a robust interfacial self-healing capacity in terms of shear resistance.These findings contribute to the design of the bentonite buffer and facilitate the evaluation of its safe operation at specified disposal ages.展开更多
Thermal conductivity, water retention curve and swelling behavior of two MX80 bentonite samples were studied in the present work. The difference obtained from these two MX80 bentonite samples was then analyzed in term...Thermal conductivity, water retention curve and swelling behavior of two MX80 bentonite samples were studied in the present work. The difference obtained from these two MX80 bentonite samples was then analyzed in terms of mineralogical effects （effects of the proportion of quartz and montmorillonite）. It was concluded that the mineralogical effect was significant on the thermal conductivity and the swelling capacity; on the contrary, it was negligible on the water retention property.展开更多
A methodology for identifying and calibrating the material parameters for a coupled hydro-mechanical problem is presented in this pape r.For validation purpose,a laboratory-based water infiltration test was numericall...A methodology for identifying and calibrating the material parameters for a coupled hydro-mechanical problem is presented in this pape r.For validation purpose,a laboratory-based water infiltration test was numerically simulated using finite element method(FEM).The test was conducted using a self-designed column-type experimental device,which mimicked the wetting process of a candidate backfill material in a nuclear waste repository.The real-time measurements of key state variables(e.g.water content,relative humidity,temperature,and total stresses)were performed with the monitoring sensors along the height of cylindrical soil sample.For numerical simulation,the modified Barcelona Basic Model(BBM)along with soil-water retention model for compacted bentonite was used.It shows that the identified model parameters successfully captured the moisture migration process under an applied hydraulic gradient in a bentonite-based compacted soil sample.A comparison between the measured and predicted values of total stresses both in axial and lateral directions along with other state variables revealed that heterogeneous moisture content was distributed along the hydration-path,resulting in non-uniform stress-deformation characteristics of soil.展开更多
基金the support of the National Natural Science Foundation of China(Grant Nos.42030714,42177138 and 41907239).
文摘Investigation of thermal effects on the strain rate-dependent properties of compacted bentonite is crucial for the long-term safety assessment of deep geological repository for disposal of high-level radioactive waste.In the present work,cylindrical GMZ01 bentonite specimens were compacted with suction-controlled by the vapor equilibrium technique.Then,a series of temperature-and suction-controlled stepwise constant rate of strain(CRS)tests was performed and the rate-dependent compressibility behavior of the highly compacted GMZ01 bentonite was investigated.The plastic compressibility parameterλ,the elastic compressibility parameterκ,the yield stress p0,as well as the viscous parameterαwere determined.Results indicate thatλ,κandαdecrease and p0 increases as suction increases.Upon heating,parametersλ,αand p0 decrease.It is also found that p0 increases linearly with increasing CRS in a double-logarithm coordinate.Based on the experimental results,a viscosity parameterα(s,T)was fitted to capture the effects of suction s and temperature T on the relationship between yield stress and strain rate.Then,an elastic-thermo-viscoplastic model for unsaturated soils was developed to describe the thermal effects on the rate-dependent behavior of highly compacted GMZ01 bentonite.Validation showed that the calculated results agreed well to the measured ones.
基金supported by the National Science Fund for Distinguished Young Scholars of China(Grant No.42125701)Innovation Program of Shanghai Municipal Education Commission(Grant No.2023ZKZD26)the Fundamental Research Funds for the Central Universities,and Top Discipline Plan of Shanghai Universities-Class I.
文摘In the high-level radioactive waste(HLW)deep geological repository,bentonite is compacted uniaxially,and then arranged vertically in engineered barriers.The assembly scheme induces the initial anisotropy,and with hydration,it develops more evidently under chemical conditions.To investigate the anisotropic swelling of compacted Gaomiaozi(GMZ)bentonite and the further response to saline effects,a series of constant-volume swelling pressure tests were performed.Results showed that dry density enhanced the bentonite swelling and raised the final anisotropy,whereas saline inhibited the bentonite swelling but still promoted the final anisotropy.The final anisotropy coefficient(ratio of radial to axial pressure)obeyed the Boltzmann sigmoid attenuation function,decreasing with concentration and dry density,converging to a minimum value of 0.76.The staged evolution of anisotropy coefficient was discovered,that saline inhibited the rise of the anisotropy coefficient(Dd)in the isotropic process greater than the valley(d1)in the anisotropic process,leading to the final anisotropy increasing.The isotropic stage amplified the impact of soil structure rearrangement on the macro-swelling pressure values.Thus,a new method for predicting swelling pressures of compacted bentonite was proposed,by expanding the equations of Gouy-Chapman theory with a dissipative wedge term.An evolutionary function was constructed,revealing the correlation between the occurrence time and the pressure value due to the structure rearrangement and the former crystalline swelling.Accordingly,a design reference for dry density was given,based on the chemical conditions around the pre-site in Beishan,China.The anisotropy promoted by saline would cause a greater drop of radial pressure,making the previous threshold on axial swelling fail.
基金supported by the National Natural Science Foundation of China(Grant Nos.51179136 and 51222903)
文摘An effective thermal conductivity model was proposed for unsaturated compacted bentonites with consideration of the bimodal shape of pore size distribution curves. The pores of soils were grouped into two dominant pore size modes corresponding to the intra- and inter-particle pores, and were simulated with randomly distributed spheroidal inclusions of different aspect ratios. With the assumption of preferential invasion of the wetting fluid （water） into pores of smaller sizes and by virtue of the analyt- ical solution to the inhomogeneous inclusion problem in heat conduction, the model was developed using the Mori-Tanaka （MT）, Ponte Castafieda-Willis （PCW） and self-consistent （SC） homogenization approaches for different considerations of the interactions between pores and the solid phase. The proposed model is functions of the thermal conductivities of the solid, liq- uid and gas phases, porosity, the degree of saturation, the aspect ratios of pores and/or soil particles, and the fraction of the smaller group of pores. The proposed model was validated against five sets of laboratory measurement data on the MX-80, FEBEX, KunigeI-V1 and GMZ01 bentonites, showing a good agreement between the model predictions and the laboratory measurements. The responses of the model with respect to the geometries of pores and solid particles were examined. Com- pared to series-parallel structural models, the proposed model may overall exhibit better performance if proper homogenization schemes are adopted, but as an advantage, the model has clearer physical mechanisms and a smaller number of parameters.
基金supported by the National Natural Science Foundation of China (Grant Nos.42125701 and 41977232)China Postdoctoral Science Foundation (Grant No.2021M702234).
文摘The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces,with particular emphasis on the self-healing(automatic healing upon wetting)of assembled bentonite-bentonite interfaces.This study determined the shear resistance(including the peak shear strength and secant modulus)of densely compacted Gaomiaozi(GMZ)bentonite and its assembled interface after confined water saturation.The effect of bentonite dry density and saturation time on the shear resistance of saturated healed interfaces was elucidated,and the interfacial self-healing capacity was assessed.The results indicate that the shear resistance of the saturated healed interfaces increased with the bentonite dry density but had a non-monotonic correlation with the saturation time.For a given dry density of the bentonite,the saturated healed interface exhibits a lower peak shear strength than the saturated intact bentonite but a higher peak shear strength than the saturated separated interface.The saturated healed and separated interfaces have comparable shear moduli(secant moduli),which are lower than that of the saturated intact bentonite.The saturated healed interfaces display smooth shear failure planes,while the saturated assembled interfaces and intact bentonite exhibit comparable frictional angles.This indicates that interfacial self-healing plays a pivotal role in enhancing interfacial peak shear strength by facilitating microstructural bonding at the assembled interface.Finally,it can be stated that densely compacted GMZ bentonite has a robust interfacial self-healing capacity in terms of shear resistance.These findings contribute to the design of the bentonite buffer and facilitate the evaluation of its safe operation at specified disposal ages.
文摘Thermal conductivity, water retention curve and swelling behavior of two MX80 bentonite samples were studied in the present work. The difference obtained from these two MX80 bentonite samples was then analyzed in terms of mineralogical effects （effects of the proportion of quartz and montmorillonite）. It was concluded that the mineralogical effect was significant on the thermal conductivity and the swelling capacity; on the contrary, it was negligible on the water retention property.
基金the German Research Foundation(DFG)for the financial support(Grant No.SCHA 675/17-1)。
文摘A methodology for identifying and calibrating the material parameters for a coupled hydro-mechanical problem is presented in this pape r.For validation purpose,a laboratory-based water infiltration test was numerically simulated using finite element method(FEM).The test was conducted using a self-designed column-type experimental device,which mimicked the wetting process of a candidate backfill material in a nuclear waste repository.The real-time measurements of key state variables(e.g.water content,relative humidity,temperature,and total stresses)were performed with the monitoring sensors along the height of cylindrical soil sample.For numerical simulation,the modified Barcelona Basic Model(BBM)along with soil-water retention model for compacted bentonite was used.It shows that the identified model parameters successfully captured the moisture migration process under an applied hydraulic gradient in a bentonite-based compacted soil sample.A comparison between the measured and predicted values of total stresses both in axial and lateral directions along with other state variables revealed that heterogeneous moisture content was distributed along the hydration-path,resulting in non-uniform stress-deformation characteristics of soil.
