Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric...Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes.The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz.At each level of confining pressure,the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed.Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude(i.e.critical dynamic deviatoric stress)beyond which the jointed rock specimens yielded.The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress.At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress,minor cumulative residual axial strains were observed,resulting in hysteretic damping.However,for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress,the plastic strains increased promptly,and the resilient moduli degraded rapidly during the initial loading cycles.Cyclic triaxial test results showed that at higher confining pressures,the ultimate residual axial strain attained by the jointed rock specimen decreased,the steadystate dissipated energy density and steady-state damping ratio per load cycle decreased,while steadystate resilient moduli increased.展开更多
Based on three dimensional （3D） Discrete Element Method （DEM）, the paper presents simulation results of undrained tests on loose assemblies of polydisperse spheres under axisymmet- ric compression and plane strain...Based on three dimensional （3D） Discrete Element Method （DEM）, the paper presents simulation results of undrained tests on loose assemblies of polydisperse spheres under axisymmet- ric compression and plane strain conditions using a periodic cell. In the present work, undrained tests were modelled by deforming the samples under constant volume conditions. The undrained （effective） stress paths are shown to be qualitatively similar to experimental results in literature. A microscopic parameter in terms of redundancy factor （RF） is used to identify the onset of lique- faction （or temporary liquefaction）, with the condition of RF equal to unity defining the transition from ＇solid-like＇ to ＇liquid-like＇ behaviour. It is found that the undrained behaviour is governed by the evolution of redundancy factor under both undrained axisymmetric compression and plane strain conditions, and a reversal of deviatoric stress in stress path for medium loose systems oc- curs due to the fact that the system becomes a structural mechanism （RF 〈 1） transiently at the microscopic level during the evolution.展开更多
In recent years,finite element analysis is increasingly being proposed in slope stability problems as a competitive method to traditional limit equilibrium methods(LEMs)which are known for their inherent deficiencies....In recent years,finite element analysis is increasingly being proposed in slope stability problems as a competitive method to traditional limit equilibrium methods(LEMs)which are known for their inherent deficiencies.However,the application of finite element method(FEM)to slope stability as a strength reduction method(SRM)or as finite element limit analysis(FELA)is not always a success for the drawbacks that characterize both methods.To increase the performance of finite element analysis in this problem,a new approach is proposed in this paper.It consists in gradually expanding the mobilized stress Mohr’s circles until the soil failure occurs according to a prescribed non-convergence criterion.The present approach called stress deviator increasing method(SDIM)is considered rigorous for three main reasons.Firstly,it preserves the definition of the factor of safety(FOS)as the ratio of soil shear strength to the mobilized shear stress.Secondly,it maintains the progressive development of shear stress resulting from the increase in the principal stress deviator on the same plane,on which the shear strength takes place.Thirdly,by introducing the concept of equivalent stress loading,the resulting trial stresses are checked against the violation of the actual yield criterion formed with the real strength parameters rather than those reduced by a trial factor.The new numerical procedure was encoded in a Fortran computer code called S^(4)DINA and verified by several examples.Comparisons with other numerical methods such as the SRM,gravity increasing method(GIM)or even FELA by assessing both the FOS and contours of equivalent plastic strains showed promising results.展开更多
The present-day lithospheric stress state of the Qinghai-Tibetan Plateau and neighboring areas is controlled by both the lithosphere itself and the underlying mantle.In other words,the stress is affected by the gravit...The present-day lithospheric stress state of the Qinghai-Tibetan Plateau and neighboring areas is controlled by both the lithosphere itself and the underlying mantle.In other words,the stress is affected by the gravitational potential energy(GPE)difference caused by the change in the density distribution within the lithosphere and the drag force on the base of the lithosphere caused by mantle convection.The study of the lithospheric stress state plays an important role in further understanding the dynamic background and mechanism for the evolution of the Qinghai-Tibetan Plateau.In this study,the Crust1.0 crustal density model combined with the S40RTS mantle shear wave velocity variation model was used to calculate the GPE.The EGM2008gravity field model was used to calculate the drag force from mantle convection at the base of the lithosphere.The lithospheric and joint stress fields of the two sources were obtained by solving the force balance under the thin sheet approximation.This way,we could comprehensively analyze the characteristics of the stress state within the Plateau.Six regions were classified according to the GPE stress field,mantle drag force stress field,the relative magnitude of the two stress fields,and correlation between the two stress fields and surface deformation.The lithospheric stress fields of the Tarim Basin and other stable blocks are mainly controlled by the GPE difference.The lithospheric stress field in the collision zone between the Indian Plate and the QinghaiTibetan Plateau is predominantly controlled by the deep mantle drag force.The lithospheric stress field in the interior of the Plateau is controlled by both GPE and mantle drag.The correlation between the lithospheric stress field and surface deformation at the southeast margin of the Qinghai-Tibetan Plateau is poor.It is hypothesized that the presence of lower crustal flow with lower effective viscosity leads to crust-mantle decoupling,and the mantle drag force has a weaker influence on the shallow crust,resulting in the inconsistency between the average lithospheric stress field and surface deformation.展开更多
The dynamic characteristics of compacted loess are of great significance to the seismic construction of the Loess Plateau area in Northwest China,where earthquakes frequently occur.To study the change in the dynamic m...The dynamic characteristics of compacted loess are of great significance to the seismic construction of the Loess Plateau area in Northwest China,where earthquakes frequently occur.To study the change in the dynamic modulus of the foundation soil under the combined action of vertical and horizontal earthquakes,a hollow cy-lindrical torsion shear instrument capable of vibrating in four directions was used to perform two-way coupling of compression and torsion of Xi'an compacted loess under different dry density and deviator stress ratios.The results show that increasing the dry density can improve the initial dynamic compression modulus and initial dynamic shear modulus of compacted loess.With an increase in the deviator stress ratio,the initial dynamic compression modulus increases,to a certain extent,but the initial dynamic shear modulus decreases slightly.The dynamic modulus gradually decreases with the development of dynamic strain and tends to be stable,and the dynamic modulus that reaches the same strain increases with an increasing dry density.At the initial stage of dynamic loading,the attenuation of the dynamic shear modulus with the strain development is faster than that of the dynamic compression modulus.Compared with previous research results,it is determined that the dynamic modulus of loess under bidirectional dynamic loading is lower and the attenuation rate is faster than that under single-direction dynamic loading.The deviator stress ratio has a more obvious effect on the dynamic compression modulus.The increase in the deviator stress ratio can increase the dynamic compression modulus,to a certain extent.However,the deviator stress ratio has almost no effect on the dynamic shear modulus,and can therefore be ignored.展开更多
文摘Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes.The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz.At each level of confining pressure,the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed.Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude(i.e.critical dynamic deviatoric stress)beyond which the jointed rock specimens yielded.The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress.At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress,minor cumulative residual axial strains were observed,resulting in hysteretic damping.However,for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress,the plastic strains increased promptly,and the resilient moduli degraded rapidly during the initial loading cycles.Cyclic triaxial test results showed that at higher confining pressures,the ultimate residual axial strain attained by the jointed rock specimen decreased,the steadystate dissipated energy density and steady-state damping ratio per load cycle decreased,while steadystate resilient moduli increased.
基金supported by the Guangdong Natural Science Foundation, China (No. 10151503101000006)the Engineering and Physical Sciences Research Council, UK (No. GR/R91588)
文摘Based on three dimensional （3D） Discrete Element Method （DEM）, the paper presents simulation results of undrained tests on loose assemblies of polydisperse spheres under axisymmet- ric compression and plane strain conditions using a periodic cell. In the present work, undrained tests were modelled by deforming the samples under constant volume conditions. The undrained （effective） stress paths are shown to be qualitatively similar to experimental results in literature. A microscopic parameter in terms of redundancy factor （RF） is used to identify the onset of lique- faction （or temporary liquefaction）, with the condition of RF equal to unity defining the transition from ＇solid-like＇ to ＇liquid-like＇ behaviour. It is found that the undrained behaviour is governed by the evolution of redundancy factor under both undrained axisymmetric compression and plane strain conditions, and a reversal of deviatoric stress in stress path for medium loose systems oc- curs due to the fact that the system becomes a structural mechanism （RF 〈 1） transiently at the microscopic level during the evolution.
文摘In recent years,finite element analysis is increasingly being proposed in slope stability problems as a competitive method to traditional limit equilibrium methods(LEMs)which are known for their inherent deficiencies.However,the application of finite element method(FEM)to slope stability as a strength reduction method(SRM)or as finite element limit analysis(FELA)is not always a success for the drawbacks that characterize both methods.To increase the performance of finite element analysis in this problem,a new approach is proposed in this paper.It consists in gradually expanding the mobilized stress Mohr’s circles until the soil failure occurs according to a prescribed non-convergence criterion.The present approach called stress deviator increasing method(SDIM)is considered rigorous for three main reasons.Firstly,it preserves the definition of the factor of safety(FOS)as the ratio of soil shear strength to the mobilized shear stress.Secondly,it maintains the progressive development of shear stress resulting from the increase in the principal stress deviator on the same plane,on which the shear strength takes place.Thirdly,by introducing the concept of equivalent stress loading,the resulting trial stresses are checked against the violation of the actual yield criterion formed with the real strength parameters rather than those reduced by a trial factor.The new numerical procedure was encoded in a Fortran computer code called S^(4)DINA and verified by several examples.Comparisons with other numerical methods such as the SRM,gravity increasing method(GIM)or even FELA by assessing both the FOS and contours of equivalent plastic strains showed promising results.
基金supported by the National Natural Science Foundation of China(Grant No.42074092)the Youth Innovation Promotion Association of the Chinese Academy of Sciences(Grant No.2016064)。
文摘The present-day lithospheric stress state of the Qinghai-Tibetan Plateau and neighboring areas is controlled by both the lithosphere itself and the underlying mantle.In other words,the stress is affected by the gravitational potential energy(GPE)difference caused by the change in the density distribution within the lithosphere and the drag force on the base of the lithosphere caused by mantle convection.The study of the lithospheric stress state plays an important role in further understanding the dynamic background and mechanism for the evolution of the Qinghai-Tibetan Plateau.In this study,the Crust1.0 crustal density model combined with the S40RTS mantle shear wave velocity variation model was used to calculate the GPE.The EGM2008gravity field model was used to calculate the drag force from mantle convection at the base of the lithosphere.The lithospheric and joint stress fields of the two sources were obtained by solving the force balance under the thin sheet approximation.This way,we could comprehensively analyze the characteristics of the stress state within the Plateau.Six regions were classified according to the GPE stress field,mantle drag force stress field,the relative magnitude of the two stress fields,and correlation between the two stress fields and surface deformation.The lithospheric stress fields of the Tarim Basin and other stable blocks are mainly controlled by the GPE difference.The lithospheric stress field in the collision zone between the Indian Plate and the QinghaiTibetan Plateau is predominantly controlled by the deep mantle drag force.The lithospheric stress field in the interior of the Plateau is controlled by both GPE and mantle drag.The correlation between the lithospheric stress field and surface deformation at the southeast margin of the Qinghai-Tibetan Plateau is poor.It is hypothesized that the presence of lower crustal flow with lower effective viscosity leads to crust-mantle decoupling,and the mantle drag force has a weaker influence on the shallow crust,resulting in the inconsistency between the average lithospheric stress field and surface deformation.
基金the National Natural Science Foundation of China(No.41272320,52108342)the Key Scientific Research Projects of Higher Education Institutions in Henan Province,China(No.21A560009).
文摘The dynamic characteristics of compacted loess are of great significance to the seismic construction of the Loess Plateau area in Northwest China,where earthquakes frequently occur.To study the change in the dynamic modulus of the foundation soil under the combined action of vertical and horizontal earthquakes,a hollow cy-lindrical torsion shear instrument capable of vibrating in four directions was used to perform two-way coupling of compression and torsion of Xi'an compacted loess under different dry density and deviator stress ratios.The results show that increasing the dry density can improve the initial dynamic compression modulus and initial dynamic shear modulus of compacted loess.With an increase in the deviator stress ratio,the initial dynamic compression modulus increases,to a certain extent,but the initial dynamic shear modulus decreases slightly.The dynamic modulus gradually decreases with the development of dynamic strain and tends to be stable,and the dynamic modulus that reaches the same strain increases with an increasing dry density.At the initial stage of dynamic loading,the attenuation of the dynamic shear modulus with the strain development is faster than that of the dynamic compression modulus.Compared with previous research results,it is determined that the dynamic modulus of loess under bidirectional dynamic loading is lower and the attenuation rate is faster than that under single-direction dynamic loading.The deviator stress ratio has a more obvious effect on the dynamic compression modulus.The increase in the deviator stress ratio can increase the dynamic compression modulus,to a certain extent.However,the deviator stress ratio has almost no effect on the dynamic shear modulus,and can therefore be ignored.
