Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines.Their long-term stability is critical to the utilization of underground structures,such as underground reservoirs at ...Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines.Their long-term stability is critical to the utilization of underground structures,such as underground reservoirs at coal mines.In this study,a modified rock property testing system was used to explore the mechanical properties of coal specimens under quasi-static combined compression-shear loading conditions.The acoustic emission technique was applied to investigating the microcrack fracturing of coal specimens at various inclination angles.The experimental results show that specimen inclination has remarkable effects on the microcrack initiation,microcrack damage and ultimate failure of the coal specimen.The failure mode of the coal specimen tends to transit from axial splitting to shear failure with increasing specimen inclination,and its peak strength is closely associated with the microcrack damage threshold.In practice,it is recommended to consider coal strength under combined compression-shear loading when using empirical pillar strength formulae so that the effect of pillar inclination can be included.展开更多
The occurrence of geological hazards and the instability of geotechnical engineering structures are closely related to the time-dependent behavior of rock.However,the idealization boundary condition for constant stres...The occurrence of geological hazards and the instability of geotechnical engineering structures are closely related to the time-dependent behavior of rock.However,the idealization boundary condition for constant stress in creep or constant strain in relaxation is not usually attained in natural geological systems.Therefore,generalized relaxation tests that explore the simultaneous changes of stress and strain with time under different stress levels with constant pore-water pressure are conducted in this study.The results show that in area Ⅰ,area Ⅱ,and area Ⅲ,the stress and strain both change synchronously with time and show similar evolutionary laws as the strain-time curve for creep or the stress-time curve for relaxation.When the applied stress level surpasses the δ_(ci) or δ_(cd) threshold,the variations in stress and strain and their respective rates of change exhibit a significant increase.The radial deformation and its rate of change exhibit greater sensitivity in response to stress levels.The apparent strain deforms homogeneously at the primary stage,and subsequently,gradually localizes due to the microcrack development at the secondary stage.Ultimately,interconnection of the microcracks causes the formation of a shear-localization zone at the tertiary stage.The strain-time responses inside and outside the localization zone are characterized by local strain accumulation and inelastic unloading during the secondary and tertiary stages,respectively.The width of the shear-localization zone is found to range from 4.43 mm to 7.08 mm and increased with a longer time-to-failure.Scanning electron microscopy(SEM)reveals a dominant coalescence of intergranular cracks on the fracture surface,and the degree of physiochemical deterioration caused by water-rock interaction is more severe under a longer lifetime.The brittle sandstone’s time-dependent deformation is essentially controlled by microcrack development during generalized relaxation,and its expectancy-life is determined by its initial microstructural state and the rheological path.展开更多
The recent argument about nucleation phase of earthquakes reminds us to completely study the concept of earthquake nucleation. The original meaning of nucleation includes concentration, nucleation and initiation of th...The recent argument about nucleation phase of earthquakes reminds us to completely study the concept of earthquake nucleation. The original meaning of nucleation includes concentration, nucleation and initiation of the eruptive processes. Thus, it is needed to discuss how to exactly translate the word 'nucleation' into Chinese in different fields. The basic concept of earthquake nucleation refers to microcrack concentration in rock. It causes local weakening and instability of the rock. The narrow sense of nucleation theory of friction constitutive is significant in friction of fault surfaces, but should not abuse everywhere unconditionally. In terms of thermodynamics, nucleation actually means the variety processes of multiple state parameters of rock. The nucleation is a project that covers multiple courses. In this paper, the studies of damage theory, fracture, earthquake rupture dynamics and constitutive of friction and their implication to earthquake nucleation are remarked. The recently developments are introduced, including the influence of tectonic on the earthquake nucleation process, the method of measuring medium anisotropy, especially shear wave splitting led by concentration and orientation of microcracks. and the experimental study of remote sensing of infrared and microwave radiation related to the nucleation, etc. This paper also discusses the characteristic of large earthquake nucleation, and the implication of above studies to precursors of Strong earthquakes.展开更多
基金supported by the Fundamental Research Funds for the Central Universities(Grant No.2018QNA31)。
文摘Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines.Their long-term stability is critical to the utilization of underground structures,such as underground reservoirs at coal mines.In this study,a modified rock property testing system was used to explore the mechanical properties of coal specimens under quasi-static combined compression-shear loading conditions.The acoustic emission technique was applied to investigating the microcrack fracturing of coal specimens at various inclination angles.The experimental results show that specimen inclination has remarkable effects on the microcrack initiation,microcrack damage and ultimate failure of the coal specimen.The failure mode of the coal specimen tends to transit from axial splitting to shear failure with increasing specimen inclination,and its peak strength is closely associated with the microcrack damage threshold.In practice,it is recommended to consider coal strength under combined compression-shear loading when using empirical pillar strength formulae so that the effect of pillar inclination can be included.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.52304099,52172625)Shenzhen Science and Technology Program(Grant No.RCYX20221008092903013).
文摘The occurrence of geological hazards and the instability of geotechnical engineering structures are closely related to the time-dependent behavior of rock.However,the idealization boundary condition for constant stress in creep or constant strain in relaxation is not usually attained in natural geological systems.Therefore,generalized relaxation tests that explore the simultaneous changes of stress and strain with time under different stress levels with constant pore-water pressure are conducted in this study.The results show that in area Ⅰ,area Ⅱ,and area Ⅲ,the stress and strain both change synchronously with time and show similar evolutionary laws as the strain-time curve for creep or the stress-time curve for relaxation.When the applied stress level surpasses the δ_(ci) or δ_(cd) threshold,the variations in stress and strain and their respective rates of change exhibit a significant increase.The radial deformation and its rate of change exhibit greater sensitivity in response to stress levels.The apparent strain deforms homogeneously at the primary stage,and subsequently,gradually localizes due to the microcrack development at the secondary stage.Ultimately,interconnection of the microcracks causes the formation of a shear-localization zone at the tertiary stage.The strain-time responses inside and outside the localization zone are characterized by local strain accumulation and inelastic unloading during the secondary and tertiary stages,respectively.The width of the shear-localization zone is found to range from 4.43 mm to 7.08 mm and increased with a longer time-to-failure.Scanning electron microscopy(SEM)reveals a dominant coalescence of intergranular cracks on the fracture surface,and the degree of physiochemical deterioration caused by water-rock interaction is more severe under a longer lifetime.The brittle sandstone’s time-dependent deformation is essentially controlled by microcrack development during generalized relaxation,and its expectancy-life is determined by its initial microstructural state and the rheological path.
文摘The recent argument about nucleation phase of earthquakes reminds us to completely study the concept of earthquake nucleation. The original meaning of nucleation includes concentration, nucleation and initiation of the eruptive processes. Thus, it is needed to discuss how to exactly translate the word 'nucleation' into Chinese in different fields. The basic concept of earthquake nucleation refers to microcrack concentration in rock. It causes local weakening and instability of the rock. The narrow sense of nucleation theory of friction constitutive is significant in friction of fault surfaces, but should not abuse everywhere unconditionally. In terms of thermodynamics, nucleation actually means the variety processes of multiple state parameters of rock. The nucleation is a project that covers multiple courses. In this paper, the studies of damage theory, fracture, earthquake rupture dynamics and constitutive of friction and their implication to earthquake nucleation are remarked. The recently developments are introduced, including the influence of tectonic on the earthquake nucleation process, the method of measuring medium anisotropy, especially shear wave splitting led by concentration and orientation of microcracks. and the experimental study of remote sensing of infrared and microwave radiation related to the nucleation, etc. This paper also discusses the characteristic of large earthquake nucleation, and the implication of above studies to precursors of Strong earthquakes.
