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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 σci or σcd 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.
Chen et al. (Sun,) studied this question.