The loaded rock experiences multiple stages of deformation. It starts with the formation of microcracks at low stresses (crack initiation, CI) and then transitions into unstable crack propagation (crack damage, CD) near the ultimate strength. In this study, both the acoustic emission method (AEM) and the ultrasonic testing method (UTM) were used to examine the characteristics of AE parameters (b-value, peak frequency, frequency-band energy ratio, and fractal dimension) and ultrasonic (ULT) properties (velocity, amplitude, energy attenuation, and scattering attenuation) of bedded shale at CI, CD, and ultimate strength. The comparison involved analyzing the strain-based method (SBM), AEM, and UTM to determine the thresholds for damage stress. A fuzzy comprehensive evaluation model (FCEM) was created to describe the damage thresholds and hazard assessment. The results indicate that the optimal AE and ULT parameters for identifying CI and CD stress are ringing count, ultrasonic amplitude, energy attenuation, and scattering attenuation of the S-wave. Besides, damage thresholds were detected earlier by AE monitoring, ranging from 3 MPa to 10 MPa. CI and CD identified by UTM occurred later than SBM and AEM, and were in the range of 12 MPa. The b-value, peak frequency, energy ratio in the low-frequency band (0-62.5 kHz), correlation dimension, and sandbox dimension showed low values at the peak stress, while the energy ratio in a moderate-frequency band (187.5-281.25 kHz) and amplitude showed high values. The successful application of FCEM to laboratory testing of shales has demonstrated its ability to quantitatively identify AE/ULT precursors of seismic hazards associated with rock failure. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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 s ci or s cd threshold, the variations in stress and strain and their respective rates of change exhibit a signi ficant 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. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Artificial frozen sandy gravel exhibits the characteristics of wide distribution of particle size and complex composition, which are quite distinct from frozen fine-grained soils such as clay and silt. It may be more accurate to use both macroscopic and microscopic scales to evaluate the damage of artificial frozen sandy gravel. Therefore, this paper proposes an investigation on the macro-plastic damage and micro-crack damage of artificial frozen sandy gravel through triaxial compression and X-ray CT scanning tests. The two types of damage are obtained from completely different macro-plastic and micro-crack damage theoretical calculation methods. It can be concluded that the evolution law of the two damages is similar, but the value is different. Moreover, the defined cross-scale modified damage which is fitted through the calculated macro-plastic damage and micro-crack damage is proposed. The fitting functions reveal the evolution law of frozen sandy gravel damage more accurate, which is beneficial to the safety of the artificial ground freezing project and provides a valuable reference for subsequent numerical simulations of the frozen sandy gravel constitutive relationship.