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This study aims to investigate the influence of length-to-diameter (L/D) ratio on the strain energy storage and evolution characteristics of rock materials during progressive rock failure under compression. Uniaxial compression tests and single-cycle loading–unloading uniaxial compression tests were conducted on four rock materials with two specimen L/D ratios. The uniaxial compressive strength (UCS) and peak strain of the rock specimens with L/D = 1 were significantly greater than those of the specimens with L/D = 2, thus exhibiting a strong size effect. In the single-cycle loading-unloading uniaxial compression tests, the total energy density, elastic energy density and dissipated energy density of the specimens with different L/D ratios increased nonlinearly with the actual stress level. The specimen L/D ratio had a significant impact on the nonlinear energy evolutions over various actual stress levels. Interestingly, regardless of the L/D ratio, failure pattern and type of material in the tests, highly linear relationships exist between the elastic energy, dissipated energy and total energy. Although the UCS is sensitive to the specimen L/D ratio, the energy storage coefficient (ESC) and energy dissipation coefficient (EDC) almost show a size-independent behaviour. The ESC and EDC are not affected by the L/D ratio and failure mode and can be used as physical parameters in geotechnical engineering design. [ABSTRACT FROM AUTHOR]

Size effect is a project that cannot be ignored in rock mechanics. To investigate the size effect on the energy distribution and evolution laws, several groups of uniaxial compression tests and single-cycle loading–unloading uniaxial compression tests were performed on red sandstone specimens of different sizes (diameters of 25, 37, 50, 75, and 100 mm; a constant length-to-diameter ratio of 2.0) using the INSTRON 1346 test system. Experimental results show that mechanical properties are influenced by specimen size while failure mode has no significant variation for different diameter specimens. Strain energy parameters (input strain energy, elastic strain energy, and dissipated strain energy) under each unloading stress level were calculated by integration based on the stress–strain curves. The input strain energy, elastic strain energy, and dissipated strain energy nonlinearly increase with actual unloading stress levels, expressed as unified quadratic function relationships. Furthermore, the elastic strain energy and dissipated strain energy have linear relationships with the input strain energy. Through analyzing coefficients of variation of common parameters, it was found that the energy storage capacity of rock was not sensitive to the specimen size, and therefore, the energy storage coefficient could be considered an essential property of rock materials. [ABSTRACT FROM AUTHOR]

To investigate the influence of water on stress-induced failure in D-shaped hard rock tunnels, a series of true triaxial tests were performed on naturally saturated red sandstone cube samples containing a D-shaped hole. A self-developed real-time monitoring device was used to observe and record hole failure in real time, and the process and characteristics of stress-induced failure in D-shaped holes were summarized. The influence of water on failure form, energy storage properties, surrounding rock stability, and axial stress on the stability of the surrounding rock was discussed, and the mechanism of water on stress-induced failure was revealed. The results show that the stress-induced failure in a water-bearing D-shaped tunnel can be divided into buckling deformation, tension cracking, macrocrack propagation and coalescence, and slab flaking. The dominant failure mode of the stress-induced changes from tension around the tunnel to shear of the surrounding rock in depth. A quiet period exists between two adjacent significant AE counts, and the length of the quiet period increases as the two horizontal principal stresses increase. Under the influence of water, the capacity of the surrounding rock to store elastic strain energy decreases, and the dissipated energy required for deformation and failure increases; the form of stress-induced failure changes from spalling accompanied by occasional rockburst to spalling. The increase in lateral stress can enhance the stability of the surrounding rock, and the influence of axial stress on the stability of the surrounding rock is related to the magnitude of lateral stress. [ABSTRACT FROM AUTHOR]

To explore the mechanical behaviors and energy evolution characteristics of rock materials with hole defects, eight types of red sandstone specimens with different hole numbers and arrangements were subjected to a series of single-cycle loading-unloading uniaxial compression tests. The experimental result revealed that the mechanical behaviors and energy evolution characteristics of the red sandstone specimens are significantly influenced by hole defects. As the number of boreholes increases, the compressive strength of rock specimens decreases. When the angle between drilling hole arrangement and loading direction changes from 0° to 27° and then to 90°, the compressive strength first decreases and then increases, and shear-splitting, splitting, and shear failure modes appear respectively. The experimental results also show that the energy evolution characteristics (linear energy storage and dissipation laws) of the rock specimens with hole defects were basically consistent with that of intact rock specimens, while the energy storage coefficient is different in various cases. There was a good correspondence between the compressive strength and energy storage coefficient. The larger the energy storage coefficient is, the greater the compressive strength. The hole arrangement angle shows a stronger effect on energy storage coefficient and compressive strength than the number of holes. Among all the arrangements, the rock specimens containing three holes with an angle of 27° between borehole arrangement and loading direction have the lowest energy storage coefficient and compression strength. The above research results provide a scientific basis for rock fracturing induced by drilling. [ABSTRACT FROM AUTHOR]

The quantitative determination of crack damage stress is of significant importance for investigating rock deformation and failure. In this study, a new quantitative method for determining the crack damage stress of rock materials was proposed based on acoustic emission (AE) signal detection. Ten rock materials were subjected to uniaxial compression testing, during which real-time AE signals and axial, lateral, and volumetric strain values were recorded. The AE cumulative count curves of the different rock materials were calculated based on the AE counts. These curves were divided into three stages as the axial stress increased: the slowly increasing stage, the steady stage, and the sharply increasing stage. The point where the sharply increasing stage began was defined as the PR point and could be accurately determined based on a fitting optimization method. Then, the damage stress was calculated by using the crack volumetric strain model. Finally, the relationship between the R stress (axial stress corresponding to the PR point) and the crack damage stress was investigated for the ten rock materials. The results showed that there was a good correspondence between the R stress and the crack damage stress. Hence, the R stress can be used to determine the crack damage stress and the starting point of the unstable crack growth stage in rock materials. [ABSTRACT FROM AUTHOR]