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To explore the thermal damage deterioration characteristics of basalt, the evolution of physical parameters, mechanical properties and failure modes was investigated. Based on computed tomography image reconstruction techniques, the spatial distribution and morphological characteristics of the pores of basalt were explored. The results indicate that thermal damage leads to the phase transition of basalt mineral grains and uncoordinated expansion and deformation, increasing the thermal deterioration of rock specimens. The temperature of 800 °C is the threshold for rapidly deteriorated physical properties of basalt, which has deformation characterized by the transition from ductility to brittleness. With the increase in temperature, basalt specimens transit from shear failure to tensile-shear combined failure, and then to tensile splitting failure. Meanwhile, irregular block-shaped collapse is transformed to strip-shaped rock fragments spalling. The crack width based on CT technology and three-dimensional (3D) image reconstruction of the crack volume can quantify the structural deterioration characteristics of basalt induced by thermal damage. When the temperature increases: 25 °C → 600 °C → 1000 °C, the corresponding porosity of the rock changes from 6.86% → 7.04% → 18.02%, exhibiting an evolution from low-speed development to high-speed growth. The thermal damage sensitivity of different lithologies at high temperatures differs, among which, the thermal damage sensitivity of marble is the highest, followed by granite and then basalt, the sensitivity of sandstone is the lowest. [ABSTRACT FROM AUTHOR]

Strainburst often occurs in the loading process of tangential stress concentration from surrounding rocks after excavation and unloading of deep rock mass. The concentrated tangential stress is relatively larger on the tunnel walls, which decreases towards the interior of surrounding rocks with a certain gradient. To explore the fracture evolution characteristics of surrounding rocks under different tangential gradient stresses and understand various phenomena in the strainburst process, a large‐scale true‐triaxial gradient and hydraulic‐pneumatic composite loading rockburst test device was adopted to carry out the strainburst tests under four different gradient stresses. Based on the macroscopic failure phenomena, distribution of rockburst debris, macro‐micro morphology of failure cross section, and monitored data of acoustic emission (AE), the fracture evolution of rockburst specimens was analyzed under different stress gradient loadings. The results indicate that the stress gradient has an obvious influence on the fracture characteristics, resulting in different rockburst phenomena. With the increase of stress gradient, the failure stress of rockburst decreases, while the dynamic failure characteristics of rockburst trends to be significant. The increase of stress gradient contributes to higher fragmentation degree of debris, more uniform distribution and better continuity. The total number of cracks in the model decreases with the rise of the stress gradient, and the proportion of shear failure increases, which is the main reason for the enhancement of the rockburst intensity. [ABSTRACT FROM AUTHOR]

Rockburst is a safety problem that needs to be solved urgently in deep rock and mineral exploitation. The stress gradient is an important factor affecting rockburst. To explore the influence mechanism of stress gradient on rockburst, the loading and unloading tests of rockburst physical model under different stress gradients are carried out using the self-developed gas–liquid composite rockburst test device. Based on the strain test results of rockburst model, the spatial distribution characteristics of elastic energy are analyzed. The particle size distribution characteristics and throwing kinetic energy of debris after the rockburst of model are studied. The following conclusions are obtained: (1) the value of the stress gradient coefficient m reflected by the strain within the range of the model loading rockburst gradient 1 affects the temporal and spatial distribution of energy release process; (2) With the increase of the vertical loading stress gradient, the rockburst debris gradually changes from large plate debris to small flake, block and powder debris. The maximum ejection distance of rockburst debris gradually increases, and the dynamic failure phenomenon of rockburst becomes more and more obvious; (3) As the loading stress gradient increases, the throwing kinetic energy of rockburst debris per unit mass shows an increasing trend. The research results are of great significance to tunnel safety construction and rockburst early warning. Highlights: The stress gradient is an important factor affecting rockburst in deep rock mass engineering. The value of the stress gradient coefficient affects the temporal and spatial distribution of energy release process. As the loading stress gradient increases, the rockburst debris gradually changes from large plate debris to small flake, block and powder debris. With the increase of the stress gradient, the maximum ejection distance of debris gradually increases, and the dynamic failure tends to be severe. [ABSTRACT FROM AUTHOR]

To study the influence of fracture dip angle on the mechanical properties and fracture evolution mechanism of granite under triaxial stress state, MTS 815 mechanics test system was used to conduct triaxial tests on granite with different fracture dip angles, and PCI–II acoustic emission (AE) system was used to monitor the whole process information. The results show that the brittle characteristics of fractured samples with 30°, 45° and 60° dip angles are obviously weakened, while the plastic characteristics are enhanced. The fractures destroy the structural integrity of rock, resulting in the reduction of rock resistance to load and deformation. With the increase in fracture dip angles from 0° to 90°, the compressive strength and elastic modulus of rock samples show a nearly "U"-shaped changing trend of decreasing first and then increasing, and the deterioration ratio coefficients are 7.8 ~ 43.3% and 7.5 ~ 66.9%, respectively. Due to the difference of fracture dip angles, the failure of granite sample shows two modes: "through-cutting fracture surface failure" and "shear failure along fracture surface." It mainly depends on the angle between fracture surface and maximum principal stress. The fracture dip angle affects the variation law of AE signals during the fracture process. Especially for the fractured rock samples with 30°, 45° and 60° dip angles, the AE ring count and energy show obvious "migration" phenomenon, and the signal concentration distribution area is widened and moved backward. With the increase in dip angle, the active degree of rock fracture decreases at first and then increases. [ABSTRACT FROM AUTHOR]

Among the factors affecting rock mass failure, the stress state has the greatest influence on the mechanical behavior of rock. The mechanical behavior of rock depends on the change of stress path, and the stress of rock mass is closely related to deformation and failure. In order to study the influence of surrounding rock stress state on the failure mode of surrounding rock, based on the theory of elastic mechanics and fracture mechanics, this paper analyzed the stress state of surrounding rock element and crack propagation of surrounding rock, and estimated the potential failure mode of surrounding rock in different areas of a phosphate mine in Yichang. The results showed that: the surrounding rock element is affected by the gradient stress, and a larger stress gradient corresponds to a larger principal stress ratio and variation direction of the surrounding rock. The failure mode of surrounding rock is affected by the coupling effect of principal stress ratio and principal stress direction. Under the action of certain crack angle and appropriate confining pressure, different fracture mechanisms may occur in the surrounding rock compression. But the partition of surrounding rock failure mode region can predict the surrounding rock failure mode to a certain extent. [ABSTRACT FROM AUTHOR]

In order to explore the specimen failure characteristics during rock-burst under different gradient stress conditions, in this paper, a novel experimental technique was proposed; a common series of tests under two gradient stress paths were conducted on rock similar material specimens using the true-triaxial gradient and hydraulic-pneumatic combined test apparatus. And plaster was used as the rock similar material. In the experimental process, several rock-burst debris with area sizes of 100 mm2 were collected, and the fractal dimensions of typical detrital section crystal contours were analyzed and calculated using a scanning electron microscopy (SEM) method. The results showed that the specimens' failure characteristics which had been induced by the two gradient stress processes were various. Also, the mesoscopic morphology of the rock-burst detrital section had effectively reflected its macroscopic failure characteristics. It was found that the fractal dimensions of the crystal contours of the specimen's detrital section had fractal characteristics, and the box-counting dimension based on the SEM image could quantitatively describe the rock-burst failure characteristics. Furthermore, under the same magnification, the fractal dimensions of the crystal contours of the splitting failures were found to be relatively smaller than those of the shearing failures. [ABSTRACT FROM AUTHOR]