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The existence of joints has a significant influence on the failure characteristics of rock mass. In this paper, based on two-dimensional (2D) particle flow code (PFC2D), uniaxial compression tests are conducted on the granite specimens with a single joint at different dip angles to investigate the evolution laws of macroscopic and mesoscopic failure characteristics. The results demonstrate that macro-mechanical characteristics of rock are influenced by the dip angle of the joint. The peak strength, peak deformation, and elastic modulus are positively correlated with the dip angle of the joint, while Poisson's ratio is negatively correlated. With the increase in the dip angle of the joint, mechanical characteristics of rock finally tend to be consistent with those of an intact rock specimen. In addition, when the dip angle of the joint is small, the rock specimens show obvious plastic characteristics before reaching the peak and there are multiple peaks being found on stress–strain curves. As the dip angle of the joint rises, the number of stress fluctuations gradually decreases and the curve near the peak becomes straighter and sharper. Moreover, the rock specimens exhibit more obvious brittle characteristics. The increase in the dip angle of the joint enhances the tendency of azimuth angle of microcracks to develop along the direction of the maximum principal stress. In the meanwhile, the distribution of the azimuth angle is gradually similar to that of microcracks in the intact specimen. With the rise of the joint dip angle, the failure modes of rock specimens change from tensile failure to shear failure to tensile failure, and finally tend to be consistent with the intact rock. [ABSTRACT FROM AUTHOR]

Uniaxial and triaxial compression tests are commonly used in rock engineering to achieve strength and shape variations parameters. However, loading conditions in real projects like excavation, underground reservoirs, and even high-magnitude earthquakes play a vital role in these experiments. Therefore, to represent the actual loading conditions on Earth, a true triaxial test ( σ 1 ≥ σ 2 ≥ σ 3 ) is performed on rock samples. In this regard, choosing the right shape of samples has significant effects on stress distribution. Cylindrical samples are more common in experimental tests because they represent better behaviour under stress and can compensate for some cubic drawbacks. Accordingly, this paper proposes a new simulation of a true triaxial test on cylindrical samples to present an efficient and economical procedure to understand the behaviour of rocks in a true triaxial state. Particle flow code in three dimensions (PFC3D) based on the discrete element method (DEM) and flat joint model were selected in this study. First, based on the data from experimental tests, a simulation of rock was performed in PFC3D software. Then, the micro-parameters were calibrated based on the macro-parameters of Inada granite test results, and the effect of those micro-parameters in this step was investigated. Next, the cylindrical true triaxial test machine was simulated in PFC3D, and loading conditions on the sample were simulated by applying various stresses in x-, y-, and z-directions. At last, the effect of unequal confinement pressure on the ultimate sample strength and failure has been investigated to predict the sample behaviour under true triaxial conditions. [ABSTRACT FROM AUTHOR]

The physical and mechanical properties of the surrounding rock on the geothermal wellbore will be deteriorated by high temperature, resulting in reduced stability. In order to investigate the tensile properties and damage mechanism of sandstone after the action of different temperatures, direct tensile tests were conducted on square sandstone after the action of 20 °C, 200 °C, 600 °C, and 1000 °C and tested the change rule of wave velocity in the process of rock gradual cracking and analyzed the influence of temperature on roughness. Meanwhile, based on the Weibull distribution and the statistical damage principle, a statistical damage constitutive model for square sandstone, considering high temperature and tensile effects, is constructed, and the parameter expressions are determined. The result showed that the tensile stress–strain curve of square sandstone has four-stage properties, and the tensile strength and elastic modulus of square sandstone gradually decrease with the increase of temperature. The tensile strength and elastic modulus of the square sandstone gradually decrease, and they decrease by 35.55% and 76.43% after the action of 1000 °C relative to that of 20 °C. The overall wave velocity of the rock also tends to decrease with strain and temperature. And roughness gradually decreases with increasing temperature. The comparison of the model curves reveals that the calculated curve and the test curve have a high degree of agreement, indicating that the constitutive model can reflect the tensile stress–strain process of the square sandstone and can reflect the reasonableness of the model. The coupled damage variables D after different temperature effects showed an S-shaped monotonic increasing trend with strain. The research results of this paper can provide some theoretical guidance for the stability assessment of geothermal mining engineering. [ABSTRACT FROM AUTHOR]

The deep surrounding rock is in a three-directional stress state before mining. The process of roadway excavation is the process of confining pressure unloading due to the engineering needs. Therefore, the confining pressure effect plays an indispensable and important role in deep engineering. Based on the above engineering background, the current research status and development trend on the confining pressure effect are introduced. Besides, combined with the results of rock mechanics tests, the basic mechanical parameters, strength and deformation evolution law, energy evolution law, damage evolution law, and failure characteristics of coal and rock under different confining pressures are revealed. Meanwhile, four new insights are proposed: (1) It is urgent to establish the detailed evaluation index system to quantitatively and qualitatively characterize the brittleness and ductility of rock under the confining pressure effect; (2) It is necessary to strengthen the research on the time-dependent deformation mechanism and the construction of time-dependent strength evolution model of coal and rock, so that the localization of time effect is gradually transferred from rheological mechanics to other mechanical fields; (3) It is valuable to strengthen the research on the mechanical properties of coal and rock under the confining pressure effect from the microscopic perspective; (4) It is meaningful to strengthen the creep mechanical experimental research on coal and rock with the new stress path of "three stages" loading, which includes initial high in-situ stress state reduction, constant axial pressure with unloading confining pressure, and constant confining pressure with loading axial pressure corresponding to three stages of the stress variation characteristics of the surrounding rock around the roadway of deep mines: before excavation, being excavated, and after excavation. [ABSTRACT FROM AUTHOR]

To study the mechanical behavior of apparent fissures in salt rock, tests on the rock mechanics were conducted on an RMT-150B multifunctional system. The effects of the apparent fissures on the strength and deformation, as well as the apparent fissure propagation behavior of the salt rock and its coalescence pattern, were analyzed. The propagation evolution of the apparent fissures was divided into two types. The main fissures extending from the initial apparent fissures of the salt rock specimens comprised the first type. The second type was that the initial apparent fissures of specimens were closed. Then, new microfissures initiated from the initial apparent fissures gathered together, and formed main fissures. Simultaneously, the secondary fissures grew slowly. The tensile coalescence pattern and tension-shear coalescence pattern are the main coalescence patterns of salt rock with apparent fissures. Pressured coalescence patterns and other coalescence patterns did not appear. Owing to the compactness of the structure, the paths of fissure coalescence present an obvious zigzag line. The influence of vertical transfixion fissures on the deformation modulus of salt rock is greater than the influence of inclined fissures. [ABSTRACT FROM AUTHOR]

The mechanical properties of shale rock have a great significance on drilling engineering and the establishment of safety protection facilities in the process of shale gas exploitation. Hence, in this article, the deformation characteristics and the strength criterion of the Longmaxi formation shale rock are examined through uniaxial direct tension tests, uniaxial compression tests, and triaxial compression tests. The results show that shale rock experienced linear elastic stage, yield stage, and failure stage, under the direct tensile state and triaxial compression state. The shale rock experienced initial compaction stage, linear elastic stage, yield stage, and failure stage under uniaxial compression. The fracture surface of shale rock is smooth horizontal section under uniaxial tension. Under uniaxial compression, it is mainly manifested as splitting failure with shear failure in part. Under low confining pressure of triaxial compression, the shale rock show shear-dilatancy failure, and under high confining pressure, the shale rock show local shear failure along the primary fracture. The six strength criteria are used to describe the strength characteristics of shale rock. The results show that the Mohr-Coulomb criterion and Hoek-Brown strength criterion can well describe the shale rock under relative low confining pressure. It is found that both the generalized Hoke-Brown criterion and the power function strength criterion can better describe the strength characteristics of shale rock under different confining pressures than the other four strength criteria. The research results can provide experimental basis and mechanism understanding for the analysis of mechanical properties and engineering application of shale rock. [ABSTRACT FROM AUTHOR]

Rock energy theory and its application have an important significance on the analysis of the essential reason of rock failure, and the optimization of design and construction of rock engineering. Recently, research studies on the rock mechanics based on the energy principle and rock mechanics from energy theory have been done. Aiming at the main contents of this field, combining with the experimental results, theoretical analysis, and numerical simulation, and according to the rock energy reserve, release, and dissipation mechanism, the energy conversion characteristics of rock during the deformation and failure are summarized. Then, the various energy damage constitutive models of rock are introduced. Finally, the research on rock energy strength criteria and engineering applications of rock energy theory are reviewed. Moreover, the scope of their applications is clarified, and the defects and application prospect of these studies are proposed. However, there are still many key scientific problems in rock energy theory and its application research. Based on above, the prospects for future research in this field are provided. [ABSTRACT FROM AUTHOR]

Arc fissure is one of the basic forms of defective rock, where the expansion and evolution mechanism plays an important role in the stability of engineering rock mass under the external load action. Uniaxial compression experiments of sandstone samples that contained various angles of arc fissures (sandstone sample was 80 mm × 160 mm × 30 mm) were performed in order to investigate the effect that arc angle α had on the mechanical properties, the failure mode, and the fracture evolution process of sandstone. The results showed that when arc angle α was increased, the peak strength and the strain of the sandstone samples initially decreased before increasing and the minimum peak strength and strain were reached when α = 15°. The deterioration of the bearing capacity and the number of cracks that appeared during the sandstone loading process decreased as the arc angle of the fissure increased. The arc fissure destruction was primarily initiated from the fragile area of the arch tip. The tensile cracks appeared on the fissure tip and non-tip as the axial force increased. The various arc angle α played an important role in the initiation stress and the rupture evolution of the specimen. [ABSTRACT FROM AUTHOR]