To investigate the influence of mesoscopic heterogeneity and confining pressure on the mechanical properties of granite under cyclic loading, a series of biaxial compression tests were conducted on granite samples with different mineral crystal size distributions using Particle Flow Code (PFC) software. A grain-based model was established to explore the effects of crystal size distribution-induced mesoscopic heterogeneity, confining pressure, and cyclic loading on rock mechanics parameters. The mesoscopic damage mechanism was analyzed from a microscopic perspective. The results showed that confining pressure and heterogeneity significantly affected the mechanical behavior and damage mechanisms of rock samples under both compressive and cyclic loading. As confining pressure increased, the peak strength, elastic modulus, and strain of the rock gradually increased, but the growth rate slowed down, mainly due to the micro-crack closure and compaction effects induced by the confining pressure. The increase in heterogeneity led to a gradual decrease in the peak stress and elastic modulus of the rock, and the influence of heterogeneity on these mechanical parameters weakened as confining pressure increased. The number of micro-cracks in the rock passed through three stages: slow growth, lag growth, and accelerated growth. Under high confining pressure, the failure mode of the rock tended to be ductile, accompanied by plastic deformation and crack closure, while heterogeneity accelerated crack propagation. The mechanical properties of granite were significantly affected by its mineral composition and hardness differences. The existence of a hardness gradient intensified local crack propagation and affected the overall mechanical behavior and damage evolution of granite.

In underground construction, especially tunnel building, a detailed knowledge of the geological conditions ahead of the tunnel face is essential for both safety and efficiency of work. Many tunnel collapses have been reported to occur because of a lack of accurate evaluation of the geological conditions. While conventional horizontal boring conducted from a tunnel face provides good accuracy, it is infrequently conducted due to its high cost and time-consuming nature. In addition, the tunnel seismic reflection method has limited practicality owing to the complexity of data processing. This paper presents a new approach based on the specific energy of the mechanical drilling rig to evaluate the geological conditions ahead of the tunnel face. In order to assess the geological conditions, rock mass quality index, buried depth of the tunnel and tunnel deformation investigation were undertaken, and the obtained data were compared to the specific energy. Results from the comparison reveal the evaluation criterion that if the distribution of specific energy in some areas deviates from the distribution of buried depth, it is considered that abnormal geological conditions exist in this area. This work can greatly contribute to the accurate and effective evaluation of the geological conditions ahead of the tunnel face.

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. At the same time, PFC numerical simulation software is used to establish relevant models to study the evolution of microcracks and the distribution characteristics of the force chain on the rock discontinuity during the shear process. The results show that the shear mechanical properties of irregular serrated rock discontinuities are affected by normal stress, undulating angle, and undulating height. The shear strength increases with the increase of normal stress and undulating height, and decreases with the increase of undulating angle. The numerical simulation results show that the irregular structural surface cracks under different undulation angles, which first start at the near force end serration root on both sides and further evolve to the adjacent serrations, while the irregular structural surface cracks under different undulation heights, which first start at the serration root with the lowest height and expand to the adjacent serrations. At the same time, the number of cracks increases with the increase of normal stress and the force chain is mainly distributed near the sawtooth surface. The force chain is more concentrated near the near force end sawtooth and at the tip and root of the rest of the sawtooth. At the same time, the direction of the force chain is approximately perpendicular to the force surface of the sawtooth. The research results are helpful in further understanding the shear mechanical properties and differences of irregular serrated rock discontinuities.

An appropriate understanding of the hydraulic characteristics of the two-phase flow in the rock fracture network is important in many engineering applications. To investigate the two-phase flow in the fracture network, a study on the two-phase flow characteristics in the intersecting fractures is necessary. In order to describe the two-phase flow in the intersecting fractures quantitatively, in this study, a gas-water two-phase flow experiment was conducted in a smooth 3D model with intersecting fractures. The results in this specific 3D model show that the flow structures in the intersecting fractures were similar to those of the stratified wavy flow in pipes. The nonlinearity induced by inertial force and turbulence in the intersecting fractures cannot be neglected in the two-phase flow, and the Martinelli-Lockhart model is effective for the two-phase flow in intersecting fractures. Delhaye’s model can be adapted for the cases in this experiment. The turbulence of the flow can be indicated by the values of in Delhaye’s model, but resetting the appropriate range of the values of is necessary.