Understanding the shear properties of joints of rock masses is of great importance for engineering disaster prevention and control. In this paper, a systematic study of the macroscopic shear properties of joints of rock masses with different strengths is carried out using a combination of indoor tests and PFC^2D numerical simulations. The results show that (i) the shear stress curve of low-strength rock joints is strain-softening type, while high-strength rock joints are strain-hardening type, and high-strength rock joints are more sensitive to the change of roughness. (ii) With the increase of JRC, the damage mode of different strength rock joints gradually changes from abrasion to abrasion+gnawing, and the damage characteristics of the surface of high-strength rock joints are more significant. (iii) The contact force between particles is mainly concentrated on the joints. At the beginning of shear, the contact force is mainly distributed on the second-order roughness and gradually concentrated on the first-order roughness as the shear progresses. Compared with the low-strength rock joints, the contact force on the high-strength rock joints is larger and more widely distributed. (iv) Due to the change of contact force, the cracks keep expanding and the particle rotation arc keeps changing. The particles with larger rotational arcs are consistent with the location of crack distribution, and the cumulative number of cracks on the joints of high-strength rock is higher. (v) The total input energy and dissipation energy increase continuously with the shear, and the elastic energy tends to increase at the beginning of shear and then starts to decrease and gradually tends to be constant near the peak of shear stress. The total input energy and dissipation energy of the joints of the high-strength rock are larger, while the peak elastic energy of it is smaller.

This paper carries out a study on the numerical simulation of borehole instability based on the disturbance state concept. By introducing the disturbance damage factor into the classical Mohr–Coulomb yield criterio, we establish a finite element hydro-mechanical coupling model of borehole instability and program the relevant field variable by considering elastic–plastic deformation in borehole instability, the distribution of the damage disturbance area, the variation of porosity and permeability with the disturbance damage factor, etc. Numerical simulation shows that the borehole stability is related to the action time of drilling fluid on the wellbore, stress anisotropy, the internal friction angle of rock, and borehole pressure. A higher horizontal stress difference helps suppress shear instability, and a higher rock internal friction angle enhances shear failure around the borehole along the maximum horizontal principal stress. When considering the effect of the internal friction angle of rock, the rock permeability, disturbance damage factor, and equivalent plastic strain show fluctuation characteristics. Under the high internal friction angle of rock, a strong equivalent plastic strain area and disturbance damage area occur in the direction of the maximum horizontal principal stress. Their cloud picture shows the mantis shape, where the bifurcation corresponds to the whiskers of the shear failure area in borehole instability. This study provides a theoretical basis for solving the problem of borehole instability during drilling engineering.

In order to study the macro-meso shear mechanical characteristics of natural gas hydrate-bearing sediments, the direct shear simulations of natural gas hydrate-bearing sediment specimens with different saturations under different normal stress boundary conditions were carried out using the discrete element simulation program of particle flow, and the macro-meso shear mechanical characteristics of the specimens and their evolution laws were obtained, and their shear damage mechanisms were revealed. The results show that the peak intensity of natural gas hydrate-bearing sediments increases with the increase in normal stress and hydrate saturation. Hydrate particles and sand particles jointly participate in the formation and evolution of the force chain, and sand particles account for the majority of the force chain particles and take the main shear resistance role. The number of cracks produced by shear increases with hydrate saturation and normal stress. The average porosity in the shear zone shows an evolutionary pattern of decreasing and then increasing during the shear process.

To study the shear deformation and failure characteristics of a wellbore and the interaction mechanism with its surrounding rocks induced by a layer slip during natural gas hydrates (NGHs) extraction, this paper conducted a numerical simulation study of wellbore shear induced by a layer slip using ABAQUS software and carried out a laboratory experiment of wellbore shear to verify the accuracy of the numerical model. The results show that the shear force–displacement curves obtained from the laboratory experiments and numerical simulations are consistent with five stages, including the compaction stage, linear stage, plastic stage, strain-softening stage and residual stage. The wellbore shows a “Z”-shaped deformation characteristic after its shear breakage. The shear force of the wellbore is maximum at the shear surface, and it is distributed in an approximate “M” shape along the shear surface. The axial force of the wellbore is small and uniformly distributed in the initial stage of the shear. The wellbore bending moment is minimum at the shear surface, with a value of 0, and it is distributed in a skew–symmetric wave shape along the shear surface. During the shearing, the evolution of the wellbore axial force and shear force can be classified into the distribution pattern along the radial direction on the shear surface and the pattern along the axial direction. The combination of the wellbore axial force and shear force causes the tensile–shear compound failure of the wellbore. During shearing, the wellbore and rock body gradually enter the plastic state with the increase in the shear displacement. When the entire cross-section of the wellbore is in the plastic state, a “necking” phenomenon of the wellbore begins to appear. During the shearing, the frictional dissipation energy and plastic dissipation energy increase constantly. In addition, the elastic strain energy increases to a peak and then decreases to a certain value, which remains unchanged along with the work conducted by the shear force.

In the deep mining of metal mines, the stability of pillar–backfill composite materials (PBCMs) under cyclic loading is crucial for preventing dynamic disasters in goafs. Although previous studies have extensively investigated backfill materials under static loading, the damage evolution mechanism of PBCM under cyclic disturbance—particularly the coupled effects of pillar width and disturbance amplitude—remains insufficiently understood. To address this gap, this study explored the mechanical properties and damage evolution of PBCM under cyclic loading using an indoor testing system. Tests were conducted on composite specimens with varying pillar widths (6, 9, 12, 15 mm) and disturbance amplitudes (3, 4, 5 MPa), combined with acoustic emission (AE), digital image correlation (DIC), and scanning electron microscopy (SEM). Results show that wide-pillar specimens (≥12 mm) exhibit significantly improved bearing strength and deformation modulus, with increases of nearly 90% and over 40%, respectively, compared to narrow-pillar specimens. Notably, wide pillars maintain over 95% strength stability even under 5 MPa cyclic disturbances. Narrow pillars are prone to localized damage concentration with high-frequency AE signals and shear failure, while wide pillars exhibit uniform damage development. Failure morphology confirms that pillar size dictates failure mode: narrow pillars undergo sudden through failure, whereas wide pillars display progressive composite failure, with fewer damage-induced cavities and directional crack propagation along maximum shear stress. These findings provide a theoretical basis for stope structure optimization and dynamic disaster prevention in deep mines.