Geological dynamic hazards during coal mining can be caused by the structural instability of a composite system of roof rock and coal layers, and joints in coal play a vital role in this structural instability. In this paper, uniaxial compression simulation tests on roof rock–coal combined bodies with a single joint at different angles in coal were conducted using PFC2D software. In particular, the effects of joint angle on the uniaxial compressive strength (UCS), acoustic emission (AE) and failure characteristics in a combined body were analysed. The results show the following. (1) The joint in coal causes a decrease in the UCS and crack initiation stress, and has promoting effects on the AE occurrence of the combined body. With an increase in the included angle α between the loading direction and joint plane direction, the UCS, AE occurrence time and crack initiation stress first decrease and then increase, presenting ‘V-shaped’ curves. The UCS and crack initiation stress at α of 45° are the lowest, and the AE occurrence time at α of 30° is the earliest. (2) The AE characteristics at α of 0°, 75° and 90° are basically consistent with those of the intact combined body, whereas at α between 15° and 60°, the intensity and frequency of AE events in the macro-failure stage are reduced by the joint in coal. (3) The failure of the combined body mainly occurs within the coal, whereas no apparent failure is observed for the roof rock. Only when α is between 15° and 45°, the roof rock near the bedding plane is destroyed by crack propagation in the coal. (4) Three typical failure modes (i.e. V-shaped shear failure cutting across the coal, shear failure partly along the joint plane direction and shear failure along the joint plane direction) are found for combined bodies with a single joint in coal.

The existence of mudstone weak interlayers has a significant impact on the stability of open-pit coal mine slopes. Under the combined influence of rainfall and groundwater, the mechanical properties of the mudstone of weak interlayers deteriorate, leading to a local loss of bearing capacity of the slope and further accelerating the overall instability of the slope. In order to investigate the changes of macroscopic and mesoscopic structures, mechanical failure behavior, and the damage evolution mechanism of water-immersed mudstone, non-destructive water immersion experiments and uniaxial compression experiments were conducted. The results indicate that the main causes of macroscopic structure failure of water-immersed mudstone are the initiation, propagation, and mutual penetration of micro cracks. The mesoscopic structure characteristics of water-immersed mudstone are primarily manifested by increased surface smoothness, increased occurrence of small-scale pores, the presence of a dense network of fissures on the surface, and fusion of mineral unit boundaries. With the increasing immersion time, the quality, relative water content, and peak strain increase, while the uniaxial mechanical parameters and energy parameters decrease. In addition, a statistically damaged constitutive model for mudstone considering the coupling damage of water immersion and low-stress loading was established, and the model is consistent with experimental results. Finally, the water-softening characteristics of mudstone are caused by the propensity of clay minerals to expand and disintegrate upon water contact, changes in pore structure, variations in mineral types and distributions, and the presence of pore water pressure. This study provides valuable insights into the water–rock deterioration mechanism of mudstone and the stability of slopes containing weak interlayers.

There is an obvious impact effect of on-site blasting on the slope coal mass of open-pit mines, so it is of great significance to study the dynamic mechanical response characteristics of coal rock for slope stability control. In this paper, first, the mineral composition and microstructure of lignite from open-pit mine are analyzed, and it is found that the content of non-organic minerals in lignite such as clay accounts for more than 24.40%; meanwhile, the rock sample has obvious horizontal bedding characteristics and mainly micro pores and transition pores inside; further, there are obvious banded areas with high water content in the rock, which has the same extending direction as the beddings. Based on the SHPB test system, the dynamic compression tests of lignite with different impact velocities are carried out. The results show that there is a significant hardening effect caused by the increase of strain rate on the dynamic mechanical parameters of rock samples, and the stress–strain curve has obvious “double peak” characteristics; meanwhile, the macroscopic crack of the rock appears at the first stress peak and disappears after further compression until the interlayer fracture occurs; further, the fracture fractal dimension of lignite increases linearly with the impact velocity, revealing that the fragmentation of rock samples increases gradually. In addition, with the increase of impact velocity, the input energy and dissipated energy of rock samples increase linearly, while the elastic property increases slowly and at a low level. The bedding characteristics of lignite and the wave impedance difference between the layers cause the high-reflection phenomenon in the process of stress-wave propagation, and then produce the obvious tensile stress wave in the rock sample, which finally results in the interlayer fracture failure of the rock.

The blasting at a site can cause impact disturbances to an open‐pit mine slope. For further study the dynamic mechanical properties of rock masses in open‐pit mine slope, in this paper, the mudstone of an open‐pit slope in Inner Mongolia Autonomous Region of China was taken as research object. Through an indoor split‐Hopkinson impact test and a finite difference method and discrete element method coupling simulation (FDM‐DEM), the macro and micro impact mechanical response of mudstone under different impact velocities was studied. The results showed that under dynamic load, mudstone exhibited significant strain rate effects. The postpeak plasticity varied in exponentially increasing changes. The crack propagation process in mudstone can be divided into undamaged, initiation, propagation, and rupture stages. As the impact velocity increased, the initiation stage exhibited more microcracks, and the cracks opening in the rupture stage became larger. The 3D coupling numerical model can satisfy stress effectiveness during the dynamic impact process. During the impact process, microcracks increased sharply before the peak stress, and there was a strain lag between the maximum point of crack increment and the peak point of stress. A large number of internal microcracks developed during the postpeak stage, and the cumulative crack increment exhibited a reverse “Z” shape.