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A b s t r a c tThe objective of this work is to provide a new insight into the spatial distribution of physical, chemical structure of the filter cake layer. The spatial distribution was characterized by various analytical techniques, including high-temperature high-pressure (HTHP) fluid loss test, scanning electron microscopy (SEM), energy dispersive spectrum (EDs), particle size distribution (PSD), fluorescence test, pore size, porosity, density, water and polymer content measurement etc.. Specific cake resistance and the permeability of the filter cake were measured. The results obtained show that the spatial distribution of filter cake layer structure changes from the top layer to bottom layer. A large amount of deformable colloid particles in the filter cake layer is beneficial to the reducing of the cake permeability. The polymer and water content of the cake layer decreased from the top cake layer to the bottom cake layer. Through the SEM images, cake layer was found to have a loose outer surface when compared with the bottom cake layer.

On August 2, 2019, a catastrophic rockburst disaster occurred in Tangshan mine, causing death of 7 miners. After the investigation, the coal mine is facing reproduction. Taking the 0291 panel as the engineering background, this paper studies the coal seam blasting pressure relief and ground stress monitoring technology in working face retreat. During the roadway development and working face excavation, coal seam blasting was adopted to transfer the high ground stress of coal seam to the deep ground of the coal body. The blasting operation is presented in detail in this paper. In the working face retreat stage, drilling powder method, hydraulic shield resistance monitoring, roof displacement, and vibration monitoring methods are implemented. The results show that the pressure relief range of coal seam is 4–12 m in the coal mass after blasting. The shield working resistance is stable at 20–30 MPa. The range of relative displacement of the roof is about −1.0 to 2.5 mm, and the maximum vertical vibration velocity is in the range of 7–11 cm/s, up to 12 cm/s. The measured parameters are acceptable, so it is concluded that 0291 panel can be safely mined. This study provides a reference for the coal seam blasting design for rockburst coal mine and provides a technical means for the analysis of pressure release effect and dynamic pressure monitoring during working face retreating.

To investigate the failure mechanisms of coal bursts in the longwalls of underground coal mines, a case study based on the real coal burst in the Tangshan coal mine occurring on 2 August 2019 was carried out to analyze the cause and basic theories of the coal burst and, thus, propose approaches and methods to mitigate the damage. The coal burst occurred mainly due to the sliding of coal from the longwall ribs and the simultaneous uplifting of the floor. As a result of the longwall retraction, the roof above the coal seam was too stable to collapse, forming a long hanging roof behind the longwall. Because the longwall acted as a fulcrum, the hanging roof acted like a saw, exerting massive loads on the longwall. As a result, the roof in front of the longwall tended to slope upward, leaving the underlying coal seam unconfined. Due to the horizontal stresses, the longwall ribs collapsed into the longwall, causing the coal seam to break out. These findings could help improve the fundamental understanding of the failure mechanisms of coal breakouts and, thus, aid in the development of measures to prevent such occurrences.

Rock burst poses a serious threat to mining, and its mechanism still needs to be studied. Statistics show that more than 85% of rock bursts occur in mining roadways. The authors investigated the mechanism of rock burst in the roadways by creating a finite elastic long beam dynamic model in longwall mining based on the elastic foundation theory. In this study, the fracture failure of the basic roof and the related dynamic effects are analyzed. The results indicate that the occurrence of the free vibration of the basic roof is caused by fracturing during the first face advance. It is also found that the vibration frequency increases and the period decreases with the increase in roof thickness. Furthermore, the strength and elastic modulus increase with the increase in roof hardness. The effect of a thick, hard roof on the coal mass is mainly reflected in the vibration of the roof above the coal seam. During free vibration of the roof, the stress applied by the roof on the coal mass decreases. Under the action of horizontal stress, the coal mass slips along the interface between the roof and coal mass, which may result in rock burst. The mode of the basic roof vibration and the viscoelastic properties of the coal mass significantly influence the vibration–slippage impact. With the increase in the strength, thickness, and horizontal stress of the roof, the possibility of sliding between the coal seam and the roof also increases, which may potentially result in rock burst. This mechanism may explain the occurrence of rock burst in large-scale coal bodies that move out in the roadway, which could provide a reference for the prevention and control of such phenomena.