To analyze the fractal characteristics of coal rock failure under unloading conditions, triaxial unloading confining pressure tests were carried out on coal and sandstone rock samples under different unloading rates and initial confining pressures. We examined the distribution of the surface cracks and fragmentation of the coal and sandstone samples that failed under different triaxial unloading confining pressure tests. The results showed that the fractal dimension of the surface cracks in coal and sandstone decreased as the initial unloading confining pressure increased. Thus, shear failure is more obvious in coal or sandstone with high‐stress conditions caused by unloading confining pressure than in coal or sandstone with low‐stress conditions. However, the fractal dimension of the surface cracks increased with the unloading rates. Additionally, the fractal dimension of the fragmentation in the coal and sandstone samples had a negative correlation with the initial unloading confining pressure. When the initial confining pressure was relatively low, the samples underwent splitting and shear failure; when the initial confining pressure was higher, the failure mode was mostly shear failure and the fragmentation of the samples was less homogeneous. In contrast, the fractal dimension of the fragmentation in the coal and sandstone increased with higher unloading rates. The lithology had a significant effect on the fractal dimension of the surface cracks and on the fragmentation. Samples with more internal fissures had more surface cracks and the fragmentation was more homogeneous when the rock failed compared with samples with less fissures under the same experimental conditions.

In order to understand the influence of unloading path on the mechanical properties of coal, triaxial unloading confining pressure tests with different initial confining pressure and different unloading rate were carried out. The test results show that the triaxial unloading strength of coal samples under different test conditions is lower than conventional triaxial tests, but the brittleness characteristics are more obvious. This result indicates that the coal samples are easily damaged under unloading conditions. In the axial loading stage of the confinement unloading tests, the axial strain plays a leading role. However, during the confining pressure unloading stage, the circumferential deformation is large, which is the main deformation in this stage. Higher unloading rates of confining pressure are associated with shorter times between the peak stress position and sample complete failure. This shows that samples are more easily destroyed under higher unloading rates and the samples are more difficultly destroyed under lower unloading rates. In addition, with increasing unloading rate, the peak principal stress difference and confining pressure at failure decrease gradually, whereas the confining pressure difference at failure increases gradually. Compared with conventional triaxial compression tests, the cohesion of coal is reduced and the internal friction angle is increased under the condition of triaxial unloading test.

With the mining depth getting deeper and deeper, surrounding rock control technology has become a difficult issue in the development of modern mining, especially the mining roadways affected by mining. It is very difficult to control the surrounding rock due to the intense mining of the last working faces. In order to alleviate the impact of dynamic pressure on the gob-side entry retained, solve the ventilation difficulties caused by the commonly used method of the gob-side entry retained without leaving coal pillars, and solve the disadvantages of the inability to effectively isolate water and harmful gases, this paper proposes a small coal pillar gob-side entry retained technology based on the key stratum, that is, cutting the key stratum above the top plate, transferring lateral support pressure from the workings further up the seam to the depths of the coal seam, and then protecting gob-side entry retained method. In the final method of precracking and cutting joints by blasting with D-type polytunnel, we directionally cut the lower key stratum above the roadway in the designed direction and position. The theoretical analysis, numerical simulation and field test, and the method of the blasting broken roof pressure release effect are analyzed. Compared to the unloaded pressure method, the relatively complete direct roof is conducive to the maintenance of the surrounding rock on the gob-side entry retained. At the same time, the pressure relief caused by the fracture of the lower key stratum of the coal roof significantly improves the stress environment. The stress is no longer concentrated on the small coal pillars and the gob-side entry retained, and the required roadway support strength is greatly reduced and no longer needs to carry out the construction of roadside support system. This technology reduces labor intensity and production costs.

In order to study the acoustic emission characteristics and Felicity effect in the process of coal fatigue failure and reveal the internal relationship between the fatigue damage evolution law and the acoustic emission activity, with the help of MTS815.02 electrohydraulic servo rock mechanics test system and PCI‐2 acoustic emission detection and analysis system, a triaxial cycling loading acoustic emission test was carried out on the coal samples. The results show that the higher the upper limit stress is, the more obvious the degree of fatigue damage will be caused by coal samples. At the same time, the more active acoustic emission signal will appear. The coal samples under linear loading are on the initial damage state, and slight fatigue, moderate fatigue, deep fatigue, and ultimate fatigue failure under cyclic loading. The acoustic emission shows the “L‐” type development evolution law in any previous stress level range, while at the last stress level, it shows the obvious “U‐” type development evolution law. The higher the frequency of the cyclic loading is, the higher the rate of initiation and expansion of the microcrack will be, while the more obvious acoustic emission phenomenon will appear. Furthermore, the ringing counting rate is basically the same as that of the energy counting rate. Under triaxial cyclic loading, a shear failure mode that extends along different directions of fracture surface will be presented. The acoustic emission in the range of different stress levels shows a different degree of Felicity effect. In contrast, it is more reasonable to use the principal stress difference as a parameter to study the Felicity effect of coal under cyclic loading.