Roof accidents seriously affect the safe and efficient mining of the working faces. Therefore, it is necessary to assess and identify the possible and influencing factors on the occurrence of roof risk in a fully mechanized mining workface. In this study, based on the analytic hierarchy process and fuzzy comprehensive evaluation, a comprehensive standard cloud model was established through constructing a quantitative grade interval and calculating the weight of each index to achieve the aim of a roof risk assessment and identification. The accuracy of risk assessment was ensured by using the comprehensive analyses of various aspects, such as cloud digital features, risk assessment cloud image and standard cloud image. This showed that the main influencing factors on the occurrence of roof accidents were roof separation distance, weighting intensity and rib spalling followed by the coal body stress concentration, initial support force and geological conditions. Taking 42,115 fully mechanized working faces in the Yushen coal mining area as an engineering background, this model was adopted to assess and identify the risk of roof accidents through generating comprehensive assessment cloud images and introducing the Dice coefficient to calculate the similarity degree. The results showed that the overall risk of roof accidents in 42,115 working faces was regarded as grade II (general risk) through the overall index of comprehensive risk evaluation and a similarity degree of 0.8606. The impact of roof condition was mainly influenced by the risk of roof accidents, while the support status, personal working status and coal body condition had a limited effect on the risk of roof accidents. The comprehensive standard cloud model proposed in this study had strong visibility and discovered the key parts of risk indexes easily to solve the problems of ambiguity and quantitative identification in traditional roof risk evaluation methods. Therefore, this model was worth promoting, because it laid the foundation for the intelligent identification and early warning system of roof accident risk in a fully mechanized mining workface.

In order to solve the problems of roadway stability and easy instability under repeated mining of close-distance coal seam groups, the mechanism and control technology of surrounding rock instability under repeated mining were studied via indoor testing, field testing, physical similarity simulation experiment, and numerical simulation. The results show that the surrounding rock of roadway has low strength, low bearing capacity, and poor self-stabilization ability, and it is vulnerable to engineering disturbances and fragmentation. Affected by the disturbance under repeated mining, the roadway surrounding rock cracks are developed and the sensitivity is strong, and it is prone to large-scale loose and destroyed. The location of the roadway is unreasonable, and the maximum principal stress of the roadway is 3.1 times of the minimum principal stress, which is quite different. Thus, under a large horizontal stress, the surrounding rock undergoes long-range expansion deformation. On the basis of this research, the direction and emphasis of stability control of roadway surrounding rock under repeated mining of coal seam groups in close-distance are shown. A repair scheme (i.e., long bolt + high-strength anchor cable + U-shaped steel + grouting) is proposed, and reduces the risk of roadway instability.

Aiming at the problem that it is difficult to achieve accurate laying of model and precise excavation of roadways in special surrounding rock structure roadway according to conventional physical similarity simulation, which reduces the reliability of experimental results. An accurate laying of model and precise excavation of roadway method, named “labeling positioning and drawing line, presetting roadway model” (LPDLPRM), was proposed. The physical similarity simulation of deformation and failure characteristics of surrounding rock of coal-rock rise, under the influence of repeated mining in close distance coal seams, was carried out based on the method and infrared detection. The results show that the coal-rock rise in close distance coal seams was affected by repeated mining disturbances, and the surrounding rock of coal-rock rise was characterized by obvious asymmetric deformation, specific for the stress and strain near the coal pillar were higher than that of other parts, and cracks near the coal pillar were denser than other parts; when the coal seam is mined in which the coal-rock rise is located, the stress concentration of the surrounding rock near the rise was weakened by mining pressure relief in the upper coal seam; the stress concentration of the surrounding rock near the rise increases when the coal and the lower coal seam are mined, and the stress on the right side (coal pillar side) near the coal-rock rise was the most concentrated. Therefore, it is important to take measures to strengthen support near the coal pillar and to control asymmetric deformation when the coal-rock rise is influenced by repeated mining.

In order to study the influence of roof stress and fracture distribution characteristics on the stability of stope surrounding rock under repeated mining in close-distance coal seams, the research background of this paper is the close-distance coal seam mining of a mine in Guizhou province, China. The dynamic evolution characteristics of stope roof caving and the stress environment change law under repeated mining are studied by using the method of similar simulation experiment, numerical simulation, and field verification. The results show that the roof stage under repeated mining can be divided into four stages: normal mining, roof deterioration, end face roof leaks, and support crushing. The tip-to-face distance, support height, and support working condition are the main influencing factors in end face roof control. The main reason for the support crushing is that the roof above the support is broken, the main roof cannot form the self-stable structure, and the support force is insufficient. The roof dynamic load is high, and the subsidence of the end face roof is large, which is prone to roof caving during the stope roof weighting. Moreover, the roof weighting is frequent under repeated mining, which leads to the broken roof and the support cannot be supported, resulting in the occurrence of the support crushing accident. The control strategies for stope roof under repeated mining are provided based on the thorough study findings. The working face 17101 roof is successfully controlled by the aforesaid procedures, which provides the foundation for the management of stope surrounding rock under repeated loading.

This study aims to effectively solve the problem of the destruction of the coal roof and floor overlying rock after mining the protective layer, and determines whether the gas in the protected layer can be effectively released. We do this based on the engineering background of the Weng’an Coal Mine; research and analysis of the movement of the roof and floor overlying rocks; the evolution of cracks and the pressure relief characteristics of the protected layer after mining the protective layer; and through theoretical analysis and similar simulation experiments. Through numerical simulation, it was found that the protected layer was depressurized due to the mining of the protective layer, and the decompression rate of the protected layer was 0.2–0.8. In addition, the overall expansion rate of the protected layer was greater than the requirements of the “Detailed Rules for Prevention and Control of Coal and Gas Outbursts” for coal mines, and the investigation of the residual gas pressure and content of the protected layer revealed that the protected coal seam had been mined in the upper protective layer coal face. The gas pressure dropped to 50.7% of the original coal seam gas pressure, the rate of decrease was 49.3%, the residual gas content dropped by 68.67%, and the gas concentration in the return airway was 0.31% on average, meeting the national regulations that require its value to be less than or equal to 1%. The study comprehensively demonstrates that mining of the protective layer is beneficial to the release of gas from the protected layer, and provides a practical reference for coal and gas outbursts in mines.