## 1 Introduction  
China is rich in coal resources, among which the reserves of thick and extra-thick coal seams account for more than 40% of China's coal reserves [1, 2]. Thick and extra-thick coal seams are the main mining coal seams in China at this stage. With the continuous improvement of fully mechanized caving surrounding rock control systems [3, 4, 5, 6] and the continuous improvement of underground mechanization and intelligence [7, 8, 9, 10], fully mechanized top coal caving mining is widely used in underground thick coal seam mining because of its low cost and high yield, effectively reducing the number of working face moves and other advantages. However, there are many problems in the final mining stage of extra thick coal seam, such as the coal recovery rate being low [11; 12], the working face being under severe pressure, and the support withdrawal being blocked [13; 14]. Many scholars and experts have conducted research on the overburden structure [15; 16; 17], the law of mine pressure behavior [18; 19; 20], and the final mining withdrawal process [21] of the extra-thick fully mechanized caving face. In order to solve the problem of serious floor heave in water-bearing soft rock roadway, [PERSON] et al. [22] used an indoor test, numerical simulation, and other research methods to study the mechanism of floor heave and the difference in grouting schemes with five different reinforcement depths, and finally obtained the reliable conclusion that the grouting depth is determined by floor heave and water content. Aiming at the problems of complex overburden rock migration and the difficult control of surrounding rock in steeply inclined and extra-thick coal seam mining, [PERSON] et al. [23] used a variety of research methods to analyze the characteristics of overburden rock migration under geological conditions, and the research results achieved safe and efficient mining of fully mechanized caving. [PERSON] et al. [24] used a variety of research methods to demonstrate that surface filling is a reliable means to control the influence of strong mining in the thick coal seam under a complex overburden environment. In the technology of gob-side entry retaining in fully mechanized caving mining, [PERSON] et al. [25] established a mine pressure model, numerical model, and similar model of keeping an alley along empty space under the condition of top coal caving mining, and summarized the stress evolution characteristics between the top coal and filling body and the influence of different top coal conditions on the stability of keeping an alley along empty space. [PERSON] et al. [26] studied the injury degrees of surrounding rock in a shallow fully mechanized caving mining face by using borehole peeping and CT scanning technology. [PERSON] et al. [27] established a mechanical model in the fully mechanized top-coal caving mining of an extra-thick island working face and deduced the mechanical equation of mining stress distribution and failure depth of coal rock under this geological condition. [PERSON] et al. [28] determined the distribution range of an internal and external stress field and the reasonable layout position of roadway by studying the overburden structure and stress evolution characteristics of extra-thick fully mechanized caving mining. Based on the problem of asymmetric deformation and failure of gob-side entry in a fully mechanized caving face, [PERSON] et al. [29] used a variety of research methods to analyze the stress evolution and plastic deformation mode of coal pillar roadway, and put forward the asymmetric high pre-tightening force support scheme. From different research perspectives, the above scholars have performed effective research on the overburden structure, mine pressure behavior laws, and support control of mining roadway in extra-thick fully mechanized caving face using different research methods. The final mining working face involves the problem of safe withdrawal.