## 1 Introduction  
China is a major coal producer, producing more than half of the world's coal annually [1; 2]. Among the measured coal reserves in China, thick and extra-thick coal seams account for about 45% [3; 4]. With the application and promotion of large mining height and top coal caving technology, thick and extra-thick coal seam become the main mined seam due to its high yield and efficiency [5; 6]. Compared with traditional coal mining methods, large mining height and top coal caving methods have higher requirements for the sections of the underground strap joint chamber and washing chamber, etc. [7; 8]. With the above two coal mining methods, the area of the chamber section is generally over 70-100 m\({}^{2}\). When the large-section chamber is arranged in the coal seam, it can not only speed up the excavation, but also increase the coal output and reduce the gangue discharge [9]. However, when excavatingthe large-section chamber in the coal seam, the coal body often breaks off from the chamber dome and creates a roof fall hazard due to the long span and high height of the chamber and the relatively low strength of the coal body. There is usually no precursor to the failure of the chamber dome. Once the roof fall hazard occurs, it is often sudden. As shown in Figure 1, the roof fall hazard not only causes casualties, but also brings great psychological pressure to the workers in workforce, damages mechanical equipment and delays production, which adversely affects the normal production of coal mines [10]. Therefore, it is extremely important to control the roof fall hazard of the full seam chamber with ultra-large section. In order to control the roof fall hazard, it is necessary to uncover its mechanism, based on which, the corresponding control technology can then be proposed. In view of the roof fall hazard of ordinary roadways under different geological conditions, scholars have conducted in-depth research from three aspects: prediction method; roof fall mechanism and control technology. In terms of roof fall prediction, [PERSON] et al. [11] proposed a new method for predicting roof separation from the full-seam roadway based on theoretical analysis. [PERSON] et al. [12] proposed the use of artificial neural networks to predict roof stability of the roadway. [PERSON] et al. [13] proposed a kind of monitoring system for roof separation. In terms of the leakage mechanism, [PERSON] et al. used UDEC and RS\({}^{2}\) software to reveal the roof fall mechanism for large span open-off cut under a rich aquifer [14; 15]. [PERSON] et al. [16] used numerical simulation methods to reveal the mechanism of roof fall hazard of the roadway under weak immediate roof conditions. [PERSON] and [PERSON] [17] used the boundary element method to study the roof fall induced by gas outburst. [PERSON] et al. [18] studied the influence of foundation stiffness of the immediate roof on the stability of roadway roof. In terms of roof fall control technology, [PERSON] et al. [19] proposed a method for strengthening the chamber dome by U-shaped steel support for the roadway in the weak coal seam. [PERSON] et al. [20] proposed an inner stagger layout to control the roadway roof fall for the layout of the lower coal seam roadway. [PERSON] et al. [21] studied the effect of multi-echelon support of deep roadways in a kilometer-deep well based on experiments. By a systematic monitoring and evaluation of the seismic events, [PERSON] et al [22] pointed out that rock burst was closely related to the lignite extraction. In addition, the support type was put forward to decrease the deformation. The above researches mainly focus on the prediction, mechanism and control technology of the roof fall hazard for ordinary roadway under different geological conditions, but none of them investigates the roof fall hazard mechanism of the full seam chamber with ultra-large section.