## 1 Introduction  
With the rapid development of the world economy, the consumption of mineral resources is increasing, which has put forward higher requirements for the development of the mining industry [1, 2]. However, with the increasing depletion of mineral resources and the growing importance of environmental protection, there is an urgent need to solve the problem of how to develop and utilize mineral resources safely and efficiently [3, 4, 5]. Due to the advantages of high safety, high recovery rate and low damage to the surface environment, the filling mining method is preferred by more and more mines [6, 7, 8]. This research focuses on the underhand drift cut-and-fill stopping [9, 10]. This method is mainly used for metal deposits where both the ore and the surrounding rock are unstable or where the ore is of high value and high grade [11, 12]. Although the underhand drift cut-and-fill stopping has the outstanding advantages of flexible stope layout and excellent applicability to complex fractured rocks, it also has the problems of relatively high mining costs and low mining efficiency [13, 14]. Since miners need to work under the filling body, if the filling body is too strong, it can increase the mining cost, and if the filling body is too weak, it can be a safety hazard [15, 16]. It is generally accepted that the larger the size of the stope the higher the strength of the filling body required, but there is no scientific basis for matching the relationship between stope size and filling body parameters [17, 18, 19, 20]. Therefore, it is important to study the relationship between the size of different mining fields and the strength of the filling body, and to propose a reasonable design method for the size of the mining field and the strength of the filling body for safe and efficient recovery of the mine. Most existing studies on the strength design of filling bodies focus on theoretical calculations and numerical simulations [21; 22; 23; 24]. The thin plate theory model and beam theory model are the most commonly used force models for the top plate of the filling body [25]. [PERSON] et al. derived the relationship between the thickness of the CPB roof, CPB tensile strength and overlying load q based on the combined beam theory, and obtained the minimum thickness to satisfy the stability requirement of the CPB roof [26]. [PERSON] et al. constructed a computational model of the tensile forces on the CPB roof based on the elastic thin plate theory, and obtained that the damage mode of the CPB roof is dominated by tensile damage, which mainly occurs in the middle of the lower surface of the CPB roof. In addition, the study also shows that the tensile strength of the CPB roof is linearly and positively correlated with the overlying load q [27]. [PERSON] et al. show that when the thickness-to-span ratio of CPB roofs is larger than 1/4, large errors occur when using the elastic thin plate theory as well as the elastic beam theory for the strength design of the filling body, and other methods need to be used for the relevant design at this time [28]. The results of existing studies show that the main factors affecting the strength of the filling body are structural parameters (size of the stope, thickness and span of the CPB roof) and physical and mechanical parameters of the ore body and overlying load. In engineering practice, the structural parameters and the physical and mechanical parameters of the ore body are easy to obtain, but the overlying load is difficult to determine [29]. By summarizing the existing studies, it is found that the overlying load of the CPB roof cannot be accurately calculated yet, which would negatively affect the determination of the strength of CPB roof. Based on this, this study is the first to accurately determine the overlying load of the CPB roof by in situ measurement, which can provide data support for reasonable determination of stope size and strength of the filling body.