## 2 Influencing Factors of the Height of WCFZ  
Based on extensive theoretical and field research findings, some factors affecting the development height of WCFZ mainly include the following aspects:  
1. Mining height Mining height is the only parameter contributing to the empirical formula in the 'Code for Coal Pillar Setting and Mining' jointly formulated by four national departments. It is the main factor affecting the height development of WCFZ. Some engineering cases show that with an increase in mining height, the range of the plastic zone in overlying strata will also increase accordingly; that is, the height of the caving zone and WCFZ will increase.
2. Mining depth The damaged degree and range of the surrounding rock after excavation are often affected by the stress of the rock mass near the excavation space. The original stress of rock mass includes gravity stress and tectonic stress. In general, the greater the mining depth, the greater the initial stress due to gravity stress, and the more serious damage degree of overlying rock after excavation will appear. Therefore, the development height of WCFZ is positively correlated with the mining depth.  
Panel width is also one of the important factors affecting the development height of WCFZ. Overlying strata can be regarded as rock beams fixed at both ends. When the mining span of the working face is larger, the rock beams will bend downward under the gravity action, resulting in a greater development height of WCFZ. However, with an increase in panel width, the height of the fissure zone increases slowly and finally reaches a stable level and does not increase with the increase in panel width.  
The structure type and compressive strength of the combined rock strata also affect the development height of WCFZ. Under normal circumstances, the higher the strength of the overlying rock, the higher the development height of WCFZ. However, it is difficult to quantify the roof strata whole strength and the combination structure; therefore, we introduce a parameter ratio of hard rock \(b\) referring to the ratio of the accumulated thickness of hard rock in the mining influence range, which is 15 to 20 times of the mining thickness in most cases [32]. Thus, the empirical formula is as follows:  
\[b=\frac{\sum h}{(15\sim 20)M} \tag{1}\]  
where \(b\) is the ratio of hard rock, \(M\) is the mining thickness, and \(\sum h\) is the accumulated thickness of hard rock within the mining influence range.  
Coal seam dip  
Due to the differences in the occurrence characteristics of coal seams, coal seams can be divided into nearly horizontal coal seams (<8\({}^{\circ}\)), gently inclined coal seams (8-25\({}^{\circ}\)), inclined coal seams (25-45\({}^{\circ}\)), and sharply inclined coal seams (>45\({}^{\circ}\)) according to the coal seam dip. Because the sliding state of rock strata after the formation of mining cracks will be affected by the inclination degree of rock strata, that is, when the inclination of coal seam changes greatly, there will be great differences in the development process, distribution pattern, and maximum height of WCFZ.  
The descriptive information of the above-mentioned influencing factors is shown in Figure 1a,b.  
Based on the previous studies, the five factors such as mining thickness, mining depth, panel width, the ratio of hard rock, and coal seam dip are selected as the main parameters for the prediction height of WCFZ. Through on-site investigation and reference to relevant literature, a total of 129 field-measured data sets of WCFZ and influence factors are collected, as shown in Figure 2. In total, 129 data sets are presented in detail in Appendix A.  
Figure 1: Underground mining section (**a**) and plan (**b**) schematic diagram.