## 2 Engineering Background  
The experimental site considered in this study was the 21309 roadway of the Huojitu coal mine. The Huojitu coal mine is owned by the China Shenhua Shendong Coal Group Corporation Limited. It is located on the bank of the Ulan Mulun River at the southern end of Daliuta town in Shenmu County, Shaanxi Province, China. The annual production capacity of the Huojitu coal mine is 5 million tons, the minefield area is 63 km\({}^{2}\), the geological reserves are 950 million tons, and the recoverable reserves are 624 million tons. Its location is presented in Figure 1.  
The 21309 roadway is located in the No. 1-2 upper coal seam, with a buried depth of 120 m. The thickness of the No. 1-2 upper coal seam in the Huojitu coal mine is 2.95-4.16 m, with an average thickness of 3.7 m. The dip angle of the coal seam is nearly horizontal, the structure is relatively simple, and it generally does not contain coal gauge. The thickness of the immediate roof is 0.94-3.4 m, with an average of 2.16 m. It is mainly composed of  
Figure 1: Location of the Huojitu coal mine.  
fine-grained sandstone, and there is a false roof with a thickness of 0.1 m in some areas. The basic roof is mainly coarse-grained sandstone, and some areas are siltstone or sandy mudstone. The immediate floor is mainly coarse-grained sandstone, with a thickness of 0.7-9.54 m and an average of 2.85 m; the basic floor is mainly medium-grained sandstone, and some areas are fine-grained sandstone or siltstone. The 21309 roadway was used as the material haulage roadway of the 21308 working face and the coal haulage roadway at the 21309 working face. Thus, it is affected by the mining influences of the two working faces. The location of the 21309 roadway and a generalized stratigraphy column are presented in Figure 2.  
With regard to design dimensions, the 21309 roadway has a width of 5.4 m and a height of 3.3 m. Its cross section was designed to be rectangular, and the roadway lies along the coal seam floor. The average mining height of the 21308 working face is 3.7 m, and the length of the working face is 340 m. The design size of the coal pillar between working faces is 15 m. The previous support design for the roadway was based on the engineering analogy method; in other words, the support design of adjacent mines with similar geological conditions was adopted. The previous support design for the 21308 roadway is illustrated in Figure 3. The roof is supported by a rebar bolt, steel mesh, and steel strip. Five rebar bolts were arranged in each row, with a diameter of 20 mm, length of 1800 mm, spacing of 1100 mm, and row spacing of 1200 mm.  
To elucidate the rock mechanics parameters of the surrounding rock of the 21309 roadway in the Huojitu mine, two groups of roof and floor rock samples of the No. 1-2 upper coal seam with a length of 12.8 m were obtained at the open-off cut of 21305(2) working face. The obtained rock samples were processed into standard sample sizes and tested in the laboratory. This testing process is presented in Figure 4, and the obtained rock mechanical parameters are listed in Table 1.  
Figure 3: Previous support design.  
Figure 2: Location of the 21309 roadway and generalized stratigraphy column.