## 2 Field Sampling Analysis of Roadway Rock Bolts  
### Engineering Background  
The coal seam mined at the 41072 working face of Changping Mine is the main mined 3\({}^{\#}\) coal seam of the mine. The average burial depth is 500 m. The average dip angle is 6\({}^{\circ}\), belonging to the nearly horizontal coal seam. The thickness of the coal seam ranges from 4.60 to 6.83 m, with an average thickness of 5.86 m. The length of the 41072 working face is 300 m, and the designed mining height is 5.86 m. Fully mechanized mining with full-seam extraction is adopted. The average advancing speed is 2.54 m/d. The treatment method of the goat is the total saving method. Furthermore, according to the geological data of the mine, the hydrogeological conditions of the mine are simple: the coal seam is not prone to spontaneous combustion, and the coal dust has no explosiveness. The mine belongs to a low-impact tendency mine. However, due to the large mining height and burial depth of the working face, the periodic weighting of the mine is intense. The 41072 roadway in Changping Mine was originally the return airway of the 4307 working face. After the mining of 4307 was completed, the roadway was retained to serve the 4107 working face. The layout of roadway 41072 is shown in Figure 1. The 4107 working face mainly mines the 3\({}^{\#}\) coal seam, with an average thickness of 5.75 m. The average uniaxial compressive strength of the coal body is distributed between 12.8 and 15.4 MPa. The immediate roof is sandy mudstone, with a thickness of 1.57 m. The fissures are developed, the rock is soft, and the strength is mainly concentrated between 27.6 and 32.6 MPa. The main roof is medium sandstone, with a thickness of 9.26 m. The rock is hard, the strength is between 70.0 and 80.0 MPa, and the uniaxial compressive strength is large. The immediate floor is sandy mudstone, with a thickness of 6.89 m. The strength is between 25.8 and 34.9 MPa, and the uniaxial compressive strength is small. The old floor is sandstone, with a thickness of 2.20 m, hard texture, and an average strength of 70.5 MPa. The measurement results of in situ stress in the fourth panel area of Changping Mine show that the maximum \(\sigma_{\rm H}\) is 10.81 MPa and the minimum is 8.85 MPa. In terms of magnitude, the magnitude of the in situ stress field in the fourth panel area is moderately low. The lateral pressure coefficient \(\sigma_{\rm H}/\sigma_{\rm V}\) is greater than 1. The in situ stress in the panel area is dominated by horizontal stress and belongs to the type of tectonic stress field. The maximum horizontal principal stress is generally in the north-east-east direction. The geological and hydrological conditions in the fourth panel area are relatively simple. The 41072 roadway is excavated along the floor, with 2 m thick top coal at the top and a layer of gangue 500 mm below the immediate roof. The 41072 roadway adopts the combined support of rock bolts and cables. The rock bolts have a diameter of 22 mm and a length of 2400 mm. The steel grade is BHRB500, where 'B' is the abbreviation of the manufacturer, 'HRB' is the abbreviation of 'Hot Rolled Ribbed Bars', and '500' indicates a yield strength of 500 MPa. The thread at the tail of the bolt is M24, where 'M' represents the common thread and '24' indicates that the major diameter (nominal diameter) of the thread is 24 mm. It is often used for fastening mechanical connection components. There are six rock bolts in each row, with a row spacing of 900 mm and a spacing of 1000 mm. The top corner rock bolts are 300 mm away from the roadway side and are all set perpendicularly to the roof. The resin-extended anchorage uses two anchoring agents, one with a specification of MSK2335 and the other with a specification of MSZ2360. Among them, 'MS' represents 'Resin Anchoring Agent', '23' is the diameter of the anchoring agent, and both are 23 mm. The length of the anchoring agent is denoted as '60', which is 350 mm and 600 mm, respectively.