## 4 Study on Stress Transfer of Overlying Strata in Deep Goal
### Vertical Stress Transfer Characteristics of Stope  
With the extraction of the coal body, the roof and floor boundary of the goat roof loses its constraint, and the original stress equilibrium state is destroyed. To seek a new mechanical equilibrium state, the overburden stress of the goat roof is transferred to the goat, resulting in the increase in the bearing body pressure, thus forming a bearing pressure increase zone, as shown in Figure 15.  
To further explain the reason of working face advancing on overburden pressure transfer and facilitate intuitive analysis, the characteristics of stress transfer are analyzed by advancing 25 m (initial weighting step), 40 m (periodic weighting step 15 m), 55 m, 70 m, 85 m, 120 m (the first square of working face), 160 m, 240 m (the second square of  
Figure 15: Stress distribution around goat. working face), and the characteristics of stress transfer at different distances of working face advancing are obtained, as shown in Figure 16.  
With the increase in the advancing distance, the stress is transferred to the surrounding area of the goal, and the magnitude and range of the stress transfer are different. In addition, the peak stress generally shows an increasing trend, and the scope of the influence is getting larger and larger. The abutment pressure of the solid coal side of the gob-side roadway also generally shows an increasing trend. In general, the stress of the surrounding rock in front of the working face and the solid coal side of the gob-side roadway is large, which should be the key control area of the surrounding rock. In the range of 0-30 m in front of coal wall, the advance abutment stress increases first and then decreases. The position of peak stress is 4 m from coal wall, and the value is 39.9 MPa, indicating that this stage is greatly affected by the mining activity, and the side roadway of the goat should be strengthened. The distance from the coal wall is more than 30 m, and the advance abutment stress tends to be gentle. It shows that mining has little effect on the surrounding rock starting from this position. In a range 0-15 m away from the coal wall behind the working face, the goat roof beam movement is strong due to the influence of advanced mining. Additionally, the goat roof appears the phenomenon of rapid stress unloading, indicating that the goat roof is broken and collapsed at this stage, but the collapsed gangue at this stage is not filled with the goat. The bending and sinking of the overlying rock beam do not affect the collapsed gangue in the goat. With the increase in the distance from the coal wall, the abutment stress of the goat first decreases to the minimum and then increases, indicating that the caving gangue fills the goat space and then gradually compacts, supporting the overlying rock beam; thus, the stress gradually increases. From Figure 17, the peak value and spread range of the abutment stress on the solid coal side of the lower roadway are significantly larger than the upper, which is resulted from the dip angle of the coal seam. In the inclined direction, the range of 0-20 m from the coal side of the roadway, the solid coal stress increases. With the further increase in the distance, i.e., when it is greater than 20 m, the abutment stress tends to be fixed and close to the original rock stress. Figure 16: Working face advancing different distance stress transfer characteristics. The peak stress value and position data of abutment stress under different advancing distances are extracted to obtain Table 2.  
From Table 2, the advancing distance is from 0 to 85 m, and the stress is transferred to the solid coal side of the gob-side roadway, which is caused by the increasing area of the suspended roof in the goaf. With the further increase, the overburden stress in the goaf fluctuates in a small range, which is caused by the further fracture of the goaf roof and the gradual compaction of the goaf gangue.