## 5 Results and Discussion
### Discussion  
Before the mining of working face 1511, in the overlying strata of strip goaf 1513, only KS1 breaks and rotates, KS2 shows a certain degree of deflection subsidence (Figures 7a,b and 12a), and other KS only show a small degree of deflection subsidence (Figure 7c-g). Obviously, the rotation degree of the KS is different (decrease from KS1 to KS7). When the working face 1511 is gradually mined towards the right to 30 m, the KS of overlying strata does not break or rotate due to the small mining length. However, during this process, the overlying strata stress of goaf 1511 transfers to coal pillar B and coal rib along the direction of working face 1511 (Figure 12b). At this time, the pressure of coal pillar B will increase and the overall subsidence will occur (Figure 13b). The overall subsidence of coal pillar B will result in the overall subsidence of the rock strata in the \"T\" type area, which further leads to the overall subsidence of KS1 and other KS in the strip goaf 1513 (Figure 12b). Figure 11: Change law of surface displacement of BHR 1513 with mining in working face 1511. Figure 12: Collapse characteristics of KS in overlying strata during mining of strip goat 1513 and working face 1511: (**a**) strip goat 1513 mining finished, working face 1511 unmined; (**b**) small mining distance of working face 1511; and (**c**) long mining distance of working face 1511. When the working face 1511 is gradually mined towards the right to 60-90 m, the compression and deformation of coal pillar B occur due to continuous pressure (Figure 13c,d), resulting in the penetration trend between strip goaf 1513 and goaf 1511, which forms a larger goaf. The overlying strata of goaf break up and collapse in turn, resulting in the breaking and rotation of the original unbroken high KS above strip goaf 1513, which forms dynamic block A and block B (Figure 12c). The broken block A and block B in KS1 above coal pillar A continued to rotate towards strip goaf 1513 under the breaking and rotating effect of high KS (Figure 14), making each KS in the lateral roof of BHR1513 begin to increase the rotation degree (Figure 7). Therefore, the vertical displacement of the 5 measuring points in the roof above coal pillar A begins to change (Figure 9a), showing a trend of rotation towards the strip goaf 1513. The further rotation and collapse of the broken block A and B in KS1 above coal pillar A towards the strip goaf 1513 will exert a disturbance effect on BHR 1513. The law of roadway surface displacement also reveals that when the mining distance of working face 1511 exceeds 60 m, the displacement velocity of the coal pillar sidewall begins to significantly accelerate (Figure 11), which is due to the overall displacement of the coal pillar sidewall towards the roadway space under the rotation of the broken rock in KS1 (Figure 14). When the mining distance of working face 1511 arrives at 90 m, coal pillar B is eventually crushed (Figure 13d), and goaf 1511 and strip goaf 1513 connect to form a larger goaf. At this time, all 7 KS are broken and rotated with an increased rotation degree, causing stronger disturbance to BHR 1513. Figure 14: Comparison of orientation of lateral main roof and attitude of coal pillar A of BHR 1513 before and after mining of working face 1511. Figure 13: Deformation characteristics of coal pillars A and B due to stress during mining of working face 1511. It is worth noting that before the 1511 working face is mined, the KS1 lateral broken rock above coal pillar B is inclined towards the strip goaf 1513. Therefore, when working face 1511 is mined to 30-60 m, the measuring line segment of KS1 gradually changes from an incline towards strip goaf 1513 to rotate towards strip goaf 1511. Therefore, the curve in Figure 8 changes from an inclination towards the left to flat, showing a trend of overall subsidence.