## 5 Discussion  
The height of the caved zone and fractured zone is significant in terms of water in-rush and gas drainage. These data of height obtained from the physical modeling are shown in Figure 12.  
Figure 12 shows that at the very beginning of the extraction, the height of the caved zone was equal to that of the fractured zone. When the face advanced 45 m to where the first weighting occurred, the height of the two zones was about 3.4 m above the coal seam. When the face advanced 60 m to where the first periodic weighting occurred, the height of the two zones reached 10.1 m. Following that, the rate of increase of the height of the two zones was faster than that of the caved zone. When the face advanced 105 m, the height of the caved zone was 15 m, and that of the two zones reaches 25.5 m. At that time, the increase rate of the caved is still very slow while that of the two zones increased faster. When the face advanced 127.5 m, the height of caved did not increase while that of the two zones was as high as 51.45 m. When the face advanced 165 m, the height of caved zone still remained unchanged, while a value of 66.45 m of the height of the two zones was achieved. It was not until the face advanced 180 m that the height of the caved zone increased to 25.5 m, but the height of the two zone remained the same. After that, when the face advanced 195 m, the caved zone hardly developed, while the height of the two zones began to increase dramatically, again reaching 90 m. At last, when the face advanced 200 m, the caved zone was 25.5 m high and the height of two zones was 90 m.  
In order to present the results quantitatively, the data recorded by the three measuring lines was plotted, as shown in Figures 13-15. Figure 12: The height of the caved zone and fractured zone with respect to face advance. Figure 13 shows the displacement of line 1 where the main roof is located 1 m above the coal seam. Before 30 m, the roof only slightly deflected. When extraction reached 50 m, the maximum displacement was at the center (25 m away from the set up room), with a value of 2.05 m. This indicated that the roof did not cave completely. For extraction of 70 m, still, the maximum displacement of the roof was located 25 m away from the set up room, with a value of 3.76 m. This suggests that the main roof caved almost completely and filled the caved zone. After that, the maximum displacement value hardly changed. Figure 14 presents the displacement of line 2, 31 m above the coal seam, where fine-grained sandstone stratum was located. It is also demonstrated that before 30 m of extraction, the roof hardly moved downward. When extraction reached 50 m, the maximum displacement was also at the center (25 m away from the set up room), but with a value of 0.83 m. This also indicated that the roof did not cave completely. For extraction of 70 m, the maximum displacement of the roof was located 30 m.  
m away from the set up room, with a value of 3.08 m. When the face advanced 90 m, the maximum displacement value remained unchanged, but the range with the maximum displacement value of 3.08 m covers about 25 m. When the face advanced 120 m, the maximum displacement reached 3.46 m. The peak point was located around 57 m away from the set up room. After that, the maximum displacement value only slightly changed. Figure 15 presents the displacement of line 3, 88.5 m above the coal seam where the 2# coal seam stratum was located. It is indicated that before 50 m of extraction, the roof hardly moved downward. This reflects that line 3 was not subjected to the influence of mining, as it was far from the coal seam.