## 3 The Occurrence Mechanism of SCWID
### Engineering Verification of Overburden Breakage's Features  
The mechanism of key stratum broken entirely during mining below UCA can be further verified through the water level variation of long-term hydrology detecting hole and the actual water flowing fractured zone of Qidong coal mine.  
To monitor the water level variation of the aquifer, many long-term observation holes were constructed around the Qidong mine. For example, in 7114 working face, No.SQ1 long-term observation hole is about 738 m away from 7114 working face, as shown in Figure 8, and the water level can be influenced during coal mining. Meanwhile, water inflow at 714 working face was monitored. The field data show that No.SQ1 long-term hydrology detecting hole kept stable from 7 July 2004 to 27 July 2004, and the water level elevation was about \(-\)20.5 m (ground elevation was about +22 m). However, when the SCWID happened on 29 July, the water level elevation decreased rapidly to \(-\)23 m, while the water inflow in working face did not increase suddenly, but increased step by step, then reached 60 m\({}^{3}\)/h and kept stable until 3 August 2004. Figure 9 reveals the correspondence between water inflow of 7114 working face, the water level decline of the aquifer and the supports crushing during coal mining.  
Figure 8: Location of detecting borehole in 714 working face.  
Obviously, the rapid decline of water level in No.SQ1 long-term observation hole was not caused by the water inflow in 714 working face. This is because only when the large-area falling-over of the roof appears, the rapid mobility and recharge of confined water can be caused. From the correspondence between the dynamic change of water level and water inflow variation before and after support crushing in 714 working face, it can be seen that when support crush disaster happens, the bedrock under UCA breaks entirely. This is consistent with [PERSON] [22], who analyzed and explained the relationship between the change of water level and the change of roof pressure, and believed that the rapid decline of water level was the advanced reflection of rock instability movement.  
According to the conventional method ([2], pp. 73-78), the maximum height of water flowing fracture is expected to 45 m in medium-hard rock layer, the bed rook thickness is far larger than the theoretical value. Therefore, in theory, water inrush was less likely to happen. Furthermore, in order to find out the actual height of water flowing fissures at the water inrush position of 714 working face, two ground vertical exploration boreholes were constructed near the water inrush position. D1 drill was 18 m inside the goat near the ventilating roadway of 714 working face, D2 drill was 16 m inside the goat near the haulage roadway. Figure 10 shows the difference between the results. The height of water flowing fracture of D1 and D2 is 62.0 m and 102.3 m respectively [18]. The real detection result is far greater than the estimated value, and eventually the water inrush accidents happen, which indicate that the water flowing fractured zone has developed and connected the aquifer. Meanwhile, the difference of the height of water flowing fracture around the drill location is basically the same as the difference of bedrock thickness, which further shows that when SCWID happen in 714 working face, the overlying strata broke entirely. The detection results confirmed the mechanism of the SCWID beneath UCA explained above.  
Figure 9: Relationship between water inflow and water-level variation in 714 working face.