Observation program design
The maximum height of the water flowing fractured zone calculated by the above prediction formula is 49.79 m. In order to ensure that the observation boundary exceeds the height of the actual failure zone, the vertical depth of the borehole is extended by 11 ~ 15 m. The dip angle of 91105 working face is 3° on average, belonging to the near-horizontal coal seam. It is expected that its stable form is that the starting point is within the coal body, and the boundary gradually shifts to the coal body, and the highest point is in the saddle shape of the inner side of the goaf of the mining boundary24.
Considering the stability of surrounding rock, convenience of construction and limitation of construction conditions at the observation position (the inclination angle of borehole is between 45° and 55°), the borehole field is arranged in the transport contact roadway about 41 m away from the final mining line of the working face, and a pre-mining borehole (CQ1) and two post-mining boreholes (CH1 and CH2) are designed and constructed. of which the CH1 borehole is used to observe the middle position of the saddle top, and the CH2 borehole is used to observe the highest point of the saddle top. The layout of the observation borehole is shown in Fig. 3.
During the observation, a contrast borehole is arranged before the mining of the working face to observe the development of primary fractures in cover rock, two post-mining boreholes were arranged after the mining of the working face to observe the development of fractures in cover rock after mining. Due to the influence of mining, the overlying strata movement and deformation, borehole is difficult to form, considering the influence of lithology of overlying rock and mining thickness, the reasonable observation time of water flowing fractured zone height should be one month after the end of working face mining.
3.3 Analysis of observation results
By sorting out the observation data of each borehole, the distribution of water injection leakage in borehole is shown in Fig. 4. Comparing the change of water injection leakage in each hole section, the height of water flowing fractured zone is determined.
(1) CQ1 pre-mining borehole
From the Fig. 4, it can be seen that the average water injection leakage of the Pre-mining contrast borehole CQ1 is about 2.5 L/min under the condition that the overlying strata on the working face is not affected by mining, indicating that the rock mass structure of the roof is complete before mining the working face. The local leakage changes in the range of 10.2 ~ 12.3 L/min, indicating that there are relatively developed primary microcracks or caves in local rock strata.
(2) CH1 post-mining borehole
Compared with water injection leakage of pre-mining borehole, the leakage of water injection in the post-mining borehole of CH1 is 19.8 ~ 32.5 L/min at the borehole depth of 61 ~ 73 m (vertical height 43.13 ~ 51.62 m), and the leakage is significantly increased, indicating that the rock in this section is seriously damaged. The borehole has penetrated the fracture zone, and a large number of new fractures were produced in the rock affected by mining. In the range of borehole depth 74.5 ~ 89 m (vertical height 52.68 ~ 62.93 m), there are two abnormal boreholes, namely borehole depth 82 m (vertical height 57.98 m) and borehole depth 86.5 m (vertical height 61.16 m), and the leakage is 11.8 ~ 13.4 L/min. Compared with the pre-mining borehole analysis, it is considered that the hole section is the original micro-fracture area and the non-mining fracture area. The leakage of other hole sections is 2.1 ~ 3.5 L/min, indicating that the height of fracture zone is not developed here. Therefore, the maximum development height of the water flowing fractured zone in the working face determined by the CH1 borehole is at the borehole depth of 74.5 m, developing to mudstone 52.68 m away from the roof of working face.
(3) CH2 post-mining borehole
Compared with water injection leakage of pre-mining borehole, the leakage of water injection in the post-mining borehole of CH2 is 15.8 ~ 28.3 L/min at the borehole depth of 61 ~ 68.5 m (vertical height 46.73 ~ 52.47 m), and the leakage is significantly increased, indicating that this section is the top of the water flowing fractured zone. In the area of 70 ~ 82 m in borehole depth (53.62 ~ 62.82 m in vertical height), the leakage of most hole sections is about 2.7 L/min, which is not significantly different from that of the corresponding section of the pre-mining borehole, indicating that the rock stratum of this hole section is relatively complete. Other local leakage is 9.1 ~ 11.0 L/min, which is obviously higher than that of the same hole section of the pre-mining borehole, indicating that there are small primary fractures in this section. Therefore, the maximum development height of the water flowing fractured zone in the working face determined by the CH2 borehole is at the depth of 70 m, developing to mudstone 53.62 m away from the roof of working face.
In summary, the height of water flowing fractured zone measured by CH1 borehole is 52.68 m, and that measured by CH2 borehole is 53.62 m. Taking the maximum value of the two, the height of water flowing fractured zone in 91105 working face is 53.62 m, and the fracture mining ratio is 12.32. The top boundary of water flowing fractured zone is located in the mudstone 53.62 m away from the roof of working face. According to the observation results and the borehole layout position, it is determined that the water flowing fractured zone of 91105 working face presents the characteristics of saddle shape, as shown in Fig. 5.