Study on mining area of roof surrounding rock stability of mining fissure development

The “double end sealing device of drilling drilling water injection leakage” section, to the different depth of sealing hole drilling water injection test, observation in different directions and different drilling water Kong Shen missed development in mining fracture analysis at different depth of stratum loss, combined with different directions borehole sections of fissure, preliminary judgment the test site of mining roof surrounding rock fracture stability region boundary of mining; use of field test data, analysis of Yu Yang coal mine N21110 mining fissure development scope and the degree of fracture development in mining area of roof stability.


Introduction
With the advance of the working face, the overlying strata are affected by mining, and the stress failure produces the mining fracture zone with dynamic evolution distribution, which provides the main passage and space for the pressure-relief gas flow and storage in the coal and rock strata, it also provides the main passage for gas seepage to Goaf and working face [1][2]. How to accurately determine the development height of overlying rock fracture zone after mining is of great significance to gas extraction [3][4][5][6]. This research adopts "drilling Hole double-end plugging and leak detection device" to carry out the drilling step-by-step water injection method, that is, drilling holes to the roof of the coal seam under the well, and testing water injection into the holes at different depths, the development of mining fractures in rock layers of different depths is analyzed by observing boreholes in different directions and water leakage in different depths, so as to judge the development range of mining fractures in roof surrounding rock of mining stable area at the test site, as shown in Figure 1.

General Situation of test working face
The test was carried out at N21110 working face of Yuyang coal mine, Chongqing Songzao Coal Power Co. , Ltd. . The working face has a strike length of 177m, an inclined length of 1187m, an average inclined angle of 5.5°and an average thickness of 0.65 m, the working face roof is shown in Table1.  Figure 2.

3.
Step-by-step water injection test

Test plan
The test was carried out in N21112 return air roadway of Yuyang Coal Mine, and 75mm drill bit was used to drill the roof hole in the direction of N21110 transport roadway. The final hole was located above the roof of mining stable area of N21110 working face.
(1) Lateral test drilling of roof in mining stable area In the N21110 mining stability area, three lateral test boreholes were constructed, the spacing of the holes was 0.5 m, the spacing of the holes was 0.2 m, and the boreholes were on the same horizontal line. Drilling parameters and layout are shown in Table 2 and Figure 3.

Experimental data observation
There are 3 lateral test boreholes and 2 Longitudinal Test Boreholes in the roof of the mining stable area. Some information about water injection and leakage in the test borehole is shown in Table 3~7.
(1) Lateral test drilling of roof in mining stable area   According to the data, the variation curve of drilling leakage in lateral test in mining stability area is obtained, as shown in Figures 6~8.   It can be seen from the diagram that the variation regularity of the test boreholes in different azimuth angles is quite consistent, indicating that the fracture height is relatively consistent in the horizontal direction. On the whole, the testing hole leakage shows the changing trend of size and size. In the shallow part (3.7 ~ 4m) of the test drilling hole, the leakage is slightly larger (2 ~ 4L/min) , this is because the shallow part of the test drilling hole is located in the loose zone of the roadway and the protective coal pillar, and the protective coal pillar is affected by the mining of the working face, and the fractures are relatively developed; In the interval of 4 ~ 6 m, the loss of borehole is reduced, which shows that the borehole enters the area above the coal pillar which is less affected by mining, and the degree of fracture development is low in this area; When the vertical height of the Borehole Exceeds 6M, the leakage of the Borehole survey points remains at a high level within the range of 6 ~ 14.1m, which is more than 4L/min, indicating that the survey points of the borehole are in the area where the fractures are well developed in the N21110 working face, this area is affected by the mining of this working face, and the longitudinal fracture is highly developed. When the vertical height of the Borehole exceeds 14.1 m, the GRET 2021 IOP Conf. Series: Earth and Environmental Science 859 (2021) 012008 IOP Publishing doi:10.1088/1755-1315/859/1/012008 7 leakage of the measured point decreases to less than 2 L/min, which indicates that the degree of fracture development in this area is low and that the borehole has passed through the fracture zone.
(2) Longitudinal test drilling of roof in mining stable area According to the data in Table 3    4 #,7 # and 8 # boreholes (corresponding dip angles of 50 °, 60 ° and 45 °) enter the relatively developed fracture region near the vertical heights of 5.9 m, 6.1 m and 5 m respectively, and the corresponding horizontal distances are 7 m, 6.2 m and 5.1 m, it shows that the development range of the mined-out area is not vertical, but arc-shaped, and the whole shape is semi-saddle shape, the development scope of mining fracture in Goaf is shown in Figure 12.

Conclusion
Based on the analysis of the geological conditions and mining technical conditions of N21110 working face in Yuyang coal mine, the reliable underground observation method is selected on the basis of reasonable prediction of the development range of fractures, in-situ tests and data analysis on the leakage of sectional water injection have been carried out in 5 mining stable zones, and the main conclusions are as follows: (1) The variation law of Borehole leakage measured in mining stable area is generally consistent, and the variation trend of the magnitude is generally presented. In the shallow part of the test hole (3.7~4m) , the leakage is slightly larger (2 ~ 4L/min) , and then the leakage decreases in the interval of 4~6m, which shows that the test hole enters the area above the coal pillar which is less affected by mining, the degree of fracture development is low in this area; In the range of 6 ~ 14.1 m, the leakage rate of Borehole survey points is higher than 4 L/min, which shows that the survey points of Borehole survey are located in the area where the roof rock fractures are relatively developed, and this area is affected by mining in this working face, it is located in the range of fracture zone; When the vertical height of the Borehole exceeds 14.1 m, the leakage of the Borehole survey point decreases to less than 2 L/min, which indicates that the degree of fracture development in this area is low and that the borehole survey point has passed through the fracture zone.
(2) With the increase of the testing hole angle, the boundary coordinates of the fractured zone in the roof of the working face decrease transversely and increase vertically, and the upper limit of the fractured zone is about 14.1m. The development range of mined-out fracture in Goaf is not vertical distribution, but arc distribution, and the whole shape is semi-saddle shape.
(3) In the case of intact rock strata, the soft rock is more plastic, can withstand greater bending deformation, is easy to lose crack energy, and is not easy to produce cracks; moreover, most soft rock expands when exposed to water, is easy to plug cracks, and has a restraining effect on mining cracks, hard Rock can stop the crack from spreading more than soft rock. Therefore, the best combination of rock layers to prevent the development of roof fractures is hard rock first, then soft rock, then hard rock, hard and soft rock superimposed accordingly.
(4) Due to the perfection of the current testing methods and the accuracy of the parameter-value trade-off, it may affect the accuracy of the results of the study area. Especially when there is a small fault in the working face, it may cause the High Anomaly of the overlying rock fracture near the fault.