In order to directly reflect the migration and destruction of the overlying rock, the development and evolution of the fissures in the shallow-buried and short-distance coal seam mining of the Lijiahao Coal Mine, the two-dimensional physical similarity simulation is first used to study.
3.1 Model making
(1) Material ratio
River sand and mica are used as aggregates, and lime and gypsum are used as cements to construct a similar simulation model of shallow-buried and short-distance coal seam mining in Lijiahao Coal Mine, and the experimental platform is simulated by planar similar materials with length, width and height of 3000mm, 300mm and 2000mm.
The model similarity constant is determined according to the site conditions and the similarity law, in which the geometric similarity ratio is 1:100, the bulk density similarity ratio is 1:1.6, and the time similarity ratio is 1:10. According to the physical and mechanical parameters of the actual coal stratum in the coal mine, the ratio number is obtained by conversion and comparison with the ratio table of similar simulated materials. The physical and mechanical parameters of the rock formation are shown in Table 1.
Table 1
Physical and mechanical parameters of rock stratum.
No.
|
Layer
|
Thickness
(m)
|
Simulated thickness (cm)
|
Number of layers
|
Compressive strength (MPa)
|
Simulated compressive strength (MPa)
|
Apparent density
(g/cm3)
|
Simulated apparent density
(g/cm3)
|
1
|
Fine-grained sandstone
|
4
|
4
|
2
|
44
|
0.28
|
2.7
|
1.8
|
2
|
Sandy mudstone
|
13.6
|
14
|
7
|
25
|
0.16
|
2.55
|
1.7
|
3
|
3 − 1 Coal
|
5.7
|
5
|
3
|
22
|
0.20
|
2.55
|
1.7
|
4
|
Fine-grained sandstone
|
26.7
|
26
|
13
|
44
|
0.28
|
2.7
|
1.8
|
5
|
Sandy mudstone
|
2.4
|
2
|
1
|
25
|
0.16
|
2.5
|
1.67
|
6
|
2–2 Coal
|
3.1
|
3
|
2
|
17
|
0.08
|
2.55
|
1.7
|
7
|
Sandy mudstone
|
2.7
|
3
|
2
|
28
|
0.18
|
2.45
|
1.63
|
8
|
Coal
|
2.1
|
2
|
1
|
25
|
0.16
|
2.55
|
1.7
|
9
|
Siltstone
|
1.6
|
2
|
1
|
44
|
0.28
|
2.7
|
1.8
|
10
|
Mudstone
|
8.1
|
8
|
4
|
12
|
0.08
|
2.55
|
1.7
|
11
|
Siltstone
|
8
|
8
|
4
|
20
|
0.13
|
2.72
|
1.81
|
12
|
Siltstone
|
3.2
|
3
|
2
|
18
|
0.08
|
2.55
|
1.7
|
13
|
Fine-grained sandstone
|
28
|
28
|
14
|
21
|
0.13
|
2.7
|
1.8
|
14
|
Fine-grained sandstone
|
2.8
|
3
|
2
|
19
|
0.08
|
2.72
|
1.81
|
15
|
Sandy mudstone
|
3.4
|
3
|
2
|
12
|
0.20
|
2.45
|
1.63
|
16
|
Siltstone
|
12
|
12
|
6
|
25
|
0.16
|
2.55
|
1.7
|
The thickness of the simulated rock layer obtained from the geometric similarity constant cannot reflect all the histograms, and the method of equivalent stress loading is applied to the overlying rock layer that cannot be stacked. According to the self-weight stress field of the overlying strata, the stress similarity constant is used to convert it into a simulated stress value. The vertical pressure that needs to be loaded onto the model is 15kPa.
(2) Measuring point layout
The coal seam mining will cause displacement and stress redistribution in the rock formation around the mining space. Therefore, in the process of model excavation, the stress and displacement of the overlying rock need to be detected. The vertical and horizontal lines were drawn on the front of the model, and the distance between the horizontal and vertical lines was 10cm. Non-coding points were set at the intersections of the lines. The XJTUDP three-dimensional optical photogrammetry system was used to observe the displacement changes of the non-coding points of the overlying rock. A stress measurement line parallel to the coal seam was arranged in the rock layers 10cm above the coal seams of models 2–2 and 3 − 1, respectively, to detect the real-time changes of the vertical stress of the overlying rock during the coal seam mining process. A total of 12 survey points are arranged in the two survey lines, the number of survey points above the 3 − 1 coal seam is from 1 to 6, and the number of survey points above the 2–2 coal seam is from 7 to 12. Among them, the distance between the measurement points at both ends of the stress measurement line is 50 cm from the model boundary, and the distance between the adjacent measurement points on each measurement line is 40 cm. Before the model is mined, the value of each stress measurement point must be cleared, that is, the original rock stress of each measurement point must be at the 0 scale line. If the stress value becomes positive during the mining process, it means that the measurement point is in a pressurized state. If it becomes negative, it is in a pressure-relief state. The actual layout of the model is shown in Fig. 1.
3.3 Experimental results
(1) Characteristics of overburden caving and fracture development
In the mining process, the model has successively experienced three stages: "upper coal seam mining", "gob mining", and "solid coal mining." The "upper coal seam mining" stage is the "2–2 coal seam mining stage," and the characteristics of overburden collapse and fissure development are similar to those of general single coal seam mining. When the basic roof collapses for the first time, the roof rock layer of the goaf begins to separate and collapse under the action of its own gravity and the pressure of the overlying rock layer, forming a caving zone and a fissure zone structure, as shown in Fig. 2a. During the subsequent advancement of the working face, the overlying rock periodically caves in the form of "cantilever - articulation - collapse - stabilisation" and the cracks are periodically generated, closed, and compacted, resulting in the phenomenon of cracks expanding upwards and towards the working face. Every time the overlying rock collapses, a layer-separated fissure zone will be formed above the goaf, the caving fissure zone will be formed at the bottom, and the middle part will be the compaction zone. Above the side of the opening and the stop line, the area where the rock blocks are hinged vertical fissure zone is formed, as shown in Fig. 2b. At the beginning of 3 − 1 coal seam mining, although the roof did not collapse, the stress balance state of the overall surrounding rock changed due to the mining operation, and the height of the fissure zone in the gob of the upper coal seam was further developed upward. After that, the working face entered the mining stage under the goaf. When it advanced to 68cm, the roof of the 3 − 1 coal seam collapsed, and obvious separation cracks were formed above it, as shown in Fig. 2c. When the working face continues to advance to 80cm, the key layer between the coal seams is broken, the upper and lower coal seam gobs are connected, and the collapsed rock layers in the original 2–2 coal seam gobs will undergo secondary caving, and the cracks are criss-crossed at this time. The shape is extremely complex, as shown in Fig. 2d. During the subsequent advancement of the working face, the key layers between the coal seams were broken many times, the "masonry beam" structure continued to evolve with the advancement of the working face, and the pressure changes in the working face were frequent and periodically reduced. After passing through the upper coal seam goaf, a permanent vertical fracture is formed in the overlying rock above the 2–2 coal seam stop line, as shown in Fig. 2e. After that, the working face resumes periodic pressure, the frequency of pressure decreases, and the overlying rock collapses. The development of falls and fissures has returned to the characteristics of single coal seam mining. When the main key layer of the coal seam bedrock is broken, the layer separation fissure above the fissure zone is closed, the topsoil layer collapses simultaneously, and the fissure penetrates the surface, as shown in Fig. 2f.
(2) Stress distribution characteristics
The distribution of rock formation stress has been in dynamic change with coal seam mining, and its stress evolution process can also be divided into three stages: the upper coal seam mining stage, the gob mining stage, and the solid coal mining stage. Figures 3 and 4, respectively, show the stress distribution of the overlying rock in the key mining stages of the upper and lower coal seams. The blue line represents the stress distribution of the 2–2 coal seam roof stress line, and the orange line is the 3 − 1 coal seam roof stress line stress distribution. Comparing Fig. 3 and Fig. 4, it can be seen that there is a significant difference between the stress evolution process of short-distance coal seam mining and single coal seam mining.
After the mining of the 2–2 coal seam, the stress value at the measuring point on the side of the goaf decreases rapidly, the advanced bearing stress is concentrated in the rock layer near the coal wall side of the working face, and the stress value at the measuring point increases. When the stress is higher than the yield strength of the rock mass After that, the roof stratum yields and breaks, and the stress value at the measuring point decreases in a leap, and cracks are generated in this process. After the roof rock layer has been fully collapsed and recompacted, the stress value near the collapsed rock layer in the goaf is restored to the original rock stress level, and most of the cracks are also closed at this time. During the whole mining process of the 2–2 coal seam, the stress changes on the roof and floor show three changing processes of and the corresponding areas are: the pressure-boosting area, the pressure-relieving area, the stress-recovery area, and the overall stress after mining. The distribution conforms to the "saddle-shaped" distribution characteristic.
After the mining of the 3 − 1 coal seam, the overburden stress has further changed from the previous distribution state, but during the mining process of the coal seam under the goaf, the stress values of the upper and lower measurement lines no longer change synchronously. The 3 − 1 coal seam roof measurement the variation of linear stress is similar to the variation of the stress value of the roof rock stratum in single coal seam mining, but the linear stress value of the 2–2 coal seam roof measurement has unique variation characteristics. Before the key layer between the coal seams is broken, in the overlying rock in front of the 3 − 1 coal seam working face, the stress value of the measuring point on the orange measuring line changes from a negative value to a positive value, indicating that the stress is concentrated, and after the working face passes the measuring point, the stress value of the measuring point decreases again, but the stress value of the blue measuring line measuring point at the same position does not change significantly during this process. After the key layer is broken and collapsed, the stress value of the upper measuring point suddenly decreases, and the stress value of the lower measuring point suddenly increases, until the working face is far away from the measuring point and the stress value slowly returns to the original rock stress level. After that, every time the key layer collapses, the stress value of the measuring point of the upper secondary caving rock layer will decrease significantly, and the stress value of the measuring point of the lower rock layer will increase. When the 3 − 1 coal seam working face is close to the 2–2 coal seam stop mining line, the leading stress value of the orange measuring line reaches 0.015MPa, which has exceeded the stress value of the blue measuring line below 0.004MPa. At this time, the stress value distribution along the upper and lower measuring lines is also roughly close to "saddle shape". Later, during the mining process of the working face under the solid coal, the stress changes of the upper and lower survey lines returned to the same consistency. It is worth noting that during the whole mining process, the stress value of the measuring point near the open-cut side has been low, which is the same as the low stress value on the side of the gob of the working face because the rock formation here is always in the mining process. It is not compacted, so the rock formation at the measuring point has been in a pressure relief state.
Comparing the similar simulation crack distribution with the roof stress distribution, it can also be seen that the "vertical fracture crack" in the similar simulation results is in the same position as the pressure relief area, and the "caving crack area" is in the same position as the stress recovery area.
(3) Characteristics of overburden subsidence
Through the extraction and analysis of the rock stratum displacement measuring point data after the mining of the similar simulation experiment, it can be judged that the mining fractures are mainly enriched in the areas where the displacement values of the adjacent measuring points are significantly different. The displacement vector of the overlying stratum in the abscission fissure area is downward, and the displacement change of the overlying stratum is not synchronous, resulting in a stratum-separated fissure, which is analysed to be mainly generated by rock tension. The vector direction of the displacement field of the rock formation in the compaction area is also downward, and the displacement is basically the same. The displacement of the rock layers on both sides of the vertical fracture is different, and the fracture is generated by the shear fracture of the rock layer.
Figure 5 shows the settlement of the rock formation near the goaf after the coal seam mining is completed. The abscissa represents the distance between the displacement measuring point in the horizontal direction and the stop line of the 3 − 1 coal seam working face, and the blue line represents the settlement of the displacement measuring line at 2cm above the 2–2 coal seam, the orange line represents the settlement of the displacement measuring line at 4 cm above the 3 − 1 coal seam, and the full length of the 2–2 coal seam working face advancement is between 1200 mm and 2400 mm on the abscissa. Figure 5a shows that the overlying rocks on the side of the 2–2 seam incision and the side of the mining stop line are in an inclined arrangement, the rock blocks form a hinged structure, and the "vertical fractures" are distributed in this overlying area. After the 3 − 1 coal seam mining is completed, the overlying rock subsides again, and the subsidence amount of the overlying rock survey line is shown in Fig. 5b. At the two ends of the two displacement lines, the subsidence changes significantly, the rock block rotates obviously, and the vertical fracture area is enriched here. The strata of the 2–2 overlying coal seam are arranged obliquely between 500mm and 1200mm. Combined with similar simulation results, the lower strata in this area have not been fully compacted, but the upper strata are relatively closely arranged, so there is no obvious fracture development in this area. The subsidence value at the stop line of the 2–2 coal seam also has obvious changes. Due to the lack of a coal seam, the overlying rock here cannot be compacted, and the vertical cracks will exist permanently. Comparing Figs. 5a and 5b, after the mining of 3 − 1 coal seam is completed, the subsidence values of the strata in the goaf of 2–2 coal seam tend to be the same, indicating that the rock strata in the caving zone of 2–2 coal seam are further compressed. In fact, the number of cracks has decreased.