3.1 Model design and construction
Based on the geological conditions of W1123 working face in Kuangou Coal Mine, a physical similar material simulation model was built. The experiment adopts a plane strain model frame of 5.0×0.3×2m, and the geometric similarity ratio (model: prototype) of the simulation experiment is 1: 200.The paving size of similar experimental model is 5.0×0.3×1.89m. Loading a layer of iron brick on the top of the model is equivalent to 40m thick overburden, and the loading stress is 0.8MPa. The physical material model construction and microseismic monitoring arrangement are shown in Fig. 3. The physical W1123 working face of the model, after 3cm open-cut, can achieve the simulation effect of the actual 4.8m/d advancing speed of the mine as much as possible with a single mining of 2.4cm. At the same time, the ground pressure phenomenon and microseismic energy evolution law associated with rock breaking are analyzed by floor pressure monitoring and microseismic monitoring.
Table 1
Simulation material ratio of physical model.
Rock stratum
|
Actual thickness /m
|
Simulated thickness /cm
|
Serial number
|
River sand /kg
|
Plaster /kg
|
big white powder /kg
|
Packing thickness and times
|
A
|
29.2
|
14.6
|
837
|
42.67
|
1.60
|
3.73
|
2cm
|
8
|
B
|
11
|
5.5
|
737
|
23.10
|
0.99
|
2.31
|
1.1cm
|
10
|
A
|
48
|
24
|
837
|
42.67
|
1.60
|
3.73
|
2cm
|
12
|
A
|
42
|
21
|
837
|
64.00
|
2.40
|
5.60
|
3cm
|
7
|
A
|
50
|
25
|
837
|
42.67
|
1.60
|
3.73
|
2cm
|
13
|
C
|
13
|
6.5
|
828
|
27.73
|
0.69
|
2.77
|
1.3cm
|
5
|
A
|
10
|
5
|
837
|
21.33
|
0.80
|
1.87
|
1cm
|
5
|
C
|
14
|
7
|
828
|
21.33
|
0.53
|
2.13
|
1cm
|
7
|
D
|
13
|
6.5
|
928
|
28.08
|
0.62
|
2.50
|
1.3cm
|
5
|
C
|
8
|
4
|
828
|
21.33
|
0.53
|
2.13
|
1cm
|
4
|
B
|
7
|
3.5
|
737
|
37.80
|
1.62
|
3.78
|
1.8cm
|
2
|
A
|
6
|
3
|
837
|
21.33
|
0.80
|
1.87
|
1cm
|
3
|
C
|
8
|
4
|
828
|
21.33
|
0.53
|
2.13
|
1cm
|
4
|
E
|
16
|
8
|
728
|
84.00
|
2.40
|
9.60
|
4cm
|
2
|
C
|
9
|
4.5
|
828
|
32.00
|
0.80
|
3.20
|
1.5cm
|
3
|
A
|
8
|
4
|
837
|
21.33
|
0.80
|
1.87
|
1cm
|
4
|
E
|
5
|
2.5
|
728
|
21.00
|
0.60
|
2.40
|
1cm
|
3
|
B4-1 coal
|
3
|
1.5
|
20:20:1:5
|
15.65
|
0.78
|
3.91
|
1.5cm
|
1
|
A
|
8
|
4
|
837
|
21.33
|
0.80
|
1.87
|
1cm
|
4
|
E
|
14
|
7
|
728
|
73.50
|
2.10
|
8.40
|
3.5cm
|
2
|
B3 coal
|
1.8
|
0.9
|
20:20:1:5
|
10.43
|
0.52
|
2.61
|
1.8cm
|
1
|
A
|
3
|
1.5
|
837
|
32.00
|
0.80
|
3.20
|
1.5cm
|
1
|
B
|
16
|
8
|
737
|
84.00
|
3.60
|
8.40
|
4cm
|
2
|
B2 coal
|
9.5
|
4.75
|
20:20:1:5
|
10.43
|
0.52
|
2.61
|
9.5cm
|
1
|
A
|
4
|
2
|
837
|
42.67
|
1.60
|
3.73
|
2cm
|
1
|
B
|
22
|
11
|
737
|
42.00
|
1.80
|
4.20
|
2cm
|
6
|
Note: A-mudstone, B-fine-grained sandstone, C-sandy mudstone, D-sandstone, E-coarse-grained sandstone |
The simulation material ratio of the physical model is shown in Table 1. Based on the overburden lithology of B2 coal seam proved by ZK201 borehole histogram of W1123 working face in Kuangou Coal Mine, the similar material ratio is formulated. The materials used in the model paving process are: river sand, large white powder, plaster of Paris and water, among which fly ash should be added when proportioning coal seams. In this experiment, the composition and strength of similar materials are little different from the actual ones, which can better simulate the actual rock strata.
3.2 Evolution law of complex spatial overlying strata structure
The evolution characteristics of different overlying strata structures in the mining process of the working face cause the microseismic difference changes in the mining process of the working face. In order to clarify the microseismic change characteristics caused by the mining process of the working face, this paper firstly analyzes the evolution law of overlying strata structures in the mining process of the working face.
The evolution characteristics of overlying strata in partial periodic weighting process of W1123 working face mining are shown in Fig. 4, in which Fig. 4(a) and Fig. 4(b) respectively show the evolution characteristics of overlying strata structure in working face mining under solid coal and gob. When the working face under the solid coal advances by 73.2cm, the overlying strata on the upper part of the working face collapse for the first time, and an obvious broken area with a height of 19.8cm is formed. During the first caving of the working face to the advancing of the working face to 145.2m, with the mining of the working face, the breaking height of overlying strata gradually increased. Among them, when advancing to the working face by 87.6cm, 106.8cm, 121.2cm and 130.8cm, the overburden failure is 33.5cm, 41.2cm, 56.9cm and 81.4cm respectively. When the working face advances to 145.2, when mining under the solid coal, the overlying strata breaking area is constantly moving forward with the advancing of the working face, but the height of the breaking area is relatively stable, with the breaking height of 95.0cm. When the working face under the gob advances by 73.2cm, the overlying strata on the upper part of the working face collapse intensively, and an obvious broken area with a height of 38.8cm is formed. In the process of the working face under the gob gradually advancing from 198.0cm to 370.8cm, with the mining of the working face, the broken height of overlying strata gradually increases. Among them, when advancing to the working face by 279.6cm, 303.6cm, 313.2cm and 342.0cm, the overburden failure is 80.5cm, 88.3cm, 96.4cm and 103.7cm respectively. When the working face is advanced to 370.8 and then mined under the gob, with the advancement of the working face, the broken area of overlying strata is constantly moving forward, but the height of the broken area is relatively stable, with the breaking height of 126.2cm.
The evolution characteristics of overlying strata in working face mining from solid coal to the critical position of gob are shown in Fig. 5. When the working face advances by 188.4cm, the roof of the working face hangs in a large area and there is obvious separation space, and the overlying strata on the upper part of the roof are relatively complete. When the working face advances by 193.2cm, the working face is located below the open cut of W1145 working face, and the hanging distance of the working face roof increases, which significantly increases the separation space. When the working face is advanced by 198.0cm, the hanging distance of the working face roof reaches the limit value, and the roof is broken in a large area with a breaking distance of about 35.2cm. When the working face is advanced by 202.8cm, the upper part of the working face is hung in a wide range. Under the action of large-scale deflection and extrusion of overlying strata, the overlying strata of the upper part of the working face under the solid coal caving in a large area, and the overlying strata breaking angle of 74 is formed. Under the influence of extrusion, the separation space of the gob at the cut-off position of the upper coal seam working face is closed.
3.3 Energy evolution characteristics of complex spatial microseisms
Microseismic monitoring can realize the time, space and energy monitoring of coal and rock fracture events in the mining process of working face, and it is a necessary monitoring means for rock burst mines. Through the statistical analysis of microseismic event data in the mining process of W1123 working face, we can analyze the quantity and energy of microseismic events in different stages of mining in complex heterogeneous space, and support the effective verification of safe mining in working face.
Figure 6 shows the microseismic energy distribution cloud map of the mining face from solid coal to the critical position of the gob, in which Fig. 6(a), Fig. 6(b), Fig. 6(c) and Fig. 6(d) are the working faces respectively Cloud map of microseismic energy distribution when advancing to 188.4cm, 193.2cm, 198.0cm and 202.8cm. When the working face advances by 188.4 cm, the microseismic energy is mainly located in the range of 166.3-195.2 cm in the advancing direction of the working face. There is an obvious large energy distribution area within the model height range of 106.6-123.5 cm, and the maximum energy in the area reaches 135 J. When the working face advances 193.2 cm, the microseismic energy is mainly located in the range of 166.3-204.3 cm in the advancing direction of the working face. There is an obvious large-energy event at the model height of 75.3 m, where the energy reaches 250 J. When the working face advances 198.0cm, the microseismic energy is mainly located in the range of 167.8 ~ 202.6cm in the working face advancing direction, and there is an obvious large-energy event at the model height of 99.7m, where the energy reaches 190 J. When the working face is advanced by 202.8cm, there are two obvious large energy accumulation areas in the range of 188.5 ~ 198.7cm in the advancing direction of the working face and the model height of 79.2 ~ 89.3cm. The energy is about 370 J.
Comparing the evolution of the overburden and its microseismic energy distribution of the mining face in Fig. 5 and Fig. 6, it can be seen that the evolution of the overburden structure during the advancing process of the working face causes different microseismic energy variation characteristics. When the working face advances to 188.4cm At this time, the change of the roof of the working face is small, and the upper overlying rock is relatively complete, so the microseismic energy generated by the propulsion process of the working face is small, and the energy value of the energy accumulation area is low. When the working face gradually advanced to 193.2cm and 198.0cm, the overlying rock changed significantly, and the roof separation space gradually developed and formed a large area of roof collapse, which made the energy value of the upper overlying rock energy accumulation area high. 135 J when the working face is advanced to 188.4cm. When the working face is advanced to 202.8cm, the central strata between the coal seam groups are broken in a concentrated manner, generating high-energy microseismic energy, and the energy value of the accumulation area reaches 370 J. With the closure of the abscission space at the incision position of the W1145 working face, its work There is a certain activation effect of overlying rock in the upper space in the range of 215.3-234.5 cm in the forward direction of the surface, which is accompanied by the generation of higher energy.
The energy and frequency of microseismic events can reflect the change of coal and rock stress. The higher the microseismic event energy and the more frequent the vibration, the greater the stress concentration of coal and rock mass and the more serious the damage. Therefore, the microseismic energy distribution and its weighting characteristics in the whole mining process of W1123 working face are drawn based on the microseismic and support pressure monitoring data, as shown in Fig. 7, in which Fig. 7(a) is the microseismic energy distribution cloud map, and Fig. 7(b) is the microseismic energy, frequency and weighting characteristics.
It can be seen from Fig. 7(a) that the microseismic energy distribution cloud map shows that the key layer of solid coal is relatively complete and can better support the concentrated effect of the upper overburden compared with the key layer of the broken overburden in the gob. The severity of overburden damage in the mining process of working face 132.5 ~ 163.3 is obviously greater than that in other areas. During the mining process of the W1123 working face, the microseismic energy is mainly located in the model width of 132.5 ~ 163.3cm, and its energy is mostly in the range of 140 ~ 220 J. It can be seen from Fig. 7(b) that the microseismic energy, frequency and the characteristics of the incoming pressure show that the microseismic and large energy events generated by the overburden fracture of the W1123 working face are mainly concentrated in the mining process of the working face under the solid coal. The energy peak positions of the four microseismic events generated by mining under the solid coal face are located at the model widths of 92.8cm, 164.8cm, 230.2cm and 227.2cm, respectively. When the working face advances to these four positions, the roof will periodically fracture and collapse when the elastic energy accumulated by the upper overburden reaches its ultimate strength, releasing the accumulated elastic energy. It is stable and easy to induce rock burst. However, the energy value of W1145 gob is relatively small during mining, and the energy of microseismic events is mostly between 0 ~ 50 J. The energy of microseismic events under solid coal is higher than that under gob, and the frequency of microseismic events under gob is higher than that under solid coal mining.
Table 2 shows the statistical table of the periodic back pressure data of the W1123 working face mining. The W1123 working face will form different back pressure characteristics when mining the solid coal of the upper B4-1 coal seam and the mined-out area. Combining with the position of the periodic pressure in Fig. 7(b), it can be seen that the initial pressure occurs at 73.2 cm below the solid coal, and the recovery occurs at 73.2 ~ 193.2cm, with a total of 8 periodic pressures. In the 193.2 ~ 432cm mining under the gob, there are 12 times of cyclic pressure, and the cycle of mining under solid coal is relatively frequent. The overlying rock structure under the gob is loose, broken and easy to move, and the arched structure is damaged and the supporting capacity is weakened, so the accumulation and release period of the microseismic energy is short. The fluctuation of the cyclic pressure value under the gob is small, and the support bears more overlying rock. Therefore, the cyclic pressure value of the W1123 working face in the mining process under the gob is slightly higher than that under the solid coal.
Table 2
Statistical table of the period to pressure data of W1123 working face mining.
Under solid coal
|
Under the gob
|
Number of times/N
|
Advance distance/cm
|
Bracket pressure /MPa
|
Number of times/N
|
Advance distance/cm
|
Bracket pressure /MPa
|
1
|
73.2
|
27.89
|
1
|
198
|
31.95
|
2
|
87.6
|
28.18
|
2
|
212.4
|
29.62
|
3
|
106.8
|
28.85
|
3
|
241.2
|
28.49
|
4
|
121.2
|
28.44
|
4
|
255.6
|
30.53
|
5
|
130.8
|
28.80
|
5
|
279.6
|
27.46
|
6
|
145.2
|
28.23
|
6
|
303.6
|
28.01
|
7
|
159.6
|
28.52
|
7
|
313.2
|
30.01
|
8
|
169.2
|
30.53
|
8
|
342
|
30.10
|
9
|
193.2
|
29.72
|
9
|
370.8
|
30.08
|
Note: During the mining process of W1123 working face, a total of 21 obvious cycles were formed.
|
10
|
390
|
29.86
|
11
|
409.2
|
29.19
|
12
|
423.6
|
30.17
|