(1) Characteristics of overburden structure after ground fracturing
The structural characteristics of the high-level KS4 and KS5 are analyzed. The structural characteristics of KS4 and KS5 before and after KS5 is fractured are shown in Fig. 4.
As shown in Figures 4(a) and (c), the high-level KS5 is not fractured, the breaking span of KS4 and KS5 are 170m and 175m respectively. The breakage of KS4 and KS5 causes an unstable and a synchronous rotation of the lower strata, and the range of the motion space in the overburden is wide. The vertical displacement of the cantilever beam structure close to the working face caused by the breakage of KS5 and KS4 is 0.48 m and 2.5 m, respectively.
After KS5 is fractured, the first breaking span of the high-level KS4 is reduced to 142 m, of which the length of the broken block A is only 47 m, and the displacement of the immediate roof at the working surface is 0.45 m, as shown in Fig. 4(b). Compared to the KS5 unfractured model, the KS4 breaking span is reduced after KS5 is fractured; this is mainly because the bearing capacity of KS5 is reduced after fracturing. Moreover, the weight of KS5 and its overlying strata is transferred to KS4, increasing the load above KS4, thus causing the breaking span to decrease. It also can be seen from Fig. 4(b) that after KS4 is broken, KS5 also undergoes a large degree of deflection and rotation by the fracturing action, but it does not rotate synchronously with KS4 and still maintains a certain structural stability.
As the working face continues mining, KS5 continues to rotate. Because of its relatively slow rotation, its breaking and unstable energy release intensities are low and have no impact on the underlying strata. The underlying key layer structure can be kept stable. The rotation of the KS5 breaking block does not generate a ground pressure, as shown in Fig. 4(d).
(2) Strata movement control effect of ground fracturing
Taking KS4 and KS5 as the research objects, their vertical displacement changes are obtained when they are broken under the conditions of occurrence and non-occurrence of KS5 fractured, as shown in Fig. 5. The abscissa is the position of the measurement point along the model mining direction.
As shown in Fig. 5(a), in the absence of KS5 fractured, when KS4 breaks, the maximum vertical displacement of KS5 is 1.2 m, and the KS5 structure remains stable. After the KS5 fractured, when KS4 breaks, the KS5 structural integrity is reduced, being affected by the weakening of the ground fracturing, and a large bending subsidence of 1.73 m occurs in KS5. However, the KS5 structure does not sink with KS4 synchronously, and it can maintain the structural stability and a certain separation space with the underlying rock formation. As the working face continues mining, KS5 slowly settles to stability.
It can be seen from Fig. 5(b) that when KS5 is not subjected to ground fracturing, its integrity is strong, and the breaking span is 175 m. The large rotary movement and high strength during KS5 breakage causes a synchronous movement of the underlying strata. Taking KS4 as an example, the KS5 breakage causes the vertical displacement of KS4 to reach 1.68 m. When KS5 is fractured, the rotation of KS5 is no longer a transient, high-speed, high-intensity movement process but a slow deflection and sinking one. The strength and energy release during the KS5 breakage is low, which has a weak impact on the underlying rock formation. The rotation of KS5 after the ground fractured will barely cause a synchronous movement of the underlying rock formation, the vertical displacement of KS4 hardly changes, and the KS4 structure remains stable. It can be seen that the ground fracturing significantly reduces the movement strength of the hard strata.
In addition, after KS5 is fractured, owing to its reduced bearing capacity, the part weight of the overburden is carried by KS4, which increases the load strength above KS4, causing the KS4 breaking span to decrease. It is found in the experiment that the KS4 breaking span reduces to 142 m, which is reduced by 28 m compared to the model of KS5 unfractured. Moreover, the degree of the strong mine pressure of KS4 breaking is reduced to a certain extent.
The vertical displacement of the immediate roof next to the supports during the breakage of KS4 and KS5 in the absence and presence of KS5 fractured is shown in Fig. 6. It can be seen from Fig. 6 that when the ground fracturing of high-level KS5 is not implemented, the breakage of KS4 results in a maximum sinking of 2.5 m in the immediate roof, which will probably cause the supports to crash on the working face. After KS5 is fractured from the ground, the vertical displacement of the immediate roof caused by the breakage of KS4 is significantly reduced to 0.45 m, and the KS5 rotation process barely causes the motion of the underlying strata. The ground fracturing of the high-level hard strata has a significant effect on reducing the strong mine pressure in the working face.
(3) Control effects of overburden movement space
The experimental research shows that the ground fracturing of high-level KS5 is beneficial for reducing the breaking span of KS5 and the low key strata, thereby reducing the influence range during the breakage of high-level thick and hard rock strata and avoiding the formation of excessive structures in the overlying large space. The degree of the strong ground pressure in the working face is reduced, and the spatial structure of the overlying rock before and after KS5 fractured are shown in Fig. 7.
It can be seen from Figures 7(a) and (c) that when KS5 is not fractured, the large structural dimensions in the overburden after high-level key layers KS4 and KS5 break are 210 × 165 m and 225 × 165 m respectively, and after KS5 is fractured, the structural size is reduced to 210 × 135 m and 225 × 120m, respectively. The ground fracturing action reduces the spatial influence range of the high-level hard strata breakage. The area of the overlying strata that acts on the working face supports is denoted as S. According to the statistics, the area, S, after the breakage of KS4 and KS5 before and after KS5 is fractured is given in Table 2.
Table 2 The statistics of overburden S area
No
|
Items
|
KS4 breakage
|
KS5 breakage
|
1
|
KS5 unfractured
|
S1— 4643.78
|
S2— 6638.08
|
2
|
KS5 fractured
|
S1'--3774.06
|
S2'--1062.97
|
3
|
Ratio (S'/S)
|
81.3%
|
16.01%
|
It is easy to see from Table 2 that the ground fracturing causes the area, S, to decrease in KS4 and KS5 breakage, and the area, S, of the KS4 breakage is reduced to 81.3%. The ground fracturing has the most significant control effect on the mine pressure caused by KS5, and the area, S, of KS5 breakage is significantly reduced to 16.01%, which highly alleviates the strength of the mining pressure.
In summary, it can be seen that after KS5 is ground fractured, the broken structure of KS5 is changed, and the stable rock stratum structure of KS5 is no longer present. The rotation process of the KS5 structure is sluggish, which significantly reducing the pressure effect of KS5. In addition, after KS5 is fractured, the load strength of the underlying strata is increased, resulting in a decrease in the breaking span. However, KS5 does not rotate synchronously with the underlying strata, and so, it does not increase the pressure strength of the underlying strata. Contrastingly, the pressure strength of the underlying hard strata (i.e., KS4) decreases with the decrease in the breaking span.
4 Engineering application
The 8218 working face of the Tashan coal mine in the Datong mining area was selected as the test site. The geological conditions of 8218 working face is similar to that of 8101 working face. The 8218 working face is 230 m long, and the strike length is 2894 m. The thickness, burial depth, and inclination of the coal seam are 15 m to 22 m, 414.5 m to 632.1 m, and 2°, respectively. The mining method of top-coal caving is used. The coal seam is covered with multiple hard rock layers. The hard rock layers are mostly lithologically compact and medium-sized and coarse-grained sandstones. Particularly in the 110 m vertical distance from the coal seam, a 20-m-thick hard sandstone is formed. According to the above research, in extra-thick coal seam mining, when the ratio of the distance between the hard strata and coal seam to the coal seam thickness is between 5.3 and 7.3, the mine pressure of the hard strata breaking is the strongest. This is the main factor causing the strong mining pressure of the working face (Yu et al. 2019; Gao 2018), based on which 20-m-thick and hard rock formations are designated as the fractured target layers.
A horizontal well can realize multistage fracturing, for which the control range is wide and the fracturing effect is good. In view of the need to achieve high-efficiency control of high-level thick and hard rock layers, a horizontal well is adopted in this experiment.
4.1 Fracturing Process and Equipment
The horizontal fracturing well consists of a vertical section, deflecting section, and horizontal section. The position of the wellhead is 99 m away from the stop line of the working face, 105 m away from the return airway, and 125 m away from the intake airway. The horizontal section of the fracturing well extends parallel to the mining direction of the working face but opposite to the mining direction. The vertical section of the fracturing well adopts a three-level structure. The first level is drilled to 30 m with a Φ-444.5-mm drill bit, and a Φ-339.7 × 9.65-mm surface casing is inserted. The second level is drilled to 120 m with a Φ-311.5-mm drill bit, and a Φ-244.5×8.94-mm intermediate casing is inserted. The third level is drilled to 650 m with a Φ-216-mm drill bit, and a Φ-139.7×7.72-mm intermediate casing is inserted. The length of the deflecting section is 330 m, and the horizontal section is 200 m long. The relative positions of the fracturing well and working face and the fracturing well structure are shown in Fig. 8.
The area around the target fractured strata was perforated by drilling numerous small holes on the walls of the fracturing well. These allow the fracturing fluid to expand, thereby achieving the fracturing purpose. The target strata were designed to be fractured by three-stages in the horizontal section. To ensure the fracturing effect, the perforation density in the fractured zone is designed to be as high as 16 per meter. When the ground fracturing is performed, five pump trucks, one sand mixer, one instrument vehicle, five liquid tank trucks, and one sand tank truck are used. The reserve fracturing water is 2000 m3. The ground fracturing site construction is shown in Fig. 9.
The maximum bursting pressure in the first stage is 12.46 MPa, and the total liquid volume is 470.9 m3. The maximum bursting pressure in the second stage is 10MPa, and the total liquid is 549 m3. The maximum bursting pressure in the third stage is 10.33 MPa, and the total liquid is 576.9 m3.
4.2 Fracturing Effect Analysis
4.2.1 Crack propagation monitoring
The detectors were placed on the ground to monitor the microseismic wave signal during the fracturing process to describe the law of hydraulic crack propagation. Taking the fracturing well as the center, the detectors were arranged around the fracturing well, and the arrangement is shown in Fig. 10.
The positions of the detectors are listed in Table 3. The detectors were accurately positioned with high-precision GPS (GPS accuracy is not more than 3.0 m), and the ground depth of the detectors was not less than 30 cm.
Table 3 Coordinates of the detectors
|
X
|
Y
|
Z
|
DX
|
DY
|
DZ
|
Wellhead
|
498150
|
4420432
|
1490
|
|
|
|
A1
|
498064
|
4420648
|
1452
|
-86
|
216
|
-38
|
A2
|
498036
|
4420649
|
1446
|
-114
|
217
|
-44
|
A3
|
498004
|
4420655
|
1438
|
-146
|
223
|
-52
|
A4
|
497976
|
4420654
|
1434
|
-174
|
222
|
-56
|
A5
|
497954
|
4420656
|
1435
|
-196
|
224
|
-55
|
A6
|
497929
|
4420672
|
1438
|
-221
|
240
|
-52
|
A7
|
497896
|
4420677
|
1434
|
-254
|
245
|
-56
|
A8
|
497860
|
4420686
|
1429
|
-290
|
254
|
-61
|
B1
|
497971
|
4420500
|
1512
|
-179
|
68
|
22
|
B2
|
497963
|
4420514
|
1514
|
-187
|
82
|
24
|
B3
|
497807
|
4420575
|
1496
|
-343
|
143
|
6
|
B4
|
497830
|
4420585
|
1482
|
-320
|
153
|
-8
|
B5
|
497936
|
4420511
|
1479
|
-214
|
79
|
-11
|
B6
|
497922
|
4420557
|
1485
|
-228
|
125
|
-5
|
B7
|
497829
|
4420463
|
1523
|
-321
|
31
|
33
|
B8
|
497848
|
4420530
|
1492
|
-302
|
98
|
2
|
C1
|
498004
|
4420417
|
1545
|
-146
|
-15
|
55
|
C2
|
497973
|
4420414
|
1540
|
-177
|
-18
|
50
|
C3
|
497937
|
4420438
|
1540
|
-213
|
6
|
50
|
C4
|
497902
|
4420443
|
1520
|
-248
|
11
|
30
|
C5
|
497885
|
4420460
|
1521
|
-265
|
28
|
31
|
C6
|
497880
|
4420484
|
1519
|
-270
|
52
|
29
|
C7
|
497878
|
4420520
|
1488
|
-272
|
88
|
-2
|
C8
|
497775
|
4420466
|
1536
|
-375
|
34
|
46
|
After the first stage of fracturing, the crack propagation pattern was monitored, as shown in Fig. 11(a). As can be seen from Fig. 11(a), the crack propagation direction is NE90°, and the expansion lengths are 134 m and 62 m, respectively. After the second stage of fracturing, the crack propagation was monitored, as shown in Fig. 11(b). It can be seen from Fig. 11(b) that the crack propagation direction is NE55°, the crack spreads in two opposite directions, and the expansion lengths are 98 m and 118 m, respectively. The total length of the crack is 216 m. After the third stage of fracturing, the crack propagation was monitored, as shown in Fig. 11(c). It can be seen from the figure that the crack propagation direction is NE50°, and the cracks extend in two opposite directions. The expansion lengths are 118 m and 98 m, respectively, and the total crack length is 216 m.
The morphological characteristics of the cracks after fracturing thrice in the horizontal section are listed in Table 4. The fracturing crack extending in the horizontal direction is 216 m, which is longer than the working face length. The expansion direction is approximately perpendicular to the horizontal section of the fracturing well. The crack extending in the vertical direction is 50 m, and the crack expansion range is wide, completely covering the thickness range of the fractured target layer.
Table 4 Crack morphology statistics of the three stages
No
|
Stage
|
Crack length (m)
|
Crack height (m)
|
Propagation direction (°)
|
left
|
right
|
upward
|
downward
|
1
|
First stage
|
134
|
62
|
22
|
21
|
NE90
|
2
|
Second stage
|
98
|
118
|
24
|
26
|
NE55
|
3
|
Third stage
|
118
|
98
|
22
|
25
|
NE50
|
4.2.2 Ground pressure control effect
To monitor the deformation of the working face after the ground fracturing in the range of crack expansion, the hydraulic supports in the middle of the working face were selected to note the resistance of the working face in the normal mining section and fractured crack extension range. The roadway deformation monitoring points were also arranged at intervals of 10 m in the normal mining section and fractured crack extension area in the roadway, and were recorded as #1–#6, respectively, to monitor the deformation of the roadway, as shown in Fig. 12.
The roadway deformation and support resistance before and after the working face enters the ground fracturing control zone are shown in Fig. 13. The deformation characteristics of the roadway ahead of the working face at 20 m of each measuring point are shown in Fig. 13(a). The roadway deformation at #1 measuring point and #3 measuring point is large, which is mainly owing to the large mining thickness of the coal seam. The roadway deformation is significant in the advanced 20 m of the working face, and the extent of the roof-to-floor and two-side convergence is more than 1500mm. The bending of the single pillar is prominent. When the working face enters the ground fracturing control zone, the roadway deformation in advance is highly controlled. Taking #5 measuring point as an example, the roadway deformation is less than 300 mm, the single props do not exhibit any bending phenomenon, and the roadway maintenance is in excellent condition.
The characteristics of the support resistance before and after the working face enters the ground fracturing control zone are shown in Fig. 13(b). It can be clearly seen from the figure that the periodic roof weighting pace of the working face is in the range from 35 m to 55 m before the working face enters the ground fracturing control zone. Affected by the breakage and instability of the hard roofs, the compressive strength is up to 43 MPa, which has an obvious influence on the working face. When the working face is mined into the ground fracturing control zone, there are no obvious weighting step characteristics. The maximum strength of the supports is reduced to 30 MPa.
In summary, after the ground fracturing of the high-level hard and thick strata, the integrity and structural characteristics of the hard strata are destroyed, which reduces the energy intensity of the strata breakage. The surrounding rock failure and compressive strength of the supports are significantly controlled. It can be seen that the ground fracturing has a positive effect on ameliorating the stress environment of the stope, controlling the deformation of the surrounding rock, and reducing the strength of the mine pressure. This is a new and powerful approach to control the hard roofs in coal mining areas.