Abstract
In Northwest China, rainfall is low, water resources are scarce, and the ecological environment is fragile. For shallow-buried and close-spaced coal seams with a thickness of upper coal bed >60∼70 m, the water-conducting fissures of the overlying rock will not penetrate the water-isolating layer after the upper coal seam is mined; the internal and external gap angles of the water-conducting fissures are not generated from the water-isolating layer. We set out to explore the critical internal and external dislocations for the second significant development of water-conducting fissures in the overlying rock after coal mining under control. A calculation model for the critical internal and external staggered distances of coal mining face in shallow-buried and close-spaced coal seams is established, the calculation formula is given, and the calculation formula for the critical seam mining ratio under the condition of internal staggered mining mode is given. Numerical simulation performed by UDEC methods: taking the overburden strata in the shallow-buried and close-spaced coal seam mining area of Shigetai Coal Mine as a prototype, it was verified that the critical internal and external offsets of the coal mining face in shallow-buried and close-spaced coal seams have a significant effect on the overlying water flow cracks in the mining of the lower coal seam. For the feasibility of developmental control, according to the engineering geological conditions of Shigetai, through the calculation method of external staggered distance, it is concluded that the distance of the open cut of the lower coal face and the upper coal face is only 21∼27 m, which is much smaller than the water barrier. It does not produce the critical distance of the water-conducting cracks. Therefore, in the process of mining the lower coal seam, the water-proof layer will produce water-conducting cracks, lose its water-proof performance, and cause water loss. This is also the cause of the water inrush accident in Shigetai Coal Mine.
1. Introduction
Taking the geological parameters that affect the development of water-conducting fissures in the overlying rock as the basic conditions, starting with the mining parameters, find the control methods. There is a soft rock water barrier in the shallow-buried and close-spaced Shendong coal mining area. Once the water barrier is stretched and damaged and eventually water-conducting cracks are formed, water resources will be lost. Looking for a control method for the development of water-conducting fissures in the overlying rock in the repeated disturbance zone is to find a control method that does not produce water-conducting fissures in the water barrier [1–6].
For the first type of shallow-buried and close-spaced coal seams in Northwest China (the thickness of the upper seam’s bedrock <50–60 m), when the full-thickness long-wall mining method is adopted to mine the coal seam, the overburden water-conducting fissures will penetrate the ground surface, causing water resource loss. For such shallow-buried and close-spaced coal seams, even if the full-mining and full-filling coal mining method is adopted in the mining of the lower coal seam, the loss of water resources cannot be prevented, unless the overburden water-conducting fissures are grouted and blocked. For the second type of shallow-buried and close-spaced coal seams (the thickness of the upper seam’s bedrock >60∼70 m), when the full-thickness long-wall mining method is adopted to mine the coal seam, after the upper coal seam is mined, due to the expansion of the water-resistant layer existence, the water-conducting cracks in the overlying rock will not penetrate the water-impermeable layer in the end. When mining the lower coal seam, whether the water-conducting cracks in the overlying rock will penetrate the water-resistant layer is mainly related to the control method adopted. Therefore, exploring the control methods for the development of water-transmitting fissures in the overlying rock in shallow-buried and close-spaced coal seams will be carried out for the second type of shallow-buried and close-spaced coal seams.
Starting from the mining layout (the inner and outer staggered layouts of the mining face of the upper and lower coal seams), the effective way to is find the second significant development of the overlying water-conducting fissures in the smallest area after the mining of the lower coal seam.
2. The Critical Inner and Outer Distances of the Water Barrier That Do Not Produce Water-Conducting Cracks
Coal mining will cause overlying strata to move. Under sufficient mining conditions, the surface above the mined-out area will sink to form a sinking basin. In the main section of the surface subsidence basin, the angle between the line from the edge point to the boundary point of the corresponding mined-out area and the horizontal line is called the strata movement boundary angle. The center of the sinking basin is a flat, no-deformation zone, and the angle between the line from the edge point to the boundary point of the corresponding mined-out area and the horizontal line is called the full-mining angle of rock formation. For shallow-buried and close-spaced coal seams, the boundary angles and full-mining angles of the upper and lower coal seams during mining can be used to determine the critical inner and outer displacements of the aquifer without water-conducting cracks.
2.1. Internal Staggered Layout
When the upper and lower coal mining face adopts the internal staggered layout, if the development of the overlying water-conducting fissures near the upper coal seam’s mining boundary can be maintained at the original stable state and no longer continue to develop after the lower coal seam is mined, then this type of internal error distance under the conditions is the critical internal error distance. After the coal seam is mined, the overlying rock layer is broken, forming a rock layer breaking line. The angle between the rock fracture line and the horizontal line is the rock fracture angle, generally 60–78°. The fracture angle of the rock formation is often greater than the full-mining angle of the rock formation movement [7].
When adopting the internal staggered layout, if the moving edge point of the water barrier (the intersection of the boundary line of the rock formation and the top surface of the water barrier) during the mining of the lower coal seam is located in the no-deformation zone of the sinking basin of the water barrier after the mining of the upper coal seam, then, during the mining process of the lower coal seam, the secondary development of water-conducting fissures in the overlying rock on the side of the cut-off cut in the upper coal seam will not occur. When mining shallow-buried and close-spaced coal seams, the critical internal staggered layout of the upper and lower coal mining faces is shown in Figure 1.
According to the calculation model of the critical internal offset of the coal mining face in shallow-buried and close-spaced coal seams (as shown in Figure 2), the calculation formula of the critical internal offset iswhere is the total thickness of the rock above the upper coal seam and below the aquifer, m; is the full-mining angle after the upper coal seam is mined; is stratum movement boundary angle after mining of the lower coal seam; is the maximum subsidence value of the aquifer when the upper coal seam is mined, m; is the thickness of interbedded rock in shallow-buried and close-spaced coal seams, m.
2.2. External Staggered Distance
Figure 3 shows the overall external staggered layout of the coal face under the shallow-buried and close-spaced coal seam. When the shallow-buried and short-distance coal seam adopts the external staggered layout, if the boundary point of the no-deformation zone of the sinking basin of the lower coal seam is located at the boundary point of the water-resisting layer movement during the mining of the upper coal seam (the boundary line of the rock layer movement and the water barrier except for the intersection point of the top surface of the seam), the mining of the lower coal seam will not cause the secondary development of water-conducting fissures in the upper coal seam on the side of the cut.
The calculation model of the critical distance for shallow-buried and close-spaced coal seams is shown in Figure 4. The calculation formula for the critical distance is as follows:where is the thickness of interlayer rock, m; is the upper seam’s thickness, m; is the full-mining angle when mining the lower coal seam; and is stratum movement boundary angle during upper coal mining.
The seam mining ratio is proposed for the mining of close-spaced coal seams, and its meaning is the ratio of the thickness of the interlayer rock layer to the mining height of the lower coal seam, η; that is, η = hcj/Mx.
When the external staggered layout is adopted, the development of water-transmitting fissures in the overlying rock during the mining process of the shallow-buried and close-spaced coal seam can be regarded as a single coal seam (lower coal seam) mining for research. In some cases, due to the restriction of the production geological conditions, the internal staggered layout is required. Then, it is necessary to study the critical stratum mining ratio that does not produce water-conducting cracks in the water barrier.
2.2.1. The Minimum Value of the Maximum Subsidence Value When the Water Barrier Does Not Produce Water-Conducting Cracks
According to existing research results, the relationship between the crack width of the aquifer and its maximum subsidence value iswhere d is the water barrier crack width; is the maximum subsidence value when the water barrier does not produce water-conducting cracks.
The nonhydrophilic water barrier material measured by the lateral restraint expansion experiment is = 0.605 mm. To ensure that water resources are not lost, ; that is, >2 = 1.210 mm. Substituting formula (3), we have ≤ 1.649 m.
Therefore, = 1.649 m is the minimum maximum subsidence value when the water barrier does not produce water-conducting cracks. When it is > 1.649 m, water-conducting fissures will occur in the water barrier.
2.2.2. Critical Layer Mining Ratio Where the Water Barrier Does Not Produce Water-Conducting Cracks
Shallow-buried and close-spaced coal seams adopt internal staggered arrangement. When the internal staggered distance is greater than the critical inner staggered distance, the area where the overlying strata moved and the cracks developed due to the mining of the lower coal seam is located in the no-deformation zone of the sinking basin after the upper coal seam is mined; that is to say, before the mining of the lower coal seam, the rock layers in the repeated disturbance zone were in a horizontal state. The hard rock layers were broken into blocks and squeezed and closed horizontally, while the soft rock water barrier did not undergo horizontal tensile deformation and did not produce water-conducting cracks.
Whether there are water-conducting cracks in the water barrier is mainly related to the height of the effective sinking space below, in addition to its physical and mechanical properties and hydraulic properties. The factors that determine the height of the effective sinking space are the thickness of the underlying rock layer and its residual swelling coefficient. According to existing research results, for close-spaced coal seams, after the lower coal seam is mined, the residual breaking expansion coefficient of the rock strata in the upper coal seam mining caving zone will become smaller, but the magnitude is not large, and the rock strata in the upper coal seam mining fracture zone will enter the caving zone, and the residual breaking expansion coefficient will increase, but the amplitude is not very large. On the whole, after the lower coal seam is mined, the residual breaking expansion coefficient of the overlying rock of the upper coal seam can be regarded as unchanged, and the shallow-buried and close-spaced coal seams also conform to this law. Therefore, for shallow-buried and close-spaced coal seams, when the internal fault distance is greater than the critical internal fault distance, the lower coal seam can be regarded as a single coal seam mining, and the thickness of the overlying rock layer is the layer spacing. Under the condition of the internal staggered arrangement, the critical condition for the water barrier not to produce water-conducting cracks iswhere is the average residual breaking expansion coefficient of the interlayer rock formation.
It is concluded that the critical layer mining ratio for the water barrier without water-conducting cracks is
At the same time, according to formulas (5) and (6), it can also be determined that the reasonable mining height of the lower coal seam when the water-resistant layer does not produce water-conducting cracks is
3. Numerical Simulation Analysis
Taking the overburden strata in the shallow-buried and close-spaced coal mining area of Shigetai Coal Mine as a prototype, the overburden parameters are shown in Table 1 [8–11].
Using UDEC’s stress-seepage coupling system, simulation calculation, and analysis of the development law and seepage characteristics of the internal and external offsets to the overburden seepage fissures during the mining of shallow-buried and close-spaced coal seams, according to the measured physical and mechanical parameters of the overburden, the model parameters are assigned. The rock mechanical parameters are shown in Table 2, and the rock joint parameters are shown in Table 3. The boundary conditions are as follows: the left and right sides and the lower part of the model are displacement boundaries and nonseepage boundaries. The simulated solid-liquid coupling adopts SET steady flow, and the joint characteristics adopt the default setting (the second type). The initial pore pressure is set as pp = 0.125 MPa.
3.1. The Influence of Internal Staggered Distance on the Water-Conducting Fissures
Numerical models of different internal error distances are established, and the simulation results are shown in Figure 5.
(a)
(b)
(c)
From the analysis of Figure 5, it can be seen that, with the increase of the internal staggered distance, the secondary development degree of the water-conducting fissures on the side of the cut-off cut in the upper coal seam gradually decreases. When the internal staggered distance is less than 80 m, the mining of the lower coal seam will cause the water-conducting fissures on the side of the open cut to develop twice; when the internal staggered distance is 80 m, the water-conducting fissures on the side of the open cut will not be affected by the mining of the lower coal seam. Without secondary development, it can maintain the original stable state after the upper coal seam is mined (before the lower coal seam is mined).
3.2. The Influence of External Staggered Distance on the Water-Conducting Fissures of the Overlying Rock
Numerical models of different external error distances are established, and the simulation results are shown in Figure 6.
(a)
(b)
(c)
(d)
It can be seen from the analysis of Figure 6 that, with the increase of the external staggered distance, the development degree of the overlying rock water-conducting fissures on the side of the cut-off cut in the upper coal seam gradually decreases. When the offset distance is less than 80 m, the mining of the lower coal seam will cause the secondary development of the water-conducting fissures on the open cut side of the upper coal seam; and when the external staggered distance is greater than 90 m, the water-conducting fissures on the open cut side of the upper coal seam will not develop. Affected by the mining of the lower coal seam, new water-conducting fissures are generated in the overlying rock near the mining boundary of the lower coal seam.
4. Reverse Verification of Project Examples
At 5:50 in the morning on August 2, 2010, the head of the lower coal seam was advanced by 17.5 m and the tail was advanced by 21.5 m. A roof water gushing accident occurred. The total amount of water gushing was about 47,000 m3, causing the equipment in the fully mechanized mining face to be flooded. The location of the water gushing is shown in Figure 7, and the actual photo of the scene is shown in Figure 8 [12].
According to the geological conditions of the water gushing site in Shigetai Coal Mine, the distance between the upper coal seam and the lower coal seam is 2.6 m, and the bedrock thickness of the upper coal seam is 76.37 m. According to the calculation method of the critical external offset, = 60°, = 73°, = 2.6 m, = 76.37 m, and = 0.86 m, substituted into equations (1)–(6), and the calculation is = 68.12 m. At the site of the roof water gushing accident, the distance between the lower seam face and the upper seam face is only 21∼27 m, which is far smaller than the critical outer staggered distance where the water barrier does not produce water-conducting cracks. Therefore, in the process of mining the lower coal seam, the water-proof layer will produce water-conducting cracks, lose the water-proof performance, and cause water loss.
5. Conclusion
(1)For the second type of shallow-buried and close-spaced coal seams (the thickness of the base rock of the upper coal seam >60∼70 m), under the conditions of repeated disturbance and multiple mining, the lower water-conducting cracks of the overburden water-proof layer are given. The calculation formula for the critical inner and outer staggered distances of the coal seam determines the calculation method for the critical stratum mining ratio that does not produce water-conducting cracks in the overburden water-resistant layer under the condition of inner staggered layout.(2)The roof water gushing accident at the fully mechanized coal mining face of Shigetai Coal Mine’s coal seam 12 caused the equipment in the working face to be flooded. The main reason was that the distance between the open cut of the lower coal face and the upper coal face was only 21∼27 m, which is smaller than the critical distance of the water barrier that does not produce water-conducting cracks.Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no known conflicts of interest that could influence the work reported in this paper.
Acknowledgments
This paper was supported by Guizhou Province Basic Research (Science and Technology Fund) Project (Qiankehe Foundation [2020] 1Y215).