Safe mining method for water-rich gently inclined coal seam

Abstract

A mining method for a water-rich gently inclined coal seam includes steps such as preparation of cut hole strips, construction of a local drainage system, mining of the cut hole strips, run-of-mine coal cross-cut mining, construction of a continuous drainage system, construction of a bulkhead dam, inner dumping follow-up, and mining cycling. A coal mass and a roller-compacted bulkhead dam are used to alternately support an inner dump, so as to ensure slope stability of the inner dump. Prolonged three-sided dewatering of the coal mass, especially bedding dewatering in an advancing direction, is conducive to dewatering and quality improvement of run-of-mine coal. The coal seam can ne mined in a centralized manner from the middle toward both sides along the advancing direction, thereby shortening an inner dumping haul distance of the overburden materials along the end slopes on both sides.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This patent application claims the benefit and priority of Chinese Patent Application No. 202510230041.8, filed with the China National Intellectual Property Administration on February 28, 2025, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.TECHNICAL FIELD

[0002] The present disclosure relates to a coal seam mining method and, in particular embodiments, to a safe mining method for a water-rich gently inclined coal seam.BACKGROUND

[0003] When open-pit mining is used, a gently inclined coal seam refers to a coal seam having a floor with an inclination angle of 5°~15°. During the mining process, the following problems are encountered. First, the inclination angle is large. Although there are specific inner dumping conditions, a dump still tends to slide along the floor, which seriously affects slope safety and effective capacity of an inner dump. Second, water from end slopes and the inner dump may be collected in a mining working face and a coal mining face is soaked for a long time. For open-pit mine with a large quantity of water inflow, operation safety of a device during mining is affected, and a water content of mined run-of-mine coal may also be increased to affect heat production of run-of-mine coal.SUMMARY

[0004] In view of the foregoing problems in the prior art, the present disclosure provides a safe mining method for a water-rich gently inclined coal seam, so as to reduce a water content of run-of-mine coal while ensuring slope safety of an inner dump.

[0005] The present disclosure provides technical solutions for a safe mining method for a water-rich gently inclined coal seam. Embodiments include preparation of cut hole strips, construction of a local drainage system, mining of the cut hole strips, run-of-mine coal cross-cut mining, construction of a continuous drainage system, construction of a bulkhead dam, inner dumping follow-up, and mining cycling.

[0006] Preparation of cut hole strips: At positions that are located on two symmetrical sides of a coal seam central axis on a working slope coal seam, cut hole strips are divided along an open-pit mine advancing direction, and a width of the cut hole strip in the open-pit mine advancing direction is less than an annual advance. An overburden bench above a coal seam roof of the cut hole strip advances ahead to expose a coal seam in the cut hole strip completely.

[0007] Construction of a local drainage system: At a junction of the coal seam in the cut hole strip and the overburden bench in an upper part, a trench below the coal seam floor is dug down as a dewatering trench. With the coal seam central axis as a middle point, the dewatering trench is arranged along the coal seam floor in a pseudo-inclined layout toward end slopes on both sides respectively to form a 0.5%~1% flow gradient, a position near the end slopes on both sides is a low point, and a water collection pit in communication with the dewatering trench is separately dug at the low point. In a position of the coal seam central axis, a trench below the coal seam floor is dug down along the open-pit mine advancing direction as a diversion trench, and the diversion trench is in communication with the dewatering trench to form a "T"-shaped drainage structure.

[0008] Mining of the cut hole strips: The coal seam in the cut hole strip is divided for small-bench mining along the open-pit mine advancing direction. The floor is exposed after the coal seam is mined, after a width of an exposed area that moves upward along the coal seam is more than four times a thickness of the coal seam, rubble is laid in areas of the drainage trench and the diversion trench below the coal seam floor, and a straw curtain or gauze is overlaid to form a dewatering blind trench and a diversion blind trench.

[0009] Run-of-mine coal cross-cut mining: After the cut hole strip is completed, benches on both sides of the cut hole strip in an advancing direction are widened to form two cross-cut mining working faces, the cross-cut mining working faces advance in directions of the end slopes, the working slope coal seam is mined, and the overburden bench in an upper part of the coal seam advances ahead of the cross-cut mining working face.

[0010] Construction of a continuous drainage system: As the cross-cut mining working faces advance constantly in directions of the end slopes on both sides, the coal seam floor is constantly exposed. The dewatering blind trench also extends synchronously towards both sides, and a construction position of the water collection pit also changes synchronously, but the cross-cut mining working face is always located at an end point position on a side of the dewatering blind trench near the end slope. Staged diversion blind trenches in parallel with the diversion blind trench are spaced 100~200m apart on the coal seam floor, and are in communication with the dewatering blind trench.

[0011] Construction of a bulkhead dam: In a run-of-mine coal cross-cut mining process, in a goaf after the coal seam in the cut hole strip is mined, open-pit mine overburden materials are discharged and roller-compacted in layers along an edge of a side of the dewatering trench close to an inner dump, so as to form a bulkhead dam. In addition, a permeable material is backfilled between the working slope coal seam and the bulkhead dam as a permeable layer.

[0012] Inner dumping follow-up: After the bulkhead dam reaches a design height, and when the bulkhead dam extends beyond widths of two open-pit mine lanes in the directions of the end slopes on both sides, with the coal seam central axis as a starting point, a conveyor bridge in communication with the inner dump and the working slope coal seam is constructed above the bulkhead dam. With the conveyor bridge as a starting point, the inner dump performs inner dumping follow-up respectively towards the end slopes on both sides and the open-pit mine advancing direction.

[0013] Mining cycling: When run-of-mine coal cross-cut mining reaches the end slopes, on both sides and the inner dump synchronously follows the end slopes, this round of mining ends. In this case, the permeable layer is recovered, and then a next round of cut hole strip is delimited. A next round of mining is performed based on the foregoing steps, so that mining is cyclically similarly.

[0014] A length of the cut hole strip in a direction perpendicular to the open-pit mine advancing direction might not be less than L, and L=2B min+Harctanβ, where B min is a minimum pit bottom width, H is a coal seam height, β is a coal seam slope angle, and a vertical drawdown is less than one bench height.

[0015] The dewatering trench may have a depth of more than 2 m below the coal seam floor.

[0016] The diversion trench may have a depth of more than 2 m below the coal seam floor.

[0017] Small-bench division in a mining step of the cut hole strip may be horizontal slicing in a direction from a working slope to the inner dump, and each bench height does not exceed 3 m.

[0018] The bulkhead dam may keep follow-up advancing at a distance of the minimum pit bottom width from the cross-cut mining working face with the coal seam central axis as a starting point. A total height of the bulkhead dam may be equal to one-time drawdown of open-pit mine, with a slicing height of 1~2 m.

[0019] In the inner dumping follow-up step, as the inner dump may advance toward the directions of the end slopes on both sides. A second conveyor bridge in communication with the inner dump and an upper bench of the working slope coal seam may be constructed near the coal seam central axis.

[0020] Compared with the prior art, in the present disclosure, a coal mass and a roller-compacted bulkhead dam are used to alternately support an inner dump, so as to ensure slope stability of the inner dump. Prolonged three-sided dewatering of the coal mass, especially bedding dewatering in an advancing direction, is conducive to dewatering and quality improvement of run-of-mine coal. The coal seam is mined in a centralized manner from the middle toward both sides along the advancing direction, thereby shortening an inner dumping haul distance of the overburden materials along the end slopes on both sides. In addition, a rapidly extended conveyor bridge shortens an inner dumping haul distance of the overburden materials in the middle. Without changing an overall open-pit mine advancing direction, the construction technology is simple, used devices are original mine devices, and the costs are controllable.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic diagram of preparation of cut hole strips and a local drainage system according to the present disclosure;

[0022] FIG. 2 is a schematic diagram of a local drainage system and mining of cut hole strips according to the present disclosure;

[0023] FIG. 3 is a schematic diagram of run-of-mine coal cross-cut mining and construction of a continuous drainage system according to the present disclosure;

[0024] FIG. 4 is a schematic diagram of construction of a bulkhead dam according to the present disclosure; and

[0025] FIG. 5 is a schematic diagram of inner dumping follow-up according to the present disclosure.

[0026] Reference numerals: 1: working slope coal seam; 2: open-pit mine advancing direction; 3: cut hole strip; 4: coal seam central axis; 5: overburden bench; 6: dewatering trench; 7: end slope; 8: water collection pit; 9: diversion trench; 10: dewatering blind trench; 11: diversion blind trench; 12: cross-cut mining working face; 13: staged diversion blind trench; 14: inner dump; 15: bulkhead dam; 16: permeable layer; and 17: conveyor bridge.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The present disclosure is further described below in combination with the attached figures.

[0028] The following clearly and completely describes technical solutions in embodiments of the present disclosure with reference to drawings in the embodiments of the present disclosure. Apparently, the described embodiments are merely a part rather than all of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art on the basis of the embodiments of the present disclosure without requiring the exercise of inventive effort fall within the scope of protection of the present disclosure.

[0029] As shown in FIG. 1 to FIG. 5, the present disclosure provides a technical solution, including steps such as preparation of cut hole strips, construction of a local drainage system, mining of the cut hole strips, run-of-mine coal cross-cut mining, construction of a continuous drainage system, construction of a bulkhead dam, inner dumping follow-up, and mining cycling.Preparation of cut hole strips

[0030] At positions that are located on two symmetrical sides of a coal seam central axis 4 on a working slope coal seam 1, cut hole strips 3 are divided along an open-pit mine advancing direction 2, and a width of the cut hole strip 3 in the open-pit mine advancing direction 2 is less than an annual advance. A length of the cut hole strip 3 in a direction perpendicular to the open-pit mine advancing direction 2 is not less than L, and L=2B min+Harctanβ, where B min is a minimum pit bottom width, generally not less than 50 m, H is a coal seam height, and β is a coal seam slope angle. A vertical drawdown is less than one bench height, and the vertical drawdown is a height difference between starting and ending points of the cut hole strip 3 corresponding to the coal seam.

[0031] An overburden bench 5 above a coal seam roof of the cut hole strip 3 advances ahead to expose a coal seam in the cut hole strip 3 completely. Benches above the coal seam roof adopt a mining mode of combined benches and steep working slopes, so as to reduce ahead overburden engineering amount.Construction of a local drainage system

[0032] At a junction of the coal seam in the cut hole strip 3 and the overburden bench 5 in an upper part, a high-front shovel is used to dig down a trench having a depth of more than 2 m below the coal seam floor down as a dewatering trench 6. With the coal seam central axis 4 as a middle point, the dewatering trench 6 is arranged along the coal seam floor in a pseudo-inclined layout toward end slopes 7 on both sides respectively to form a 0.5%~1% flow gradient. The dewatering trench 6 is integrally of a structure that is high in the middle and low at both ends. A position of the dewatering trench 6 near the end slopes 7 on both sides is a low point. A water collection pit 8 in communication with the dewatering trench 6 is separately dug at the low point.

[0033] In a position of the coal seam central axis 4, a trench having a depth of more than 2 m below the coal seam floor is dug down along the open-pit mine advancing direction 2 as a diversion trench 9, and the diversion trench 9 is in communication with the dewatering trench 6 to form a "T"-shaped drainage structure. Water at the bottom of the pit is collected in the water collection pits 8 at both ends of the dewatering trench 6 to be discharged.Mining of the cut hole strips

[0034] The coal seam in the cut hole strip 3 is divided for small-bench mining along the open-pit coal mine advancing direction 2, and is dug down and loaded by using a hydraulic backhoe. A small-bench division principle is that horizontal slicing is performed in a direction from the working slope to the inner dump 14, and each bench height does not exceed 3 m, so as to adapt a change in a dip angle of the coal seam to the maximum extent.

[0035] The floor is exposed after the coal seam is mined, after a width of an exposed area that moves upward along the coal seam is more than four times a thickness of the coal seam, rubble is laid in areas of the dewatering trench 6 and the diversion trench 9 below the coal seam floor, and a straw curtain or gauze is overlaid to form a dewatering blind trench 10 and a diversion blind trench 11. Unwatering and dewatering effects of the coal seam can be achieved continuously.Run-of-mine coal cross-cut mining

[0036] After the cut hole strip 3 is completed, benches on both sides of the cut hole strip 3 in an advancing direction are widened to form two cross-cut mining working faces 12, the cross-cut mining working faces 12 advance in directions of the end slopes 7, the working slope coal seam 1 is mined, and the overburden bench 5 in an upper part of the coal seam advances ahead of the cross-cut mining working face 12, so as to accelerate run-of-mine coal recycling.Construction of a continuous drainage system

[0037] As the cross-cut mining working faces 12 advance constantly in directions of the end slopes 7 on both sides, the coal seam floor is constantly exposed, the dewatering blind trench 10 also extends synchronously towards both sides, and a construction position of the water collection pit 8 also changes synchronously, but the cross-cut mining working face 12 is always located at an end point position on a side of the dewatering blind trench 10 near the end slope 7. Staged diversion blind trenches 13 in parallel with the diversion blind trench 11 are spaced 100~200m apart on the coal seam floor, and are in communication with the dewatering blind trench 10. The water in the inner dump 14 is dredged to the dewatering blind trench 10 and collected to the water collection pit 8.Construction of a bulkhead dam

[0038] In a run-of-mine coal cross-cut mining process, in a goaf after the coal seam in the cut hole strip 3 is mined, open-pit mine overburden materials are discharged and roller-compacted in layers along an edge of a side of the dewatering trench 6 close to an inner dump 14, so as to form a bulkhead dam 15 that supports the inner dump 14. The bulkhead dam 15 keeps follow-up advancing at a distance of the minimum pit bottom width from the cross-cut mining working face 12 with the coal seam central axis 4 as a starting point, a total height of the bulkhead dam 15 is equal to one-time drawdown of open-pit mine, with a slicing height of 1~2 m. A specific height is determined based on an effective compaction depth of a rolling device.

[0039] In addition, a permeable material such as glutenite is backfilled between the working slope coal seam 1 and the bulkhead dam 15 as a permeable layer 16, which can support the bulkhead dam 15. In addition, it is ensured that a slope surface of the working slope coal seam 1 normally seeps water into the dewatering blind trench 10.Inner dumping follow-up

[0040] After the bulkhead dam 15 reaches a design height, and when the bulkhead dam 15 extends beyond widths of two open-pit mine lanes in the directions of the end slopes 7 on both sides, with the coal seam central axis 4 as a starting point, a conveyor bridge 17 in communication with the inner dump 14 and the working slope coal seam 1 is constructed above the bulkhead dam 15, and with the conveyor bridge 17 as a starting point, the inner dump 14 performs inner dumping follow-up respectively towards two directions such as the end slopes 7 on both sides and the open-pit mine advancing direction 2.

[0041] To accelerate inner dumping follow-up, as the inner dump 14 advances toward the direction of the end slopes 7 on both sides, a second conveyor bridge 17 in communication with the inner dump 14 and an upper bench of the working slope coal seam 1 is constructed near the coal seam central axis 4.Mining cycling

[0042] When run-of-mine coal cross-cut mining reaches the end slopes 7, on both sides and the inner dump 14 synchronously follows the end slopes 7, this round of mining ends. In this case, the permeable layer 16 is recovered, and then a next round of cut hole strip 3 is delimited. A next round of mining is performed based on the foregoing steps, so that mining is cyclically similarly.

[0043] For those skilled in the art, the present disclosure is not limited to the details of the exemplary embodiment, and the present disclosure can be achieved in other specific forms without departing from the spirit or essential characteristics of the present disclosure. Therefore, for every point, the embodiments should be regarded as exemplary embodiments and are unrestrictive, the scope of the present disclosure is restricted by the claims appended hereto, and therefore, all changes, including the meanings and scopes of equivalent elements, of the claims are aimed to be included in the present disclosure. Any mark of accompanying drawings in the claims should not be regarded as limitation to the involved claims.

[0044] The foregoing descriptions are merely exemplary embodiments of the present disclosure, but are not intended to limit the present disclosure. Any tiny modification, equivalent replacement, or improvement made for the above embodiments based on technical essences of the present disclosure shall fall within the protection scope of the present disclosure.

Claims

1. A mining method comprising:preparing cut hole strips, wherein at positions that are located on two symmetrical sides of a coal seam central axis on a working slope coal seam, cut hole strips are divided along an open-pit mine advancing direction, and a width of the cut hole strip in the open-pit mine advancing direction is less than an annual advance, and wherein an overburden bench above a coal seam roof of the cut hole strip advances ahead to expose a coal seam in the cut hole strip completely;constructing a local drainage system, wherein at a junction of the coal seam in the cut hole strip and the overburden bench in an upper part, a trench below a coal seam floor is dug down as a dewatering trench, wherein, with the coal seam central axis as a middle point, the dewatering trench is arranged along the coal seam floor in a pseudo-inclined layout toward end slopes on both sides respectively to form a 0.5%~1% flow gradient, wherein a position near the end slopes on both sides is a low point, wherein a water collection pit in communication with the dewatering trench is separately dug at the low point, wherein in a position of the coal seam central axis, a trench below the coal seam floor is dug down along the open-pit mine advancing direction as a diversion trench, and wherein the diversion trench is in communication with the dewatering trench to form a "T"-shaped drainage structure;mining the cut hole strips, wherein the coal seam in the cut hole strip is divided for small-bench mining along the open-pit mine advancing direction, wherein a floor is exposed after the coal seam is mined, wherein, after a width of an exposed area that moves upward along the coal seam is more than four times a thickness of the coal seam, rubble is laid in areas of the dewatering trench and the diversion trench below the coal seam floor, and wherein a straw curtain or gauze is overlaid to form a dewatering blind trench and a diversion blind trench;performing run-of-mine coal cross-cut mining, wherein after the cut hole strip is completed, benches on both sides of the cut hole strip in an advancing direction are widened to form two cross-cut mining working faces, wherein the cross-cut mining working faces advance in directions of the end slopes, wherein the working slope coal seam is mined, and wherein the overburden bench in an upper part of the coal seam advances ahead of the cross-cut mining working face;constructing a continuous drainage system, wherein, as the cross-cut mining working faces advance constantly in directions of the end slopes on both sides, the coal seam floor is constantly exposed, the dewatering blind trench also extends synchronously towards both sides, and a construction position of the water collection pit also changes synchronously, but the cross-cut mining working face is always located at an end point position on a side of the dewatering blind trench near the end slope, and wherein staged diversion blind trenches in parallel with the diversion blind trench are spaced 100~200m apart on the coal seam floor and are in communication with the dewatering blind trench;constructing a bulkhead dam, wherein in a run-of-mine coal cross-cut mining process and in a goaf after the coal seam in the cut hole strip is mined, open-pit mine overburden materials are discharged and roller-compacted in layers along an edge of a side of the dewatering trench close to an inner dump, so as to form a bulkhead dam, and, in addition, a permeable material is backfilled between the working slope coal seam and the bulkhead dam as a permeable layer;performing an inner dumping follow-up, wherein after the bulkhead dam reaches a design height and when the bulkhead dam extends beyond widths of two open-pit mine lanes in the directions of the end slopes on both sides, with the coal seam central axis as a starting point, a conveyor bridge in communication with the inner dump and the working slope coal seam is constructed above the bulkhead dam, and, with the conveyor bridge as a starting point, the inner dump performs inner dumping follow-up respectively towards the end slopes on both sides and the open-pit mine advancing direction; andperforming mining cycling, wherein when run-of-mine coal cross-cut mining reaches the end slopes, on both sides and the inner dump synchronously follows the end slopes, this round of mining ends and the permeable layer is recovered, and then a next round of cut hole strip is delimited, and wherein a next round of mining is performed based on the foregoing steps, so that mining is cyclically similarly.

2. The mining method according to claim 1, wherein a length of the cut hole strip in a direction perpendicular to the open-pit mine advancing direction is not less than L and L=2Bmin+Harctanβ wherein Bmin is a minimum pit bottom width, H is a coal seam height, β is a coal seam slope angle, and a vertical drawdown is less than one bench height.

3. The mining method according to claim 1, wherein the dewatering trench has a depth of more than 2 m below the coal seam floor.

4. The mining method according to claim 1, wherein the diversion trench has a depth of more than 2 m below the coal seam floor.

5. The mining method according to claim 1, wherein small-bench division in a mining step of the cut hole strip is horizontal slicing in a direction from a working slope to the inner dump, and wherein each bench height does not exceed 3 m.

6. The mining method according to claim 1, wherein the bulkhead dam keeps follow-up advancing at a distance of a minimum pit bottom width from the cross-cut mining working face with the coal seam central axis as a starting point, and wherein a total height of the bulkhead dam is equal to one-time drawdown of open-pit mine, with a slicing height of about 1 to 2 m.

7. The mining method according to claim 1, wherein in performing the inner dumping follow-up, as the inner dump advances toward the directions of the end slopes on both sides, a second conveyor bridge in communication with the inner dump and an upper bench of the working slope coal seam is constructed near the coal seam central axis.

8. A method of mining, the method comprising:establishing cut hole strips on two symmetrical sides of a coal seam central axis of a working slope coal seam, wherein the cut hole strips are oriented along an open-pit mine advancing direction, and wherein an overburden bench above a coal seam roof of each cut hole strip is removed to expose the coal seam in the cut hole strips;constructing a T-shaped drainage structure by:digging a dewatering trench below a floor of the exposed coal seam at a junction between the coal seam and the overburden bench, wherein the dewatering trench extends from the coal seam central axis toward end slopes on both sides with a pseudo-inclined layout that forms a flow gradient between 0.5% and 1%, wherein the dewatering trench is positioned such that portions near the end slopes are lower than a portion at the coal seam central axis;digging a water collection pit at each low point of the dewatering trench near the end slopes, wherein each water collection pit is in fluid communication with the dewatering trench; anddigging a diversion trench below the coal seam floor along the open-pit mine advancing direction at the coal seam central axis, wherein the diversion trench is in fluid communication with the dewatering trench to form the T-shaped drainage structure;mining the coal seam in the cut hole strips by dividing the coal seam into small benches oriented along the open-pit mine advancing direction;converting the dewatering trench and the diversion trench into subsurface drainage structures by placing rubble in the dewatering trench and the diversion trench and overlaying a permeable covering material on the rubble after the coal seam floor is exposed over an area having a width that is more than four times a thickness of the coal seam, thereby forming a dewatering blind trench and a diversion blind trench;performing cross-cut mining by widening benches on both sides of the cut hole strips to form two cross-cut mining working faces that advance toward the end slopes to mine the working slope coal seam;extending the dewatering blind trench synchronously with advancement of the cross-cut mining working faces toward the end slopes while maintaining each cross-cut mining working face at an end point position of the dewatering blind trench near a respective end slope;constructing staged diversion blind trenches on the coal seam floor parallel to the diversion blind trench and spaced 100-200 meters apart, wherein the staged diversion blind trenches are in fluid communication with the dewatering blind trench;constructing a bulkhead dam by discharging and roller-compacting open-pit mine overburden materials in layers in a goaf created after mining the coal seam in the cut hole strips, wherein the bulkhead dam is formed along an edge of the dewatering trench closest to an inner dump;backfilling a permeable material between the working slope coal seam and the bulkhead dam to form a permeable layer; andperforming inner dumping by constructing a conveyor bridge above the bulkhead dam after the bulkhead dam reaches a design height and extends beyond widths of two open-pit mine lanes in directions of the end slopes, wherein the conveyor bridge provides communication between the inner dump and the working slope coal seam, and wherein the inner dump performs follow-up dumping toward the end slopes and along the open-pit mine advancing direction.

9. The method of claim 8, further comprising repeating the mining process by recovering the permeable layer and delimiting a next round of cut hole strips after the cross-cut mining reaches the end slopes and the inner dump synchronously follows the end slopes.

10. The method of claim 8, wherein a length of each cut hole strip in a direction perpendicular to the open-pit mine advancing direction is at least L, where L=2Bmin+H×arctan(β), wherein Bmin is a minimum pit bottom width, H is a height of the coal seam, and β is a slope angle of the coal seam.

11. The method of claim 10, wherein a vertical drawdown of each cut hole strip is less than one bench height.

12. The method of claim 8, wherein the dewatering trench has a depth of at least 2 meters below the coal seam floor, and wherein the diversion trench has a depth of at least 2 meters below the coal seam floor.

13. The method of claim 8, wherein mining the coal seam in the cut hole strips comprises horizontal slicing from a working slope toward the inner dump, and wherein each bench height does not exceed 3 meters.

14. The method of claim 8, wherein the bulkhead dam maintains a follow-up advancing distance equal to a minimum pit bottom width from the cross-cut mining working faces, as measured from the coal seam central axis.

15. The method of claim 8, wherein a total height of the bulkhead dam equals a one-time drawdown height of the open-pit mine, and wherein the bulkhead dam is constructed with a slicing height between 1 and 2 meters.

16. The method of claim 8, wherein the permeable covering material comprises a straw curtain or gauze.

17. The method of claim 8, wherein the permeable material backfilled to form the permeable layer comprises glutenite.

18. The method of claim 8, further comprising constructing a second conveyor bridge near the coal seam central axis as the inner dump advances toward the end slopes, wherein the second conveyor bridge provides communication between the inner dump and an upper bench of the working slope coal seam.

19. The method of claim 8, wherein the overburden bench in an upper part of the coal seam advances ahead of each cross-cut mining working face during the cross-cut mining.

20. The method of claim 8, wherein the coal seam is a gently inclined coal seam having a floor with an inclination angle between 5 degrees and 15 degrees.

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