Method for judging overhead longwall mining of overlying coal seam of shallow-buried room and pillar type goaf

Abstract

The invention discloses a shallow-buried room-and-pillar goaf overlying coal seam overhead longwall mining judgment method, which comprises the following steps of: 1, drilling and coring a room-and-pillar goaf by combining a mine drilling histogram, the occurrence condition of an overlying coal seam of the room-pillar type goaf, the formation condition of a rock stratum between the room-pillar type goaf and an overlying overhead coal seam and the damage condition of remaining coal pillars and a roof of the room-pillar type goaf are ascertained; 2, carrying out shallow-buried room-pillar type goaf overlying coal seam overhead mining to carry out feasibility evaluation; the method is simple and easy to master and use by engineers, the mining rate of a coal mine is increased, a large amount of funds are saved, non-renewable resources are effectively and reasonably utilized on the premise that safety production is achieved, and the method has the advantages of being high in practicability and high in practicability aiming at specific mining conditions of shallow coal seam lower room-pillar type goaf mining. The rock stratum structure is fully considered, and the feasibility of the overhead mining can be quantitatively and pertinently judged.

Description

Technical Field

[0001] The present invention relates to the technical field of mining determination, in particular to a method for determining longwall mining of overlying coal seams in shallow room-and-pillar goaf areas. Background Art

[0002] With the continuous decline of coal resources in central and eastern my country, western my country has become the primary coal mining base. Mining areas in western China are mostly concentrated in shallow areas, buried within 200 meters. These areas are characterized by a base load ratio (JZ) less than 1, a single key stratum structure in the roof, and significant dynamic loading. Longwall mining offers many advantages over room-and-pillar mining, including higher recovery rates, fewer roof accidents, and improved ventilation. However, due to relatively backward production technology and equipment, and a low degree of coal mining intensification, many coal mines have adopted room-and-pillar mining. This has left numerous small coal pillars within the goaf, leaving the roof in a state of either partial collapse, partial roof hanging, or incomplete collapse. The loads borne directly by the overlying coal strata and topsoil are borne by the coal pillars in the goaf. Over time, these pillars exhibit creep properties, increasing their deformation. Under natural conditions, this can lead to instability and collapse, resulting in overburden movement and impacting the recovery of the coal seam above them. The mainstream mining technology and methods for coal seam clusters in China currently utilizes a descending sequential mining method. However, in some special cases, some mines employ an ascending mining method, whereby the lower coal seam is mined first, and then tunnels and working faces are laid in the upper coal seam after stabilization. In existing operating mines, many mined-out areas still contain substantial recoverable coal seams with considerable economic value. Recovering this resource is of vital economic and strategic significance.

[0003] At present, since most of the existing domestic technologies use the "key layer" theory and the "three-zone" theory as the basis for determining whether goaf mining can be carried out, they are not well applicable to room-and-pillar goaf areas in shallow coal seams with only a single main key layer, resulting in low practical feasibility, low resource recovery rate, large amount of governance engineering and high mining costs in specific engineering practices.

[0004] Therefore, there is an urgent need for a method and system for determining longwall mining of overlying coal seams in shallow room-and-pillar goafs that can solve the above-mentioned problems, so that mines with this problem can achieve safe production. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for determining longwall mining of overlying coal seams in shallow room-and-pillar goaf areas, so as to solve the problem that the existing domestic determination methods have low applicability in longwall mining of overlying coal seams in room-and-pillar goaf areas of shallow coal seams, and in actual engineering, they can only rely on experience to determine whether longwall mining can be carried out.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for determining longwall mining of overlying coal seams in shallow room-and-pillar goafs comprises the following steps:

[0008] The first step is to drill and core the room-and-pillar goaf in combination with the mine borehole histogram to determine the occurrence of the overlying coal seams, the composition of the rock strata between the room-and-pillar goaf and the overlying coal seams, and the damage to the coal pillars and roof remaining in the room-and-pillar goaf.

[0009] The second step is to conduct a feasibility assessment on the mining of the overlying coal seam in the shallow room-and-pillar goaf.

[0010] The third step is to conduct stress and strength analysis on the coal pillars left in the room-and-pillar goaf;

[0011] The fourth step is to determine the instability conditions of the coal pillars left in the room-and-pillar goaf;

[0012] The fifth step is to qualitatively analyze the stability of the room-and-pillar goaf based on the residual coal pillars and roof damage in the proven room-and-pillar goaf, and classify them. There are four states of the room-and-pillar goaf: the residual coal pillars and roof are both stable; the residual coal pillars and roof are both damaged, and the ground surface slowly sinks; the residual coal pillars are stable, and the roof partially collapses; the residual coal pillars are damaged, and the roof is stable.

[0013] The sixth step is to determine the feasibility of mining the overlying coal seam in the room-and-pillar goaf. If mining is not theoretically feasible, the next coal seam will be mined first, and the mining of the overlying coal seam will be carried out after the mine pressure destroys and compacts the coal pillars in the room-and-pillar goaf. If mining is theoretically feasible, the safety of mining will be determined.

[0014] The seventh step is to conduct a safety assessment of the overlying coal seam in the room-and-pillar goaf. If the safety does not meet the requirements, reasonable control methods should be adopted to ensure that the safety meets the requirements before mining can proceed. If the safety meets the requirements, the maximum inclined length of the goaf longwall working face can be determined by the depth of the floor failure zone.

[0015] Beneficial effects of the present invention:

[0016] The method for determining the feasibility of goaf mining is simple and easy for engineers to master and apply. On the premise of achieving safe production, it improves the coal mine recovery rate, saves a lot of money, and effectively and reasonably utilizes non-renewable resources. This determination method is aimed at the specific mining conditions of goaf mining in room-and-pillar goaf areas under shallow coal seams, fully considers the rock structure, and can quantitatively and specifically determine the feasibility of goaf mining. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To facilitate understanding by those skilled in the art, the present invention is further described below with reference to the accompanying drawings.

[0018] Figure 1 This is the flow chart of longwall mining in shallow room-and-pillar goaf;

[0019] Figure 2 This is a schematic diagram of the coal pillar bearing capacity in room-and-pillar goaf using the subordinate area method;

[0020] Figure 3 This is a schematic diagram of the instability form of room-and-pillar goaf;

[0021] Figure 4 Develop a roadmap for determining the feasibility of air mining;

[0022] Figure 5 This is the stress distribution diagram along the working face;

[0023] Figure 6 This is the distribution map of the coal pillars left behind in the underlying room-and-pillar goaf when the coal seam is advanced;

[0024] Notes in the figure: W represents the width of the coal pillar; B represents the width of the coal room; l1 represents the descending distance after the peak value of the support stress increase area; l2 represents the ascending distance before the peak value of the support stress increase area; l3 represents the distance between the mining wall and the goaf; l4 represents the distance between the support stress reduction area; γ represents the rock density; H represents the burial depth; h1 represents the depth of the water-conducting damage zone of the floor of the longwall working face; b represents the height of the natural equilibrium arch of the coal pillar left in the room-and-pillar goaf; h represents the distance between coal seams. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0026] like Figure 1-6 As shown in the figure, a method for determining longwall mining of overlying coal seams in shallow room-and-pillar goaf is provided. The specific steps are as follows:

[0027] Step 1: Drill and coring the goaf area based on the mine borehole histogram to determine the occurrence of the goaf coal seam, the composition of the rock strata between the room-and-pillar goaf and the goaf coal seam, and the damage to the coal pillars and roof of the room-and-pillar goaf. When determining the composition of the rock strata between the room-and-pillar goaf and the goaf coal seam, thinner rock strata can be treated as weak interlayers, while thicker rock strata can be artificially stratified based on the natural joints within them. Generally, the natural layers are used as the stratification boundaries.

[0028] Step 2: Conduct feasibility assessment on mining the open coal seam based on the proven geological conditions;

[0029] Step 3: Conduct stress and strength analysis on the coal pillars left in the room-and-pillar goaf;

[0030] (1) Stress analysis of the residual coal pillar in the room-and-pillar goaf. The “subordinate area method” is used to calculate the load of the residual coal pillar in the goaf. The load of the overlying strata in the goaf is mainly borne by a single coal pillar and a structure consisting of half the size of the coal room. The average stress calculation formula acting on the residual coal pillar in the form of uniformly distributed load is:

[0031]

[0032] Where: Sp - residual coal pillar stress, MPa;

[0033] ρ——average density of overburden, t / m 3 ;

[0034] H——coal seam burial depth, m;

[0035] W——coal pillar width, m;

[0036] B——coal room width, m;

[0037] (2) Strength analysis of coal pillars left in room-and-pillar goaf;

[0038] The strength of the coal pillar is calculated using the Bieniawsk formula: The strength of the coal pillar is calculated using the Obert-Duvall formula: The strength of the coal pillar is calculated using the Holland formula: The coal pillar strength is calculated using the Salamaon-Munro formula:

[0039] Where: σ p ——coal pillar strength, MPa;

[0040] σ c ——Uniaxial compressive strength of coal pillar, MPa;

[0041] W——coal pillar width, m;

[0042] H g ——coal pillar height, m;

[0043] n——constant, >5, n = 1.4; <5, n = 1;

[0044] The strength and deformation characteristics of coal pillars are key to maintaining the stability of the goaf roof. After obtaining the calculated results, the coal pillar strength calculation results are compared to see if they are close. Taking into account a certain safety factor, the average value of the close calculated results is taken as the ultimate strength of the coal pillar left in the room-and-pillar goaf.

[0045] Step 4: Determine the instability conditions of the coal pillars left in the room-and-pillar goaf.

[0046] The strength and stress of the coal pillar itself are the basis of coal pillar stability analysis. When the stress of the coal pillar exceeds its ultimate bearing strength, it will cause the coal pillar to become unstable and fail. The ratio of the ultimate strength of the room-and-pillar coal pillar to the stress transmitted by the overburden strata to the residual coal pillar is called the coal pillar stability safety factor F. S , and its calculation formula is: Where: F s ——Safety factor of coal pillar stability;

[0047] S p ——Stress of the remaining coal pillar, MPa;

[0048] σ p ——coal pillar strength, MPa;

[0049] According to theoretical experience, the general coal pillar stability safety factor F S >1.5, it is believed that the coal pillar can maintain long-term stability; when the coal pillar stability safety factor F S <1.5, it is considered that the coal pillar cannot maintain long-term stability; when the coal pillar stability safety factor F S When ≈1.5, it is believed that the coal pillar can remain stable for a certain period of time, but the remaining coal pillar will creep under the action of long-term roof load, and the two sides of the coal pillar will gradually yield and peel off, the effective bearing area will be reduced, and the stress concentration will increase, the overall stability will decrease, and eventually large-scale instability will occur;

[0050] Step 5: Based on the ascertained geological conditions and the two structural elements of the residual coal pillar damage and roof damage in the room-and-pillar goaf, the instability of the roof and residual coal pillar in the goaf can be generally summarized into four states. The system can be classified into one of them according to the specific geological conditions. These four states are: State 1: Both the residual coal pillar and the roof are stable; State 2: Both the residual coal pillar and the roof are damaged, and the ground surface sinks slowly; State 3: The residual coal pillar is stable, and the roof partially collapses; State 4: The residual coal pillar is damaged, and the roof is stable.

[0051] Step 6: Determine the feasibility of mining by stepping on the coal pillars and taking different measures according to the different compositions of the coal pillars and roof left in the room-and-pillar goaf. The specific measures are as follows:

[0052] State 1: The remaining coal pillars and roof of the room-and-pillar goaf are both stable. In this case, the overburden and the entire ground surface of the room-and-pillar goaf are in an elastic state, with no obvious subsidence or damage. All the remaining coal pillars in the goaf can well bear the entire load of the overburden and topsoil layers. The roof can be regarded as a plate with fixed supports on all four sides and pillars in the middle, with good mechanical properties and in an elastic state. There are no cracks between the room-and-pillar goaf and the stepped coal seam, and the floor of the stepped coal seam has not undergone step deformation. Therefore, the longwall working face can be directly arranged in the stepped coal seam.

[0053] For state 2: both the remaining coal pillars and the roof are destroyed. During the destruction of the coal pillars, the roof rock strata are broken and gradually develop upward, and eventually the ground slowly sinks. In this case, the overlying rock strata in the goaf will inevitably move and deform, resulting in caving zones and fracture zones. Regardless of the interlayer spacing between the goaf coal seam and the room-and-pillar goaf, the room-and-pillar goaf needs to be processed before goaf mining. At this time, there are two states in the room-and-pillar goaf: the room-and-pillar goaf has been compacted and the room-and-pillar goaf has not been completely compacted. At this time, it is necessary to verify whether the room-and-pillar goaf has been re-compacted in combination with the drilling situation. If the goaf has been re-compacted, then Arranging a longwall working face in a coal seam will not cause safety accidents due to disturbance to the lower room-and-pillar goaf. Therefore, the longwall working face can be arranged directly in the hollow coal seam. If the goaf is not fully compacted, arranging the longwall working face directly in the hollow coal seam will disturb the coal pillars left in the room-and-pillar goaf, further accelerating the destruction and crack development of the rock strata between the goaf and the hollow coal seam, and even causing the rock strata to sink. Therefore, it is necessary to grout the room-and-pillar goaf to cement the loose and broken rock strata into a whole, forming a "stone body" with certain mechanical strength and chemical stability to meet the needs of arranging a longwall working face in the hollow coal seam.

[0054] For state 3, that is, the remaining coal pillar is stable and the roof partially collapses; in this case, the broken roof rock layer forms a self-supporting balanced arch structure, and the surface subsidence is caused by the plastic deformation of the roof rock layer; at this time, the roof rock layer undergoes plastic deformation and cracks develop, but the floor of the goaf coal seam does not produce step deformation. Regardless of the interlayer distance between the goaf coal seam and the room-and-pillar goaf, the room-and-pillar goaf needs to be processed before goaf mining can be carried out; therefore, the room-and-pillar goaf should be filled first, and then the longwall working face should be arranged in the goaf coal seam;

[0055] For state 4, that is, the remaining coal pillar is destroyed and the roof is stable; this situation occurs when the load imposed by the overlying rock strata on the coal pillar exceeds the bearing capacity of the remaining coal pillar in the room-and-pillar goaf, resulting in the destruction of the coal pillar; among the four occurrence states, this state has the most serious impact on the goaf. At this time, once the coal pillar is unstable, the roof rock strata will lose support and cause large-scale collapse, leading to catastrophic instability; when performing goaf mining in this situation, it is necessary to use the surrounding rock balance method, ratio method and "two-zone" height discrimination method to determine the feasibility of mining; if the surrounding rock balance method, ratio method and "two-zone" height discrimination method are all theoretically feasible for mining, then the safety of goaf mining is determined; if it is not theoretically feasible, mining should be carried out after the mine pressure compacts the remaining coal pillar in the room-and-pillar goaf;

[0056] The specific implementation methods for determining mining feasibility using the surrounding rock balance method, ratio method, and "two-zone" height discrimination method are as follows:

[0057] (1) Surrounding rock balance method. Since room-and-pillar mining destroys the original rock stress equilibrium state and causes the surrounding rock stress to be redistributed, it will inevitably cause the overlying rock strata to deform and destroy in the horizontal and vertical directions. Among them, the overlying rock strata have shear stress in the vertical direction, which causes the rock strata to produce shear deformation. Specifically, the coal seam undergoes step dislocation, destroying the coal seam structure, resulting in the inability to arrange the longwall working face in the empty coal seam. During the mining process, the rock stratum that can form a balanced rock stratum structure without step dislocation is called the balanced rock stratum. The height from the lower coal seam roof to the balanced rock stratum roof is called the surrounding rock balance height. The minimum surrounding rock balance height required for empty mining is calculated as follows:

[0058]

[0059] Where: H is the distance between the goaf and the room-and-pillar goaf, m;

[0060] M——mining height of lower coal seam, m;

[0061] K——the expansion coefficient of the roof strata in room-and-pillar goaf;

[0062] h p ——Thickness of the equilibrium rock layer itself, m;

[0063] The basic principle of mining is: if there is a hard rock layer in the overlying stratum and the coal seam is located above the equilibrium rock layer closest to the lower coal seam, the coal seam is considered to be feasible for mining; if the overlying stratum in the mining area is all soft rock and the coal seam is located in the fracture zone, the coal seam is considered to be unfeasible for mining and mining should be carried out after the rock layer caused by mining of the lower coal seam is stabilized;

[0064] (2) Ratio method: Whether the mining can be carried out depends mainly on the ratio of the distance between the two coal seams to the thickness of the lower coal seam (mining impact multiple). After the lower coal seam is mined, the feasibility of the mining can be judged by the size of the ratio K, that is:

[0065] ① Under the conditions of mining a single coal seam in the lower part, the calculation formula for the mining impact multiple is:

[0066]

[0067] Where: H——coal seam spacing, m;

[0068] M——mining height of lower coal seam, m;

[0069] ② If the lower part is multi-seamed mining, the calculation formula for the mining impact multiple is:

[0070]

[0071] Where: K1, K2, K n are the mining impact multiples of the first, second and nth coal seams respectively. After the coal seam is mined, the basic principle of its mining is: if the interlayer is hard rock, when K>7, the mining feasibility of the coal seam is considered; if the interlayer is medium-hard rock, when 5<K<7, the mining feasibility of the coal seam is considered;

[0072] (3) “Two-belt” height determination method:

[0073] As the coal pillars left in the room-and-pillar goaf are destroyed, the roof rock strata lose their support and will collapse on a large scale. The overlying rock strata will move and slide, eventually forming relatively regular collapse zones, fracture zones, and curved subsidence zones. The collapse zones and fracture zones are collectively called water-conducting fracture zones. Based on the rock mass damage characteristics of the collapse zones and fracture zones, when using the "two-zone" height method to determine the feasibility of upward mining in longwall goaf, the minimum interlayer spacing required for goaf mining is calculated as follows:

[0074]

[0075] Where: H li ——Height of water-conducting fracture zone in overlying strata of longwall goaf, m;

[0076] H k ——Height of the collapse zone of the overlying rock strata in the longwall goaf, m;

[0077] The basic principle of the mining of the coal seams is: when the distance between two coal seams is greater than the minimum distance between the two seams, the coal seams are considered feasible for mining;

[0078] Step 7: When the mining feasibility of the hollow coal seam is theoretically established, a safety assessment is conducted on the mining of the hollow coal seam. If the safety meets the requirements of hollow mining, the maximum inclined length of the longwall working face can be determined by the depth of the floor damage zone, and hollow mining can be completed. If the safety does not meet the requirements of hollow mining, a reasonable control method should be adopted based on the safety factor of the stability of the coal pillars left in the room-and-pillar goaf to conduct hollow mining. The specific methods for safety assessment and calculation of the maximum inclined length of the longwall working face of the hollow coal seam are as follows:

[0079] (1) Method for determining the safety of air mining:

[0080] The horizontal stress distribution along the working face after mining of the goaf coal seam is shown in the figure. Ignoring the influence of the support base pressure on the floor, the overburden load on the floor of section CD is 0. Considering the time effect of rock compaction in the goaf, it is assumed that the overburden load on the goaf of section DE increases linearly with distance from the coal wall. Section AC is the support pressure influence area, and the stress increase coefficient is k. By simplifying the triangular and trapezoidal loads, the stress analytical formula of any point (x0, y0) on the floor under uniform load is obtained:

[0081]

[0082] Where: q is the uniformly distributed load on any point of the base plate, MPa;

[0083] x1, x2——starting points;

[0084] Then the maximum and minimum principal stresses at (x0, y0) are:

[0085]

[0086] To prevent the chain instability of room-and-pillar goafs caused by push-out mining, leading to dynamic rock pressure and overburden collapse, it is necessary to determine whether the key pillars in room-and-pillar goafs are unstable based on the "key pillar theory." According to the "key pillar" theory, the underlying room-and-pillar goaf bears the greatest vertical stress during push-out mining in the upper coal seam. The coal pillar with the largest stress concentration coefficient usually appears directly below the center of the longwall working face along the inclination direction and on the coal pillar closest to the front along the advancing direction.

[0087]

[0088] Where: F s ——Safety factor of coal pillar stability;

[0089] σ p ——coal pillar strength, MPa;

[0090] σ1——maximum principal stress of key column, MPa;

[0091] When the safety factor of coal pillar stability F SWhen the safety factor F of coal pillar stability is greater than 1.5, it is considered that the coal pillar can maintain long-term stability and its safety meets the requirements for free-fall mining, so free-fall mining can be carried out. S When the safety factor FS of coal pillar stability is less than 1.5, it is considered that the coal pillar cannot remain stable for a long time and its safety does not meet the requirements for goaf mining. It is necessary to mine the next coal seam first and wait until the mine pressure destroys and compacts the coal pillars left in the room-and-pillar goaf before mining the goaf coal seam. When the safety factor FS of coal pillar stability is ≈ 1.5, it is considered that the coal pillar can remain stable for a certain period of time and it is necessary to fill the room-and-pillar goaf before goaf mining.

[0092] (2) Calculation method for the maximum inclined length of the longwall working face in the open coal seam:

[0093] During the longwall mining process, due to the mining pressure, the floor rock layer loses its support due to the removal of the coal seam and is subjected to the pressure of the overlying rock layer, causing deformation, fracture, and even shattering, forming a water-conducting damage zone. According to the fracture mechanics formula, the theoretical calculation of the depth of the water-conducting damage zone is:

[0094]

[0095] Where: h1 is the depth of the water-conducting damage zone in the bottom plate of the longwall working face, m;

[0096] γ——average bulk density of rock strata between the goaf and room-and-pillar goaf, t / m3;

[0097] H——mining depth, m;

[0098] R c ——Rock mass compressive strength, generally taken as 0.15 times the uniaxial compressive strength of the rock, MPa;

[0099] In order to ensure the normal operation of goaf mining, the depth of the water-conducting damage zone of the floor of the goaf longwall working face and the height of the natural equilibrium arch between the coal pillars left in the room-and-pillar goaf must meet the following requirements:

[0100] k(h1+b)≤h

[0101] Where: k is the interlayer height coefficient of the free-fall mining, which is 1.3 to 1.5;

[0102] h1——depth of water-conducting damage zone in the bottom plate of the longwall working face, m;

[0103] b——height of natural equilibrium arch of coal pillar left in room-and-pillar goaf, m;

[0104] h——coal seam spacing, m;

[0105] The maximum inclined length L of the longwall working face in the open coal seam is determined as follows:

[0106]

[0107] Where: L is the inclined length of the longwall working face in the open coal seam, m;

[0108] R c 2 ——Rock mass compressive strength, generally taken as 0.15 times the uniaxial compressive strength of the rock, MPa;

[0109] k——interlayer height coefficient of free-fall mining, taken as 1.3~1.5;

[0110] γ——Average bulk density of rock strata between the goaf and room-and-pillar goaf, t / m 3 ;

[0111] h——coal seam spacing, m;

[0112] H——mining depth, m;

[0113] In order to prevent instability caused by the natural balance arch conduction between the water-conducting damage zone of the floor of the goaf coal seam and the remaining coal pillars in the underlying room-and-pillar goaf, the inclined length of the goaf longwall working face should not exceed the maximum inclined length within its allowable range during design.

[0114] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for determining longwall mining of overlying coal seams in shallow room-and-pillar goafs, characterized in that: The following steps are involved: The first step is to drill and core the room-and-pillar goaf in combination with the mine borehole histogram to determine the occurrence of the overlying coal seams, the composition of the rock strata between the room-and-pillar goaf and the overlying coal seams, and the damage to the coal pillars and roof remaining in the room-and-pillar goaf. The second step is to conduct a feasibility assessment on the mining of the overlying coal seam in the shallow room-and-pillar goaf. The third step is to conduct stress and strength analysis on the coal pillars left in the room-and-pillar goaf; The fourth step is to determine the instability conditions of the coal pillars left in the room-and-pillar goaf; The fifth step is to qualitatively analyze the stability of the room-and-pillar goaf based on the remaining coal pillars and roof damage in the proven room-and-pillar goaf, and classify them. There are four states of the room-and-pillar goaf: the remaining coal pillars and roof are both stable; the remaining coal pillars and roof are both damaged, and the ground surface slowly sinks; the remaining coal pillars are stable, and the roof partially collapses; the remaining coal pillars are damaged, and the roof is stable; The sixth step is to determine the feasibility of mining the overlying coal seam in the room-and-pillar goaf. If mining is not theoretically feasible, the next coal seam will be mined first, and the mining of the overlying coal seam will be carried out after the mine pressure destroys and compacts the coal pillars in the room-and-pillar goaf. If mining is theoretically feasible, the safety of mining will be determined. The seventh step is to conduct a safety assessment of the overlying coal seam in the room-and-pillar goaf. If the safety does not meet the requirements, reasonable control methods should be adopted to ensure that the safety meets the requirements before mining can proceed. If the safety meets the requirements, the maximum inclined length of the goaf longwall working face can be determined by the depth of the floor failure zone.

Images