Arrangement and construction method for re-mining working face of coal resources left in strip mining of thick coal seam

Abstract

The invention discloses an arrangement and construction method of a re-mining working face for coal resources left in strip mining of a thick coal seam, and belongs to the technical field of re-mining of coal. The problem of cooperative prevention and control of rock burst induction, gas accumulation and spontaneous combustion multi-source disasters in the island coal pillar re-mining process is solved. The method comprises the steps that firstly, mine geological data are investigated, and the remining range of remaining coal is determined; the original roadway is repaired, the exploration roadway is tunneled to the position in front of the mining stopping line, drilling, sampling and gas extraction are conducted, and the goaf state is explored; establishing a three-dimensional numerical model, combining the three-dimensional numerical model with a similar material simulation method and the like, analyzing the movement state and the compaction condition of overlying strata in a goaf, and determining a roadway arrangement mode according to a result; digging a mining roadway; punching is conducted on the top of the roadway while the roadway is excavated, and a foaming material is injected; and finally, drilling and nitrogen injection are performed on the working face while tunneling is performed to extract gas. The resource recovery rate in the re-mining process can be increased, and the disaster risks such as gas accumulation and spontaneous combustion are reduced.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, specifically to a method for the layout and construction of a mining face for coal resources left over from thick coal seam strip mining. Background Technology

[0002] In the field of coal resource development, the isolated coal pillars and goaf remnants left by thick coal seam strip mining have enormous resources. However, due to complex geological conditions and mining technology bottlenecks, they have long faced the dual challenges of "high disaster risk and low resource recovery rate". As the mining depth continues to increase, the abandoned coal pillars form high stress concentration areas under the superposition of stress from multiple rounds of mining, which can easily induce problems such as rock bursts during the remining process. The complex fracture network formed by the abandoned coal and broken rock strata in the goaf not only becomes the main space for gas accumulation, but also provides oxygen channels for coal oxidation and spontaneous combustion. Conventional grouting and sealing technology cannot achieve coordinated control of gas seepage and spontaneous combustion hazards due to the limited diffusion range. In addition, traditional remining technology relies on experience-based design and lacks accurate assessment of the overburden movement law and coal pillar stability in the goaf, resulting in a resource recovery rate of generally less than 60%, and safety hazards such as roadway excavation and goaf ventilation and water accumulation threats.

[0003] Current technologies for remining isolated coal pillars still face challenges such as rockburst induction, gas accumulation, and spontaneous combustion. Therefore, existing technologies require further improvement. Thus, a three-pronged technical system integrating "grouting isolation in front of the excavation roadway, backfilling for roadway protection, and dynamic grouting reinforcement" is needed to solve the engineering challenges of safe and efficient remining of residual coal resources in deep, high-stress environments. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the layout and construction of a mining face for the re-mining of coal resources left over from strip mining of thick coal seams. This method achieves balanced control of coal pillar stress and coordinated prevention and control of multi-source disasters, which can improve the resource recovery rate in the re-mining process and reduce the risk of disasters such as gas accumulation and spontaneous combustion.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for the layout and construction of a longwall face for the remining of coal resources left over from strip mining of thick coal seams, comprising the following steps:

[0007] a. Investigate the geological data of the mine to clarify the scope of remining of abandoned coal;

[0008] b. Repair the original roadway and excavate the exploration roadway to the front of the stop line, drill, sample, extract gas and explore the condition of the goaf;

[0009] c. Establish a three-dimensional numerical model and combine it with similar material simulation methods and rock mechanical properties to analyze the overburden movement state and compaction of the goaf. Based on the results, determine the roadway layout method. The roadway layout method includes two schemes, Scheme A and Scheme B. Scheme A is "grouting and filling of the existing roadway + repair and excavation of the existing roadway", and Scheme B is "grouting and filling of the goaf center + excavation of the goaf".

[0010] The selection between Scheme A and Scheme B is based on the following steps:

[0011] c1. Obtain rock cores from the top and sides of the existing tunnel by drilling.

[0012] c2. The mechanical parameters of the obtained rock core were measured, namely: uniaxial compressive strength, elastic modulus and Poisson's ratio and porosity.

[0013] c3. Based on the number of boreholes and the mechanical parameter measurement results of the rock core, a three-dimensional numerical model is established to analyze the state of the goaf. The three-dimensional numerical model includes a geometric module, a constitutive module, and a boundary condition and loading module. The geometric module includes a coal seam module and a layered roof and floor strata module. The constitutive module includes an elastic zone module and a plastic zone module. The boundary condition and loading module is used to record the changes in stress field and displacement field to simulate the mining process.

[0014] c4. Conduct similar simulation experiments on schemes A and B, and obtain the degree of agreement between the fracture development height and the subsidence curve through similar simulation experiments;

[0015] c5. Assess cavity risk by core recovery rate;

[0016] c6. Determine the collapse situation of the goaf and further determine the layout plan of the re-mining face. Determine the collapse situation of the goaf according to formula (1).

[0017] Overall score = (Indicator Score) ×weight (1);

[0018] The indicators in Equation (1) are porosity, plastic zone ratio, damage variable, crack development height, sinking curve fit, and safety risk, respectively.

[0019] If the overall score is ≥8.0: the collapse is sufficient and stable, so adopt scheme B;

[0020] Overall score < 8.0: No local collapse or moderate risk exists, adopt option A;

[0021] If the overall score is less than 5.0, it is not recommended to carry out re-collection work;

[0022] d. Excavate the mining roadway to form the mining face;

[0023] e. While excavating the tunnel, drill holes in the top of the tunnel and inject foam material;

[0024] f. When excavating the roadway, nitrogen is injected into the working face to extract gas while excavating. Steps d and e are repeated in sequence until a mining working face is formed.

[0025] The above-mentioned method for the layout and construction of a working face for the re-mining of coal resources left over from the strip mining of thick coal seams, in step a, the investigation of mine geological data mainly includes: investigating historical mining data, determining the mine hydrological type, analyzing the deformation of the original roadways, and formulating a gas drainage plan.

[0026] The above-mentioned method for the layout and construction of a working face for the remining of coal resources left over from the strip mining of thick coal seams mainly involves the following aspects in the historical mining data survey: analyzing the thickness and dip angle of the remaining coal seam, understanding the spatial distribution of the remaining coal and the ash, sulfur, and calorific value of the coal; determining the mine hydrological type mainly involves estimating the range and volume of water accumulation in the goaf, understanding the groundwater flow direction, permeability coefficient, and drainage costs during remining; and analyzing the deformation of the original roadways mainly involves analyzing the roof collapse and floor bulging in the goaf, the stability of the remaining coal pillars and the risk of rock bursts, and assessing the difficulty of controlling the surrounding rock during remining.

[0027] In the above-mentioned method for the layout and construction of a working face for the remining of coal resources left over from the strip mining of thick coal seams, step b involves first filling the surrounding area of ​​the working face with grouting and filling technology to repair the original roadway. The exploration roadway is then excavated obliquely along the original track roadway towards the middle of the stop line. After reaching 5-10m in front of the middle of the stop line, boreholes are drilled into the goaf area to collect rock cores in sections to analyze the degree of rock fragmentation, determine the height of the collapse zone and the compaction state, and monitor the gas concentration and spontaneous combustion of the goaf area through the boreholes.

[0028] In the above-mentioned method for the layout and construction of a working face for the remining of coal resources left over from the strip mining of thick coal seams, in step c1, core samples are extracted by drilling in sections in the exploration roadway towards the roadway, the goaf, and both sides of the roadway, until the old roof above the goaf that has not fractured is reached.

[0029] In the above-mentioned method for the layout and construction of a working face for the re-mining of coal resources left over from the strip mining of thick coal seams, in step c2, the uniaxial compressive strength, elastic modulus, Poisson's ratio, and porosity are calculated according to equations (1), (2), (3), and (4), respectively:

[0030] (1);

[0031] In equation (1), Uniaxial compressive strength, The failure load of the rock core. This represents the cross-sectional area of ​​the rock core.

[0032] (2);

[0033] In equation (2), For elastic modulus, For stress increment, For strain increment;

[0034] (3);

[0035] In equation (3), Poisson's ratio, For lateral strain, For axial strain;

[0036] (4);

[0037] In equation (4), The porosity of the sample. The pore volume in the rock sample. The total volume of the rock sample. The dry density of the rock sample. This represents the true density of the rock.

[0038] In the above-mentioned method for the layout and construction of a working face for the remining of coal resources left over from the strip mining of thick coal seams, in step c5, when the core recovery rate is ≥85%, it indicates that the core is continuous and intact, reflecting that the rock strata are dense and free of voids; when 70%≤core recovery rate<85%, it indicates that the core is locally broken, with small cracks or isolated small voids; when the core recovery rate<70%, it indicates that the core is severely missing, with large voids or strongly fractured zones.

[0039] The above-mentioned method for the layout and construction of a mining face for the re-mining of coal resources left over from the strip mining of thick coal seams includes the following weightings: porosity (20%), plastic zone (30%), damage variable (10%), fracture development height (10%), sinking curve fit (10%), and safety risk (20%).

[0040] The above-mentioned method for the layout and construction of a remining face for coal resources left over from strip mining of thick coal seams includes the following steps in step d: First, a grouting process is used to fill the surrounding area of ​​the remining face in all directions to form a grouting isolation zone; second, a grouting filling process is used to fill the area around the excavated roadway to protect the excavated roadway; finally, the excavated roadway is carried out entirely within the filling body, and when the working face advances, pre-grouting filling is carried out simultaneously on the next section of the goaf to form an advanced protection zone.

[0041] In the above-mentioned method for the layout and construction of a working face for the re-mining of coal resources left over from the strip mining of thick coal seams, in step f, the nitrogen injection flow rate is calculated according to formula (5):

[0042] (5);

[0043] In equation (5), For nitrogen injection flow rate, The volume of the goaf. The concentration of methane in the goaf. For the target gas concentration, For replacement efficiency, This refers to the nitrogen injection time.

[0044] Compared with the prior art, the present invention brings the following beneficial technical effects:

[0045] This invention utilizes techniques such as grouting ahead of excavated roadways in areas with abandoned coal pillars to form isolation zones, employing a simultaneous backfilling and excavation process to excavate roadways within the backfill while simultaneously providing support and sealing, and arranging grouting holes along the coal pillar edge or goaf for segmented grouting reinforcement of fractured coal and rock. These techniques, combined with fly ash-cement composite backfill materials to seal gas seepage channels, form a three-in-one technical system: grouting ahead of excavated roadways to isolate disaster sources, backfill protection during roadway excavation, and dynamic grouting reinforcement. The design and implementation of this invention achieves balanced stress control of coal pillars and coordinated prevention and control of multi-source disasters, improving resource recovery rates in remining processes and reducing the risks of gas, spontaneous combustion, and other disasters.

[0046] This invention first investigates the feasibility of remining based on mine geological data, then excavates exploratory roadways to 5-10m ahead of the stop line to investigate the goaf's condition. Next, it uses numerical simulation and other methods to analyze the overlying rock movement and compaction in the goaf to determine the roadway layout. A method of grouting ahead of the excavated roadways and simultaneous backfilling and excavation is adopted to isolate the gas in the goaf and excavate the recovery roadways. During roadway excavation, foaming material is injected into the top through drilling to prevent gas leakage. Finally, before recovery, nitrogen is injected into the working face through drilling until the recovery working face is formed. Two roadway layout schemes, Scheme A and Scheme B, are proposed, determined through core sampling to determine mechanical parameters, numerical simulation, and similarity simulation experiments. The roadways adopt a rectangular shape and are supported by anchor mesh cables. In processes such as grouting ahead of excavated roadways, reasonable grouting parameters are adopted. The grout diffusion radius is calculated using a columnar diffusion model suitable for fractured rock strata. When injecting foamed material through perforations above the roadway, the grouting pressure must be controlled, and its diffusion radius is calculated using a modified spherical diffusion radius formula while considering the expansion effect. When injecting nitrogen to extract gas, the nitrogen injection flow rate and pressure must be determined based on the volume of the goaf. This method, through the synergy of multiple technologies, achieves the safe and efficient re-mining of legacy coal resources. Attached Figure Description

[0047] The present invention will be further described below with reference to the accompanying drawings:

[0048] Figure 1 This refers to the possible state of the goaf and coal pillars predicted based on geological data before any remining of the remaining coal is carried out.

[0049] Figure 2 A schematic diagram of grouting reinforcement near the excavation exploration tunnel and the stop mining line;

[0050] Figure 3 This is a schematic diagram of the layout for Scheme A;

[0051] Figure 4 This is a schematic diagram of the layout for Scheme B;

[0052] Figure 5 This is a top view of the working face roadway during construction;

[0053] Figure 6 This is a plan view of the working face roadway during construction;

[0054] Figure 7 This is a side view of the working face roadway during construction;

[0055] Figure 8 This is a flowchart of the method layout of the present invention.

[0056] In the diagram: 1. Remaining strip coal pillar; 2. Collapsed old roof; 3. Goaf; 4. Exploration roadway; 5. Original working face roadway; 6. Stopped mining line; 7. Grouting slurry; 8. Drilling exploration; 9. Newly excavated working face roadway; 10. Repaired original roadway; 11. Grouting hole; 12. Grouting pipeline; 13. Foaming material. Detailed Implementation

[0057] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0058] In the description of this application, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. The "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists alone, A and B exist simultaneously, and B exists alone.

[0059] The technical solution of this application will be further described in detail below with reference to the accompanying drawings.

[0060] The main technical concept of this invention is as follows: by simultaneously grouting and tunneling, the technical challenge of coordinated prevention and control of multiple disasters in the re-mining of isolated coal pillars is systematically solved; by using segmented casing grouting technology, the diffusion range of grout in broken coal and rock is precisely controlled, reducing the error between the grouting reinforcement effect and the actual engineering requirements; and by using a multi-scale experimental method combining numerical simulation and similar material simulation, the stress field and displacement field distribution of the model working condition are made consistent with the prototype re-mining face.

[0061] Combination Figures 1 to 7 As shown in the figure, the following are included: 1. Existing strip coal pillar; 2. Collapsed old roof; 3. Goaf; 4. Exploration roadway; 5. Original working face roadway; 6. Stopped mining line; 7. Grouting slurry; 8. Drilling exploration; 9. Newly excavated working face roadway; 10. Repaired original roadway; 11. Grouting hole; 12. Grouting pipeline; and 13. Foaming material.

[0062] The sources of the parameters mentioned in this invention are as follows:

[0063] The proportion of the plastic zone directly reflects the extent and stability of rock mass failure and is a core indicator of collapse status. In the safety assessment of re-mining, the degree of expansion of the plastic zone determines the difficulty of support and the risk of roof collapse, therefore it has the highest weight.

[0064] Porosity: Characterizes the degree of compaction of the collapsed mass, affecting permeability and the risk of secondary settlement. Measured porosity is the most direct evidence for determining whether the goaf has fully collapsed, and its weight is secondary.

[0065] Safety risks: These include immediate safety hazards such as cavities, micro-seismic events, and rock bursts.

[0066] The proportion of the plastic zone, porosity, and safety risks belong to the category of engineering practice and have priority.

[0067] Damage variables: obtained through numerical simulation and CT scans, the data has high accuracy, but it is an indirect indicator.

[0068] Fracture development height: Similar simulation results are affected by the scaling effect and need to be cross-validated with borehole data. The reliability is moderate.

[0069] Sinking curve fit: mainly used to verify the accuracy of the model, its contribution to the judgment of the actual collapse state is indirect.

[0070] Damage variables, fracture development height, and the fit of the subsidence curve are considered reliable parameters in experimental and simulation data, and thus account for a relatively small proportion.

[0071] The following is combined Figure 8 This invention provides a detailed description of the layout and construction method for remining coal resources left over from thick coal seam strip mining, specifically including the following steps:

[0072] The first step is to investigate the mine's geological data and analyze the possible states of the goaf. These include: the occurrence state of the remaining strip coal pillar 1, the collapse state of the collapsed old roof 2 above goaf 3, and the compaction state of the coal gangue in goaf 3.

[0073] Specifically: (1) Investigate historical mining data, analyze the thickness and dip angle of the remaining coal seam, understand the spatial distribution of the remaining coal and the ash, sulfur and calorific value of the coal, and determine the type of remaining coal based on historical mining technology. (2) Determine the mine hydrological type, the range and volume of water accumulation in the goaf, the direction of groundwater flow, the permeability coefficient, and the drainage cost during remining. (3) Analyze the deformation of the original roadway, the collapse and bulging of the roof in the goaf, the stability of the remaining coal pillar, the risk of rock bursts, and assess the difficulty of controlling the surrounding rock during remining. (4) Understand the mine gas and spontaneous combustion, analyze the gas content in the remining area and the goaf, and formulate a gas extraction plan. Analyze the sealing situation and the integrity of the sealing, and determine whether there is a risk of spontaneous combustion or gas accumulation. (5) Based on the above conditions and the level of mining technology and equipment, analyze the feasibility of remining the remaining coal and determine the economic value of remining.

[0074] The second step is to fill the surrounding area of ​​the working face with grouting through the filling and grouting process, repair the original roadway, and dig an exploration roadway along the original roadway to the front of the stop line. Then, continue to excavate into the goaf or into the original roadway to find out the collapse situation of the goaf and the occurrence conditions of the coal pillar.

[0075] Specifically: such as Figure 2 As shown, (1) an exploration roadway is excavated according to historical mining conditions. The exploration roadway is excavated obliquely along the original track roadway to the middle of the stop mining line. After reaching 5-10m in front of the middle of the stop mining line, boreholes are drilled into the goaf area to collect core samples in sections to analyze the degree of rock fragmentation, determine the height of the collapse zone and the compaction state, and monitor the gas concentration and spontaneous combustion of the goaf area through the boreholes. (2) The working face layout scheme is further selected according to the goaf area conditions.

[0076] The third step is to establish a three-dimensional numerical model and combine it with similar material simulation methods and rock mechanical properties to analyze the overlying rock movement and compaction in the goaf. Based on the results, the roadway layout will be determined. Figure 3 and Figure 4 As shown, there are two roadway layout methods: Scheme A and Scheme B. Scheme A is "grouting and filling of the existing roadway + repair and excavation of the existing roadway", and Scheme B is "grouting and filling of the goaf area + excavation of the goaf area".

[0077] Choose between Option A and Option B based on the following steps:

[0078] (1) After the grout injected into the goaf solidifies, an exploration tunnel is excavated into the goaf. In the exploration tunnel, core samples are drilled in sections towards the top of the tunnel, towards the goaf, and towards both sides of the tunnel (simulating the stress on the tunnel roof and sides) until the old roof above the goaf is intact. The obtained core samples are used for subsequent laboratory testing and geological condition assessment.

[0079] (2) The mechanical parameters of the obtained rock core were measured, namely: uniaxial compressive strength, elastic modulus and Poisson's ratio and porosity;

[0080] Uniaxial compressive strength, elastic modulus, Poisson's ratio, and porosity are calculated according to equations (1), (2), (3), and (4), respectively:

[0081] (1);

[0082] In equation (1), Uniaxial compressive strength, The failure load of the rock core. This represents the cross-sectional area of ​​the rock core.

[0083] (2);

[0084] In equation (2), For elastic modulus, For stress increment, For strain increment;

[0085] (3);

[0086] In equation (3), Poisson's ratio, For lateral strain, For axial strain;

[0087] (4);

[0088] In equation (4), The porosity of the sample. The pore volume in the rock sample. The total volume of the rock sample. The dry density of the rock sample. This represents the true density of the rock.

[0089] (3) Based on the number of boreholes and the results of core mechanical parameter measurements, a three-dimensional numerical model was established to analyze the state of the goaf.

[0090] Specifically, it includes:

[0091] Geometric module: Includes coal seams and layered (1-5m) roof and floor strata;

[0092] Constitutive modules include an elastic region module with parameters of elastic modulus and Poisson's ratio, and a plastic region module with parameters of cohesion and internal friction angle.

[0093] Boundary and Loading Module: Used to record stress and displacement field changes to simulate the mining process; fix the bottom of the model and apply horizontal stress laterally to simulate the coal seam excavation process step by step.

[0094] Record the changes in stress field and displacement field during the simulation process, and calculate them in detail, as shown in equations (5) and (6):

[0095] (5);

[0096] In equation (5), The proportion of the plastic zone, This represents the number of elements in the numerical model that undergo plastic yielding (shear or tensile failure). This represents the total number of units in the model.

[0097] Porosity mapping based on empirical relationship between rock stratum damage variables and porosity:

[0098] (6);

[0099] In equation (6), The porosity of the sample. The initial porosity, The damage-porosity coefficient, For damage variables.

[0100] (4) Similarity simulation experiments were conducted on the two tunnel layout methods respectively. Geometric, density, and strength similarity ratios were designed. Then, based on the mechanical parameters and the results obtained from numerical simulation, models were fabricated and monitored. Similar materials were laid according to the rock strata sequence, and each layer was compacted to the designed density. Displacement sensors were placed on the model surface to record the subsidence of the overburden, and strain gauges were embedded in key strata to capture the elastoplastic strain distribution. Finally, high-speed photography was used to record the collapse process and fracture propagation. The similarity ratio was set as follows:

[0101] Geometric similarity ratio: (7);

[0102] In the formula: For geometric similarity ratio, For model dimensions, This refers to the actual prototype dimensions.

[0103] Density similarity ratio: (Made from a sand-plaster mixture) (8);

[0104] In the formula, This represents the density similarity ratio.

[0105] Strength similarity ratio: (9);

[0106] In the formula, This represents the intensity similarity ratio.

[0107] Finally, the fracture development height was obtained through similar simulation experiments. Alignment with sinking curve .

[0108] (5) Based on the above experiments, determine the collapse situation of the goaf and further determine the working face layout plan. The collapse situation of the goaf is determined according to the following criteria.

[0109] The weights were determined based on the experimental results, with porosity accounting for 20%, the plastic zone for 20%, damage variables for 15%, crack development height for 10%, settlement matching curve for 10%, safety risk for 15%, and resource recovery rate for 10%. The specific methods for judging each indicator are as follows:

[0110] Porosity: ≤10%, 10 points; 10% < ≤25%, 6 points; >25%, 2 points.

[0111] Percentage of plastic zone: ≤30%, 10 points; 30% < ≤60%, 5 points; >60%, 3 points.

[0112] Damage variables: ≤0.3, 10 points; 0.3 < ≤0.6, 6 points; >0.6, 2 points.

[0113] Crack development height: ≤1.2 10 points; 1.2 < ≤1.5 5 points; >1.5 2 points.

[0114] Subsidence anastomosis curve: ≥0.9, 10 points; 0.7 < ≤0.9, 6 points; ≤0.7, 2 points.

[0115] Safety risks: No risk, 10 points; Local risk, 5 points; High risk, 0 points.

[0116] Resource recovery rate: High efficiency ≥75%: reflects good coal body integrity after backfilling and low mining loss, 10 points; Low efficiency <75%: indicates high mining cost or safety hazards, 6 points.

[0117] The final overall evaluation score is determined by the following formula:

[0118] Overall score = (Indicator Score) ×weight )

[0119] If the overall score is ≥8.0: the collapse is sufficient and stable, so adopt scheme B;

[0120] Overall score < 8.0: No local collapse or moderate risk exists, adopt option A;

[0121] If the overall score is less than 5.0, it is not recommended to carry out re-collection work;

[0122] If drilling reveals large-scale cavities (bore diameter ≥ 1m and continuously distributed), it is not recommended to carry out re-mining.

[0123] The fourth step is to excavate the mining roadway and use the filling and grouting process to form a grouting isolation zone around the mining face to prevent gas leakage and spontaneous combustion. At the same time, advance grouting is carried out into the goaf or the edge of the coal pillar to form a protective layer of filling body. Finally, the roadway is excavated in the filling body by filling and tunneling at the same time, including roadway and cut-out, to ensure that the tunneling process is isolated from the goaf and further form the mining face.

[0124] The specific steps for excavating roadways within the aforementioned filling body are as follows:

[0125] (1) The filling grouting process is used to fill the surrounding area of ​​the working face in all directions to form an isolation zone, avoid wind leakage, and prevent gas and spontaneous combustion disasters during mining.

[0126] (2) Grouting is used to fill the area around the excavated roadway to protect the excavated roadway.

[0127] (3) The excavated roadways are carried out entirely within the filling body to ensure that there is no cross-ventilation with the goaf. When the working face advances, the next section of the goaf is pre-grouted and filled simultaneously to form an advanced protection zone to ensure that there is no risk of water permeability and gas accumulation.

[0128] Combination Figure 5 , Figure 6 , Figure 7As shown, the grouting holes are arranged in rows of 6, with a depth of 10-20m and a grouting section interval of 4m. A segmented grouting process is adopted, with each hole constructed in 2-3 sections, each section being 2-3m long, and grouting proceeding from bottom to top. The final hole pressure is not less than 5.0MPa.

[0129] The diffusion radius of the slurry was determined using a columnar diffusion model suitable for fractured rock formations.

[0130] (10);

[0131] In equation (10), Where is the diffusion radius, For grouting pressure, For grouting time, The rock permeability coefficient, The dynamic viscosity of the slurry. The porosity is the density of the rock mass.

[0132] Fifth step: While excavating the tunnel, drill holes in the top of the tunnel and inject foam material to prevent gas leakage;

[0133] Foamed material is injected into the roof through holes drilled above the roadway. Grouting is then performed using a borehole grouting method, drilling to the roof, inserting grouting pipes, and grouting from the roof to the roadway. The borehole is perpendicular to the roof or inclined outwards at 5–10° to ensure it avoids anchor bolt support, and ensures the borehole penetrates the immediate roof to reach 1–2 meters into the existing roof. When the borehole enters the fractured rock strata of the collapsed immediate roof, the drilling torque decreases by 10%–30%; when the borehole enters the dense rock strata of the existing roof, the torque increases significantly by 10%–30%.

[0134] Grouting pressure control:

[0135] (11);

[0136] In equation (11), It represents the tensile strength of the rock strata.

[0137] The diffusion radius of the injected foaming material is determined using a modified spherical diffusion radius formula, taking into account the expansion effect of the foaming material.

[0138] (12);

[0139] In equation (12), Where is the diffusion radius, The coefficient of thermal expansion of the foamed material is typically taken as 8 to 15. For grouting time, The rock mass fracture rate is taken as 5% to 10%. This is because part of the immediate roof had collapsed during the remining process.

[0140] Step 6: While excavating the roadway, inject nitrogen into the working face to extract gas, and repeat steps S5 and S6 until a mining face is formed.

[0141] Nitrogen injection flow rate:

[0142] (13);

[0143] In equation (13), For nitrogen injection flow rate, The volume of the goaf. The concentration of methane in the goaf. For the target gas concentration, The replacement efficiency is determined based on the equipment. This refers to the nitrogen injection time.

[0144] Nitrogen injection pressure is determined based on geological conditions. Static pressure nitrogen injection is used in shallow goaf areas, with a pressure of 0.1–0.3 MPa. Pressurized nitrogen injection is used in deep mines or for long-distance transportation, with a pressure of 0.5–1.0 MPa.

[0145] The concentration of methane gas is monitored in real time by installing sensors, and the purity of nitrogen gas must be ≥97%.

[0146] In summary, the present invention provides a method for the layout and construction of a working face for the remining of residual coal resources in thick coal seam strip mining. This method effectively achieves the balanced control of coal pillar stress and the coordinated prevention and control of multi-source disasters, significantly improves the recovery rate of residual coal resources and the safety of remining operations, and promotes the sustainable development of mineral resources.

[0147] Any parts not mentioned in this invention can be achieved by referring to existing technologies.

[0148] Those skilled in the art should recognize that the above embodiments are only used to illustrate this application and are not intended to limit this application. Any appropriate changes and variations made to the above embodiments within the essential spirit and scope of this application fall within the scope of protection claimed in this application.

Claims

1. A method for the layout and construction of a longwall face for the remining of coal resources left over from strip mining of thick coal seams, characterized in that, The steps are as follows: a. Investigate the geological data of the mine to clarify the scope of remining of abandoned coal; b. Repair the original roadway and excavate the exploration roadway to the front of the stop line, drill, sample, extract gas and explore the condition of the goaf; c. Establish a three-dimensional numerical model and combine it with similar material simulation methods and rock mechanical properties to analyze the overburden movement state and compaction of the goaf. Based on the results, determine the roadway layout method. The roadway layout method includes two schemes, Scheme A and Scheme B. Scheme A is "grouting and filling of the existing roadway + repair and excavation of the existing roadway", and Scheme B is "grouting and filling of the goaf center + excavation of the goaf". The selection between Scheme A and Scheme B is based on the following steps: c1. Obtain rock cores from the top and sides of the existing tunnel by drilling. c2. The mechanical parameters of the obtained rock core were measured, namely: uniaxial compressive strength, elastic modulus and Poisson's ratio and porosity. c3. Based on the number of boreholes and the mechanical parameter measurement results of the rock core, a three-dimensional numerical model is established to analyze the state of the goaf. The three-dimensional numerical model includes a geometric module, a constitutive module, and a boundary condition and loading module. The geometric module includes a coal seam module and a layered roof and floor strata module. The constitutive module includes an elastic zone module and a plastic zone module. The boundary condition and loading module is used to record the changes in stress field and displacement field to simulate the mining process. c4. Conduct similar simulation experiments on schemes A and B, and obtain the degree of agreement between the fracture development height and the subsidence curve through similar simulation experiments; c5. Assess cavity risk by core recovery rate; c6. Determine the collapse situation of the goaf and further determine the layout plan of the re-mining face. Determine the collapse situation of the goaf according to formula (1). Overall score = (Indicator Score) ×weight (1); The indicators in Equation (1) are porosity, plastic zone ratio, damage variable, crack development height, sinking curve fit, and safety risk, respectively. If the overall score is ≥8.0: the collapse is sufficient and stable, so adopt scheme B; Overall score < 8.0: No local collapse or moderate risk exists, adopt option A; If the overall score is less than 5.0, it is not recommended to carry out re-collection work; d. Excavate the mining roadway to form the mining face; e. While excavating the tunnel, drill holes in the top of the tunnel and inject foam material; f. When excavating the roadway, nitrogen is injected into the working face to extract gas while excavating. Steps d and e are repeated in sequence until a mining working face is formed.

2. The method for layout and construction of a longwall face for remining coal resources left over from strip mining of thick coal seams according to claim 1, characterized in that: In step a, the main aspects of investigating mine geological data include: investigating historical mining data, determining the mine hydrological type, analyzing the deformation of existing roadways, and formulating a gas extraction plan.

3. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 2, characterized in that: The historical mining data surveyed mainly includes: analyzing the thickness and dip angle of the remaining coal seams, understanding the spatial distribution of residual coal and the ash, sulfur, and calorific value of the coal; determining the mine hydrological type mainly includes: estimating the range and volume of water accumulation in the goaf, understanding the groundwater flow direction, permeability coefficient, and drainage costs during remining; analyzing the deformation of the original roadways mainly includes: analyzing the roof collapse and floor bulging in the goaf, the stability of the remaining coal pillars and the risk of rock bursts, and assessing the difficulty of controlling the surrounding rock during remining.

4. The method for layout and construction of a longwall face for remining coal resources left over from strip mining of thick coal seams according to claim 1, characterized in that: In step b, the working face is first filled in all directions by grouting and filling process to repair the original roadway. The exploration roadway is then excavated obliquely along the original track to the middle of the stop line. After reaching 5-10m in front of the middle of the stop line, boreholes are drilled into the goaf to collect rock cores in sections to analyze the degree of rock fragmentation, determine the height of the collapse zone and the compaction state, and monitor the gas concentration and spontaneous combustion of the goaf through the boreholes.

5. The method for layout and construction of a longwall face for remining coal resources left over from strip mining of thick coal seams according to claim 1, characterized in that: In step c1, core samples are extracted by drilling in sections along the exploration tunnel, towards the top of the tunnel, towards the goaf, and towards both sides of the tunnel, until the old roof above the goaf is intact.

6. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 1, characterized in that: In step c2, the uniaxial compressive strength, elastic modulus, Poisson's ratio, and porosity are calculated according to equations (1), (2), (3), and (4), respectively: （1）； In equation (1), Uniaxial compressive strength, The failure load of the rock core. This represents the cross-sectional area of ​​the rock core. （2）； In equation (2), For elastic modulus, For stress increment, For strain increment; （3）； In equation (3), Poisson's ratio, For lateral strain, For axial strain; （4）； In equation (4), The porosity of the sample. The pore volume in the rock sample. The total volume of the rock sample. The dry density of the rock sample. This represents the true density of the rock.

7. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 1, characterized in that: In step c5, when the core recovery rate is ≥85%, it indicates that the core is continuous and intact, reflecting that the rock strata are dense and free of voids; when 70%≤core recovery rate<85%, it indicates that the core is locally broken, with small cracks or isolated small voids; when the core recovery rate<70%, it indicates that the core is severely missing, with large voids or strongly fractured zones.

8. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 1, characterized in that: Porosity accounts for 20% of the weight, the plastic zone accounts for 30%, damage variables account for 10%, crack development height accounts for 10%, the fit of the sinking curve accounts for 10%, and safety risk accounts for 20%.

9. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 1, characterized in that: Step d specifically includes the following steps: First, a grouting process is used to fill the surrounding area of ​​the re-mining face in all directions to form a grouting isolation zone; second, a grouting filling process is used to fill the area around the excavated roadway to protect the excavated roadway; finally, the excavated roadway is carried out in the filling body at all times, and when the working face advances, pre-grouting filling is carried out simultaneously on the next section of the goaf to form an advanced protection zone.

10. The method for layout and construction of a longwall face for remining residual coal resources from thick coal seam strip mining according to claim 1, characterized in that: In step f, the nitrogen injection flow rate is calculated according to equation (5): (5)； In equation (5), For nitrogen injection flow rate, The volume of the goaf. The concentration of methane in the goaf. For the target gas concentration, For replacement efficiency, This refers to the nitrogen injection time.

Images