Column mining coal goaf suspended roof risk evaluation method

Abstract

The invention discloses a column type mining coal goaf hanging top danger evaluation method, and belongs to the field of mine engineering geology and the field of mining engineering. The method comprises the following steps: measuring the initial concentration of radon gas before coal mining in the same mining area, determining a key layer of a goaf, sampling the key layer, measuring the tensile strength, calculating the crack density of an evaluation object through radon gas concentration measurement data, evaluating the tensile strength by combining the current crack density of the key layer, and further calculating the ultimate tensile strength of the key layer. And evaluating the safety of the goaf suspended roof. The method does not depend on a large amount of drilling engineering, can be quickly and economically implemented on the ground or underground, realizes quantitative evaluation of the stability of the key layer, and provides a scientific basis for prevention and control of the coal goaf roof hanging disaster.

Description

Technical Field

[0001] This invention belongs to the fields of mine engineering geology and mining engineering, and more specifically relates to a method for evaluating the risk of roof collapse in coal goaf areas of pillar mining. Background Technology

[0002] In the early stages of mining shallow coal resources, due to limitations in mining methods and equipment, coal recovery rates were low, and many coal mining areas adopted pillar mining. This method often leaves behind large amounts of unrecovered coal pillars, forming goafs. In some areas, the roof failed to collapse in time, resulting in "overhanging roofs" of varying sizes. With the continuous advancement of coal mining technology and equipment, the underlying coal seams have been gradually developed and utilized. During this process, the overhanging roof areas have become a major hidden danger affecting safe production. Especially for overhanging roof areas that have existed for a long time, their stability is extremely difficult to assess directly. If the overhanging roof becomes unstable, it can easily trigger severe mine pressure disasters, posing a great threat to mine safety. Currently, the detection and assessment of the overhanging roof hazards in goafs mainly rely on traditional methods such as drilling, high-density electrical resistivity tomography, and three-dimensional laser scanning.

[0003] However, existing technologies have significant limitations. Drilling can only obtain information from a single borehole location, making it difficult to comprehensively reveal the overall fracture development and spatial distribution characteristics of the overhanging roof. Furthermore, the highly random nature of the fractures means that boreholes can easily miss critical hazards. While 3D laser scanning can acquire spatial morphological information of coal pillars within the goaf, it cannot effectively identify and evaluate the structure and stability of key layers, especially the core control layer for the generation and development of the overhanging roof—the key layer—leading to limited overall evaluation effectiveness. High-density electrical resistivity tomography (EDS) is typically used to delineate the goaf area and make a preliminary assessment of the overhanging roof's hazard, but it lacks the ability to quantify the overhanging roof's risk. Therefore, existing geophysical and drilling technologies are insufficient in terms of the comprehensiveness, quantification, and specificity of overhanging roof hazards, making it difficult to meet the needs of accurate evaluation.

[0004] In addition, in the past, radon gas measurements were mostly used as an auxiliary means to delineate the scope of goaf areas. There was a lack of quantitative analysis methods for the stability of key layers, and it was not widely applied to the specific assessment of roof hazard, resulting in limited overall effectiveness.

[0005] To address the aforementioned issues, there is an urgent need to develop a new, economical, efficient, and quantitative assessment technology for assessing the risk of roof collapse in coal goaf areas, in order to overcome the shortcomings of traditional methods in terms of accuracy, universality, and quantification. Summary of the Invention

[0006] This invention addresses the problems in current coal goaf hazard assessment processes, such as the difficulty of drilling and traditional geophysical exploration methods in comprehensively revealing the overall situation of the roof, the difficulty in quantitatively assessing the stability of key strata, and the low efficiency and high cost of related methods. It proposes a rapid, economical, and ground-based method for assessing the hazard of roof overhangs in coal goafs, which enables quantitative analysis of the stability of key strata, thereby improving the scientificity and accuracy of roof overhang disaster prevention and control.

[0007] To achieve the above objectives, the present invention employs the following technical solution: the method includes: The initial radon concentration was measured before coal mining in the same mining area; Identify the key strata in the goaf area to be assessed; Sampling was conducted on the identified key strata, and the tensile strength of the key strata was measured before coal mining. Radon concentration was measured in the goaf area of ​​the coal mining face to be evaluated. The crack density of the object being evaluated is calculated, and the development density of the crack field determines the concentration of radon gas measured. Assess the current tensile strength of the critical layer; crack density determines the tensile strength of the critical layer. Calculate the ultimate tensile strength of the critical layer; The safety of the suspended roof in the goaf is evaluated, and whether the key layer of the column structure is damaged determines the danger of the suspended roof. Different measures should be taken to ensure safety for different types of safety assessments.

[0008] In one approach, when determining the initial radon concentration before coal mining in the same mining area, a radon meter is used to measure the concentration at multiple representative points within the area to determine a reasonable background radon concentration C0, thereby enhancing the representativeness and accuracy of the measurement.

[0009] In one approach, when determining the key strata of the goaf to be evaluated, the key strata theory is adopted. By comprehensively judging the geometric, mechanical, deformation and fracture characteristics of each rock stratum, the main key strata and the specific location of the key strata are determined.

[0010] In one approach, samples are taken from the identified critical strata. When determining the tensile strength of the critical strata before coal mining, a well-known rock mechanics test is used to measure the tensile strength σ0 of the samples, which serves as the basis for subsequent safety calculations.

[0011] In one approach, when measuring the radon concentration in the goaf area of ​​the coal face to be evaluated, a radon meter is used. Three or more points are selected within a range of 0 to 50 meters from the midpoint between adjacent coal pillars to obtain the radon concentration C in the area.

[0012] In one approach, when calculating the crack density of the evaluation object, the relationship between crack density and radon detection concentration is established through a diffusion-convection coupling model, and the crack density of the corresponding area is calculated using the measured radon concentration data.

[0013] In one approach, when assessing the current tensile strength of the critical stratum, an engineering simplification model is used. This model combines the tensile strength σ0 measured before coal mining with the current fracture density to infer the current tensile strength of the critical stratum.

[0014] In one approach, the critical layer theory is used to calculate the ultimate tensile strength of the critical layer. The calculation is based on the relationship between the ultimate collapse distance L determined by the critical layer and the tensile strength, combined with the current mechanical parameters of the critical layer.

[0015] In one approach, after evaluating the safety of the suspended roof in the goaf, the goaf is classified into three types based on the evaluation results: relatively safe, hazardous, and major hazardous. Continuous monitoring, safe roof caving, and backfilling are implemented for each type, and the criterion for termination of remediation is the reduction of radon concentration to a level suitable for the safe type.

[0016] Beneficial effects of this invention: Compared with the prior art, the beneficial effects of this invention are as follows: 1) This project does not require a large amount of drilling work, radon measurement is carried out on the ground, each working face can be measured quickly, and it is simple and easy to implement; 2) The radon measurement method has high application efficiency in coal mine goaf areas, so it is economical and reasonable; 3) By constructing the relationship between radon concentration and crack density, the limit of key layer failure can be quantitatively evaluated, realizing the quantitative evaluation of overhang, and the effect is better; 4) It avoids the uncertainty of drilling and has a wider range of applications. Attached Figure Description

[0017] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0018] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Typical embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

[0019] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. To facilitate understanding, the invention will now be described more fully with reference to the accompanying drawings. Typical embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to make the disclosure of the invention more thorough and complete.

[0020] The general idea of ​​this invention is as follows: First, coal seam mining creates fracture fields, which are the main channels for radon gas leakage, thus providing a basis for radon measurement. Second, the development density of the fracture field determines the concentration of radon gas measured; a diffusion-convection coupling model was used to establish the relationship between fracture density and radon gas detection concentration. Third, fracture density determines the tensile strength of the critical stratum; a simplified engineering model was used to establish the relationship between fracture density and the tensile strength of the critical stratum. Fourth, whether the columnar critical stratum fails determines the risk of roof collapse; the risk of roof collapse can be assessed by using the relationship between the ultimate collapse distance L and tensile strength in critical stratum theory.

[0021] like Figure 1 As shown, a method for assessing the risk of roof overhang in coal goaf mining using pillar-type methods includes the following steps: Step 1: Measure the initial radon concentration in the same mining area before coal mining. C 0. Radon gas originates from deep within the Earth and escapes to the surface through various fissures; therefore, step one detected the background values ​​in this area.

[0022] Step Two: Identify the key strata in the goaf to be evaluated. The key strata are determined using well-known key strata theory. The so-called main key strata refer to the strata that control all rock strata. The determination of key strata is mainly based on the geometric, mechanical, deformation, and fracture characteristics of each rock stratum. Key strata are characterized by relatively thick layers, high compressive (tensile) strength, deformation that is coordinated with the surrounding rock strata, and fracture leading to large-scale fracture and movement of the overlying rock strata. After fracture, if the SR structural stability criterion for rock block structure is met, the main characteristics are the evolution from a beam structure to a masonry beam structure.

[0023] Step 3: Take samples from the identified key strata and determine the tensile strength of the key strata before coal mining. s 0. Well-known rock mechanics experiments were used for testing; the radon diffusion coefficient D was determined in the key strata after coal mining using well-known transient methods. : Established methods were used to determine radon diffusion and mechanical parameters, providing key parameters for subsequent calculations.

[0024] Step Four: Radon concentration is measured in the goaf area of ​​the coal face to be evaluated. The result is denoted as C. The measurement method involves selecting three or more points within a 0-50 meter range between the midpoints of adjacent coal pillars, measuring the concentration using a radon meter, and taking the average value. Coal seam mining creates fracture fields, which are the main channels for radon gas leakage; therefore, radon measurement provides a basis for this process. Thus, the results after coal mining are primarily controlled by the coal mining fracture fields compared to Step One.

[0025] Step 5: Calculate the crack density ρ of the evaluation object. The development density of the crack field determines the concentration of radon gas measured. A diffusion-convection coupling model was used to establish the relationship between crack density and radon gas detection concentration.

[0026] Calculate using the following formula

[0027] In the formula, ρ is the crack density, m / m 2 C represents the current radon concentration at the coal face, obtained through step four, in Bq / m³; C0 represents the initial radon concentration before coal mining, obtained through step one, in Bq / m³; β represents the fracture connectivity correction coefficient, ranging from 0 to 1; D represents the radon diffusion coefficient, obtained through step three, in m² / s; τ represents the radon half-life, taken as 3.3 × 10⁻⁶. 5 s.

[0028] Step Six: Assess the current tensile strength of the critical layer s t The following formula is used to calculate the tensile strength of the critical layer, and a simplified engineering model is adopted to establish the relationship between crack density and tensile strength of the critical layer.

[0029]

[0030] In the formula, s 0 represents the initial tensile strength of the critical layer, obtained through step three, in MPa; α is an empirical coefficient, ranging from 0.1 to 0.5, in m; ρ is the crack density, obtained through step five, in m / m. 2 .

[0031] Step 7: Calculate the ultimate tensile strength of the critical layer s m The ultimate tensile strength of the critical layer was calculated using the masonry beam model in structural mechanics to determine the ultimate collapse strength, and the following formula was used for the calculation.

[0032]

[0033] In the formula, s m1 is the ultimate tensile strength of the key layer, MPa; L is the spacing between adjacent coal pillars, obtained through mining design, m; h is the thickness of the key layer, obtained through borehole columnar section, m; q is the overburden load of the key layer, obtained through borehole columnar section and density calculation of each rock layer, MPa.

[0034] Step 8: Evaluate the safety of the suspended roof in the goaf. Whether the critical stratum of the column structure is damaged determines the danger of the suspended roof. By using the relationship between the ultimate tensile strength and the existing tensile strength in the critical stratum theory, the danger of roof failure can be assessed. The evaluation results are divided into three types: relatively safe type (a ≤ 0.8), hazardous hazard type (a = 0.8~1), and major hazard type (a ≥ 1). 'a' is the safety factor, calculated according to the following formula:

[0035] Step Nine: Take corresponding measures to ensure safety based on different safety assessment types. For relatively safe types, continuous monitoring does not require safety measures; for hazardous hazard types, manual safety caving of the key roof layers is required to prevent strong mine pressure accidents; for major hazard types, the goaf needs to be filled, and radon levels should be measured after filling until the radon concentration C drops to a safe level.

[0036] Example: In the early stages of mining, a coal mine in western my country adopted room-and-pillar mining due to limited mining technology. During the investigation of hidden disaster-causing factors, a large area of ​​overhanging roof was discovered, requiring an assessment of the hazard posed by the overhanging roof. Traditional assessment methods rely on whether the overhanging roof area delineated by geophysical exploration exceeds 1 km². 2 The standard assessment cannot provide quantitative guidance for addressing subsequent hidden disaster-causing factors. Therefore, this technology is adopted and implemented as follows: Step 1: Measure the initial radon concentration in the same mining area before coal mining. C 0 = 112 Bq / m³.

[0037] Step 2: Determine the key stratum of the goaf to be evaluated. The key stratum is determined by calculation using the well-known key stratum theory, which is the medium sandstone located 38 meters above the coal seam in pillar mining.

[0038] Step 3: Take samples from the identified key strata and determine the tensile strength of the key strata before coal mining. s The pressure was 0 = 11.6 MPa, tested using well-known rock mechanics experiments; the radon diffusion coefficient D of the key strata after coal mining was determined to be 5.5 × 10⁻⁶. - 7 m² / s, measured using well-known transient methods.

[0039] Step 4: Measure the radon concentration in the goaf area of ​​the coal face to be evaluated. The result is C=135Bq / m³. The measurement method is to select 3 points within a range of 0~50 meters from the midpoint between adjacent coal pillars, and use a radon meter to measure and take the average values ​​of 133Bq / m³, 139Bq / m³ and 133Bq / m³.

[0040] Step 5: Calculate the crack density ρ of the object being evaluated. Use the following formula to calculate...

[0041] In the formula, ρ is the crack density, m / m 2 C = 135 Bq / m³ is the current radon concentration at the coal face, obtained through step four, in Bq / m³; C0 = 112 Bq / m³ is the initial radon concentration before coal mining, obtained through step one, in Bq / m³; β is the fracture connectivity correction coefficient, taken as 0.5; D = 5.5 × 10⁻⁶. -7 m² / s is the radon diffusion coefficient, obtained through step three; τ is the radon half-life, taken as 3.3 × 10⁻⁶. 5 s.

[0042] Step Six: Assess the current tensile strength of the critical layer s t The following formula is used for calculation.

[0043]

[0044] In the formula, s 0 = 11.6 MPa is the initial tensile strength of the key layer, obtained through step three; α is an empirical coefficient, taken as 0.2 m; ρ = 0.96 m / m 2 The crack density, obtained through step five, is m / m. 2 .

[0045] Step 7: Calculate the ultimate tensile strength of the critical layer s m The following formula is used for calculation.

[0046]

[0047] In the formula, s m q = 10 m is the ultimate tensile strength of the key layer, MPa; L = 10 m is the spacing between adjacent coal pillars, obtained through mining design, m; h = 5 m is the thickness of the key layer, obtained through borehole columnar section, m; q = 7.1 MPa is the overburden load of the key layer, obtained through borehole columnar section and density calculation of each rock layer, MPa.

[0048] Step 8: Evaluate the safety of the suspended roof in the goaf. The evaluation results are divided into three types. α=0.66 is the safety factor, indicating a safe type. The calculation is performed according to the following formula:

[0049] Step Nine: Take appropriate measures to ensure safety based on different safety assessment types. For relatively safe assessments, continuous monitoring does not require safety measures.

[0050] Although this mine uses room-and-pillar mining for the 2-2 coal seam, the underlying coal seam can be mined normally because the key strata have not yet weathered and deteriorated to the point of instability. Practical verification has shown that no strong mine pressure has occurred in the coal mine, and the evaluation results are consistent with the actual situation.

[0051] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. The storage medium can be a magnetic disk, optical disk, read-only memory (ROM), or random access memory (RAM), etc.

[0052] It should be understood that the above detailed description of the technical solutions of the present invention with reference to preferred embodiments is illustrative and not restrictive. Those skilled in the art can modify the technical solutions described in the embodiments or make equivalent substitutions for some of the technical features based on reading this specification; however, these modifications or substitutions do not cause the essence of the corresponding technical solutions to depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for assessing the risk of roof collapse in coal goaf areas during pillar mining, characterized in that: The method includes: The initial radon concentration was measured before coal mining in the same mining area; Identify the key strata in the goaf area to be assessed; Sampling was conducted on the identified key strata, and the tensile strength of the key strata was measured before coal mining. Radon concentration was measured in the goaf area of ​​the coal mining face to be evaluated. The crack density of the object being evaluated is calculated, and the development density of the crack field determines the concentration of radon gas measured. Assess the current tensile strength of the critical layer; crack density determines the tensile strength of the critical layer. Calculate the ultimate tensile strength of the critical layer; The safety of the suspended roof in the goaf is evaluated, and whether the key layer of the column structure is damaged determines the danger of the suspended roof. Different measures should be taken to ensure safety for different types of safety assessments.

2. The method for assessing the risk of roof collapse in coal goaf mining using pillar-type mining as described in claim 1, characterized in that: When determining the initial radon concentration before coal mining in the same mining area, a radon meter is used to measure multiple representative points in the area to determine a reasonable background radon concentration C0, thereby enhancing the representativeness and accuracy of the measurement.

3. The method for assessing the risk of roof collapse in coal goaf mining using pillar-type mining as described in claim 1, characterized in that: When determining the key strata of the goaf to be evaluated, the key strata theory is adopted. By comprehensively judging the geometric, mechanical, deformation and fracture characteristics of each rock stratum, the main key strata and the specific location of the key strata are determined.

4. The method for assessing the risk of roof collapse in coal goaf mining according to claim 1, characterized in that: Sampling was conducted on the identified critical strata. When determining the tensile strength of the critical strata before coal mining, well-known rock mechanics experiments were used to measure the tensile strength σ0 of the samples, which was then used as the basis for subsequent safety calculations.

5. The method for assessing the risk of roof collapse in coal goaf mining according to claim 1, characterized in that: When measuring the radon concentration in the goaf area of ​​the coal mining face to be evaluated, a radon meter is used. Three or more points are selected within a range of 0 to 50 meters from the midpoint between adjacent coal pillars to obtain the radon concentration C in the area.

6. The method for assessing the risk of roof collapse in coal goaf mining according to claim 1, characterized in that: When calculating the crack density of the evaluation object, the relationship between crack density and radon gas detection concentration is established through a diffusion-convection coupling model, and the crack density of the corresponding area is calculated using the measured radon gas concentration data.

7. The method for assessing the risk of roof collapse in coal goaf mining according to claim 1, characterized in that: When assessing the current tensile strength of the critical stratum, an engineering simplification model is used. The tensile strength σ0 measured before coal mining and the current crack density are combined to infer the current tensile strength of the critical stratum through the model.

8. The method for assessing the risk of roof collapse in coal goaf mining using pillar-type mining as described in claim 1, characterized in that: When calculating the ultimate tensile strength of the critical layer, the critical layer theory is adopted, and the calculation is carried out based on the relationship between the ultimate collapse distance L determined by the critical layer and the tensile strength, combined with the current mechanical parameters of the critical layer.

9. The method for assessing the risk of roof collapse in coal goaf mining according to claim 1, characterized in that: After evaluating the safety of the suspended roof in the goaf, the goaf is divided into relatively safe, dangerous and hidden danger, and major danger types according to the evaluation results. Continuous monitoring, safe roof caving and filling treatment measures are taken respectively, and the criterion for termination of treatment is the decrease of radon concentration to the safe type.

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