A safety assessment and prevention method for coal pillar dam considering fracture connectivity

Abstract

The connectivity of fractures within the coal pillar dam has a significant impact on the dam's stability. During the operation of the coal mine underground water reservoir, when the dam is under different saturations, different water pressures, and different numbers of water storage and release cycles, the different connectivity of fractures will lead to different safety states of the dam. The coal pillar dam is monitored in real time through an artificial system, and the functional expression between the data obtained from physical and mechanical tests and sensor monitoring is optimized. At the same time, a coal pillar dam model is established using numerical simulation software, and the parameters obtained from the optimized expression are assigned to the model and simulated. For areas where cracks have appeared in the dam or where cracks are showing a development trend, a safety assessment is conducted based on two evaluation methods: the current pressure and the theoretical compressive strength of the area, or the current fracture connectivity rate and the fracture connectivity rate at failure. Appropriate prevention and control measures are taken for dangerous areas according to the risk level.

Description

Technical field:

[0001] The present invention relates to the technical field of safety detection methods for coal pillar dams of underground reservoirs in coal mines, and in particular to a safety assessment and prevention method for coal pillar dams that takes fracture connectivity into consideration. Background technology:

[0002] The safe management of underground reservoir coal pillar dams is a particularly critical aspect of coal mining. The coal pillar dam not only supports the overlying rock strata and prevents surface collapse, but also ensures safety within the mine. Current technology lacks consideration for the long-term operation of underground reservoirs in coal mines. During operation, the reservoir undergoes cycles of water storage and release, which inevitably impacts the safety of the coal pillar dam. Furthermore, the number of water storage and release cycles and the reservoir's water level cause the coal pillar dam to be in varying states and experience varying degrees of pressure. Under this pressure, the increased connectivity of the internal fractures in the dam can lead to further structural degradation and ultimately to its destruction.

[0003] The connectivity state of cracks inside the coal pillar dam has a significant impact on the stability of the dam. However, acoustic emission technology and microseismic monitoring technology cannot effectively monitor the connectivity state of cracks inside the dam. In addition, the existing technology lacks a specific explanation of the safety assessment steps for the coal pillar dam of coal mine underground reservoirs, and lacks standard assessments and reasonable response measures for the classification of risk levels and prevention and control measures for the coal pillar dam of coal mine underground reservoirs.

[0004] Through safety inspection and assessment of underground reservoir coal pillar dams, cracks, pores, or other defects in the coal pillar dams can be discovered as early as possible. Rapid and reasonable prevention and control measures for problem areas can effectively prevent the problem from worsening and avoid accidents. This can not only effectively extend the service life of the coal pillar dam, but also reduce unnecessary time and money consumption in the later stage. The safety assessment and prevention of coal pillar dams can not only effectively improve the overall safety of coal mine projects, but also lay a solid foundation for achieving sustainable development goals. Therefore, it is very necessary to study the safety assessment and prevention methods of coal pillar dams and propose a safety assessment and prevention method for coal pillar dams that takes into account the fracture connectivity rate. Summary of the invention:

[0005] The connectivity state of the internal fractures in the coal pillar dam of a coal mine underground reservoir is closely related to the safety status of the dam. During the operation of the coal mine underground reservoir, when the dam is under different saturation, different water pressure and different storage and discharge cycle conditions, the fracture connectivity state will be different, which will cause the dam to be in different safety states. The safety assessment of the dam is carried out by considering the fracture connectivity state of the coal pillar dam in different states. The following steps are included:

[0006] Step 1: Conduct physical and mechanical tests on standard specimens to obtain the compressive strength relationship of the specimens under different saturations, different water pressures, and different numbers of water storage and discharge cycles, as well as the crack connectivity relationship when the specimens are destroyed, and establish functional expressions for compressive strength and crack connectivity when destroyed.

[0007] Step 2: Conduct seepage tests on samples with different fracture connectivity rates, obtain the permeability of each sample, and establish a functional expression between fracture connectivity and permeability.

[0008] Step 3: Place stress sensors on the inside of the coal pillar dam of the underground reservoir and seepage monitors in the middle to obtain the pressure and permeability of the dam in its current state, and connect the stress sensors and seepage monitors to the artificial intelligence system via data cables.

[0009] Step 4: Optimize the function expression of compressive strength, the function expression of fracture connectivity rate at failure, and the function expression between fracture connectivity rate and permeability through the artificial intelligence system, and determine the compressive strength of the coal pillar dam body in the current state, the fracture connectivity rate at failure, and the fracture connectivity rate of the coal pillar dam body.

[0010] Step 5: Use numerical simulation software to build a model of the coal pillar dam, assign the parameters obtained from the optimized expression to the model and perform simulation. For areas where cracks have appeared in the dam or where cracks have a development trend, conduct a safety assessment based on the current pressure of the coal pillar dam and the range of crack connectivity in the area.

[0011] Step 6: For the areas after the safety assessment is completed, different prevention and control measures are proposed according to different risk levels. Description of the drawings:

[0012] Figure 1 It is a flow chart of the implementation of the method of the present invention;

[0013] Figure 2 is the stress sensor arrangement diagram;

[0014] Figure 3 It is the layout diagram of the seepage monitor;

[0015] Figure 4 It is the risk level classification and prevention and control measures diagram of the present invention;

[0016] Figure 5 This is a diagram of the artificial intelligence monitoring system.

[0017] In the figure: 1-coal pillar dam; 2-water storage area; 3-stress sensor; 4-seepage monitor; 5-overlying coal seam; 6-floor; 7-information collection equipment; 8-information processing equipment. Specific implementation method:

[0018] The present invention will be further described below with reference to the accompanying drawings.

[0019] Step 1: Obtain coal samples from the rock layer where the underground reservoir coal pillar dam is located, and make standard cylindrical specimens with a diameter of 50 mm and a height of 100 mm for physical and mechanical experiments. By simulating the state of the in-situ underground reservoir coal pillar dam, the most realistic mechanical parameters are detected and the functional expressions between the parameters are accurately established. The specific test conditions are as follows:

[0020] The saturation S is set to 0, 25%, 50%, 75%, 100%. The number of water storage and release cycles C is set to 0, 0.2C. max , 0.4C max , 0.6C max , 0.8C max , C max Group 6, C max C is the total number of times of water storage and release in the service life of the underground reservoir, and its calculation formula is max = Number of times water is stored and released per year × service life. Water pressure P is set to 0, 0.2P max , 0.4P max , 0.6P max , 0.8P max , P max Six groups, P max The water pressure is the pressure when the underground reservoir is full. The hydraulic pressure test was conducted only when the saturation was 100%. A total of 60 sets of mechanical loading tests were conducted under different conditions. The test path was to apply axial pressure and confining pressure to the existing stress state of the coal pillar dam body, then load the specimen at a loading rate of 0.1 MPa / s until failure. The compressive strength under different conditions was recorded, and a functional expression for the compressive strength was established:

[0021] (1) The compressive strength function expression of the specimen when the saturation S is 0, 25%, 50%, and 75%:

[0022] σ c =σ o -nS (1)

[0023] σ p1 =bC -t σ c =bC -t (σ o -nS) (2)

[0024] Among them, σ p1 is the compressive strength of the sample under different saturations and different numbers of water storage and discharge cycles, σ o is the compressive strength of the sample in dry state, σ c is the compressive strength of the sample at different saturation states, and n, b, and t are fitting parameters.

[0025] (2) The compressive strength function expression of the sample when the saturation S is 100%:

[0026] σ p2 =bC -t σ c -mP=bC -t (σ o -nS)-mP (3)

[0027] Among them, σ p2 is the compressive strength of the sample under different saturations, different numbers of water storage and discharge cycles and different water pressures, σ o is the compressive strength of the sample in dry state, σ c is the compressive strength of the sample at different saturation states, and n, b, t, and m are fitting parameters.

[0028] The crack characteristics of the specimen at failure were obtained by CT scanning, and the crack connectivity rate at failure was calculated. The functional expression of the crack connectivity rate at failure was established:

[0029] (1) When the saturation S is 0, 25%, 50%, and 75%, the functional expression of the crack connectivity rate when the sample is destroyed is:

[0030]

[0031] in, is the fracture connectivity rate of the specimen when it is destroyed under the combined action of different saturations and different numbers of water storage and discharge cycles, e represents the base of the natural logarithm function, γ is the total number of fractures in the specimen obtained by CT scanning, and θ and λ are fitting parameters.

[0032] (2) When the saturation S is 0, 25%, 50%, and 75%, the functional expression of the crack connectivity rate when the sample is destroyed is:

[0033]

[0034] in, is the crack connectivity rate of the sample when it is destroyed under the combined action of different saturations, different water pressures and different numbers of water storage and discharge cycles, γ is the total number of cracks in the sample obtained by CT scanning, e is the base of the natural logarithm function, θ, λ, δ are fitting parameters.

[0035] Step 2: Conduct seepage tests on samples with different fracture connectivity rates, obtain the permeability k of each sample, and establish the fracture connectivity rate The expression between and permeability k:

[0036]

[0037] Where k is the permeability, is the crack connectivity rate.

[0038] Step 3: If Figure 2 、 Figure 3 and Figure 5 As shown in the figure, the coal pillar dam of the underground reservoir in the coal mine is gridded. Near the inside of the underground reservoir, the dam is divided into a 15m×15m plane grid. Stress sensor monitoring points are placed at the center of the grid lines in the monitoring area. Following the same grid division, a seepage monitor is placed in the middle of the dam to obtain the current pressure σ1 and permeability k of the dam. The stress sensor and seepage monitor are connected to the artificial intelligence system via data cables. The artificial intelligence system monitors the coal pillar dam continuously and in real time throughout the day. The data and results of the physical and mechanical tests on the specimens are also entered into the artificial intelligence system, paving the way for subsequent optimization of the various function expressions using the artificial intelligence system.

[0039] Step 4: The artificial intelligence system is used to optimize the compressive strength function expression, the fracture connectivity function expression at failure, and the function expression between fracture connectivity and permeability, including deleting data that deviates from the fitting expression and filling the missing data with the average value to replace the missing value. At the same time, the accuracy of the artificial intelligence system in optimizing the function expression is visually verified by drawing. The saturation, water pressure, and number of water storage and release cycles of the current coal pillar dam body are substituted into formula (2) or formula (3) to obtain the compressive strength σ of the coal pillar dam body in the current state. p , substitute into formula (4) or formula (5) to obtain the crack connectivity rate at the time of failure Substitute the permeability k obtained by field monitoring into formula (6) to obtain the fracture connectivity rate of the coal pillar dam body:

[0040] Step 5: Use numerical simulation software to build a model of the dam body. The parameters used are accurate and in line with the actual situation, including geometric parameters, material properties, load conditions, boundary conditions, seepage parameters, etc. The parameters obtained after optimizing the expression are assigned to the model and simulated. For areas where cracks have appeared in the dam body or where cracks have a development trend, the current pressure σ1 of the coal pillar dam body in this area or the crack connectivity rate are used. The safety assessment is carried out on the scope of the project and the risk level is divided into four levels: Level 1, Level 2, Level 3 and Level 4. Figure 4 shown.

[0041] When σ1<0.4σ p or When , the dam risk level in the detection area is determined to be level 1.

[0042] when or 0.4 When , the dam risk level in the detection area is determined to be level 2.

[0043] When 0.6σ p <σ1<0.8σ p or 0.6 When , the dam risk level in the detection area is determined to be level 3.

[0044] When 0.8σ p <σ1 or 0.8 When , the dam risk level in the detection area is determined to be level 4.

[0045] Step 6: For the area after the safety assessment is completed, different prevention and control measures are proposed according to different risk levels. Figure 4 As shown, prevention and control measures include:

[0046] When the risk level is level 1, the underground reservoir coal pillar dam is in a stable and safe state and no intervention is required;

[0047] When the risk level is level 2, the water resource utilization plan of the groundwater reservoir is optimized to reduce the number of water storage and release;

[0048] When the risk level is level 3, the number of water storage and release is reduced while reducing the damage to the coal pillar dam caused by the underground reservoir water storage height and water pressure cycle;

[0049] When the risk level is 4, two grouting pipes are driven from the outside of the coal pillar dam toward the inspection area, and one grouting pipe is driven into the adjacent inspection area. The depth of the grouting holes should exceed 75% of the width of the coal pillar dam. Inverted wedge-shaped metal anchors are driven into the damaged unit every 5 meters. The anchor driving depth must not be less than 90% of the anchor length to avoid exposure or insufficient depth.

[0050] The grouting materials are made from local fine tailings, fly ash, slag powder, etc. The industrial solids produced by coal mining are used as raw materials to make cement slurry. The ratio of cement and water glass double slurry is between 1:0.45-1:0.65, and the ratio of industrial resin and acid curing agent is between 4:1-5:1. The production of environmentally friendly grouting materials for grouting reinforcement not only reduces material loss but also facilitates the utilization of industrial waste.

Claims

1. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity, characterized in that: Specifically include the following steps: Step 1: Conduct physical and mechanical tests on standard specimens to obtain the compressive strength relationship of the specimens under different saturations, different water pressures, and different numbers of water storage and discharge cycles, as well as the crack connectivity relationship when the specimens are destroyed; The compressive strength function expression of the sample when the saturation S is 0%, 25%, 50%, and 75% is: s p1 =bC -t s c =bC -t (s o -nS) The compressive strength function expression of the sample when the saturation S is 100% is: s p2 =bC -t s c -mP=bC -t (s o -nS)-mP Among them, σ p1 is the compressive strength of the sample at saturation of 0%, 25%, 50%, and 75% and under different water storage and discharge cycles, σ p2 is the compressive strength of the sample when the saturation is 100% and under different water storage and discharge cycles and different water pressures, σ o is the compressive strength of the sample in dry state, σ c is the compressive strength of the sample at different saturation states, n, b, t, and m are fitting parameters, C is the number of water storage and discharge cycles, and P is the water pressure; When the saturation S of the sample is 0%, 25%, 50%, and 75%, the function expression of the crack connectivity rate when the sample is destroyed is: When the saturation S of the sample is 100%, the functional expression of the crack connectivity rate when the sample is destroyed is: in, is the crack connectivity rate when the sample is destroyed at saturation of 0%, 25%, 50%, and 75% and under the combined action of different water storage and discharge cycles, is the crack connectivity rate of the sample when the saturation is 100% and under the combined action of different water storage and discharge cycles and different water pressures; γ is the total number of cracks in the sample obtained by CT scanning; θ, λ, δ are all fitting parameters, C is the number of water storage and release cycles, and P is the water pressure; Conduct seepage tests on samples with different fracture connectivity rates, obtain the permeability k of each sample, and establish the fracture connectivity rate Functional expression between and permeability k: Step 2: Place stress sensors and seepage monitors in the coal pillar dam of the underground reservoir to obtain the pressure and permeability of the dam in its current state; Step 3: Use the artificial intelligence system to optimize the compressive strength function expression, the fracture connectivity function expression at failure, and the function expression between fracture connectivity and permeability, and substitute the current saturation, water pressure, and number of storage and release cycles of the coal pillar dam into the compressive strength function expression to obtain the compressive strength σ of the coal pillar dam in the current state. p , substitute the fracture connectivity rate function expression at failure to obtain the fracture connectivity rate at failure Substitute the permeability k obtained by field monitoring into the functional expression between fracture connectivity and permeability to obtain the fracture connectivity of the coal pillar dam body. Step 4: Build a model of the coal pillar dam using numerical simulation software. The parameters used include geometric parameters, material properties, load conditions, boundary conditions, and seepage parameters. For areas where cracks have appeared or where cracks are developing, a safety assessment is conducted based on the current pressure σ1 of the coal pillar dam or the range of the crack connectivity rate in the area.

2. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step 2, the arrangement of stress sensors and seepage monitors requires first dividing the underground reservoir coal pillar dam into a 15×15 grid, and arranging the stress sensor monitoring point at the center of the monitoring area grid line near the inside of the underground reservoir coal pillar dam, and arranging the seepage monitor monitoring point at the center of the monitoring area grid line in the middle of the coal pillar dam.

3. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step four, the current pressure and theoretical compressive strength are compared and analyzed, the current fracture connectivity rate and the fracture connectivity rate at the time of failure are compared and analyzed, and the risk level is evaluated. The risk level is divided into four levels: level 1, level 2, level 3 and level 4. Different prevention and control measures are taken according to the different risk levels of the coal pillar dam.

4. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 2, characterized in that: The saturation S is set to five groups: 0, 25%, 50%, 75%, and 100%; the number of water storage and discharge cycles C is set to 0, 0.2C max , 0.4C max , 0.6C max , 0.8C max 、C max Group 6, C max C is the total number of times of water storage and release in the service life of the underground reservoir, and its calculation formula is max = Number of water storage and release times per year × service life; water pressure P is set to 0 and 0.2P max , 0.4P max , 0.6P max , 0.8P max 、P max Six groups, P max This is the water pressure when the underground reservoir is full of water. The water pressure test is only carried out when the saturation is 100%. There are 60 groups of mechanical tests under different conditions.

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