Coal pillar dam body safety assessment and prevention method considering fracture connectivity rate

Abstract

A fracture communication state in a coal pillar dam body has an important influence on the stability of the dam body, and when the dam body is under the conditions of different saturations, different water pressures and different water storage and drainage circulation times during the operation period of a coal mine underground reservoir, the different fracture communication states can be caused, so that the dam body is in different safety states. A coal pillar dam body is monitored in real time through an artificial system, and a function expression between data obtained through a physical and mechanical test and data obtained through sensor monitoring is optimized. And meanwhile, establishing a coal pillar dam body model through numerical simulation software, endowing each parameter obtained by optimizing the expression into the model, performing simulation, and determining a crack position or an area where the crack has a development trend in the dam body. Safety assessment is carried out according to two judgment methods of current pressure and theoretical compressive strength of the area or current fracture connectivity rate and fracture connectivity rate during damage, and corresponding prevention and control measures are taken for the dangerous area according to the risk level.

Description

Technical field:

[0001] The 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 taking fracture connectivity into consideration. Background technology:

[0002] The safety management of the coal pillar dam of the underground reservoir is a particularly important aspect in the coal mining process. The coal pillar dam not only supports the overlying rock layer and prevents the surface from collapsing, but also ensures the safety inside the mine. The current technology lacks consideration for the long-term operation of underground reservoirs in coal mines. During the operation of underground reservoirs in coal mines, the cycle of water storage and water discharge in the reservoir will inevitably affect the safety of the coal pillar dam. In addition, the number of water storage and discharge cycles in the underground reservoir and the water storage height of the reservoir will cause the coal pillar dam of the underground reservoir in the coal mine to be in different states and bear different degrees of pressure. Under the action of pressure, the increase in the connectivity of the internal fractures of the dam will lead to further deterioration of the structure and ultimately the destruction of the dam.

[0003] The connectivity state of the cracks inside the coal pillar dam has an important impact on the stability of the dam. Since acoustic emission technology and microseismic monitoring technology cannot effectively monitor the connectivity state of the cracks inside the dam, and the existing technology lacks specific explanations on the safety assessment steps of the coal pillar dam of the coal mine underground reservoir, and lacks standard assessments and reasonable response measures for the classification of risk levels and prevention and control measures of the coal pillar dam of the coal mine underground reservoir.

[0004] Through the safety inspection and evaluation of the coal pillar dam of the underground reservoir, cracks, pores or other defects in the coal pillar dam can be discovered as early as possible. Rapidly taking reasonable prevention and control measures for the problem area 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 the coal pillar dam can not only effectively improve the overall safety of coal mine engineering, but also lay a solid foundation for achieving sustainable development goals. For this reason, it is very necessary to study the safety assessment and prevention methods of the coal pillar dam and propose a safety assessment and prevention method for the coal pillar dam that takes into account the fracture connectivity rate. Summary of the invention:

[0005] The fracture connectivity state inside the coal pillar dam of the underground reservoir of the coal mine is closely related to the safety condition of the dam. During the operation of the underground reservoir of the coal mine, when the dam is under different saturation, different water pressure and different storage and release 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 at 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 inner side 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: The artificial intelligence system is used to optimize the function expression of compressive strength, the function expression of fracture connectivity at failure, and the function expression between fracture connectivity and permeability, and the compressive strength of the coal pillar dam body under the current state, the fracture connectivity at failure, and the fracture connectivity of the coal pillar dam body are determined.

[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 tendency to develop, 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 that have completed the safety assessment, propose different prevention and control measures based on 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 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 It is a diagram of 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-bottom plate; 7-information collection equipment; 8-information processing equipment. Specific implementation method:

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

[0019] Step 1: Obtain coal samples from the rock layer where the coal pillar dam of the underground reservoir 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 coal pillar dam of the underground reservoir in situ, the most realistic mechanical parameters are detected, and the function 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% in five groups. 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 each 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 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 loading tests under different conditions. The test path is to load the axial pressure and confining pressure to the stress state of the coal pillar dam body, and then load it at a loading rate of 0.1MPa / s until the sample is destroyed, record the compressive strength under different conditions, and establish the functional expression of the compressive strength:

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

[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, n, b, and t are all 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, n, b, t, and m are all fitting parameters.

[0028] The crack characteristics of the specimen at the time of failure are obtained through CT scanning, and the crack connectivity rate at the time of failure is calculated, and the functional expression of the crack connectivity rate at the time of failure is established:

[0029] (1) When the saturation S of the sample 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 sample 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 sample obtained by CT scanning, and θ and λ are fitting parameters.

[0032] (2) When the saturation S of the sample 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 fracture 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 fractures in the sample obtained by CT scanning, e is the base of the natural logarithm function, θ, λ, δ are all fitting parameters.

[0035] Step 2: Perform 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 of the coal mine is gridded, and the coal pillar dam is divided into a 15m×15m plane grid near the inner side of the coal pillar dam of the underground reservoir. The stress sensor monitoring point is arranged at the center of the grid line of the monitoring area. According to the same grid division, the seepage monitor is arranged in the middle of the coal pillar dam to obtain the current pressure σ1 and permeability k of the coal pillar dam at this time. The stress sensor and the seepage monitor are connected to the artificial intelligence system through a data cable, and the coal pillar dam is monitored in real time all day long by the artificial intelligence system. The data and results of the physical and mechanical tests of the samples are entered into the artificial intelligence system to prepare for the subsequent optimization of each function expression through 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 the deletion of data that deviates from the fitting expression and the use of the average to replace the missing data. 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 Substituting 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 the area or the crack connectivity rate are used. The safety assessment is carried out in 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 risk level of the dam body in the detection area is determined to be level 1.

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

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

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

[0045] Step 6: For the areas after the safety assessment, propose different prevention and control measures according to different risk levels. Figure 4 As shown, prevention and control measures include:

[0046] When the risk level is level 1, the coal pillar dam of the underground reservoir 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 water 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 to the detection area, and one grouting pipe is driven into the adjacent detection area. The depth of the grouting hole should exceed 75% of the width of the coal pillar dam, and an inverted wedge-type metal anchor bolt is driven into the damaged unit every 5m. The driving depth of the anchor bolt shall not be less than 90% of the anchor bolt length to avoid exposure or insufficient depth.

[0050] The grouting materials are made of 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 to water glass double slurry is between 1:0.45-1:0.65, and the mixture 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 process waste.

Claims

1. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity, characterized in that: Specifically, the following steps are included: Step 1: Conduct physical and mechanical tests on standard samples to obtain the compressive strength relationship of the samples under different saturations, different water pressures and different storage and discharge cycle times, as well as the fracture connectivity relationship when the samples are destroyed, and conduct seepage tests to obtain the relationship between fracture connectivity and permeability; Step 2: Arrange 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 numerical simulation software to build a model of the coal pillar dam body, assign the parameters obtained from the expression optimized by the artificial intelligence system to the model, and conduct safety assessment and prevention and control according to the risk level judgment standards for areas where cracks have appeared in the dam body or where cracks have a tendency to develop.

2. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step one, when conducting the seepage test, coal samples with different fracture connectivity rates should be selected for testing.

3. 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 that the coal pillar dam of the underground reservoir be divided into a 15×15 grid, and the stress sensor monitoring points be arranged at the center of the grid line of the monitoring area near the inner side of the coal pillar dam of the underground reservoir, and the seepage monitor monitoring points be arranged at the center of the grid line of the detection area in the middle of the coal pillar dam.

4. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step three, a model of the coal pillar dam is established through numerical simulation software. The parameters used are accurate and consistent with the actual situation, including geometric parameters, material properties, load conditions, boundary conditions, seepage parameters, etc.

5. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step three, the artificial intelligence system is used to optimize each function expression, including deleting data that deviates from the fitted expression and filling in missing data by replacing missing values ​​with the average, and using graphics to visually verify the accuracy of the artificial intelligence system in optimizing the function expression.

6. A method for safety assessment and prevention of coal pillar dams considering fracture connectivity according to claim 1, characterized in that: In step three, the risk level evaluation criteria include comparing and analyzing the current pressure and theoretical compressive strength, and comparing and analyzing the current crack connectivity rate and the crack connectivity rate at the time of destruction. The classification of the danger level only needs to meet one of the two evaluation criteria. 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 dam body.

7. 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%, and 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 water storage and release times during the service life of the underground reservoir, and its calculation formula is max = Annual water storage and release times × 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 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 loading tests under different conditions.

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