Method for measuring time-space evolution of goaf void ratio and permeability in roof-cutting along-void remaining roadway

Abstract

The present application relates to the field of coal mining and disaster prevention, and particularly relates to a method for measuring the spatial and temporal evolution of the void ratio and permeability of a goaf in a top-cut gob-side entry retaining mining area. The method comprises: determining the pressure distribution function of the goaf in the top-cut gob-side entry retaining mining area; determining the geometric size and void ratio of the overburden rock in the caving zone of the goaf when the overburden rock is not under pressure; loading the caving zone of the goaf in the top-cut gob-side entry retaining mining area into a sample compression box, and finely adjusting the initial void ratio to be completely consistent with the goaf; vertically compressing the experimental sample using a pressure head with a scale; calculating the void ratio according to the initial void ratio, initial height and depth of the sample box; stopping the load when the pressure value at the bottom of the sample reaches the maximum value of the goaf pressure; calculating the binary function relationship between the void ratio of the sample and the pressure value; calculating the void ratio distribution function using the binary function relationship between the goaf pressure and the void ratio; and calculating the permeability, viscous resistance coefficient and inertial resistance coefficient of the goaf.

Description

Technical Field

[0001] This invention relates to the field of coal mining and disaster prevention, and specifically to a method for determining the spatiotemporal evolution of porosity and permeability in goaf areas where roof cutting and goaf retention are carried out. Background Technology

[0002] The roof-cutting and gob-side-entry pillarless mining technology represents a comprehensive upgrade from traditional gob-side-entry mining methods. This technology actively cuts off roof pressure transmission and automatically forms gob-side entry channels, effectively solving key problems in traditional gob-side-entry mining such as stress concentration and difficulties in constructing the gob-side filling wall. It also addresses the significant drawbacks of traditional mining methods, such as large roadway excavation volumes and low coal recovery rates. A major challenge in using this technology is the large-scale air leakage in the goaf and the resulting gas and spontaneous combustion hazards. The gob-side entry channel automatically formed by roof cutting and pressure relief is directly connected to the goaf's collapse zone, drastically reducing the friction and local resistance along the air leakage channels within the goaf, increasing the leakage range and volume, and significantly exacerbating the risk of spontaneous combustion. Simultaneously, because the goaf formed by roof cutting and gob-side entry channels is relatively open, harmful gases such as methane can easily leak into the gob-side entry channel under negative pressure, increasing the difficulty of managing harmful gases within the roadway.

[0003] Currently, both domestically and internationally, goaf areas are generally considered as porous media, with porosity and permeability being key parameters characterizing the fracture field of these porous media. During the mining process, the porosity and permeability of the goaf are dynamically changing due to factors such as mining pressure and time, further increasing the difficulty of research. The porosity and permeability of goaf areas in roof-cutting and goaf-side-retaining methods differ significantly from those in traditional mining methods, resulting in significant changes to the flow field and gas concentration field, making the prevention and control of disasters such as gas and coal spontaneous combustion more difficult. Currently, theoretical calculations and numerical simulations are commonly used to obtain the porosity and permeability parameters of goaf areas, but these methods have significant errors and are difficult to achieve satisfactory research results. Therefore, there is an urgent need to propose a method that can accurately obtain the porosity and permeability of goaf areas in roof-cutting and goaf-side-retaining methods. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for determining the spatiotemporal evolution of porosity and permeability in goaf areas where roof cutting and goaf retention are carried out.

[0005] This invention adopts the following technical solution: a method for determining the spatiotemporal evolution of porosity and permeability in goaf areas of roof-cutting and goaf-retaining roadways, comprising: S100: Determine the pressure distribution function of the goaf area in the cut-off roadway; S200: Determine the geometric dimensions and porosity of the overlying strata in the goaf when they are not under pressure; S300: Load the uncompressed broken rock from the caving zone of the goaf in the cut-off roadway into a sample compression box, and set the initial porosity. Fine-tune it to perfectly match the goaf area, and record the initial height of the broken rock in the sample compression chamber as . h ; S400: Uses a scaled indenter to compress the test sample vertically downwards, and records the depth of the indenter into the sample chamber in real time during the compression process. and the corresponding pressure value ; S500: Based on the initial porosity Initial height h and the depth of the sample chamber The calculated pressure is porosity ; S600: When the pressure value at the bottom of the sample reaches the maximum pressure value of the goaf area. Stop ballasting at that time; S700: The porosity of the sample was calculated based on experimental data. With pressure value The bivariate functional relationship between them ; S800: Based on the pressure distribution function of the goaf area in the cutting-off-roof-and-leaving-goaf-retaining roadway, the porosity distribution function is calculated using the bivariate functional relationship between air pressure and porosity. S900: Calculated permeability of the goaf area ,in, The average grain size of the collapsed overburden in the goaf area; S1000: Calculated viscous resistance coefficient and inertial drag coefficient .

[0006] Step S100 includes: S101: After slotting at the bottom of the working face, a pressure sensor is placed. The pressure sensor wire is protected by a sleeve and extends to the roadway left along the goaf after the roof is cut. As the working face advances, the pressure sensor is buried at the bottom of the goaf and the pressure distribution in the goaf is monitored in real time to obtain a scatter plot of the pressure distribution in the goaf. S102: Based on the actual geological data of the working face with roof cutting and goaf retention, a calculation model is established using numerical simulation software, and the pressure distribution function of the goaf during the working face advancement process is obtained through numerical analysis.

[0007] In some embodiments, step S200 includes: installing an explosion-proof camera in the goaf-side roadway to capture a high-definition image of the overlying roof of the goaf collapse zone when it first collapses and is not under pressure; performing binarization processing on the image; and obtaining the geometric dimensions and initial porosity of the overlying roof of the goaf collapse zone when it is not under pressure after statistical analysis.

[0008] Compared with existing technologies, this invention addresses the difficulty of directly obtaining the porosity and permeability distribution of goaf areas. Combining the large-scale openness of goaf areas in roof-cutting and goaf-retaining roadways, it proposes to obtain the pressure distribution function of goaf areas through field testing and numerical simulation. It also utilizes a self-developed large-scale pressure-bearing experimental system for overburden in goaf caving zones to determine the binary functional relationship between goaf pressure and porosity, cleverly transforming the porosity and permeability distribution function of goaf areas in roof-cutting and goaf-retaining roadways into a more easily implemented pressure distribution function for research. Attached Figure Description

[0009] Figure 1 This is a site layout diagram for testing the pressure distribution and initial void ratio of the tunnel along the cut-off roof; Figure 2 This is a schematic diagram of the working principle of a large-scale pressure-bearing test on the overlying rock of the goaf caving zone; Figure 3 This is a schematic diagram of a large-scale pressure-bearing experimental platform for the overlying rock of the goaf collapse zone; In the diagram, 1-top-cutting and goaf-retention mining face, 2-track roadway, 3-belt roadway, 4-advance blasting pre-splitting hole, 5-working face hydraulic support, 6-top-cutting line, 7-top-cutting and goaf-retention roadway, 8-pressure sensor I, 9-open goaf area, 10-explosion-proof camera, 11-large hydraulic equipment support, 12-three-cylinder four-column hydraulic system, 13-control box, 14-pressure head with scale, 15-uncompressed crushed rock in the caving zone, 16-sample compression box, 17-pressure sensor II. Detailed Implementation

[0010] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0011] Taking the actual roof-cutting and goaf-retention roadway working face 1 as an engineering example, pressure sensor I8 was placed after a groove was cut at the bottom of the working face. The conductor of pressure sensor I8 was protected by a steel sleeve and extended into the roof-cutting and goaf-retention roadway 7. As the working face advanced, after pressure sensor I8 was buried at the bottom of the goaf, it monitored the pressure distribution of the goaf in real time online, and obtained a scatter plot of the pressure distribution in the goaf.

[0012] like Figure 1The diagram shows the site layout for testing the pressure distribution and initial porosity of the roof-cutting goaf-retaining roadway. The roof-cutting goaf-retaining roadway working face 1 has a track roadway 2 and a belt conveyor roadway 3 on either side. A pre-blasting pre-splitting hole 4 is installed on one side of the track roadway 2. The track roadway 2 connects to the roof-cutting goaf-retaining roadway 7 on one side of the open goaf area 9. Pressure sensors I8 are installed within the open goaf area 9.

[0013] Based on the actual geological data of the roof-cutting and goaf-retention working face 1, a calculation model was established using FLAC3D numerical simulation software. The pressure distribution in the goaf area during the working face advancement process was numerically analyzed. Combining field tests and numerical analysis, the pressure distribution function of the goaf area in the roof-cutting and goaf-retention working face was finally determined.

[0014] An explosion-proof camera 10 was installed in the goaf-side roadway to capture high-definition images of the overlying roof of the goaf collapse zone when it first collapsed and was not under pressure. The images were then binarized using ImageJ software, and statistical analysis was performed to obtain the geometric dimensions and initial porosity of the overlying roof of the goaf collapse zone when it was not under pressure.

[0015] The uncompressed crushed rock 15 from the top-cutting and goaf-retaining roadway collapse zone was transported from underground to the surface laboratory.

[0016] The uncompressed crushed rock 15 from the collapse zone of the goaf along the cut-off roadway was placed into the sample compression box 16, and the initial porosity was determined. Fine-tune it to perfectly match the goaf area, and record the initial height of the broken rock in the sample compression chamber as . h .

[0017] The experimental sample is compressed vertically downward using a three-cylinder four-column hydraulic system 12 that drives a scale-equipped pressure head 14.

[0018] like Figure 3 As shown, the bottom of the sample compression box 16 is a pressure sensor II17, and the scaled pressure head 14 is located directly above the sample compression box 16. The scaled pressure head 14 is driven by a three-cylinder four-column hydraulic system 12, which is mounted on a large hydraulic equipment support 11. Both the three-cylinder four-column hydraulic system 12 and the pressure sensor II17 are connected to the control box 13.

[0019] The depth of the indenter inserted into the sample chamber is recorded in real time during the ballast process. and the corresponding pressure value .

[0020] Based on the initial porosity of the fractured rock Initial height h and the depth of the sample chamber The calculated pressure is porosity .

[0021] When the pressure value at the bottom of the sample reaches the maximum pressure value of the goaf area. Stop ballasting at that time.

[0022] Based on the experimental data, the porosity of the sample was calculated. With stress The bivariate functional relationship between them .

[0023] Based on the pressure distribution function of the goaf area in the cut-off roadway, the porosity distribution function is calculated using the bivariate functional relationship between air pressure and porosity.

[0024] The permeability of the goaf was obtained using the Blake-Kozeny formula. ,in, The average grain size of the collapsed overburden in the goaf is obtained by binarizing high-resolution images. This is the porosity distribution function.

[0025] The viscous resistance coefficient is obtained from the Forchheimer formula for non-Darcy seepage. and inertial drag coefficient .

[0026] Taking a specific roof-cutting and goaf-retention roadway working face as the engineering background, this paper analyzes the goaf pressure distribution during the roof-cutting and goaf-retention roadway mining process by combining field measurements and FLAC3D numerical simulation, and obtains the goaf pressure distribution function.

[0027] High-definition explosion-proof cameras were used to take high-definition pictures of the goaf area in the goaf roadway. The images were then binarized using ImageJ software. After statistical analysis, the geometric dimensions and initial porosity of the overlying rock in the goaf collapse zone when it was not under pressure were obtained.

[0028] Using a self-developed large-size fractured rock mass compression test device, with the geometric dimensions and initial porosity of the overlying strata in the caving zone before it is under pressure as initial conditions, and the pressure distribution parameters in the goaf as experimental variables, a compression test of the overlying strata in the caving zone of the goaf in the roof-cutting and goaf-retaining roadway was carried out. The change law of porosity during the pressure process of the fractured overlying strata was analyzed, and a binary function relationship between the pressure and porosity of the fractured overlying strata was established. The porosity monitoring of the goaf in the roof-cutting and goaf-retaining roadway, which is relatively difficult to operate, was transformed into pressure monitoring, which is easier to implement. The porosity distribution function of the goaf in the roof-cutting and goaf-retaining roadway, which conforms to the actual situation on site, was obtained.

[0029] By utilizing the porosity parameter of the goaf area in the roof-cutting and goaf-retaining roadway, and combining the relationship between porosity and permeability, the permeability that plays a decisive role in the gas flow in the goaf area is further calculated, resulting in a permeability distribution function of the goaf area in the roof-cutting and goaf-retaining roadway that conforms to the actual situation on site.

[0030] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the spatiotemporal evolution of porosity and permeability in goaf areas of roadways with roof cuts and goaf retention, characterized in that, include: S100: Determine the pressure distribution function of the goaf area in the cut-off roadway; S200: Determine the geometric dimensions and porosity of the overlying strata in the goaf when they are not under pressure; S300: The uncompressed crushed rock (15) in the collapse zone of the goaf in the cut-off roadway is loaded into the sample compression box (16), and the initial porosity is set. Fine-tune it to perfectly match the goaf area, and record the initial height of the broken rock in the sample compression chamber as . h ; S400: The test sample is compressed vertically downward using an indenter (14) with a scale, and the depth of the indenter into the sample box is recorded in real time during the compression process. and the corresponding pressure value ; S5 00: Based on the initial porosity Initial height h and the depth of the sample chamber The calculated pressure is porosity ; S600: When the pressure value at the bottom of the sample reaches the maximum pressure value of the goaf area. Stop ballasting at that time; S700: The porosity of the sample was calculated based on experimental data. With pressure value The bivariate functional relationship between them ; S800: Based on the pressure distribution function of the goaf area in the cutting-off-the-roof-and-leaving-goaf-retaining roadway, the porosity distribution function is calculated using the bivariate functional relationship between air pressure and porosity. S900: Calculated permeability of the goaf area ,in, The average grain size of the collapsed overburden in the goaf area; S1000: Calculated viscous resistance coefficient and inertial drag coefficient .

2. The method for determining the spatiotemporal evolution of porosity and permeability in goaf areas by roof cutting and goaf retention according to claim 1, characterized in that, Step S100 includes: S101: After slotting at the bottom of the working face, a pressure sensor (8) is placed in the working face. The wire of the pressure sensor (8) is protected by a sleeve and extends to the roadway left along the top cut (7). As the working face advances, the pressure sensor (8) is buried at the bottom of the goaf and the pressure distribution of the goaf is monitored in real time online to obtain a scatter plot of the goaf pressure distribution. S102: Based on the actual geological data of the working face (1) with the cut-off and goaf-keeping, a calculation model was established using numerical simulation software, and the pressure distribution function of the goaf during the working face advancement process was obtained through numerical analysis.

3. The method for determining the spatiotemporal evolution of porosity and permeability in goaf areas by roof cutting and goaf retention according to claim 1, characterized in that, Step S200 includes: installing an explosion-proof camera (10) in the goaf-side roadway to capture a high-definition image of the overlying strata roof of the goaf collapse zone when it first collapses and is not under pressure; performing binarization processing on the image; and obtaining the geometric dimensions and initial porosity of the overlying strata roof of the goaf collapse zone when it is not under pressure after statistical analysis.

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