A method for exploring and evaluating a collapse column in a north China type coalfield based on ground directional drilling technology

Abstract

The application discloses a North China type coalfield collapse column exploration and evaluation method based on ground directional drilling technology, which comprises the following steps: determining the center position of the collapse column in the coal seam floor contour map within the three-dimensional seismic delineation as the opening position of the main hole for exploration; determining the number, direction and horizontal displacement of the directional branch holes for collapse column exploration; calculating the distance between the coal seam floor and the Ordovician limestone aquifer and the water head height of the Ordovician limestone aquifer; calculating the safety aquifer thickness M of the coal seam floor; calculating the final hole depth of the main hole for collapse column exploration and the final hole elevation of the branch hole; determining the deflecting radius and trajectory of the branch hole; performing the main hole construction, determining the top boundary position of the collapse column according to the core and performing the hydrogeological test; performing the branch hole construction, determining the upper interface and the side interface of the collapse column and performing the hydrogeological test; delineating the spatial form of the collapse column and analyzing and evaluating the water enrichment and water conductivity of the collapse column. The application provides a scientific basis for the water prevention and control design of the mining field and the mine, and ensures the safe and efficient production of the mine.

Description

Technical Field

[0001] This invention relates to the field of collapse column detection technology. Specifically, it is a method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology. Background Technology

[0002] The existence of collapse columns not only disrupts the stability and continuity of coal seams, reducing coal reserves and resulting in significant coal resource losses, but also affects the conventional layout of working faces, reduces the efficiency of fully mechanized mining machinery, and poses a major adverse impact on coal mine safety and economic benefits. Insufficient investigation of water-conducting collapse columns can lead to mine flooding and other major accidents, posing a significant threat to mine safety. Karst collapse columns are one of the main channels for karst groundwater activity, often connecting major aquifers and introducing Ordovician limestone water into the mine, causing flooding accidents. Therefore, it is of great significance to employ effective technical methods to determine the scale and water conductivity of collapse columns in production areas and provide effective technical support for coal mine safety. Production practice has proven that using a combination of geophysical exploration and drilling is the key to effectively identifying collapse columns.

[0003] All geophysical exploration methods interpret the differences in certain physical property parameters (such as gravity, electrical parameters, magnetic parameters, and seismic wave parameters). Therefore, each method has its own advantages, disadvantages, and applicable scope. Practice has proven that using integrated geophysical exploration techniques for comprehensive interpretation and analysis, with various technical results mutually verifying each other and complementing each other's strengths, is an effective way to improve the reliability and accuracy of geophysical data interpretation results. 3D seismic exploration and transient electromagnetic methods are commonly used geophysical exploration methods in coalfields. 3D seismic exploration technology is more sensitive to the structure of coalfields (faults, collapse columns, etc.), while transient electromagnetic methods are more sensitive to the water content within the structure. The integrated application of 3D seismic exploration and transient electromagnetic methods is an effective means of exploring coal seam occurrence and structure, and solving hydrogeological problems in coal mines.

[0004] Because collapse columns are isolated geological bodies with irregular spatial shapes (their projections on the horizontal plane or coal seam floor contour maps are generally elliptical, circular, or elongated), varying sizes, heights, and are highly concealed, their detection is technically challenging. In the late 1980s, 3D seismic exploration technology was introduced to the North my country coalfields. Since then, coal mines have seen significant improvements in observation system design, data acquisition, data processing, and data interpretation using this technology. However, due to the influence of many factors, including the accuracy of geophysical instruments, geological conditions (such as topographic relief, thickness of overlying loose layers, dip angle variations of coal-bearing strata, and structural complexity), the performance of data processing software, and the experience of data interpreters, the structural interpretation results and coal seam thickness predictions are still not entirely compatible with the control precision required for coal mine production. According to incomplete statistics from coal mine exploration and mining comparisons, currently, the accuracy rate of 3D seismic technology in interpreting collapse columns with a major axis diameter of 25 m or more is only 40%–50%. Transient electromagnetic detection results primarily reflect volumetric effects, often only providing qualitative interpretations of collapse columns and their water-bearing properties, with relatively low accuracy. Therefore, it is necessary to develop a method for detecting and evaluating collapse columns in North China-type coalfields based on ground-based directional drilling technology to improve the accuracy of interpreting collapse columns with a major axis diameter exceeding 25 m. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to provide a method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology, so as to solve the problem that the existing methods for detecting and evaluating collapse columns have low accuracy in interpreting collapse columns with a major axis diameter of more than 25 m.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0007] A method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology includes the following steps:

[0008] (1) Using three-dimensional seismic exploration to delineate the extension and distribution of the collapse column on the contour map of the bottom plate of the mineable coal seam, determine the center point of the collapse column on the contour map of the bottom plate of the mineable coal seam. O ( x , y , z The location of the main hole is determined based on the center point.

[0009] (2) Determine the number of branch holes for the collapse column, and determine the horizontal projection orientation and horizontal displacement (or horizontal length) of the branch holes.

[0010] (3) Analyze and calculate to determine the distance between the coal seam floor near the collapse column and the Ordovician limestone (hereinafter referred to as Ordovician limestone) aquifer, as well as the water head height of the Ordovician limestone aquifer;

[0011] (4) Determine the critical water inrush coefficient and calculate the thickness of the safe water-resistant layer on the coal seam floor. M ;

[0012] (5) Calculate the final hole depth of the main hole for the collapse column exploration and determine the final hole elevation of each directional branch hole.

[0013] (6) Determine the directional branch hole directional drilling section directional radius or dog leg angle, and design the trajectory of each directional branch hole (including side drilling elevation, final hole elevation, etc.).

[0014] (7) Construct the main borehole. After core sampling in the main borehole, determine the location of the top boundary of the collapse column based on the core sample and conduct hydrogeological tests on the collapse column section.

[0015] (8) Construct directional branch holes, determine the upper interface and the edge interface of the collapse column, and conduct hydrogeological tests on the collapse column section;

[0016] (9) Based on the determined location of the top boundary of the collapse column, the upper interface and the edge interface of the collapse column, determine the area below the bottom of the coal seam. M The actual spatial morphology of the collapse column, extending to the top boundary of the collapse column, is compared and analyzed with the location of the collapse column delineated by 3D seismic exploration. Based on the results of hydrogeological tests, the water-bearing capacity, water conductivity, and risk of water inrush to the surrounding coal seam mining of the collapse column are analyzed and evaluated.

[0017] This invention utilizes surface directional drilling technology and borehole hydrogeological tests (pumping tests, or water injection tests, water pressure tests) to explore and evaluate collapse columns in North China-type coalfields. Based on geophysical exploration to delineate the anomaly zone of the collapse column, this method further clarifies the spatial morphology, water-bearing capacity, and water-conductivity of the collapse column, avoiding the risk of water inrush during underground drilling and providing a scientific basis for the design of water control in mining areas and mines.

[0018] The above-mentioned method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology, in step (1), the opening position of the main borehole is the center point of the collapse column delineated by three-dimensional seismic exploration. O ( x , y , z The projection point of the point on the ground; the main borehole is a vertical borehole;

[0019] In step (2), the number of directional branch holes and the azimuth and horizontal displacement (or horizontal length) of the horizontal projection of the directional branch holes are determined based on the extension range of the collapse column on the contour map of the mineable coal seam floor delineated by the three-dimensional seismic exploration. The number of directional branch holes is 4 to 8. Each directional branch hole is drilled from the side of the exploration main hole. The horizontal projection length of the directional branch hole exceeds the projection boundary of the collapse column on the coal seam floor by 10 to 20 m.

[0020] The formula for calculating the horizontal displacement of each directional branch hole is as follows:

[0021] (1);

[0022] In formula (1): H i —Horizontal displacement of the directional branch hole ( i =1, 2, 3...), m;

[0023] —Horizontal distance, in meters, from the center of the collapse column projected onto the contour map of the coal seam floor to the intersection of the directional branch hole and the boundary of the collapse column;

[0024] l —Directional branch holes exceed The length, in meters; l Take 10-20;

[0025] In this invention, for a collapse column with a major axis diameter less than 100m, the center point of its planar projection is used as the opening position of the main exploration hole on the ground. Four directional branch holes are drilled sideways at a certain depth of the main hole, and the horizontal projections of the four directional branch holes are respectively in the directions of the major axis and the minor axis. For a near-elliptical collapse column with a major axis diameter greater than 100m, the center point of its planar projection is used as the opening position of the main exploration hole on the ground. Eight directional branch holes are drilled sideways at a certain depth of the main hole. The orientation of the first branch hole is the orientation of the major axis with an angle of less than 90° between the horizontal projection and the due north direction. One directional branch hole is designed every 45° in a clockwise (or counterclockwise) direction. For a collapse column with a major axis diameter greater than 100m and an irregular shape, the number of directional branch holes is designed according to its planar projection shape, so that the projection spacing of the final hole positions of each adjacent directional branch hole on the horizontal plane is basically equal.

[0026] In step (3), the distance between the bottom of the normally mineable coal seam near the collapse column and the underlying Ordovician limestone aquifer, as well as the water head height of the Ordovician aquifer at the center of the collapse column, are calculated based on the hydrogeological data of the coal mine. The calculation method is explained as follows: The distance between the normal mineable coal seam floor near the collapse column and the underlying Ordovician limestone aquifer can be calculated by drawing a contour map of the distance between the coal seam floor and the Ordovician limestone aquifer based on the coordinates of each borehole near the collapse column and the distance between the coal seam floor and the Ordovician limestone aquifer at each borehole, using interpolation (or extrapolation). The water head height of the Ordovician aquifer at the center of the collapse column can be calculated by drawing an Ordovician limestone aquifer water level contour map based on the coal seam floor contour map, and using the interpolation method (or extrapolation method). Coal mine geological and hydrogeological data include coal seam floor contour maps, borehole columnar sections, mine water-bearing maps, Ordovician limestone water level contour maps, and Ordovician limestone roof contour maps, etc.

[0027] In the above-mentioned method for the exploration and evaluation of collapse columns in North China coalfields based on ground directional drilling technology, in step (4), the critical water inrush coefficient for the section where the coal seam floor is damaged by structural damage is 0.06 MPa / m, and the critical water inrush coefficient for the section where the water-resistant layer of the coal seam floor is intact and there is no structural damage is 0.10 MPa / m.

[0028] Coal seam floor safety waterproof layer thickness M The calculation formula is:

[0029] (2);

[0030] In formula (1): P S —Safe head value of the bottom slab waterproof layer, in meters;

[0031] T S —Critical inrush coefficient, MPa / m; A 0 — Safety factor, with a value of 1.2 to 1.5.

[0032] In the above-mentioned method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology, step (5) involves calculating the elevation of the coal seam floor when the collapse column is not developed based on the contour lines of the minable coal seam floor near the collapse column. c M Intersection of the horizontal projection of each directional branch hole and the boundary of the collapse column C i ( i Elevation of the coal seam floor at positions (=1, 2, 3...) C Ci ( i =1, 2, 3...), and according to M and C Ci Calculate the final depth of the main hole and determine the final elevation of each directional branch hole. FCi Note: The original elevation of the coal seam floor without collapse columns at each point was obtained by interpolation based on the contour map of the coal seam floor.

[0033] ① The formula for calculating the final depth of the main hole is:

[0034] (3);

[0035] In formula (3): d M —Final depth of the main hole, in meters;

[0036] z M —Ground elevation of the main borehole opening location, in meters;

[0037] c M —Elevation of the coal seam floor when the collapse column is not developed at the main borehole location, in meters;

[0038] M —Thickness of the safety waterproof layer on the bottom of the coal seam;

[0039] ②The formula for calculating the final elevation of each directional branch hole is:

[0040] (4);

[0041] In equation (4): C Ci —Intersection of the horizontal projection of the directional branch hole and the boundary of the collapse column C i ( i Elevation of the coal seam floor at positions (=1, 2, 3...), in meters; F Ci — The final elevation of the branch hole, in meters.

[0042] In the above-mentioned method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology, step (6) is as follows:

[0043] ① The formula for calculating the vertical displacement of each directional branch hole is as follows:

[0044] (5);

[0045] In formula (5): V i —Vertical displacement of directional branch holes ( i =1, 2, 3...), m;

[0046] r—The directional branch hole drilling radius shall be ≥200 m, i.e., the dogleg angle ≤8.60° / 30 m; H i —Horizontal displacement of the directional branch hole ( i =1, 2, 3...), m; the larger the build-up radius, the smaller the curvature, that is, the smaller the dogleg angle, and the easier the construction; the smaller the build-up radius, the larger the curvature, the higher the requirements for the strength (such as bending strength) of the drilling tool material, and the greater the construction difficulty; the build-up radius can be infinitely large, that is, a straight hole, in which case the dogleg angle is 0° / 30 m).

[0047] ② Length of each directional branch hole L i The calculation formula is:

[0048] (6);

[0049] In formula (6): L i —Length of directional branch hole ( i =1, 2, 3...), m; r —Radius of directional branch hole drilling, m;

[0050] ③ Sidetracking elevation of each directional branch hole S i The calculation formula is:

[0051] (7);

[0052] In equation (7): S i —Side drilling elevation of directional branch holes ( i =1, 2, 3...), m;

[0053] C Ci —Intersection of the horizontal projection of the directional branch hole and the boundary of the collapse column C i ( i Elevation of the coal seam floor at positions (=1, 2, 3...), in meters; M —Thickness of the safety waterproof layer on the bottom of the coal seam.

[0054] The above-mentioned method for detecting and evaluating collapse columns in North China coalfields based on ground directional drilling technology, in step (7), the main borehole is a three-section structure borehole: the first section, with a diameter of... F 311.1 mm, hole depth 30–50 m, lowered F 244.5×8.94 mm casing, and cemented the entire length of the casing to the borehole opening with cement grout; second opening, borehole diameter F215.9 mm, borehole depth 10–20 m below bedrock surface, lowered into F 177.8×8.05 mm casing, with the entire length of the casing secured to the borehole opening using cement grout; three openings, borehole diameter... F 152.4 mm, bare hole;

[0055] After drilling into the bedrock, core samples were taken from the entire borehole to determine the location of the top boundary of the collapse column. I ( x I , y I , z I After entering the collapse column, observe the core characteristics and the changes in borehole leakage and water level after drilling stops; core characteristics include the continuity of the strata, the shape of the rock blocks, striations, and filling materials;

[0056] A hydrogeological test is conducted every 10–50 m of drilling. This test includes one or more of the following: pumping test, water injection test, and water pressure test, to obtain hydrogeological parameters. These parameters include the single-hole yield. q Permeability coefficient K And permeable rate q P .

[0057] In the above-mentioned method for detecting and evaluating collapse columns in North China coalfields based on ground directional drilling technology, step (7) involves using a pumping test to determine the unit water inflow of the borehole. q and permeability coefficient K hour:

[0058] A single-hole pumping test was conducted according to the "Pumping Test Specification" -- YS / T 5215-2021; the pumping rate was determined based on the inflow rate during pumping. Q and water level drawdown S Based on the data, determine the curve using the least squares method or graphical method, according to... QS The curve determines the flow rate of the pumping hole when the water level drawdown is 10 m. Then, the flow rate when the hole diameter is 91 mm is calculated using formula (8). Finally, the flow rate is divided by 10 m to obtain the unit flow rate. q ;

[0059] (8);

[0060] In equation (8): Q 91 、R 91 and r 91 —These represent the water inflow, radius of influence, and borehole radius for a 91mm borehole; Q、R andr h —These represent the water inflow, radius of influence, and borehole radius to be converted;

[0061] Permeability coefficient K Calculate using the following formula: (9);

[0062] In equation (9): K —Permeability coefficient of the pumping section of the collapse column, m / d; Q —The proposed conversion of borehole water inflow, in meters. 3 / d; r h —The borehole radius to be converted, in meters; S —Drawdown, in meters.

[0063] The above-mentioned method for the exploration and evaluation of collapse columns in North China coalfields based on ground directional drilling technology, in step (7) of the water injection test, the permeability is calculated according to the following formula;

[0064] (10);

[0065] In formula (10): q P —Permeability of the test section, Lu or L / (min•MPa•m);

[0066] Q I —Stable flow rate during the injection water test, in meters. 3 / d; H —Test water head height difference, m; L — The length of the test section, in meters.

[0067] In the above-mentioned method for detecting and evaluating collapse columns in North China coalfields based on ground directional drilling technology, step (7) involves conducting a water pressure test on the collapse column exposed by the borehole according to the "Water Pressure Test Procedure" -- YS / T 5216-2020. The water pressure test is conducted in three pressure levels and five stages, namely... p 1→ p 2→ p 3→ p 4 ( =p 2) → p 5 ( p 1) ，p 1 ＜p 2 ＜p 3, of which p 1 、p 2 、p3. The three pressure levels were 0.3 MPa, 0.6 MPa, and 1.0 MPa, respectively. The unit water absorption was measured. oh Permeability of the test section q P ;

[0068] (11);

[0069] (12);

[0070] In equations (11) and (12): oh —Unit water absorption, L / (min•m) 2 ); Q 3—Stable flow rate in the third stage of the test, L / min; L —Length of the test section, in meters; H 3—Third-stage test head, m; q P —Permeability of the test section, Lu or L / (min•MPa•m); P 3 — The test pressure for the third stage, in MPa;

[0071] Permeability coefficient K Calculate using the following formula:

[0072] (a) When the depth of the bottom of the test section from the waterproof layer is greater than the length of the test section:

[0073] (13);

[0074] (b) When the depth of the bottom of the test section from the waterproof layer is less than the length of the test section:

[0075] (14);

[0076] (c) The test section is located below the groundwater level, with low permeability and a pressure-flow rate (P / F) ratio. P - Q When the curve is laminar:

[0077] (15);

[0078] In equations (13), (14), and (15): K —Permeability coefficient, m / d; Q P —Push-in flow rate, m 3 / d; H —Test water head height difference, m; L —Length of the test section, in meters; r h— The borehole radius to be converted, in meters.

[0079] The above-mentioned method for detecting and evaluating collapse columns in North China coalfields based on ground directional drilling technology, in step (8), optimizes the side-drilling depth and final depth of the directional branch boreholes based on the actual top boundary burial depth of the collapse column revealed by the main borehole and the coal seam burial depth; each directional branch borehole is a bare borehole with the same borehole diameter as the main borehole three-way borehole; according to the opening position of each directional branch borehole in the main borehole, each branch borehole is constructed sequentially from bottom to top, and the location of the collapse column drilled in on the trajectory of each directional branch borehole is determined by comprehensive borehole logging, drilling rock powder analysis and comprehensive drilling pressure. I i ( x i , y i , z i () i =1, 2, 3...) and the location of the drilled-out collapse column. O j ( x j , y j , z j () j =1, 2, 3...);

[0080] After each directional branch borehole enters the collapse column, a hydrogeological test is conducted every 10–50 m of drilling. The test procedures and evaluation methods are the same as those for the main borehole. Based on the leakage of each directional branch borehole, the changes in the water level in the borehole after drilling stops, and the hydrogeological tests, the hydrogeological parameters at different locations of the collapse column are obtained.

[0081] The technical solution of the present invention achieves the following beneficial technical effects:

[0082] This invention, prior to the design of coalfield mining areas in North China, utilizes drilling and hydrogeological tests to further investigate the spatial morphology and hydrogeological characteristics of collapse columns delineated by surface geophysical exploration. This provides a scientific basis for the design of mining areas and mine water control, ensuring safe and efficient mine production. The method for detecting and evaluating collapse columns in North China coalfields based on surface directional drilling technology significantly improves the accuracy of collapse column interpretation, especially for collapse columns with a major axis diameter exceeding 25m. Attached Figure Description

[0083] Figure 1 A flowchart illustrating the method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology in this embodiment of the invention;

[0084] Figure 2 A plan view of the collapse column and its associated coal seam floor contour lines, geological structures, preliminary mining area design, and exploratory borehole layout determined by three-dimensional seismic analysis in this embodiment of the invention.

[0085] Figure 3 A schematic diagram illustrating the spatial relationship between the collapse column, coal seam, exploratory borehole, and Ordovician limestone aquifer in an embodiment of the present invention;

[0086] Figure 4 Schematic diagram of the drilling structure for detecting a collapse column in an embodiment of the present invention;

[0087] Figure 5 A schematic diagram of a single-hole confined water incomplete well (hole) pumping test in an embodiment of the present invention. Detailed Implementation

[0088] The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology in this embodiment includes the following steps:

[0089] (1) Analyze the extension characteristics of the collapse column on the contour map of the bottom plate of the mineable coal seam in the three-dimensional seismic exploration delineation, and determine the center position of the collapse column on the contour map of the bottom plate of the mineable coal seam. O ( x , y , z The projection of this point on the ground is the opening location of the main borehole for the exploration of the collapse column, and the main borehole is a vertical borehole.

[0090] (2) Based on the three-dimensional seismic exploration delineation of the extension range of the collapse column on the contour map of the bottom plate of the mineable coal seam, determine the number of directional branch holes (4 to 8) of the collapse column exploration holes and the azimuth, horizontal displacement or horizontal length of the horizontal projection of the directional branch holes. Each directional branch hole is drilled from the side of the main hole, and its horizontal projection length exceeds the projection boundary of the collapse column on the bottom plate of the coal seam by 10 to 20 m.

[0091] Horizontal displacement of each directional branch hole:

[0092] (1);

[0093] In the formula: H i —Horizontal displacement of the directional branch hole ( i =1, 2, 3...), m.

[0094] — The horizontal distance, in meters, from the center of the collapse column projected onto the contour map of the coal seam floor to the intersection of the directional branch hole and the boundary of the collapse column.

[0095] l —Directional branch holes exceed The length, in meters;l Take 10-20;

[0096] In this embodiment, for a collapse column with a major axis diameter less than 100m, the center point of its planar projection is used as the opening position of the main exploration hole on the ground. Four directional branch holes are drilled at a certain depth of the main hole, and the horizontal projections of the four directional branch holes are respectively in the major axis and minor axis directions. For a near-elliptical collapse column with a major axis diameter greater than 100m, the center point of its planar projection is used as the opening position of the main exploration hole on the ground. Eight directional branch holes are drilled at a certain depth of the main hole. The orientation of the first branch hole is the major axis orientation where the angle between the horizontal plane projection and the due north direction is less than 90°. A directional branch hole is designed every 45° in a clockwise (or counterclockwise) direction. For a collapse column with a major axis diameter greater than 100m and an irregular shape, the number of directional branch holes is designed according to its planar projection shape, so that the projection spacing of the final hole positions of each adjacent directional branch hole on the horizontal plane is basically equal.

[0097] (3) Based on the coal mine hydrogeological data (such as coal seam floor contour map, mine water-bearing map, Ordovician limestone contour map and Ordovician limestone roof contour map), calculate the distance between the normally mineable coal seam near the collapse column and the underlying Ordovician limestone aquifer, as well as the water head height of the Ordovician limestone aquifer; calculation method explanation: The distance between the normal mineable coal seam floor near the collapse column and the underlying Ordovician limestone aquifer can be calculated by drawing a contour map of the distance between the coal seam floor and the Ordovician limestone aquifer based on the coordinates of each borehole near the collapse column and the distance between the coal seam floor and the Ordovician limestone aquifer at each borehole, using interpolation (or extrapolation). The water head height of the Ordovician aquifer at the center of the collapse column can be obtained by drawing an Ordovician limestone aquifer water level contour map based on the coal seam floor contour map, and then calculating it using interpolation (or extrapolation).

[0098] (4) Refer to the provisions of the "Detailed Rules for Prevention and Control of Water in Coal Mines" regarding the "critical water inrush coefficient". T The s-value should be determined based on local data. Generally, it is calculated at 0.06 MPa / m for sections where the base plate is structurally damaged, and at 0.1 MPa / m for sections where the aquitard is intact and structurally undamaged. The critical inrush coefficient is selected based on the structural complexity near the collapse column. T The s-value was calculated, and the thickness of the floor aquitard near the collapse column to ensure safe mining of the coal seam was determined. M :

[0099] (2);

[0100] In the formula: M —Thickness of the waterproof layer on the bottom plate, in meters;

[0101] P S —Safe head value of the bottom slab waterproof layer, in meters;

[0102] T S —Critical inrush coefficient, MPa / m; A 0 — Safety factor, with a value of 1.2 to 1.5.

[0103] (5) Calculate the elevation of the original coal seam floor without a developed collapse column at point O, the center position of the collapse column, based on the contour lines of the minable coal seam floor near the collapse column [i.e., the elevation of the coal seam floor when the collapse column is not developed at the main borehole location]. c M 】, Intersection of the horizontal projection of each directional branch hole and the boundary of the collapse column C i ( i Elevation of the coal seam floor at positions (=1, 2, 3...) C Ci ( i =1, 2, 3...), and according to M , C Ci Calculate the final depth of the main hole and determine the final elevation of each directional branch hole. F Ci Note: The original elevation of the bottom plate of the coal seam without collapse columns at each point was obtained by interpolation based on the contour map of the coal seam bottom plate.

[0104] ① Final depth of main hole:

[0105] (3);

[0106] In formula (3): d M —Final depth of the main hole, in meters;

[0107] z M —Ground elevation of the main borehole opening location, in meters;

[0108] c M —Calculate the elevation of the coal seam floor when the collapse column is not developed at the location of the main borehole, in meters.

[0109] ② Final elevation of each directional branch hole:

[0110] (4);

[0111] In equation (4): C Ci —Intersection of the branch hole and the boundary of the collapse column C i ( iElevation of the coal seam floor at positions (=1, 2, 3...), in meters; F Ci —Elevation of the final branch hole, in meters.

[0112] (6) Based on the performance of the directional drilling equipment, select the directional branch hole directional section directional radius (or dog leg angle) and design the trajectory of each directional branch hole.

[0113] ① Vertical displacement of each directional branch hole:

[0114] (5);

[0115] In the formula: V i —Vertical displacement of directional branch holes ( i =1, 2, 3...), m; r —The directional branch hole drilling radius is generally taken as ≥200 m, that is, the dog leg angle is ≤8.60° / 30 m;

[0116] ② Length of each directional branch hole:

[0117] (6);

[0118] In the formula: L i —Length of directional branch hole ( i =1, 2, 3...), m; r —The directional branch hole inclination radius shall be ≥200 m, i.e., the dogleg angle ≤8.60° / 30 m.

[0119] ③ Side drilling elevation of each directional branch hole:

[0120] (7);

[0121] In the formula: S i —Design the side drilling elevation of the branch hole ( i =1, 2, 3...), m;

[0122] C Ci —The elevation of the coal seam at the intersection of the projection line of the branch hole on the contour line of the coal seam floor and the boundary of the collapse column ( i =1, 2, 3...), m.

[0123] (7) Construction of the main hole

[0124] The main hole is a three-section drilling structure: Section 1, hole diameter... F311.1 mm, hole depth 30–50 m, lowered F 244.5×8.94mm casing, and cemented the entire length of the casing to the borehole opening with cement grout; second opening, borehole diameter F 215.9 mm, borehole depth 10–20 m below bedrock surface, lowered into F 177.8×8.05 mm casing, with the entire length of the casing secured to the borehole opening using cement grout; three openings, borehole diameter... F 152.4 mm, bare hole. The main hole structure can be adjusted according to formation conditions, drilling rig and drilling tool performance, etc.

[0125] After drilling into the bedrock, core samples were taken from the entire borehole to determine the location of the top boundary of the collapse column. I ( x I , y I , z I After entering the collapse column, carefully observe the core characteristics (such as the continuity of the strata, the shape of the rock blocks, striations, filling materials, etc.), borehole leakage, and changes in the water level in the borehole after drilling stops; conduct a hydrogeological test (such as pumping test, water injection sample, water pressure test, etc.) every 10-50m; obtain hydrogeological parameters (such as single-hole water yield, permeability coefficient, water permeability, etc.) based on borehole leakage, changes in the water level in the borehole after drilling stops, and hydrogeological tests, and evaluate the water-bearing and water-conducting properties of the hydrogeological test section of the collapse column.

[0126] ① Pumping test

[0127] When the hydrogeological test section of the main borehole collapse column is water-bearing, a pumping test can be conducted.

[0128] First, determine the stable water level of the collapse column;

[0129] Secondly, in accordance with the "Specification for Pumping Tests" (YS / T 5215-2021), a single-hole pumping test was conducted, and the unit inflow rate of the borehole was measured. q Permeability coefficient K ;

[0130] (i) Single-hole flow rate q Measurement

[0131] According to the "Detailed Rules for Water Prevention and Control in Coal Mines," "When evaluating the water-bearing capacity of an aquifer, the unit water inflow rate of the borehole should be based on a borehole diameter of 91 mm and a drawdown of 10 m. If the borehole diameter and drawdown do not match the above, they should be converted before comparing the water-bearing capacity. The conversion method is as follows: first, calculate the water inflow rate during pumping." Q and descent depth S Based on the data, determine the curve using the least squares method or graphical method, according to... QSThe curve determines the inflow rate of the pumping hole at a drawdown of 10 m. Then, the inflow rate for a 91 mm orifice diameter is calculated using the following formula. Finally, dividing by 10 m gives the unit inflow rate. q .

[0132] (8);

[0133] In the formula: Q 91 、R 91 、r 91 —Water inflow, radius of influence, and borehole radius for a 91mm diameter borehole;

[0134] Q、R、r h —The proposed conversion of borehole water inflow, radius of influence, and borehole radius.

[0135] (ii) Permeability coefficient K Measurement

[0136] Since a pumping test is conducted every 10–50 m after the exploratory borehole enters the collapse column, the pumping test is a single-hole, confined water, incomplete well pumping test, and its permeability coefficient is... K It is advisable to calculate using the following formula:

[0137] (9);

[0138] In the formula: K —Permeability coefficient of the pumping section of the collapse column, m / d;

[0139] Q —Water flow rate from the pumping hole, in meters (m) 3 / d;

[0140] r h —The borehole radius to be converted, in meters;

[0141] S —Drawdown, in meters.

[0142] Finally, referring to the water-bearing capacity classification standard of aquifer in Appendix 1 of the "Detailed Rules for Water Prevention and Control in Coal Mines" (Table 1) and the "Rock Permeability Classification Table" in the "Coal Mine Water Prevention and Control Handbook", the water-bearing capacity and permeability (or water conductivity) of the hydrogeological test section of the collapse column were evaluated.

[0143] Table 1. Criteria for classifying the water-bearing capacity of formations

[0144]

[0145] Table 2. Classification of Rock Permeability

[0146]

[0147] ② Water injection test

[0148] Water injection testing is a method for approximating the permeability coefficient of a rock formation when the groundwater level is relatively deep or the test layer is a permeable but aquifer. The test method is the opposite of pumping testing; water is injected into a borehole to raise the water level, causing water to flow from the borehole towards the surrounding aquifer, forming an inverted funnel-shaped surface centered on the borehole. After injecting a certain amount of water into the borehole to raise the water level to a certain height, and when the water level and injection volume stabilize, the permeability coefficient K of the rock formation can be calculated using the water injection well formula.

[0149] Water injection tests were conducted on the borehole-exposed collapse column in accordance with the "Specifications for Curtain Grouting in Mines" (DZ / T 0285-2015), "Test Procedures for Water Injection in Water Conservancy and Hydropower Projects" (SL345-2007), and "Test Procedures for Water Injection" (YS / T 5214-2021). The permeability of the test section was... q P Calculate using the following formula:

[0150] (10);

[0151] In the formula: q P —Permeability of the test section, Lu or L / (min•MPa•m);

[0152] Q I —Stable flow rate during water injection test, m 3 / d;

[0153] H —The effect on the water level rise in the test section, i.e., the test head height difference, in meters;

[0154] L — The length of the test section, in meters.

[0155] ③ Pressure test

[0156] A water pressure test is an in-situ permeability test that uses a plug to isolate a certain length of borehole and pressurize it with water. The relationship between pressure and flow rate is used to determine the permeability of the rock mass. Its main task is to measure the unit water absorption and permeability of the rock mass, and then calculate the permeability coefficient to illustrate the permeability and fracture nature of fractured rock masses.

[0157] According to the "Specification for Pressure Water Test" (YS / T 5216-2020), a pressure water test is conducted on the collapsed column exposed in the borehole. The pressure water test is generally carried out in three pressure levels and five stages, namely... p 1→ p 2→p 3→ p 4 （=p 2 ） → p 5 （p 1 ），p 1 ＜p 2 ＜p 3, of which p 1 、 p 2 、p 3. The three pressure levels were 0.3 MPa, 0.6 MPa, and 1.0 MPa, respectively. The unit water absorption was measured. oh Permeability of the test section q :

[0158] (11);

[0159] (12);

[0160] In the formula: oh —Unit water absorption, L / (min•m) 2 );

[0161] Q 3—Stable flow rate in the third stage of the test, L / min;

[0162] L —Length of the test section, in meters;

[0163] H 3—Third-stage test head, m;

[0164] q P —Permeability of the test section, Lu or L / (min•MPa•m);

[0165] P 3 — The test pressure for the third stage, in MPa.

[0166] At the same time, based on the collapse column revealed by the borehole, the permeability coefficient can be determined. K The following formula can be used for approximate calculation:

[0167] (a) When the depth of the bottom of the test section from the waterproof layer is greater than the length of the test section:

[0168] (13);

[0169] (b) When the depth of the bottom of the test section from the waterproof layer is less than the length of the test section:

[0170] (14);

[0171] (c) The test section is located below the groundwater level, with low permeability and a pressure-flow ratio (P / F). P - Q When the curve is laminar:

[0172] (15);

[0173] In the formula: K —Permeability coefficient, m / d;

[0174] Q P —Push-in flow rate, m 3 / d;

[0175] H —Test water head height difference, m;

[0176] L —Length of the test section, in meters;

[0177] r h — Drilling radius, in meters.

[0178] The permeability of the test section of the collapse column can be evaluated with reference to the table below.

[0179] Table 3. Classification of permeability of soil and rock masses

[0180]

[0181] (8) Construction of each directional branch hole

[0182] Based on the actual collapse column conditions revealed by the main borehole (e.g., the top boundary depth of the collapse column, the coal seam depth, etc.), optimize the directional branch borehole parameters (e.g., side-drilling opening depth, final borehole depth, etc.). Each branch borehole is a bare hole, with the same borehole diameter as the main borehole's three-section borehole. Drill each branch borehole sequentially from bottom to top, according to its opening (side-drilling) position in the main borehole. Since coring is difficult in the branch boreholes, comprehensive borehole logging (e.g., electrical logging, sonic logging, and radioactive logging), drilling rock powder analysis, and drilling pressure are used to comprehensively determine the location of the collapse column along the branch borehole trajectory. I i ( x i , y i , z i () i =1, 2, 3...), the location of drilling out of the sinkhole pillar O j ( x j , y j ,z j () j =1, 2, 3...).

[0183] After each branch borehole enters the collapse column, a hydrogeological test (such as pumping test, water injection sample, water pressure test, etc.) is conducted every 10-50m of drilling. The test procedures and evaluation methods are the same as those for the main borehole. Based on the leakage of each directional branch borehole, the change of water level in the borehole after drilling stops, and the hydrogeological tests, hydrogeological parameters (such as single-hole inflow, permeability coefficient, permeability, etc.) at different locations of the collapse column are obtained.

[0184] (9) The location of the top boundary of the collapsed column revealed by the main borehole. I ( x I , y I , z I ), the location of each branch borehole drilled into the collapse column I i ( x i , y i , z i ) and the location of the drilling site collapse column O j ( x j , y j , z j ), delineate the area below the floor of the normal coal seam near the collapse column using 3D seismic exploration. M The actual spatial form of the collapsed column, extending from its depth to the top of the collapsed column.

[0185] Based on the hydrogeological test results of the main borehole and branch boreholes in each segment of the collapse column, contour maps of various hydrogeological parameters in the collapse column (such as single borehole inflow, permeability coefficient, water permeability, etc.) are drawn. Referring to Tables 1 to 3, the water-bearing capacity, water conductivity and water inrush risk of the collapse column to the mining of surrounding coal seams are comprehensively evaluated; the accuracy and reliability of the three-dimensional seismic exploration are compared and analyzed.

[0186] In this embodiment, the major axis of the collapse column is 172m. The coalfield collapse column detection and evaluation method based on ground directional drilling technology in this embodiment can effectively improve the accuracy of collapse column interpretation.

[0187] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology, characterized in that, The steps include the following: (1) Using three-dimensional seismic exploration to delineate the extension and distribution of the collapse column on the contour map of the bottom plate of the mineable coal seam, determine the center point of the collapse column on the contour map of the bottom plate of the mineable coal seam. O ( x , y , z The location of the main borehole is determined based on the center point. (2) Determine the number of directional branch holes for the collapse column, and design the horizontal projection orientation and horizontal displacement of the directional branch holes; (3) Analyze and calculate to determine the distance between the coal seam floor near the collapse column and the Ordovician limestone aquifer, as well as the water head height of the Ordovician limestone aquifer; (4) Determine the critical water inrush coefficient and calculate the thickness of the safe water-resistant layer on the coal seam floor. M ; (5) Calculate the final depth of the main borehole for the collapse column exploration and determine the final elevation of the directional branch borehole; (6) Determine the directional branch hole directional drilling radius or dogleg angle, and the trajectory of each directional branch hole; (7) Construct the main borehole. After core sampling in the main borehole, determine the location of the top boundary of the collapse column based on the core sample and conduct hydrogeological tests on the collapse column section. (8) Construct directional branch holes, determine the upper interface and the edge interface of the collapse column, and conduct hydrogeological tests on the collapse column section; (9) Based on the determined location of the top boundary of the collapse column, the upper interface and the edge interface of the collapse column, determine the area below the bottom of the coal seam. M The actual spatial morphology of the collapse column, extending to the top boundary of the collapse column, is compared and analyzed with the location of the collapse column delineated by 3D seismic exploration. Based on the results of hydrogeological tests, the water-bearing capacity, water-conductivity of the collapse column, and the risk of water inrush to the mining of surrounding coal seams are analyzed and evaluated. In step (1), the opening location of the main borehole is the center point of the collapse column delineated by the three-dimensional seismic exploration. O ( x , y , z The projection point on the ground; the main borehole is a vertical borehole; In step (2), the number of directional branch holes and the azimuth and horizontal displacement of the horizontal projection of the directional branch holes are determined based on the extension range of the collapse column on the contour map of the mineable coal seam floor delineated by the three-dimensional seismic exploration. The number of directional branch holes is 4 to 8. Each directional branch hole is drilled from the side of the exploration main hole. The horizontal projection length of the directional branch hole exceeds the projection boundary of the collapse column on the coal seam floor by 10 to 20 m. In step (3), the distance between the bottom of the normally mineable coal seam near the collapse column and the underlying Ordovician limestone aquifer is calculated based on the coal mine geological data and hydrogeological data, as well as the water head height of the Ordovician limestone aquifer at the center of the collapse column; the coal mine geological data and hydrogeological data include the coal seam bottom contour map, borehole columnar section, mine water-bearing map, Ordovician limestone aquifer contour map and Ordovician limestone roof contour map; The formula for calculating the horizontal displacement of each directional branch hole is as follows: （1）； In formula (1): H i —Horizontal displacement of the directional branch hole, i =1, 2, 3, ..., m; —Horizontal distance, in meters, from the center of the collapse column projected onto the contour map of the coal seam floor to the intersection of the directional branch hole and the boundary of the collapse column; —Directional branch holes exceed The length, in meters; Take 10-20; For a collapse column with a major axis diameter of less than 100m, four directional branch holes are drilled on the side of the main hole section. The horizontal projections of the four directional branch holes are in the major axis and minor axis directions, respectively. For a near-elliptical collapse column with a major axis diameter greater than 100m, eight directional branch holes are drilled on the side of the main borehole section. The orientation of the major axis with an angle of less than 90° between the horizontal projection of the collapse column and the due north direction is taken as the orientation of the first branch hole. A directional branch hole is designed every 45° in a clockwise or counterclockwise direction. For irregularly shaped collapse columns with a major axis diameter greater than 100m, the number of directional branch holes is designed according to the planar projection shape of the collapse column, so that the projection distance between the final hole positions of any two adjacent directional branch holes on the horizontal plane is equal.

2. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 1, characterized in that, In step (4), the critical water inrush coefficient for the section where the coal seam floor is damaged by tectonic activity is 0.06 MPa / m, and the critical water inrush coefficient for the section where the coal seam floor aquitard is intact and there is no tectonic damage is 0.10 MPa / m. Coal seam floor safety waterproof layer thickness M The calculation formula is: （2）； In formula (2): P S —Safe head value of the bottom slab waterproof layer, in meters; T S —Critical inrush coefficient, MPa / m; A 0 — Safety factor, with a value of 1.2 to 1.

5.

3. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 1, characterized in that, In step (5), the elevation of the coal seam floor when the collapse column is not developed is calculated based on the contour lines of the minable coal seam floor near the collapse column. c M Intersection of the horizontal projection of each directional branch hole and the boundary of the collapse column C i Elevation of the coal seam floor at the location C Ci , i =1, 2, 3... and based on the thickness of the safety waterproof layer of the coal seam floor. M and C Ci Calculate the final depth of the main hole and determine the final elevation of each directional branch hole. F Ci ; The formula for calculating the final depth of the main hole is: （3）； In formula (3): d M —Final depth of the main hole, in meters; z M —Ground elevation of the main borehole opening location, in meters; c M —Elevation of the coal seam floor when the collapse column is not developed at the main borehole location, in meters; M —Thickness of the safety waterproof layer on the bottom of the coal seam; The formula for calculating the final elevation of each directional branch hole is as follows: （4）； In equation (4): C Ci —Intersection of the horizontal projection of the directional branch hole and the boundary of the collapse column C i Elevation of the coal seam floor at the location, in meters; i =1, 2, 3...; F Ci — Final elevation of the directional branch hole, in meters; M —Thickness of the safety waterproof layer on the bottom of the coal seam.

4. The North China-type coalfield collapse column based on ground directional drilling technology according to claim 1 Explore The method of investigation and evaluation is characterized by, In step (6): ① Vertical displacement of each directional branch hole V i The calculation formula is: （5）； In equation (5): V i —— Vertical displacement of the directional branch hole, in meters (m). i =1, 2, 3...; r —The directional branch hole's slant radius is greater than or equal to 200 m, i.e., the dogleg angle is less than or equal to 8.60° / 30 m; H i —Horizontal displacement of the directional branch hole, in meters; i =1，2，3……； ② Length of each directional branch hole L i The calculation formula is: （6）； In formula (6): L i — Length of directional branch hole, in meters. i =1, 2, 3...; r — The deflection radius of the directional branch hole, in meters; H i —Horizontal displacement of the directional branch hole, in meters; ③ Side drilling elevation of each directional branch hole S i The calculation formula is: （7）； In equation (7): S i —Elevation of the side drilling of the directional branch hole, in meters; i =1，2，3……， C Ci —Intersection of the horizontal projection of the directional branch hole and the boundary of the collapse column C i The elevation of the coal seam floor at the location, in meters. i =1, 2, 3...; M —Thickness of the safety waterproof layer on the bottom of the coal seam; V i —— Vertical displacement of the directional branch hole, in meters (m). i =1，2，3……。 5. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 1, characterized in that, In step (7), the main hole is a three-section drilling structure: the first section has a diameter of... Φ 311.1 mm, hole depth 30–50 m, lowered Φ 244.5×8.94 mm casing, and cemented the entire length of the casing to the borehole opening with cement grout; second opening, borehole diameter Φ 215.9 mm, borehole depth 10–20 m below bedrock surface, lowered into Φ 177.8×8.05 mm casing, with the entire length of the casing secured to the borehole opening using cement grout; three openings, borehole diameter... Φ 152.4 mm, bare hole; After drilling into the bedrock, core samples were taken from the entire borehole to determine the location of the top boundary of the collapse column. I ( x I , y I , z I After entering the collapse column, observe the core characteristics and the changes in borehole leakage and water level after drilling stops; core characteristics include the continuity of the strata, the shape of the rock blocks, striations, and filling materials; A hydrogeological test is conducted every 10–50 m of drilling. This test includes one or more of the following: pumping test, water injection test, and water pressure test, to obtain hydrogeological parameters. These parameters include the single-hole yield. q Permeability coefficient K and permeability q P .

6. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 5, characterized in that, In step (7), when the unit water inflow of the borehole is determined by a pumping test... q and permeability coefficient K hour: A single-hole pumping test was conducted according to the "Pumping Test Specification" -- YS / T 5215-2021; the pumping rate was determined based on the inflow rate during pumping. Q and water level drawdown S Based on the data, determine the curve using the least squares method or graphical method, according to... QS The curve determines the flow rate of the pumping hole when the water level drawdown is 10 m. Then, the flow rate when the hole diameter is 91 mm is calculated using formula (8). Finally, the flow rate is divided by 10 m to obtain the unit flow rate. q ; （8）； In equation (8): Q 91 、R 91 and r 91 —These represent the water inflow, radius of influence, and borehole radius for a 91mm borehole; Q, R and r h —These represent the water inflow, radius of influence, and borehole radius to be converted; Permeability coefficient K Calculate using the following formula: （9）； In equation (9): K —Permeability coefficient of the pumping section of the collapse column, m / d; Q —The proposed conversion of borehole water inflow, in meters. 3 / d; r h —The borehole radius to be converted, in meters; S —Drawdown, meters; In step (7), the water permeability is calculated using the following formula during the water injection test; （10）； In formula (10): q P —Permeability of the test section, Lu or L / (min•MPa•m); Q I —Stable flow rate during water injection test, m 3 / d; H —Test water head height difference, m; L — The length of the test section, in meters (m).

7. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 5, characterized in that, In step (7), a water pressure test is conducted on the borehole exposed collapse column according to the "Water Pressure Test Procedure" -- YS / T 5216-2020. The water pressure test is carried out in three pressure levels and five stages, namely... p 1→ p 2→ p 3→ p 4 or p 2→ p 5 or p 1 ，p 1 ＜p 2 ＜ p 3, of which p 1 、p 2 、p 3. The three pressure levels were 0.3 MPa, 0.6 MPa, and 1.0 MPa, respectively. The unit water absorption was measured. ω Permeability of the test section q P ; （11）； （12）； In equations (11) and (12): ω —Unit water absorption, L / (min•m) 2 ); Q 3—Stable flow rate in the third stage of the test, L / min; L —Length of the test section, in meters; H 3—Third-stage test head, m; q P —Permeability of the test section, Lu or L / (min•MPa•m); P 3 — The test pressure for the third stage, in MPa; Permeability coefficient K Calculate using the following formula: (a) When the depth of the bottom of the test section from the waterproof layer is greater than the length of the test section: （13）； (b) When the depth of the bottom of the test section from the waterproof layer is less than the length of the test section: （14）； (c) The test section is located below the groundwater level, with low permeability and a pressure-flow rate (P / F) ratio. P - Q When the curve is laminar: （15）； In equations (13), (14), and (15): K —Permeability coefficient, m / d; ω —Unit water absorption, L / (min•m) 2 r—the deflection radius of the directional branch hole, in meters; Q P —Push-in flow rate, m 3 / d; H —Test water head height difference, m; L —Length of the test section, in meters; r h —The borehole radius to be converted, in meters.

8. The method for detecting and evaluating collapse columns in North China-type coalfields based on ground directional drilling technology according to claim 1, characterized in that, In step (8), based on the actual burial depth of the top boundary of the collapse column and the burial depth of the coal seam revealed by the main borehole, the side drilling opening depth and final drilling depth of the directional branch boreholes are optimized; each directional branch borehole is a bare hole, and the borehole diameter is the same as that of the three-section borehole of the main borehole; according to the opening position of each directional branch borehole in the main borehole, each branch borehole is constructed sequentially from bottom to top, and the location of the collapse column drilled on the trajectory of each directional branch borehole is determined by comprehensive borehole logging, drilling rock powder analysis and drilling pressure. I i ( x i , y i , z i ), i =1, 2, 3... and the location where the collapse pillar was drilled out. O j ( x j , y j , z j ), j =1, 2, 3...; After each directional branch borehole enters the collapse column, a hydrogeological test is conducted every 10–50 m of drilling. The test procedures and evaluation methods are the same as those for the main borehole. Based on the leakage of each directional branch borehole, the changes in the water level in the borehole after drilling stops, and the hydrogeological tests, the hydrogeological parameters at different locations of the collapse column are obtained.

Images