Square hole drilling equipment based on lelo triangle principle and deep high stress hard roof blasting weakening method

Abstract

The embodiment of the application provides square hole drilling equipment based on the Lelos triangle principle and a deep high ground stress hard roof blasting weakening method, and relates to the field of deep rock mass drilling and blasting excavation. The method comprises the following steps: determining the hole parameters of the square hole arranged on the hard roof according to the square hole blasting crack propagation growth law under different ground stress states and the thickness of the hard roof; drilling multiple groups of square holes by using a square hole drill bit according to the hole parameters; initiating each group of holes, and forming a stress field superposition area along the extension line direction of the sharp end of the square hole in the propagation process of the explosion stress wave; guiding and controlling the first directional main crack to grow in the extension line direction of the sharp end of the square hole by using the formed stress field superposition area; and forming a second directional main crack in the extension line direction of the opposite sharp end of the adjacent hole, which grows towards the first directional main crack to form a mutual penetration fracture network. The blasting weakening effect of the hard roof can be improved.

Description

Square borehole drilling equipment based on the Reuleaux triangle principle and a method for weakening the hard roof during deep high-stress blasting. Technical Field

[0001] This invention relates to the field of deep rock drilling and blasting, and particularly to a square blast hole drilling device based on the Reuleaux triangle principle and a method for weakening the hard roof under high ground stress in deep rock. Background Technology

[0002] Although my country has been vigorously promoting the use of new energy sources in recent years, coal still dominates the national economic development, accounting for about 67% of the energy structure. The proportions of nuclear power, hydropower, and solar energy remain relatively low. It is foreseeable that coal will continue to be a major energy source for decades to come. Currently, coal mining has entered deep-sea operations, and many problems associated with deep mining are becoming increasingly prominent. For example, high ground stress is a particularly challenging issue in deep coal mining. Coal safety during deep mining is of paramount importance and cannot be ignored. Roof collapse accidents account for a large proportion of coal mine accidents, especially when the coal seam roof is located on a thick layer of hard sandstone or other solid rock.

[0003] Among them, a hard roof refers to a roof with strong self-bearing capacity located above the coal seam to be mined. Its main characteristics are a relatively large roof thickness, good overall strength and integrity, and poor development of weak surfaces such as joints and fissures. When the roof of a large-area goaf cannot collapse naturally for a long time, the pressure in the rock mass above the mining area will be highly concentrated. When the pressure on the roof exceeds the support strength, the roof will undergo shear failure under high pressure. At the same time, due to the large self-weight of the hard roof, when the self-weight of the roof exceeds the strength of the upper part of the roof, tensile stress will be generated, causing tensile failure. When a large area of ​​suspended roof suddenly collapses, the rock strata break and generate strong dynamic loads, causing a large number of working face supports to be pushed over or damaged, resulting in working face collapse and cave-in accidents. This can lead to a series of secondary disasters, such as the formation of extremely destructive hurricanes and rock bursts. The huge kinetic energy carried by the hurricane's airflow often destroys supports, air doors, and brick wall seals in the mining face and its adjacent roadways. More seriously, it may cause gas to gush out instantly, causing major gas accidents.

[0004] Weakening a hard roof using blasting is a highly technical operation, the key being how to control the collapse volume of the waste rock to ensure it fully fills the goaf. To control the roof collapse area and surrounding rock stability, a pre-splitting blasting control technique based on circular boreholes is typically employed. However, for blasting weakening of deep, high-stress hard roofs, the influence of ground stress on crack propagation cannot be ignored, and this is a crucial factor that needs to be considered during the blasting process. Summary of the Invention

[0005] In view of this, the deep high-stress hard roof blasting weakening method based on square blast holes provided by the embodiments of the present invention, which takes into account the influence of ground stress on the blasting crack direction of hard roof in deep high-stress rock tunnels, can improve the blasting weakening effect of hard roof.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] A square blast hole drilling device based on the Reuleaux triangle principle and a method for weakening the hard roof in deep high ground stress by blasting include the following steps: determining the ground stress state of the hard roof in the roadway and the blasting treatment height;

[0008] Based on Reuleaux's triangle principle, a square borehole drill bit is designed so that the drill bit's rotation and revolution are in opposite directions.

[0009] Based on the obtained crack propagation and growth patterns of square blast holes under different geostress states and the thickness of the hard top plate, the blast hole parameters for laying square blast holes on the hard top plate are determined.

[0010] According to the aforementioned borehole parameters, multiple sets of square boreholes are drilled on a hard top plate using the aforementioned square borehole drill bit. Each set of square boreholes includes multiple square boreholes, which are arranged in a square pattern. The tips of adjacent boreholes within a set are positioned opposite each other, and the tips of adjacent boreholes between sets are positioned opposite each other.

[0011] Explosives were loaded into each set of square blast holes, and detonators were installed.

[0012] When the square boreholes are detonated, the explosive stress wave forms a stress field superposition zone along the extension line of the tip of the square borehole during its propagation.

[0013] By utilizing the stress field superposition zone formed, the growth of the first directional main crack is guided and controlled to extend along the extension line of the tip of the square borehole; at the same time, the second directional main cracks formed by extending along the relative extension lines of the tips of adjacent boreholes grow towards the first directional main cracks, forming an interconnected fracture network to weaken the hardness of the hard top plate.

[0014] Optionally, the borehole parameters include: the spacing between adjacent boreholes laterally and the row spacing between adjacent boreholes longitudinally;

[0015] The process of determining the borehole parameters for laying square boreholes on a hard top plate based on the obtained crack propagation growth law of square boreholes under different ground stress states and the thickness of the hard top plate includes: conducting experimental research on the relationship between different ground stress states and the corresponding crack propagation growth law of square boreholes in advance, and analyzing the correlation between different ground stress states and the corresponding crack propagation growth law of square boreholes.

[0016] The geostress state at the location of the rigid top plate is measured, including the geostress in the horizontal axis direction and the geostress in the vertical axis direction.

[0017] Based on the correlation obtained from the analysis and the stress state of the location of the hard top plate, the spacing between adjacent lateral boreholes and the row spacing between adjacent longitudinal boreholes that need to be laid on the hard top plate are determined.

[0018] When the measured ground stress in the horizontal axis direction is equal to the ground stress in the vertical axis direction, the spacing between the lateral adjacent boreholes and the row spacing between the longitudinal adjacent boreholes are arranged to be equal.

[0019] When the measured horizontal stress is equal to the vertical stress, the spacing between adjacent blast holes in the lateral direction is arranged to be greater than the spacing between adjacent blast holes in the longitudinal direction.

[0020] When the measured stress in the horizontal axis direction is small

[0021] When the ground stress is equal in the vertical axis direction, the spacing between adjacent lateral boreholes should be reduced.

[0022] The arrangement of adjacent boreholes in the longitudinal direction;

[0023] Furthermore, based on the magnitudes of the horizontal and vertical axial stresses, the spacing between adjacent lateral boreholes and the row spacing between adjacent longitudinal boreholes are determined, so that after each group of boreholes is detonated, the directional main cracks formed by adjacent groups of boreholes interconnect to form a directional fracture network.

[0024] Optionally, the spacing between adjacent lateral boreholes is set to 2l. d +D, the longitudinal spacing between adjacent boreholes is 2lc+D, where l d lc represents the length of the directional main crack along the horizontal axis, lc represents the length of the directional main crack along the vertical axis, and D represents the length of the diagonal of the square borehole.

[0025] Optionally, the borehole parameters further include: borehole depth;

[0026] The step of determining the borehole parameters for laying square boreholes on a hard top plate based on the obtained crack propagation growth law of square boreholes under different ground stress states and the thickness of the hard top plate includes: measuring and obtaining the thickness of the hard top plate.

[0027] Based on the thickness of the rigid roof, the depth of the square blast hole is determined according to the depth selection condition formula, wherein the depth selection condition formula is: a≧0.7*H, where a is the depth of the square blast hole and H is the thickness of the rigid roof above the goaf.

[0028] Optionally, the borehole parameters also include: the included angle between adjacent groups of boreholes, the borehole elevation angle and the borehole side length. The multiple boreholes are distributed in a fan shape. The included angle between adjacent groups of boreholes is 10 to 15 degrees. The borehole elevation angle is 29 to 74 degrees and is inclined upward in the direction opposite to the mining face. The borehole side length is 80 to 100 mm, and the charging method in the borehole is fully coupled charging.

[0029] Optionally, the drill bit includes: a disc-shaped body, with a mounting hole at the center of the disc-shaped body, and a cutting edge on the edge of the disc-shaped body. The outline of the cutting edge includes: three sets of circular arc segments that are connected to each other and arranged radially on the edge of the disc-shaped body. Each set of circular arc segments includes a first arc and a second arc connected to the first arc. The endpoints of the three sets of circular arc segments are located on the same circular envelope.

[0030] In this set of arc segments, the first arc is established with the central hole as the center of the coordinate system, with the center of the arc relative to the central hole as the origin, and the position with coordinates (-20.6785, -2.8367) relative to the origin as the first center.

[0031] A first arc is formed with the location of the first circle's center as the center and a predetermined radius.

[0032] And, with the location of the first circle center as the center, the position relative to the origin coordinates (-7.1173, 5.7975) is the location of the second circle center, and the radius is a predetermined radius to form a second arc;

[0033] The first arc is connected to the second arc, but their concave directions are opposite.

[0034] Optionally, drilling multiple sets of square blast holes in a hard top plate using the square blast hole drill bit includes: mounting the drill bit on a drilling machine that can rotate and revolve, making the rotation and revolution directions of the drill bit opposite, with a speed ratio of 1:3, and drilling square blast holes in deep rock mass through rotation and propulsion.

[0035] Optionally, the square bore can be a square bore or a rhombus bore, wherein the four tips of the rhombus bore are located on the x and y axes respectively, and the four tips of the square bore are located in the positive and negative 45° directions of the x and y axes respectively.

[0036] Optionally, the measurement of the geostress state at the location of the rigid roof includes:

[0037] Drill a separate square hole in the hard top plate, load it with the same amount of explosives as those used in the subsequent blasting, and place a detonator to detonate it, while monitoring the distribution of cracks in the top plate.

[0038] Based on the crack distribution, the stress state of the hard roof in the tunnel is determined.

[0039] Optionally, determining the stress state of the hard roof in the tunnel based on the crack distribution includes: if the directional main crack extends along the extension line of the tip of the square blast hole, and the directional main crack does not deflect at a predetermined angle, and the shape of the directional main crack is approximately × when the square blast hole is a square blast hole, or approximately + when the square blast hole is a rhombus blast hole, then the stress σh1 in the horizontal axis direction is equal to the stress σh2 in the vertical axis direction.

[0040] If the directional main crack extends along the extension line of the tip of the square borehole and deflects at a predetermined angle in the x-axis direction, or if the directional main crack grows in the x-direction, then it is determined that the ground stress σh1 in the horizontal axis direction is greater than the ground stress σh2 in the vertical axis direction.

[0041] If the directional main crack extends along the extension line of the tip of the square borehole and deflects in the y direction, or if the directional main crack grows in the y direction, then the stress σh1 in the horizontal axis direction is less than the stress σh2 in the vertical axis direction.

[0042] The present invention provides a square blast hole drilling device based on the Reuleaux triangle principle and a method for weakening the hard roof in deep, high-stress rock tunnels. Targeting the hard roof in deep, high-stress rock tunnels, it comprehensively considers the influence of ground stress on the direction of blasting fractures in the hard roof. This is achieved by determining the ground stress state and blasting height of the hard roof in the tunnel; designing a square blast hole drill bit based on the Reuleaux triangle principle, with the drill bit's rotation and revolution directions opposite; determining the blast hole parameters for laying square blast holes in the hard roof based on the obtained fracture propagation and growth patterns of square blast holes under different ground stress states and the thickness of the hard roof; and drilling multiple sets of square blast holes in the hard roof using the square blast hole drill bit according to the blast hole parameters. Each set of square blast holes includes… The device includes multiple square blast holes arranged in a square pattern, with the tips of adjacent blast holes within a group facing each other, and the tips of adjacent blast holes between groups facing each other. Explosives are loaded into each group of square blast holes, and detonators are installed. The blast holes are detonated, and during the propagation of the explosive stress wave, a stress field superposition zone is formed along the extension line of the tip of the square blast hole. This stress field superposition zone is used to guide and control the growth of a first directional main crack along the extension line of the tip of the square blast hole. Simultaneously, a second directional main crack, formed by the growth of adjacent blast holes along their relative tip extension lines, grows towards the first directional main crack, forming a mutually interconnected directional fracture network. This weakens the hardness of the rigid roof, improving the blasting weakening effect. Attached Figure Description

[0043] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 is a schematic diagram of the crack distribution formed by the blasting of a circular blast hole.

[0045] Figures 2A and 2B are schematic diagrams showing the location and condition of the rigid roof in the tunnel;

[0046] Figure 3 shows the distribution of directional principal cracks in a square borehole (generally a square borehole) under the same horizontal and vertical stresses.

[0047] Figure 4 shows the distribution of directional main cracks in a square borehole under the action of in-situ stress in the horizontal axis direction greater than that in the vertical axis direction.

[0048] Figure 5 shows the distribution of directional main cracks in a square borehole under the action of ground stress in the horizontal axis direction being smaller than that in the vertical axis direction.

[0049] Figure 6 shows the distribution of directional main cracks in a rhomboid borehole (a special square borehole) under the same horizontal and vertical stresses.

[0050] Figure 7 shows the distribution of directional main cracks in a rhomboid borehole under the action of geostress in the horizontal axis direction greater than that in the vertical axis direction.

[0051] Figure 8 shows the distribution of directional main cracks in a rhomboid borehole under the action of ground stress in the horizontal axis direction being smaller than that in the vertical axis direction.

[0052] Figure 9 is a schematic diagram of the drill bit structure in one embodiment of the present invention;

[0053] Figure 10 is a schematic diagram illustrating the layout analysis of an embodiment of the rhomboid blast hole;

[0054] Figure 11 is a schematic diagram of an embodiment of blast hole layout on a hard top plate;

[0055] Figure 12 is a schematic diagram of the process of a square blasting hole drilling device based on the Reuleaux triangle principle and a method for weakening the hard roof plate under high ground stress in deep soil according to an embodiment of the present invention. Detailed Implementation

[0056] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0057] It should be understood that numerous technical details are described in the following specific embodiments to more clearly illustrate the present invention. Those skilled in the art should understand that the present invention can be implemented even without some of these details. Furthermore, to highlight the main points of the present invention, some methods, means, components, and applications well-known to those skilled in the art are not described in detail; however, this does not affect the implementation of the present invention. The embodiments described herein are merely some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0058] As resource extraction shifts from shallow to deep, coal mining depths have reached 1500m, geothermal and non-ferrous metal mining depths have exceeded 3000m and 4350m respectively, and oil and gas resource mining depths have reached 7500m. The term "deep" or "deep rock mass" lacks a clear and specific definition within the industry; it is used in contrast to "shallow" or "shallow rock mass." One of the main characteristics of deep rock masses is high ground stress. Some countries use several hundred meters as a boundary, while others use thousands of meters; the difference lies only in the stress values, but all share the characteristic of high ground stress in deep rock masses.

[0059] As shown in Figure 2A, during coal mining, as the working face advances, the hard roof in the goaf is difficult to collapse in time, resulting in large-area overhangs. When the vertical stress and horizontal thrust generated by the roof's rotation and subsidence act on the surrounding rock and backfill, the surrounding rock and backfill undergo large deformations and lose their load-bearing capacity. Figure 2A shows that without blasting pre-splitting of the roof, the hard roof fractures above the coal pillar and subsides. Due to the large impact force after fracture and subsidence, it quickly falls into the goaf, damaging the surrounding rock of the roadway.

[0060] As shown in Figure 2B, when blasting technology is used to weaken the hard roof, the hard roof fractures promptly at the roof cutting line, significantly reducing the exposed length of the roof. This releases the pressure acting on the surrounding rock of the roadway, ensuring roadway stability, optimizing the stress and structure of the remaining roadway surrounding rock, alleviating stress concentration problems, and reducing roadway surrounding rock deformation. Therefore, weakening the hard roof to cause it to collapse becomes a key point in solving mining safety hazards.

[0061] The square blasting equipment based on the Reuleaux triangle principle and the blasting weakening method for hard roofs in deep high-stress rock tunnels provided in this invention are applicable to blasting weakening and roof caving projects in deep high-stress rock tunnels.

[0062] The square blasting hole drilling equipment based on the Reuleaux triangle principle and the deep high-stress hard roof blasting weakening method provided by the embodiments of the present invention target the hard roof in deep high-stress rock tunnels. Taking into account the influence of ground stress on the direction of blasting fractures in the hard roof, the square blasting hole groups are reasonably arranged to form an interconnected directional fracture network, so as to weaken the hardness of the hard roof and improve the blasting weakening effect of the hard roof.

[0063] Prior to this application, in engineering practice, technologies such as the pre-splitting blasting control technology for hard roof loosening all used circular blasting holes to weaken the roof through blasting. However, as shown in Figure 1, the crack distribution in circular blasting holes lacks clear directionality, and the crack formation angle is random. Therefore, it is not conducive to controlling the direction of pre-splitting blasting weakening of hard roof loosening, and the blasting weakening effect needs to be improved.

[0064] The inventors of this application discovered in engineering practice that during the blasting of square boreholes, stress waves form superposition zones along the extended lines of the four apexes of the square borehole, resulting in stress concentration. This concentration guides the formation of blast cracks. By arranging borehole groups at different positions of the four apexes of the square borehole, the generation of blast cracks can be controlled in different directions, thus adapting to the blasting weakening of hard roofs under different working conditions. However, compared to circular boreholes, square boreholes present challenges in processing, including greater difficulty in machining, fewer drilling tools, and more complex processing techniques. This application addresses these issues by incorporating the unique geometric properties of the Reuleaux triangle into the design of drill bits for square boreholes, providing feasibility for blasting weakening of hard roofs with deep ground stress based on square boreholes.

[0065] The Reuleaux triangle, generally referring to the Reuleaux triangle, was first discovered and named after Franz Reuleaux (1829–1905), a renowned German mechanical engineer and kinematician. It is a curvilinear triangle formed by three arcs, each centered at a vertex of an equilateral triangle and with its side length as its radius.

[0066] Since the Reuleaux triangle is a curve of fixed width, meaning it can move between parallel lines of fixed width, if another set of parallel lines with the same spacing is set perpendicular to this set of parallel lines, the Reuleaux triangle can move within the square enclosed by the two sets of parallel lines. Based on this principle, a drill bit based on the shape of the Reuleaux triangle can be made to drill square blast holes.

[0067] Based on the above principles, a Reuleaux triangle drill bit is designed for drilling square boreholes, with the drill bit's dimensions corresponding to the side length of the square casing. To more intuitively aid in understanding the design of the square borehole drill bit, it is assumed that the machining dimensions of the square borehole are 40mm × 40mm. Therefore, the distance between two adjacent cutting edges of the drill bit is 40mm. The projections of the three end cutting edges are all formed by the envelopes of two circles with diameters φ of 47.2mm and 22.8mm.

[0068] Please refer to Figure 9. In some embodiments, the drill bit includes: a disc-shaped body with a mounting hole at the center of the disc-shaped body, and a cutting edge on the edge of the disc-shaped body. The outline of the cutting edge includes: three sets of arc segments that are connected to each other and arranged radially on the edge of the disc-shaped body. Each set of arc segments includes a first arc and a second arc connected to the first arc. The endpoints of the three sets of arc segments are located on the same circular envelope. The distance between the endpoints of adjacent sets of arc segments is approximately the same as the side length of the square blast hole to be drilled.

[0069] In this set of arc segments, the first arc is established with the central hole as the center of the coordinate system, with the center of the arc relative to the central hole as the origin, and the position with coordinates (-20.6785, -2.8367) relative to the origin as the first center.

[0070] A first arc is formed with the location of the first circle's center as the center and a predetermined radius.

[0071] And, with the location of the first circle center as the center, the position relative to the origin coordinates (-7.1173, 5.7975) is the location of the second circle center, and the radius is a predetermined radius to form a second arc;

[0072] The first arc is connected to the second arc, but their concave directions are opposite.

[0073] In the other two sets of arc segments, the center coordinates of the first arc are (9.9503, 15.2894) and (10.6265, -20.2164), respectively, and the center coordinates of the second arc are (-3.6285, -13.1836) and (10.5720, -0.5634), respectively. The radius of the first arc is 11.2, and the radius of the second arc is 23.6.

[0074] After solving the problem of drilling square blast holes, a large amount of related research was conducted on the crack distribution after blasting square blast holes in deep, high-stress rock tunnels.

[0075] For deep, unexcavated rock masses, they are generally in a triaxial stress state, i.e., horizontal stresses σh1 and σh2 and vertical stress σ v However, for deep rock tunnels beneath a hard roof, the stress in the vertical direction perpendicular to the roof is zero. Therefore, the roof of a deep rock tunnel is under bidirectional stress, i.e., subjected to horizontal stresses σh1 and σh2, which can be simplified to a two-dimensional plane strain problem.

[0076] Please refer to Figures 3 to 8. There are three cases of ground stress in the deep hard top plate: (1) σh1=σh2; (2) σh1>σh2; (3) σh1<σh2; where σh1 is the ground stress in the horizontal axis direction and σh2 is the ground stress in the vertical axis direction.

[0077] 1. Typical square blast holes (example of horizontally arranged square blast holes)

[0078] (1) σh1=σh2

[0079] Figure 3 shows the distribution of blast cracks in a square borehole under bidirectional isobaric stress conditions. As can be seen from the figure, since the horizontal stresses σh1 and σh2 are equal, the crack distribution is not significantly different from that under no-stress conditions. Four directional main cracks are formed along the four tips of the square borehole. However, since the bidirectional isobaric stress has a significant inhibitory effect on crack propagation in all directions, the crack length is significantly shorter than that under no-stress conditions.

[0080] (2) σh1>σh2

[0081] Figure 4 shows the distribution of blast cracks in a square borehole under the condition that the horizontal stress σh1 is greater than the horizontal stress σh2 (where the x-direction stress σh1 is the maximum principal stress). Compared with the conditions of no stress and bidirectional isobaric stress, the lengths of the four directional principal cracks in the square borehole are no longer straight, but rather arc-shaped. The formation of the directional principal cracks is the result of the combined action of the blast load and the ground stress in the square borehole. The directional principal cracks gradually deflect towards the direction of the maximum principal stress (x-direction), making the entire crack propagation path arc-shaped. In the early stage of crack propagation, the intensity of the blast load is much greater than that of the ground stress, so the crack propagation in the early stage is basically along the extension line of the tip of the square borehole. The crack propagation in this stage is approximately linear, and its length is denoted as l. d 1; In the later stage of crack propagation, the explosive load decays rapidly, and the guiding effect of the maximum principal stress in the geostress field on the directional main crack gradually becomes apparent, causing a significant deflection of crack propagation. The crack propagation in this stage is a distinctly curved shape, and its length is denoted as l. d 2. The turning point from straight to curved on the directional principal crack is denoted as M. Secondary cracks in other directions propagate towards the direction of maximum principal stress.

[0082] In addition, l d1 and l d2 The relative length of the explosive is determined by three main factors: explosive properties, rock properties, and geostress state. Therefore, in practical engineering, it is necessary to conduct preliminary experiments in situ to determine the length in advance. d1 and l d2 The length of the boreholes and the relative magnitude of these two values ​​are crucial for the realization of refined blasting weakening of hard roofs. Therefore, the rationality of the selection of borehole spacing directly affects whether the influence of ground stress on crack growth can be effectively utilized, and thus affects the effect of refined blasting weakening of hard roofs.

[0083] (3) σh1<σh2

[0084] Figure 5 shows the distribution of blast cracks in a square borehole under the condition that the horizontal stress σh1 is less than the horizontal stress σh2 (the stress σh2 in the y-direction is the maximum principal stress). The four directional principal cracks in the square borehole deflect towards the direction of the maximum principal stress (y-direction), and the secondary cracks in other directions also extend towards the direction of the maximum principal stress (y-direction). The specific explanation is similar to (2), and can be found above.

[0085] 2. (Rhombus) Square blast hole

[0086] (1) σh1=σh2

[0087] Figure 6 shows the distribution of blast cracks in a (rhomboid) square borehole under bidirectional isobaric stress conditions. Four directional main cracks form along the extension line of the borehole tip, while several shorter secondary cracks form in directions other than the tip extension line. Compared with conditions without ground stress, bidirectional isobaric stress restricts crack propagation, reduces the extent of blast-induced cracking, and significantly reduces the crack length of both directional main cracks and secondary cracks.

[0088] (2) σh1>σh2

[0089] Figure 7 shows the distribution of blast cracks in a square borehole (rhomboid shape) under the condition that the horizontal stress σh1 is greater than the horizontal stress σh2 (the stress σh1 in the x-direction is the maximum principal stress). Compared with the conditions of no stress and bidirectional isobaric stress, the length l of the directional principal crack in the x-direction is... d The length of the directional principal crack lc in the y-direction increases, while the length of secondary cracks in other directions deflects towards the direction of maximum principal stress (x-direction). The blasting effect under this stress state is objective, because the stress increases the length of the directional crack in the x-direction. In practical engineering, the borehole spacing can be appropriately increased or the charge amount reduced to lower the construction cost.

[0090] (3) σh1<σh2

[0091] Figure 8 shows the distribution of blast cracks in a (rhomboid) square borehole under the condition that the horizontal in-situ stress σh1 is less than the horizontal in-situ stress σh2 (where the y-direction stress σh2 is the maximum principal stress). Compared with the condition without in-situ stress, the directional principal crack length in the y-direction of the (rhomboid) square borehole increases, while the directional principal crack length in the x-direction decreases. Secondary cracks in other directions deflect towards the direction of the maximum principal stress. Under this condition, the effect of in-situ stress promotes the propagation of the directional principal crack in the y-direction, resulting in an increase in crack length.

[0092] The above description helps to further understand the technical solutions provided in the embodiments of the present invention.

[0093] After conducting the aforementioned engineering practice and theoretical research and obtaining corresponding research results, the inventors proposed a blasting weakening scheme that organically applies square blast holes to the hard roof in deep, high-stress rock tunnels, as shown in Figure 12. In some embodiments, the square blast hole drilling equipment based on the Reuleaux triangle principle and the blasting weakening method for hard roofs in deep, high-stress rock tunnels provided by the present invention include the following steps:

[0094] S10. Determine the geostress state of the hard roof in the roadway and the blasting height;

[0095] In this embodiment, the horizontal axial stress σh1 and vertical axial stress σh2 of the hard roof, as well as the thickness H of the hard roof above the goaf, can be measured based on factors such as the location, direction, and depth of the roadway. The blasting treatment height a is determined based on the thickness H of the hard roof, ensuring that a ≥ 0.7 * H to guarantee the weakening effect of the blasting.

[0096] In some embodiments, measuring the geostress state at the location of the rigid roof includes:

[0097] Drill a separate square hole in the hard top plate, load it with the same amount of explosives as those used in the subsequent blasting, and place a detonator to detonate it, while monitoring the distribution of cracks in the top plate.

[0098] Based on the crack distribution, the stress state of the hard roof in the tunnel is determined.

[0099] Specifically, judging the stress state of the hard roof in the tunnel based on the crack distribution includes: if the directional main crack extends along the extension line of the tip of the square blast hole, and the directional main crack does not deflect at a predetermined angle, and the shape of the directional main crack is approximately × when the square blast hole is a square blast hole, or the shape is approximately + when the square blast hole is a rhombus blast hole, then the stress σh1 in the horizontal axis direction is equal to the stress σh2 in the vertical axis direction.

[0100] If the directional main crack extends along the extension line of the tip of the square borehole and deflects at a predetermined angle in the x-axis direction, or if the directional main crack grows in the x-direction, then it is determined that the ground stress σh1 in the horizontal axis direction is greater than the ground stress σh2 in the vertical axis direction.

[0101] If the directional main crack extends along the extension line of the tip of the square borehole and deflects in the y direction, or if the directional main crack grows in the y direction, then the stress σh1 in the horizontal axis direction is less than the stress σh2 in the vertical axis direction.

[0102] S20. Based on Reuleaux's triangle principle, design a square borehole drill bit so that the drill bit's rotation and revolution are in opposite directions.

[0103] As mentioned earlier, the Reuleaux triangle is a curve of constant width, meaning its width remains the same regardless of the curve's direction. Utilizing this characteristic, a technical solution for a drill bit capable of drilling square boreholes was proposed earlier. Of course, the size and shape of the drill bit can be adjusted as needed to accommodate different borehole side lengths and elevation angles. The drill bit can be mounted on a drilling rig capable of both rotation and revolution, with the rotation and revolution directions opposite, at a speed ratio of 1:3. Thus, when drilling into deep rock, the drill bit will cut a square outline on the rock surface, which widens with increasing depth.

[0104] It is important to note that during actual drilling, the drilling location needs to be determined comprehensively based on the site conditions of the roof, the location of key layers, and the blasting strata. A square blast hole should be drilled into the roof at a distance h from the floor, as shown in Figure 11. Simultaneously, the positions of the anchor bolts and cables within the roof should be fully considered during drilling, and overlapping positions should be avoided.

[0105] S30. Based on the obtained crack propagation and growth patterns of square blast holes under different geostress states and the thickness of the hard top plate, determine the blast hole parameters for laying square blast holes on the hard top plate.

[0106] S40. According to the blast hole parameters, use the square blast hole drill bit to drill multiple sets of square blast holes on the hard top plate. Each set of square blast holes includes multiple square blast holes, which are arranged in a square. The tips of adjacent blast holes within a set are set opposite each other, and the tips of adjacent blast holes between sets are set opposite each other.

[0107] In some embodiments, each group of square boreholes includes four square boreholes.

[0108] S50. Load explosives into each group of square blast holes and install detonators;

[0109] S60, when the square boreholes are detonated, the explosion stress wave forms a stress field superposition zone along the extension line of the tip of the square borehole during the propagation process.

[0110] During detonation, each group of four boreholes forms a detonation group, and the boreholes in the same group are loaded with explosives simultaneously to ensure that the explosive stress waves are effectively superimposed between the boreholes, forming a stress field superposition zone, and promoting the propagation of cracks between the boreholes, thereby improving the effect of fine blasting weakening of hard roofs.

[0111] The boreholes in each group are detonated sequentially according to design requirements. For example, different types of non-electric time-delay detonators can be used to achieve micro-delay detonation between different borehole groups. In a feasible specific embodiment, the detonation between groups includes the following steps:

[0112] Square blast holes are laid out on a hard top plate. According to the predetermined parameters of the horizontal spacing between adjacent blast holes, the vertical spacing between adjacent blast holes, and the length of the directional main crack after detonation, the square blast holes are divided into multiple blast hole groups, and each blast hole group contains multiple square blast holes.

[0113] Two non-electric time-delay detonators of different segments are installed in each square borehole. One detonator controls the detonation sequence of square boreholes within the same borehole group, while the other controls the detonation sequence between different borehole groups. The detonating cords of the two non-electric time-delay detonators are connected to two independent detonation networks to improve the reliability of the networks.

[0114] A non-electric delay detonator is installed outside each borehole group to control the detonation time interval between different borehole groups. The non-electric delay detonator is connected to the non-electric delay detonators of adjacent borehole groups through connecting elements, forming a ring-shaped detonation network.

[0115] Cross lines are added to the ring-shaped detonation network to prevent network interruptions. These cross lines consist of non-electric detonating cords and connecting elements, linking non-electric delay detonators at different locations.

[0116] Using a capacitive detonator or a high-energy electromagnetic induction detonator, any non-electric delay detonator in the ring detonation network is activated, causing it to propagate simultaneously in two directions to other non-electric delay detonators, and then triggering the non-electric delay detonators inside each borehole group, thus achieving micro-delay detonation between different borehole groups.

[0117] In this embodiment, by using non-electric delay detonators of different segments to achieve micro-delay initiation, the types and quantities of required equipment are reduced, thus lowering costs. Ring and cross circuits are used to increase the reliability of the initiation network and prevent circuit breaks and initiation failures. Furthermore, combining it with square blast holes can improve the blasting weakening effect on hard roofs.

[0118] S70. Utilizing the stress field superposition zone formed, guide and control the growth of the first directional main crack in the direction of the extension line of the tip of the square borehole; at the same time, the second directional main crack formed in the direction of the extension line of the relative tips of adjacent boreholes grows in the opposite direction to the first directional main crack, forming a mutually penetrating fracture network to weaken the hardness of the hard top plate.

[0119] The method of using the stress field superposition zone to guide and control the growth of the first directional main crack in the direction of the extension line of the tip of the square borehole includes: using the geometric structural abrupt changes that occur at the tips of two or more directional main cracks to make the stress overlap and superimpose in the same direction space, resulting in a sharp increase in stress, or even the occurrence of singularity (i.e., the phenomenon that the stress tends to be infinite in theory).

[0120] By utilizing the rapidly increasing stress or stress singularity, crack propagation and spread occur in the hard top plate region near the tip of the directional master crack, thereby forming a larger fracture network and improving the effect of explosive weakening of the hard top plate.

[0121] In steps S60 and S70, each group of blast holes adopts a synchronous detonation system, which simultaneously detonates the detonators in the four square blast holes, so that the high-pressure gas and shock wave generated by the explosion produce tensile stress on the wall of the square blast hole.

[0122] By utilizing the resulting stress field superposition zone, the growth of a first directional main crack is guided and controlled along the extension direction of the tip of a square borehole. Simultaneously, a second directional main crack, growing along the relative extension direction of the tips of adjacent boreholes, grows in the opposite direction to the first directional main crack, forming an interconnected fracture network to weaken the hardness of the rigid roof. The above steps are repeated, and multiple sets of square boreholes arranged along the length and width of the rigid roof are detonated according to a predetermined detonation sequence and delay time, so that the fracture network covers the entire rigid roof area. Through the action of the fracture network, the rigid roof collapses locally or entirely along a predetermined direction or within a predetermined range, achieving the purpose of refined blasting and weakening of the rigid roof.

[0123] This invention addresses the issue of hard roofs in deep, high-stress rock tunnels. It comprehensively considers the influence of ground stress on the direction of blasting fractures in the hard roof. The stress state of the hard roof and the blasting height are determined. Based on the Reuleaux triangle principle, a square blasting drill bit is designed so that its rotation and revolution are opposite. Based on the obtained fracture propagation patterns of square blasting holes under different stress states and the thickness of the hard roof, the blasting parameters for laying square blasting holes on the hard roof are determined. According to these parameters, multiple sets of square blasting holes are drilled on the hard roof using the square blasting drill bit. Each set of square blasting holes includes multiple square blasting holes arranged in a square pattern. Assume that the tips of adjacent blast holes within a group are positioned opposite each other, and the tips of adjacent blast holes between groups are positioned opposite each other. Explosives are loaded into each group of square blast holes, and detonators are installed. Each group of square blast holes is detonated. During the propagation of the explosive stress wave, a stress field superposition zone is formed along the extension line of the tip of the square blast hole. Using the formed stress field superposition zone, the growth of a first directional main crack along the extension line of the tip of the square blast hole is guided and controlled. Simultaneously, a second directional main crack, formed by the growth of adjacent blast holes along their relative tip extension lines, grows towards the first directional main crack, forming a mutually interconnected directional fracture network. This weakens the hardness of the hard roof and improves the blasting weakening effect.

[0124] In some embodiments, as previously described, the crack propagation and growth patterns of square blast holes under different geostress states were investigated. To obtain these patterns, experimental studies were conducted beforehand to analyze the correlation between different geostress states and the corresponding crack propagation and growth patterns of square blast holes. Specifically, in-situ drilling and blasting experiments were performed on-site to observe and record the crack distribution on the rock samples after blasting. Based on the observation results, the relationship between crack propagation and growth patterns under different geostress states and parameters such as the spacing between adjacent blast holes laterally, the spacing between adjacent blast holes longitudinally, and the length of the directional main crack can be obtained. Based on these relationships, combined with the measured geostress state and thickness of the hard roof, the parameters such as the spacing between adjacent blast holes laterally, the spacing between adjacent blast holes longitudinally, and the length of the directional main crack that need to be installed on the hard roof are determined.

[0125] Specifically, the borehole parameters include: the spacing between adjacent boreholes laterally and the row spacing between adjacent boreholes longitudinally; the arrangement of these borehole parameters can be optimized according to the following scheme:

[0126] The process of determining the borehole parameters for laying square boreholes on a hard top plate based on the obtained crack propagation growth law of square boreholes under different ground stress states and the thickness of the hard top plate includes: conducting experimental research on the relationship between different ground stress states and the corresponding crack propagation growth law of square boreholes in advance, and analyzing the correlation between different ground stress states and the corresponding crack propagation growth law of square boreholes.

[0127] The geostress state at the location of the rigid top plate is measured, including the geostress in the horizontal axis direction and the geostress in the vertical axis direction.

[0128] In most cases, the magnitudes of horizontal in-situ stresses in deep rock masses are not equal, i.e., σh1 ≠ σh2. For the convenience of describing the embodiments of the present invention, in one embodiment, it is assumed that the in-situ stress state is σh1 > σh2.

[0129] Based on this, a square borehole blasting experiment was conducted. Two square boreholes, with the same depth as those used in a normal hard roof weakening blasting, were drilled into the roof. The two square boreholes used two different borehole shapes, as described above. The same amount of explosive as in a normal blasting was placed inside the square boreholes. The first borehole was a standard square borehole, and the second was a rhomboid borehole. The crack distribution of the two boreholes under this specific geostress condition was obtained. The relevant crack distribution has already been described in detail above with reference to the attached figures, and will not be repeated here.

[0130] Based on the correlation obtained from the analysis and the stress state of the location of the hard top plate, the spacing between adjacent lateral boreholes and the row spacing between adjacent longitudinal boreholes that need to be laid on the hard top plate are determined.

[0131] When the measured ground stress in the horizontal axis direction is equal to the ground stress in the vertical axis direction, the spacing between the lateral adjacent boreholes and the row spacing between the longitudinal adjacent boreholes are arranged to be equal.

[0132] When the measured ground stress in the horizontal axis direction is greater than the ground stress in the vertical axis direction, the spacing between the adjacent blast holes in the lateral direction is arranged to be greater than the row spacing between the adjacent blast holes in the longitudinal direction.

[0133] When the measured stress in the horizontal axis direction is small

[0134] When the ground stress is equal in the vertical axis direction, the spacing between adjacent lateral boreholes should be reduced.

[0135] The arrangement of adjacent boreholes in the longitudinal direction;

[0136] Furthermore, based on the magnitudes of the horizontal and vertical axial stresses, the spacing between adjacent lateral boreholes and the row spacing between adjacent longitudinal boreholes are determined, so that after each group of boreholes is detonated, the directional main cracks formed by adjacent groups of boreholes interconnect to form a directional fracture network.

[0137] In this embodiment of the invention, based on the crack propagation and growth patterns of two different morphologies of square blast holes under different geostress distribution conditions, a directional fracture network for blasting hard roofs under various working conditions can be achieved through optimized arrangement of the pointed ends of the square blast holes. By drilling square blast holes into the hard roof above the goaf, based on the unique directional crack control performance of square blast holes, and with reasonable parameters such as blast hole spacing and row spacing, the directional cracks between the blast holes can be interconnected, forming a directional fracture network, thereby improving the effect of weakening the hard roof.

[0138] In some embodiments, the spacing between the lateral adjacent boreholes is set to 2l. d +D, the longitudinal spacing between adjacent boreholes is 2lc+D, where l d lc represents the length of the directional main crack along the horizontal axis, lc represents the length of the directional main crack along the vertical axis, and D represents the length of the diagonal of the square borehole.

[0139] In this embodiment, the measured length of the directional main crack along the extension line of the tip of a typical square borehole is denoted as (l). d1 +l d2 The length of the straight line segment is l. d1 The length of the arc segment is l d2 The length of the directional main crack along the x-direction of the rhomboid borehole is l. d The length of the directional main crack along the y-direction is l cBased on the above data, the spacing of the blasting holes for weakening the hard roof was calculated. The reasonable arrangement of the blasting hole spacing allows the directional main cracks to penetrate each other, so that the weakening effect of the blasting on the rock is optimal, which is more conducive to the fracture of the roof and achieves the purpose of pressure relief, thus ensuring the stability of the surrounding rock of the roadway.

[0140] The borehole parameters also include: borehole depth;

[0141] The step of determining the borehole parameters for laying square boreholes on a hard top plate based on the obtained crack propagation growth law of square boreholes under different ground stress states and the thickness of the hard top plate includes: measuring and obtaining the thickness of the hard top plate.

[0142] Based on the thickness of the rigid roof, the depth of the square blast hole is determined according to the depth selection condition formula, wherein the depth selection condition formula is: a≧0.7*H, where a is the depth of the square blast hole and H is the thickness of the rigid roof above the goaf.

[0143] The square blast hole can be a square blast hole or a rhombus blast hole, wherein the four tips of the rhombus blast hole are located on the x and y axes respectively, and the four tips of the square blast hole are located at ±45° directions on the x and y axes respectively.

[0144] Given the varying difficulty of charging explosives in blasting holes, deep-hole pre-splitting blasting holes for mining the hard roof above the coal seam need to be set at a certain angle. Therefore, the blasting hole parameters also include: the included angle between adjacent groups of blasting holes, the blasting hole elevation angle, and the blasting hole side length. Multiple blasting holes are distributed in a fan shape. The included angle between adjacent groups of blasting holes is 10 to 15 degrees. The blasting hole elevation angle is 29 to 74 degrees and is inclined upwards in the direction opposite to the mining face. The blasting hole side length is 80 to 100 mm, and the charging method inside the blasting hole is fully coupled charging.

[0145] Based on practical experience, the borehole elevation angle is set to 29–74° to facilitate charging. The borehole length 'a' is calculated using the formula mentioned earlier. Of course, the range of borehole length can also be determined by considering the goaf treatment method and the location of the roof.

[0146] The blast holes are arranged in a fan shape, and the included angle between each group of fan-shaped blast holes is preferably 13°, and the side length of the blast holes is preferably 90mm.

[0147] The method for weakening a hard roof based on square blast holes provided in this invention, considering the influence of deep high ground stress characteristics on the propagation and growth of blast cracks, uses reasonable blast hole spacing, blast hole row spacing, and blast hole angle arrangement to make directional cracks between blast holes interconnected, forming a directional fracture network. During detonation, stress waves can be effectively used to form a stress field superposition zone along the extension line of the tip of the square blast hole, thereby creating stress concentration during propagation. This promotes crack propagation from the tip extension line, reduces the pulverization zone, and increases the corresponding crack development zone and propagation zone, thus achieving the effect of weakening the hard roof. This method can significantly improve the blasting weakening effect of hard roofs under high ground stress.

[0148] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for weakening deep, high-stress, hard roof blasting based on square blast holes, characterized in that, The square blast hole method involves the following steps: determining the stress state of the hard roof in the tunnel and the blasting height; designing a square blast hole drill bit based on the Reuleaux triangle principle, ensuring the drill bit's rotation and revolution are opposite; determining the blast hole parameters for laying square blast holes on the hard roof based on the obtained crack propagation growth patterns of square blast holes under different stress states and the thickness of the hard roof; and drilling multiple sets of square blast holes on the hard roof using the square blast hole drill bit according to the blast hole parameters. Each set of square blast holes includes multiple square blast holes arranged in a square pattern, with the tips of adjacent blast holes within the set facing each other. The tips of adjacent blast holes in the groups are positioned opposite each other; explosives are loaded into each group of square blast holes, and detonators are installed; each group of square blast holes is detonated, and during the propagation of the explosive stress wave, a stress field superposition zone is formed along the extension line of the tip of the square blast hole; the stress field superposition zone is used to guide and control the growth of the first directional main crack along the extension line of the tip of the square blast hole; at the same time, the second directional main crack formed by the growth of the adjacent blast holes along the extension line of the relative tips grows in the opposite direction to the first directional main crack, forming an interconnected fracture network to weaken the hardness of the hard top plate; the horizontal axis direction is the x-axis direction, and the vertical axis direction is the y-axis direction.

2. The method for weakening a hardened roof during blasting according to claim 1, characterized in that, The borehole parameters include: the spacing between adjacent boreholes laterally and the spacing between adjacent boreholes longitudinally; the determination of borehole parameters for laying square boreholes on the hard roof based on the obtained crack propagation growth law of square boreholes under different geostress states and the thickness of the hard roof includes: conducting experimental research on the relationship between different geostress states and the corresponding crack propagation growth law of square boreholes, and analyzing the correlation between different geostress states and the corresponding crack propagation growth law of square boreholes; measuring the geostress state at the location of the hard roof, which includes: geostress in the horizontal axis direction and geostress in the vertical axis direction; and determining the spacing between adjacent boreholes laterally and longitudinally on the hard roof based on the obtained correlation and the geostress state at the location of the hard roof. The spacing between adjacent boreholes is determined as follows: when the measured horizontal stress is equal to the vertical stress, the spacing between adjacent boreholes in the lateral direction is equal to the spacing between adjacent boreholes in the longitudinal direction; when the measured horizontal stress is greater than the vertical stress, the spacing between adjacent boreholes in the lateral direction is greater than the spacing between adjacent boreholes in the longitudinal direction; when the measured horizontal stress is less than the vertical stress, the spacing between adjacent boreholes in the lateral direction is less than the spacing between adjacent boreholes in the longitudinal direction; and, based on the magnitudes of the horizontal and vertical stresses, the spacing between adjacent boreholes in the lateral direction and the spacing between adjacent boreholes in the longitudinal direction are determined so that after each group of boreholes is detonated, the directional main cracks formed in adjacent groups of boreholes interconnect to form a directional fracture network.

3. The method for weakening a hardened roof during blasting according to claim 2, characterized in that, The spacing between adjacent lateral boreholes is set to 2l. d +D, the longitudinal spacing between adjacent boreholes is 2lc+D, where l d lc represents the length of the directional main crack along the horizontal axis, lc represents the length of the directional main crack along the vertical axis, and D represents the length of the diagonal of the square borehole.

4. The method for weakening a hardened roof by blasting according to claim 2 or 3, characterized in that, The borehole parameters also include: borehole depth; the process of determining the borehole parameters for laying square boreholes on a hard top plate based on the obtained crack propagation growth law of square boreholes under different ground stress states and the thickness of the hard top plate includes: measuring and obtaining the thickness of the hard top plate; and determining the depth of the square borehole based on the thickness of the hard top plate according to the formula for selecting the depth of the square borehole, wherein the formula for selecting the depth is: a≧0.7 H, where a is the depth of the square blast hole and H is the thickness of the hard roof above the goaf.

5. The method for weakening a hardened roof during blasting according to claim 1, characterized in that, The parameters of the blast holes also include: the included angle between adjacent groups of blast holes, the elevation angle of the blast holes and the side length of the blast holes. The multiple groups of blast holes are distributed in a fan shape. The included angle between adjacent groups of blast holes is 10 to 15 degrees. The elevation angle of the blast holes is 29 to 74 degrees and is inclined upward in the opposite direction to the mining face. The side length of the blast holes is 80 to 100 mm and the charging method in the blast holes is fully coupled charging.

6. The method for weakening a hardened roof during blasting according to claim 1, characterized in that, The drill bit includes: a disc-shaped body with a mounting hole at its center, and a cutting edge on the edge of the disc-shaped body. The profile of the cutting edge includes three sets of interconnected, radially arranged arc segments on the edge of the disc-shaped body. Each set of arc segments includes a first arc and a second arc connected to the first arc. The endpoints of the three sets of arc segments lie on the same circular envelope. The first arc in one set of arc segments establishes a coordinate system with the center of the mounting hole as the origin, and its center is located at (-20.6785, -2.8367) relative to the origin. The first arc has a predetermined radius. The second arc has a predetermined radius and a position relative to the origin at (-7.1173, 5.7975) as its center. The second arc has a predetermined radius. The first arc and the second arc are connected, but their concave directions are opposite.

7. The method for weakening a hardened roof during blasting according to claim 6, characterized in that, The method of drilling multiple sets of square blast holes in a hard top plate using the square blast hole drill bit includes: mounting the drill bit on a drilling machine that can rotate and revolve, so that the rotation and revolution of the drill bit are opposite in direction, with a speed ratio of 1:3, and drilling square blast holes in deep rock mass through rotation and propulsion.

8. The method for weakening a hardened roof during blasting according to claim 1, characterized in that, The four tips of the square borehole are located on the x-axis and y-axis, respectively.

9. The method for weakening a hardened roof during blasting according to claim 2, characterized in that, The method for measuring the geostress state at the location of the hard roof includes: drilling a separate square hole into the hard roof, loading it with the same amount of explosives as those used in subsequent blasting, inserting a detonator to detonate, and monitoring the distribution of cracks in the roof; and determining the geostress state at the location of the hard roof in the roadway based on the crack distribution.

10. The method for weakening a hardened roof during blasting according to claim 9, characterized in that, The determination of the geostress state at the location of the hard roof in the roadway based on the crack distribution includes: if the directional main crack extends along the extension line of the tip of the square blast hole and the shape of the directional main crack is approximately +, then the geostress σh1 in the horizontal axis direction is equal to the geostress σh2 in the vertical axis direction; if the directional main crack extends along the extension line of the tip of the square blast hole and grows in the x-direction, then the geostress σh1 in the horizontal axis direction is greater than the geostress σh2 in the vertical axis direction; if the directional main crack extends along the extension line of the tip of the square blast hole and grows in the y-direction, then the geostress σh1 in the horizontal axis direction is less than the geostress σh2 in the vertical axis direction.

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