阅读说明：本技术 一种顶板断裂型冲击地压分级预警方法 (Roof fracture type rock burst grading early warning method ) 是由 高厚 赵武胜 陈卫忠 钟坤 秦长坤 解佩瑶 于 2021-09-13 设计创作，主要内容包括：本发明公开了一种顶板断裂型冲击地压分级预警方法,步骤是：①采用光纤光栅应力传感器对坚硬顶板进行应力解除试验；②测量坚硬顶板的弹性模量、泊松比；③获取坚硬顶板极限储存能与围压间的函数关系；④利用应力解除数据计算坚硬顶板的初始应力；⑤采用光纤光栅应力传感器在线实时监测坚硬顶板的相对应力；⑥叠加初始和相对应力,计算坚硬顶板的实时应力；⑦利用实时应力,计算坚硬顶板的实时弹性能密度U-(e)和在应力水平下的实时极限储存能U-(t)；⑧通过比较U-(e)和U-(t)对顶板断裂型冲击地压进行分级预警。根据坚硬顶板能量状态的监测结果,实现了顶板断裂型冲击地压的分级预警,可有效避免或减少顶板断裂型冲击地压灾害的发生,保证矿井的安全生产。(The invention discloses a graded early warning method for roof fracture type rock burst, which comprises the following steps: firstly, a stress relief test is carried out on a hard top plate by adopting a fiber bragg grating stress sensor; measuring the elastic modulus and Poisson ratio of the hard top plate; thirdly, acquiring a functional relation between the ultimate storage energy of the hard top plate and confining pressure; fourthly, calculating the initial stress of the hard top plate by utilizing the stress relief data; adopting a fiber bragg grating stress sensor to monitor the relative stress of the hard top plate on line in real time; sixthly, overlapping the initial stress and the relative stress, and calculating the real-time stress of the hard top plate; seventhly, calculating the real-time elastic energy density U of the hard top plate by using the real-time stress e And real time ultimate stored energy U at stress level t (ii) a Through comparison of U e And U t And carrying out grading early warning on the top plate fracture type rock burst. According to hard top plate energyAccording to the monitoring result of the state, the grading early warning of the roof fracture type rock burst is realized, the occurrence of the roof fracture type rock burst disaster can be effectively avoided or reduced, and the safety production of a mine is ensured.)

1. A graded early warning method for roof fracture type rock burst comprises the following steps:

(1) and (3) carrying out a stress relief test on the hard top plate by adopting a fiber bragg grating stress sensor: the hollow core type or aperture deformer type fiber grating stress sensor is a stress sensor taking fiber gratings as strain measuring elements, wherein the fiber gratings are passive filter devices;

(2) the modulus of elasticity, poisson's ratio of the hard top plate were measured: performing a uniaxial compression test on the core obtained at the hard top plate in the step (1) to obtain the elastic modulus and Poisson's ratio of the hard top plate;

(3) obtaining a functional relation between the limit storage energy of the hard top plate and confining pressure: performing a conventional triaxial compression test on the rock core obtained at the hard top plate in the step (1), fitting an optimal functional relation between the limit storage energy and the confining pressure, and obtaining a functional relation between the limit storage energy and the confining pressure of the hard top plate:

U₀＝f(σ_cp) (1)

wherein, U₀To limit the storage energy, σ_cpFor confining pressure, the function y ═ f (x) is an optimal functional relation between the limit storage energy and the confining pressure obtained by fitting the results of a conventional triaxial compression test;

(4) the initial stress of the hard top plate was calculated using stress relief data: solving the initial stress of the hard top plate by using the stress relief data obtained in the step (1) through a three-dimensional stress calculation formula;

(5) the relative stress of the hard top plate is monitored on line in real time by adopting a fiber bragg grating stress sensor: installing the fiber bragg grating stress sensor in the drill hole of the hard top plate again, and then monitoring the relative stress of the hard top plate in real time on line by using the fiber bragg grating stress sensor;

(6) stacking the initial stress and the relative stress, and calculating the real-time stress of the hard top plate: after the initial stress and the relative stress of the hard top plate are obtained, the initial stress and the relative stress are superposed to obtain the real-time stress of the hard top plate;

(7) calculating the real-time elastic energy density U of the hard top plate by using the real-time stress_eAnd real time ultimate stored energy U at that stress level_t: after the real-time stress of the hard top plate is obtained, the real-time elastic energy density U of the hard top plate is calculated_eAnd real time ultimate stored energy U at that stress level_t；

Real-time elastic energy density U_eThe calculation formula of (2) is as follows:

wherein E is the elastic modulus in the step (2), μ is the Poisson's ratio in the step (2), and σ is₁、σ₂And σ₃The first, second and third principal stresses which are the real-time stresses in the step (6) respectively;

real-time limited stored energy U_tThe calculation formula of (2) is as follows:

U_t＝f(σ₃) (3)

wherein, the function y ═ f (x) is the functional relation in the step (3), and σ₃A third principal stress which is the real-time stress in the step (6);

(8) will U_eAnd U_tAnd comparing, and performing grading early warning on the roof fracture type rock burst by using a comparison result: by comparing real-time elastic energy density U_eAnd real-time limited stored energy U_tAnd the grading early warning of the roof fracture type rock burst is realized.

2. The graded early warning method for roof fracture type rock burst according to claim 1, characterized in that: and (3) carrying out grading early warning on the top plate fracture type rock burst in the step (8), and realizing the grading early warning by the following mode:

comparing the real-time elastic energy density U in the step (7)_eAnd real time ultimate stored energy U at that stress level_tObtaining the risk index of rock burst;

secondly, according to the corresponding relation between the rock burst danger index and the rock burst danger, grading early warning is carried out on the top plate fracture type rock burst.

Technical Field

The invention relates to the technical field of rock burst monitoring and early warning, in particular to a graded early warning method for roof fracture type rock burst, which is suitable for monitoring and early warning of rock burst of coal mines, metal mines and construction wells.

Background

With the annual increase of the mining depth of coal resources, the rock burst disaster is increasingly aggravated, becomes the most main disaster influencing the safety production of coal mines in China, and seriously threatens the safety of underground personnel and equipment. The roof fracture type rock burst is a typical rock burst disaster, is hard to collapse in time due to the hardness of the roof, causes stress concentration and energy accumulation due to large-area suspended roof to cause the occurrence of the rock burst, and has the characteristics of high occurrence frequency, wide damage range and the like. Therefore, the effective roof fracture type rock burst early warning technology is established, and the method has important engineering practical significance. The generation mechanism of the roof fracture type rock burst is as follows: along with the working face extraction, the hard top plate is not easy to collapse in time, so that a large amount of elastic energy is accumulated in the top plate, once the top plate is subjected to instability fracture damage, huge elastic energy is released instantly, thereby inducing coal body fracture, generating outward projection motion and finally causing rock burst. The origin of the mechanism of roof fracture type rock burst is known to be the large amount of elastic energy released upon unstable fracture of the hard roof. Therefore, in roof fracture type rock burst, the hard roof is a key disaster site, and unstable fracture of the hard roof is a dominant factor. Therefore, when the roof fracture type rock burst is monitored and early warned, the fracture danger of the hard roof is monitored and early warned in an important mode, and when the fracture danger exists in the hard roof, the occurrence of the rock burst danger is indicated.

After the root is reached, the fracture of the top plate is a state instability phenomenon under the drive of energy. Therefore, by monitoring the energy state of the hard top plate, the risk of breakage of the hard top plate can be determined, and the risk of occurrence of rock burst can be determined. At present, the main technologies used for monitoring and early warning of roof fracture type rock burst at home and abroad comprise a vibration monitoring technology, an electromagnetic monitoring technology, a coal seam stress monitoring technology, a charge monitoring technology, a drilling cutting monitoring technology and the like. The monitoring and early warning technologies indirectly analyze the stress state or the energy state of the coal rock mass by monitoring corresponding signals, so that early warning is carried out on the roof fracture type rock burst. However, these monitoring and early warning technologies cannot realize direct and accurate monitoring of the energy state of the hard roof, so the early warning precision is low. The CN 201910774013.7 can directly monitor the stress state of the roof rock, but cannot directly monitor the energy state of the roof rock, so that the early warning precision of the roof fracture type rock burst is not enough.

Disclosure of Invention

The invention aims to solve the technical problem of providing a graded early warning method for roof fracture type rock burst, which is easy to implement and simple and convenient to operate, establishes graded early warning indexes for judging the occurrence of the rock burst danger through the roof fracture danger based on an energy theory, realizes graded early warning of the roof fracture type rock burst, effectively avoids or reduces the occurrence of roof fracture type rock burst disasters, and ensures safe production of mines.

In order to achieve the purpose, the invention adopts the following technical measures:

a graded early warning method for roof fracture type rock burst comprises the following steps:

(1) and (3) carrying out a stress relief test on the hard top plate by adopting a fiber bragg grating stress sensor: a hollow core type or aperture deformer type fiber grating stress sensor is a stress sensor using fiber grating as a measuring strain element. The fiber grating is a passive filter device (common), the refractive index of the fiber core of the fiber grating is subjected to axial periodic modulation by a certain method, and the fiber grating has the advantages of small volume, electromagnetic interference resistance, stable and sensitive signal, compatibility with the fiber and the like, and the resonance wavelength of the fiber grating is sensitive to the change of external parameters such as temperature, strain and the like. Therefore, the fiber grating can be used for manufacturing a fiber grating stress sensor.

(2) The modulus of elasticity, poisson's ratio of the hard top plate were measured: and (3) carrying out uniaxial compression test on the core obtained at the hard top plate in the step (1), so as to obtain the elastic modulus and Poisson ratio of the hard top plate. Wherein, uniaxial compression test: test of axial compression of the test specimens to failure under uniaxial loading.

(3) Obtaining a functional relation between the limit storage energy of the hard top plate and confining pressure: performing a conventional triaxial compression test on the rock core obtained at the hard top plate in the step (1), and fitting an optimal functional relation between the limit storage energy and the confining pressure, so as to obtain a functional relation between the limit storage energy and the confining pressure of the hard top plate:

U₀＝f(σ_cp) (1)

wherein, U₀To limit the storage energy, σ_cpFor confining pressure, the function y ═ f (x) is the best functional relationship between ultimate stored energy and confining pressure obtained by fitting the results of conventional triaxial compression tests.

(4) The initial stress of the hard top plate was calculated using stress relief data: and (3) solving the initial stress of the hard top plate by using the stress relief data obtained in the step (1) through a three-dimensional stress calculation formula.

(5) The relative stress of the hard top plate is monitored on line in real time by adopting a fiber bragg grating stress sensor: and the fiber bragg grating stress sensor is installed in the drill hole of the hard top plate again, and then the relative stress of the hard top plate is monitored in real time on line by using the fiber bragg grating stress sensor.

(6) Stacking the initial stress and the relative stress, and calculating the real-time stress of the hard top plate: and after the initial stress and the relative stress of the hard top plate are obtained, the initial stress and the relative stress are superposed to obtain the real-time stress of the hard top plate.

(7) Calculating the real-time elastic energy density U of the hard top plate by using the real-time stress_eAnd real time ultimate stored energy U at that stress level_t: after the real-time stress of the hard top plate is obtained, the real-time elastic energy density U of the hard top plate can be calculated_eAnd real time ultimate stored energy U at that stress level_t。

Real-time elastic energy density U_eThe calculation formula of (2) is as follows:

wherein E is the elastic modulus in the step (2), μ is the Poisson's ratio in the step (2), and σ is₁、σ₂And σ₃The first, second and third principal stresses of the real-time stress in step (6) are respectively.

Real-time limited stored energy U_tThe calculation formula of (2) is as follows:

U_t＝f(σ₃) (3)

wherein, the function y ═ f (x) is the functional relation in the step (3), and σ₃The third principal stress which is the real-time stress in the step (6).

(8) Will U_eAnd U_tAnd comparing, and performing grading early warning on the roof fracture type rock burst by using a comparison result: by comparing real-time elastic energy density U_eAnd real-time limited stored energy U_tAnd the grading early warning of the roof fracture type rock burst is realized.

The fiber grating stress sensor in the step (1) has the performances of water resistance, explosion resistance, electromagnetic interference resistance and good long-term stability.

And (3) carrying out grading early warning on the top plate fracture type rock burst in the step (8), and specifically realizing the grading early warning by the following modes:

comparing the real-time elastic energy density U in the step (7)_eAnd real time ultimate stored energy U at that stress level_tObtaining the risk index of rock burst;

secondly, according to the corresponding relation between the rock burst danger index and the rock burst danger, grading early warning is carried out on the top plate fracture type rock burst.

As known from a roof fracture type rock burst generation mechanism, the fracture risk of a hard roof is characterized by the occurrence risk of rock burst, and the roof fracture is fundamentally a state instability phenomenon under energy drive. Therefore, the method is an effective way to establish the graded early warning index of the roof fracture type rock burst based on the energy theory. The ultimate stored energy is the maximum elastic energy that can be stored per volume of rock and can be used to characterize the ability of the rock to accumulate elastic energy. Thus, the roof can be made harder by comparisonThe elastic energy density and the limit storage energy of the plate are used for judging the fracture risk of the hard top plate, so that the occurrence risk of the top plate fracture type rock burst is judged. Based on the method, firstly, a fiber bragg grating stress sensor with the performances of water resistance, explosion resistance, electromagnetic interference resistance, good long-term stability and the like is adopted to perform a stress relief test on the hard top plate, then a uniaxial compression test is performed on a rock core obtained at the position of the hard top plate, the elastic modulus and the Poisson ratio of the hard top plate are measured, then a conventional triaxial compression test is performed on the rock core, and the functional relation between the limit storage energy and the confining pressure of the hard top plate is obtained. Then, using the stress relief data, the initial stress of the hard top plate is calculated. And then, monitoring the relative stress of the hard top plate on line in real time by adopting a fiber bragg grating stress sensor, and superposing the relative stress with the initial stress to obtain the real-time stress of the hard top plate. Then, real-time stress is utilized to calculate real-time elastic energy density U of the hard top plate_eAnd real time ultimate stored energy U at that stress level_t. Finally, by comparing U_eAnd U_tAnd carrying out grading early warning on the top plate fracture type rock burst.

Among the above 8 steps, step (3), step (7), and step (8) are key steps. Wherein, the step (3) obtains the function relation between the limit storage energy and the confining pressure of the hard top plate, and the step (7) obtains the real-time elastic energy density U of the hard top plate_eAnd real time ultimate stored energy U at that stress level_tStep (8) by comparing U_eAnd U_tAnd carrying out grading early warning on the top plate fracture type rock burst. The detailed process for realizing the graded early warning of the roof fracture type rock burst is as follows: first, by comparing the real-time elastic energy density U_eAnd real-time limited stored energy U_tObtaining the risk index R ═ U of rock burst_e/U_t(ii) a And then, carrying out grading early warning on the top plate fracture type rock burst according to the corresponding relation between the rock burst risk index R and the rock burst risk. Wherein, the corresponding relation between the rock burst danger index R and the rock burst danger can be determined by the following formula:

according to the formula (4), when the rock burst risk index R is less than 0.3, no rock burst risk exists on site, and no early warning is given; when the rock burst danger index R is more than or equal to 0.3 and less than 0.4, weak rock burst danger exists on site, and I-level early warning is immediately sent out; when the rock burst danger index R is more than or equal to 0.4 and less than 0.5, a medium rock burst danger exists on site, and a II-level early warning is immediately sent out; when the rock burst danger index R is more than or equal to 0.5, a strong rock burst danger exists on site, and III-level early warning is immediately sent out.

Of course, in field application, the limit value of the formula (4) can be adjusted according to actual conditions.

One of the technical bottlenecks of the graded early warning of the roof fracture type rock burst is to determine a proper early warning index and a limit value thereof. Based on an energy theory, the invention judges the fracture danger of the hard top plate by comparing the elastic energy density and the limit storage energy of the hard top plate, thereby judging the occurrence danger of rock burst and establishing a top plate fracture type rock burst grading early warning index. Compared with common stress indexes, the index is more scientific, and the grading early warning accuracy of the roof fracture type rock burst is effectively improved. According to the coal mine rock burst prevention and control rule and a large amount of indoor test data, the formula (4) divides the early warning result into four grades and provides a judgment standard of the early warning grade, so that the graded early warning of the roof fracture type rock burst is realized. In field application, the limit value of the formula (4) can be adjusted according to actual conditions so as to be more pertinent. The grading early warning of the roof fracture type rock burst provides more detailed early warning information for mine managers, and the mine managers can adopt different danger relieving measures according to different early warning grades, so that the condition that the expenditure cost is too high and even the normal production is influenced due to the adoption of excessive danger relieving measures is effectively avoided, and the aims of relieving the rock burst danger and not wasting the cost are fulfilled.

Compared with the prior art, the invention adopts the fiber bragg grating stress sensor as the monitoring sensor, realizes the direct and long-term monitoring of the elastic energy density of the underground roof rock, and fills the blank in the industry; based on an energy theory, a grading early warning index for judging the danger of rock burst through the roof fracture danger is established, the grading early warning accuracy of the roof fracture type rock burst is effectively improved, and more accurate and detailed early warning information is provided for mine responsible persons.

The invention has been tested on site on a coal face with a hard roof. In this field test, a monitoring section is arranged. Through a conventional triaxial compression test, the limit storage energy U of a hard top plate at a monitored section is obtained₀With confining pressure σ_cpFunctional relationship between: u shape₀＝261.84exp(0.0305σ_cp) Where y is exp (x) is an exponential function. Monitoring the real-time stress (sigma) of the hard top plate at the fracture surface along with the advance of the working surface₁，σ₂，σ₃) Real-time elastic energy density U_eReal-time limited storage energy U_tThe change curves of (a) are shown in fig. 1, fig. 2 and fig. 3, respectively. By comparing real-time elastic energy density U_eAnd real-time limited stored energy U_tThe risk index R ═ U of rock burst can be obtained_e/U_tThe curve as the working surface advances is shown in fig. 4. As can be seen from fig. 4, during the face extraction, the rock burst risk indicator R at the monitored fracture surface is always less than 0.3, indicating that there is no rock burst risk at this position. In actual conditions, the working face safely and smoothly passes through the monitoring section, and the grading early warning result of the invention is consistent with the actual conditions on site. In a field test, a monitoring signal is stable and sensitive and is anti-electromagnetic interference, the direct and long-term monitoring of the elastic energy density of the hard roof is realized, the danger of the roof fracture type rock burst is accurately judged, detailed early warning information is provided, and the safety production of a mine is effectively guided.

The technical scheme of the invention has the following beneficial effects:

1. the fiber bragg grating stress sensor is used as a monitoring sensor, monitoring signals are stable, sensitive and anti-electromagnetic interference, direct and long-term monitoring of the elastic energy density of the underground roof rock is achieved, and the blank in the industry is filled.

2. Based on an energy theory, a grading early warning index for judging the danger of rock burst through the roof fracture danger is established, the grading early warning accuracy of the roof fracture type rock burst is effectively improved, and a reliable monitoring and grading early warning technology is provided for the roof fracture type rock burst disaster.

3. The grading early warning of the roof fracture type rock burst provides more detailed early warning information for mine managers, and the mine managers can take different danger relieving measures according to different early warning grades, so that the condition that the expenditure cost is too high and even the normal production is influenced due to the excessive danger relieving measures is effectively avoided.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a graph of real-time stress of a hard top plate at a monitoring section along with the advancing of a working face in a field test;

FIG. 2 is a graph of real-time elastic energy density of a hard top plate at a monitoring section along with the advancing of a working face in a field test;

FIG. 3 is a graph of real-time ultimate storage energy of a hard top plate at a monitoring section along with the advance of a working face in a field test;

FIG. 4 is a graph showing the variation of the risk index of rock burst at the monitored cross section with the advancement of the working face in a field test;

FIG. 5 is a flowchart of an implementation of a graded warning method for roof fracture type rock burst;

FIG. 6 is a three-dimensional stress distribution diagram of a borehole wall rock;

FIG. 7 is a schematic diagram of the installation position of a fiber grating stress sensor, wherein (a) is a schematic plan view and (b) is a schematic P-P section;

FIG. 8 is a graph of the fit between the limiting stored energy and the confining pressure where y is e^xIs an exponential function and can also be expressed as y ═ exp (x).

Detailed Description

In order that those skilled in the art will better understand the disclosure, the invention will be described in further detail with reference to the accompanying drawings and specific embodiments. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1:

as can be seen from fig. 5, a top plate fracture type rock burst grading and early warning method includes the steps of:

and S1, performing a stress relief test on the hard top plate by using the fiber bragg grating stress sensor.

The fiber grating stress sensor is a sensor using fiber grating as a strain element. The fiber grating is a passive filter device, the refractive index of a fiber core of the fiber grating is subjected to axial periodic modulation by a certain method, and the fiber grating has the advantages of small volume, electromagnetic interference resistance, stable and sensitive signal, compatibility with the fiber and the like, and the resonance wavelength of the fiber grating is sensitive to the change of external parameters such as temperature, strain and the like. Therefore, the fiber grating can be used for manufacturing a fiber grating stress sensor. Specifically, the specific working principle, internal structure, using method, etc. of the fiber grating stress sensor can refer to the related descriptions of the existing empty core matrix type or aperture deformer type fiber grating stress sensor (utility model patent CN 201720754878.3, utility model patent CN 201720754879.8, utility model patent CN 201821255732.5, utility model patent CN201720750949.2), and are not repeated here.

The fiber grating stress sensor has the performances of water resistance, explosion resistance, electromagnetic interference resistance and good long-term stability. In general, when the top plate is broken, the tensile stress monitored by a sensor is close to 20 MPa; during face mining, the maximum compressive stress in the real time stress of the roof is close to 100 MPa. Therefore, in order to better monitor the real-time stress of the hard top plate, a fiber grating stress sensor with the monitoring range of the maximum tensile stress not less than 20MPa and the maximum compressive stress not less than 100MPa is preferably adopted.

In the embodiment of the invention, the tested and monitored roof rock can be a hard roof under a coal mine. Of course, the roof fracture type rock burst grading early warning method provided by the invention can also be used for monitoring other mines or hard roofs under construction wells, so that the roof fracture type rock burst is graded early warned, and the safe production or construction under the wells is ensured. In the embodiment of the invention, the underground coal mine is taken as an example for detailed description, and reference can be made to the classification early warning of the roof fracture type rock burst in other underground environments.

And S2, measuring the elastic modulus and the Poisson ratio of the hard top plate.

The uniaxial compression test was performed on the core obtained at the hard roof in step S1, thereby obtaining the elastic modulus and poisson' S ratio of the hard roof.

Wherein, uniaxial compression test: test of axial compression of the test specimens to failure under uniaxial loading.

Specifically, the core obtained at the hard top plate in step S1 was prepared into a standard cylindrical sample having a height of about 100mm and a diameter of about 50mm, and then subjected to a uniaxial compression test to measure the elastic modulus and poisson' S ratio of the sample.

And S3, acquiring the functional relation between the limit storage energy of the hard top plate and the confining pressure.

Performing a conventional triaxial compression test on the core obtained from the hard top plate in step S1, fitting an optimal functional relation between the ultimate stored energy and the ambient pressure, thereby obtaining a functional relation between the ultimate stored energy and the ambient pressure of the hard top plate:

U₀＝f(σ_cp) (1)

wherein, U₀To limit the storage energy, σ_cpFor confining pressure, the function y ═ f (x) is between the limiting stored energy and confining pressure obtained by fitting the results of conventional triaxial compression testsThe optimal functional relation.

Wherein, conventional triaxial compression test: and (3) axially compressing the sample to a destructive test under a certain confining pressure.

The maximum stored energy is the maximum elastic energy which can be stored in the unit volume of rock and can be obtained by calculation through a conventional triaxial compression test or a triaxial confining pressure test. The ultimate storage capacity depends only on the confining pressure and the unloading rate, and is independent of the loading and unloading path. In the process of pushing the working face, the stress of the top plate rock in front of the working face is gradually increased and is always in a loading state until the top plate is broken. Therefore, the ultimate storage energy of the hard top plate can be obtained through a conventional triaxial compression test.

Specifically, the core obtained at the hard top plate in step S1 is made into a standard cylindrical sample with a height of about 100mm and a diameter of about 50mm, then at least 3 different confining pressure levels are set, a conventional triaxial compression test is performed, after the limit storage energy of each sample is obtained, the optimal functional relation between the limit storage energy and the confining pressure is fitted, and thus the functional relation between the limit storage energy and the confining pressure of the hard top plate is obtained.

And S4, calculating the initial stress of the hard top plate by using the stress relief data.

Determining the initial stress of the hard top plate by the three-dimensional stress calculation formula using the stress relief data obtained in step S1

And S5, monitoring the relative stress of the hard top plate in real time on line by adopting a fiber bragg grating stress sensor.

The fiber bragg grating stress sensor is installed in the drill hole of the hard top plate again, and then the relative stress (delta sigma) of the hard top plate is monitored in real time on line by the fiber bragg grating stress sensor₁，Δσ₂，Δσ₃)。

And S6, superposing the initial stress and the relative stress, and calculating the real-time stress of the hard top plate.

After the initial stress and the relative stress of the hard top plate are obtained, the initial stress and the relative stress are superposed to obtain the hard top plateReal time stress (σ)₁，σ₂，σ₃)。

In order to facilitate better understanding of those skilled in the art, how to obtain real-time stress of the rigid top plate by using the fiber grating stress sensor in the present embodiment, the steps S4, S5 and S6 are combined.

According to the illustration in fig. 6, a borehole is constructed in an infinite rock mass (e.g. a complete rock mass above a down-hole passage, i.e. a small non-independent rock mass), and a rectangular coordinate system and a cylindrical coordinate system are established around the borehole, wherein the z-axes of the rectangular coordinate system and the cylindrical coordinate system are consistent. In cylindrical coordinates, the angle θ counts counterclockwise from the x-axis as positive. Assuming that the rock mass is an elastic homogeneous body, when the drill hole is under the action of the three-dimensional stress of the rock mass at the infinite position, the stress distribution formula of the surrounding rock around the drill hole is as follows:

wherein σ_x，σ_y，σ_z，τ_xy，τ_yz，τ_zxAs a rectangular coordinate systemThe three-dimensional stress of the rock mass below; sigma_r，σ_θ，σ_z'，τ_rθ，τ_θz，τ_zrThe hole edge surrounding rock stress under a cylindrical coordinate system; a is the borehole radius.

According to the theory of elastic mechanics, the relationship among stress, displacement and strain in the cylindrical coordinate is as follows:

wherein epsilon_r、ε_θ、ε_z' is a positive strain, γ_rθ、γ_θz、γ_zrFor shear strain, u is the radial displacement, v is the circumferential displacement, w is the axial displacement, E is the modulus of elasticity, μ is the Poisson's ratio,

from equation (11) to equation (16), it can be found that the strain or displacement at the borehole wall has a corresponding relationship with the hole-surrounding rock stress. From the formulas (5) to (10), it can be seen that the three-dimensional stress of the rock body can be obtained through the hole edge surrounding rock stress. Therefore, the three-dimensional stress of the rock body can be obtained by measuring the strain or displacement at the hole wall.

According to the wavelength data of the fiber bragg grating stress sensor, the strain or displacement of the hole wall of the drill hole can be calculated, and therefore the three-dimensional stress of the rock body can be obtained. In the process of obtaining the real-time stress of the hard top plate, firstly, calculating the initial stress of the hard top plate according to stress relief data (namely wavelength data of a fiber bragg grating stress sensor in the stress relief test process); then installing the fiber bragg grating stress sensor in the drill hole of the hard top plate again, and calculating the relative stress of the hard top plate according to the wavelength data of the fiber bragg grating stress sensor; and finally, superposing the initial stress and the relative stress to obtain the real-time stress of the hard top plate.

S7, calculating the real-time elastic energy density U of the hard top plate by using the real-time stress_eAnd real time ultimate stored energy U at that stress level_t。

After the real-time stress of the hard top plate is obtained, the real-time elastic energy density U of the hard top plate can be calculated_eAnd real time ultimate stored energy U at that stress level_t。

Real-time elastic energy density U_eThe calculation formula of (2) is as follows:

where E is the elastic modulus in step S2, μ is the Poisson' S ratio in step S2, σ₁、σ₂And σ₃First, second and third principal stresses, respectively, of the real-time stress in step S6.

Real-time limited stored energy U_tThe calculation formula of (2) is as follows:

U_t＝f(σ₃) (3)

where the function y ═ f (x) is the functional relationship in step S3, and σ₃The third principal stress which is the real-time stress in step S6.

S8, mixing U_eAnd U_tPerforming comparison by using the comparison junctionAnd (3) carrying out grading early warning on the top plate fracture type rock burst: the grading early warning of the roof fracture type rock burst can be realized by the following specific means:

comparing the real-time elastic energy density U in step S7_eAnd real time ultimate stored energy U at that stress level_tObtaining the risk index of rock burst;

secondly, according to the corresponding relation between the rock burst danger index and the rock burst danger, grading early warning is carried out on the top plate fracture type rock burst.

For convenience of description, the above two steps will be described in combination.

Real-time elastic energy density U_eAnd real-time limited stored energy U_tComparing the obtained results to obtain a rock burst risk index R ═ U_e/U_tAnd then, carrying out grading early warning on the top plate fracture type rock burst according to the corresponding relation between the rock burst risk index R and the rock burst risk.

Wherein, the corresponding relation between the rock burst danger index R and the rock burst danger can be determined by the following formula:

of course, in field application, the limit value can be adjusted according to actual conditions.

The following description is given with reference to specific examples.

The length of a coal face to be early warned is about 180m, the propelling length is about 1200m, the thickness of a coal layer is about 4m, the direct top of the face is a mudstone and siltstone interbed, the thickness is about 3m, and the Prussian hardness is 2-6; the basic top is fine sandstone and medium sandstone, the thickness is about 15m, and the Prussian hardness is 8-10. The basic top rock layer is large in thickness, hard in lithology and strong in elastic energy accumulation capacity. During the face extraction, roof fracture type rock burst disasters may occur.

According to the field geological data, 4 monitoring sections are planned to be arranged on the whole working face. The stress sensor of the hollow inclusion type fiber bragg grating is selected, the measuring range is [ -30MPa, 150MPa ] (the pressure is positive), and the measuring range requirement is met. The selected hollow core type fiber bragg grating stress sensor has the performances of water resistance, explosion resistance, electromagnetic interference resistance and good long-term stability.

In the embodiment of the present invention, 4 monitoring sections are arranged. Of course, in other embodiments, the number of the monitoring sections is not limited, and an appropriate number of monitoring sections may be arranged according to actual field conditions.

In the embodiment of the invention, the specific implementation process of the method for realizing the graded early warning of the roof fracture type rock burst comprises the following steps:

the step 1: a geological core drill is adopted, and a drill hole (called a large hole) is constructed from a working face to be pre-warned to a hard top plate in an auxiliary transportation roadway, namely a basic top, wherein the diameter of the large hole is about 130mm, and the length of the large hole is 10-30 m; then constructing small holes with the diameter of about 38mm at the bottom of the large hole, wherein the length of the small holes is 0.3-0.5 m; after cleaning and wiping the small hole, installing the fiber bragg grating stress sensor in the small hole; and (4) carrying out a stress relief test after the sensor is firmly bonded with the wall of the small hole. Specifically, the installation position of the fiber grating stress sensor at a certain monitoring section is shown in fig. 7. And constructing a drill hole at the monitoring section, and installing a fiber bragg grating stress sensor. The height of the drilled hole is 3m, the horizontal projection is vertical to the coal wall, the elevation angle is 30 degrees, the length of the big hole is 19.6m, and the length of the small hole is 0.4 m.

For convenience of description, the following embodiments will be developed with respect to the above-mentioned monitoring sections, and other embodiments of monitoring sections may be referred to herein.

The step 2: the core obtained at the hard top plate in step 1 was prepared into a standard cylindrical sample having a height of about 100mm and a diameter of about 50mm, and then subjected to uniaxial compression test to measure the elastic modulus (E ═ 22.15GPa) and poisson's ratio (μ ═ 0.23) of the hard top plate.

The step 3: and (3) manufacturing the core obtained from the hard top plate in the step (1) into a standard cylindrical sample with the height of about 100mm and the diameter of about 50mm, and then performing a conventional triaxial compression test. In the conventional triaxial compression test, the method is applied according to the siteForce levels 5 different ambient pressure levels were set, including 10MPa, 15MPa, 20MPa, 30MPa and 40MPa, and 3 samples were compressed under each ambient pressure level for a total of 15 samples tested. And analyzing the test data, and fitting an optimal functional relation between the maximum storage energy and the confining pressure after the maximum storage energy of each sample is obtained. In this embodiment, data fitting was performed using software Excel. In the fitting process, the x axis represents the confining pressure, the y axis represents the limit storage energy, a scatter diagram is drawn, then a trend line is added, and the optimal functional relation between the limit storage energy and the confining pressure is found out by setting different function types for the trend line. The trend line is set to be in an exponential function form, a linear function form, a logarithmic function form, a polynomial function form and a power function form in sequence, so that the fitting relation of the exponential function form is found to be best (namely R)²Maximum value of (c). Therefore, a trend line function in the form of an exponential function is selected as the optimal functional relationship between the ultimate stored energy and the confining pressure, as shown in fig. 8, where y is e^xIs an exponential function, and can also be expressed as y ═ exp (x), so as to obtain the functional relationship between the ultimate storage energy of the hard top plate and the confining pressure:

U₀＝273.12exp(0.0296σ_cp) (17)

wherein, U₀To limit the storage energy, σ_cpFor confining pressure, the function y exp (x) is an exponential function.

The step 4: solving the initial stress of the hard top plate by using the stress relief data obtained in the step 1 through a three-dimensional stress calculation formulaAs shown in table 1.

TABLE 1 initial stress of hard top plate

Wherein the stress is positive by pressure, the azimuth angle north is positive clockwise, and the inclination angle is positive from the horizontal plane upwards.

And step 5: in step 1, the method should beAfter the force release test was completed, a small hole having a diameter of about 38mm and a length of 0.4m was constructed again in the bottom of the borehole, and then the fiber grating stress sensor was mounted again in the small hole. After the sensor is installed, the stress relief test is not carried out any more, and the relative stress (delta sigma) of the hard top plate is monitored in real time on line by using the fiber bragg grating stress sensor₁，Δσ₂，Δσ₃)。

And 6: after the initial stress and the relative stress of the hard top plate are obtained, the initial stress and the relative stress are superposed to obtain the real-time stress (sigma) of the hard top plate₁，σ₂，σ₃) E.g. real time stress (σ) calculated at a time₁＝79.35MPa，σ₂＝46.73MPa，σ₃＝17.25MPa)。

The step 7: using the real-time stress (σ) in step 6₁＝79.35MPa，σ₂＝46.73MPa，σ₃17.25MPa), the real-time elastic energy density (U) of the hard top plate was calculated_e＝137.06kJ/m³) And real time ultimate stored energy (U) at that stress level_t＝455.10kJ/m³)。

Wherein the real-time elastic energy density U_eThe calculation process of (2) is as follows:

where E and μ are the elastic modulus (E ═ 22.15GPa) and poisson's ratio (μ ═ 0.23) measured in step 2, respectively.

Wherein the real-time limit storage energy U_tThe calculation process of (2) is as follows:

U_t＝273.12exp(0.0296σ₃)＝455.10kJ/m³ (19)

wherein the function y is 273.12exp (0.0296x) which is a functional relationship between the limit stored energy of the hard top plate obtained in step 3 and the confining pressure.

The step 8: the real-time elastic energy density (U) obtained in the step 7_e＝137.06kJ/m³) And real-time ultimate stored energy (U)_t＝455.10kJ/m³) Comparing the obtained results to obtain a rock burst risk index R ═ U_e/U_tWhen 137.06/455.10 is 0.3012, the early warning grade is I-grade early warning, and the grade of the rock burst danger is weak rock burst danger according to the corresponding relation between the rock burst danger index R and the rock burst danger. The correspondence between the rock burst risk index R and the rock burst risk is shown in formula (4).

By the above-mentioned specific technical measures, the following specific advantages and effects are obtainable:

1. the fiber bragg grating stress sensor is used as a monitoring sensor, so that the direct, long-term and real-time monitoring of the elastic energy density of the underground hard top plate is realized, and the change condition of the energy state of the hard top plate is mastered in time.

2. Real time stress (σ) when hard top plate₁，σ₂，σ₃) When the value is (79.35MPa, 46.73MPa, 17.25MPa), the real-time elastic energy density U at that time_e＝137.06kJ/m³Real time ultimate stored energy U at this stress level_t＝455.10kJ/m³And the rock burst danger index R is 0.3012, and according to the formula (4), the weak rock burst danger exists in the area of the monitored section, and an I-level early warning signal is immediately sent out, so that the grading early warning of the top plate fracture type rock burst is realized, and the safety production of the working face to be early warned is ensured.

3. The more detailed early warning information is provided instead of simple early warning, the mine responsible person formulates danger solving measures suitable for I-level early warning according to early warning grades, the condition that the expenditure cost is too high and normal production is influenced due to excessive danger solving measures (such as danger solving measures suitable for II-level early warning or III-level early warning) is avoided, and the production cost is effectively saved.