Coal rock damage stress threshold value determination method

Abstract

The invention provides a coal rock damage stress threshold determination method. The method continuously monitors the coal rock microcrack propagation process by using acoustic emission technology, and captures the changes near the stress threshold. The acoustic emission data node determination method based on derivative analysis avoids the errors caused by manual determination of node position, makes the node results more objective and reliable, and realizes scientific processing of data. A coal rock damage stress threshold database is established, combined with interpolation algorithm to quickly find the coal rock damage stress threshold, and the stress threshold result is quickly obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of underground engineering safety, and in particular to a coal rock damage stress threshold determination method. BACKGROUND

[0002] In the process of underground resource development and underground space utilization, with the continuous increase of mining depth, the problem of coal rock stability is increasingly prominent. During the implementation of underground engineering, blasting and excavation operations will inevitably change the stress state of the surrounding coal rock, and the coal rock is in a complex mechanical environment. The change of stress state has a profound and complex influence on the threshold of coal rock. Stress changes can act on coal rock in various ways, such as loading, unloading or dynamic stress process. With the increase of stress, the internal structure of coal rock gradually changes, resulting in changes in its bearing capacity and deformation characteristics. At the same time, the long-term effect of stress may also trigger the rheological behavior of coal rock, further affecting its threshold. The reduction or unloading of stress may cause the threshold of coal rock to recover or adjust to varying degrees. In summary, stress changes have an important influence on the change of coal rock damage stress threshold.

[0003] In the field of deep underground engineering in China, coal rock stability evaluation is an important means to ensure the safety of underground engineering. Changes in the damage stress threshold of coal rock will bring great difficulty and instability factors to the evaluation of coal rock stability. Therefore, determining the damage stress threshold of coal rock is an important technology for evaluating coal rock stability and realizing the safety of deep underground engineering. Relevant research results show that the coal rock failure evolution process can be divided into initial crack closure stage, elastic stage, crack stable expansion stage and crack unstable expansion stage according to the closure stress, crack initiation stress, damage stress and peak stress. The threshold points in each stage represent the nodes of coal rock damage development and change. Determining the damage stress threshold of coal rock can help understand the stress state at which coal rock begins to be damaged, so as to predict potential dangers that may be faced in underground resource development and underground space utilization.

[0004] However, the existing coal rock stress threshold determination method usually has low sensitivity in identifying stress threshold, and can only discover the macroscopic damage of coal rock after the stress reaches a certain threshold, which is difficult to detect the occurrence of damage in the early stage, and there is a certain error in obtaining the damage stress. In addition, the International Society for Rock Mechanics has not yet recommended a method for determining the coal rock stress threshold. At the same time, the research results of coal rock damage stress threshold are scattered, and the data is difficult to obtain and integrate systematically, which cannot be fully utilized in practical application.

[0005] Therefore, it is of great significance to provide a coal rock damage stress threshold determination method. SUMMARY

[0006] The purpose of the present application is to provide a coal rock damage stress threshold determination method to solve the problems in the prior art.

[0007] The technical solution adopted to achieve the purpose of the present application is as follows: a coal rock damage stress threshold determination method comprises the following steps:

[0008] 1) The target coal rock is made into a coal rock sample.

[0009] 2) The coal rock sample is subjected to a one-time loading experiment, a cyclic loading and unloading compression experiment, a shear experiment and a tensile experiment, respectively. In the experiment process, an acoustic emission sensor is used to continuously collect acoustic emission signals generated by the damage and rupture of the coal rock sample, and the coal rock microcrack expansion and slip process is monitored.

[0010] 3) According to the axial stress and acoustic emission ringing cumulative count data of the coal rock sample, an acoustic emission ringing cumulative count-axial stress initial curve is drawn.

[0011] 4) The acoustic emission ringing cumulative count-axial stress initial curve is subjected to preliminary visual inspection, and the curve form is judged. The curve form includes an S-shaped Boltzmann curve, an exponential growth curve and a double S-shaped curve.

[0012] 5) The acoustic emission ringing cumulative count-axial stress initial curve is fitted. Based on derivative analysis, the acoustic emission data node is determined as the damage stress threshold.

[0013] 6) Different types of coal rock samples are prepared, and steps 2) to 5) are repeated to obtain the damage stress thresholds of various types of coal rocks under different mechanical conditions. Through an interpolation algorithm, a coal rock damage stress threshold database is constructed.

[0014] 7) According to the actual working conditions and coal rock conditions, the actual working stress conditions and the types of coal rocks are input into the database to obtain the damage stress thresholds of the coal rocks in the field under the corresponding working conditions.

[0015] Further, in step 1), the large rock sample is processed into a cylindrical raw rock sample with a size of 50x100mm through cutting, grinding and polishing.

[0016] Further, when the curve conforms to the S-shaped Boltzmann curve form, step 5 specifically comprises the following sub-steps:

[0017] A5.1) The initial curve is further fitted into an S-shaped Boltzmann curve.

[0018] A5.2) Let the acoustic emission cumulative count be N, the axial stress be σ, the data point sequence be (σ1, N1), (σ2, N2), … (σn, Nn), and the slope of a point (σi, Ni) be Second derivative Let the second derivative of the curve be zero Finding the inflection point of the stable growth phase (x0-d x ·ln(2), ) and the inflection point of the failure phase (x0+d x ·ln(2), ).

[0019] A5.3) After determining the positions of the two inflection points, draw the tangent lines at the two inflection points, and find the intersection point (dσ+σ0, ) of the two tangent lines, which is the damage stress threshold of coal and rock.

[0020] Further, step A5.1) specifically includes the following sub-steps:

[0021] A5.1.1) Import the data into Origin software.

[0022] A5.1.2) Use the nonlinear curve fitting in Origin software, and select the Boltzmann function model.

[0023] A5.1.3) According to the data range and physical meaning, set reasonable initial values and constraint conditions for the fitting parameters A1, A2, x0 and d x .

[0024] A5.1.4) Check the goodness of fit R 2 in the fitting report. If R 2 ≤ 0.95, adjust the values of A1, A2, x0 and d x until R 2 meets the requirements.

[0025] Further, when the curve conforms to the exponential growth curve form, step 5 specifically includes the following sub-steps:

[0026] B5.1) Further fit the initial curve as an exponential growth curve.

[0027] B5.2) Let the acoustic emission cumulative number be N, the axial stress be σ, the data point sequence be (σ1, N1), (σ2, N2), … (σ n , N n ), and the slope at any point (σ i , N i ) be

[0028] B5.3) When a point k i = 90%·k max , confirm that the horizontal coordinate of the point is the damage stress threshold.

[0029] Further, step B5.1) specifically includes the following sub-steps:

[0030] B5.1.1) Import data into Origin software.

[0031] B5.1.2) Use nonlinear curve fitting in Origin software, select ExpDecay2 function model.

[0032] B5.1.3) Set reasonable initial values for fitting parameters A, k according to data range and physical meaning.

[0033] B5.1.4) Check goodness of fit R 2 in the fitting report. If R 2 ≤ 0.95, adjust the values of A, k until R 2 meets the requirements.

[0034] Further, when the curve conforms to the double S-shaped curve form, step 5 specifically comprises the following sub-steps:

[0035] C5.1) Further fit the initial curve into a double S-shaped curve.

[0036] C5.2) Let the acoustic emission cumulative number be N, the axial stress be σ, the data point sequence be (σ1, N1), (σ2, N2), … (σ n , N n ). The slope at any point (σ i , N i ) is The second derivative is

[0037] C5.3) The last point where the second derivative of the second segment of the curve is zero is the end inflection point, and the horizontal coordinate of the point is confirmed as the damage stress threshold.

[0038] Further, step C5.1) specifically comprises the following sub-steps:

[0039] C5.1.1) Import data into Origin software.

[0040] C5.1.2) Use Open Dialog in Origin software, and define a custom function y = A1 / (1+exp(-(x-x01) / dx1))+A2 / (1+exp(-(x-x02) / dx2)) using New Function.

[0041] C5.1.3) Set reasonable initial values for fitting parameters A1, A2, x 01 , x 02 , dx1 and dx2 according to data range and physical meaning.

[0042] C5.1.4) Check goodness of fit R 2 in the fitting report. If R2 ≤0.95, then adjust the values of A1, A2, x 01 , x 02 , dx1 and dx2 until R 2 meets the requirements.

[0043] The technical effects of the present application are self-evident:

[0044] A. In view of the problem that the stress threshold of coal rock damage is difficult to determine, the advantages of acoustic emission sensitivity to micro damage and high numerical fitting accuracy are fully combined, the effective information related to coal rock damage is more clearly identified, the coal rock damage characteristics are simply represented, and thus the stress threshold of coal rock damage is accurately obtained, thereby providing support for realizing the safety of deep underground engineering.

[0045] B. In view of the problem that the stress threshold of coal rock is affected by the type of coal rock and loading conditions, an interpolation algorithm is effectively used to construct a damage stress threshold database of various types of coal rock under different mechanical conditions, and the damage stress threshold of coal rock can be quickly found according to the actual working conditions and environment. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 A flow chart for determining the stress threshold of coal rock damage.

[0047] Figure 2 A schematic diagram of the arrangement of acoustic emission probes. DETAILED DESCRIPTION

[0048] The present application will be further described below in conjunction with examples, but should not be understood as limiting the above-mentioned subject matter of the present application to the following examples. Various substitutions and modifications can be made according to ordinary technical knowledge and conventional means in the art without departing from the above-mentioned technical idea of the present application, and all such substitutions and modifications should be included in the protection scope of the present application.

[0049] Example 1:

[0050] Referring to Figure 1 , the present embodiment provides a method for determining the stress threshold of coal rock damage, comprising the following steps:

[0051] 1) The target coal rock is made into a coal rock sample.

[0052] 2) The coal rock sample is subjected to compression, shear and tension tests. The compression, shear and tension tests all include a one-time loading compression test and a cyclic loading and unloading test, and the tests include different stress paths. During the test process, an acoustic emission sensor continuously collects acoustic emission signals generated by the damage and rupture of the coal rock sample, and monitors the micro crack propagation and slip process of the coal rock. The acoustic emission sensor is attached to the surface of the fixed pressure head or the surface of the coal rock sample.

[0053] 3) According to the axial stress and acoustic emission ring count data of the coal rock sample, an acoustic emission ring count-axial stress initial curve is drawn.

[0054] 4) The acoustic emission ring count-axial stress initial curve is preliminarily visually inspected to determine the curve form. The curve form includes an S-shaped Boltzmann curve, an exponential growth curve and a double S-shaped curve.

[0055] 5) The acoustic emission ring count-axial stress initial curve is fitted. Based on derivative analysis, the acoustic emission data node is determined as a damage stress threshold.

[0056] 6) Different types of coal rock samples are prepared, and steps 2) to 5) are repeated to obtain the damage stress thresholds of various types of coal rocks under different mechanical conditions. An interpolation algorithm is used to construct a coal rock damage stress threshold database.

[0057] 7) According to the actual working conditions and coal rock conditions, the actual working stress conditions and coal rock types are input into the database to obtain the damage stress thresholds of the coal rocks under the corresponding working conditions.

[0058] The embodiment provides a clear and systematic method for conveniently and accurately determining the stress threshold of coal rocks, which has extremely important practical significance for engineering construction, design, construction and stability evaluation.

[0059] Embodiment 2:

[0060] The main content of the embodiment is the same as that of embodiment 1, and in step 1), the large rock sample is processed into a cylindrical original rock sample with a size of 50*100 mm through cutting, grinding and polishing. The acoustic emission sensor arrangement position is shown in Figure 2 The numbers in the figure represent the acoustic emission probe numbers.

[0061] Embodiment 3:

[0062] The main content of the embodiment is the same as that of embodiment 1 or 2, and when the curve conforms to the S-shaped Boltzmann curve form, step 5 specifically includes the following sub-steps:

[0063] A5.1) The initial curve is further fitted as an S-shaped Boltzmann curve. Step A5.1) specifically includes the following sub-steps:

[0064] A5.1.1) The data is imported into the Origin software.

[0065] A5.1.2) The nonlinear curve fitting in the Origin software is used, and the Boltzmann function model is selected.

[0066] A5.1.3) According to the data range and physical meaning, the fitting parameters A1, A2, x0 and dx Set reasonable initial values and constraints.

[0067] A5.1.4) Check the goodness of fit R in the fitting report 2 . If R 2 ≤ 0.95, adjust the values of A1, A2, x0 and d x until R 2 meets the requirements.

[0068] A5.2) Let the acoustic emission cumulative number be N, the axial stress be σ, the data point sequence be (σ1, N1), (σ2, N2), … (σn, Nn), and the slope at a point (σi, Ni) be The second derivative Let the second derivative of the curve be zero Find the inflection point (x0-d x ·ln(2), ) in the stable growth stage of the curve and the inflection point (x0+d x ·ln(2), ) in the failure stage.

[0069] A5.3) After determining the positions of the two inflection points, draw the tangent lines at the two inflection points and find the intersection point (dσ+σ0, ) of the two tangent lines, which is the coal rock damage stress threshold.

[0070] When the curve conforms to the exponential growth curve form, step 5 specifically includes the following sub-steps:

[0071] B5.1) Further fit the initial curve as an exponential growth curve. Step B5.1) specifically includes the following sub-steps:

[0072] B5.1.1) Import the data into the Origin software.

[0073] B5.1.2) Use the nonlinear curve fitting in the Origin software and select the ExpDecay2 function model.

[0074] B5.1.3) Set reasonable initial values for the fitting parameters A and k according to the data range and physical meaning.

[0075] B5.1.4) Check the goodness of fit R in the fitting report 2 . If R 2 ≤ 0.95, adjust the values of A and k until R 2 meets the requirements.

[0076] B5.2) Let the acoustic emission cumulative number be N, the axial stress be σ, the data point sequence be (σ1, N1), (σ2, N2), … (σ n , N n), the slope at any point (σ i , N) is i

[0077] B5.3) When a point k i = 90% k max , the abscissa of the point is confirmed as the damage stress threshold. When the curve conforms to the double S-shaped curve form, step 5 specifically comprises the following sub-steps:

[0078] C5.1) Further fitting the initial curve as a double S-shaped curve. Step C5.1) specifically comprises the following sub-steps:

[0079] C5.1.1) Import the data into the Origin software.

[0080] C5.1.2) Use Open Dialog in the Origin software to define a custom function y = A1 / (1+exp(-(x-x01) / dx1))+A2 / (1+exp(-(x-x02) / dx2)) using New Function.

[0081] C5.1.3) Set reasonable initial values for the fitting parameters A1, A2, x 01 , x 02 , dx1 and dx2 according to the data range and physical meaning.

[0082] C5.1.4) Check the goodness of fit R 2 in the fitting report. If R 2 ≤ 0.95, adjust the values of A1, A2, x 01 , x 02 , dx1 and dx2 until R 2 meets the requirements.

[0083] C5.2) Set the acoustic emission cumulative number as N, the axial stress as σ, and the data point sequence as (σ1, N1), (σ2, N2), … (σ n , N n ). The slope at any point (σ i , N i ) is The second derivative is

[0084] C5.3) The last point of the second curve where the second derivative is zero is the end inflection point, and the abscissa of the point is confirmed as the damage stress threshold.

[0085] ​The acoustic emission technology is used to continuously monitor the micro crack propagation and slip process of coal and rock, capture the change near the stress threshold, and make up for the insufficient information provided by traditional methods (such as destructive test method and stress-strain curve). The acoustic emission data node determination method based on derivative analysis is proposed to avoid the error caused by the artificial determination of node position, make the node result more objective and reliable, and realize the scientific processing of data. The coal and rock damage stress threshold database is established, the interpolation algorithm is combined to quickly find the coal and rock damage stress threshold, and the quick acquisition of stress threshold result is realized.

[0086] Embodiment 4

[0087] In the step A5.1.3), A1 can be initially set as the maximum value of the acoustic emission cumulative count, A2 can be initially set as the minimum value of the acoustic emission cumulative count, x0 can be initially set as the middle value of the stress data, and d x can be initially set as a small positive number or set as At the same time, the constraint conditions of the parameters can be set according to the actual situation, such as A1>A2, d x >0, etc., to ensure the rationality of the fitting result.

[0088] In the step B5.1.3), A is set as the maximum value of the acoustic emission ring count, and k is set as 0.1-1.

[0089] In the step C5.1.3), A1 and A2 are initially set as half of the maximum value of the acoustic emission ring count data, x 01 and x 02 are initially set as the center points of the two stages of growth, and dx1 and dx2 are set as 5 or other small values.

Claims

1. A method for determining a coal rock damage stress threshold, characterized in that: The following steps are involved: 1) preparing target coal rock into coal rock samples; 2) Compression, shear, and tensile tests were performed on the coal and rock samples. During the experiments, acoustic emission sensors were used to continuously collect acoustic emission signals generated by damage and fracture of the coal and rock samples to monitor the expansion process of microcracks in the coal and rock. 3) Based on the axial stress and acoustic emission ringing cumulative number data of the coal and rock samples, draw the acoustic emission ringing cumulative number-axial stress initial curve; 4) Perform a preliminary visual inspection of the acoustic emission ringing cumulative number-axial stress initial curve to determine the curve form; the curve forms include S-shaped Boltzmann curve, exponential growth curve, and double S-shaped curve; 5) Fitting the acoustic emission ringing cumulative number-axial stress initial curve; determining the acoustic emission data node as the damage stress threshold based on derivative analysis; 6) Prepare different types of coal and rock samples, repeat steps 2) to 5) to obtain the damage stress thresholds of various types of coal and rock under different mechanical conditions; and construct a coal and rock damage stress threshold database using an interpolation algorithm; 7) According to the actual working conditions and coal rock conditions on site, the actual working stress conditions and coal rock types are input into the database to obtain the damage stress threshold of the coal rock on site under the corresponding working conditions.

2. The method for determining the coal rock damage stress threshold according to claim 1, characterized in that: In step 1), the large rock sample is processed into a cylindrical original rock sample with a size of 50×100 mm by cutting, grinding and polishing.

3. The method for determining the coal rock damage stress threshold according to claim 1, characterized in that: In step 2), the compression, shear and tensile tests all include a single loading compression test and a cyclic loading and unloading test, and the experiments include different stress paths.

4. The method for determining the coal rock damage stress threshold according to claim 1, characterized in that: When the curve conforms to the S-shaped Boltzmann curve, step 5 specifically includes the following sub-steps: A5.1) further fitting the initial curve into an S-shaped Boltzmann curve; A5.2) Let N be the cumulative number of acoustic emissions, σ be the axial stress, and the data point sequence be (σ1, N1), (σ2, N2), … (σn, Nn). The slope at a point (σi, Ni) is Second-order derivative Set the second derivative of the curve to zero Find the inflection point of the curve in the stable growth stage Inflection point of destruction stage A5.3) After determining the locations of the two inflection points, draw the tangent lines at the two inflection points and find the intersection of the two tangent lines. This is the stress threshold of coal rock damage.

5. A method for determining a coal rock damage stress threshold according to claim 4, characterized in that: Step A5.1) specifically includes the following sub-steps: A5.1.1) Import the data into Origin software; A5.1.2) Use nonlinear curve fitting in Origin software and select the Boltzmann function model; A5.1.3) According to the data range and physical meaning, the fitting parameters A1, A2, x0 and d x Set reasonable initial values ​​and constraints; A5.1.4) Check the goodness of fit R in the fit report 2 If R 2 ≤0.95, then adjust A1, A2, x0 and d x The value of R 2 Meets the requirements.

6. The method for determining the coal rock damage stress threshold according to claim 1, characterized in that: When the curve conforms to the exponential growth curve form, step 5 specifically includes the following sub-steps: B5.1) Further fit the initial curve into an exponential growth curve; B5.2) Let N be the cumulative number of acoustic emissions, σ be the axial stress, and the sequence of data points be (σ1, N1), (σ2, N2), … (σ n , N n ), any point (σ i , N i ) is B5.3) When a point k i =90%·k max , it is confirmed that the horizontal coordinate of this point is the damage stress threshold.

7. The method for determining the coal rock damage stress threshold according to claim 6, characterized in that: Step B5.1) specifically includes the following sub-steps: B5.1.1) Import the data into Origin software; B5.1.2) Use nonlinear curve fitting in Origin software, selecting the ExpDecay2 function model; B5.1.3) Set reasonable initial values ​​for the fitting parameters A and k based on the data range and physical meaning; B5.1.4) Check the goodness of fit R in the fit report 2 If R 2 ≤0.95, then adjust the values ​​of A and k until R 2 Meets the requirements.

8. The method for determining the coal rock damage stress threshold according to claim 1, characterized in that: When the curve conforms to the double S-shaped curve form, step 5 specifically includes the following sub-steps: C5.1) further fitting the initial curve into a double S-shaped curve; C5.2) Let N be the cumulative number of acoustic emissions, σ be the axial stress, and the sequence of data points be (σ1, N1), (σ2, N2), … (σ n , N n ); any point (σ i , N i ) is The second-order derivative is C5.3) The last point in the second segment of the curve where the second derivative is zero is the final inflection point. Confirm that the horizontal coordinate of this point is the damage stress threshold.

9. The method for determining the coal rock damage stress threshold according to claim 8, characterized in that: Step C5.1) specifically includes the following sub-steps: C5.1.1) Import the data into Origin software; C5.1.2) Using the Open Dialog in Origin, use the New Function function to define the custom function y = A1 / (1+exp(-(x-x01) / dx1))+A2 / (1+exp(-(x-x02) / dx2)); C5.1.3) According to the data range and physical meaning, the fitting parameters A1, A2, x 01 、x 02 , dx1 and dx2 set reasonable initial values; C5.1.4) Check the goodness of fit R in the fit report 2 If R 2 ≤0.95, then adjust A1, A2, x 01 、x 02 , dx1 and dx2 values, until R 2 Meets the requirements.

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