Surrounding rock brittle failure index evaluation method based on true triaxial total stress-strain curve

Abstract

The invention discloses a surrounding rock brittle failure index evaluation method based on a true triaxial total stress-strain curve, and belongs to the technical field of rock mechanical property evaluation.The surrounding rock brittle failure index evaluation method comprises the steps that S1, a rock sample is manufactured, and the shape of the rock sample is a cuboid; s2, the manufactured rock sample is installed in a true triaxial testing machine, a displacement sensor is installed between the surface of the rock sample and the testing machine, and an acoustic emission probe is installed between the bottom of the rock sample and the testing machine; s3, drawing a total stress-strain curve according to displacement changes in the three stress directions; the brittleness indexes of # imgabs 0 #, # imgabs 1 # and # imgabs 2 # are obtained; s4, acquiring acoustic emission signal data, calculating to obtain a shear / tensile crack propagation evolution characteristic curve, and calculating to obtain a brittleness index # imgabs3 # through the characteristic curve; s5, the brittleness index # imgabs4 #, the brittleness index # imgabs5 #, the brittleness index # imgabs6 # and the brittleness index # imgabs7 # are subjected to a multiplication weight synthesis method, and the brittleness index # imgabs8 # is obtained; according to the surrounding rock brittleness failure index evaluation method based on the true triaxial total stress-strain curve, the obtained brittleness index structure is more reliable.

Description

Technical Field

[0001] The invention relates to the technical field of rock mechanical property evaluation, and in particular to a surrounding rock brittle failure index evaluation method based on a true triaxial full stress-strain curve. Background Art

[0002] Excavating underground caverns in high-in-situ stress hard rock areas may encounter rock bursts, spalling and other damage. Rock bursts are caused by the concentration of rock stress, which causes the rock to suddenly break and release a large amount of energy, causing the rock to fly out or collapse. It is usually accompanied by strong noise and vibration, and has great destructive power, posing a serious threat to construction personnel and equipment. Spalling usually manifests itself as layer-by-layer peeling of the rock surface or small pieces of collapse. The scale is relatively small, but if not handled in time, it may develop into a more serious geological disaster. Rock brittleness evaluation is an important part of studying the destructive behavior of rocks under stress. It is one of the research and investigation tasks required before excavating caverns in medium and high in-situ stress areas. Brittleness evaluation is of great significance for predicting the form of rock failure and formulating reasonable support design and construction plans.

[0003] Previous studies have mostly studied the uniaxial condition ( ) or under pseudo triaxial conditions ( ) to evaluate the brittleness of the surrounding rock. The existing patent can refer to the application number CN201710137576.6, named a method for obtaining a rock brittleness index and a method for evaluating rock brittleness, which is a rock brittleness evaluation method established under pseudo-triaxial conditions by changing deformation and strength parameters before and after destruction; or the application number is 201911399355.1, named a shale brittleness index evaluation method based on energy evolution, which is also a rock brittleness evaluation method based on energy evolution under uniaxial conditions or pseudo-triaxial conditions. During the excavation of underground caverns, the actual stress to which the rock is subjected is true triaxial ( ), using the uniaxial condition ( ) or under pseudo triaxial conditions ( )Evaluating the brittleness of surrounding rock may reduce the reliability of the obtained results, bringing unnecessary risks to construction and design. Therefore, it is necessary to establish a rock brittle failure index evaluation method under true triaxial stress using the stress, strain, and acoustic emission signal data obtained from true triaxial tests. Summary of the invention

[0004] The purpose of the present invention is to provide a method for evaluating the brittle failure index of surrounding rock based on true triaxial full stress-strain curve to solve the problems existing in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides a method for evaluating the brittle failure index of surrounding rock based on true triaxial full stress-strain curve, comprising the following steps: S1. Prepare a rock sample in the shape of a cuboid; S2. Install the prepared rock sample into a true triaxial testing machine so that the three-axis stress is vertically loaded on the six surfaces of the rock sample respectively. Install a displacement sensor between the surface of the rock sample and the testing machine to monitor the displacement change of the rock sample in the three-axis stress direction; install an acoustic emission probe between the bottom of the rock sample and the testing machine to monitor the acoustic emission signal of the rock sample when strain occurs; S3. According to the displacement changes in the three stress directions, the strain of the rock sample in the three stress directions is obtained. , , , and draw the total stress-strain curve; based on the total stress-strain curve, obtain the brittleness index , , ; S4. Acquire acoustic emission signal data, calculate the shear / tensile crack extension evolution characteristic curve based on the acoustic emission signal data, and calculate the brittleness index considering crack evolution through the characteristic curve ; S5, Brittleness Index , , and The brittleness index considering the true triaxial full stress-strain curve and acoustic emission signal is obtained by using the multiplication weight synthesis method. .

[0006] Preferably, the three-dimensional stress in step S2 includes the maximum principal stress applied in the vertical direction , the intermediate principal stresses applied in two mutually orthogonal directions on the horizontal plane and minimum principal stress .

[0007] Preferably, step S3 specifically includes: According to the displacement changes in the three principal stress directions, the strain of the rock sample in the three principal stress directions is obtained. , , , and plot the maximum principal stress The full stress-strain curve of the strain in the three principal stress directions and the calculation of the crack volume strain , and draw the crack volume strain-axial strain curve, find the strain value corresponding to the minimum crack volume strain in the crack volume strain-axial strain curve as the crack initiation strain, and at the maximum principal stress With strain The maximum principal stress corresponding to the crack initiation strain in the total stress-strain curve is the crack initiation stress. , find the stress value when the maximum principal stress is the peak value in the three full stress-strain curves as the peak stress , find the maximum principal stress after the peak stress With strain The maximum principal stress value in the full stress-strain curve of the residual stress corresponds to the maximum principal stress value of the smooth section where the maximum principal stress value drops to , according to the peak stress The three total stress-strain curves are divided into the pre-peak crack extension section and the post-peak stress steep drop section, and the maximum principal stress in the pre-peak crack extension section and the post-peak stress steep drop section on the three total stress-strain curves is calculated. With each strain The area enclosed by the peak After the peak , after passing the peak With Fengqian The ratio of the three strain directions is obtained. , , , ,for .

[0008] Preferably, the crack volume strain is calculated , specifically: First, calculate the volume strain of the rock sample at a certain moment when the three principal stresses are applied : ; Calculation of elastic volume strain using Hooke's law : ; in, and Represent the elastic modulus and Poisson's ratio, respectively, and are obtained through the elastic segment of the total stress-strain curve; crack volume strain for: .

[0009] Preferably, step S4 specifically includes: S41. Calculate the rise angle of the acoustic emission signal by the ratio of the rise time and the maximum amplitude of the original signal monitoring data recorded by the acoustic emission probe : ; in, is the rise time; is the maximum value; S42, calculating the average frequency of the acoustic emission signal by the ratio of the ring count and duration of the original signal monitoring data recorded by the acoustic emission probe : ; in, Count the rings; for duration; S43, calculate and obtain the rising angle and the average frequency After that, the ratio of each signal monitoring data is obtained : ; when When , the acoustic emission signal is classified as a tensile fracture signal; when When the AE signal is classified as a shear fracture signal, the tensile and shear fracture signals are plotted over time, and the time when the rock sample reaches the cracking stress and peak stress is determined, and the brittleness index is calculated: ; In the formula, The tensile fracture signal count corresponding to the crack initiation stress is reached; It is the tensile fracture signal count corresponding to the peak stress; The tensile fracture signal counts corresponding to the residual stress; The shear fracture signal count corresponding to the crack initiation stress is reached; is the shear fracture signal count corresponding to the peak stress; The corresponding shear fracture signal counts when the residual stress is reached; , are the weight coefficients of the tensile and shear fracture signals, respectively, and are both set to 0.5.

[0010] Preferably, the brittleness index in step S5 is for: + + + ; in, , , Represents the brittleness index , , , The weight coefficient of = = =0.8; = 0.2; and , , .

[0011] Therefore, the present invention adopts the above-mentioned surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve, which has the following beneficial effects: (1) The brittleness index is obtained by considering the full stress-strain curve of the rock in three directions; (2) By combining the acoustic emission signal parameters, the brittleness index considering the evolution of the fracture signal is obtained; (3) Compared with evaluating the brittleness of the surrounding rock under uniaxial or pseudo-triaxial conditions, the brittleness index obtained by the above method is more reliable, making the construction and design more in line with the real environment and safer.

[0012] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a flow chart of the surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve of the present invention; Figure 2 It is a schematic diagram of a true triaxial total stress-strain curve and a crack volume-axial strain curve of an embodiment of a surrounding rock brittle failure index evaluation method based on a true triaxial total stress-strain curve of the present invention; Figure 3 It is a schematic diagram of a curve showing a change of the maximum principal stress with the test time and a curve showing a change of the shear fracture signal with the time in an embodiment of the present invention; Figure 4 It is a schematic diagram of a curve showing a change of the maximum principal stress with the test time and a curve showing a change of the tensile rupture signal with the time in an embodiment of the present invention; Figure 5 It is a position direction distribution diagram of three orthogonal principal stress directions outside the rock sample according to an embodiment of the present invention; Figure 6 This is a diagram of the brittleness index of altered rocks under different intermediate principal stresses according to an embodiment of the present invention. DETAILED DESCRIPTION

[0014] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0015] like Figure 1As shown, a method for evaluating the brittle failure index of surrounding rocks based on true triaxial full stress-strain curve is provided. In this embodiment, altered granite is used as the implementation object. The brittleness of the rock is evaluated by applying stress-strain and acoustic emission data obtained by true triaxial tests in three orthogonal directions to the rock sample: intermediate principal stress 20MPa, minimum principal stress 10MPa, and maximum principal stress, respectively. The method includes the following steps: Step 1: Prepare a granite cuboid rock sample with a length and width of 50 mm and a height of 100 mm according to the test standard; Step 2: Install the prepared rock sample into the true triaxial testing machine, install displacement sensors in three orthogonal principal stress directions, and install an acoustic emission probe at the bottom of the rock sample to monitor the acoustic emission signal. The true triaxial testing machine is used to test the rock sample in two orthogonal directions (also called circumferential directions) on the horizontal plane according to the set intermediate principal stress. is 20MPa, the minimum principal stress The true triaxial testing machine also applies the maximum principal stress to the rock sample in the vertical direction (also called axial direction) perpendicular to the horizontal plane. Until the rock sample is compressed to failure, the displacement and stress changes in the three orthogonal principal stress directions are monitored in real time, and the changes in the acoustic emission signals are monitored. The position and direction distribution of the three orthogonal principal stress directions outside the rock sample are as follows: Figure 5 shown.

[0016] Step 3: After the test, the full stress-strain curve of the true triaxial test is drawn according to the displacements and stresses obtained. Based on the full stress-strain curve obtained, the brittleness index is obtained. , , .

[0017] The calculation is as follows: First, the maximum principal stress The strain curves in the three principal stress directions are plotted as follows: Figure 2 The black line on the right side of a is the maximum principal stress Strain in the direction of maximum principal stress The axial strain variation curve is as follows: Figure 2 The solid black line with a circular pattern in the figure is the maximum principal stress. Strain in the direction of minimum principal stress The hoop strain curve is as follows: Figure 2 The black solid line with a triangular pattern in the figure is the maximum principal stress. Strain in the direction of the intermediate principal stress The circumferential strain curve shows that the rock cracks are open when not compressed, and the axial strain and hoop strain in two horizontal orthogonal directions , When the rock sample is subjected to isotropic stress, it begins to compress and moves in the axial direction with the maximum principal stress. becomes larger, and the axial compression causes the axial strain increases; in two horizontal orthogonal directions with the maximum principal stress becomes larger, generating new cracks, causing annular expansion. Figure 2 The left side shows the strain , Become smaller.

[0018] Then, by calculating the crack volume strain, a curve of crack volume strain-axial strain is drawn, where the rock sample volume strain at a certain moment of true triaxial stress loading is: ; In the formula, are the strains in the three principal stress directions respectively.

[0019] The elastic modulus and Poisson's ratio can be obtained from the elastic section of the uniaxial compression stress-strain curve ; ; in , , , are the compressive stress and the corresponding strain of the rock in the elastic stage during uniaxial compression, , are the lateral strain and axial strain of uniaxial compression, respectively.

[0020] The volume strain of rock samples can be divided into elastic volume strain and plastic volume strain caused by cracking or extension of primary cracks. The elastic volume strain can be calculated by Hooke's law: ; In the formula, is the elastic volume strain, is the volume strain of the rock sample; is the maximum principal stress, which is the value at a certain moment of loading; is the intermediate principal stress, which is a fixed value of 20MPa; is the minimum principal stress, which is a fixed value of 10 MPa.

[0021] Therefore, the volume strain of the test crack can be obtained as , the crack volume strain calculated by ,like Figure 2 As shown by the black solid line in the figure, the crack volume strain-axial strain curve is drawn. When not compressed, the rock crack is open. After being subjected to all-directional stress, it begins to compress. The crack closure causes the axial strain to increase, while the crack volume strain decreases. Figure 2 The right curve in the middle reaches the highest point (the minimum point of crack volume strain), and further compression produces new cracks that open, resulting in axial compression and outward expansion in the annular direction. The annular expansion capacity is greater than the axial compression. Therefore, the crack volume strain-axial strain curve gradually decreases from the highest point, and the crack volume strain is increasing. The axial strain corresponding to the boundary where the curve changes from compression to expansion (i.e., the highest point of the crack volume strain-axial strain curve) is found from the crack volume strain-axial strain curve as the crack initiation axial strain.

[0022] Taking the axial strain change curve as an example, the axial stress corresponding to the axial strain of crack initiation is found in the axial strain change curve as the crack initiation stress is 174.156MPa, and then find the peak stress of the axial stress rock sample corresponding to the highest point from the axial strain change curve : 380.732 MPa, and then find the axial stress corresponding to the smooth section where the axial strain change curve drops to after the peak stress from the axial strain change curve, which is the residual stress of the curve : 155.08 MPa.

[0023] In the circumferential strain curve, the crack initiation stress The same as in the axial strain change curve, the peak stress is the axial stress corresponding to the highest point of the annular strain change curve, and the residual stress is the axial stress corresponding to the smooth section to which the annular strain change curve drops after the peak stress; in the annular strain change curve, the peak stress is the axial stress corresponding to the highest point of the annular strain change curve, and the residual stress is the axial stress corresponding to the smooth section to which the annular strain change curve drops after the peak stress.

[0024] Then, the axial strain change curve, the annular strain change curve, and the annular strain change curve can be divided into the pre-peak crack extension section and the post-peak stress steep drop section by the peak stress, which can be calculated by the following formula: Stress in the post-peak stress drop section With strain The enclosed area, Pre-peak crack extension stress With strain The enclosed area, : ; ; The brittleness index determined by the maximum principal stress, intermediate principal stress, and minimum principal stress is: ; Step 4: Based on the acquired acoustic emission signal data, the shear / tensile crack growth evolution characteristic curve is calculated, and the brittleness index considering the crack evolution is calculated through the curve. : The acoustic emission signal data processing calculation method in step 4 is as follows: The rise angle of the acoustic emission signal is calculated by the ratio of the rise time and the maximum amplitude. : ; in, is the rise time; The maximum amplitude can be directly monitored by the original signal recorded by the acoustic emission probe.

[0025] Then through the ring count ( ) and duration ( ) to calculate the average frequency of the acoustic emission signal : ; Among them, the ring count ( ) and duration ( ) can monitor data directly from the raw signal recorded by the acoustic emission probe. The ring count is the number of oscillations in the acoustic emission signal waveform that exceed the preset threshold. That is, the number of times the amplitude exceeds the preset threshold (also called the acoustic emission threshold).

[0026] In the calculation, the rise angle and the average frequency Then we can get the ratio of each signal monitoring data : ; The existing technology can use the acoustic emission signal of granite The values ​​can be distinguished into fracture and stretch signals. When , the acoustic emission signal is classified as a tensile fracture signal; when When , the acoustic emission signal is classified as a shear fracture signal; The values ​​are expressed as follows: ; ; Through the above discrimination, the monitored acoustic emission signals can be divided into shear fracture signals and tensile fracture signals, and the cumulative amount of shear and tensile fracture signals over time can be further obtained, and the cumulative amount of shear and tensile fracture signals over time can be plotted, respectively. Figure 3 and Figure 4 . At the same time Figure 3 and Figure 4The maximum principal stress is plotted over time, and then the tensile fracture signal counts and shear fracture signal counts corresponding to the time when the maximum principal stress reaches the crack initiation stress, peak stress, and residual stress are calculated.

[0027] Specific The tensile fracture signal count corresponding to the crack initiation stress is reached; It is the tensile fracture signal count corresponding to the peak stress; The tensile fracture signal counts corresponding to the residual stress; The shear fracture signal count corresponding to the crack initiation stress is reached; is the shear fracture signal count corresponding to the peak stress; To count the shear fracture signals corresponding to the residual stress, Figure 3 and Figure 4 In the above, we can know that: Therefore, the brittleness index calculated from the acoustic emission parameters is as follows: ; In the above formula, the evolution characteristics of shear and tensile fracture signals are different and need to be explained separately, but they are equally important and have equal weights, so , Both are 0.5.

[0028] Step 5: Brittleness Index , , and The brittleness index considering the true triaxial full stress-strain curve and acoustic emission signal is obtained by using the multiplication weight synthesis method.

[0029] In summary, the brittleness index evaluation considering the true triaxial full stress-strain curve and acoustic emission signal is as follows: ; in, , Consider each principal stress as 80% of the ratio of the principal stress invariant. Consider a weight of 20%.

[0030] , , ; Figure 6This is the result of evaluating the brittleness index of altered rock. Under four different intermediate principal stresses, the same minimum principal stress is maintained, and the true triaxial full stress-strain curve and acoustic emission signal data are obtained. The brittleness index B is obtained according to the above brittleness evaluation method. It can be seen from the figure that as the intermediate principal stress increases, the brittleness of granite increases, which is consistent with the results of rock brittleness research in a true triaxial environment using most existing technologies.

[0031] Therefore, the present invention adopts the above-mentioned surrounding rock brittle failure index evaluation method based on the true triaxial full stress-strain curve, and the brittle index structure obtained is more reliable.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for evaluating surrounding rock brittle failure index based on true triaxial full stress-strain curve, characterized in that: The following steps are involved: S1. Prepare a rock sample in the shape of a cuboid; S2. Install the prepared rock sample into a true triaxial testing machine so that the three-axis stress is vertically loaded on the six surfaces of the rock sample respectively. Install a displacement sensor between the surface of the rock sample and the testing machine to monitor the displacement change of the rock sample in the three-axis stress direction; install an acoustic emission probe between the bottom of the rock sample and the testing machine to monitor the acoustic emission signal of the rock sample when strain occurs; S3. According to the displacement changes in the three stress directions, the strain of the rock sample in the three stress directions is obtained. , , , and draw the total stress-strain curve; based on the total stress-strain curve, obtain the brittleness index , , ; S4. Acquire acoustic emission signal data and calculate the shear / tensile crack extension evolution characteristic curve, and calculate the brittleness index through the characteristic curve ; S5, Brittleness Index , , and The fragility index is obtained by using the multiplication weight synthesis method. .

2. The surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve according to claim 1 is characterized by: The three-dimensional stress in step S2 includes the maximum principal stress applied in the vertical direction , the intermediate principal stresses applied in two mutually orthogonal directions on the horizontal plane and minimum principal stress .

3. The surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve according to claim 2 is characterized in that: Step S3 specifically includes: According to the displacement changes in the three principal stress directions, the strain of the rock sample in the three principal stress directions is obtained. , , , and plot the maximum principal stress The full stress-strain curve of the strain in the three principal stress directions and the calculation of the crack volume strain , and draw the crack volume strain-axial strain curve, find the strain value corresponding to the minimum crack volume strain in the crack volume strain-axial strain curve as the crack initiation strain, and at the maximum principal stress With strain The maximum principal stress corresponding to the crack initiation strain in the total stress-strain curve is the crack initiation stress. , find the stress value when the maximum principal stress is the peak value in the three full stress-strain curves as the peak stress , find the maximum principal stress after the peak stress With strain The maximum principal stress value in the full stress-strain curve of the residual stress corresponds to the maximum principal stress value of the smooth section where the maximum principal stress value drops to , according to the peak stress The three total stress-strain curves are divided into the pre-peak crack extension section and the post-peak stress steep drop section, and the maximum principal stress in the pre-peak crack extension section and the post-peak stress steep drop section on the three total stress-strain curves is calculated. With each strain The area enclosed by the peak After the peak , after passing the peak With Fengqian The ratio of the three strain directions is obtained. , , , ,for .

4. The surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve according to claim 3 is characterized in that: Calculation of crack volume strain , specifically: First, calculate the volume strain of the rock sample at a certain moment when the three principal stresses are applied : ； Calculation of elastic volume strain using Hooke's law : ； in, and Represent the elastic modulus and Poisson's ratio, respectively, and are obtained through the elastic segment of the total stress-strain curve; crack volume strain for: 。 5. The surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve according to claim 1 is characterized in that: Step S4 specifically includes: S41. Calculate the rise angle of the acoustic emission signal by the ratio of the rise time and the maximum amplitude of the original signal monitoring data recorded by the acoustic emission probe : ； in, is the rise time; is the maximum value; S42, calculating the average frequency of the acoustic emission signal by the ratio of the ring count and duration of the original signal monitoring data recorded by the acoustic emission probe : ； in, Count the rings; for duration; S43, calculate and obtain the rise angle and the average frequency After that, the ratio of each signal monitoring data is obtained : ； when When , the acoustic emission signal is classified as a tensile fracture signal; when When the AE signal is classified as a shear fracture signal, the tensile and shear fracture signals are plotted over time, and the time when the rock sample reaches the cracking stress and peak stress is determined, and the brittleness index is calculated: ； In the formula, The tensile fracture signal count corresponding to the crack initiation stress is reached; It is the tensile fracture signal count corresponding to the peak stress; The tensile fracture signal counts corresponding to the residual stress; The shear fracture signal count corresponding to the crack initiation stress is reached; is the shear fracture signal count corresponding to the peak stress; The corresponding shear fracture signal counts when the residual stress is reached; , are the weight coefficients of the tensile and shear fracture signals, respectively, and are both set to 0.

5.

6. The surrounding rock brittle failure index evaluation method based on true triaxial full stress-strain curve according to claim 3 is characterized in that: Brittleness index in step S5 for: + + + ； in, , , Represents the brittleness index , , , The weight coefficient of = = =0.8; =0.2; , , .

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