Rock characteristic stress identification method based on wave impedance characteristics

Abstract

The application discloses a rock characteristic stress identification method based on wave impedance characteristics, and comprises the following steps: S1, preparing a labeled rock sample; S2, collecting the longitudinal wave velocity under the uniaxial compressive strength condition; S3, calculating the rock density at each longitudinal wave velocity collection time; S4, calculating the rock wave impedance according to the data collected in S2 and S3; and S5, calculating the axial stress according to the axial load corresponding to each longitudinal wave velocity measurement time. The application provides a reliable and convenient quantitative identification method for the closed stress, cracking stress and damage stress of brittle rock. The method overcomes the limitations of the crack volume strain method, such as the solving precision and reliability, and the influence of the rock elastic modulus, Poisson's ratio value and natural fissure development degree.

Description

Technical Field

[0001] This invention relates to a method for identifying characteristic stresses in rocks based on wave impedance characteristics. Background Technology

[0002] Characteristic stress in rocks is a core concept in rock mechanics and engineering geology. It is crucial for elevating the mechanical behavior of geological bodies from qualitative description to quantitative calculation, enabling engineering decisions to be based on verifiable and repeatable mechanical foundations. It serves as a bridge between laboratory testing and field engineering, and is also the cornerstone of theoretical analysis and numerical simulation, ultimately contributing to the safety, economy, and reliability of engineering projects. Based on the deformation and failure modes of rocks, its research objects mainly include brittle rocks and ductile rocks.

[0003] Compared to ductile rocks, brittle rocks exhibit low strain and sudden failure characteristics. This is typically determined based on the closure stress. Cracking stress With damage stress The magnitude of these stresses quantitatively divides the pre-failure stage of brittle rocks into four phases: microcrack closure, elastic deformation, stable microcrack propagation, and unstable microcrack fracture. The closure stress reflects the initial damage level (microcrack density) and marks the transition from nonlinear compaction to linear elastic deformation. The initiation stress characterizes the transition from elastic deformation to damage accumulation, representing the critical stress value at which newly formed cracks begin stable propagation within the rock. The damage stress indicates the critical stress value at which the damage accumulation rate exceeds the material's self-healing ability, leading to the unstable propagation stage of internal cracks. Therefore, determining the closure stress, initiation stress, and damage stress of brittle rocks is a crucial prerequisite for studying the microscopic mechanisms of brittle rock failure and analyzing its engineering stability.

[0004] Currently, the most commonly used method for determining the closure stress, crack initiation stress, and damage stress of brittle rocks is the crack volumetric strain method. This method requires collecting axial stress, axial strain, and radial strain data of the specimen under uniaxial compression conditions. Then, the volumetric strain, elastic modulus, and Poisson's ratio are calculated. Based on this, the crack volumetric strain is calculated. Finally, the closure stress, crack initiation stress, and damage stress of the brittle rock are determined graphically. Figure 8 ).

[0005] The technical problems of solving for characteristic stresses using the crack volumetric strain method are as follows:

[0006] (1) The values ​​of elastic modulus and Poisson's ratio are subject to certain subjective arbitrariness.

[0007] The rationality and accuracy of the characteristic stress values ​​are closely related to the values ​​of the elastic modulus and Poisson's ratio. In the "Test Methods for Rock Mass in Engineering" (GB / T 50266-2013), the elastic modulus of rock is the slope corresponding to the "straight line segment" on the stress-strain curve, and the Poisson's ratio is the ratio of radial strain to axial strain corresponding to the "straight line segment." However, the standard does not provide a quantitative method for identifying approximately straight line segments. Therefore, the values ​​of the elastic modulus and Poisson's ratio are somewhat subjective and arbitrary, reducing the reliability of the characteristic stress solution results.

[0008] (2) The prerequisite for solving the characteristic stress using the crack volumetric strain method is to accurately collect the values ​​of axial and radial strain during the failure process of brittle rock. However, due to the natural nature of rock, there are a large number of micropores and microcracks distributed inside it. The axial and radial strains collected in the experiment may be distorted, which will lead to the "approximate" straight line segment on the crack volumetric strain curve not being obvious, or the crack volumetric strain curve having an "approximate" straight line segment, but the volumetric strain curve not having an "inflection point", making it impossible to solve for the characteristic stress point. Summary of the Invention

[0009] This invention addresses the problems existing in the prior art by providing a method for identifying rock closure stress, crack initiation stress, and damage stress applicable to brittle rocks with axial strain less than 3% before failure under uniaxial compression conditions.

[0010] The technical solution adopted in this invention is: a method for identifying characteristic stresses in rocks based on wave impedance characteristics, comprising the following steps: S1, preparing labeled rock specimens; S2, collecting longitudinal wave velocities under uniaxial compressive strength conditions. S3, Calculate the rock density at each P-wave velocity acquisition time. S4, Calculate the rock wave impedance based on the data collected in S2 and S3. S5, based on the axial load corresponding to each longitudinal wave velocity measurement time. Calculate axial stress S6, Plot the axial stress-wave impedance curve, with axial stress as the X-axis and wave impedance as the Y-axis; S7, Smooth the axial stress-wave impedance curve; S8, Solve for the first and second derivatives of the smoothed axial stress-wave impedance curve, and plot the axial stress-first derivative curve and the axial stress-second derivative curve; S9, Define the wave impedance stable region (Ⅰ) as the axial stress interval corresponding to the absolute value of the first derivative of the wave impedance being greater than 0 and less than 1, and the axial stress corresponding to the starting point of the interval is the closing stress. The axial stress corresponding to the end point of the interval is the crack initiation stress. S10, Define the wave impedance instability region (II) as when the axial stress is greater than the crack initiation stress. In the region of wave impedance instability (II), along the direction of increasing axial stress, the axial stress corresponding to the first minimum point of the second derivative of the wave impedance is taken as the damage stress. .

[0011] As a further improvement of the present invention, in step S1, the rock specimen is a cylindrical specimen with a diameter of 45-55 mm and a height of 95-105 mm. To ensure the uniformity and standardization of the rock specimens, the rock specimens are prepared according to the "Standard for Testing Methods of Engineering Rock Mass" (GB / T50266-2013).

[0012] As a further improvement of the present invention, the diameter error of the cylindrical specimen is controlled within 0.3 mm, the unevenness of the end face is controlled within 0.05 mm, and the perpendicularity error between the end face of the specimen and the specimen axis is less than 0.25°.

[0013] As a further improvement of the present invention, the number of acoustic emission sensors for collecting longitudinal wave velocity is 8.

[0014] As a further improvement of the present invention, two treatment lines are drawn from the upper and lower ends of the rock specimen, and generatrices L1, L2, L3, and L4 are made on its surface to connect the upper and lower ends; nine points are precisely measured along the generatrices, each 10 mm apart; these points are arranged in a circumferential pattern on generatrices L1, L2, L3, and L4; sensor No.1 is placed on generatrice L1 at a distance of 10 mm from the lower end, sensor No.2 is placed on generatrice L2 at a distance of 20 mm from the lower end, sensor No.3 is placed on generatrice L3 at a distance of 30 mm from the lower end, and sensor No.4 is placed on generatrice L4 at a distance of 10 mm from the lower end. At a distance of 40mm from the bottom; place sensor No.5 on bus L1 at a distance of 40mm from the top, sensor No.6 on bus L2 at a distance of 30mm from the top, sensor No.7 on bus L3 at a distance of 20mm from the top, and sensor No.8 on bus L4 at a distance of 10mm from the top; set sensor No.1 as the acoustic wave emitting sensor and sensors No.2-No.8 as the acoustic wave receiving sensors; record the longitudinal wave velocity values ​​collected by sensors No.1 to No.2-No.8 respectively, and take the average of the above 7 wave velocity values. The longitudinal wave velocity values ​​were taken as the test values ​​under the current axial load conditions. Considering the heterogeneity of the rock, the average of the above 7 wave velocity values ​​was used. This serves as the test longitudinal wave velocity value under the current axial load conditions.

[0015] As a further improvement of the present invention, the uniaxial compression test loading method adopts displacement control or force control, and the loading rate is 0.2KN / s.

[0016] Closure stress, crack initiation stress, and damage stress are used to describe the evolution characteristics of microcracks during the failure of brittle rocks. Laboratory and theoretical studies have shown that the longitudinal wave velocity during brittle rock failure is closely related to the microcrack evolution characteristics and rock density. Generally, the more micropores and microcracks distributed within the rock, the lower the longitudinal wave velocity, and vice versa; correspondingly, the higher the rock density, the higher the longitudinal wave velocity. In rock mechanics, wave impedance can be defined as the product of rock density and longitudinal wave velocity, used to describe the rock's resistance to wave propagation. Therefore, for the same brittle rock, a lower wave impedance indicates more microcracks within the rock; a higher wave impedance indicates fewer microcracks within the rock.

[0017] This invention provides a reliable and convenient method for quantitatively identifying the closure stress, crack initiation stress, and damage stress in brittle rocks. It overcomes the limitations of the crack volumetric strain method in terms of accuracy and reliability, which is easily affected by the rock's elastic modulus, Poisson's ratio, and the degree of development of natural fractures.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] (1) Wave impedance was applied to the identification of closure stress, crack initiation stress and damage stress in brittle rocks. By analyzing the characteristics of the first and second derivatives of the wave impedance curve during the failure process of brittle rocks, a quantitative identification method for closure stress, crack initiation stress and damage stress was given.

[0020] (2) Since this method does not involve solving for the elastic modulus and Poisson's ratio of the rock, it simplifies the characteristic stress identification steps and reduces the influence of human factors and the naturalness of the rock on the identification results. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of existing technology.

[0022] Figure 2 This is a schematic diagram of a rock specimen according to an embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of sensor arrangement according to an embodiment of the present invention.

[0024] Figure 4 This is a schematic diagram of sensor transmission and reception according to an embodiment of the present invention.

[0025] Figure 5 This is a waveform impedance-axial stress curve after smoothing treatment, according to an embodiment of the present invention.

[0026] Figure 6 This is an axial stress-first derivative curve of an embodiment of the present invention.

[0027] Figure 7 This is an axial stress-second derivative curve of one embodiment of the present invention.

[0028] Figure 8 This is a comparative schematic diagram disclosed in this invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings, embodiments, and comparative examples.

[0030] To verify the reliability of the method of the present invention, a uniaxial compression test was performed on the same standard specimen, and the characteristic stress was solved by the method of the present invention (Example 1) and the crack volume strain method (Comparative Example 1).

[0031] Example 1

[0032] 1. Preparation of standard rock specimens

[0033] Typical brittle rock—granite—was selected as the test object. Drilling, cutting, and grinding were performed according to the "Standard for Test Methods of Engineering Rock Mass" (GB / T50266-2013). A typical specimen was used as an example (…). Figure 2 The measured initial diameter of the specimen was 49.8 mm, the initial height was 100.1 mm, the initial mass was 194.88 g, and the initial density was 2.62 g·cm⁻³.

[0034] 2. Impedance Testing of Rock Failure Process under Uniaxial Compression

[0035] 2.1 Instruments and Equipment

[0036] The testing instruments and equipment included: a rigid press, an acoustic emission instrument, an acoustic emission sensor, a strain gauge, and strain plates. The rigid press employed a GDS VIS 400 kN HPTAS triaxial rheometer, which automatically acquires axial stress (force) and axial deformation data. The acoustic emission instrument used was a PCI-2 acoustic emission testing system manufactured by PAC Technologies, USA, with a Nano30 sensor operating in the 125–750 kHz frequency range. The strain gauge was a YJZ-16 intelligent digital static resistance strain gauge, equipped with a BFH120-3AA-R1-D150 strain plate.

[0037] 2.2 Arrangement of acoustic emission sensors

[0038] A total of 8 acoustic emission sensors were deployed, and the deployment steps are as follows:

[0039] (1) Draw two vertical lines on the upper and lower ends of the specimen with a pencil, and draw line L1, L2, L3 and L4 on the surface to connect the upper and lower ends.

[0040] (2) Use vernier calipers to accurately measure 9 points along the generatrix direction, with each point 10mm apart;

[0041] (3) A ring-shaped sensor is deployed on bus L1, L2, L3 and L4 respectively; sensor No.1 is placed on bus L1 at a distance of 10mm from the bottom, sensor No.2 is placed on bus L2 at a distance of 20mm from the bottom, sensor No.3 is placed on bus L3 at a distance of 30mm from the bottom, and sensor No.4 is placed on bus L4 at a distance of 40mm from the bottom.

[0042] (4) Similarly, place sensor No.5 on bus L1 at a distance of 40mm from the top, place sensor No.6 on bus L2 at a distance of 30mm from the top, place sensor No.7 on bus L3 at a distance of 20mm from the top, and place sensor No.8 on bus L4 at a distance of 10mm from the top.

[0043] (5) Finally, with the center of the bottom surface of the specimen as the origin O, calculate and record the coordinates of the center points of the above 8 sensors. See the sensor layout diagram. Figure 3 and Figure 4 The coordinates of the transmitting sensor are shown in the table below.

[0044] Acoustic emission sensor coordinates

[0045]

[0046] 2.3 Parameter settings during the experiment

[0047] 2.3.1 Loading Method and Loading Rate

[0048] According to the "Standard for Test Methods of Engineering Rock Mass" (GB / T50266-2013), the force-controlled loading rate is 0.2 kN / s.

[0049] 2.3.2 Setting Acoustic Emission Acquisition Parameters

[0050] The acoustic emission instrument's preamplifier gain is set to 40dB, sampling rate to 2MSPS, sampling length to 2K, PDT to 200µs, HDT to 800µs, and HLT to 1000µs. The main acoustic emission parameter settings are shown in the table below.

[0051] Key parameters for acoustic emission 3.

[0053] 4. Test Procedure

[0054] 3.1 The uniaxial compression test loading rate was set to 0.2 kN / s. During the test, the loading rate was increased every 5000 N.

[0055] Collect the longitudinal wave velocity once, and record the longitudinal wave velocity values ​​collected by sensors No.1 to No.2-No.8 respectively.

[0056] 3.2 The specimen was continuously loaded, and the longitudinal wave velocity and strain gauge data were collected in real time until the specimen failed.

[0057] 3.3 After the test, the calculation yielded the result corresponding to the axial load as follows: Rock longitudinal wave velocity at time N:

[0058] ;

[0059] In the formula: V represents the number of longitudinal wave velocity measurements. 2i -V 8i The values ​​represent the longitudinal wave velocity collected by sensors No.1 through No.2-No.8, in units of... ; For the axial load, At N, the longitudinal wave velocity of the rock, in units of N. ;

[0060] 3.4 Calculate the rock density corresponding to each P-wave velocity acquisition time:

[0061]

[0062] In the formula: The density of the rock corresponding to each P-wave velocity acquisition moment, in units of ;

[0063] The initial mass of the rock is given in units of 1. ; The initial radius of the specimen, in units of ; The initial height of the specimen, in units of ; This represents the axial deformation of the specimen collected by the press at each longitudinal wave velocity acquisition time, expressed in units of... ;

[0064] 3.5 Corresponding to each P-wave velocity acquisition time, calculate the rock wave impedance:

[0065]

[0066] In the formula: The rock wave impedance corresponds to the moment of each P-wave velocity acquisition, in units of ;

[0067] 3.6 Calculate the axial stress corresponding to each longitudinal wave velocity measurement moment.

[0068]

[0069] In the formula: The axial stress corresponds to the moment of each P-wave velocity acquisition, in units of... ;

[0070] The wave impedance data for each axial stress are shown in the table below.

[0071] Wave impedance values ​​during the test

[0072]

[0073] 3.7 Based on the data in the table above, plot the wave impedance-axial stress curve and smooth it using an FFT filter in Origin software. The processed wave impedance-axial stress curve is shown below. Figure 5 .

[0074] 3.8 Solve for the first derivative of the smoothed axial stress-wave impedance curve and plot the axial stress-first derivative curve. Determine the wave impedance stability region (Ⅰ) based on a first derivative value greater than 0 and less than 1, see [reference needed]. Figure 6 .

[0075] 3.9 Determining the starting point of the wave impedance stability region (Ⅰ) For the axial stress. Draw a line through point A. axis parallel lines ,straight line and The value corresponding to the x-coordinate of the intersection point of the axes ( ) is the closing stress ,See Figure 6 .

[0076] 3.10 Determine the starting point of the stable wave impedance region (Ⅰ) For the axial stress. Draw a line through point B. axis parallel lines ,straight line and The value corresponding to the x-coordinate of the intersection point of the axes ( ) is the crack initiation stress ,See Figure 6 .

[0077] 3.11 Solve for the second derivative of the smoothed axial stress-wave impedance curve and plot the axial stress-second derivative curve. Assume the axial stress is greater than the crack initiation stress ( The region corresponding to this is defined as the wave impedance instability region (II), see Figure 7 .

[0078] 3.12 In the wave impedance instability region (II), determine the first minimum point along the direction of increasing axial stress. .Pass Point of work axis parallel lines ,straight line and The value corresponding to the x-coordinate of the intersection point of the axes ( Damage stress ,See Figure 7 .

[0079] Comparative Example 1

[0080] 1. Rock specimens are shown in Comparative Example 1.

[0081] 2. Strain Arrangement

[0082] Strain gauges are used to collect the axial and radial strain of the specimen during failure and are used to solve for the characteristic stress using the crack volumetric strain method. In the experiment, to reduce the risk of characteristic stress calculation failure due to data distortion from the strain gauges, two sets of strain gauges were arranged in the middle of the specimen, with one strain gauge in each set along the specimen's axial direction and one along its circumference.

[0083] 3. Experimental Procedure

[0084] Axial strain acquired using YJZ-16 intelligent digital static resistance strain gauge and

[0085] Radial strain data is used to identify characteristic stresses. Figure 8 Showing the closure stress of the crack using the volumetric strain method The identification result is 42.47 MPa, the initiation stress. The identification result is 67.41 MPa, damage stress. The identification result is 83.33 MPa.

[0086] Example 1 and Comparative Example 1 were analyzed.

[0087] Define the characteristic stress identification error of the wave impedance method for:

[0088]

[0089] In the formula, The characteristic stress identification error of the wave impedance method is expressed in units of %.

[0090] The table below shows the identification of closure stress using the wave impedance method. Cracking stress Damage stress The errors were 5.71%, 3.58%, and 0.10%, respectively, and the overall identification results were the same as those of the crack volumetric strain method. It should be noted that the longitudinal wave velocity sampling interval in Example 1 was 5000N. Reducing the sampling interval would help to further reduce the characteristic stress identification error of the wave impedance method.

[0091] Identification results of wave impedance method and crack volume strain method

[0092]

[0093] Those skilled in the art should understand that the protection scheme of the present invention is not limited to the above embodiments, and various arrangements, combinations and transformations can be made on the basis of the above embodiments. Without departing from the spirit of the present invention, all transformations made to the present invention fall within the protection scope of the present invention.

Claims

1. A method for identifying characteristic stresses in rocks based on wave impedance characteristics, characterized by: Includes the following steps: S1, Prepare labeled rock specimens; S2, Collect longitudinal wave velocity under uniaxial compressive strength conditions ; S3, Calculate the rock density at each P-wave velocity acquisition time. The calculation formula is: ; In the formula, The density of the rock corresponding to each P-wave velocity acquisition moment, in units of ; The initial mass of the rock is expressed in units of... ; The initial radius of the specimen, in units of ; The initial height of the specimen, in units of ; This represents the axial deformation of the specimen collected by the press at each longitudinal wave velocity acquisition time, expressed in units of... ; S4. Calculate the rock wave impedance based on the data collected in S2 and S3. The calculation formula is as follows: ； In the formula, The rock wave impedance corresponds to the moment of each P-wave velocity acquisition, in units of ; To correspond to axial load Longitudinal wave velocity of rock, in units of ; S5, based on the axial load corresponding to each longitudinal wave velocity measurement time. The formula for calculating axial stress is as follows: ； In the formula, The axial stress corresponds to the moment of each P-wave velocity acquisition, in units of... ; This is the axial load force, expressed in N. The initial radius of the specimen, in units of ; S6, Plot the axial stress-wave impedance curve, with axial stress as the X-axis and wave impedance as the Y-axis; S7, smoothing the axial stress-wave impedance curve; S8. Solve for the first and second derivatives of the smoothed axial stress-wave impedance curve, and plot the axial stress-first derivative curve and the axial stress-second derivative curve. S9, define the wave impedance stability region (Ⅰ) as the axial stress interval corresponding to the absolute value of the first derivative of the wave impedance being greater than 0 and less than 1, and the axial stress corresponding to the starting point of the interval is the closure stress. The axial stress corresponding to the end point of the interval is the crack initiation stress. ; S10, the wave impedance instability region (II) is defined as the region where the axial stress is greater than the crack initiation stress. In the region of wave impedance instability (II), along the direction of increasing axial stress, the axial stress corresponding to the first minimum point of the second derivative of the wave impedance is taken as the damage stress. .

2. The identification method according to claim 1, characterized in that: In S1, the rock specimen is a cylindrical specimen with a diameter of 45-55 mm and a height of 95-105 mm.

3. The identification method according to claim 2, characterized in that: The cylindrical specimen has a diameter error controlled within 0.3 mm, an unevenness of the end face controlled within 0.05 mm, and a perpendicularity error between the end face and the specimen axis less than 0.25°.

4. The identification method according to claim 1, characterized in that: In S2, there are 8 acoustic emission sensors for collecting longitudinal wave velocity.

5. The identification method according to claim 4, characterized in that: Two treatment lines are drawn from the top and bottom of the rock specimen, and generatrices L1, L2, L3, and L4 are constructed on its surface to connect the top and bottom. Nine points are precisely measured along the generatrices, each 10 mm apart. These points are then arranged in a ring around generatrices L1, L2, L3, and L4. Sensor No. 1 is placed on generatrice L1 at a distance of 10 mm from the bottom; sensor No. 2 is placed on generatrice L2 at a distance of 20 mm from the bottom; sensor No. 3 is placed on generatrice L3 at a distance of 30 mm from the bottom; and sensor No. 4 is placed on generatrice L4 at a distance of 40 mm from the bottom. At position m; sensor No. 5 is placed on bus L1 at a distance of 40 mm from the top, sensor No. 6 is placed on bus L2 at a distance of 30 mm from the top, sensor No. 7 is placed on bus L3 at a distance of 20 mm from the top, and sensor No. 8 is placed on bus L4 at a distance of 10 mm from the top; sensor No. 1 is set as the acoustic wave emitting sensor, and sensors No. 2-No. 8 are acoustic wave receiving sensors; the longitudinal wave velocity values ​​collected by sensors No. 1 to No. 2-No. 8 are recorded respectively, and the average value of the above 7 wave velocity values ​​is taken. This serves as the test longitudinal wave velocity value under the current axial load conditions.

6. The identification method according to any one of claims 1-5, characterized in that: The uniaxial compression test loading method adopts displacement control or force control, and the loading rate of force control is 0.2KN / s.

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