A method for determining rock damage using crack closure energy

Abstract

The present invention discloses a method for determining rock damage by using crack closure energy. Crack closure energy is defined as the energy required for the rock to effectively close the initial microcracks inside the rock under the action of a compressive load. To determine rock damage by using this method, firstly, a uniaxial compression test is performed on the rock, and the corresponding stress-strain curve is recorded; then, the crack closure stress of the rock is determined according to the stress-strain curve; the crack closure energy is marked in the stress-strain curve according to the crack closure stress; equidistant vertical auxiliary lines are drawn within the crack closure energy area to divide it into a number of small trapezoids; the area of ​​each small trapezoid is calculated and then summed; the total area obtained is the crack closure energy of the rock. The method of the present invention uses crack closure energy to determine the initial microcrack damage of the rock more objectively, is easy to program to realize batch operation, and can effectively evaluate the initial microcrack damage inside the rock.

Description

Technical Field

[0001] The present invention belongs to the field of rock mechanics and geotechnical engineering technology, and specifically relates to a method for determining rock damage, and in particular to a method for determining the degree of initial microcrack damage in engineering rock masses, which can be widely used in deep mineral resource mining, deep buried tunnel construction and slope engineering management. Background Art

[0002] Engineering rock mass always exists in a certain geological environment. Under the action of weathering or dynamic disturbance (excavation unloading, blasting vibration, seismic activity, etc.), the in-situ stress state of the rock mass will change, resulting in changes in the stress field of the engineering rock mass, causing the development of structural surfaces of varying sizes (including faults, fissures, joints, microcracks, etc.) inside the engineering rock mass. The developed microcracks will cause greater damage inside the rock mass, which will directly reduce the mechanical parameters of the rock mass such as compressive strength and elastic modulus. Therefore, it is very necessary to determine the initial microcrack damage size inside the rock mass to ensure the reliability of engineering stability evaluation.

[0003] There are many methods for evaluating engineering rock mass damage, which can be generally divided into two categories according to the different test locations: field test methods and indoor test methods. The field test method is mainly a method of evaluating the degree of rock mass damage by testing the parameters of the rock mass at the engineering site. Commonly used test parameters include rock mass strength, sonic wave velocity, etc. The test of rock mass strength relies on the use of large-scale instruments on site, which is often time-consuming and labor-intensive, and the cost is high, so it is rarely used in engineering; relatively speaking, the test of sonic wave velocity is relatively simple and is currently widely used. However, the sonic wave velocity test often has a very large amount of data, and the data is relatively discrete, requiring specialized technicians to process the data.

[0004] Indoor test methods are also often used to evaluate the degree of damage to engineering rock mass. According to different test methods, indoor test methods can be divided into mesoscopic methods and macroscopic methods. The mesoscopic method is a method of directly observing the development of microscopic microcracks inside the rock by mesoscopic testing of rock samples collected on site. At present, the commonly used methods for mesoscopic testing include microscopic slice microscope observation, electron microscope scanning, CT scanning, nuclear magnetic resonance, etc. Although these test methods have high accuracy, they have a large workload and high equipment usage costs. Relatively speaking, the test of macroscopic methods is relatively simple and low in cost, so they are used more. The macroscopic method mainly tests the macroscopic mechanical properties of rocks, including deformation characteristics, strength characteristics, fracture characteristics, etc. to evaluate the degree of rock damage. Commonly used indicators include rock strength, elastic modulus, degree of rock fragmentation after destruction, crack closure stress, etc. Although rock macroscopic mechanical testing is relatively simple, there is still a lack of a simple and reliable indicator to evaluate the degree of rock damage. Therefore, it is necessary to propose a simple indicator to determine rock damage. Summary of the invention

[0005] In view of this, the purpose of the present invention is to propose a simple, easy and intuitive method for determining rock damage by using crack closure energy. The method adopts a simple macro-indicator "crack closure energy" and provides the method of using this indicator to determine rock damage. The method of using this indicator to determine rock damage is more objective than existing methods and can effectively evaluate the initial microcrack damage inside the rock.

[0006] To achieve the above-mentioned object of the present invention, a method for determining rock damage using crack closure energy is implemented by the following steps:

[0007] First, the crack closure energy is defined as the energy required for the rock to effectively close the initial microcracks inside the rock under compressive load; then, the following steps are used to determine it:

[0008] S1: Perform a uniaxial compression test on the rock and record the corresponding axial stress-axial strain curve; the origin of the coordinate on the axial stress-axial strain curve is set as point O;

[0009] S2: According to the axial stress-axial strain curve obtained in step S1, the crack closure stress of the rock is calculated and determined by the axial strain response method, and the crack closure stress point is determined on the axial stress-axial strain curve. The crack closure stress point is set as point A, and the axial strain coordinate point corresponding to point A is set as point E;

[0010] S3: Plot the crack closure energy, i.e., OAE area, on the axial stress-axial strain curve;

[0011] S4: Draw equidistant vertical auxiliary lines within the crack closure energy OAE area to divide it into several small trapezoids;

[0012] S5: Calculate the area of ​​each small trapezoid and then sum them up. The total area obtained by summing up is the crack closure energy of the rock. The size of the initial microcrack damage of the rock can be intuitively evaluated based on the crack closure energy value.

[0013] In order to ensure the test precision and accuracy, at least three sets of parallel tests are performed on the rock in uniaxial compression test in step S1.

[0014] In step S4, when drawing equidistant vertical auxiliary lines, the distance between the auxiliary lines should be as small as possible, so as to ensure that the error is small enough to be ignored when using the trapezoid to calculate the area of ​​each small area. Therefore, the number of vertical auxiliary lines should be no less than 200.

[0015] Furthermore, in step S4, it can be automatically completed by exporting the stress-strain curve test data. This is because the strain is recorded according to equal time intervals. It is only necessary to set the time interval small enough to ensure that relatively dense stress-strain data is automatically generated, thereby automatically generating a sufficient number of vertical auxiliary lines.

[0016] Furthermore, in step S2, the specific steps of calculating and determining the crack closure stress of rock using the axial strain response method are as follows:

[0017] 1) The damage stress corresponding to the unstable extension of cracks inside the rock is obtained by volume strain method;

[0018] 2) Draw a straight reference line connecting the origin and the damage stress point;

[0019] 3) Calculate the axial strain difference between the axial stress-axial strain curve and the reference line;

[0020] 4) Draw the axial strain difference-axial stress curve;

[0021] 5) Determine the maximum value of the axial strain difference-axial stress curve, which is the crack closure stress.

[0022] Furthermore, in step S5, the calculation and summation of the areas of the small trapezoids are calculated using data programming processing.

[0023] Compared with the prior art, the method of determining rock damage by using crack closure energy in the present invention has the following characteristics and advantages:

[0024] First: The physical meaning of the crack closure energy parameter defined by this method is clear, and it is more intuitive to use this indicator to evaluate the initial microcrack damage of rocks. The reason why there is a certain amount of initial damage inside the rock is mainly because more microcracks will develop inside the rock under weathering or dynamic disturbance. The existence of microcracks will significantly increase the nonlinearity of the initial stage of the stress-strain curve during rock compression. This method defines crack closure energy as the energy required for the rock to effectively close the initial microcracks inside the rock under compression load. This definition is directly related to the main factors causing initial microcrack damage inside the rock, and it is more intuitive and reasonable to use it to evaluate the initial microcrack damage inside the rock.

[0025] Second: Using crack closure energy to determine the initial microcrack damage of rock is more objective. Existing methods have strong human experience in determining the initial microcrack damage of rock. For example, the initial microcrack damage inside the rock is evaluated by observing the microcracks developed inside the rock slice through microscopic slices. Although the method is intuitive, the judgment of microcrack density has strong human interference. This method can eliminate the influence of human factors by calculating the numerical value of crack closure energy to judge the initial microcrack damage of rock, so it has strong objectivity.

[0026] Third: Because the method is highly objective, it is easy to program and operate, especially the calculation of crack closure energy, which can be implemented through simple codes in existing programming languages, ensuring the efficiency of batch work. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of a typical stress-strain curve of a rock under compression in a method of determining rock damage using crack closure energy according to the present invention;

[0028] Figure 2 for Figure 1 Crack closure energy magnification diagram determined based on the area decomposition method;

[0029] Figure 3 Schematic diagram of crack closure energy at different sampling depths in the embodiment;

[0030] Figure 4 for Figure 3 Schematic diagram of the relationship between the crack closure ability determined in and the sampling depth. DETAILED DESCRIPTION

[0031] To describe the present invention, a method for determining rock damage using crack closure energy of the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0032] Figure 1 A schematic diagram of a typical stress-strain curve of a rock under compression in a method of determining rock damage using crack closure energy according to the present invention; Figure 2 for Figure 1 The crack closure energy magnification diagram determined based on the area decomposition method. Figure 1 , Figure 2 It can be seen that the method of the present invention for determining rock damage by using crack closure energy specifically adopts the following steps to obtain the crack closure energy and then determine the size of rock damage:

[0033] S1: Perform a uniaxial compression test on the rock and record the corresponding axial stress-axial strain curve; the origin of the coordinate on the axial stress-axial strain curve is set as point O;

[0034] S2: According to the axial stress-axial strain curve obtained in step S1, the axial strain response method is used to calculate and determine the crack closure stress of the rock. The crack closure stress point is determined on the axial stress-axial strain curve. The crack closure stress point is set as point A. The point where point A intersects with the axial strain coordinate (abscissa) is Figure 1 Point E shown in ;

[0035] S3: Figure 1 The crack closure energy is marked in the figure. According to the definition of crack closure energy, the horizontal axis is the axial strain and the vertical axis is the axial stress. The product of the two is the energy. Therefore, Figure 1 The area of ​​OAE in is the crack closure energy;

[0036] S4: Draw equidistant vertical auxiliary lines within the crack closure energy OAE area to divide it into several small trapezoids, such as Figure 2 As shown, in order to ensure that the OA segment of the axial stress-axial strain curve can be approximated as a straight line in the small trapezoid, the distance between the auxiliary lines should be as small as possible. In the embodiment of the present invention, it is recommended to draw more than 200 auxiliary lines;

[0037] S5: Calculate the area of ​​each small trapezoid and then sum them up. The total area obtained by summing up is the crack closure energy of the rock. Figure 2 The area of ​​each strip in the test rock can be approximately equivalent to a trapezoid. The area of ​​each small trapezoid is calculated, and then the total area of ​​the OAE region is calculated by summing up. This area is the crack closure energy of the test rock. According to this value, the size of the initial microcrack damage of the rock can be intuitively evaluated.

[0038] Depend on Figure 1 As shown in the schematic diagram of a typical stress-strain curve of a rock under compression in a method of determining rock damage using crack closure energy of the present invention, the OA segment in the figure represents the crack closure stage of the rock during the loading process. At this time, the axial stress-axial strain curve shows obvious nonlinear characteristics, that is, the axial stress-axial strain curve presents a concave feature. Point A indicates that the microcracks inside the rock have been effectively closed during the loading process. At this time, the corresponding axial stress is the crack closure stress. After point A, the rock compression enters the linear elastic deformation stage, and the axial stress-axial strain curve shows the characteristics of a straight line segment. It can be seen that the crack closure energy corresponds to the work done by the rock compressor when the axial stress-axial strain curve reaches point A. Since the horizontal coordinate of the axial stress-axial strain curve is the axial strain (axial displacement) and the vertical coordinate is the axial stress, the crack closure energy can be represented by the area of ​​the illustrated OAE, that is, the size of the shaded area in the illustration.

[0039] Example

[0040] When sampling at deep depths, high stress concentration at deep depths will lead to large initial microcrack damage inside the core. This example is explained by experimental data of rock samples drilled at different depths. The rock obtained by drilling is granite, and the sampling depths are 0m, 240m, and 420m respectively. Figure 3 Axial stress-axial strain curves measured for rock samples at different depths. Figure 4 Calculated crack closure energy for rock samples at different depths.

[0041] Step 1: Draw the axial stress-axial strain curves of rocks sampled at different depths according to the test results (see Figure 3 As shown in the figure, the axial strain response method is used to calculate and determine the crack closure stress of the rock sampled at different depths. The crack closure stress of the rock sampled at different depths of 0m, 240m, and 420m are marked as A, B, and C respectively;

[0042] Step 2: Draw a vertical auxiliary line along the crack closure stress point to the horizontal axis. The points where the auxiliary line of the rock sampled at different depths of 0m, 240m, and 420m intersects with the horizontal axis are D, E, and F, respectively. Then, the crack closure energy of the rock can be marked in the figure. The crack closure energy of the rock sampled at different depths of 0m, 240m, and 420m is OAD, OBE, and OCF, respectively.

[0043] Step 3: Draw equidistant vertical auxiliary lines inside the determined crack closure energy area to obtain the size of the area calibrated by the crack closure energy. The crack closure energies of rocks sampled at different depths determined by this method are 15.75 kPa, 28.23 kPa, and 46.49 kPa respectively;

[0044] Step 4: Based on the obtained crack closure energy, the initial microcrack damage of different rocks is evaluated, and the crack closure energy evolution curves of rocks sampled at different depths are plotted (see Figure 4 As shown in the figure, it can be seen that the crack closure energy of the rock is the largest when sampling at 420m, indicating that the initial microcrack damage inside the rock sample is the largest when sampling at this depth, and the crack closure energy of the rock is the smallest when sampling at 0m, indicating that the initial microcrack damage inside the rock sample is the smallest when sampling on the surface.

[0045] During deep sampling, due to the large ground stress, the drill bit will generate a large stress concentration inside the core during drilling, thereby causing microcracks inside the rock sample to develop and cause microcrack damage to the rock sample. This phenomenon is consistent with the evaluation results of the present invention, indicating that the method for determining rock damage based on the crack closure energy method provided by the present invention is feasible; in addition, the greater the initial microcrack damage in the rock sample, the lower the rock strength and elastic modulus obtained by the test, which is also consistent with the results of the present invention. Figure 3 The results shown in are in good agreement, which also verifies the reliability of the method of the present invention.

[0046] The above description is only a preferred embodiment of the present invention. The points O, A, B, C, D, E, F, and M given in the present invention are only for the convenience of description and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the technical ideas and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for determining rock damage using crack closure energy, characterized in that: The crack closure energy is defined as the energy required to effectively close the initial microcracks inside the rock under compressive load, and is determined by the following steps: S1: Perform a uniaxial compression test on the rock and record the corresponding axial stress-axial strain curve; the origin of the coordinate on the axial stress-axial strain curve is set as point O; S2: According to the axial stress-axial strain curve obtained in step S1, the crack closure stress of the rock is calculated and determined by the axial strain response method, and the crack closure stress point is determined on the axial stress-axial strain curve. The crack closure stress point is set as point A, and the axial strain coordinate point corresponding to point A is set as point E; S3: Plot the crack closure energy, i.e., OAE area, on the axial stress-axial strain curve; S4: Draw equidistant vertical auxiliary lines within the crack closure energy OAE area to divide it into several small trapezoids; S5: Calculate the area of ​​each small trapezoid and then sum them up. The total area obtained by the sum is the crack closure energy of the rock. The initial microcrack damage degree of the rock can be intuitively evaluated according to the crack closure energy value, that is, the larger the crack closure energy value is, the larger the initial microcrack damage of the rock is; the smaller the crack closure energy value is, the smaller the initial microcrack damage of the rock is.

2. A method for determining rock damage using crack closure energy as claimed in claim 1, characterized in that: In step S1, at least three sets of parallel tests are performed on the rock in uniaxial compression test.

3. A method for determining rock damage using crack closure energy as claimed in claim 2, characterized in that: In step S4, the number of the vertical auxiliary lines is no less than 200.

4. A method for determining rock damage using crack closure energy as claimed in claim 3, characterized in that: In step S4, a sufficient number of vertical auxiliary lines are automatically generated by exporting stress-strain curve test data.

5. A method for determining rock damage using crack closure energy as claimed in claim 1, 2, 3 or 4, characterized in that: In step S2, the specific steps of calculating and determining the crack closure stress of rock using the axial strain response method are as follows: 1) The damage stress corresponding to the unstable extension of cracks inside the rock is obtained by volume strain method; 2) Draw a straight reference line connecting the origin and the damage stress point; 3) Calculate the axial strain difference between the axial stress-axial strain curve and the reference line; 4) Draw the axial strain difference-axial stress curve; 5) Determine the maximum value of the axial strain difference-axial stress curve, which is the crack closure stress.

6. A method for determining rock damage using crack closure energy as claimed in claim 5, characterized in that: In step S5, the calculation and summation of the areas of the small trapezoids are calculated using data programming processing.

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