Brittleness evaluation method of weathered granite under different confining pressures

Abstract

A brittle evaluation method of weathered granite under different confining pressures, comprising: drilling fresh granite rock samples from underground mine, and making standard test pieces under natural environment for 7 years of weathering; respectively performing triaxial compression synchronous acoustic emission test of the standard test pieces under different confining pressures, obtaining stress-strain relationship diagram, acoustic emission energy-cumulative energy-stress-time relationship diagram of each standard test piece; defining brittle index of weathered granite by acoustic emission cumulative energy B 3. Quantitative evaluation of the brittleness of weathered granite under different confining pressures. The present application does not completely depend on the change of stress-strain behavior, directly obtains from the acoustic emission signal parameters, is convenient and fast, has simple calculation process, improves the certainty of the brittle evaluation result, and has good application prospect for studying the deformation and failure of such mine surrounding rock and stable support in mine operation.

Description

Technical Field

[0001] This invention belongs to the field of mine surrounding rock stability support and safe mining technology, specifically involving a method for evaluating the brittleness of weathered granite under different confining pressures. Background Technology

[0002] Underground mining is influenced by various geological conditions, and the rock masses underground are usually in a humid, water-rich, and easily weathered environment. For some granite surrounding rocks in mines, due to alteration and argillaceization during diagenesis, they contain a high amount of clay minerals. Under the influence of water, they are prone to disintegration and softening, weakening their weathering resistance. After further weathering, their mechanical properties are significantly reduced, posing numerous disadvantages to the stability of the mine's surrounding rock support and personnel safety. Therefore, studying the engineering characteristics of such weathered granite, especially its deformation and failure characteristics under triaxial compression similar to its actual stress state, is of great significance for solving the problems of mine surrounding rock stability and safe mining.

[0003] The brittleness index is an important indicator describing the deformation and failure characteristics of rocks. Current methods for evaluating rock brittleness mainly include: direct evaluation using elastic or strength parameters from the stress-strain curve; applying energy balance-based theories to the stress-strain curve; and evaluation through rock failure modes. The first two methods both require the stress-strain curve, and there are three evaluation approaches: 1. performing brittleness assessment only based on characteristics before the peak of the stress-strain curve; 2. performing brittleness assessment only based on characteristics after the peak of the stress-strain curve; 3. combining information from both pre-peak and post-peak characteristics of the stress-strain curve before performing a brittleness assessment. Although pre-peak and post-peak deformation or failure characteristics of the stress-strain curve can be considered individually as indicators of rock brittleness, the latter is used most frequently because the post-peak characteristics of the stress-strain curve represent the deformation of the rock when it is about to fail, and the post-peak mechanical response reflects its self-sustaining ability. After rock failure, the post-peak stress decreases and strain softens significantly, making it easier to obtain key information in brittleness assessment. However, for weakly brittle and ductile rocks, their brittleness is difficult to express through the post-peak characteristics of the stress-strain curve, making it impossible to calculate post-peak brittleness parameters, including residual stress and strain. Furthermore, when using energy balance theory to calculate energy from the stress-strain curve, it is usually necessary to assume that there is no heat exchange between the rock sample and the external environment during loading, satisfying the first law of thermodynamics for easier energy calculation. However, in reality, this is only an ideal state; the absence of heat exchange between the rock sample and the external environment does exist. Energy calculated based on this assumption contains errors, affecting the accuracy of the brittleness assessment.

[0004] Methods for evaluating the brittleness of rocks based on their failure modes generally fall into four categories: complex shear failure, splitting failure, shear failure, and plastic failure. Complex shear failure includes shear slip or failure along multiple planes, while splitting failure includes failure along the loading direction. As the brittleness of the rock increases, obvious fractures are more likely to be observed on the rock surface; therefore, multi-plane and splitting shear failure modes are most likely to occur. For semi-brittle and weakly brittle rocks, single-plane shear failure is more likely, while for ductile rocks, plastic failure is most likely. Of course, there are also cases where rocks undergo large deformations but show no obvious fractures on their surface. Therefore, this method has a certain degree of uncertainty, which significantly reduces the accuracy of its brittleness evaluation results.

[0005] In summary, current methods for evaluating rock brittleness based on stress-strain curves or failure modes of rock samples are often affected by the physical and mechanical properties of the rock. Furthermore, when calculating energy using stress-strain curves, theoretical and model assumptions about the sample are required, resulting in numerous uncertainties and inaccurate evaluation results.

[0006] Acoustic emission (AE), as a technique for real-time non-destructive monitoring of internal deformation and failure of materials, more directly reflects the initiation, propagation, and penetration of cracks within rock samples, as well as the occurrence of fracture. Therefore, it is necessary to apply energy balance and related theories to acoustic emission technology, that is, to study the deformation and failure of rocks through acoustic emission energy, and to establish a brittleness index evaluation method based on the cumulative energy of acoustic emission. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies and provide a method for evaluating the brittleness of weathered granite under different confining pressures by defining the cumulative acoustic emission energy as the brittleness index and quantitatively analyzing the effect of confining pressure on the brittleness of weathered granite. This method does not rely entirely on changes in stress-strain behavior, is quick and convenient, and provides highly accurate evaluation results. It has excellent application prospects for studying the deformation and failure of surrounding rocks in such mines and for stable support in mining operations.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for evaluating the brittleness of weathered granite under different confining pressures includes the following steps:

[0010] Step 1: Preparation of standard specimens for weathered granite: Fresh granite samples are drilled from the mine and weathered under natural conditions for 7 years. The weathered granite samples are then made into standard specimens.

[0011] Step 2: Conduct triaxial compression synchronous acoustic emission tests on the standard specimens described in Step 1 under different confining pressures to obtain stress-strain relationship diagrams and acoustic emission energy-cumulative energy-stress-time relationship diagrams for each standard specimen;

[0012] Step 3: Define and calculate the brittleness index B3 of weathered granite standard specimens under different confining pressures. Specific steps include:

[0013] For the standard specimen before failure, a series of plastic deformations occur inside the specimen, including crack initiation, propagation, and penetration, releasing some strain energy. The energy consumed before time t1 corresponds to the energy dissipation during this process. The higher the proportion of energy dissipated during this process to the total energy, the more pronounced the brittle characteristics become. Therefore, the brittleness index based on dissipated energy can be calculated by the following formula:

[0014] (1)

[0015] In the formula, E1 is the dissipated energy and E2 is the accumulated strain energy.

[0016] For a standard rock specimen that tends towards complete failure with increasing load, the time interval corresponding to this process is t1-t2, during which a large amount of accumulated strain energy is released. If it is assumed that the faster the strain energy release rate during this process, the higher the brittleness, then the brittleness index based on the strain energy release rate can be calculated by the following formula:

[0017] (2)

[0018] In the formula, t1 and t2 are the peak stress time and the time of complete failure of the rock standard specimen, respectively.

[0019] Therefore, the calculation formula for the brittleness index based on the cumulative energy of acoustic emission can be obtained through equations (1) and (2):

[0020] (3)

[0021] Step 4: Summarize the brittleness index values ​​of weathered granite under different confining pressures obtained in Step 3, quantitatively compare and analyze the influence of confining pressure on the brittleness of weathered granite, and judge the brittleness change trend of weathered granite under different confining pressures based on the change law of brittleness evaluation index B3.

[0022] Furthermore, the standard specimens mentioned in step one are all cylinders with a diameter of 50 mm and a height of 100 mm, and the loaded standard specimens all meet the requirements of intact appearance, uniform texture, and no obvious joints or cracks.

[0023] Furthermore, the loading process of the triaxial compression synchronous acoustic emission test described in step two is divided into two stages: the first stage is the simultaneous loading of axial load and confining pressure, which is force-controlled loading with a confining pressure rate of 0.5 MPa / s until the confining pressure reaches the specified value; the second stage is to maintain the stability of the confining pressure, and then load is controlled by displacement with a loading rate of 0.01 mm / s, continuing to apply axial pressure until the standard specimen is destroyed; in addition, in order to filter out the influence of external noise and ensure the authenticity of the data, the acoustic emission threshold value is set to 50 dB and the sampling rate is set to 3 MSPS.

[0024] The beneficial effects of this invention are:

[0025] This invention conducts triaxial compression synchronous acoustic emission tests on standard specimens of weathered granite under different confining pressures, defines the brittleness index of weathered granite under different confining pressures by using the cumulative acoustic emission energy of the standard specimens, and quantitatively analyzes the effect of confining pressure on the brittleness of weathered granite.

[0026] This invention directly obtains the acoustic emission energy parameters of standard specimens under different confining pressures by conducting triaxial compression synchronous acoustic emission tests on weathered granite standard specimens. Then, using the calculation formula in step three, the brittleness index B1 based on dissipated energy and the brittleness index B2 based on strain energy release rate are calculated using acoustic emission energy and time parameters, respectively. The quotient B3 of the two is then used as the brittleness evaluation index. The trend of brittleness change of weathered granite specimens under different confining pressures is judged based on the change law of brittleness evaluation index B3. Compared with the commonly used rock brittleness evaluation methods based on stress-strain curves or failure modes of rock specimens, this method can timely display and monitor the trend of brittleness change of rocks.

[0027] This invention does not rely entirely on changes in stress-strain behavior, but obtains the data directly from acoustic emission signal parameters, which is convenient, quick, and the calculation process is simple and accurate, thus improving the accuracy of the evaluation results.

[0028] The reliability of this invention has been verified by experimental phenomena. In actual mine rock stabilization and support engineering, this method can quickly and accurately assess the brittleness of weathered granite surrounding rock, understand its deformation and failure characteristics, and provide corresponding solutions for mine rock stabilization and safe mining.

[0029] This invention takes an energy perspective and applies energy balance to acoustic emission energy, while also taking into account the changes in acoustic emission energy parameters before and after the peak value. This approach is more comprehensive than methods that only analyze characteristic parameters before or after the peak value. Attached Figure Description

[0030] Figure 1 This is a stress-strain diagram of standard weathered granite specimens under different confining pressures according to embodiments of the present invention;

[0031] Figure 2 This is a graph showing the acoustic emission energy-cumulative energy-stress-time relationship of weathered granite standard specimen a under a confining pressure of 0 MPa according to an embodiment of the present invention.

[0032] Figure 3 This is a graph showing the acoustic emission energy-cumulative energy-stress-time relationship of weathered granite standard specimen b under confining pressure of 2 MPa in Embodiment 2 of the present invention.

[0033] Figure 4 This is a graph showing the acoustic emission energy-cumulative energy-stress-time relationship of weathered granite standard specimen c under confining pressure of 4 MPa in Example 4 of the present invention.

[0034] Figure 5 This is a graph showing the acoustic emission energy-cumulative energy-stress-time relationship of the weathered granite standard specimen d under a confining pressure of 6 MPa according to Embodiment 6 of the present invention.

[0035] Figure 6 This is a graph showing the acoustic emission energy-cumulative energy-stress-time relationship of weathered granite standard specimen e under a confining pressure of 8 MPa according to Embodiment 8 of the present invention.

[0036] Figure 7 This is a schematic diagram illustrating the energy partitioning principle of an embodiment of the present invention;

[0037] Figure 8 This is a diagram showing the failure morphology of weathered granite standard specimen a under a confining pressure of 0 MPa according to an embodiment of the present invention.

[0038] Figure 9 This is a diagram showing the failure morphology of weathered granite standard specimen b under confining pressure of 2 MPa in Example 2 of this invention.

[0039] Figure 10 This is a diagram showing the failure morphology of standard weathered granite specimen c under confining pressure of 4 MPa in Example 4 of this invention.

[0040] Figure 11 This is a diagram showing the failure morphology of the weathered granite standard specimen d under a confining pressure of 6 MPa according to Example 6 of the present invention.

[0041] Figure 12 This is a diagram showing the failure morphology of the weathered granite standard specimen e under confining pressure of 8 MPa in Example 8 of the present invention. Detailed Implementation

[0042] A method for evaluating the brittleness of weathered granite under different confining pressures includes the following steps:

[0043] Step 1: Preparation of standard specimens for weathered granite: Fresh granite samples are drilled from the mine and weathered under natural conditions for 7 years. The weathered granite samples are then made into standard specimens.

[0044] The granite samples used in this embodiment were all collected from a thick tantalum-niobium granite ore body in a mine in Jiangxi Province. Their mineral composition mainly consists of quartz, feldspar, biotite, and some clay minerals such as montmorillonite, illite, and kaolinite. Quartz has a dense structure, thus exhibiting good mechanical properties, followed by feldspar. Biotite is soft with a prominent porous structure. The clay minerals are indirectly produced during the rock formation process through alteration and mudification; they easily swell upon contact with water, and weathering increases the clay mineral content within the samples, leading to increased porosity, loosening of the structure, and, to some extent, promoting weathering, thus weakening the mechanical properties of the rock samples. Fresh rock samples were drilled from a depth of 200 m at the mine site and allowed to weather naturally on the surface for 7 years. Following international rock mechanics testing requirements, the samples were processed into standard specimens in the laboratory. The standard specimen for triaxial compression testing was a cylinder with a diameter of 50 mm and a height of 100 mm.

[0045] Step 2: Conduct triaxial compression synchronous acoustic emission tests on the standard specimens described in Step 1 under different confining pressures, and obtain stress-strain relationship diagrams and acoustic emission energy-cumulative energy-stress-time relationship diagrams for each standard specimen;

[0046] Depend on Figure 1 It can be seen that the stress-strain curves of weathered granite under different confining pressures can be divided into four stages: compaction stage A, elastic deformation stage B, plastic development stage C, and post-peak failure stage D. In the compaction stage, some weathered and original pores inside the standard specimen close with the application of the initial load, and the stress-strain curve in this stage shows an upward concave growth trend. Afterward, the standard specimen gradually enters the elastic deformation stage. In this stage, the pores inside the standard specimen are further compacted, and a small number of microcracks are generated, but the standard specimen as a whole does not change significantly, and its stress-strain curve is approximately a straight line. As the load continues to be applied, the standard specimen gradually enters the plastic development stage. In this stage, some irreversible plastic deformation mainly occurs inside the weathered granite standard specimen, which is manifested by the large-scale generation, development, and propagation of new cracks. With continued application of load, the standard specimen enters the post-peak failure stage. Before reaching the peak value, large-scale generation, development, propagation, and penetration of new and old cracks occur inside the weathered granite standard specimen, and some macroscopic cracks can be gradually seen on the surface of the standard specimen. When the stress reaches near the peak value, a large amount of strain energy accumulated inside the standard specimen is released, accompanied by a crisp sound, and the standard specimen undergoes instability and failure. From its stress-strain relationship diagram, it can be seen that the stress does not decrease linearly after the peak, but decreases slowly with the increase of strain. This indicates that after instability and failure, the standard specimen still has a certain load-bearing capacity and undergoes further failure after the peak.

[0047] Depend on Figure 2, Figure 3 , Figure 4 , Figure 5 , Figure 6 It can be seen that in the stages before the peak stress, the cumulative acoustic emission energy curves of each standard specimen under different confining pressures all show a slow growth trend, corresponding to a low level of acoustic emission energy. Near the peak stress, the cumulative acoustic emission energy curves of each standard specimen show an inflection point, corresponding to a higher energy level, which means that the acoustic emission energy has an accelerated release trend and is released in a concentrated manner within a short period of time. In the stage after the peak stress, the cumulative acoustic emission energy curves of each standard specimen show a high growth rate trend, corresponding to a high level of acoustic emission energy, reaching a peak at a certain point. The above phenomena further indicate that before the stress peak, friction-type and small-scale fracture-type events mainly occur inside the standard specimens, while large-scale friction-type and fracture-type events mainly occur inside the standard specimens at and after the stress peak. In addition, when the confining pressure increases, the monitored acoustic emission energy value, especially the energy value after the stress peak, also continuously increases, which indirectly reflects that under higher confining pressure conditions, the deformation and failure of weathered granite requires more energy.

[0048] Step 3: Define and calculate the brittleness index B3 of weathered granite standard specimens under different confining pressures. Specific steps include:

[0049] From an energy perspective, brittle materials accumulate strain energy through elastic deformation, and then release most of this accumulated strain energy upon instability and failure. Therefore, the energy parameters obtained from acoustic emission also need to consider the energy changes before and after the peak failure stage. Thus, the total accumulated acoustic emission energy can be divided into two parts, such as... Figure 7 As shown. That is:

[0050] For the standard specimen before failure, a series of plastic deformations occur inside the specimen, including crack initiation, propagation, and penetration. A portion of the strain energy is released. The energy consumed before time t1 corresponds to the energy dissipation during this process. The higher the proportion of dissipated energy to total energy, the more pronounced the brittle characteristics. Therefore, the brittleness index based on dissipated energy can be calculated using the following formula:

[0051]

[0052] (1) In the formula, E1 is the dissipated energy and E2 is the accumulated strain energy.

[0053] For a standard rock specimen that tends towards complete failure with increasing load, the time interval corresponding to this process is t1-t2, during which a large amount of accumulated strain energy is released. If it is assumed that the faster the strain energy release rate during this process, the higher the brittleness, then the brittleness index based on the strain energy release rate can be calculated by the following formula:

[0054] (2)

[0055] In the formula, t1 and t2 are the peak stress time and the time of complete failure of the rock standard specimen, respectively.

[0056] Therefore, the calculation formula for the brittleness index based on the cumulative energy of acoustic emission can be obtained through equations (1) and (2):

[0057] (3)

[0058] Step 4: Summarize the brittleness index values ​​of the weathered granite standard specimens obtained in Step 3 under different confining pressures, and quantitatively compare and analyze the influence of confining pressure on the brittleness of weathered granite.

[0059] Table 1 summarizes the brittleness evaluation results of weathered granite standard specimens obtained based on the cumulative acoustic emission energy. As shown in Table 1, when the confining pressure is 0 MPa, 2 MPa, and 4 MPa, the corresponding brittleness index values ​​B3 are 4.55, 2.65, and 1.84, respectively, showing a decreasing trend. However, when the confining pressure is 6 MPa and 8 MPa, the obtained brittleness index values ​​B3 are 4.65 and 5.22, respectively, showing an increasing trend after the confining pressure reaches 4 MPa. Therefore, as the confining pressure increases from 0 MPa to 8 MPa, the overall brittleness of the weathered granite shows a decreasing and then increasing trend. Figure 8 , Figure 9 , Figure 10 , Figure 11 , Figure 12 It can be seen that when the standard specimen is in the range of 0MPa to 4MPa, it is observed that it is in the process of splitting failure and the number of corresponding macroscopic fracture cracks gradually decreases. This indicates that the increase of confining pressure causes the mineral grains to gradually adhere more tightly, thereby inhibiting the generation of some fracture cracks and gradually reducing its brittleness. However, when the standard specimen is in the range of 4MPa to 8MPa, its failure mode transitions from splitting failure to shear failure, and it is observed that the number of macroscopic fracture cracks on the surface of the standard specimen gradually increases. This is different from the results of general rock studies. This is because weathering has caused inhomogeneity inside the weathered granite standard specimen. As a result, when the confining pressure is higher, while it inhibits the propagation of some cracks in certain directions, it also promotes the propagation of some cracks in other directions. However, the overall result is still shear failure, so its brittleness increases.

[0060] Table 1. Brittleness evaluation results of weathered granite standard specimens based on cumulative acoustic emission energy.

[0061]

[0062] In Table 1, E1 is the dissipated energy in mV*ms, E2 is the accumulated energy in mV*ms; t1 is the time when the specimen reaches the peak stress in seconds; t2 is the time when the specimen completely fails in seconds; B1 is the brittleness index based on dissipated energy; B2 is the brittleness index based on strain energy release rate; and B3 is the brittleness index based on acoustic emission cumulative energy.

[0063] This invention conducts triaxial compression synchronous acoustic emission tests on standard specimens of weathered granite under different confining pressures. The cumulative acoustic emission energy of each standard specimen is used to define the brittleness index of weathered granite under different confining pressures. The influence of confining pressure on the brittleness of weathered granite is quantitatively analyzed. Experimental results demonstrate the reliability of the method. In actual mine surrounding rock stabilization and support engineering, this method can be used to effectively assess the brittleness of weathered granite surrounding rock, thereby understanding its deformation and failure characteristics and providing corresponding solutions. The above-disclosed embodiments are merely preferred embodiments of the invention and should not be construed as limiting the scope of the invention. Therefore, equivalent variations made within the scope of this invention are still within the scope of this invention.

Claims

1. A method for evaluating the brittleness of weathered granite under different confining pressures, characterized in that, Includes the following steps: Step 1: Preparation of weathered granite standard specimens: Fresh granite rock samples are drilled from the mine and weathered under natural conditions for 7 years to prepare standard specimens; Step 2: Conduct triaxial compression synchronous acoustic emission tests on the standard specimens described in Step 1 under different confining pressures to obtain stress-strain relationship diagrams and acoustic emission energy-cumulative energy-stress-time relationship diagrams for each standard specimen; Step 3: Define and calculate the brittleness index B3 of weathered granite standard specimens under different confining pressures. Specific steps include: For a standard specimen before failure, the brittleness index of dissipated energy is calculated by the following formula: （1） In the formula, E1 is the dissipated energy and E2 is the accumulated strain energy; For standard specimens that tend to fail completely with increasing load, the brittleness index of strain energy release rate is calculated by the following formula: （2） In the formula, t1 and t2 are the peak stress time and the time of complete failure of the rock standard specimen, respectively. The brittleness index calculation formula based on the cumulative acoustic emission energy is obtained through equations (1) and (2): （3） Step 4: Summarize the brittleness index values ​​of weathered granite under different confining pressures obtained in Step 3, and evaluate the brittleness change trend of weathered granite under different confining pressures based on the variation law of brittleness evaluation index B3.

2. The method for evaluating the brittleness of weathered granite under different confining pressures according to claim 1, characterized in that, The standard specimen mentioned in step one is a cylinder with a diameter of 50 mm and a height of 100 mm. The standard specimen must be complete in appearance, have a uniform texture, and be free of obvious joints and cracks.

3. A method for evaluating the brittleness of weathered granite under different confining pressures according to claim 1 or 2, characterized in that, The loading process of the triaxial compression synchronous acoustic emission test described in step two is divided into two stages: the first stage is the simultaneous loading of axial load and confining pressure, which is force-controlled loading with a confining pressure rate of 0.5 MPa / s until the confining pressure reaches the specified value; the second stage is to maintain the stability of the confining pressure by displacement-controlled loading with a loading rate of 0.01 mm / s, and to continue applying axial pressure until the standard specimen fails.

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