Method for determining rock mass integrity index Kv under high ground stress condition

Abstract

The invention discloses a rock mass integrity index Kv determination method under a high crustal stress condition, and relates to the technical field of rock mass basic quality evaluation, and the method comprises the following steps: carrying out field crustal stress test to obtain the crustal stress of a rock mass and the elastic longitudinal wave velocity of the rock mass; if the result of the field crustal stress test is judged to be extremely high crustal stress or high crustal stress, determining a rock mass integrity index Kv according to the following method; a high-pressure rock triaxial dynamic test system is used for carrying out a standard cylinder sample wave velocity stress test for recovering the confining pressure of an original ground stress environment, and the elastic longitudinal wave velocity of the standard cylinder sample considering the confining pressure is obtained. The rock mass integrity index Kv is calculated based on the longitudinal wave velocity of the rock mass and the elastic longitudinal wave velocity of the standard cylinder sample, the rock core unloading effect caused by high ground stress is considered, the actual integrity of the rock mass is accurately reflected, and the problem that the Kv value is too high in a traditional method is solved. By more accurately calculating the Kv value, the rock mass quality is more scientifically evaluated, and a more reliable basis is provided for engineering design and construction.

Description

Technical Field

[0001] The present invention relates to the technical field of basic rock mass quality evaluation, and in particular to a method for determining the rock mass integrity index Kv under high in-situ stress conditions. Background Art

[0002] The hierarchical evaluation of rock mass quality is an essential basic task in the field of geological engineering. Through systematic analysis of the physical and mechanical properties of rock masses, it provides a scientific basis for various geotechnical engineering projects, which is not only directly related to the rationality and economy of engineering design, but also plays an irreplaceable role in ensuring the safety during construction and the long-term stability after completion. In addition, through the effective classification of rock mass quality, it can also provide important reference information for environmental protection and disaster prevention.

[0003] Common rock mass quality classification systems include the RMR system method, HC method, GSI classification, Q system method, BQ classification method, etc. These systems and methods have their own focuses and are applicable to different engineering backgrounds and geological conditions. Correct application of these classification systems can greatly improve the success rate and safety of engineering projects. Among many rock mass quality evaluation methods, the rock mass integrity index Kv is one of the important indicators for evaluating rock mass quality and plays a key role in rock mass quality evaluation. The definition of the rock mass integrity index Kv is the square of the ratio of the elastic longitudinal wave velocity of the in-situ rock mass to the elastic longitudinal wave velocity of the rock block. Since there are joints and fractures in the in-situ rock mass, the wave velocity of the in-situ rock mass is always less than the wave velocity of the rock block in the laboratory, and the rock mass integrity index Kv is always less than 1. However, in the acoustic wave test of the bedrock of the second bridge of a certain bridge, it was found that the logging wave velocity of the rock mass was greater than the wave velocity of the rock sample. The same phenomenon was also found during the acoustic wave test of the bedrock of a certain bridge, and it was considered that the unloading disturbance effect of the rock core affected the result of the wave velocity test. Through experiments, it was proved that in-situ stress (confining pressure) is an important factor affecting the wave velocity test result and further affecting the value of the rock mass integrity index Kv, and it was found that with the decrease of the rock mass weathering degree, the influence of in-situ stress gradually increases. Since the calculation of the rock mass integrity index is closely related to the test results of the longitudinal wave velocities of the rock mass and the rock block, the value of the rock mass integrity index Kv may be difficult to reflect the true integrity of the rock mass.

[0004] Based on this, many scholars have proposed many optimization algorithms for calculating the rock mass integrity index Kv. The advantages and disadvantages of calculating the rock integrity coefficient using shear waves and longitudinal waves are introduced, and improvement measures for calculating the rock mass integrity index Kv are proposed. It is found that there is a positive linear correlation between the rock mass quality index Kv and the elastic modulus of the rock mass. Considering the unloading effect of rock blocks and the frequency difference of the measured wave velocities, it is proposed to use the unloading index and the frequency correction coefficient to correct the calculation of the rock mass integrity index Kv. Using geophysical means, the inverted resistivity is combined with the rock mass quality index Kv to establish a 2D / 3D rock mass quality assessment method. The relationship between the rock damage variable of granite and numerical calculation and the damage constitutive curve is established, and a method for evaluating the rock mass integrity coefficient is proposed based on this. A supplementary algorithm for calculating the rock mass integrity index Kv value through the first wave amplitude of the effective modal component of shear waves is established based on empirical integral mode decomposition processing.

[0005] In addition, some scholars have proposed to weaken the influence of the rock mass integrity index Kv value in the rock mass quality classification by modifying other rock mass quality classification methods. In view of the high in-situ stress environment and the existing rockburst problems, the RMR system method, the HC method, and the Q system method are modified, and a comprehensive classification scheme for surrounding rocks with multiple methods is established.

[0006] Regarding the problems and calculation methods of the rock mass integrity index Kv value, many scholars have put forward optimization suggestions. Many scholars have also made supplementary corrections to other rock mass quality classifications. However, as the fastest and simplest rock mass quality evaluation method, the fundamental problems existing in calculating the rock mass integrity index Kv value through rock sample wave velocity tests have not been solved. For high in-situ stress areas, how to solve the influence of abnormal wave velocity test results and thus calculate the correct rock mass integrity index Kv value is still an urgent problem to be solved. Aiming at the problem that calculating the rock mass integrity index Kv in high in-situ stress areas cannot truly reflect the integrity degree of the rock mass. Summary of the Invention

[0007] In order to solve the technical problems of basic rock mass quality evaluation, the present invention provides a method for determining the rock mass integrity index Kv under high in-situ stress conditions. The following technical solutions are adopted: A method for determining the rock mass integrity index Kv under high in-situ stress conditions includes the following steps: Step 1: Divide the in-situ stress into four grades: extremely high in-situ stress, high in-situ stress, medium in-situ stress, and low in-situ stress according to the ratio of the maximum principal stress to the maximum initial in-situ stress; Step 2: Conduct on-site in-situ stress tests to obtain the in-situ stress of the rock mass and the elastic longitudinal wave velocity of the rock mass; Step 3: If it is judged that the result of the on-site in-situ stress test is extremely high in-situ stress or high in-situ stress, then determine the rock mass integrity index Kv according to the following method; Step 31: Process the rock mass sample into a standard cylindrical specimen with a set size; Step 32: Conduct a wave velocity - stress test on the standard cylindrical specimen for restoring the confining pressure of the in - situ stress environment using a high - pressure rock triaxial dynamic testing system to obtain the elastic longitudinal wave velocity of the standard cylindrical specimen considering the confining pressure; Step 33: Calculate the rock mass integrity index Kv based on the longitudinal wave velocity of the rock mass and the elastic longitudinal wave velocity of the standard cylindrical specimen.

[0008] Optionally, the formula for calculating the rock mass integrity index Kv in Step 33 is: ; where, is the elastic longitudinal wave velocity of the rock mass, is the elastic longitudinal wave velocity of the standard cylindrical specimen considering the confining pressure.

[0009] By adopting the above - mentioned technical solution, using the in - situ stress classification method in the "Standard for Engineering Rock Mass Classification" (GB50218 - 2014), the in - situ stress is divided into four grades: extremely high in - situ stress, high in - situ stress, medium in - situ stress, and low in - situ stress according to the ratio of the maximum principal stress to the maximum initial in - situ stress. First, it is necessary to judge whether the engineering area belongs to high stress or extremely high stress. If so, the original in - situ stress environment of the rock mass sample needs to be considered in the determination of the rock mass integrity index Kv; First, it is necessary to conduct on - site in - situ stress testing to obtain the in - situ stress and the elastic longitudinal wave velocity of the rock mass. The in - situ stress of the rock mass provides a parameter basis for the wave velocity - stress test under the same subsequent confining pressure, and the elastic longitudinal wave velocity of the rock mass provides a parameter basis for the subsequent calculation of the rock mass integrity index Kv; After obtaining the in - situ stress of the rock mass, conduct a wave velocity - stress test on the standard cylindrical specimen for restoring the confining pressure of the in - situ stress environment using a high - pressure rock triaxial dynamic testing system. The high - pressure rock triaxial dynamic testing system can provide a wave velocity - stress test under the confining pressure for restoring the in - situ stress environment.

[0010] The scientific nature of the calculation method for the rock mass integrity index Kv in high in-situ stress areas is verified. The method of calculating the rock mass integrity index Kv based on the longitudinal wave velocity of the rock mass and the longitudinal wave velocity of the standard cylindrical specimen takes into account the core unloading effect caused by high in-situ stress, more accurately reflects the actual integrity of the rock mass, and avoids the problem of the Kv value being too high in the traditional method. By calculating the Kv value more accurately, the rock mass quality can be evaluated more scientifically, providing a more reliable basis for engineering design and construction, and reducing engineering risks. An accurate Kv value can more effectively guide engineering design and construction, take corresponding measures to improve engineering safety, and avoid accidents such as rock mass instability. Through a more accurate Kv value, engineering materials and construction methods can be selected more reasonably, over-design can be avoided, and engineering costs can be reduced. The optimized Kv calculation method can be combined with other rock mass quality evaluation methods to further improve the accuracy of rock mass quality evaluation.

[0011] Optionally, in step 2, the hydraulic fracturing method is used for in-situ stress testing, including the following steps: Step 21, using a pair of rubber packers, sealing a section of the borehole at the depth where the predetermined rock mass sample is located, and then pumping in liquid to pressurize the borehole, and obtaining the in-situ stress of the rock mass according to the pressure characteristic value of the fracturing process curve; Step 22, using an intelligent engineering logging system to measure the longitudinal wave velocity of the rock stratum where the rock mass sample is located according to the principle of single-hole seismic source excitation method .

[0012] By adopting the above technical solution, the hydraulic fracturing method is one of the recommended methods for measuring rock stress promulgated by the Testing Methods Committee of the International Society for Rock Mechanics in 1987 and 2003. This recommended method also includes the borehole diameter deformation measurement method, the borehole wall strain measurement method, and the rock mass surface stress measurement method. Compared with the other three measurement methods, the hydraulic fracturing method has the following outstanding advantages: large measurement depth; no need for rock elastic parameters to participate in the calculation during data processing, which can avoid errors caused by inaccurate values of rock elastic parameters; a wide stress range on the rock wall (long pressure-bearing section of the borehole), which can avoid the limitations of the "point" stress state and the influence of geological condition inhomogeneity; simple operation and short test period. Therefore, the hydraulic fracturing method is widely used in various fields of rock engineering in industries such as water conservancy and hydropower, transportation, and mining, as well as in geodynamics research. The in-situ stress testing principle of the hydraulic fracturing method is to use a pair of expandable rubber packers to seal a section of the borehole at the predetermined test depth, and then pump in liquid to pressurize this section of the borehole, and calculate the in-situ stress according to the pressure characteristic value of the fracturing process curve.

[0013] Optionally, in step 22, the longitudinal wave velocity of the measured rock stratum ; where is the distance between two sound sources on the probe tube; The time difference of the first arrival refracted waves of the same type of acoustic waves reaching two receivers.

[0014] By adopting the above technical solution, the in-situ wave velocity test uses the JGS-1B intelligent engineering logging system. The in-situ wave velocity data is obtained through the S523 type acoustic velocity and amplitude logging probe. The downhole detection part of the acoustic logging instrument consists of an ultrasonic transmitter and a receiver. The ultrasonic transmitter is connected to the pulse signal source on the ground, and the receiver is connected to the recording device composed of electronic circuits. The distance between the sound sources is 0.5 m, and the double-receiver spacing is 0.2 m. According to the principle of the single-hole seismic source excitation method, it is considered that the borehole diameter can be constant within the range of the distance between the two sound sources. Therefore, the longitudinal wave velocity of the measured rock formation is: ; wherein, is the distance between the two sound sources on the probe, with the unit of (m); is the time difference of the first arrival refracted waves of the same type of acoustic waves reaching two receivers, with the unit of (μm).

[0015] Optionally, in step 31, a cutting and grinding device is used to process the rock mass sample into a standard cylindrical specimen with a diameter d = 50 mm and a height h = 100 mm.

[0016] Optionally, in step 32, the GCTSRTR-2000 high-pressure rock triaxial dynamic test system is used for the experiment. The GCTSRTR-2000 high-pressure rock triaxial dynamic test system provides a maximum axial pressure of 2000 kN, a maximum confining pressure of 140 MPa, and a maximum pore pressure of 140 MPa; The measurement of the longitudinal and transverse wave velocities is realized by the ultrasonic measurement unit equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic test system; The standard cylindrical specimen is clamped on a high-rigidity loading frame. At the beginning of the test, a conventional uniaxial compressive strength test is first carried out, and the elastic longitudinal wave velocity of the standard cylindrical specimen without load is measured. Subsequently, the confining pressure is gradually applied to restore the in-situ stress environment during rock block sampling, and the elastic longitudinal wave velocity of the standard cylindrical specimen under the restored confining pressure is measured.

[0017] Optionally, in step 32, after the measurement of the elastic longitudinal wave velocity of the standard cylindrical specimen is completed, the confining pressure is gradually increased until the standard cylindrical specimen fails. During this period, the acoustic emission events and the changes in the elastic transverse and longitudinal wave velocities of the standard cylindrical specimen are continuously collected.

[0018] By adopting the above technical solution, the rock block wave velocity-stress test uses the GCTSRTR-2000 high-pressure rock triaxial dynamic test system. This system is mainly used for testing the physical parameters of rocks in normal temperature and pressure environments and high temperature and high pressure environments, and can simultaneously measure the mechanical parameters, acoustic parameters, and permeability properties of specimens under the conditions of temperature and pressure changes.

[0019] For the stress test, a high-rigidity loading frame equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic test system is used. The load rigidity reaches 10 MN / mm, which can provide a maximum axial pressure of 2000 kN, a maximum confining pressure of 140 MPa, and a maximum pore pressure of 140 MPa.

[0020] For the measurement of the longitudinal and transverse wave velocities, the ultrasonic measurement unit ULT-100 equipped with this device is used. Its ultrasonic acquisition unit, ULT-100, has ultrasonic emission and digital acquisition functions, and is equipped with corresponding 1 MHz ultrasonic emission and reception transducers and corresponding visualization software. Corresponding acquisition programs can be compiled according to the test requirements to automatically control the acquisition process.

[0021] For the acoustic emission test, the PCI–II acoustic emission system of Physical Acoustics Corporation (PAC) in the United States and the GCTS electro-hydraulic servo loading system in the United States are used. The sampling frequency of the PCI–II acoustic emission system is as high as 40 MHz, with 18-bit analog-to-digital conversion, and has the ability to record continuous waveforms. It can collect 20 characteristic parameters, including AE events, energy, ringing, rise time, etc. In addition, the PCI–II also has 8 external parameters, and the stress values, strain values, etc. of the GCTS can be introduced into the AE acquisition system to keep time synchronization records. During the test, 2 NANO acoustic emission probes are pasted around the cylindrical rock sample to record the parameters and waveforms of AE events.

[0022] This rock block wave velocity-stress test is a constant confining pressure and increasing axial pressure test, aiming to determine the wave velocity value of the specimen under the condition of restoring the confining pressure, as well as the changes in the elastic longitudinal and transverse wave velocities and acoustic emission events of the rock at different stress levels. The test adopts stress control and gradually applies axial pressure and confining pressure according to the hydrostatic pressure condition (rate 0.05 MPa / s). At the beginning of the test, a conventional uniaxial compressive strength test is first carried out, and the elastic longitudinal wave velocity of the rock block without load is measured. Subsequently, after gradually applying axial pressure and confining pressure, the in-situ stress environment at the time of rock block sampling is restored, and the elastic longitudinal wave velocity of the rock block under the restored confining pressure is measured. Continue to increase the confining pressure until the specimen fails. During this period, acoustic emission events and changes in the elastic longitudinal and transverse wave velocities of the rock are continuously collected.

[0023] In summary, the present invention includes at least one of the following beneficial technical effects: The present invention can provide a method for determining the rock mass integrity index Kv under high in-situ stress conditions. The in-situ stress provides a parameter basis for subsequent wave velocity-stress tests under the same confining pressure, and the elastic longitudinal wave velocity of the rock mass provides a parameter basis for subsequent calculation of the rock mass integrity index Kv; after obtaining the in-situ stress of the rock mass, a wave velocity-stress test on a standard cylindrical specimen with the confining pressure restored to the in-situ stress environment is carried out using a high-pressure rock triaxial dynamic test system, and the high-pressure rock triaxial dynamic test system can provide a wave velocity-stress test under the confining pressure restored to the in-situ stress environment.

[0024] The scientific nature of the calculation method for the rock mass integrity index Kv in high in-situ stress areas is verified. The method for calculating the rock mass integrity index Kv based on the longitudinal wave velocity of the rock mass and the longitudinal wave velocity of the standard cylindrical specimen takes into account the core unloading effect caused by high in-situ stress, more accurately reflects the actual integrity of the rock mass, and avoids the problem of the Kv value being too high in the traditional method. By calculating the Kv value more accurately, the rock mass quality can be evaluated more scientifically, providing a more reliable basis for engineering design and construction, and reducing engineering risks. An accurate Kv value can more effectively guide engineering design and construction, take corresponding measures to improve engineering safety, and avoid accidents such as rock mass instability. By calculating the Kv value more accurately, engineering materials and construction methods can be selected more reasonably, over-design can be avoided, and engineering costs can be reduced. The optimized Kv calculation method can be combined with other rock mass quality evaluation methods to further improve the accuracy of rock mass quality evaluation. Description of the Drawings

[0025] Figure 1 is a schematic flow chart of the method for determining the rock mass integrity index Kv under high in-situ stress conditions of the present invention; Figure 2 is a schematic diagram of the stress-strain curve of the rock sample in a specific embodiment of the present invention; Figure 3 is a schematic diagram of the relationship between the longitudinal wave velocity and the axial strain in a specific embodiment of the present invention. Detailed Embodiment

[0026] The present invention will be further described in detail below with reference to the accompanying drawings.

[0027] The embodiment of the present invention discloses a method for determining the rock mass integrity index Kv under high in-situ stress conditions.

[0028] Referring to Figures 1 - 3 , the method for determining the rock mass integrity index Kv under high in-situ stress conditions includes the following steps: Step 1: Classify the in-situ stress into four grades: extremely high in-situ stress, high in-situ stress, medium in-situ stress, and low in-situ stress according to the ratio of the maximum principal stress to the maximum initial in-situ stress; Step 2: Conduct in-situ stress tests on site to obtain the in-situ stress of the rock mass and the longitudinal wave velocity of the rock mass; Step 3: If it is determined that the result of the in-situ stress test on site is extremely high in-situ stress or high in-situ stress, then determine the rock mass integrity index Kv according to the following method; Step 31: Process the rock mass sample into a standard cylindrical specimen with a set size; Step 32: Use a high-pressure rock triaxial dynamic test system to conduct a wave velocity stress test on the standard cylindrical specimen with the confining pressure restored to the in-situ stress environment to obtain the longitudinal wave velocity of the standard cylindrical specimen considering the confining pressure; Step 33: Calculate the rock mass integrity index Kv based on the longitudinal wave velocity of the rock mass and the longitudinal wave velocity of the standard cylindrical specimen.

[0029] The formula for calculating the rock mass integrity index Kv in Step 33 is: ; where, is the longitudinal wave velocity of the rock mass elasticity, is the longitudinal wave velocity of the standard cylindrical specimen considering the confining pressure.

[0030] Adopt the in-situ stress classification method in the "Standard for Classification of Engineering Rock Masses" (GB50218-2014), and classify the in-situ stress into four grades: extremely high in-situ stress, high in-situ stress, medium in-situ stress, and low in-situ stress according to the ratio of the maximum principal stress to the maximum initial in-situ stress. First, judge whether the engineering area belongs to high stress or extremely high stress. If so, the original in-situ stress environment of the rock mass sample needs to be considered in the determination of the rock mass integrity index Kv; First, it is necessary to conduct in-situ stress tests on-site to obtain the in-situ stress and the longitudinal wave velocity of the rock mass elasticity of the rock mass. The in-situ stress of the rock mass provides a parameter basis for the wave velocity-stress test under the same subsequent confining pressure, and the longitudinal wave velocity of the rock mass elasticity provides a parameter basis for the subsequent calculation of the rock mass integrity index Kv; After obtaining the in-situ stress of the rock mass, conduct a wave velocity-stress test on the standard cylindrical specimen with the confining pressure restored to the original in-situ stress environment using a high-pressure rock triaxial dynamic testing system. The high-pressure rock triaxial dynamic testing system can provide a wave velocity-stress test under the confining pressure restored to the original in-situ stress environment.

[0031] The scientificity of the calculation method of the rock mass integrity index Kv in high in-situ stress areas is verified. The method of calculating the rock mass integrity index Kv based on the longitudinal wave velocity of the rock mass and the longitudinal wave velocity of the standard cylindrical specimen takes into account the core unloading effect caused by high in-situ stress, more accurately reflects the actual integrity of the rock mass, and avoids the problem of the Kv value being too high in the traditional method. By calculating the Kv value more accurately, the rock mass quality can be evaluated more scientifically, providing a more reliable basis for engineering design and construction, and reducing engineering risks. An accurate Kv value can more effectively guide engineering design and construction, take corresponding measures, improve engineering safety, and avoid accidents such as rock mass instability. By a more accurate Kv value, engineering materials and construction methods can be selected more reasonably, over-design can be avoided, and engineering costs can be reduced. The optimized Kv calculation method can be combined with other rock mass quality evaluation methods to further improve the accuracy of rock mass quality evaluation.

[0032] In Step 2, the hydraulic fracturing method is used for in-situ stress testing, including the following steps: Step 21: Using a pair of rubber packers, a section of the borehole is sealed at a predetermined depth where the rock sample is located. Liquid is then pumped into the borehole to apply pressure, and the in-situ stress of the rock mass is obtained based on the pressure characteristic value of the fracturing process curve. Step 22: Using the intelligent engineering logging system, the elastic longitudinal wave velocity of the rock layer where the rock sample is located is measured according to the principle of single-hole source excitation method. .

[0033] Hydraulic fracturing is one of the recommended methods for measuring rock stress, published in 1987 and 2003 by the Test Methods Committee of the International Society for Rock Mechanics. This recommended method also includes borehole diameter deformation measurement, borehole wall strain measurement, and rock surface stress measurement. Compared to the other three measurement methods, hydraulic fracturing offers the following advantages: a large measurement depth; no rock elastic parameters are required for data compilation, thus avoiding errors caused by inaccurate rock elastic parameter values; a wide range of rock wall stresses (long borehole pressure-bearing sections), thus avoiding the limitations of "point" stress states and the influence of geological heterogeneity; simple operation, and a short testing cycle. Therefore, hydraulic fracturing is widely used in various fields of rock engineering, including water conservancy, hydropower, transportation, and mining, as well as in geodynamic research. The principle of hydraulic fracturing in-situ stress testing is to use a pair of expandable rubber packers to isolate a section of the borehole at a predetermined test depth. Liquid is then pumped into the borehole to apply pressure, and in-situ stress is calculated based on the pressure characteristic values of the fracturing process curve.

[0034] In step 22, the longitudinal wave velocity of the measured rock layer ; in is the distance between the two sound sources on the probe; It is the time difference between the first arrival and the refracted waves of the same type of sound wave reaching the two receivers.

[0035] The on-site wave velocity test uses the JGS-1B intelligent engineering logging system. The in-situ wave velocity data is obtained through the S523 sound velocity and amplitude logging probe. The well detection part of the acoustic wave measuring instrument consists of an ultrasonic transmitter and a receiver. The ultrasonic transmitter is connected to the pulse signal source on the ground, and the receiver is connected to the recording device composed of electronic circuits. The sound source distance is 0.5m and the double-receiver spacing is 0.2m. According to the principle of the single-hole source excitation method, it is believed that the well diameter can be unchanged within the range of the distance between the two sound sources. Therefore, the longitudinal wave velocity of the measured rock formation for: ; in, is the distance between the two sound sources on the probe, in meters; It is the time difference between the first arrival and the refracted waves of the same type of sound wave reaching the two receivers, in μm.

[0036] In step 31, a cutting and grinding device is used to process the rock sample into a standard cylindrical specimen with a diameter of d = 50 mm and a height of h = 100 mm.

[0037] In step 32, the experiment is conducted using a GCTSRTR-2000 high-pressure rock triaxial dynamic testing system, which provides a maximum axial pressure of 2000 kN, a maximum confining pressure of 140 MPa, and a maximum pore pressure of 140 MPa. The P- and S-wave velocity measurements were achieved using the ultrasonic measurement unit equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic testing system; The standard cylindrical specimen is clamped in a high-rigidity loading frame. At the beginning of the test, a conventional uniaxial compressive strength test is carried out first, and the elastic longitudinal wave velocity of the standard cylindrical specimen is measured when it is not loaded. Then, confining pressure is gradually applied to restore the in-situ stress environment when the rock block is sampled, and the elastic longitudinal wave velocity of the standard cylindrical specimen under the restored confining pressure is measured.

[0038] In step 32, after completing the measurement of the elastic longitudinal wave velocity of the standard cylindrical specimen, the confining pressure is gradually increased until the standard cylindrical specimen is destroyed, during which acoustic emission events and changes in the elastic transverse and longitudinal wave velocities of the standard cylindrical specimen are continuously collected.

[0039] The rock wave velocity-stress test uses the GCTSRTR-2000 high-pressure rock triaxial dynamic testing system, which is mainly used for rock physical parameter testing under normal temperature and pressure environments and high temperature and high pressure environments. It can simultaneously measure the mechanical parameters, acoustic parameters and permeability properties of the sample under temperature and pressure changes.

[0040] The stress test uses the high-rigidity loading frame equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic testing system, with a load stiffness of 10MN / mm, which can provide a maximum axial pressure of 2000kN, a maximum confining pressure of 140MPa, and a maximum pore pressure of 140MPa.

[0041] The longitudinal and shear wave velocities are measured using the ultrasonic measurement unit ULT-100 that comes with the equipment. The ultrasonic acquisition unit, ULT-100, has ultrasonic emission and digital acquisition functions, and is equipped with a corresponding 1MHz ultrasonic emission and reception transducer and corresponding visualization software. The corresponding acquisition program can be compiled according to test needs to automatically control the acquisition process.

[0042] The acoustic emission test adopted the PCI–II acoustic emission system of Physical Acoustics Corporation (PAC) in the United States and the GCTS electro-hydraulic servo loading system in the United States. The PCI–II acoustic emission system has a sampling frequency as high as 40 MHz, 18-bit analog-to-digital conversion, and the ability to record continuous waveforms. It can collect 20 characteristic parameters, including AE events, energy, ringing, rise time, etc. In addition, the PCI–II also has 8 external parameters, which can introduce stress values, strain values, etc. of the GCTS into the AE acquisition system to keep time synchronization records. During the test, 2 NANO acoustic emission probes were pasted around the cylindrical rock sample to record the parameters and waveforms of AE events.

[0043] This rock block wave velocity-stress test was a constant confining pressure and increasing axial pressure test, aiming to determine the wave velocity value of the specimen under the condition of restoring the confining pressure, as well as the changes in the elastic transverse and longitudinal wave velocities and acoustic emission events of the rock at different stress levels. The test adopted stress control and gradually applied axial pressure and confining pressure according to the hydrostatic pressure condition (rate 0.05 MPa / s). At the beginning of the test, a conventional uniaxial compressive strength test was first carried out, and the elastic longitudinal wave velocity Vrp of the rock block without load was measured. Subsequently, after gradually applying axial pressure and confining pressure, the in-situ stress environment at the time of rock block sampling was restored, and the elastic longitudinal wave velocity Vrcp of the rock block under the restored confining pressure was measured. The confining pressure was continuously increased until the specimen failed, and during this period, the acoustic emission events and the changes in the elastic transverse and longitudinal wave velocities of the rock were continuously collected.

[0044] The following uses specific implementation cases to illustrate the implementation principle of the method for determining the rock mass integrity index Kv under high in-situ stress conditions of the present invention: The tunnel site area is located in a tectonically denuded high-middle mountain landform area, with the ground elevation of the central axis ranging from 2777.0 to 4113.0 m, a relative elevation difference of 1336.0 m, large terrain undulations, strong rock weathering, developed glaciers and glacial landforms, and the deposits are mainly composed of Quaternary glaciofluvial accumulated crushed block stone soil. The tunnel is designed as a separated tunnel with a clear span of 11.0 m (width) × 5.0 m (height). The length of the left-line tunnel is 22105.00 m, and the maximum buried depth of the tunnel bottom is about 1115.03 m; the length of the right-line tunnel is 22006.7 m, and the maximum buried depth of the tunnel bottom is about 1122.0 m.

[0045] To explore the relationship between the rock mass integrity index Kv and the confining pressure under high in-situ stress conditions, the in-situ stress test and wave velocity test were carried out on borehole SKTS14 in a certain tunnel engineering area.

[0046] The SKTS14 borehole is located at the bottom slope on the left side of the NW-trending gully that intersects the tunnel alignment at a large angle. The schematic diagram of the borehole profile is shown. The elevation of the borehole collar is 3587.12 m, and the depth of the borehole is 658.00 m. The mountains on both sides are high and steep, with the elevation of the mountain tops ranging from 3950 m to 4000 m. The main surrounding rock of the SKTS14 borehole is slightly weathered granodiorite. The description of the core within the test range is as follows: from 203.2 m to 270.9 m and from 275.6 m to 649.4 m are grayish-white slightly weathered granodiorite, with the core being complete and mostly columnar; from 270.9 m to 275.6 m and from 649.4 m to 658.0 m are grayish-green slightly weathered diorite, with the core being relatively complete and mostly columnar, and locally fragmented.

[0047] In-situ stress tests are carried out using the hydraulic fracturing method. The hydraulic fracturing method is one of the recommended methods for measuring rock stress promulgated by the Commission on Testing Methods of the International Society for Rock Mechanics in 1987 and 2003. This recommended method also includes the borehole diameter deformation measurement method, the borehole wall strain measurement method, and the rock mass surface stress measurement method. Compared with the other three measurement methods, the hydraulic fracturing method has the following outstanding advantages: (1) large measurement depth; (2) no need for rock elastic parameters to participate in the calculation during data processing, which can avoid errors caused by inaccurate values of rock elastic parameters; (3) a relatively wide range of stress on the rock wall (long borehole pressure-bearing section), which can avoid the limitations of the "point" stress state and the influence of geological condition inhomogeneity; (4) simple operation and short test period. Therefore, the hydraulic fracturing method is widely applied in various fields of rock engineering in industries such as water conservancy and hydropower, transportation, and mining, as well as in geodynamic research.

[0048] The principle of in-situ stress measurement by the hydraulic fracturing method is to use a pair of expandable rubber packers to seal a section of the borehole at the predetermined test depth, and then pump liquid to apply pressure to this section of the borehole. The in-situ stress is calculated based on the pressure characteristic values of the fracturing process curve. In-situ wave velocity tests are carried out using the JGS-1B intelligent engineering logging system. In-situ wave velocity data are obtained through the S523 type acoustic velocity and amplitude logging probe. The downhole detection part of the acoustic logging instrument consists of an ultrasonic transmitter and a receiver. The ultrasonic transmitter is connected to the pulse signal source on the ground, and the receiver is connected to the recording device composed of electronic circuits. The distance between the sound sources is 0.5 m, and the double-receiver spacing is 0.2 m. According to the principle of the single-hole seismic source excitation method, it is considered that the borehole diameter can be constant within the range of the distance between the two sound sources. Therefore, the longitudinal wave velocity of the measured rock formation is (Equation 1) where is the distance between the two sound sources on the probe, with the unit of (m); is the time difference of the first arrival refracted waves of the same type of acoustic waves reaching the two receivers, with the unit of (μm).

[0049] The in-situ stress and wave velocity test results are shown in Table 1.

[0050] Table 1 In-situ stress test results by hydraulic fracturing method in borehole SKTS14

[0051] Note: Pb is the rock fracture pressure, Pr is the crack reopening pressure, Ps is the instantaneous closure pressure, P0 is the rock pore pressure, σt is the rock tensile strength, σH is the maximum horizontal principal stress, σh is the minimum horizontal principal stress, σz is the self-weight stress, is the lateral pressure coefficient in the direction of the maximum horizontal principal stress (σH / σz), is the longitudinal wave velocity of the rock mass. The fracture pressure, reopening pressure and closure pressure are all the orifice pressure values of the measuring points. The rock unit weight is taken as 27 kN / m3. The depth of the groundwater level in the borehole during the test is about 161 m. The measured values of the "*" measuring points are on the low side and are not included in the result analysis.

[0052] In order to study the elastic longitudinal wave velocity value of high in-situ stress rock under the condition of restoring in-situ stress and the characteristics of the change of elastic transverse and longitudinal wave velocities with stress, the indoor test of restoring in-situ stress state was carried out to measure the elastic wave velocity of the rock. The selected specimen is granodiorite at a depth of 627.0 - 627.4 m in borehole SKTS14. Using cutting and grinding equipment, the granodiorite was processed into standard cylindrical specimens with a diameter d = 50 mm and a height h = 100 mm. The natural density measurement test of the rock specimens was carried out, and it was obtained that in the natural state, the mass of the rock specimen is 510.37 mg and the density is 2.621 g / cm3.

[0053] The rock block wave velocity-stress test uses the GCTSRTR-2000 high-pressure rock triaxial dynamic test system. This system is mainly used for testing the physical parameters of rocks in normal temperature and pressure environments and high temperature and pressure environments, and can simultaneously measure the mechanical parameters, acoustic parameters and permeability properties of specimens under the change of temperature and pressure.

[0054] The stress test uses a high-rigidity loading frame equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic test system. The load stiffness reaches 10 MN / mm, and it can provide a maximum axial pressure of 2000 kN, a maximum confining pressure of 140 MPa, and a maximum pore pressure of 140 MPa. The longitudinal and transverse wave velocity measurement uses the ultrasonic measurement unit ULT-100 equipped with this equipment. Its ultrasonic acquisition unit, ULT-100 has ultrasonic emission and digital acquisition functions, and is equipped with corresponding 1 MHz ultrasonic emission and receiving transducers and corresponding visualization software, and can compile corresponding acquisition programs according to the test needs to automatically control the acquisition process.

[0055] This rock block wave velocity-stress test is a constant confining pressure and increasing axial pressure test, aiming to determine the wave velocity value of the specimen under the restored confining pressure and the changes in the elastic transverse and longitudinal wave velocities and acoustic emission events of the rock at different stress levels. The test uses stress control and gradually applies the axial pressure (σ1) and confining pressure (σ3) according to the hydrostatic pressure condition (rate 0.05 MPa / s). At the beginning of the test, a conventional uniaxial compressive strength test is first carried out, and the elastic longitudinal wave velocity of the rock block when not loaded is measured. Subsequently, σ1 and σ3 are gradually applied to 13 MPa, then σ3 is kept constant, and σ1 is increased to 19 MPa to restore the in-situ stress environment when the rock block is sampled, and the elastic longitudinal wave velocity of the rock block under the restored confining pressure is measured. Continuing to keep σ3 constant and increasing σ1, with the axial pressure loading rate under stress control being 0.5 MPa / s, σ1 is gradually increased until the specimen fails. During this period, the acoustic emission events and the changes in the elastic transverse and longitudinal wave velocities of the rock are continuously collected. The test results are shown in Table 2.

[0056] Table 2 Test results of in-situ stress wave velocity of rock samples

[0057] To explore the changes in the rock mass integrity index Kv under high in-situ stress conditions, mainly the sample 1-W in the natural state is discussed and analyzed. During the loading process of the triaxial test under the restored in-situ stress environment, the deformation process of the 1-W sample can be roughly divided into four stages, namely the compaction stage, the elastic deformation stage, the plastic deformation stage, and the failure stage.

[0058] (1) Compaction stage OA: As the axial pressure continuously increases, the particle compaction degree inside the rock block continuously improves, the porosity of the rock sample continuously decreases, and the wave velocity of the rock sample continuously increases. In this stage, the wave velocity-axial strain curve increases linearly, and the strain in this stage is mainly axial strain, indicating that the changes in this stage are mainly affected by the porosity of the rock block.

[0059] (2) Elastic deformation stage AB: As the pores and microcracks inside the rock block are gradually compacted, the rock block enters the elastic deformation stage. In this stage, the axial strain first increases and then decreases with the change of axial pressure, and the rate of the wave velocity of the rock block also first increases rapidly and then slowly increases with the volumetric strain. And when the axial strain reaches 0.1339%, the rock block is at the critical point of the elastic-plastic deformation stage, and the longitudinal wave velocity reaches the maximum value of 5779.96 m / s. Different from the longitudinal wave, before the critical point of the elastic-plastic deformation stage, when the axial strain reaches the maximum value of 0.1497%, the shear wave velocity reaches the maximum value of 3138.99 m / s. In this stage, the longitudinal wave velocity accumulatively increases by 950.74 m / s, and the shear wave velocity accumulatively increases by 288.45 m / s. The increasing rate of the longitudinal wave velocity is significantly faster than that of the shear wave velocity. This indicates that the change of the wave velocity of the rock block in this stage is mainly affected by the volumetric strain of the rock block, and for the longitudinal wave, it is more significantly affected by the volumetric strain of the rock block.

[0060] (3) Plastic deformation stage BC: As the axial stress continues to increase, the rock block enters the plastic deformation stage. Microcracks are generated inside the rock block and the cracks continue to grow. At this time, the wave velocity - axial strain curve is concave, indicating that the wave velocity gradually decreases in this stage and decreases gradually with the increase of axial strain. In this stage, it can be found that when at the critical point between the plastic deformation stage and the failure stage (i.e., point C), the longitudinal wave velocity is 5585.89 m / s, a decrease of 194.07 m / s, while the shear wave velocity is 2765.10 m / s, a decrease of 373.89 m / s. It can be seen that the change rate of the shear wave is significantly faster than that of the longitudinal wave. This phenomenon shows that the shear wave is a shear wave and its propagation is more significantly affected by obvious cracks. It indicates that the change of the wave velocity of the rock block in this stage may be mainly affected by the microcracks inside the rock block, and the influence on the shear wave is greater than that on the longitudinal wave.

[0061] (4) After point C in the failure stage: As the axial stress reaches the peak bearing capacity of the rock sample, the internal structure of the rock block is completely damaged, and the cracks rapidly develop into macroscopic cracks and fracture surfaces. The stress - axial strain curve and the wave velocity - volumetric strain curve also rapidly decline, and the wave velocity rapidly decreases. Finally, the longitudinal wave velocity is 5524.07 m / s, with a cumulative decrease of 61.82 m / s in this stage, and the shear wave velocity will drop below the initial wave velocity to 2628.92 m / s, with a cumulative decrease of 136.18 m / s. It indicates that the change of the wave velocity of the rock block in this stage may be mainly affected by the macroscopic cracks and fracture surfaces inside the rock block, and it proves that the macroscopic cracks and fracture surfaces have a greater influence on the shear wave.

[0062] The rock mass integrity index Kv is an important index for evaluating the rock mass quality classification, and its definition is (Equation 2) Among them, is the longitudinal elastic wave velocity of rock mass, is the longitudinal elastic wave velocity of rock block.

[0063] Defining the rock mass integrity index Kv as the square of the ratio of the longitudinal elastic wave velocity of rock mass to that of rock block has not only been widely recognized by scholars worldwide but also adopted by many specifications. Theoretically, due to the influence of joints and fractures in in-situ rock mass, and the rock samples for indoor longitudinal wave velocity tests are usually selected without obvious joints and fractures, so the longitudinal wave velocity of rock mass should be less than that of indoor rock samples. The rock mass integrity index Kv value should always be less than 1, and the more fragmented the rock mass is, the smaller the Kv value of the rock mass integrity index.

[0064] However, in the rock mass quality classification and evaluation work of a certain tunnel construction project, it was found that there were a large number of phenomena where the longitudinal wave velocity of rock mass was greater than that of indoor rock samples in acoustic wave tests. For example, the on-site acoustic wave test results of borehole SKTS14 showed that the average longitudinal wave velocity Vmp of the rock mass at 627.0 - 627.4m of the borehole was 4640m / s, while the elastic longitudinal wave velocity Vrp of the rock block without load in the natural state was 4145m / s as measured by sampling for indoor wave velocity test. Obviously, the longitudinal wave velocity of the rock mass is greater than that of the rock block. And according to the formula defined by the rock mass integrity index Kv, it is calculated that the rock mass integrity index Kv at 627.0 - 627.4m of borehole SKTS14 is 1.25, which does not conform to the traditional understanding.

[0065] Regarding the above situation, Duan Shiwei et al. believe that there are three possible reasons for the longitudinal elastic wave velocity of rock mass exceeding that of rock block, resulting in the integrity coefficient being greater than 1: 1. The small-sample rock mass represented by acoustic logging is more complete than the rock block, resulting in a higher wave velocity; 2. The states of the rock samples in acoustic wave tests are different. For example, the wave velocity of the rock mass in the saturated state is faster than that of the dry rock block; 3. The stress release effect after the core is taken out in the high in-situ stress area makes the measured wave velocity of the rock sample much lower than that of the in-situ rock block. The rock samples for this acoustic wave test are located at 627.0 - 627.4m of the borehole. The overall rock mass of the borehole is relatively complete, and the rock mass and the rock block in the acoustic wave test are in the same water content state. Through comprehensive judgment, the main factor affecting the results of this acoustic wave test is the in-situ stress factor.

[0066] The above tests show that the test result of the longitudinal elastic wave velocity of rock mass in the rock mass integrity index Kv is affected by the in-situ stress, while the test result of the elastic longitudinal wave velocity of indoor rock block does not consider the influence brought by the in-situ stress. Therefore, under high in-situ stress conditions, when the rock block is drilled from the ground, due to the stress release, an unloading effect occurs, resulting in the generation of new fractures in the rock block after drilling, thus affecting the elastic longitudinal wave velocity of indoor rock block Test results. Therefore, when evaluating the rock mass integrity index in high in-situ stress areas, the influence of in-situ stress should be considered during the rock block wave velocity test, and in-situ stress measurement work should be carried out to obtain a more accurate rock mass integrity index value. Optimization process for calculating the rock mass integrity index Kv.

[0067] Regarding the classification problem of the magnitude level of in-situ stress, there is no unified standard at home and abroad so far, and there is even no special quantification method. It is only discussed in rock mass classification or engineering geological evaluation methods. As a well-known rock mass classification method at home and abroad, the Q-system classification method proposed by N. Barton in 1974 uses the ratio of rock mass uniaxial compressive strength to stress as an index to measure the stress level, which has been widely accepted in the engineering field. Most of the subsequent related evaluation works have followed this index. For example, in the "Standard for Classification of Engineering Rock Masses" (GB50218-2014), there are only differences in the specific stress characteristic values and index interval values adopted. Among them, Rc is the saturated uniaxial compressive strength of the rock, is the maximum principal stress, is the maximum initial in-situ stress.

[0068] It can be seen that the classification indicators for each in-situ stress level are relatively consistent. This calculation process uses the "Standard for Classification of Engineering Rock Masses" (GB50218-2014) to classify and distinguish the in-situ stress area of the rock mass.

[0069] For low in-situ stress areas, conventional rock block wave velocity tests can be carried out, and the calculation of the rock mass integrity index Kv can continue to be used; for high in-situ stress areas, a triaxial wave velocity test under the restored in-situ stress environment should be carried out to obtain the elastic longitudinal wave velocity of the rock block considering confining pressure , and the rock mass integrity index Kv value under the restored in-situ stress is calculated using the formula.

[0070] ; Among them, is the elastic longitudinal wave velocity of the rock mass, is the elastic longitudinal wave velocity of the rock block considering confining pressure.

[0071] Through the study of the in-situ stress field in a certain tunnel engineering area, based on the regression inversion results of the stress field in the engineering area, the stress distribution law of the right-line tunnel axis is interpolated, including the isochromatic cloud diagrams of the maximum horizontal principal stress σH, the vertical stress σZ, and the minimum horizontal principal stress σh. According to the stress distribution diagram and the in-situ stress classification method in the "Standard for Classification of Engineering Rock Masses" (GB50218-2014), the in-situ stress field in a certain tunnel engineering area is classified. For Class IV surrounding rock, when the tunnel burial depth is generally less than 700m, the surrounding rock stress is mainly at a high stress level; when the tunnel burial depth is greater than 700m, the surrounding rock stress is at an extremely high stress level. For Class III surrounding rock, when the tunnel burial depth is generally less than 600m, the surrounding rock stress is at a high stress level; when the tunnel burial depth is greater than 600m, the surrounding rock stress is mainly at an extremely high stress level. The rock samples taken in the test are located at 627.0 - 627.4m in the SKTS14 borehole, belonging to Class IV surrounding rock, so the surrounding rock stress is at a high stress level.

[0072] The above are all preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereby. Therefore, all equivalent changes made according to the structure, shape, and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. Method for determining rock mass integrity index Kv under high in-situ stress conditions, characterized in that, The following steps are involved: Step 1: Classify the ground stress into four levels: extremely high ground stress, high ground stress, moderate ground stress, and low ground stress according to the ratio of the maximum principal stress to the maximum initial ground stress; Step 2: Conducting on-site geostress testing to obtain the geostress and elastic longitudinal wave velocity of the rock mass; Step 3: If the result of the on-site in-situ stress test is determined to be extremely high in-situ stress or high in-situ stress, the rock integrity index Kv is determined according to the following method; Step 31, processing the rock sample into a standard cylindrical specimen of a set size; Step 32: Using a high-pressure rock triaxial dynamic testing system, a wave velocity stress test is performed on a standard cylindrical specimen with the original stress environment and confining pressure restored to obtain the elastic longitudinal wave velocity of the standard cylindrical specimen taking the confining pressure into account; Step 33: Calculate the rock mass integrity index Kv based on the rock mass longitudinal wave velocity and the elastic longitudinal wave velocity of the standard cylindrical specimen.

2. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 1, characterized in that, The formula for calculating the rock integrity index Kv in step 33 is: ； Among them, is the longitudinal wave velocity of the rock mass in elastic state, is the longitudinal wave velocity of the standard cylindrical specimen considering the confining pressure.

3. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 2, characterized in that, In step 2, a hydraulic fracturing method is used to conduct an on-site geostress test, including the following steps: Step 21: Using a pair of rubber packers, a borehole is sealed at a predetermined depth where the rock sample is located. Liquid is then pumped into the borehole to apply pressure, and the in-situ stress of the rock mass is obtained based on the pressure characteristic value of the fracturing process curve. Step 22: Use an intelligent engineering logging system to measure the elastic longitudinal wave velocity of the rock stratum where the rock mass sample is located according to the principle of the single-hole seismic source excitation method .

4. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 3, characterized in that, In step 22, the longitudinal wave velocity of the measured rock formation ; wherein is the distance between two sound sources on the probe is the time difference between the first arrival refracted waves of the same type of sound waves reaching two receivers 5. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 1, characterized in that, In step 31, a cutting and grinding device is used to process the rock sample into a standard cylindrical specimen with a diameter of d = 50 mm and a height of h = 100 mm.

6. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 1, characterized in that, In step 32, the experiment is conducted using a GCTSRTR-2000 high-pressure rock triaxial dynamic testing system, which provides a maximum axial pressure of 2000 kN, a maximum confining pressure of 140 MPa, and a maximum pore pressure of 140 MPa. The P- and S-wave velocity measurements were achieved using the ultrasonic measurement unit equipped with the GCTSRTR-2000 high-pressure rock triaxial dynamic testing system; The standard cylindrical specimen is clamped in a high-rigidity loading frame. At the beginning of the test, a conventional uniaxial compressive strength test is carried out first, and the elastic longitudinal wave velocity of the standard cylindrical specimen is measured when it is not loaded. Then, confining pressure is gradually applied to restore the in-situ stress environment when the rock block is sampled, and the elastic longitudinal wave velocity of the standard cylindrical specimen under the restored confining pressure is measured.

7. The method for determining the rock mass integrity index Kv under high in-situ stress conditions according to claim 6, characterized in that, In step 32, after completing the measurement of the elastic longitudinal wave velocity of the standard cylindrical specimen, the confining pressure is gradually increased until the standard cylindrical specimen is destroyed, during which acoustic emission events and changes in the elastic transverse and longitudinal wave velocities of the standard cylindrical specimen are continuously collected.

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