A tunnel rock burst prevention and control method combining advanced stress release hole and hydraulic fracturing

Abstract

The application discloses a tunnel rock burst prevention and control method combining an advanced stress release hole and hydraulic fracturing, and belongs to the technical field of tunnel rock burst prevention and control, and comprises the following steps: S1, the advanced stress release hole is used to prevent and control rock burst for three frequently-occurring rock burst sections; a microseismic monitoring system is used to monitor micro-fracture information in the surrounding rock in a real-time manner during excavation, and the prevention and control effect of the advanced stress release hole on rock burst is evaluated by comparing the number, energy and rock burst grade of microseisms of the drilled section and the adjacent non-drilled section; S2, a deep-buried tunnel rock burst prevention and control method combining the advanced stress release hole and the hydraulic fracturing is used, and a numerical simulation method is used to verify the feasibility of the method; and S3, the influence of the diameter, number and position parameters of the advanced stress release hole on the rock burst prevention and control effect of the hydraulic fracturing is analyzed. The application reduces the probability and grade of rock burst, avoids drilling and blasting disturbance to the surrounding rock, and utilizes the dual effects of water softening the surrounding rock and reducing the strength of the surrounding rock.

Description

Technical Field

[0001] This invention belongs to the field of tunnel rockburst prevention and control technology, specifically relating to a tunnel rockburst prevention and control method that combines advanced stress relief holes and hydraulic fracturing. Background Technology

[0002] Rockbursts have become a major geological hazard of concern in deep engineering excavation. They occur due to the sudden release of elastic energy accumulated within rock, resulting in the ejection or throwing of rock fragments. Rockbursts are characterized by their sudden onset, random spatial occurrence, and strong destructive power, severely restricting the development and utilization of underground space and resources. Therefore, research on rockburst prevention and control methods is of great significance for practical engineering applications.

[0003] Although the occurrence of rockburst is a dynamic phenomenon, the incubation process of rockburst is a static process. According to rock mechanics, after the excavation of a deep-buried tunnel, the original triaxial stress balance is destroyed, the bearing capacity around the goaf is reduced, and stress concentration and energy accumulation will occur. When the strength limit of the rock mass is reached, the danger of rockburst may occur. Usually, passive measures such as shotcrete, prestressed anchor rods and steel mesh are used to reinforce the surrounding rock to prevent minor or moderate rockbursts. However, under high ground stress conditions, it is difficult to prevent strong or extremely strong rockbursts with only passive measures. Therefore, it is necessary to reduce the energy accumulation near the working face or transfer the stress concentration area to the deeper part of the rock mass, that is, to take active measures to prevent rockbursts. Active measures include (1) pressure relief blasting, (2) pilot tunnel excavation, (3) water injection into the surrounding rock, and (4) drilling stress relief boreholes. (1) Pressure relief blasting is considered to be able to effectively alleviate the stress concentration of the surrounding rock and reduce the risk of rockburst, but it will cause strong disturbance to the surrounding rock. Especially when using TBM tunnel construction, the decompression blasting method may cause damage to the TBM. Existing technology indicates that instantaneous blasting can produce a certain range of fracture zone. If the amount of explosive is not reasonable, it may lead to insufficient decompression or release a large amount of blasting energy, or even induce rock bursts. (2) In practice, the role of pilot tunnels in rock burst prevention has been reported from both theoretical and construction perspectives. Studies have shown that the top pilot tunnel scheme can better reduce the intensity of strong rock bursts than the central pilot tunnel scheme. However, the top pilot tunnel will cause uneven load on the cutterhead during TBM excavation, affecting the safety of the main beam. In addition, the rock mass at the boundary of the top pilot tunnel may damage the TBM cutter and reduce its service life. (3) Injecting water into the surrounding rock can soften the rock and reduce or transfer the stress in the surrounding rock; however, water will also reduce the strength of the rock. When the rock is close to the critical state of failure, water injection will accelerate the rock failure and increase the possibility of rock bursts. Or, under high ground stress conditions, the fissures or structural surfaces in the hard rock mass may be lubricated by water injection, which may also induce rock bursts. (4) The borehole stress relief method has been widely used in coal mine engineering. However, borehole stress relief also has limitations. Actual cases have shown that annular stress concentrations are generated around the borehole, which restricts the gas drainage capacity. In addition, many studies are conducted at the rock sample scale in rock mechanics tests, and most are based on uniaxial compression tests, which differ significantly from engineering practice. Although these studies have made great progress in rockburst prevention measures, they have not fundamentally solved the rockburst problem in deep-buried tunnels. We conducted advanced stress relief borehole tests in the Hanjiang-to-Weihe River Water Diversion Project and found that not every borehole test can reduce or prevent rockbursts, and may even increase the occurrence of rockbursts. This indicates that conventional rockburst prevention measures are ineffective in deep-buried tunnel sections with extremely high stress. In view of this situation, we further prevented rockbursts by using hydraulic fracturing technology on the basis of advanced stress relief borehole measures. Currently, hydraulic fracturing technology has been applied in the prevention of rockbursts in coal mines.However, there are no reports on the application of hydraulic fracturing to prevent rockbursts in deeply buried tunnels.

[0004] In this invention, during the tunnel excavation of the Hanjiang-to-Weihe River Water Diversion Project, three sections prone to rockbursts were selected, and on-site advanced stress relief borehole tests were conducted. Then, using a microseismic monitoring system, parameters such as the number, energy, and rockburst severity of microseismic events in the drilled sections and adjacent un-drilled sections were statistically analyzed, and the effectiveness of the advanced stress relief boreholes in preventing rockbursts was assessed. To further improve the rockburst prevention effect, a method combining advanced stress relief boreholes and hydraulic fracturing was proposed. Hydraulic fracturing was performed within the stress relief boreholes using numerical simulation, creating fractures around each borehole and forming an interconnected stress relief zone, effectively weakening or controlling rockbursts. Finally, the impact of different stress relief borehole schemes on hydraulic fracturing for rockburst prevention was discussed. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a tunnel rockburst prevention method combining advanced stress relief holes and hydraulic fracturing, comprising the following steps: S1, using advanced stress relief holes to prevent rockbursts in three tunnel sections prone to rockbursts; using a microseismic monitoring system to monitor micro-fracture information within the surrounding rock during excavation in real time, and evaluating the effectiveness of the advanced stress relief holes in preventing rockbursts by comparing the number, energy, and rockburst level of microseismic events in the drilled section and adjacent un-drilled sections; S2, employing a deep-buried tunnel rockburst prevention method combining advanced stress relief holes and hydraulic fracturing, and verifying the feasibility of the method using numerical simulation.

[0006] S3. Analyze the influence of the diameter, number, and location parameters of the pre-stress relief holes on the effectiveness of hydraulic fracturing in preventing rockburst.

[0007] The beneficial effects of this invention are as follows: This invention reduces the probability and severity of rockbursts, avoids the threat to TBM safety caused by drilling and blasting disturbance of the surrounding rock, and utilizes the dual effects of water softening of the surrounding rock and reducing the strength of the surrounding rock. Attached Figure Description

[0008] Figure 1 This is a diagram showing the drilling locations at the tunnel face under Case I conditions of this invention.

[0009] Figure 2 This is a diagram showing the drilling locations at the tunnel face under Case II conditions of this invention.

[0010] Figure 3 This is a diagram showing the drilling locations at the tunnel face under Case III conditions of this invention.

[0011] Figure 4 This invention relates to the effect of borehole pressure relief on microseismic activity characteristics under Case I conditions.

[0012] Figure 5 This invention relates to the effect of borehole depressurization under Case II conditions on microseismic activity characteristics.

[0013] Figure 6 This invention relates to the effect of borehole depressurization under Case III conditions on microseismic activity characteristics.

[0014] Figure 7 This invention relates to the effect of borehole depressurization under Case I conditions on the spatiotemporal distribution of microseismic events.

[0015] Figure 8 This invention relates to the effect of borehole depressurization under Case II conditions on the spatiotemporal distribution of microseismic events.

[0016] Figure 9 This invention relates to the effect of borehole depressurization under Case III conditions on the spatiotemporal distribution of microseismic events.

[0017] Figure 10 Schematic diagram of the numerical model.

[0018] Figure 11 This is the hydraulic fracturing process described in Case I of the present invention.

[0019] Figure 12 This describes the hydraulic fracturing process in Case II of this invention.

[0020] Figure 13 This describes the hydraulic fracturing process in Case III of this invention.

[0021] Figure 14 This is a contour map of the maximum stress distribution in Step 350 under Case I of the present invention.

[0022] Figure 15 This is a contour map showing the maximum stress distribution in Step 350 under Case II of this invention.

[0023] Figure 16 This is a cloud map showing the maximum stress distribution in Step 350 under Case III of this invention.

[0024] Figure 17 This invention relates the cumulative quantity and cumulative energy of acoustic emissions in three scenarios to the injected water pressure.

[0025] Figure 18 This invention describes a hydraulic fracturing process with a borehole diameter of 88 mm.

[0026] Figure 19 This invention describes a hydraulic fracturing process with a borehole diameter of 95 mm.

[0027] Figure 20This invention describes a hydraulic fracturing process with a borehole diameter of 102 mm.

[0028] Figure 21 This is a cloud map showing the maximum stress distribution for a borehole with a diameter of 88mm according to the present invention.

[0029] Figure 22 This is a cloud map showing the maximum stress distribution for a borehole with a diameter of 95mm according to the present invention.

[0030] Figure 23 This is a cloud map showing the maximum stress distribution for a borehole diameter of 102 mm according to the present invention.

[0031] Figure 24 This invention relates the cumulative quantity and cumulative energy of acoustic emissions for different borehole diameters to the injected water pressure.

[0032] Figure 25 This invention describes a hydraulic fracturing process involving four boreholes.

[0033] Figure 26 This invention describes a hydraulic fracturing process involving six boreholes.

[0034] Figure 27 This is a cloud map showing the maximum stress distribution across the four boreholes of this invention.

[0035] Figure 28 This is a cloud map showing the maximum stress distribution across the six boreholes of this invention.

[0036] Figure 29 This invention relates the cumulative quantity and cumulative energy of acoustic emissions from different numbers of boreholes to the injected water pressure.

[0037] Figure 30 This invention describes a hydraulic fracturing process where the distance from the borehole to the center of the tunnel face is 0.8m.

[0038] Figure 31 This invention describes a hydraulic fracturing process where the distance from the borehole to the center of the tunnel face is 1.6m.

[0039] Figure 32 This is a cloud map showing the maximum stress distribution at a distance of 0.8m from the borehole to the center of the tunnel face in this invention.

[0040] Figure 33 This is a cloud map showing the maximum stress distribution at a distance of 1.6m from the borehole to the center of the working face in this invention.

[0041] Figure 34 This invention relates the cumulative quantity and cumulative energy of acoustic emissions at different drilling distances to the injected water pressure.

[0042] Figure 35 This invention describes a hydraulic fracturing process in which the angle between the line connecting two adjacent boreholes and the center of the working face is 12°.

[0043] Figure 36 This invention describes a hydraulic fracturing process in which the angle between the line connecting two adjacent boreholes and the center of the working face is 36°.

[0044] Figure 37 This is a cloud map showing the maximum stress distribution when the angle between the line connecting two adjacent boreholes and the center of the working face is 12°.

[0045] Figure 38 This is a cloud map showing the maximum stress distribution when the angle between the line connecting two adjacent boreholes and the center of the working face is 36°.

[0046] Figure 39 This relates the cumulative number and cumulative energy of acoustic emissions from two adjacent boreholes at an angle of 36° to the center of the working face, to the injected water pressure. Detailed Implementation

[0047] Example 1

[0048] A tunnel rockburst prevention and control method combining advanced stress relief holes and hydraulic fracturing includes the following steps: S1, using advanced stress relief holes to prevent rockbursts in three tunnel sections prone to frequent rockbursts; using a microseismic monitoring system to monitor the micro-fracture information inside the surrounding rock in real time during the excavation process, and evaluating the effectiveness of the advanced stress relief holes in preventing rockbursts by comparing the number, energy, and rockburst level of microseismic events in the drilled section and the adjacent un-drilled section;

[0049] S2. A rockburst prevention method for deep-buried tunnels combining advanced stress relief holes and hydraulic fracturing is adopted, and the feasibility of the method is verified by numerical simulation.

[0050] S3. Analyze the influence of the diameter, number, and location parameters of the pre-stress relief holes on the effectiveness of hydraulic fracturing in preventing rockburst.

[0051] In step S1, a number of boreholes are maintained in the rock mass ahead of the working face of the pre-stress relief borehole. By drilling, the stress distribution in the rock mass is changed, eliminating or mitigating the risk of rockburst. Drilling in the stress concentration area relieves pressure. On the one hand, drilling causes the stress concentration in the surrounding rock to transfer to the deeper part, changing the distribution range of the stress field of the rock mass around the working face. On the other hand, when the stress exceeds the strength of the rock mass around the borehole, the area around the borehole begins to be fractured and releases energy. As the fracture range expands, a fracture zone is eventually formed around the borehole. The fracture zones around multiple boreholes are interconnected, forming a larger fracture zone in the surrounding rock, thereby reducing the probability and severity of rockburst.

[0052] In step S1, the deepest borehole is first determined, and then corresponding distances are selected in the two un-drilled areas before and after the borehole for comparison. On a daily basis, the average number of micro-seismic events per meter is used to reflect the relative activity of micro-seismic events at different locations, the average energy released per meter of micro-seismic events is used to reflect the degree of energy release inside the rock mass, the maximum energy of a single micro-seismic event per day is used to reflect the degree of maximum energy concentration inside the surrounding rock, and the rockburst level is used as evaluation indicators.

[0053] In step S2, the numerical simulation analysis of the pre-stress relief borehole-hydraulic fracturing method is as follows: A three-dimensional numerical model is established using ANSYS. The nodes and elements of the ANSYS model are extracted through the AtoR interface developed in C language and imported into RFPA for calculation. The model adopts the Mohr-Coulomb criterion. The model is square with a side length L equal to 10 times the tunnel diameter of 6.76m, i.e., 67.6m. The excavated distance is 0.2L, and the unexcavated distance is 0.8L. The number, location, diameter, and depth of boreholes are set. The initial water pressure is set to 20MPa, and the water pressure increment is 0.2MPa. The vertical stress is obtained from the geological data of the in-situ geostress measurement results. The macroscopic mechanical parameters obtained from the geological data are used for reverse calculation and correction of the microscopic parameters in the numerical simulation. Finally, the microscopic parameters input into the RFPA numerical calculation are obtained.

[0054] In step S3, as the geostress gradually increases, the water pressure required for macroscopic cracks to appear also increases. Cracks first form in the boreholes near the arch, and all hydraulic cracks extend along adjacent boreholes because the pore pressure field formed near the boreholes controls the propagation direction of the hydraulic cracks. As the injection pressure increases, a through crack eventually forms. Hydraulic fracturing is then performed based on the pre-stress relief boreholes to form a complete pressure relief zone. This reduces the rock mass strength and the energy accumulated in the surrounding rock. On the other hand, it provides compensation space for the release of energy in the surrounding rock, acts as a buffer structure, and thus reduces the rockburst level.

[0055] Based on the relationship between the cumulative amount and energy of acoustic emissions during hydraulic fracturing and the injected water pressure, the fracturing process can be divided into the following four stages:

[0056] (I) Stress accumulation stage: During this process, the rock mass is subjected to gradually increasing water pressure, but no acoustic emission is generated and no energy is released.

[0057] (II) The crack gradually expands during this stage, during which acoustic emission occurs, and the number of acoustic emissions gradually increases, releasing more and more energy.

[0058] (III) During the rapid propagation stage of the crack, the amount of acoustic emission suddenly increases dramatically and energy is suddenly released. The change curve is almost parallel to the vertical axis; and there are two sudden increase stages in each stage.

[0059] (IV) During the stable propagation stage of the crack, as the water injection pressure increases, the number of acoustic emissions increases steadily by a certain amount. The curve is approximately a straight line. During the construction process, at least the third stage of cracking must be reached to ensure the pressure relief effect.

[0060] Among them, based on the pressure required to form the crack, the range of the pressure relief zone, the stress distribution, and the cumulative energy released, the 102mm diameter was found to have the best pressure relief effect.

[0061] Among them, the more boreholes drilled, the easier it is to generate cracks, the larger the pressure relief range formed, the better the effect of transferring compressive stress inside the surrounding rock, and the more energy is released cumulatively, and the better the effect of preventing rockbursts.

[0062] Among them, based on the maximum stress distribution cloud map, the water pressure required for macroscopic cracks, the number of acoustic emissions and energy changes, the influence of the distance between the borehole location and the center of the face on the pressure relief effect was analyzed. When the distance is 1.6m, the water pressure required to generate macroscopic cracks is small, the range of the pressure relief zone is large, the transfer of compressive stress is obvious, the cumulative number of acoustic emissions is large and the cumulative acoustic emission release energy is large. The pressure relief effect at a distance of 1.6m is better than that at 0.8m and 3.2m.

[0063] Among them, the influence of the angle between the line connecting the center of two adjacent boreholes and the center of the tunnel face on the pressure relief effect was analyzed based on the maximum stress distribution cloud map. An angle of 36° produces a large fracture zone, a good stress transfer effect, and generates the most acoustic emissions and releases the most energy. The pressure relief effect of an angle of 36° is better than that of 12° and 24°.

[0064] The influence of advanced stress relief holes on microseismic and rockburst activity characteristics:

[0065] Advanced stress relief boreholes refer to a sufficient number of boreholes maintained within the rock mass at a certain distance ahead of the working face. These boreholes alter the stress distribution within the rock mass, eliminating or mitigating the risk of rockburst. The principle behind drilling for stress relief in stress concentration areas is generally considered to be twofold: firstly, drilling causes stress concentration within the surrounding rock to shift deeper, altering the distribution of the stress field around the working face. Secondly, due to the stress field, stress concentration occurs around the borehole. When the stress exceeds the strength of the rock mass around the borehole wall, fracturing begins around the borehole, releasing energy. As the fracturing range expands, a fractured zone eventually forms around the borehole. Multiple fractured zones around the boreholes interconnect, forming a larger fractured zone within the surrounding rock, i.e., a stress relief zone, thereby reducing the probability and severity of rockburst.

[0066] The test procedure for the pre-stress relief hole is as follows:

[0067] During the construction of the Hanjiang-to-Weihe River Water Diversion Project, as the TBM advanced, the tunnel depth and ground stress gradually increased, leading to a higher frequency of rockbursts. This posed a significant safety hazard to construction personnel and equipment. To mitigate the risk of rockbursts, we conducted three advance drilling tests using a WPD80S / 108C-3000 water hammer drill rig. The drilling parameters are shown in Table 1, and the specific construction plan for the stress relief holes is as follows:

[0068] Case I: Maintenance and repairs began on December 26, 2019. Water hammer drilling was prepared for February 4, 2020, and five holes were completed between February 5 and 7, 2020. Hole 2 was only completed to 15.5 meters due to drill bit jamming, with a total depth of 159 meters. The elevation angle ranged from 7° to 9°, the hole diameter was 95 mm, the drilling pressure was 14-15 MPa, the total drilling time was 989 minutes, and the average drilling speed was 0.1595 m / min. The hole layout is as follows. Figure 1 As shown, the drilling range is within a 95° circumferential range in the upper part of the tunnel, with 5 boreholes distributed at the top of the tunnel face, and an average circumferential borehole spacing of 1.0m.

[0069] Case II: Water hammer drilling was prepared on March 25, 2020. Five holes were completed on March 26-27, and the drilling equipment was dismantled on the morning of March 28. The cumulative depth was 234m, the elevation angle range was 5°-9°, the hole diameter was 102mm, the drilling pressure was 10-15MPa, the cumulative drilling time was 2312min, and the average drilling speed of the water hammer drilling was 0.272m / min. The hole layout is as follows. Figure 2 As shown, the drilling range for this test was 85° circumferentially along the upper part of the tunnel, with an average circumferential hole spacing of 1.1m.

[0070] Case III: On the afternoon of July 26, 2020, tunneling was halted in preparation for water hammer drilling. Six boreholes were completed from July 27 to 30 (drilling distance range: K41+329~K41+377.5). The cumulative depth was 284.25m, the elevation angle ranged from 7° to 9°, the borehole diameter was 102mm, the drilling pressure was 10-15MPa, the cumulative drilling time was 3876min, and the average drilling speed of the water hammer drilling was 0.195m / min. The borehole layout is as follows. Figure 3 As shown, the drilling range for this test was an irregular distribution within a 99° circumferential radius along the upper part of the tunnel.

[0071] The impact of advanced stress relief holes on microseismic and rockburst activity

[0072] To effectively analyze the role of pre-stress relief boreholes, we first determined the deepest borehole, and then compared corresponding distances in two un-drilled areas before and after the borehole (on a daily basis). Furthermore, since the daily excavation distance varied, we used parameters such as the average number of microseismic events per meter (reflecting the relative activity of microseismic events at different locations), the average energy released per meter of microseismic events (comprehensively reflecting the degree of energy release within the rock mass), the maximum energy of a single microseismic event per day (reflecting the degree of maximum energy concentration within the surrounding rock), and the rockburst level as evaluation indicators.

[0073] Table 2

[0074]

[0075]

[0076] Notes: - No rockburst ○ Moderate rockburst Intense rock burst Extremely strong rockburst

[0077] Figure 4 Table 2 shows the experimental protocol, Case I. From Figure 4 It can be observed that the tunneling efficiency of the un-drilled section (including the pre-drilled and post-drilled sections) is significantly higher than that of the drilled section. Specifically, the pre-drilled and post-drilled sections advance 3.79m and 5.13m per day respectively, far exceeding the 2.06m per day of the drilled section. This indicates that drilling in Case I did not improve tunneling efficiency but rather reduced it. Compared to the un-drilled section, the average number of microseismic events per meter fluctuates more significantly and is greater in the drilled section as tunnel excavation progresses, indicating that microseismic activity is higher during this stage. The spatial distribution of these microseismic events is shown below. Figure 7 As shown (each sphere represents a microseismic event, its size represents energy, and color represents magnitude). Furthermore, the maximum microseismic energy is significantly reduced in the borehole section; specifically, the maximum microseismic energy distribution ranges for the pre-drilling, borehole, and post-drilling sections are 108–2390 kJ, 41.9–702 kJ, and 51.3–1310 kJ, respectively. This indicates that the stress relief holes play a crucial role in limiting the maximum energy of microseismic events. The average microseismic energy released per meter in the borehole section is greater than that in the un-drilled section. The average microseismic energy released per meter in the borehole section ranges from 22.82 to 2560.7 kJ / m, while the average microseismic energy released per meter in the pre-drilling and post-drilling sections ranges from 67.99 to 1019.65 kJ / m and 19.48 to 599.44 kJ / m, respectively.

[0078] Table 2 shows the impact of Case I on rockburst activity characteristics. Before and after drilling, there were 3 and 2 extremely strong rockbursts, 13 and 9 strong rockbursts, and 2 and 2 moderate rockbursts, respectively. No extremely strong rockbursts occurred in the drilled section, 14 strong rockbursts occurred, and 1 moderate rockburst occurred. It can be seen that Case I effectively prevented extremely strong rockbursts. Rockburst events occurred almost daily in the un-drilled section, with a maximum of six rockbursts on March 5, 2020. After drilling, from February 13 to February 22, 2020, rockbursts were largely controlled. This was mainly due to reducing the tunneling speed and drilling stress relief holes to transfer stress concentration areas to deeper parts of the surrounding rock. However, a comprehensive comparison revealed that stress relief holes did not completely control rockbursts; in fact, there were 1 and 5 more strong rockbursts compared to before and after drilling, respectively. This indicates that Case I only dispersed extremely strong rockbursts into multiple strong rockbursts through stress transfer, but did not transfer all concentrated stress to deeper parts of the surrounding rock.

[0079] Table 3

[0080]

[0081]

[0082] Notes: - No rockburst ○ Moderate rockburst Intense rock burst Extremely strong rockburst

[0083] Figure 5 Table 3 shows the experimental protocol, Case II. From... Figure 5 It can be observed that the tunneling efficiency of the drilled section is significantly higher than that of the un-drilled section, with an average daily tunneling progress of 7.41 m in the drilled section, compared to 4.34 m before drilling and 4.5 m after drilling. This indicates that drilling effectively improves tunneling efficiency in Case II. Compared to the un-drilled section, the average number of microseismic events per meter is significantly reduced in the drilled section, and its spatial distribution of microseismic events is as follows. Figure 8As shown, the average number of micro-vibrations per meter exceeded 10 in 66.67% of the pre-drilling section and 90.91% of the post-drilling section. The post-drilling section even saw an average of 85.38 micro-vibrations per meter on April 12, 2020. The average number of micro-vibrations per meter in the borehole section was below 10, ranging from 0.69 to 6.46. This indicates that the stress relief holes reduced the activity of micro-vibrations. Furthermore, the maximum energy of a single micro-vibration was also reduced in the borehole section; specifically, the maximum micro-vibration energy distribution ranges for the pre-drilling section, borehole section, and post-drilling section were 257–2270 kJ, 1.82–865 kJ, and 9.81–1100 kJ, respectively. This indicates that the stress relief holes effectively limited the maximum energy of micro-vibrations in both Case I and Case II. The average micro-vibration energy released per meter in the borehole section was also lower than that in the un-drilled section. This indicates that the pre-stress relief holes cause the stress concentration in the surrounding rock to transfer to deeper layers and reduce the activity of microseismic events.

[0084] Table 3 shows the impact of Case II on rockburst activity characteristics. Seven extremely strong rockbursts occurred before drilling, one after drilling, and 27 and 11 strong rockbursts occurred after drilling. No extremely strong rockbursts occurred in the borehole section, and two strong rockbursts occurred. It can be seen that Case II reduced both extremely strong and strong rockbursts. In the pre-drilling section, multiple rockbursts occurred almost daily, with a maximum of six strong rockbursts on March 14, 2020, and two extremely strong rockbursts on March 16 and March 22, 2020. In contrast, only one strong rockburst occurred in the borehole section on March 31 and April 3, 2020, indicating that the stress relief borehole significantly reduced the risk of rockbursts. Furthermore, no rockbursts occurred in the post-drilling section from April 4 to April 9, 2020. After April 10, 2020, rockbursts began to occur frequently again; at this point, the drilling effect had completely disappeared at a distance of approximately 30 meters from the borehole section.

[0085] Table 4

[0086]

[0087]

[0088] Notes: - No rockburst ○ Moderate rockburst Intense rock burst Extremely strong rockburst

[0089] Figure 6 Table 4 shows the experimental protocol, Case III. From... Figure 6It can be observed that the tunneling efficiency of the drilled section is slightly lower than that of the un-drilled section, with average daily tunneling progress of 4.1m before drilling, 3.97m in the drilled section, and 5.26m after drilling. Compared to the un-drilled section, the average number of microseismic events per meter is slightly increased in the drilled section, and its spatial distribution of microseismic events is as follows. Figure 9 As shown, the average number of microseismic events per meter exceeded 20 on 38.46% and 10% of the days in the pre-drilling and post-drilling sections, respectively. However, the average number of microseismic events per meter exceeded 20 on 72.73% of the days in the borehole section. This indicates that stress-relief holes increased microseismic activity. The maximum microseismic energy distribution ranges for the pre-drilling, borehole, and post-drilling sections were 384–1610 kJ, 72.4–2430 kJ, and 10.8–1030 kJ, respectively. Unlike Case I and Case II, Case III did not reduce the maximum microseismic energy. As the drilling face advances, the average energy released per meter of micro-vibration shows an overall decreasing trend before drilling, during drilling, and after drilling, with the average energy released per meter of micro-vibration ranging from 293.09 to 2255.85 kJ / m, 177.29 to 1283.17 kJ / m, and 3.52 to 784 kJ / m, respectively.

[0090] The impact of Case III on rockburst activity characteristics is shown in Table 4. Seven extremely strong rockbursts occurred in the pre-drilling section, one in the post-drilling section, sixteen in the pre-drilling section, six in the post-drilling section, and zero in the post-drilling section. The borehole section experienced seven extremely strong rockbursts, 23 strong rockbursts, and one moderate rockburst. It can be seen that rockburst activity was more active in the borehole section than in the un-drilled section in Case III, with multiple extremely strong rockbursts occurring. This indicates that the stress relief holes had little effect on rockburst control at this time, and may even have increased the risk of rockbursts.

[0091] Analysis of the principle of borehole stress relief: (i) Under conditions of great burial depth and extremely high stress, after the construction of pre-stress relief boreholes, some stress remains unchanged, a small portion of stress is transferred to the deeper parts of the surrounding rock, and some stress is concentrated around the borehole, with no cracks generated near the borehole. At this time, the effect of pre-stress relief boreholes on rockburst is weak, similar to the effect of Case III in this paper, and may even increase rockburst. (ii) Pre-stress relief boreholes cause some concentrated stress to transfer to the deeper parts of the surrounding rock, and some stress to concentrate around the borehole, generating a small number of cracks in the borehole, but the cracks between multiple boreholes are not connected. At this time, extremely strong rockburst transforms into strong rockburst, similar to the effect of Case I in this paper. (iii) After the construction of pre-stress relief boreholes, the concentrated stress around the borehole causes a large number of cracks to be generated in the rock mass, and the cracks between multiple boreholes are interconnected, forming a stress relief zone. At this time, pre-stress relief boreholes effectively reduce the probability and severity of rockburst, similar to the effect of Case II in this paper.

[0092] Drilling operations require stopping excavation, significantly increasing time and economic costs. However, microseismic monitoring results show that Case I and Case III's rockburst-limiting effects are far less effective than expected. Therefore, under extremely high stress, traditional pre-stress relief holes alone are insufficient for rockburst prevention. We propose a method combining pre-stress relief holes and hydraulic fracturing, artificially creating numerous interconnected fracture zones (pressure relief zones) around each borehole. This transfers some concentrated stress to deeper layers of the surrounding rock, preventing rapid stress release caused by disturbance. More importantly, the formation of these fracture zones releases a large amount of accumulated elastic strain energy within the rock mass and weakens its strength, reducing its capacity to store elastic energy. Even in the event of a rockburst, the fracture zone provides a compensating space for the release of accumulated energy, acting as a buffer structure and effectively reducing the rockburst severity. Furthermore, this method fully utilizes the softening and stress-transferring effects of water, reducing strength and promoting rock mass failure, without compromising the safety of the TBM equipment. We will then verify and discuss this method through numerical simulation.

[0093] Numerical Simulation Analysis of the Advanced Stress Relief Hole-Hydraulic Fracturing Method

[0094] Numerical model establishment and parameter selection

[0095] This invention utilizes ANSYS to establish a three-dimensional numerical model, extracts node and element information from the ANSYS model through the AtoR interface developed in C language, and imports it into RFPA for calculation. The model adopts the Mohr-Coulomb (MC) criterion. The numerical model is as follows: Figure 10 As shown, the Case I, Case II, and Case III models are square, with a side length (L) equal to 10 times the tunnel diameter (6.76m), i.e., 67.6m. The excavated distance is 0.2L, and the unexcavated distance is 0.8L. The number and location of boreholes for each case are as follows. Figures 1-3 As shown in Table 1, the borehole diameters are as follows. The drilling depths are 33.00 m, 46.80 m, and 47.88 m, respectively. The initial water pressure for all three cases is 20 MPa, and the water pressure increment is 0.2 MPa. Based on the geological data from the in-situ in-situ stress measurement results, the vertical stress σ in Case I is obtained. z =38.72MPa, σ x =σ y = 41.19 MPa; Case II vertical stress σ z =39.53MPa, σ x =σ y = 42.05 MPa; Case III vertical stress σ z =43.43MPa, σ x =σ y=46.18 MPa. Macroscopic mechanical parameters obtained from geological data were used for inverse calculations and to correct the mesoscopic parameters in the numerical simulation, ultimately obtaining the mesoscopic parameters input for the RFPA numerical calculation. The resulting discretized model contains 790,055 finite elements.

[0096] Table 1

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[0098]

[0099] Results Analysis

[0100] like Figures 11-13 The diagram illustrates the hydraulic fracturing processes in Cases 1, 2, and 3. It visually demonstrates the initiation, propagation, and coalescence of fractures within the rock mass, as well as the evolution of the seepage field. Figure 11 In Case I, hole #3 is located at the center of the edge of the working face, with the other four holes evenly distributed on both sides. At step 296 (water injection pressure 79.2 MPa), obvious macroscopic cracks are visible in holes #2 and #3. At step 298, the top three holes (hole #2, hole #3, hole #4) are essentially connected. At step 299, macroscopic cracks appear in the bottom two holes (hole #1, hole #5). As the water injection pressure increases, the cracks at the top and bottom extend towards each other. At step 300, hole #1 connects with the top crack, and at step 302 (water injection pressure 80.4 MPa), hole #5 connects with the top crack, at which point a essentially continuous crack is formed. Continuing to increase the water injection pressure, the crack propagation rate gradually decreases, so we terminated the calculation at step 350. Figure 12 In Case II, hole #2 is located at the center of the edge of the working face, with the borehole position slightly to the right of the shoulder. At step 316 (water injection pressure 83.2 MPa), obvious macroscopic cracks are visible in holes #2 and #3. At step 318, holes #1 to #4 are essentially continuous. As the water injection pressure increases, the crack at the top extends towards hole #5, and by step 320, all five holes have essentially formed a continuous crack. At step 323, a crack tip extending towards the center of the working face forms in hole #5. With continued increase in water pressure, the crack gradually expands towards the center of the working face. Figure 13In Case III, the six boreholes are irregularly distributed, with holes #3 and #4 at the top of the face being relatively close. At step 318 (water injection pressure 83.6 MPa), no obvious macroscopic fractures are visible in holes #3 and #4. At step 322, fractures connect holes #3 and #4, and macroscopic fractures begin to appear in holes #2 and #5. At step 327, holes #2 and #5 are connected, and the fractures gradually extend towards hole #1 with increasing water injection pressure. At step 335 (water injection pressure 87 MPa), all boreholes are essentially connected. In summary, from Case I to Case III, the geostress gradually increases, and the water pressure required for macroscopic fractures also increases. Furthermore, we found that fractures typically develop first in boreholes near the arch, and then almost all hydraulic fractures extend along adjacent boreholes. This is mainly because the pore pressure field formed near the boreholes has a significant controlling effect on the propagation direction of hydraulic fractures. With increasing water injection pressure, a continuous fracture eventually develops. This demonstrates that hydraulic fracturing based on pre-stress relief boreholes can create a complete stress relief zone. On the one hand, this reduces the rock mass strength and the energy accumulated in the surrounding rock; on the other hand, it provides a compensating space for the release of energy in the surrounding rock, acting as a buffer structure, thereby effectively reducing the rockburst intensity.

[0101] like Figures 14-16 The diagram shows the maximum stress distribution cloud map. It can be seen that stress concentration occurred at the edge of the tunnel face and the contact point with the surrounding rock after excavation in all three scenarios. With the increase of injected water pressure (step 175), tensile stress concentration formed at the center of the tunnel face and in front of the tunnel face. After the formation of the pressure relief zone at step 350, we observed that the concentrated compressive stress in front of the tunnel face shifted deeper into the surrounding rock, and was far from the tunnel to be excavated, reducing the risk of rockburst. It should be noted that pressure concentration also existed at the connection points between the tunnel face and the surrounding rock, except near the borehole, indicating that the influence range of the top borehole was limited.

[0102] like Figure 17The diagram shows the relationship between the cumulative number and energy of acoustic emissions during hydraulic fracturing and the injected water pressure. It can be seen that the fracturing process can be divided into the following four stages: (I) Stress accumulation stage: During this process, the rock mass is subjected to gradually increasing water pressure, but no acoustic emissions are generated, and no energy is released. (II) Gradual fracture propagation stage: Acoustic emissions are generated in this stage, and the number of acoustic emissions gradually increases, as does the energy released. (III) Rapid fracture propagation stage: The number of acoustic emissions suddenly increases dramatically, and energy is suddenly released; its curve is almost parallel to the vertical axis; and there are two sudden increase stages in each stage. (IV) Stable fracture propagation stage: With the increase of injection pressure, the number of acoustic emissions increases steadily by a certain amount; its curve is approximately a straight line. For example, Case II did not generate acoustic emissions in steps 1-208 (20-61.6 MPa), belonging to stage I. In steps 209-317 (61.8-83.4 MPa), the number of acoustic emissions gradually increased, belonging to stage II. From steps 318 to 323 (83.6-84.6 MPa), the number of acoustic emissions suddenly increases, belonging to stage III. From steps 324 to 350 (84.8-90 MPa), the number of acoustic emissions increases steadily, belonging to stage IV. The results indicate that in hydraulic fracturing, fracturing must reach at least stage III to ensure the formation of a fracturing zone and effectively prevent rockburst. Furthermore, at step 350, Case I produces the fewest acoustic emissions and releases the least energy. Case II and Case III release roughly the same cumulative energy at this point, but Case II produces significantly more acoustic emissions than Case III. This suggests that the average energy per acoustic emission in Case III is greater than that in Case II.

[0103] Currently, the installation of pre-stress relief boreholes at the working face has been widely applied in engineering practice. However, the placement of these boreholes still exhibits a degree of arbitrariness regarding their impact on the formation of the pressure relief zone in hydraulic fracturing. Cases I, II, and III reveal that parameters such as the radius, number, and location of the pre-stress relief boreholes have a significant impact. Therefore, it is necessary to further optimize the installation method of these stress relief boreholes and discuss the stress relief effect of hydraulic fracturing on the surrounding rock stress field and its control effect on rockburst hazards under different drilling schemes. This has important guiding significance for engineering practice. Next, we will discuss the influence of these factors in detail using Case I as an example (changing the initial water pressure of the model to 40 MPa and the water pressure increment to 0.2 MPa).

[0104] diameter

[0105] Based on Case I, Case II, and Case III, we selected three borehole diameters of 88mm, 95mm, and 102mm, and then performed hydraulic fracturing. Figures 18-20Hydraulic fracturing processes for boreholes of different diameters. When the diameter is 88mm ( Figure 18 At 88 steps (57.6 MPa), macroscopic cracks were found in hole #3 of the arch. With increasing water pressure, cracks appeared in holes #1 and #2 of the left abutment, subsequently forming two interconnected cracks. At 120, the cracks were essentially connected. When the diameter was 95 mm ( Figure 19 Macroscopic cracks appeared in holes #1 and #2 of the left abutment at 90 steps (58 MPa), followed by penetrating cracks. Finally, with increasing water injection pressure, the cracks gradually connected. When the diameter was 102 mm ( Figure 26 At step 77 (55.4 MPa), macroscopic cracks first appeared in holes #3 and #4 of the arch crown, followed by connection between holes #2 and #5. With increasing water injection pressure, the cracks were essentially complete at step 120, and the cracked area was relatively large. In summary, the water injection pressure required to generate macroscopic cracks with a diameter of 102 mm is much lower than that with hole diameters of 88 mm and 95 mm. This indicates that the diameter affects the water injection pressure required to generate macroscopic cracks; the larger the diameter, the lower the required water injection pressure, and the easier it is to form a larger pressure relief zone, and vice versa. Furthermore, the hole diameter has a significant impact on the propagation sequence of macroscopic cracks; that is, with diameters of 88 mm and 95 mm, the left abutment is connected first, while with a diameter of 102 mm, the arch crown and right abutment are connected first.

[0106] Figures 21-23 This is a contour map showing the maximum stress distribution for different borehole diameters. For diameters of 88mm and 95mm (… Figure 21 , Figure 22 After hydraulic fracturing, there is a concentration of compressive stress near the tunnel face, and tensile stress concentration around the borehole in front of the tunnel face. The concentrated compressive stress is far from the tunnel. This indicates that hydraulic fracturing transfers stress to the deeper parts of the surrounding rock and forms a significant annular zone of increased compressive stress. However, when the diameter is 102 mm ( Figure 23 There was no obvious compressive stress concentration near the tunnel face, but there was compressive stress concentration on the left and right arch shoulders ahead of the tunnel face, while the arch crown mainly experienced tensile stress concentration. In terms of stress distribution, the larger diameter tunnel had a better stress relief effect on the tunnel face, while the smaller diameter tunnel had a better stress relief effect on the surrounding rock ahead of the tunnel face. Figure 24This paper presents the relationship between the cumulative number and energy of acoustic emissions during hydraulic fracturing with different diameters and the injected water pressure. As the water pressure increases, the cumulative number of acoustic emissions follows a trend across four stages. For a diameter of 102 mm, the minimum water pressure (55.2 MPa) is required for stage III, while the required water pressure for diameters of 88 mm and 95 mm is approximately 57.5 MPa. At step 120 (64 MPa), the lowest cumulative acoustic emission energy is found for diameter 95 mm, while the highest is for diameter 102 mm. A comprehensive analysis of the pressure required for fracture formation, the range of the decompression zone, stress distribution, and the cumulative energy released indicates that the largest diameter (102 mm) provides relatively better decompression. However, during construction, it is necessary to consider the actual site conditions. Increasing the borehole diameter too much will increase the likelihood of stuck drill bits, increasing construction difficulty and project costs.

[0107] Number of holes

[0108] Based on the number of boreholes in Case I, Case II, and Case III, we selected 4, 5, and 6 boreholes for comparative analysis. When there are 4 boreholes... Figure 25 Holes #2, #3, and #4 developed macroscopic fractures at step 106 (61.2 MPa), with hole #1 being the last to develop a fracture. As water pressure increased, the fractures gradually expanded, and by step 110, the four holes were essentially connected, forming a pressure relief zone. The hydraulic fracturing process with five boreholes can be referenced. Figure 19 When there are 6 holes ( Figure 26 In the left shoulder, holes #1, #2, and #3 developed macroscopic cracks at step 86 (57.2 MPa). With increasing water pressure, a through crack first appeared in the left shoulder. Further increasing the water pressure showed that the crack was essentially complete at step 120. In summary, a greater number of holes makes it easier for macroscopic cracks to form, and a greater likelihood of creating a pressure relief zone. Furthermore, a larger number of holes and their wider distribution result in a larger pressure relief zone, which is beneficial for preventing rockbursts, and vice versa.

[0109] Figure 27-28 The maximum stress distribution contour plots are shown for different numbers of holes. When the number of holes is 4 ( Figure 27 After hydraulic fracturing, tensile stress concentrations exist on the left and right shoulders of the tunnel face, while significant compressive stress concentrations exist at the crown. Continuing excavation at this point could lead to rockbursts at the crown. The stress distribution characteristics in front of the tunnel face are opposite to those at the face; tensile stress is concentrated at the crown, while compressive stress is concentrated on the left and right shoulders. Continuing excavation at this point could also lead to rockbursts at the left and right shoulders. This indicates that when the number of boreholes is 4, hydraulic fracturing does not completely transfer stress to the deeper rock mass. When the number of boreholes is 5, the maximum stress distribution cloud map can be referenced. Figure 22 When the number of holes is 6 ( Figure 28After hydraulic fracturing, the lower half of the tunnel face experiences compressive stress concentration, while the crown and abutment experience tensile stress concentration. The surrounding rock in front of the tunnel face mainly experiences tensile stress concentration, resulting in relatively good stress relief for the rock mass. Figure 29 The figure shows the relationship between the cumulative number and energy of acoustic emissions and the injected water pressure during hydraulic fracturing with different numbers of boreholes. As the water pressure increases, the curves also conform to the changing trends of the four stages. When there are 4, 5, and 6 boreholes, the pressures required for the rapid failure stage are 61 MPa, 57.8 MPa, and 57 MPa, respectively, indicating that the more boreholes there are, the lower the injection pressure required to form the pressure relief zone. Furthermore, the number of cumulative microseismic events is highest when there are 6 boreholes. In addition, in the first 113 steps (62.6 MPa), the 6 boreholes release the most cumulative energy, while in the last 4 boreholes (after 62.8 MPa) in step 114, the last 4 boreholes release the most cumulative energy. Comprehensive comparative analysis shows that a higher number of boreholes makes it easier to generate fractures, creates a larger pressure relief range, achieves better stress transfer within the surrounding rock, releases more cumulative energy, and has a better effect on preventing rockbursts.

[0110] Distance from drilling position to the center of the working face

[0111] The drilling locations were selected at distances of 0.8m, 1.6m, and 3.2m from the center of the working face, and then hydraulic fracturing was performed. At a distance of 0.8m ( Figure 30 The boreholes were relatively close together, and macroscopic cracks appeared at step 85 (57 MPa). At step 91, the five boreholes were essentially connected, and at step 99, the connected cracks extended outwards with three fracture tips. With increasing water injection pressure, at step 120, the entire tunnel face was divided into three parts. At a distance of 1.6 m ( Figure 31 At step 79 (55.8 MPa), macroscopic cracks appeared; at step 86, the cracks between boreholes were basically continuous; and at step 97, the continuous cracks continued to extend outwards. With further increase in water injection pressure, at step 120, three cracks extending from the continuous cracks had reached the edge of the tunnel face. At a distance of 3.2 m, the cracks mainly extended along the edge of the tunnel face. (See details...) Figure 19 .

[0112] Figures 32-33 This is a contour map showing the maximum stress distribution at different borehole distances from the center of the face. At a distance of 0.8m ( Figure 32 Significant compressive stress concentration was observed at the tunnel face crown and abutment, as well as at the right abutment in front of the tunnel face. This indicates that hydraulic fracturing at this distance is less effective in preventing rockbursts. At a distance of 1.6m ( Figure 33 There is compressive stress concentration around the tunnel face. Tensile stress is the main concentration around the tunnel face, while smaller compressive stress concentrations are observed at a greater distance from the tunnel, indicating better pressure transfer at this distance. The maximum stress distribution contour map at a distance of 3.2m can be used as a reference. Figure 22 . Figure 34 The figure shows the relationship between the cumulative number and energy of acoustic emissions and the injected water pressure during hydraulic fracturing at different distances. The water pressure required for the rapid failure stage at a distance of 1.6m is the lowest (55.2 MPa), followed by 3.2m (57.8 MPa). At a distance of 0.8m, with increasing water pressure, the fractures continuously expand before step 120, and the acoustic emissions also continuously increase. A comparison reveals that the cumulative number and energy of acoustic emissions at 1.6m are significantly higher than at 0.8m and 3.2m. In summary, at a distance of 1.6m, the water pressure required to generate macroscopic fractures is lower, the pressure relief zone is larger, the transfer of compressive stress is more significant, the cumulative number of acoustic emissions is higher, and the cumulative energy released by acoustic emissions is greater, resulting in a better pressure relief effect than at 0.8m and 3.2m.

[0113] The influence of angle

[0114] The angle in this invention refers to the angle between the lines connecting two adjacent boreholes and the center of the working face. We selected angles of 12°, 24°, and 36° for hydraulic fracturing. At an angle of 12° ( Figure 35 The closer the five boreholes are, the smaller the propagation gap between adjacent boreholes, and the stronger the coupling effect between boreholes. At step 98 (59.6 MPa), macroscopic fractures formed in hole #1, and at step 101, hole #2 and hole #1 became interconnected. With increasing water pressure, hole #1 to hole #4 became interconnected at step 105. Finally, in step 120, all five holes formed a continuous fracture, which propagated sequentially from left to right. The hydraulic fracturing process at an angle of 24° can be referenced... Figure 19 When the angle is 36° ( Figure 36 At step 97 (59.4 MPa), macroscopic cracks appeared in hole #1. As the injection pressure increased, macroscopic cracks appeared in holes #2 and #4, forming a large pressure relief zone at step 120.

[0115] Figure 37 , 38 This is a contour map showing the maximum stress distribution at different angles. When the angle is 12° ( Figure 37 The tunnel face section exhibits compressive stress concentration at the arch crown and tensile stress concentration at the left and right abutments. The stress distribution in front of the tunnel face is opposite to that at the tunnel face; that is, there is significant compressive stress concentration at the left and right abutments, a small amount of tensile stress at the arch crown, and tensile stress concentration at the arch base. The maximum stress distribution at an angle of 24° is referenced... Figure 22 At an angle of 36° ( Figure 38 There is a small amount of compressive stress concentration around the tunnel face. Tensile stress concentration exists around the tunnel face, while a ring-shaped area of ​​compressive stress concentration exists further away from the tunnel. This indicates that the stress transfer effect is relatively good at this point. Figure 39The diagram shows the relationship between the cumulative number and energy of acoustic emissions and the injected water pressure during hydraulic fracturing at different angles. It can be seen that the third stage (rapid failure stage) requires the lowest water pressure (57.8 MPa) at 24°, while the required injection pressure is 59.2 MPa at 36°. At 12°, there is no rapid failure stage, and the number of acoustic emissions gradually increases within 120 steps. At 120 steps, the cumulative number and energy of acoustic emissions at 36° are significantly higher than those at 12° and 24°. The cumulative energy released at 12° and 24° is the same, but the cumulative number of acoustic emissions at 24° is greater than that at 12°. In summary, 36° produces a larger fracture zone, better stress transfer, and generates the most acoustic emissions and releases the most energy. Its pressure relief effect is better than that at 12° and 24°.

[0116] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A tunnel rock burst prevention and control method combining a stress-relief hole and hydraulic fracturing, characterized in that, It comprises the following steps: S1, for three frequency rock burst hole section, using the advanced stress release hole to prevent rock burst; using microseismic monitoring system to monitor the micro fracture information of surrounding rock in the process of excavation, comparing the number, energy and rock burst grade of microseismic in the drilling section and adjacent non drilling section to evaluate the prevention effect of advanced stress release hole on rock burst; In the step S1, first determine the deepest drilling hole, then select the corresponding distance in the front and rear sections of the drilling hole for comparison, in units of days, use the average number of microseisms per meter to reflect the relative activity of microseisms in different positions, the average energy release per meter of microseisms to reflect the energy release degree of rock mass, the maximum energy of single microseismic event per day to reflect the degree of maximum energy concentration in the surrounding rock, and the rock burst grade as the evaluation index; S2, using the advanced stress release hole and hydraulic fracturing combined deep buried tunnel rock burst prevention method, and using numerical simulation method to verify the feasibility of the method; In the step S2, the numerical simulation analysis of the advanced stress release hole-hydraulic fracturing method is as follows: a three-dimensional numerical model is established by ANSYS, the nodes and elements of the ANSYS model are extracted by AtoR interface developed by C language and imported into RFPA for calculation, the model adopts Mohr-Coulomb criterion, the model is a square with a side length L of 10 times the tunnel diameter 6.76 m, i.e. 67.6 m, the excavated distance is 0.2L, and the unexcavated distance is 0.8L, the number, position, aperture and drilling depth of the drilling hole are set, the initial water pressure is set to 20 MPa, the water pressure increment is 0.2 MPa, the vertical stress is obtained according to the in-situ stress measurement results of the geological data, the macro mechanical parameters obtained from the geological data are used for reverse calculation and correction of the mesoscopic parameters in numerical simulation, and finally the mesoscopic parameters input into the numerical calculation of RFPA are obtained; S3, analyze the influence of the diameter, number and position parameters of the advanced stress release hole on the rock burst prevention effect of hydraulic fracturing.

2. The tunnel rock burst prevention and control method of the combined advanced stress release hole and hydraulic fracturing according to claim 1, characterized in that, In the step S1, a certain number of drilling holes are kept in the rock mass in front of the working face of the advanced stress release hole, the stress distribution in the rock mass is changed by the drilling holes, and the danger of rock burst is eliminated or reduced; drilling holes are drilled in stress concentration areas to release pressure, on the one hand, the drilling holes make the stress concentration in the surrounding rock transfer to the deep part, changing the distribution range of the stress field of the rock mass around the working face; on the other hand, when the stress exceeds the strength of the rock mass around the hole wall, the surrounding rock of the drilling hole is fractured and energy is released, with the expansion of the fracturing range, a broken zone is finally formed around the drilling hole, the broken zones around multiple drilling holes are interconnected, forming a larger broken zone in the surrounding rock, thereby reducing the probability and grade of rock burst.

3. The tunnel rock burst prevention and control method of the combined advanced stress release hole and hydraulic fracturing according to claim 1, characterized in that, In step S3, with the gradual increase of ground stress, the water pressure required for the occurrence of macroscopic cracks is also increasing, and the cracks first occur in the pores near the arch, all hydraulic fractures are extended along the adjacent boreholes, because the pore pressure field formed near the borehole has a control effect on the propagation direction of the hydraulic fracture, with the increase of water injection pressure, the through crack is finally realized, on the basis of the advanced stress release hole, hydraulic fracturing is carried out, and a complete pressure relief zone is formed, which reduces the strength of the rock mass and the accumulated energy in the surrounding rock on the one hand; On the other hand, it provides compensation space for the release of energy in the surrounding rock, acts as a buffer structure, and thus reduces the grade of rock burst; Through the relationship between the cumulative number and cumulative energy of acoustic emission and the injected water pressure during the hydraulic fracturing process, the fracturing process can be divided into the following four stages: (I) Stress accumulation stage, in this process, the rock mass bears gradually increasing water pressure, but no acoustic emission occurs, and no energy is released; (II) Gradual crack expansion stage, acoustic emission occurs in this stage, and the number of acoustic emission gradually increases, and the released energy gradually increases; (III) Rapid crack expansion stage, the number of acoustic emission suddenly increases, and the energy is suddenly released, and the change curve is almost parallel to the ordinate axis; And there are two sudden increase stages; (IV) Stable crack expansion stage, with the increase of water injection pressure, the number of acoustic emission increases stably, and its curve is approximately a straight line, in the construction process, at least to the third stage, in order to ensure the pressure relief effect.

4. The combined method of advanced stress release hole and hydraulic fracturing for tunnel rock burst prevention and control according to claim 3, characterized in that, According to the pressure required to form a crack, the range of pressure relief zone, stress distribution and cumulative energy released, the best pressure relief effect is obtained for the 102 mm diameter.

5. The combined method of advanced stress release hole and hydraulic fracturing for tunnel rock burst prevention and control according to claim 3, characterized in that, The more the number of boreholes, the easier to produce cracks, the larger the pressure relief range formed, the better the internal stress transfer effect of surrounding rock, and the more the cumulative energy released, the better the rock burst prevention effect.

6. The combined method of advanced stress release hole and hydraulic fracturing for tunnel rock burst prevention and control according to claim 3, characterized in that, According to the maximum stress distribution cloud map, the water pressure required for macroscopic cracks, the number and energy of acoustic emission, the influence of the distance between the borehole position and the center of the working face on the pressure relief effect is analyzed, when the distance is 1.6 m, the water pressure required for the occurrence of macroscopic cracks is small, the range of pressure relief zone formed is large, the stress transfer is obvious, the cumulative number of acoustic emission is large, and the cumulative acoustic emission release energy is large, the pressure relief effect at a distance of 1.6 m is better than that at a distance of 0.8 m and 3.2 m.

7. The combined method of advanced stress release hole and hydraulic fracturing for tunnel rock burst prevention and control according to claim 3, characterized in that, According to the maximum stress distribution cloud map, the influence of the angle between the connecting line of the two adjacent boreholes and the center of the working face on the pressure relief effect is analyzed, the angle of 36° produces a large broken zone, the stress transfer effect is good, the number of acoustic emission produced is the most, and the pressure relief effect at an angle of 36° is better than that at an angle of 12° and 24°.

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