High-pressure liquid nitrogen multi-stage presplitting device and method for surrounding rock of water-rich soft rock roadway

Abstract

The invention discloses a high-pressure liquid nitrogen multi-stage presplitting device and method for surrounding rock of a water-rich soft rock roadway, and relates to the field of mineral engineering. The device comprises a high-pressure liquid nitrogen system and a sensor monitoring information feedback system, the high-pressure liquid nitrogen system comprises a liquid nitrogen tank, a high-pressure pump, a proppant storage bin, a conveying pipe and a pre-splitting device, the pre-splitting device is of a hollow tubular structure, and the opening number of grouting holes is adjusted through a sealing piston; the sensor monitoring information feedback system comprises an infrared monitoring sensor and a crack recognition and processing system, and can collect and analyze crack parameters in real time and feed the crack parameters back to the high-pressure pump. The method solves the problem of difficult grouting of the water-rich soft rock roadway by grading presplitting, removing water-rich interference by using liquid nitrogen to freeze water, adding a proppant to prevent fracture closing and combining real-time monitoring and adjusting parameters, improves the fracturing stability and the supporting effect, and is safe and reliable to operate.

Description

Technical Field

[0001] This invention relates to the field of tunnel support technology, and in particular to a high-pressure liquid nitrogen multi-stage pre-splitting device and method for water-rich soft rock tunnels. Background Technology

[0002] Water-rich soft rock tunnels are typically situated in complex geological environments with high permeability and intense mining disturbances. The inherent weakness, expansion, and rheological properties of soft rock make controlling the deformation of the surrounding rock in these tunnels extremely difficult, posing a serious threat to the safe and efficient exploitation of underground resources in my country for a long time. Currently, both domestically and internationally, pipe roof support and horizontal jet grouting are commonly used for these tunnels. Among these, anchor grouting support is widely applied because it can modify the structure and mechanical properties of the weak surrounding rock in deep tunnels.

[0003] Numerous studies have shown that the lithology of the surrounding rock is a key factor determining the ease of deformation control in roadways. In some soft rock roadways, the surrounding rock is exceptionally weak and has extremely low strength. After excavation, it is unable to withstand the disturbance of complex geostress, rapidly entering the rheological stage and producing significant deformation, making surrounding rock control exceptionally difficult. Reinforcement, as one of the effective means of modifying weak surrounding rock, faces many problems in water-rich soft rock roadways: the surrounding rock fissures are mostly closed, resulting in poor injectability; the effect is unsatisfactory for a considerable period after excavation; the water-rich environment severely interferes with construction, making it difficult to achieve effective support using conventional methods.

[0004] To address the aforementioned issues, some scholars have proposed borehole columnar charge blasting technology, which increases the extent of internal fracture propagation in the surrounding rock through blasting to improve injectability. However, this technology is highly effective for brittle surrounding rock, where fractures can maintain a large opening after blasting, but its effect on soft rock tunnels is less than ideal. The reason for this is that soft rock tunnels exhibit ductile failure characteristics under complex mechanical environments, and internal fractures are prone to closure. While deep-hole columnar charge blasting can open and expand fractures, the fractures quickly close in water-rich conditions after the blast impact load is removed. Reinforcement cannot be completed before the fractures close, resulting in poor application of this technology in water-rich soft rock tunnels.

[0005] Therefore, there is an urgent need for a high-pressure liquid nitrogen multi-stage pre-splitting device and method for water-rich soft rock roadways that can adapt to the characteristics of water-rich soft rock, effectively control fracture closure, and improve efficiency. Summary of the Invention

[0006] To address the aforementioned issues, this invention aims to propose a high-pressure liquid nitrogen multi-stage pre-fracture device and method for water-rich soft rock roadways. By using high-pressure liquid nitrogen for staged fracturing, liquid nitrogen to freeze water, and proppant for auxiliary support, combined with real-time information feedback, it achieves safe and efficient support for water-rich soft rock roadways, providing more options for soft rock roadway support.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock roadways includes a high-pressure liquid nitrogen system and a sensor monitoring information feedback system; The high-pressure liquid nitrogen system includes a liquid nitrogen tank, a high-pressure pump, a proppant storage chamber, a delivery pipe, and a pre-fracking device. The outlet of the liquid nitrogen tank is connected to the high-pressure pump via the delivery pipe. The output of the high-pressure pump is connected to the pre-fracking device via the delivery pipe. The high-pressure pump is equipped with a pressure gauge and a liquid injection control panel. The proppant storage chamber is connected to the slurry delivery pipe via the proppant delivery pipe, and the proppant delivery pipe is equipped with a control valve. The pre-fracking device is a hollow tubular structure with a main hole and a secondary hole on its outer surface. An axially movable sealing piston is installed inside the device, which is used to adjust the number of openings of the main hole and the secondary hole. An overflow valve is installed at the rear end of the pre-fracking device. The sensor monitoring information feedback system includes an infrared monitoring sensor and a crack identification and processing system. The infrared monitoring sensor is installed on the outer surface of the end of the pre-crack device and is used to collect crack image information inside the borehole. The crack identification and processing system is connected to the infrared monitoring sensor and the high-pressure pump via a data cable and is used to analyze the crack geometric parameters and feed them back to the high-pressure pump to adjust the parameters.

[0008] Furthermore, the liquid nitrogen tank is equipped with a replenishment port, and the infrared monitoring sensor is provided with a protective layer.

[0009] To achieve the above objectives, the present invention also provides a high-pressure liquid nitrogen multi-stage pre-fracturing method for surrounding rock in water-rich soft rock tunnels, comprising the following steps: Step 1: Arrange grouting holes at intervals in the top plate, bottom plate and two sides of the soft rock tunnel surrounding rock. The diameter of the grouting hole is 50mm±5mm and the depth of the grouting hole is 2~5m. The specific drilling depth is determined according to the range of the loosened zone of the soft rock tunnel surrounding rock. Step 2: After the grouting holes are set, clean the pre-splitting device and insert it into the grouting hole to the preset depth. After fixing it, seal the gap between the rear end of the hollow tubular structure and the rock mass. Step 3: The delivery pipe is nested into the hollow tubular structure. The hollow tubular structure and the delivery pipe are sealed and fixed by a sealing piston. The rear end of the delivery pipe is connected in sequence to the proppant storage chamber, the high-pressure pump and the liquid nitrogen tank. Step 4: Start the high-pressure pump for the first stage of pre-fracturing. Set a constant flow rate through the injection control panel to deliver high-pressure liquid nitrogen into the delivery pipe. Observe the pressure gauge. When the liquid nitrogen pressure reaches the pressure value required for the first stage of pre-fracturing, open the overflow valve to release the high-pressure liquid nitrogen. The liquid nitrogen is released and sprayed from the main and secondary holes of the hollow tubular structure, which will generate several new fractures in the surrounding rock and expand and develop the original fractures. After the first stage of pre-fracturing is completed, close the high-pressure pump and overflow valve in sequence. Step 5: The crack identification and processing system receives the borehole crack image information collected by the infrared monitoring sensor, performs quantitative analysis on the crack image, outputs the geometric and statistical parameters of the current crack, and feeds the information back to the high-pressure pump through the data line. Step 6: Reset the liquid nitrogen fracturing pressure value to perform multi-stage pre-fracturing. During this pre-fracturing process, open the valve of the proppant material chamber. The proppant material is mixed with liquid nitrogen through the proppant delivery pipe and transported into the hollow tubular structure. The high-pressure liquid nitrogen carries the proppant particles into the surrounding rock, causing further expansion of the fractures and gradually forming a more complex fracture network. After each injection, move the sealing piston a certain length towards the rear end of the hollow tubular structure to perform the next stage of grouting. The number of cycles in this step needs to be determined based on the actual situation of the surrounding rock. Step 7: After multi-stage fracturing is completed, close the liquid nitrogen tank's replenishment port, high-pressure pump, proppant storage chamber, and overflow valve, and remove the pre-fracturing device from the surrounding rock grouting hole.

[0010] Furthermore, the hollow tubular structure is made of steel structure material that is resistant to high pressure and low temperature; the number of grouting holes can be adjusted by the pre-embedded depth of the hollow tubular structure and the axial movement of the sealing piston inside the hollow tubular structure, and grouting can be carried out in stages from few to many and from shallow to deep, so as to control the grouting volume and the spacing between grouting holes and prevent the hydraulic expansion force of the surrounding rock from exceeding the bearing limit.

[0011] Furthermore, the injection volume for each level of pre-fracture is calculated using the following formula: Q=πR 2 Lnαβ, where Q is the injection volume (m³). 3 R—Liquid nitrogen diffusion radius (m) 2 L—Injection length (m), n—Porosity of the surrounding rock formation, α—Formation filling coefficient, generally taken as 0.8, β—Liquid nitrogen loss coefficient, generally taken as 1.1~1.3.

[0012] Furthermore, the pre-splitting pressure values ​​for each level are set by gradually increasing and then gradually decreasing, with the difference between adjacent pressure values ​​being 15% to 20%.

[0013] Furthermore, in each level of pre-splitting, starting from the second level, the number of pre-splitting operations needs to be determined based on the actual changes in the bearing capacity of the surrounding rock. In the multi-level pre-splitting stage until the last level of pre-splitting, proppant material needs to be mixed into liquid nitrogen to provide auxiliary support for the fractures.

[0014] Furthermore, the particle size of the proppant particles is smaller than the size of the main pore and the secondary pore to prevent blockage that could lead to liquid nitrogen backflow.

[0015] Furthermore, two infrared monitoring sensors are provided, located at both ends of the end surface of the hollow tubular structure, for real-time acquisition of crack opening image information inside the borehole.

[0016] Furthermore, the image information collected by the infrared monitoring sensor is transmitted to the crack identification and processing system for quantitative analysis to obtain geometric information on the number, opening, and distribution density of cracks, providing feedback for the injection operation to adjust the injection parameters and injection points.

[0017] Beneficial Effects: This invention employs high-pressure liquid nitrogen for staged fracturing of soft rock tunnel surroundings. During the grouting process, the liquid nitrogen rapidly freezes water, eliminating the interference of a water-rich environment on pre-fracturing. Furthermore, proppant materials are added to the liquid nitrogen to provide continuous support to the fractured surrounding rock fissures, preventing them from reopening under high ground stress and subsequent surrounding rock disturbances, thus further improving fracturing stability and providing more opportunities for grouting support in soft rock tunnels. This invention uses infrared sensors to monitor and collect fracturing data within the borehole, transmitting the image information to a crack image recognition and analysis system for processing. This system obtains geometric parameters such as the number, opening, and distribution density of fissures, providing feedback for grouting and allowing for real-time adjustment of grouting parameters to ensure energy-efficient and effective fracturing. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic diagram of the high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock tunnels according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the sensor monitoring information feedback system in the high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock tunnels according to an embodiment of the present invention. Detailed Implementation

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0020] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] Example 1 See Figure 1-2 A high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock roadways, comprising a high-pressure liquid nitrogen system and a sensor monitoring information feedback system; The high-pressure liquid nitrogen system includes a liquid nitrogen tank 1, a high-pressure pump 5, a proppant storage chamber 6, a delivery pipe 9, and a pre-cracking device. The outlet of the liquid nitrogen tank 1 is connected to the high-pressure pump 5 via the delivery pipe 9. The output end of the high-pressure pump 5 is connected to the pre-cracking device via the delivery pipe 9. The high-pressure pump 5 is equipped with a pressure gauge 4 and a liquid injection control panel 3. The proppant storage chamber 6 is connected to the slurry delivery pipe 9 via a proppant delivery pipe 8, and a control valve 7 is provided on the proppant delivery pipe 8. The pre-cracking device is a hollow tubular structure 13, with a main hole 14 and a secondary hole 12 on its outer surface. An axially movable sealing piston 11 is provided inside, which is used to adjust the number of openings of the main hole 14 and the secondary hole 12. An overflow valve 10 is provided at the rear end of the pre-cracking device. The sensor monitoring information feedback system includes an infrared monitoring sensor 16 and a crack identification and processing system 17. The infrared monitoring sensor 16 is installed on the outer surface of the end of the pre-crack device and is used to collect crack image information inside the borehole. The crack identification and processing system 17 is connected to the infrared monitoring sensor 16 and the high-pressure pump 5 through a data cable 18 and is used to analyze the crack geometric parameters and feed them back to the high-pressure pump 5 to adjust the parameters.

[0022] This embodiment uses high-pressure liquid nitrogen for graded fracturing of soft rock roadways. During the grouting process, liquid nitrogen can quickly freeze water, eliminating the interference of the water-rich environment on the pre-fracturing. Propane materials are added to the liquid nitrogen to provide continuous support for the fractured surrounding rock, preventing the fractures from reopening under high ground stress and subsequent surrounding rock disturbances, further improving fracturing stability and providing more opportunities for grouting support in soft rock roadways.

[0023] In this embodiment, the infrared sensor monitors and collects information on the cracking situation inside the borehole, and transmits the image information to the crack image recognition and analysis system for processing. The system obtains geometric parameters such as the number of cracks, their opening, and their distribution density, which provide feedback for grouting. In turn, the grouting parameters are adjusted in real time to ensure energy-saving and efficient cracking.

[0024] In a specific example, the liquid nitrogen tank 1 is provided with a replenishment port 2, and the infrared monitoring sensor 16 is provided with a protective layer 15.

[0025] Continuous liquid nitrogen supply: The liquid nitrogen tank is equipped with a replenishment port, which can replenish liquid nitrogen in a timely manner during the grouting process to avoid cracking and interruption caused by liquid interruption, and adapt to the needs of long-term and long-distance grouting.

[0026] Sensor protection: The infrared monitoring sensor is equipped with a protective layer to prevent damage to the sensor from high-pressure liquid nitrogen spray or rock debris, ensuring the stability and service life of the monitoring system.

[0027] Example 2 This embodiment also discloses a high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock roadways, including the following steps: Step 1: Arrange grouting holes at intervals in the top plate, bottom plate and two sides of the soft rock tunnel surrounding rock. The diameter of the grouting hole is 50mm±5mm and the depth of the grouting hole is 2~5m. The specific drilling depth is determined according to the range of the loosened zone of the soft rock tunnel surrounding rock. Step 2: After the grouting holes are set, clean the pre-splitting device and insert it into the grouting hole to the preset depth. After fixing it, seal the gap between the rear end of the hollow tubular structure 13 and the rock mass. Step 3: The delivery pipe 9 is nested into the hollow tubular structure 13. The hollow tubular structure 13 and the delivery pipe 9 are sealed and fixed by the sealing piston 11. The rear end of the delivery pipe 9 is connected in sequence to the proppant storage chamber 6, the high-pressure pump 5 and the liquid nitrogen tank 1. Step 4: Start the high-pressure pump 5 for the first stage of pre-fracture. Set a constant flow rate through the injection control panel 3 to deliver high-pressure liquid nitrogen to the delivery pipe 9. Observe the pressure gauge. When the liquid nitrogen pressure reaches the pressure value required for the first stage of pre-fracture, open the overflow valve 10 to release the high-pressure liquid nitrogen. The liquid nitrogen is released and sprayed from the main hole and the secondary hole of the hollow tubular structure 13, which generates a number of new fractures in the surrounding rock and expands and develops the original fractures. After the first stage of pre-fracture is completed, close the high-pressure pump 5 and the overflow valve 10 in sequence. Step 5: The crack identification and processing system 17 receives the crack image information inside the borehole collected by the infrared monitoring sensor 16, performs quantitative analysis on the crack image, outputs the geometric and statistical parameters of the current crack, and feeds the information back to the high-pressure pump 5 through the data line 18. Step 6: Reset the pressure value of liquid nitrogen fracturing to perform multi-stage pre-fracturing. During this pre-fracturing process, open the valve of the proppant material chamber. The proppant material is mixed with liquid nitrogen through the proppant delivery pipe 8 and delivered into the hollow tubular structure 13. The high-pressure liquid nitrogen carries the proppant particles into the surrounding rock, causing further expansion of the fractures and gradually forming a more complex fracture network. After each injection, move the sealing piston 11 a certain length towards the rear end of the hollow tubular structure 13 to perform the next stage of grouting. The number of cycles in this step needs to be determined based on the actual situation of the surrounding rock. Step 7: After the multi-stage fracturing is completed, close the liquid nitrogen tank 1's replenishment port 2, high-pressure pump 5, proppant storage chamber 6, and overflow valve 10, and remove the pre-fracturing device from the surrounding rock grouting hole.

[0028] This embodiment uses high-pressure liquid nitrogen for graded fracturing of soft rock roadways. During the grouting process, liquid nitrogen can quickly freeze water, eliminating the interference of the water-rich environment on the pre-fracturing. Propane materials are added to the liquid nitrogen to provide continuous support for the fractured surrounding rock, preventing the fractures from reopening under high ground stress and subsequent surrounding rock disturbances, further improving fracturing stability and providing more opportunities for grouting support in soft rock roadways.

[0029] The working principle of this embodiment is as follows: a high-pressure liquid nitrogen pump stably delivers liquid nitrogen from the storage tank to the pre-splitting device at a set flow rate and pressure. The high-pressure liquid nitrogen is released through the grouting hole and comes into contact with the surrounding rock of the soft rock tunnel. The original cracks in the borehole wall rock mass develop and expand, and new cracks are continuously generated. After the first stage of grouting is completed, the grouting system is shut down and the grouting parameters are readjusted with the pre-splitting device to carry out the next stage of grouting. The continuous staged high-pressure grouting causes the borehole wall rock mass to form a complex crack network. The proppant particles enter the cracks in the drilled rock mass along with the high-pressure liquid nitrogen to assist in supporting the cracks and prevent them from closing again under the action of surrounding rock stress and environmental disturbance. After the grouting is completed, a complete and stable pipeline has been formed in the surrounding rock, which solves the problem of difficult support of the surrounding rock in soft rock tunnels and can effectively improve the reinforcement effect of the subsequent support surrounding rock.

[0030] In a specific example, the hollow tubular structure 13 is made of steel structure material that is resistant to high pressure and low temperature; the number of grouting holes can be adjusted by the pre-embedded depth of the hollow tubular structure 13 and the axial movement of the sealing piston 11 inside the hollow tubular structure 13, and the grouting can be carried out in sections from few to many and from shallow to deep, so as to control the grouting volume and the spacing between grouting holes and prevent the hydraulic expansion force of the surrounding rock from exceeding the bearing limit.

[0031] Material compatibility: The hollow tubular structure adopts a high-pressure and low-temperature resistant steel structure, which can withstand the low temperature (-196℃) and high-pressure impact of liquid nitrogen, solving the problem of traditional devices being easily damaged in extreme environments and extending the service life of the equipment.

[0032] Precise segmented grouting: The number of grouting holes is controlled by both the pre-embedded depth and the movement of the sealing piston, strictly controlling the grouting volume and hole spacing to avoid the hydraulic expansion force exceeding the bearing limit of soft rock and prevent collapse accidents.

[0033] In a specific example, the pre-fracture injection volume for each level is calculated using the following formula: Q=πR 2 Lnαβ, where Q is the injection volume (m³). 3 R—Liquid nitrogen diffusion radius (m) 2 L—Injection length (m), n—Porosity of the surrounding rock formation, α—Formation filling coefficient, generally taken as 0.8, β—Liquid nitrogen loss coefficient, generally taken as 1.1~1.3.

[0034] Quantitative calculation of grouting volume: Provides a clear formula for grouting volume (Q=πR) 2 Lnαβ), combined with parameters such as formation porosity and filling coefficient, scientifically calculates the amount of liquid nitrogen required for each stage of pre-fracturing, avoiding insufficient fracturing due to insufficient grouting or waste caused by excessive grouting, thus achieving energy saving and high efficiency.

[0035] In a specific example, the pre-splitting pressure values ​​at each level are set by gradually increasing and then gradually decreasing, with the difference between adjacent pressure values ​​being 15% to 20%.

[0036] Dynamic pressure adaptation: The pressure setting method of "gradually increasing and then gradually decreasing" is adopted (adjacent difference 15%~20%). In the early stage, the high pressure will first break through the original fractures, and in the later stage, the pressure will be gradually reduced to consolidate the fractures with proppant. This ensures the fracturing effect and avoids excessive disturbance of soft rock by continuous high pressure, thereby improving the stability of the fractures.

[0037] In a specific example, in each level of pre-splitting, starting from the second level of pre-splitting, the number of pre-splitting times needs to be determined based on the actual changes in the bearing capacity of the surrounding rock. In the multi-level pre-splitting stage until the last level of pre-splitting, proppant material needs to be mixed into liquid nitrogen to provide auxiliary support for the fractures.

[0038] Enhanced fracture stability: Propionate is added from the second stage of pre-fracture until the last stage, and continuous auxiliary support is provided. The proppant particles are used to fill the fractures, preventing the fractures in soft rock from closing under high ground stress or water-rich disturbance. This solves the core problem of easy fracture closure in traditional grouting and extends the effective grouting window.

[0039] In one specific example, the particle size of the proppant particles is smaller than the size of the main hole 14 and the secondary hole 12 to prevent blockage that could lead to liquid nitrogen backflow.

[0040] Anti-clogging and safety: Ensure that the proppant particle size is smaller than the main hole and secondary hole size to avoid liquid nitrogen backflow caused by channel blockage, ensure stable grouting system pressure, and prevent equipment overload or local high-pressure collapse of rock mass caused by blockage, thereby improving construction safety.

[0041] In a specific example, two infrared monitoring sensors 16 are provided, located at both ends of the end surface of the hollow tubular structure 13, for real-time acquisition of crack opening image information inside the borehole.

[0042] Comprehensive monitoring: Two infrared monitoring sensors are set up and placed at both ends of the end, which can collect images of cracks inside the borehole from all directions, avoiding the blind spots of a single sensor, ensuring the integrity and accuracy of parameters such as the number and opening of cracks, and providing reliable data for feedback and adjustment.

[0043] In a specific example, the image information collected by the infrared monitoring sensor 16 is transmitted to the crack identification and processing system 17 for quantitative analysis to obtain geometric information on the number, opening, and distribution density of cracks, providing feedback for the injection operation to adjust the injection parameters and injection points.

[0044] Dynamic parameter optimization: Through quantitative analysis of image information by the crack recognition system, the geometric parameters of the crack are obtained in real time and fed back to the grouting system, realizing dynamic adjustment of parameters such as grouting pressure and flow rate, avoiding experience-based operation, and ensuring efficient cracking of water-rich soft rock under the premise of energy saving.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock tunnel surrounding rock, characterized in that, This includes a high-pressure liquid nitrogen system and a sensor monitoring and information feedback system; The high-pressure liquid nitrogen system includes a liquid nitrogen tank (1), a high-pressure pump (5), a proppant storage chamber (6), a delivery pipe (9), and a pre-cracking device; the outlet of the liquid nitrogen tank (1) is connected to the high-pressure pump (5) through the delivery pipe (9); the output end of the high-pressure pump (5) is connected to the pre-cracking device through the delivery pipe (9), and the high-pressure pump (5) is equipped with a pressure gauge (4) and a liquid injection control panel (3); the proppant storage chamber (6) is connected to the slurry delivery pipe (9) through the proppant delivery pipe (8), and the proppant delivery pipe (8) is equipped with a control valve (7); the pre-cracking device is a hollow tubular structure (13), with a main hole (14) and a secondary hole (12) on its outer surface, and a sealing piston (11) that can move axially inside, the sealing piston (11) being used to adjust the number of openings of the main hole (14) and the secondary hole (12); the rear end of the pre-cracking device is equipped with an overflow valve (10). The sensor monitoring information feedback system includes an infrared monitoring sensor (16) and a crack identification and processing system (17); the infrared monitoring sensor (16) is installed on the outer surface of the end of the pre-crack device and is used to collect crack image information inside the borehole; the crack identification and processing system (17) is connected to the infrared monitoring sensor (16) and the high-pressure pump (5) through a data cable (18) and is used to analyze the crack geometric parameters and feed them back to the high-pressure pump (5) to adjust the parameters.

2. The high-pressure liquid nitrogen multi-stage pre-fracture device for water-rich soft rock tunnels according to claim 1, characterized in that, The liquid nitrogen tank (1) is provided with a liquid replenishment port (2), and the infrared monitoring sensor (16) is provided with a protective layer (15).

3. A high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnel surrounding rock, characterized in that, Includes the following steps: Step 1: Arrange grouting holes at intervals in the top plate, bottom plate and two sides of the soft rock tunnel surrounding rock. The diameter of the grouting hole is 50mm±5mm and the depth of the grouting hole is 2~5m. The specific drilling depth is determined according to the range of the loosened zone of the soft rock tunnel surrounding rock. Step 2: After the grouting hole is set, clean the pre-splitting device and insert it into the grouting hole to the preset depth. After fixing, seal the gap between the rear end of the hollow tubular structure (13) and the rock mass. Step 3: The delivery pipe (9) is nested into the hollow tubular structure (13). The hollow tubular structure (13) and the delivery pipe (9) are sealed and fixed by the sealing piston (11). The rear end of the delivery pipe (9) is connected to the proppant storage chamber (6), the high-pressure pump (5) and the liquid nitrogen tank (1) in sequence. Step 4: Start the high-pressure pump (5) to perform the first stage of pre-fracture. Set a constant flow rate through the injection control panel (3) to deliver high-pressure liquid nitrogen to the delivery pipe (9). Observe the pressure gauge. When the liquid nitrogen pressure reaches the pressure value required for the first stage of pre-fracture, open the overflow valve (10) to release the high-pressure liquid nitrogen. The liquid nitrogen is released and sprayed from the main hole and the secondary hole of the hollow tubular structure (13), which generates several new fractures in the surrounding rock. The original fractures are expanded and developed. After the first stage of pre-fracture is completed, close the high-pressure pump (5) and the overflow valve (10) in sequence. Step 5: Receive the borehole crack image information collected by the infrared monitoring sensor (16) through the crack identification and processing system (17), perform quantitative analysis on the crack image, output the geometric and statistical parameters of the current crack, and feed the information back to the high-pressure pump (5) through the data line (18). Step 6: Reset the pressure value of liquid nitrogen fracturing and carry out multi-stage pre-fracturing. During this pre-fracturing process, open the valve of the proppant material chamber. The proppant material is mixed with liquid nitrogen through the proppant delivery pipe (8) and transported into the hollow tubular structure (13). The high-pressure liquid nitrogen carries the proppant particles into the surrounding rock, causing further expansion of the fractures and gradually forming a more complex fracture network. After each injection, move the sealing piston (11) a certain length towards the rear end of the hollow tubular structure (13) for the next stage of grouting. The number of cycles in this step needs to be determined based on the actual situation of the surrounding rock. Step 7: After the multi-stage fracturing is completed, close the liquid nitrogen tank (1) replenishment port (2), high pressure pump (5), proppant storage tank (6), and overflow valve (10), and remove the pre-fracturing device from the surrounding rock grouting hole.

4. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, The hollow tubular structure (13) is made of steel structure material that is resistant to high pressure and low temperature. The number of grouting holes can be adjusted by the pre-embedded depth of the hollow tubular structure (13) and the axial movement of the sealing piston (11) inside the hollow tubular structure (13). Grouting can be carried out in sections from less to more and from shallow to deep to control the amount of grouting and the spacing between grouting holes, so as to prevent the hydraulic expansion force of the surrounding rock from exceeding the bearing limit.

5. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, The injection volume for each level of pre-fracture is calculated using the following formula: Q=πR 2 Lnαβ, where Q is the injection volume (m³). 3 R—Liquid nitrogen diffusion radius (m) 2 L—Injection length (m), n—Porosity of the surrounding rock formation, α—Formation filling coefficient, generally taken as 0.8, β—Liquid nitrogen loss coefficient, generally taken as 1.1~1.

3.

6. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, The pre-splitting pressure values ​​for each level are set by gradually increasing and then gradually decreasing, with the difference between adjacent pressure values ​​being 15% to 20%.

7. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, In each level of pre-splitting, starting from the second level, the number of pre-splitting operations needs to be determined based on the actual changes in the bearing capacity of the surrounding rock. In the multi-level pre-splitting stage until the last level of pre-splitting, proppant material needs to be mixed into liquid nitrogen to provide auxiliary support for the fractures.

8. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, The particle size of the proppant particles is smaller than that of the main hole (14) and the secondary hole (12) to prevent blockage and backflow of liquid nitrogen.

9. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, Two infrared monitoring sensors (16) are provided, located at both ends of the end surface of the hollow tubular structure (13) respectively, for real-time acquisition of crack opening image information inside the borehole.

10. The high-pressure liquid nitrogen multi-stage pre-fracture method for water-rich soft rock tunnels according to claim 3, characterized in that, The image information collected by the infrared monitoring sensor (16) is transmitted to the crack identification and processing system (17) for quantitative analysis to obtain the geometric information of crack quantity, opening, and distribution density, and to provide feedback for the injection operation to adjust the injection parameters and injection points.

Images