A method of installing a borehole stress cell

Abstract

This invention discloses a method for installing a borehole stress gauge, comprising: drilling a borehole and simultaneously pushing the borehole stress gauge, extension cable, and grouting pipe into the borehole; using flame-retardant NBR rubber-plastic foam tubing to wrap and fix the connection joint between the borehole stress gauge and the extension cable; fitting a cable protection tube to the section of the extension cable extending inward from the borehole opening, and suspending the extension cable extending outward from the borehole opening with a length allowance; pre-embedding an vent pipe at the borehole opening, arranging the vent pipe opening along the highest point of the borehole cross-section and sealing the borehole opening end face; preparing grouting slurry by mixing water, cement, early-strength agent, and expanding agent according to a predetermined ratio; filling the grouting slurry with a grouting pump, maintaining stable grouting pressure, and then sealing the vent pipe opening. This invention achieves mechanical protection for the borehole stress gauge joint and the extension cable, avoiding the formation of a hard wrapping layer, and ensuring the accuracy of stress monitoring data and the long-term stable operation of the equipment.

Description

Technical Field

[0001] This invention relates to the field of mine safety monitoring technology, specifically a method for installing a borehole stress gauge. Background Technology

[0002] Borehole stress gauges are sensors used in mine safety monitoring to measure stress changes inside coal and rock masses. Obtaining stress data inside coal and rock masses plays an important role in early warning of mine dynamic disasters such as rockbursts and roof collapses.

[0003] In conventional underground monitoring operations in mines, the installation of borehole stress gauges requires first drilling a hole in the coal seam or rock strata, then pushing the borehole stress gauge body and the connected extension cable into the borehole. After positioning, grout is injected into the borehole. The borehole stress gauge is fixed inside the coal and rock mass by the physical strength of the solidified grout, and the solidified grout is used as a medium to transmit the stress of the coal and rock mass.

[0004] However, in underground environments, traditional installation methods suffer from the problem of easy failure of borehole stress gauges due to the lack of mechanical protection. There is a sudden change in stiffness at the connection joint between the borehole stress gauge body and the extension cable. During the grouting solidification process and when the surrounding coal and rock mass is subjected to stress deformation in the later stage, the connection joint directly comes into rigid contact with the grouting solidification body or coal and rock mass, and is damaged by shear stress. In addition, the displacement deformation of the surrounding rock at the borehole opening caused by roadway excavation will directly exert tensile force on the extension cable leading out of the borehole opening, causing the extension cable to break under tension. Traditional installation processes do not set up structural buffers for the above-mentioned weak nodes and lead-out lines, making it difficult for the borehole stress gauge to maintain stable operation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for installing a borehole stress gauge, which solves the problems of sensor connection joints being prone to shearing damage, extension cables leading out of the borehole being prone to breakage, and sensor failure caused by the solidification of grout slurry forming a hard enclosure when installing the borehole stress gauge under complex geological conditions.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for installing a borehole stress gauge, comprising the following steps: Step S1, Drilling preparation and drilling stress gauge placement: Drill a hole at the designed location and clean the residual rock powder inside the hole. Simultaneously push the drilling stress gauge, the extension cable connected to the drilling stress gauge, and the grouting pipe for subsequent processes to the designed drilling depth. Adjust the spatial orientation of the drilling stress gauge so that the sensing surface of the drilling stress gauge faces the predetermined monitoring direction. Step S2, Joint Shear Protection Treatment: Identify the connection joint between the borehole stress gauge body and the extension cable, and use flame-retardant NBR rubber and plastic foam tube to tightly wrap and fix the connection joint and the sections on both sides of the connection joint to block the transmission of rigid shear stress. Step S3, Large Deformation Protection Treatment for Extension Cables at the Borehole Opening: Install cable protection pipes on the extension cable sections located within a specific length range inward from the borehole opening and seal the cable protection pipe interfaces with polyurethane foam to prevent grout from seeping into the cable protection pipes during grouting; for extension cables leading out of the borehole opening, suspend them in a loop on the wall near the borehole opening with a length allowance to absorb the tensile stress exerted on the extension cables by subsequent surrounding rock deformation; Step S4, setting up the borehole sealing and venting system: Pre-embed an venting pipe that extends into the borehole at the borehole opening, arrange the venting pipe opening along the highest point of the borehole cross-section, use polyurethane foam to seal the end face of the borehole opening, and seal the internal space of the borehole opening while fixing the venting pipe and the extension cable. The arranged venting pipe also serves as a grout discharge indicator hole for the grouting operation. Step S5, prepare modified grouting slurry: mix water, powdered cement, early strength agent and expansion agent according to the predetermined ratio and stir with water to prepare grouting slurry with elastic modulus similar to that of coal seam and with micro-expansion and anti-bleeding properties. Step S6, Controllable pressure grouting and sealing: Gradually fill the borehole with grout from the bottom out using a grouting pump, observe the venting of the vent pipe and the continuous spraying of grout, and, combined with the stable rise of the grouting pump pressure reading, stop grouting and seal the vent pipe opening after maintaining the grouting pressure for a period of time to complete the installation of the borehole stress gauge.

[0007] The second aspect of the present invention provides a preferred technical solution for the drilling stress gauge installation method of the first aspect described above: Preferably, the raw materials for the grouting slurry include water, powdered cement, early-strength agent, and expansive agent. The mass ratio of each component is as follows: the mass ratio of water to cement is 1:1, the mass of early-strength agent is 6% to 10% of the mass of water, and the mass of expansive agent is 8% to 12% of the mass of water.

[0008] Preferably, the weighed powdered cement, early strength agent and expansion agent are mixed evenly in a dry environment; the mixed dry materials are added to water and continuous stirring is started until a uniform grout is formed, so as to avoid the powdered raw materials from clumping when they come into contact with water and prevent the grouting pipeline or borehole from being blocked in subsequent grouting operations.

[0009] Preferably, the dosage of the early strength agent is adjusted according to the time required for a single grouting process to ensure that the initial setting time of the grout is greater than the time required to complete a single grouting process; when the drilling depth is greater than 30 meters, the proportion of the early strength agent is reduced within the range of 6% to 10% of the mixing ratio to delay the initial setting time of the grout and prevent the grout from solidifying prematurely during deep hole transportation.

[0010] A third aspect of the present invention provides a further preferred technical solution for the borehole stress gauge installation method described in the first aspect: Preferably, in step S2, the wall thickness of the flame-retardant NBR foam tube is 4 mm to 6 mm, and the wrapping range of the flame-retardant NBR foam tube covers a section of 8 cm to 12 cm on each side of the connecting joint; after the wrapping operation is completed, waterproof tape is used to fix the ends of the flame-retardant NBR foam tube to prevent the flame-retardant NBR foam tube from shifting.

[0011] Preferably, in step S3, the extension cable segment extending at least 3 meters from the borehole opening into the borehole is measured and determined. A cable protection pipe is installed on the outside of the located extension cable segment. The cable protection pipe is made of steel pipe. The extension cable leading out of the borehole opening has a length margin of not less than 1 meter to absorb the tensile force generated by the deformation of the surrounding rock at the borehole opening, and to avoid the displacement deformation tensile force acting directly on the extension cable body leading out of the borehole opening, which would cause the extension cable to break.

[0012] Preferably, in step S4, the exhaust pipe is made of PVC pipe, and one end of the exhaust pipe extends into the borehole to a depth of about 1 meter; the exhaust pipe is placed at the highest point of the borehole cross-section to allow the gas inside the borehole to be fully discharged, avoiding the formation of bubbles or cavities.

[0013] Preferably, in step S6, when the drilling depth is less than or equal to 30 meters, a grouting pump with a rated pressure of not less than 5 MPa is selected; when the drilling depth exceeds 30 meters, a grouting pump with a rated pressure of 7 MPa to 8 MPa is selected for grouting operations.

[0014] Preferably, in step S6, when the grouting slurry is continuously sprayed from the exhaust pipe for 1 to 2 minutes, the change in the reading of the grouting pump pressure gauge is monitored. When the reading of the grouting pump pressure gauge stabilizes at 2 MPa to 3 MPa, the current grouting pressure is maintained for a period of time to ensure the grouting density and to fully couple the grouting solidified body with the coal and rock mass and the borehole stress gauge.

[0015] Preferably, in step S6, after the pressure holding operation is completed, the grouting pump operation is stopped, and the vent pipe opening is quickly sealed to prevent the grouting slurry inside the borehole from flowing back outward under pressure.

[0016] This invention provides a method for installing a borehole stress gauge. It has the following beneficial effects: 1. This invention uses flame-retardant NBR rubber-plastic foam tubes to wrap and buffer the connection joints of the borehole stress gauge to block the transmission of rigid shear stress. A cable protection tube is installed inside the borehole opening, and an extension cable is reserved on the outside to absorb the deformation tension of the surrounding rock at the borehole opening. This achieves mechanical protection for the borehole stress gauge from the weakest node to the lead-out line, avoiding the joint shear damage and extension cable breakage caused by grouting solidification or tunnel excavation, and ensuring the stable operation of the borehole stress gauge in complex mining environments.

[0017] 2. This invention achieves a grout elastic modulus similar to that of the coal seam by mixing water, cement, early-strength agent, and expansion agent in dry form and adding water during the grout preparation process according to mass ratio. This creates a flexible coupling medium with matching mechanical properties between the coal seam and the borehole stress gauge, preventing the cement grout from forming a hard coating layer on the outside of the borehole stress gauge. This ensures that stress changes inside the coal and rock mass are transmitted to the sensing surface of the borehole stress gauge, thus improving the accuracy of the stress monitoring data from the borehole stress gauge.

[0018] 3. This invention achieves the full discharge of residual gas inside the borehole by arranging the exhaust pipe opening along the highest point of the borehole cross-section and sealing the borehole opening end face with polyurethane foaming agent. Combined with the operation of maintaining the grouting pressure for a period of time after the grouting slurry is sprayed out of the exhaust pipe in the controllable pressure grouting process, the invention avoids the formation of air bubbles or cavities. The operation of maintaining the grouting pressure ensures the grouting density and allows the grouting solidified body to be fully coupled with the coal and rock mass and the borehole stress gauge, thus ensuring the uniformity of the grouting solidified body. Attached Figure Description

[0019] Figure 1 This is a perspective view of the present invention. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example: See attached document Figure 1 , Figure 1 This is a flowchart of a drilling stress gauge installation method according to an embodiment of the present invention.

[0022] This embodiment provides a borehole stress gauge installation method, which is applied to stress monitoring operations inside coal and rock masses in complex geological environments underground mines. The hardware foundation and working space involved in the borehole stress gauge installation method provided by this embodiment include a pre-constructed borehole, a borehole stress gauge body, an extension cable connected to the borehole stress gauge body, and a grouting pipe for installation. In order to overcome the technical defects such as the easy occurrence of hard shear failure of underground joints, the easy breakage of the lead-out cable due to large deformation of the surrounding rock at the borehole opening, and the coupling failure caused by conventional grouting hard wrapping, the borehole stress gauge installation method of this invention limits multiple protection and adaptation operations in the specific installation steps.

[0023] The aforementioned joint shear-resistant flexible buffer operation is specifically manifested in wrapping the connection joint with a flame-retardant NBR rubber and plastic foam tube to prevent the joint from rigidly contacting the subsequently injected grout or the surrounding coal and rock mass. The aforementioned extension cable large deformation protection operation specifically includes wrapping a flexible cable protection tube on the outside of the extension cable inside the borehole opening, and reserving a section for the extension cable to be suspended in a loop on the roadway wall outside the borehole opening. The aforementioned airtight venting operation specifically includes arranging a PVC vent pipe with one end inserted into the borehole close to the highest point of the borehole cross-section, and using polyurethane foam to wrap and fix the extension cable and vent pipe and fill the pores of the borehole opening. The solidified body used in the aforementioned modified grouting operation is a filler formed by mixing water, cement, early strength agent and expansion agent in a specific ratio, injecting and solidifying it.

[0024] Based on the aforementioned hardware and operational foundations, the overall workflow of the borehole stress gauge installation method of the present invention includes the following steps: Step S1: Drilling preparation and positioning of the borehole stress gauge. Drill the borehole at the designed location and clean the residual rock powder inside the borehole. Simultaneously push the borehole stress gauge, the extension cable connected to the borehole stress gauge, and the grouting pipe for subsequent processes to the designed depth of the borehole. Adjust the spatial orientation of the borehole stress gauge so that the sensing surface of the borehole stress gauge faces the predetermined monitoring direction.

[0025] Step S2, joint shear protection treatment: Identify the connection joint between the borehole stress gauge body and the extension cable, and use flame-retardant NBR rubber and plastic foam tube to tightly wrap and fix the connection joint and the two sides to block the transmission of rigid shear stress.

[0026] Step S3: Large deformation protection treatment for the extension cable at the borehole opening. For the extension cable section located in a specific length range inward from the borehole opening, a cable protection pipe is installed and the interface of the cable protection pipe is sealed with polyurethane foam to prevent grout from seeping into the cable protection pipe during grouting. For the extension cable leading out of the borehole opening, it is suspended in a loop on the wall near the borehole opening with a reserved length margin to absorb the tensile stress exerted on the extension cable by the deformation of the surrounding rock later.

[0027] Step S4, setting up the borehole sealing and venting system: pre-embed an venting pipe extending into the borehole at the borehole opening, arrange the venting pipe opening along the highest point of the borehole cross-section, use polyurethane foam to seal the end face of the borehole opening, and seal the internal space of the borehole opening while fixing the venting pipe and extension cable. The arranged venting pipe also serves as a grout discharge indicator hole for grouting operations.

[0028] Step S5: Prepare modified grouting slurry by dry mixing and adding water in the mass ratio of water:cement:early strength agent:expansion agent = 1:1:(0.06-0.10):(0.08-0.12) to prepare grouting slurry with an elastic modulus similar to that of coal seam and with micro-expansion and anti-bleeding properties.

[0029] Step S6, Controlled pressure grouting and sealing: Using a grouting pump with a rated pressure of not less than 5MPa, gradually fill the borehole with grout from the bottom outwards. Observe the venting of the vent pipe and the continuous spraying of grout for 1 to 2 minutes. Combined with the stable rise of the grouting pump pressure reading to 2 to 3MPa, maintain the grouting pressure for a period of time, then stop grouting and seal the vent pipe. If the borehole depth exceeds 30 meters, use a grouting pump with a rated pressure of 7 to 8MPa for grouting operations. Complete the installation of the borehole stress gauge.

[0030] Step S1 includes the following sub-steps: Step S101: Drill holes at the designed location and clean the residual rock powder inside the holes after drilling is completed. The cleaning operation uses compressed air equipment to blow away the rock powder inside the holes, providing a space for equipment installation free of impurities.

[0031] In step S102, the borehole stress gauge, the extension cable connected to the borehole stress gauge, and the grouting pipe for subsequent processes are slowly pushed to the designed depth position inside the borehole.

[0032] Step S103: After the borehole stress gauge reaches the designed depth position, adjust the spatial attitude of the borehole stress gauge so that the sensing surface faces the predetermined monitoring direction. The above steps complete the drilling preparation and the placement of the borehole stress gauge and grouting pipe.

[0033] There is a sudden change in stiffness at the connection joint between the borehole stress gauge body and the extension cable. The connection joint is prone to shear failure during the grouting and curing process or the deformation of the surrounding rock. To prevent shear failure, shear protection treatment needs to be performed on the connection joint. Step S2 includes the following sub-steps: Step S201: Identify the connection joint position between the borehole stress gauge body and the extension cable, and select flame-retardant NBR rubber and plastic foam tube as the buffer and protective material. The wall thickness of the flame-retardant NBR rubber and plastic foam tube is 4 mm to 6 mm. In a specific embodiment, the wall thickness of the flame-retardant NBR rubber and plastic foam tube is 5 mm.

[0034] Step S202: Wrap the flame-retardant NBR foam tube around the outside of the connector. The wrapping range of the flame-retardant NBR foam tube covers a section of 8 cm to 12 cm on each side of the connector. In a specific embodiment, the wrapping range covers a section of 10 cm on each side of the connector. After the wrapping operation is completed, use waterproof tape to fix the ends of the flame-retardant NBR foam tube.

[0035] The aforementioned joint shear protection is achieved by flame-retardant NBR rubber and plastic foam tubes arranged outside the joint. The flame-retardant NBR rubber and plastic foam tubes form a buffer layer outside the joint. When the grout injected into the borehole solidifies, or when the coal and rock mass around the borehole deforms, the flame-retardant NBR rubber and plastic foam tubes absorb the stress acting on the joint. The flame-retardant NBR rubber and plastic foam tubes prevent the joint from making rigid contact with the hardened grout or coal and rock mass, preventing the joint from breaking due to shear stress. Waterproof tape is used to fix both ends of the protective layer to prevent the wrapping from shifting.

[0036] Step S3 includes the following sub-steps: Step S301: Measure and determine the extension cable segment located at least 3 meters from the borehole opening into the borehole, and install a cable protection pipe on the outside of the located extension cable segment. In one specific embodiment, the cable protection pipe adopts a steel pipe structure.

[0037] Step S302: At each interface of the cable protection pipe, polyurethane foam is used for filling and sealing operations. The sealing operation prevents the grout from leaking into the inside of the cable protection pipe under the pump pressure during subsequent pressure grouting operations.

[0038] Step S303: For the extension cable leading out of the borehole opening, the extension cable leading out of the borehole opening is suspended by looping around the wall near the borehole opening, and a length margin of not less than 1 meter is reserved for the extension cable leading out of the borehole opening. In a specific embodiment, the extension cable leading out of the borehole opening is wrapped twice around the anchor rod near the borehole opening and led to the monitoring substation, with a reserved length margin of 2 meters.

[0039] The protection treatment for large deformation of cables at the borehole opening uses cable protection pipes to resist the squeezing effect of deformation of the surrounding rock at the borehole opening on the cable. The looping suspension section of the extended cable leading out of the borehole opening and the reserved length allowance form a relaxation buffer structure. When the surrounding rock around the borehole opening is displaced or deformed due to tunnel excavation or stress redistribution, the displacement of the surrounding rock first consumes the reserved length allowance, absorbing the tensile force generated by the deformation of the surrounding rock at the borehole opening, and avoiding the displacement deformation tensile force from directly acting on the extended cable body leading out of the borehole opening and causing it to break.

[0040] Step S4 includes the following sub-steps: Step S401: An exhaust pipe is pre-embedded at the borehole opening. The exhaust pipe is made of PVC pipe. In a specific embodiment, the exhaust pipe is a PVC pipe with an outer diameter of 10 mm and a total length of 1.2 m.

[0041] Step S402: Insert one end of the exhaust pipe into the borehole to a depth of about 1 meter, and adjust the spatial layout of the exhaust pipe to ensure that the pipe opening is arranged along the highest point of the borehole cross-section.

[0042] In grouting operations, the grouting slurry fills the borehole from the bottom upwards. The residual gas inside the borehole is squeezed upwards by the grouting slurry and gathers at the highest point of the borehole cross-section. The exhaust pipe is arranged at the highest point of the borehole cross-section to allow the gas inside the borehole to be fully discharged, avoiding the formation of air bubbles or cavities, and ensuring that the grouting slurry is tightly coupled with the surrounding rock and the borehole stress gauge.

[0043] Step S403: Use polyurethane foam to seal the end face of the drill hole. In one specific embodiment, a fast polyurethane foam is used for sealing. After the polyurethane foam cures, it seals the drill hole opening and fixes the exhaust pipe and the cable leading out of the drill hole opening at the drill hole opening.

[0044] In step S404, the vent pipe located at the highest point of the borehole is used as a grout discharge indicator hole for subsequent grouting operations after the gas inside the borehole is vented.

[0045] Step S5 includes the following sub-steps: Step S501: Weigh the raw materials required for preparing the slurry according to the set mass ratio. The slurry raw materials include water, powdered cement, early strength agent, and expansion agent. The proportions of each raw material are defined by the following mass ratio formula:

[0046] in, Represents the quality of water. Represents the quality of cement. This represents the quality of the early strength agent. This represents the quality of the expanding agent. The mass coefficient representing the early-strength agent ranges from 0.06 to 0.10. The mass coefficient of the expansive agent ranges from 0.08 to 0.12. In one specific embodiment, water is used as the solvent, P.O42.5 cement is used as the base material, calcium formate is used as the early strength agent, and UEA type expansive agent is used as the expansive agent. The mass ratio for preparation is water:P.O42.5 cement:calcium formate early strength agent:UEA type expansive agent = 1:1:0.08:0.1. The dosage of the expansive agent is controlled at 10% of the cement mass. The expansive agent is added to reduce the bleeding phenomenon of the grout, stabilize the borehole wall, and compensate for the volume shrinkage of the grout during the curing process.

[0047] Step S502: Dry mixing operation is performed on the powdered raw material components. The weighed powdered cement, early strength agent and expansion agent are placed in a dry environment and mixed evenly. In a specific embodiment, the cement, calcium formate early strength agent and UEA type expansion agent are dry mixed for 3 minutes.

[0048] Step S503: Add water and stir the mixed dry materials. Add the dry materials to the pre-weighed water and start continuous stirring until a uniform slurry is formed. In one specific embodiment, pour the dry mixture into the water and stir continuously for 5 minutes. The stirring operation is stopped when the slurry reaches the set consistency, which is manifested as the slurry forming a thin layer on the surface of the operator's gloves. The order of adding water and stirring after mixing the dry materials avoids the clumping of powdered raw materials when they come into contact with water, prevents the grouting pipeline or drill hole from being blocked in the subsequent grouting operation, and ensures the uniformity of the grouting slurry.

[0049] Step S504: Adjust the dosage of early strength agent according to the actual time required for single-hole grouting operation. The adjustment benchmark for the dosage of early strength agent is to ensure that the initial setting time of the grout is greater than the time required to complete a single grouting process. When the drilling depth is greater than 30 meters, the grouting stroke and the time consumed for single-hole grouting increase. Reduce the proportion of early strength agent within the ratio coefficient range of 0.06 to 0.10 to delay the initial setting time of the grout and prevent the grout from solidifying prematurely during deep hole transportation.

[0050] After the modified grout prepared in the aforementioned steps fills and solidifies inside the borehole, it forms a solidified grout body. After solidification, the grout prepared according to the above ratio has an elastic modulus similar to that of the coal seam, which can form a flexible coupling medium between the coal seam and the borehole stress gauge. This avoids the formation of a hard coating layer by the cement grout, ensuring that stress changes inside the coal and rock mass are transmitted to the sensing surface of the borehole stress gauge, thus improving the accuracy of stress monitoring data.

[0051] Step S6 includes the following sub-steps: Step S601: Connect the grouting pump and the grouting pipeline. When the drilling depth is less than or equal to 30 meters, a grouting pump with a rated pressure of not less than 5 MPa shall be selected. When the drilling depth exceeds 30 meters, a grouting pump with a rated pressure of 7 MPa to 8 MPa shall be selected for grouting operations.

[0052] Step S602: Start the grouting pump to perform grouting. The grout is transported to the bottom of the borehole through the grouting pipeline and gradually fills the borehole from the bottom towards the borehole opening.

[0053] Step S603: During the filling process of the grouting slurry, observe the status of the vent pipe pre-embedded at the borehole opening. After the grouting slurry fills the internal space of the borehole, it sprays out from the vent pipe. When the vent pipe continues to spray grouting slurry for 1 to 2 minutes, proceed to the next step. The operation of continuously spraying grouting slurry from the vent pipe removes residual air bubbles in the borehole through the flow of slurry, ensuring that the grouting slurry fills the internal space of the borehole. In a specific embodiment, the vent pipe starts to continuously spray grouting slurry after about 15 minutes of grouting operation, and the spraying time is maintained for 90 seconds.

[0054] In step S604, based on the jetting status of the exhaust pipe, monitor the change in the reading of the grouting pump pressure gauge. When the reading of the grouting pump pressure gauge stabilizes at 2MPa to 3MPa, maintain the current grouting pressure for a period of time. In a specific embodiment, when the reading of the grouting pump pressure gauge rises to 2.5MPa, maintain the grouting pressure at 2.5MPa for 3 minutes. Maintaining the grouting pressure can ensure the compactness of the grouting and enable the grout solidified body to fully couple with the coal and rock mass and the borehole stress gauge.

[0055] Step S605: After the pressure holding operation is completed, stop the grouting pump operation and quickly seal the vent pipe opening to prevent the grouting slurry inside the borehole from flowing back outward under pressure. This completes the installation of the borehole stress gauge. In one specific embodiment, the vent pipe opening is sealed immediately after the pump is stopped.

[0056] Specific application example: In the area of ​​coal mine rock burst hazard, borehole stress monitoring is carried out. A borehole with a diameter of 42 mm and a depth of 25 m is constructed. The rock powder inside the borehole is cleaned by compressed air equipment, and the borehole stress gauge and grouting pipe are pushed to the predetermined position inside the borehole.

[0057] At the connection joint between the borehole stress gauge and the extension cable, a flame-retardant NBR rubber foam tube with a wall thickness of 5 mm is used to wrap the connection joint, with a wrapping length of 10 cm. Waterproof tape is used to wrap and fix both ends of the flame-retardant NBR rubber foam tube to form a buffer layer.

[0058] A steel pipe is inserted into the cable section extending 3 meters from the borehole opening into the borehole. Polyurethane foam is used to seal the joints of the steel pipe. The cable leading out of the borehole opening is wrapped twice around the anchor rod near the borehole opening and led to the monitoring substation, with a 2-meter length allowance reserved for the cable.

[0059] A PVC pipe with a total length of 1.2 meters and an outer diameter of 10 millimeters is inserted into the upper part of the borehole as an exhaust pipe. The exhaust pipe extends 1 meter into the borehole. The end face of the borehole is sealed with a fast polyurethane foaming agent to fix the exhaust pipe and the extension cable bundle.

[0060] Weigh water, P.O42.5 cement, calcium formate early strength agent, and UEA type expansion agent according to a mass ratio of 1:1:0.08:0.1. Mix the weighed cement, calcium formate early strength agent, and UEA type expansion agent in a dry environment for 3 minutes. Pour the mixed dry material into water and stir continuously for 5 minutes to prepare the grouting slurry.

[0061] Connect a grouting pump with a rated pressure of 5MPa for grouting operations. After 15 minutes of grouting, start continuously spraying grout from the exhaust pipe. Maintain the spraying of grout for 90 seconds, and the pressure reading on the grouting pump will rise to 2.5MPa. Maintain the grouting pressure at 2.5MPa for 3 minutes, then stop the grouting pump, seal the exhaust pipe opening, and complete the installation of the borehole stress gauge.

[0062] After six months of monitoring, the borehole stress gauge signal transmission remained stable, with no sensor failures caused by joint shearing damage, broken extension cables, or slurry hardening. Multiple mining-induced stress fluctuation data were recorded. For the specific operation of the compressed air equipment for cleaning rock powder, the curing conditions of the polyurethane foaming agent, and the connection method of the foundation pipeline of the grouting pump, those skilled in the art can refer to the existing conventional construction specifications, which are well-known technologies in this field, and will not be elaborated here.

[0063] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for installing a borehole stress gauge, characterized in that, Includes the following steps: Step S1, Drilling preparation and drilling stress gauge placement: Drill a hole at the designed location, push the drilling stress gauge, the extension cable connected to the drilling stress gauge, and the grouting pipe to the designed drilling depth, and adjust the spatial attitude of the drilling stress gauge so that the sensing surface of the drilling stress gauge faces the predetermined monitoring direction. Step S2, Joint shear protection treatment: Tightly wrap and fix the connection joint between the borehole stress gauge body and the extension cable, as well as the sections on both sides of the connection joint; Step S3, Large Deformation Protection Treatment for Extension Cables at the Drill Hole Opening: Install cable protection tubes on the extension cable sections located within a specific length range inward from the drill hole opening; for extension cables leading out of the drill hole opening, reserve a length allowance. Step S4, setting up the borehole opening sealing and venting system: pre-embed an venting pipe extending into the borehole at the borehole opening, arrange the venting pipe opening along the highest point of the borehole cross-section, and seal the end face of the borehole opening. Step S5, preparing modified grouting slurry: Mix and stir water, cement, early strength agent and expansion agent according to the predetermined ratio to prepare grouting slurry; Step S6, Controllable pressure grouting and sealing: Use a grouting pump to fill the grouting slurry, maintain the grouting pressure for a period of time, stop grouting, and seal the vent pipe opening to complete the installation of the borehole stress gauge.

2. The drilling stress gauge installation method according to claim 1, characterized in that, In step S2, flame-retardant NBR foam tubing is used to tightly wrap and fix the connector and the sections on both sides of the connector to block the transmission of rigid shear stress. The wall thickness of the flame-retardant NBR foam tubing is 4 mm to 6 mm, and the wrapping range of the flame-retardant NBR foam tubing covers the sections on both sides of the connector, which are 8 cm to 12 cm apart. After the wrapping operation is completed, waterproof tape is used to fix the ends of the flame-retardant NBR foam tubing to prevent the flame-retardant NBR foam tubing from shifting.

3. The drilling stress gauge installation method according to claim 1, characterized in that, In step S3, the extension cable segment extending at least 3 meters into the borehole from the borehole opening is measured and determined. A cable protection pipe is installed on the outside of the located extension cable segment. The cable protection pipe is made of steel pipe and the interface of the cable protection pipe is sealed with polyurethane foam. The extension cable leading out of the borehole opening is suspended in a loop on the wall near the borehole opening with a length margin of not less than 1 meter to absorb the tensile force generated by the deformation of the surrounding rock at the borehole opening, and to avoid the displacement deformation tensile force acting directly on the extension cable body leading out of the borehole opening, which would cause the extension cable to break.

4. The drilling stress gauge installation method according to claim 1, characterized in that, In step S4, the exhaust pipe is made of PVC pipe, and one end of the exhaust pipe extends into the borehole to a depth of about 1 meter. The exhaust pipe is placed at the highest point of the borehole cross-section to allow the gas inside the borehole to be fully discharged, avoiding the formation of bubbles or cavities.

5. The drilling stress gauge installation method according to claim 1, characterized in that, In step S5, the grouting slurry raw materials include water, powdered cement, early strength agent and expansion agent. The mass ratio of each component raw material is as follows: the mass ratio of water to cement is 1:1, and the mass of early strength agent and expansion agent is 6% to 10% and 8% to 12% of the cement mass, respectively.

6. The drilling stress gauge installation method according to claim 5, characterized in that, In step S5, the weighed powdered cement, early strength agent and expansion agent are mixed evenly in a dry environment; the mixed dry material is added to water and continuous stirring is started until a uniform grout is formed, so as to avoid the powdered raw materials from clumping when they come into contact with water, and to prevent the grouting pipeline or borehole from being blocked in subsequent grouting operations.

7. The drilling stress gauge installation method according to claim 5, characterized in that, In step S5, the dosage of the accelerator is adjusted according to the time required for a single grouting process to ensure that the initial setting time of the grout is greater than the time required to complete a single grouting process. When the drilling depth is greater than 30 meters, the proportion of the accelerator is reduced within the range of 6% to 10% of the cement mass to delay the initial setting time of the grout and prevent the grout from solidifying prematurely during deep hole transportation.

8. The drilling stress gauge installation method according to claim 1, characterized in that, In step S6, when the drilling depth is less than or equal to 30 meters, a grouting pump with a rated pressure of not less than 5 MPa is selected; when the drilling depth exceeds 30 meters, a grouting pump with a rated pressure of 7 MPa to 8 MPa is selected for grouting operations.

9. A drilling stress gauge installation method according to claim 1, characterized in that, In step S6, while the grouting slurry is continuously sprayed from the exhaust pipe for 1 to 2 minutes, the change in the reading of the grouting pump pressure gauge is monitored. When the reading of the grouting pump pressure gauge stabilizes at 2 MPa to 3 MPa, the current grouting pressure is maintained for a period of time to ensure the grouting density and to fully couple the grouting solidified body with the coal and rock mass and the borehole stress gauge.

10. A drilling stress gauge installation method according to claim 9, characterized in that, In step S6, after the pressure holding operation is completed, the grouting pump operation is stopped, and the vent pipe opening is quickly sealed to prevent the grouting slurry inside the borehole from flowing back out under pressure.

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