A method for measuring three-dimensional ground stress in underground mines

Abstract

The application discloses a kind of three-dimensional ground stress measurement methods of underground mine, comprising the following steps: S1) determining the boundary of original rock stress area, S2) roof original rock drilling core, S3) determining the direction of horizontal principal stress, S4) drilling monitoring borehole, S5) installing ground stress measurement device, S6) ground stress data monitoring and calculation, the application determines the direction of maximum horizontal principal stress and minimum horizontal principal stress by the resistivity difference of different diameter directions on the circumference of unloading vertical rock core, and the accurate determination of the stress size of monitoring point in original rock mass in three directions is realized by the directional monitoring of three-dimensional ground stress measurement device.The method is simple in equipment, easy to operate, low in measurement cost, and has strong applicability.

Description

Technical Field

[0001] This invention relates to the field of geostress testing technology, and in particular to a method for measuring three-dimensional geostress in underground mines. Background Technology

[0002] In-situ stress refers to the natural stress existing in strata without engineering disturbance, including the self-weight stress caused by gravity (the weight of the overlying rock strata) and the tectonic stress caused by tectonic movement. In-situ stress is one of the important force sources causing rock mass deformation and failure, and is an important reference for the design and construction of geotechnical engineering projects such as mine shafts and tunnels. The fundamental reason why rock masses deform, fail, impact, and become unstable is that underground mining or excavation activities disrupt the stress equilibrium state of the original rock, causing high stress concentration or transient unloading in local areas, which in turn induces catastrophic changes in the rock. Therefore, accurately understanding the magnitude and distribution of in-situ stress in the original rock of a mine is crucial. However, the causes of in-situ stress are very complex, and to date, a complete theoretical calculation of in-situ stress is not possible; the stress state of the original rock mass can only be obtained through actual measurements. Practice shows that among the three-dimensional principal stresses underground, one principal stress is close to the vertical direction, namely the vertical stress caused by gravity, whose magnitude is basically equal to the weight of the overlying rock strata. Theoretically, it can be approximated by summing the products of the thickness and unit weight of each overlying rock stratum. Furthermore, the study found that tectonic stress caused by tectonic movements plays a decisive role in the magnitude of geostress, while the horizontal stress component of the rock mass is mainly controlled by tectonic stress, and its magnitude is much larger than that of vertical stress, especially in shallow rock masses. Generally speaking, tectonic movements often give rise to structural types such as folds, faults, and joints. Based on the structural features, the approximate direction of the horizontal principal stress can be determined, but its magnitude cannot be determined. In fact, before designing and constructing mining engineering projects, it is essential to understand the distribution characteristics of vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress in the rock mass, thereby providing important scientific basis for engineering layout, support decisions, and construction safety.

[0003] According to the recommendations of the International Society for Rock Mechanics and Rock Engineering, stress testing is recommended to use the stress relief method and the hydraulic fracturing method. The stress relief method first involves drilling a hole deep into the rock mass, attaching sufficient strain gauges to the hole wall, and zeroing the initial strain of these gauges. Then, a core drill bit is used to completely relieve the stress in the core, allowing the core deformation to elastically recover. The elastic recovery strain of the core tube is measured, and the rock mass stress is calculated based on these elastic recovery strains and the rock's elastic constant (measured in a confining pressure calibrating experiment). The process is: "drilling a large hole—changing the drill bit and drilling a smaller hole and cleaning—installing a triaxial strain gauge probe—drilling a large hole and releasing the core—relieving the strain." Clearly, this method has high requirements for rock mass properties and operational techniques, and is labor-intensive and costly. Hydraulic fracturing assumes that one of the principal stresses is vertical stress. During measurement, a deep borehole is drilled into the rock mass, and the upper and lower ends are sealed with packers. High-pressure fluid (water or oil) is then injected, pressurized until the borehole wall ruptures, and the pressure change over time is recorded. The fracture location is then observed using an impression device or downhole television. Based on the recorded fracture pressure, pump shut-off pressure, and fracture location, the magnitude and direction of the horizontal stress are calculated using appropriate formulas. A drawback of this method is that crack propagation during hydraulic fracturing is significantly constrained by the primary fractures within the rock mass, making it difficult to accurately determine the timing of hydraulic fractures caused by sudden changes in fluid pressure during fracturing. This method is only suitable for hard rock mines with relatively good rock mass integrity. In addition, some scholars have proposed indoor acoustic emission methods to measure in-situ stress; however, this method can only measure the maximum axial stress experienced by the core but cannot accurately reflect the actual magnitude and direction of the three-dimensional principal stresses. Given the current lack of geostress data in most metal and non-metal mines, and the reliance on experience in construction, leading to an increasing number of rock-related disasters, obtaining accurate and convenient geostress data in mines at low cost has become a top priority for mines. Therefore, it is imperative to develop a highly applicable, accurate, convenient, and economical three-dimensional geostress measurement method for underground mines. Summary of the Invention

[0004] This invention provides a method for measuring three-dimensional geostress in underground mines to solve the problems mentioned in the background art.

[0005] This invention provides a method for measuring three-dimensional geostress in underground mines. The method involves drilling a vertical core borehole in the surrounding rock of the tunnel roof to the original rock stress zone and taking at least three sets of original rock cores for resistivity testing. The direction of the maximum horizontal principal stress is determined based on the principle of resistivity extrema. Then, within a 10m range of the monitoring area, 3-5 monitoring boreholes are drilled along the direction of the maximum or minimum horizontal principal stress on both sides of the tunnel to the original rock stress zone. Geostress measuring devices are directionally installed in each monitoring borehole and grouting is performed to seal the boreholes. After the grout solidifies, the average value of the vertical stress, maximum and minimum horizontal principal stress, and stable stress at the measuring points in each monitoring borehole is measured and calculated. This average value represents the three-dimensional geostress magnitude of the monitoring area.

[0006] The solution of the present invention is: A method for measuring three-dimensional geostress in underground mines includes the following steps: S1) Determine the boundary of the original rock stress zone. Use a rock drilling rig to drill exploratory boreholes in the surrounding rock at the corner of the roadway. Use an acoustic wave tester to determine the range of the plastic zone, elastic zone and original rock stress zone of the surrounding rock of the roadway according to the difference in acoustic wave velocity of the rock mass under different fracture development and stress conditions, and determine the boundary of the original rock stress zone of the roadway. S2) Core drilling of the original rock of the roof: Vertical core drilling is carried out on the original rock mass of the roof of the roadway using directional core drilling tools. The core drilling depth is outside the boundary of the original rock stress zone. At least 3 sets of original rock cores with a length ≥10 cm and a diameter ≥50 mm are obtained, and the initial orientation of each set of original rock cores is marked. S3) Determine the direction of the horizontal principal stress. The original rock core is processed into three sets of identical rock core samples with a height of 50-100 mm and dried. Then, the samples are placed on a rotary table and the resistivity is measured between the two intersection points of the rock core diameter and the circumference using a resistivity measuring instrument. Based on the phenomenon that the number of cracks generated inside the rock core sample under different unloading stresses is different, the measurement directions of the maximum and minimum resistivity on the circumference of the rock core are obtained, which are the directions of the minimum and maximum horizontal principal stresses. S4) Drill monitoring boreholes. Using a rock drilling rig, drill 3 to 5 horizontal monitoring boreholes in the roadway within a 10 m range of the monitoring area, in the direction of the original rock stress zone on both sides, which are parallel to the direction of the maximum or minimum horizontal principal stress. After the monitoring boreholes are completed, use pneumatic tools to clean the holes in a timely manner. S5) Install the geostress measuring device. Assemble the components of the geostress measuring device on site and orient the geostress measuring device in each monitoring borehole according to the vertical stress, the maximum horizontal principal stress, and the minimum horizontal principal stress. After installation, promptly perform grouting and sealing and record the initial data. S6) Geostress data monitoring and calculation: After the slurry in the monitoring borehole solidifies, the vertical stress, maximum horizontal principal stress and minimum horizontal principal stress data of each monitoring borehole are collected periodically; after the stress data of all monitoring points tend to stabilize, the average vertical stress, maximum horizontal principal stress and minimum horizontal principal stress of each monitoring point are calculated, which is the three-dimensional geostress of the monitoring area.

[0007] As a preferred technical solution, the detection borehole needs to penetrate into the stress zone of the original rock in the roadway, with a depth of 20-40m and a diameter of 40-90mm.

[0008] As a preferred technical solution, the monitoring boreholes are arranged in the sidewalls of the roadway along the direction of the maximum or minimum horizontal principal stress, with the borehole depth being 3 to 5 m outside the boundary of the original rock stress zone of the roadway and the borehole diameter being 40 to 90 mm.

[0009] As a preferred technical solution, the geostress measurement device includes a vibrating wire three-dimensional geostress sensor, a metal casing, cables, an aviation connector, and a data acquisition unit. The vibrating wire three-dimensional geostress sensor can monitor the vibration frequency of the rock mass at the bottom of the borehole in the X, Y, and Z directions. The stress is converted according to Equation 1 below, with a maximum converted stress range of 50 MPa in each direction and a measurement accuracy of 0.01 MPa. The metal casing is used to directionally install the three-dimensional geostress sensor, arranged along the entire length of the borehole, with a diameter of 30-40 mm, each section being 1.5 m long, and interconnected by threads. Three cables are arranged together in the metal casing, one end connected to the sensor, and the other end connected to the data acquisition unit via the aviation connector, for transmitting the vibration frequency data. Equation 1:

[0010] In the formula, σ n for X , Y or Z Stress in the direction, A and B This is the sensor constant in that direction, i.e., the factory setting. f and f 0 represents the real-time frequency of the sensor in that direction and the initial installation frequency, respectively.

[0011] As a preferred technical solution, the grouting method after the installation of the geostress measuring device in the monitoring borehole is either cement grouting or chemical grouting, with a grouting pressure ≤2 MPa, to ensure that the surrounding rock of the borehole does not rupture and that the geostress monitoring device is fully coupled with the surrounding rock.

[0012] As a preferred technical solution, the rotary table includes a base, a lifting column, a control knob, a connecting plate, a bearing, and a cone. The cone is embedded in the bearing as an integral component. One component is welded to the top of the base, and the other component is welded to one end of the connecting plate. The two components are arranged symmetrically. The other end of the connecting plate is welded to the top of the lifting column. The bottom of the lifting column is connected to the base. The lifting column is equipped with a control knob. By adjusting the height of the lifting column with the control knob, the distance between the two opposing cones can be adjusted to accommodate core samples of different heights. Placing the core sample between the two opposing cones allows for free rotation of the core sample along the circumference, providing a prerequisite for the resistivity meter to measure the resistivity of the core sample in different diameter directions. The rotation angle can be read from the angle scale on the bearing.

[0013] A three-dimensional geostress measurement method for underground mines, employing the aforementioned technical solution, includes the following steps: S1) Determining the boundary of the original rock stress zone: Using a rock drilling rig, probe boreholes are drilled in the surrounding rock at the corner of the tunnel. An acoustic wave meter is used to determine the range of the plastic zone, elastic zone, and original rock stress zone of the surrounding rock under different fracture development degrees and stress states, thus determining the boundary of the original rock stress zone; S2) Core drilling of the roof: Using a directional core drilling tool, vertical core drilling is performed on the original rock mass of the tunnel roof. The core drilling depth is outside the boundary of the original rock stress zone, obtaining at least three sets of original rock cores with a length ≥10 cm and a diameter ≥50 mm, and marking the initial orientation of each set of original rock cores; S3) Determining the direction of the horizontal principal stress: The original rock cores are processed into three identical sets with a height of 50–100 mm. Core samples of mm were dried and then placed on a rotary table. A resistivity meter was used to measure the resistivity between the two points where the core diameter and circumference intersect. Based on the phenomenon that the number of fractures inside the core sample varies under different unloading stresses, the directions of maximum and minimum resistivity on the core circumference were obtained, which are the directions of minimum and maximum horizontal principal stresses. S4) Drilling monitoring boreholes were drilled, and a rock drilling rig was used in the monitoring area of ​​10 mm. Within a m-range, drill 3-5 horizontal monitoring boreholes parallel to either the direction of the maximum or minimum horizontal principal stress in the original rock stress zone on both sides of the tunnel. After the monitoring boreholes are completed, use pneumatic tools to clean them promptly. S5) Install the geostress measuring device. Assemble the components of the geostress measuring device on site and orient the device in each monitoring borehole according to the vertical stress, the direction of the maximum horizontal principal stress, and the direction of the minimum horizontal principal stress. After installation, promptly grout and seal the boreholes and record the initial data. S6) Monitor and calculate geostress data. After the grout in the monitoring boreholes solidifies, periodically collect the vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress data in each monitoring borehole. After the stress data at all monitoring points tend to stabilize, calculate the average vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress at each monitoring point, which is the three-dimensional geostress of the monitoring area.

[0014] Advantages of this invention: (1) The equipment is simple and easy to operate: This method determines the direction of the maximum and minimum horizontal principal stresses through indoor original rock core resistivity measurement experiments. Then, it only requires drilling holes in the original rock stress zone of the roadway using existing drilling equipment in the mine and burying a three-dimensional geostress sensor to obtain the three stress components of the monitoring point in real time. The required measurement equipment and procedures are simple, the operation technology requirements are low, and it does not affect the normal production of the mine. It has strong applicability in various types of mines.

[0015] (2) Accurate measurement and low cost: This geostress testing method requires minimal equipment and consumes only inexpensive three-dimensional stress sensors, resulting in a low overall cost for geostress measurement. Furthermore, the three-dimensional geostress sensor has an accuracy of 1.0%FS and a measurement precision of 0.01MPa, providing relatively accurate results. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view (AA) of the engineering layout and equipment structure of the present invention; Figure 2 This is a BB cross-sectional view of the engineering layout and equipment structure of the present invention; Figure 3 A schematic diagram of the installation of the ground stress measuring device; Figure 4 This is a schematic diagram of the resistivity test of the original rock core; In the diagram: 1-tunnel, 2-plastic zone, 3-elastic zone, 4-original rock stress zone, 5-exploratory borehole, 6-coring borehole, 7-original rock core, 8-monitoring borehole, 9-geostress measuring device, 901-three-dimensional geostress sensor, 902-metal casing, 903-cable, 904-aviation plug, 905-data acquisition unit, 10-slurry, 11-rotary table, 1101-base, 1102-lifting column, 1103-control knob, 1104-connecting plate, 1105-bearing, 1106-cone, 12-resistivity meter. Detailed Implementation

[0017] This invention provides a method for measuring three-dimensional geostress in underground mines.

[0018] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0019] Example like Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown; after the excavation of tunnel 1, based on the stress and deformation characteristics of the surrounding rock of tunnel 1, the surrounding rock mass can be divided into plastic zone 2, elastic zone 3 and original rock stress zone 4 in the radial direction.

[0020] To determine the three-dimensional geostress in the underground mine, the monitoring point was placed in the original rock stress zone 4. Therefore, a detection borehole 5 with a depth of 20-40 m and a diameter of 40-90 mm was drilled at the corner of the tunnel 1 (corresponding to the farthest boundary of the original rock stress zone) towards the surrounding rock mass. The longitudinal wave velocity of the rock mass in each section of the borehole was continuously measured from the bottom of the borehole to the opening using an acoustic wave tester.

[0021] Since the wave velocity of the rock mass is related to the degree of fissure development and the stress environment (the more developed the fissure, the lower the wave velocity; the greater the stress, the higher the wave velocity), it is obvious that the wave velocity of the rock mass in the plastic zone 2, elastic zone 3, and original rock stress zone 4 of the surrounding rock of tunnel 1 exhibits a "low-high-low" characteristic, which can be used to determine the boundary of the original rock stress zone 4. The wave velocity of the rock mass of the same lithology around the tunnel is related to the integrity of the rock mass and the magnitude of the stress it receives (the more developed the fissure, the lower the wave velocity; the greater the stress, the higher the wave velocity). The surrounding rock of the tunnel can be divided from near to far into the plastic zone (new joints and fissures caused by excavation are relatively developed and are in the unloading stress zone), the elastic zone (excavation did not cause new joints and fissures to be generated, but the secondary stress received is greater than the original rock stress), and the original rock stress zone (not affected by excavation disturbance).

[0022] Then, in tunnel 1, a vertical core drilling hole 6 is drilled into the original rock stress zone 4 using a directional coring machine, and the original rock at the top is cored. The number of original rock cores 7 is no less than 3 sets, with each set of cores 7 having a length of no less than 10 cm and a diameter of no less than 50 mm. Simultaneously, the initial orientation of each set of original rock cores 7 is recorded. The original rock cores 7 are transported indoors and processed into core samples of the same height between 50 and 100 mm. After drying (oven temperature 108℃, time 24 h), they are placed on a rotary table 11, and the resistivity between two points on the circumference in different diameter directions is measured using a resistivity meter 12. In fact, drilling the original rock core 7 under original rock stress is a stress relief process. According to rock mechanics theory, under unloading, tiny tensile cracks perpendicular to the direction of maximum stress will appear inside the core sample, and the number of cracks is related to the unloading amplitude. The rock resistivity is inversely proportional to the number of cracks. Obviously, the directions of the maximum and minimum principal stresses are the diameter directions of the core samples corresponding to the minimum and maximum resistivity. Based on the directions of the maximum or minimum horizontal principal stresses, 3 to 5 horizontal monitoring boreholes 8 with diameters of 40 to 90 mm are drilled in the rock mass 3 to 5 m away from the boundary of the original rock stress zone 4 on both sides of the tunnel 1, and the boreholes are cleaned by compressed air blowing. Then, the vibrating wire three-dimensional geostress sensor 901, metal sleeve 902, cable 903, aviation plug 904, and data acquisition device 905 are connected and assembled to form a geostress measuring device 9; subsequently, it is oriented and installed in the horizontal monitoring boreholes 8 and grouted to seal them, fully coupling them with the surrounding rock of the borehole. The three-dimensional geostress sensor 901 is used to monitor the vertical stress and the magnitude of the maximum and minimum horizontal principal stresses in three directions at the monitoring point at the bottom of the borehole. The range is 50 MPa and the resolution is 0.01 MPa. During the installation process, a multi-section metal sleeve 902 (with a diameter of 30-40 mm and extended by multiple sections, each 1.5 m long) is directionally sent into the bottom of the monitoring borehole 8. The cable 903 passes through the metal sleeve 902 and can transmit the rock vibration frequency monitored by the three-dimensional geostress sensor 901 to the data acquisition unit 905 outside the borehole via the aviation plug 904. After the installation is completed, grout 10 is injected in time for sealing. According to the initial frequency and final stable frequency of the three-dimensional geostress sensor 901 after grouting (cement or chemical grouting, grouting pressure less than 2 MPa), the three-dimensional geostress at the monitoring point can be obtained according to formula (1). The magnitude of the three-dimensional geostress in the monitoring area can be obtained by calculating the weighted average of multiple monitoring points near the monitoring area.

[0023]

[0024] The resistivity testing of the rock core samples mainly utilizes a self-made rotary table 11 and a resistivity tester 12. The rotary table 11 includes a base 1101, a lifting column 1102, a control knob 1103, a connecting plate 1104, a bearing 1105, and cones 1106. The cones 1106 are embedded in the bearings 1105 and are assembled as a single unit, appearing in pairs and able to rotate freely around the circumference. One pair is welded to the base 1101, and the other pair is welded to the end of the connecting plate 1104 and symmetrically distributed therewith. The other end of the connecting plate 1104 is connected to the top of the lifting column 1102, and the bottom of the lifting column 1102 is welded to the base 1101. A control knob 1103 is also installed on the lifting column 1102 to control the height of the lifting column 1102, thereby adjusting the distance between the two cones 1106 to accommodate rock core samples of different heights.

[0025] Additionally, bearing 1105 is engraved with angle scale lines, which can record the rotation angle of the core sample with initial orientation markings during resistivity measurement. The resistivity meter 12 can measure the resistivity at two points on the circumference of any diameter direction by aligning the probe with the core sample and rotating it in a circular motion.

[0026] The steps of the present invention are illustrated below with an experimental example: A phosphate mine has a designed production capacity of 2 million tons per year. The bedrock lithology of the mine is mainly dolomite, sandstone, shale, and mudstone, while the ore layer lithology is phosphorite. The tunnel engineering is mainly located in the upper (∈1z+∈1y) carbonate karst fissure aquifer of the Lower Cambrian Zhongyicun Formation and Yuhucun Formation. The tunnel is buried at a depth of 240m, with a net width of 4.5m, a wall height of 2.0m, and an arch height of 1.5m. The surrounding rock lithology is mainly dolomite and phosphorite. During tunnel construction, roof collapses frequently occurred, necessitating an understanding of the mine's geostress to provide a scientific basis for tunnel construction and mining design. The steps of the method of this invention for three-dimensional geostress measurement in the mine are as follows: (S1) Determine the boundary of the original rock stress zone: Using a DL-4 type rock drilling rig, drill a 30 m deep and 60 mm diameter exploratory borehole 5 in the 45° corner of the surrounding rock of transport roadway 1 (azimuth NE90°). Use a CT2 mining ultrasonic rock fracture detector to measure the longitudinal wave velocity of the surrounding rock in exploratory borehole 5. The widths of the plastic zone 2 and elastic zone 3 of the surrounding rock of roadway 1 were measured to be 3.2 m and 10.3 m, respectively. That is, the farthest boundary of the original rock stress zone 4 of roadway 1 is 13.5 m outside the roadway.

[0027] (S2) Core drilling of the original rock of the roof: The XYD-200 directional core drilling tool was used to drill vertically to core the rock mass in the stress zone 4 of the original rock of the roof of the roadway. The core drilling hole 6 was 18 m deep and 60 mm in diameter, and 3 sets of original rock cores 7 with a length of about 12 cm were obtained and the initial orientation of each set of rock cores 7 was marked. (S3) Determination of the direction of the horizontal principal stress: The original rock core 7 was processed into three sets of identical core samples with a height of 75 mm and dried to eliminate the influence of moisture. Then, the samples were placed on a self-made rotary table 11 and the resistivity between the two intersection points of the core sample diameter and the circumference was measured using an SLTZ resistivity meter 13. The diameter directions with the maximum and minimum resistivity on the circumference of the core sample, namely NE0° and NE90°, were determined, corresponding to the locations of the minimum and maximum horizontal principal stresses, respectively. 1. The present invention aims to obtain the magnitude and direction of vertical stress, maximum horizontal principal stress and minimum horizontal principal stress (by default, z in the x / y / z three-dimensional coordinate system is vertical, and x and y are on the horizontal plane).

[0028] 2. Vertical stress is caused by gravity. After determining the orientation of the maximum and minimum horizontal principal stresses on the horizontal plane, a three-dimensional stress gauge is drilled and installed according to the orientation. By aligning the three sensing elements built into the three-dimensional stress gauge with the three directions, the magnitude of the vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress can be easily determined. In this way, the magnitude of the ground stress can be determined.

[0029] 3. The vertical stress is vertical, and the directions of the maximum and minimum horizontal principal stresses (which are known to be horizontal, but whose specific orientation is unclear) are determined as follows: A vertical borehole was drilled deep into the rock mass of the tunnel roof using directional drilling, and core samples were taken. The core samples from the original stress zone were extracted, essentially a process of unloading. Under unloading, the core samples develop micro-fractures perpendicular to the stress direction; the number depends on the degree of unloading. Specifically, the most unloading fractures are generated perpendicular to the direction of the maximum horizontal principal stress, while the fewest are generated perpendicular to the direction of the minimum horizontal principal stress. After drying, the resistivity of the core samples was measured along different diameter directions by rotation. Clearly, the resistivity was lowest along the fracture development direction and highest along the non-fracture development direction. Therefore, the directions of the maximum and minimum horizontal principal stresses can be determined by measuring the resistivity of the core samples along different diameter directions.

[0030] (S4) Drilling monitoring boreholes: Using a DL-4 type rock drilling rig, drill two horizontal monitoring boreholes 8 with a depth of 18 m and a diameter of 60 mm on each side of the original rock stress zone of the roadway 1 in the monitoring area. The boreholes are spaced 5 m apart. After the monitoring boreholes 8 are completed, pneumatic tools are used to clean the boreholes in a timely manner. (S5) Installation of the ground stress measurement device: Assemble the components of the ground stress measurement device 9 on site, and align the X, Y, and Z directions of the three-dimensional ground stress sensor 901 (range 50 MPa, accuracy 0.01 MPa) with the vertical direction, NE90°, and NE0° respectively. Install the three-dimensional ground stress sensor 901 at the bottom of the monitoring borehole 8 using a 12-section, 1.5 m long, 40 mm diameter metal sleeve 902. Connect the cable 903 of the three-dimensional ground stress sensor 901 through the metal sleeve 902 and lead it outside the borehole, connecting it to the data acquisition unit 905 (which can monitor the vibration frequency of the monitoring point in real time) via an aviation connector 904. After installation, promptly use single-liquid cement grout (P.O42.5 cement, water-cement ratio 1:1, grouting pressure 1.0 MPa) for grouting and sealing, and record the initial vibration frequency data of the three-dimensional ground stress sensor 901 in the three directions. (S6) Monitoring and Calculation of Ground Stress Data: After the grout 10 in borehole 8 to be monitored solidifies, the frequency data of the vertical stress, maximum and minimum horizontal principal stress in 4 sets of monitoring borehole 8 are collected periodically and the stress is converted according to formula (1). After continuous monitoring in the field for 3 months, it was found that the data of 4 sets of three-dimensional stress sensors 901 tended to be stable. Based on the initial frequency data, the average vertical stress and the maximum and minimum horizontal principal stress of the 4 monitoring points in the monitoring area were calculated to be 6.4 MPa, 14.8 MPa and 3.7 MPa, respectively. In order to obtain the distribution law of ground stress in the mine, (S1) to (S6) are repeated to measure 5 to 6 monitoring areas at different burial depths and the data can be obtained through data analysis.

[0031] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring three-dimensional geostress in underground mines, characterized in that, Includes the following steps: S1) Determine the boundary of the original rock stress zone. Use a rock drilling rig to drill exploratory boreholes in the surrounding rock at the corner of the roadway. Use an acoustic wave tester to determine the range of the plastic zone, elastic zone and original rock stress zone of the surrounding rock of the roadway according to the difference in acoustic wave velocity of the rock mass under different fracture development and stress conditions, and determine the boundary of the original rock stress zone of the roadway. S2) Core drilling of the original rock of the roof: Vertical core drilling is carried out on the original rock mass of the roof of the roadway using directional core drilling tools. The core drilling depth is outside the boundary of the original rock stress zone. At least 3 sets of original rock cores with a length ≥10 cm and a diameter ≥50 mm are obtained, and the initial orientation of each set of original rock cores is marked. S3) Determine the direction of the horizontal principal stress. The original rock core is processed into three sets of identical rock core samples with a height of 50-100 mm and dried. Then, the samples are placed on a rotary table and the resistivity is measured between the two intersection points of the rock core diameter and the circumference using a resistivity measuring instrument. Based on the phenomenon that the number of cracks generated inside the rock core sample under different unloading stresses is different, the measurement directions of the maximum and minimum resistivity on the circumference of the rock core are obtained, which are the directions of the minimum and maximum horizontal principal stresses. S4) Drill monitoring boreholes. Using a rock drilling rig, drill 3 to 5 horizontal monitoring boreholes in the roadway within a 10 m range of the monitoring area, in the direction of the original rock stress zone on both sides, which are parallel to the direction of the maximum or minimum horizontal principal stress. After the monitoring boreholes are completed, use pneumatic tools to clean the holes in a timely manner. S5) Install the geostress measuring device. Assemble the components of the geostress measuring device on site and orient the geostress measuring device in each monitoring borehole according to the vertical stress, the maximum horizontal principal stress, and the minimum horizontal principal stress. After installation, promptly perform grouting and sealing and record the initial data. The geostress measurement device includes a vibrating wire three-dimensional geostress sensor, a metal sleeve, a cable, an aviation plug, and a data acquisition unit; The vibrating wire three-dimensional geostress sensor can monitor the vibration frequency of the rock mass at the bottom of the borehole in the X, Y, and Z directions. The stress is converted according to Equation 1 below, with a maximum converted stress range of 50 MPa in each direction and a measurement accuracy of 0.01 MPa. A metal casing is used for the directional installation of the three-dimensional geostress sensor, arranged along the entire length of the borehole. The casing is 30–40 mm in diameter, with each section being 1.5 m long, and interconnected by threads. Three cables are arranged together in the metal casing, one end connected to the sensor, and the other end connected to the data acquisition unit via an aviation connector to transmit the vibration frequency data. Equation 1: ； In the formula, σ n The stress is in the X, Y, or Z direction, A and B are the sensor constants in that direction, i.e., the factory settings, and f and f0 represent the real-time frequency and initial installation frequency of the sensor in that direction, respectively. S6) Monitoring and calculation of geostress data: After the slurry in the monitoring borehole solidifies, the vertical stress, maximum horizontal principal stress and minimum horizontal principal stress data of each monitoring borehole are collected periodically; after the stress data of all monitoring points tend to stabilize, the average vertical stress, maximum horizontal principal stress and minimum horizontal principal stress of each monitoring point are calculated, which is the three-dimensional geostress of the monitoring area. The grouting method after the installation of the geostress measuring device in the monitoring borehole is either cement grouting or chemical grouting, with a grouting pressure ≤2 MPa, to ensure that the surrounding rock of the borehole does not rupture and that the geostress monitoring device is fully coupled with the surrounding rock.

2. The method for measuring three-dimensional geostress in underground mines as described in claim 1, characterized in that: The detection borehole needs to penetrate into the stress zone of the original rock in the tunnel. The depth of the detection borehole is 20-40 m and the diameter is 40-90 mm.

3. The method for measuring three-dimensional geostress in underground mines as described in claim 1, characterized in that: The monitoring boreholes are arranged along the direction of the maximum or minimum horizontal principal stress in the sidewall of the roadway. The depth of the monitoring boreholes is 3 to 5 m outside the boundary of the original rock stress zone of the roadway, and the diameter is 40 to 90 mm.

4. The method for measuring three-dimensional geostress in underground mines as described in claim 1, characterized in that: The rotary table includes a base, a lifting column, a control knob, a connecting plate, bearings, and cones. The cones are embedded in the bearings as a single unit. One component is welded to the top of the base, and the other component is welded to one end of the connecting plate. The two components are symmetrically arranged vertically. The other end of the connecting plate is welded to the top of the lifting column. The bottom of the lifting column is connected to the base. The lifting column is equipped with a control knob. By adjusting the height of the lifting column, the distance between the two opposing cones can be adjusted to accommodate core samples of different heights. Placing the core sample between the two opposing cones allows for free rotation of the core sample along the circumference, providing a prerequisite for the resistivity meter to measure the resistivity of the core sample in different diameter directions. The rotation angle can be read from the angle scale on the bearing.

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