A fiber grating ground stress monitoring system and method based on radial-axial displacement conversion

Abstract

The present application relates to a kind of based on radial-axial displacement conversion fiber grating ground stress monitoring system and method, including multiple stress monitoring units, contact and force transmission module includes the radial contact plate being separately arranged on the two sides of shell and the push rod being fixedly connected with radial contact plate, one radial contact plate is fixedly connected with displacement transmission rod on the side away from push rod;Pre-tightening and triggering mechanism displacement-strain conversion module is arranged between two push rods, and displacement-strain conversion module includes cantilever beam, fiber grating strain sensor is pasted on cantilever beam, and one end of cantilever beam is fixedly connected with shell;Shell is provided with optical cable through hole, and optical cable is connected with multiple fiber grating strain sensors respectively, and optical cable is connected with ground fiber grating demodulator;It further includes protective sleeve and ground stress monitoring method.The present application designs stress monitoring unit, and indirectly converts rock mass radial displacement into fiber axial strain to be perceived, and in combination with distributed arrangement and all-optical signal transmission and ground stress monitoring method, realizes real-time monitoring and inversion to rock mass stress field.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering and geological disaster monitoring technology, specifically to a fiber optic grating geostress monitoring system and method based on radial-axial displacement conversion. Background Technology

[0002] In-situ stress is a core fundamental parameter for the stability analysis and disaster prevention of surrounding rock in underground engineering. As engineering projects in energy, transportation, and other fields continue to extend deeper, adverse geological conditions such as weak, fractured rock masses and fault zones under high in-situ stress are becoming increasingly common. These rock masses have low strength and poor self-stability, making them highly susceptible to sustained, large-scale plastic rheological deformation or fracture after excavation and unloading—a phenomenon known as "large deformation" disasters. This seriously threatens engineering safety and poses a severe challenge to traditional in-situ stress testing methods. Therefore, developing an in-situ stress measurement technology that can adapt to large deformation conditions in weak rock masses and achieve long-term reliable monitoring has become a critical issue urgently needing to be addressed in engineering practice.

[0003] Currently, methods for obtaining rock mass stress are mainly divided into two categories: direct measurement and indirect inversion. However, both have significant limitations when applied to long-term deformation monitoring of weak and fractured rock masses. For example, indoor testing methods based on directional rock cores (such as CN120667049A) use directional marking of rock cores within boreholes, combined with indoor ASR and DSA tests to invert in-situ stress. Essentially, this is a one-time point test, unable to achieve long-term continuous monitoring; and it highly depends on obtaining intact rock cores, making it poorly applicable in weak and fractured strata. Hydraulic pillow-type monitoring equipment (such as CN119245894A), while indirectly reflecting rock mass stress changes through grout coupling, suffers from severely insufficient reliability and lifespan due to mechanical disturbances caused by long-term large deformations and harsh underground environments. Therefore, such equipment is ill-suited for the task of long-term, stable, and direct monitoring of large rock mass deformation processes.

[0004] In summary, existing geostress measurement technologies are insufficient to meet the urgent need for long-term, stable, real-time, and distributed monitoring of large deformation processes in weak and fractured rock masses under engineering disturbances. Therefore, there is a pressing need for a new monitoring technology and equipment that can accommodate large deformation ranges, adapt to harsh environments, and accurately capture the temporal evolution of stress fields. Summary of the Invention

[0005] To address the challenge of achieving long-term, stable, and real-time monitoring of weak and fractured rock masses using existing geostress measurement technologies, a fiber optic geostress monitoring system and method based on radial-axial displacement conversion is proposed. The system designs a stress monitoring unit to indirectly convert the radial displacement of the rock mass into optical fiber axial strain for sensing. By combining distributed deployment, all-optical signal transmission, and geostress monitoring methods, long-term, stable, and high-precision real-time monitoring and inversion of the rock mass stress field, especially the large deformation process of weak rock masses, are achieved.

[0006] To achieve the above objectives, the first aspect of the present invention proposes a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion, comprising multiple stress monitoring units. Each stress monitoring unit includes a housing, a contact and force transmission module, a pre-tightening and triggering mechanism, and a displacement-strain conversion module. The contact and force transmission module includes radial contact plates disposed on both sides of the housing and push rods fixedly connected to the radial contact plates. A displacement transmission rod is fixedly connected to the side of one of the radial contact plates away from the push rod.

[0007] A pre-tensioning and triggering mechanism is provided between the two push rods. The pre-tensioning and triggering mechanism includes a pre-tensioning spring and a limiting sleeve. The pre-tensioning spring is sleeved inside the limiting sleeve, and both ends of the pre-tensioning spring are fixed to the two push rods respectively. The ends of the two push rods are inside the limiting sleeve.

[0008] The displacement-strain conversion module includes a cantilever beam, on which a fiber optic strain sensor is attached, and one end of the cantilever beam is fixedly connected to the housing.

[0009] The housing is provided with an optical cable through hole, through which an optical cable is inserted. The optical cable is connected to multiple fiber optic strain sensors and to a ground-based fiber optic demodulator.

[0010] It also includes a protective sleeve, and multiple stress monitoring units are equidistantly arranged inside the protective sleeve.

[0011] Furthermore, the displacement transmission rod has a hemispherical groove at its bottom, and the free end of the cantilever beam has a ball head that matches the hemispherical groove. A movement gap is reserved between the bottom surface of the displacement transmission rod and the upper surface of the cantilever beam.

[0012] By incorporating a hemispherical groove at the bottom of the displacement transmission rod and a matching ball head at the free end of the cantilever beam, along with a pre-reserved movement gap, a highly efficient and reliable spherical coupling structure was constructed. This structure improves the linearity and accuracy of displacement-strain conversion. The pre-reserved movement gap provides space for the relative movement of the displacement transmission rod and the cantilever beam, avoiding false triggering of the sensor due to rigid contact in the initial state, while also ensuring effective force transmission even with minor deformation of the rock mass, thus enhancing the system's sensitivity to subtle displacements.

[0013] Furthermore, the stress monitoring unit also includes a mechanical overload protection mechanism, which includes an adjustable limit screw located directly below the free end of the cantilever beam. The adjustable limit screw is threadedly connected to the bottom surface of the housing, and the length of the radial contact plate on the side closer to the adjustable limit screw is less than the length of the radial contact plate farther from the adjustable limit screw.

[0014] The distance between the top of the adjustable limit screw and the free end of the cantilever beam is adjusted by rotating the adjustable limit screw, thereby setting the maximum bending deformation of the cantilever beam. This effectively limits the maximum strain value of the fiber optic strain sensor and prevents the fiber optic grating from breaking or being damaged due to excessive bending of the cantilever beam when the rock mass undergoes large deformation.

[0015] Furthermore, the housing includes an upper housing and a lower housing, which are sealed and fixed together. The upper housing and the lower housing are combined to form a hollow cylindrical structure. The upper housing and the lower housing have annular through holes mirror-shaped on their upper and lower housings. The radial contact plate is an arc-shaped plate that fits into the annular through holes.

[0016] The optical cable through-holes are respectively opened on the upper and lower housings.

[0017] The housing is designed as a sealed and fixed structure with upper and lower shells, forming a hollow cylinder after assembly. This facilitates the installation and commissioning of internal components (contact and force transmission modules, pre-tightening and triggering mechanisms, displacement-strain conversion modules, etc.). The annular through-holes mirrored on the upper and lower shells precisely fit with the arc-shaped radial contact plate, ensuring a tight fit between the radial contact plate and the protective sleeve wall. This guarantees that the pressure generated by rock deformation is efficiently transmitted to the push rod, improving the sensitivity and accuracy of displacement monitoring.

[0018] Furthermore, multiple stress monitoring units are positioned at different angles relative to the axis, and these angles are arranged in an arithmetic sequence.

[0019] The arithmetic sequence arrangement with included angles ensures that each stress monitoring unit is evenly distributed in the circumferential direction (such as 0°, 60°, 120°, -60° and -120°), covering all directions of the monitoring holes. It can simultaneously acquire radial displacement data in at least three different directions, providing sufficient basic data for inverting two-dimensional and three-dimensional geostress fields based on elasticity theory.

[0020] The second aspect of this invention proposes a ground stress monitoring method for a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion, comprising:

[0021] Step 1: Perform laboratory calibration on each stress monitoring unit, establish the relationship curve between wavelength drift and radial displacement, connect multiple stress monitoring units to the optical cable respectively, and install them into the protective sleeve according to the arithmetic sequence of the included angles to form a string of measuring points;

[0022] Step 2: Lower the assembled measuring point string to the designed depth of the monitoring hole, with the radial contact plate tightly attached to the wall of the protective sleeve, and grout between the protective sleeve and the monitoring hole to solidify the measuring point string with the rock mass.

[0023] Step 3: If the pressure change caused by the deformation of the rock mass is transmitted to multiple stress monitoring units through the grout and protective sleeve, the stress monitoring unit converts the displacement change into the wavelength signal of the fiber optic grating. The wavelength signal is transmitted to the fiber optic grating demodulator via optical cable for real-time acquisition, calculation and storage.

[0024] Step 4: Select stable monitoring data, convert the wavelength drift of each measuring point into radial displacement value according to the calibration relationship, and establish a set of equations relating displacement and stress components based on the stress concentration theory at the orifice of the monitoring hole in elasticity.

[0025] By solving a system of equations consisting of displacement values ​​in at least three different directions, the magnitude and direction of the two-dimensional principal stress in the plane where the measuring point is located can be deduced.

[0026] The three-dimensional stress field is calculated by combining the monitoring data from multiple stress monitoring units arranged at different angles.

[0027] Furthermore, step 2 specifically includes:

[0028] Drill monitoring holes according to the designed orientation and inclination. The diameter of the monitoring hole is 1-2 mm larger than the outer diameter of the protective casing. Grout is injected between the protective casing and the monitoring hole. The grouting material includes micro-expansion cement-based grout. The grouting process is continuous until the grout returns from the orifice of the monitoring hole. After the micro-expansion cement-based grout solidifies, it forms a continuous solid with a matching elastic modulus between the rock mass and the protective casing.

[0029] The monitoring hole diameter is 1-2 mm larger than the outer diameter of the protective casing. This design ensures smooth lowering of the protective casing while providing ample space for the grout, allowing it to fully fill the gap between the monitoring hole and the protective casing, preventing issues such as voids and gaps. A micro-expansion cement-based grout is selected as the grouting material. Its micro-expansion properties after curing ensure a tight fit between the grout and the rock mass and protective casing, eliminating gaps in force transmission. Furthermore, the solid elastic modulus formed after curing matches the rock mass, effectively reducing distortion during stress transmission and ensuring that rock mass deformation is accurately and reliably transmitted to the monitoring unit. The requirement for continuous grouting until the grout returns from the borehole ensures full grouting, preventing interruptions or inaccuracies in force transmission due to incomplete grouting in certain areas.

[0030] Furthermore, a fiber optic grating ground stress monitoring method based on radial-axial displacement conversion, wherein step 4 specifically includes:

[0031] Step 4.1: Read the wavelength signal and calculate the wavelength shift based on the wavelength data. ;

[0032] Step 4.2: Based on the calibration curve, Converted into radial displacement changes of the monitoring holes in various directions ;

[0033] Step 4.3: Substitute the displacement values ​​in at least three different directions into the system of equations using the Kirch solution;

[0034] For any direction θ, the relationship between the radial displacement of the hole wall and the far-field stress is shown in equations (1) and (2):

[0035] (1);

[0036] (2);

[0037] in, Here, 'a' represents the observed value, and 'a' represents the radius of the monitoring hole. The elastic modulus of the rock mass; Poisson's ratio of the rock mass These are the normal stress components in the x and y directions, defined in the measurement coordinate system. This represents the shear stress component;

[0038] Under the boundary condition of the borehole wall (r=a), a set of polar coordinate displacement observation equations is established. Each equation corresponds to a displacement-stress relationship at a specific azimuth angle. The matrix form of the polar coordinate observation system is constructed as shown in equation (3):

[0039] (3);

[0040] Step 4.4: Solve for the stress components and calculate the maximum and minimum principal stresses at the measuring points. ) and its direction angle ( As shown in equation (4):

[0041] (4);

[0042] in, These are the maximum and minimum principal stresses; The principal stress direction angle.

[0043] The precise calculation of wavelength drift in step 4.1 provides accurate raw data for subsequent displacement conversion. Step 4.2, based on the previous laboratory calibration curve, completes the conversion of wavelength drift to radial displacement values, ensuring the quantitative accuracy of displacement data and eliminating the influence of sensor errors and environmental interference. Step 4.3 introduces the Kirchh solution from elasticity mechanics to establish a precise mathematical relationship between borehole wall radial displacement and far-field stress, organically linking displacement monitoring data with stress components and providing solid theoretical support for stress inversion. By constructing a matrix-form equation system for the polar coordinate observation system, the joint solution of multi-directional displacement data becomes more systematic and standardized, reducing the difficulty of solution and improving the efficiency of solution. Step 4.4 obtains the magnitude and direction angle of principal stress by solving the equation system, clarifying the specific calculation methods and formulas, and ensuring the accuracy and reliability of stress inversion results. The entire refinement process is logically rigorous and the steps are clear, effectively avoiding human errors in data processing and inversion, and significantly improving the accuracy of geostress inversion.

[0044] The beneficial effects of the present invention through the above technical solution are as follows:

[0045] 1. The stress monitoring unit designed in this invention includes a shell, a contact and force transmission module, a pre-tightening and triggering mechanism, and a displacement-strain conversion module. The radial contact plate and push rod in the contact and force transmission module can accurately receive and transmit the pressure generated by the radial deformation of the rock mass. The pre-tightening and triggering mechanism, composed of a pre-tightening spring and a limiting sleeve, ensures the stability of the system's initial state. The displacement-strain conversion module adopts a design combining a cantilever beam and a fiber optic strain sensor. Utilizing the inherent insulation, corrosion resistance, electromagnetic interference resistance, and low signal transmission loss characteristics of the fiber optic grating, it converts the radial displacement of the rock mass into a precisely monitorable wavelength signal, enabling the stress monitoring unit to operate for extended periods in harsh underground environments with good reliability and stability. Multiple stress monitoring units are connected in series via a single optical cable and arranged equidistantly within a protective casing, achieving a quasi-distributed deployment along the axial and circumferential directions within the same borehole. This lays the structural foundation for constructing a three-dimensional profile monitoring network, allowing for the simultaneous acquisition of displacement information from different spatial locations and directions, providing sufficient data support for inversion of the geostress field, and effectively improving the accuracy and reliability of the inversion.

[0046] 2. This invention constructs a continuous force transmission model conforming to the mechanics of continuous media through grouting technology, realizing integrated anchoring and deformation-coordinated coupling between the monitoring unit and the surrounding rock. This design not only eliminates the stress transmission distortion and long-term decoupling risks caused by point contact, but also provides a physical isolation barrier against groundwater corrosion and mechanical damage to the core sensing elements, ensuring the long-term service and stable and accurate operation of the ground stress monitoring system. Attached Figure Description

[0047] Figure 1This is one of the structural schematic diagrams of a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to the present invention;

[0048] Figure 2 This is the second schematic diagram of the structure of a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to the present invention;

[0049] Figure 3 This is the third schematic diagram of the structure of a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to the present invention;

[0050] Figure 4 This is the fourth schematic diagram of the structure of a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to the present invention;

[0051] Figure 5 This is a flowchart illustrating the steps of a fiber optic grating ground stress monitoring method based on radial-axial displacement conversion according to the present invention.

[0052] Reference numerals: 1 is the housing, 2 is the radial contact plate, 3 is the push rod, 4 is the displacement transmission rod, 5 is the preload spring, 6 is the limiting sleeve, 7 is the cantilever beam, 8 is the fiber optic strain sensor, 9 is the optical cable through hole, 10 is the optical cable, 11 is the protective sleeve, 12 is the ball head, and 13 is the adjustable limiting screw. Detailed Implementation

[0053] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0054] Example 1

[0055] like Figure 1-5 As shown, a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion includes multiple stress monitoring units. Each stress monitoring unit includes a housing 1, a contact and force transmission module, a pre-tightening and triggering mechanism, and a displacement-strain conversion module. The contact and force transmission module includes radial contact plates 2 disposed on both sides of the housing 1 and push rods 3 fixedly connected to the radial contact plates 2. A displacement transmission rod 4 is fixedly connected to the side of one of the radial contact plates 2 away from the push rod 3.

[0056] A pre-tensioning and triggering mechanism is provided between the two push rods 3. The pre-tensioning and triggering mechanism includes a pre-tensioning spring 5 and a limiting sleeve 6. The pre-tensioning spring 5 is sleeved inside the limiting sleeve 6, and both ends of the pre-tensioning spring 5 are fixed to the two push rods 3 respectively. The ends of the two push rods 3 are inside the limiting sleeve 6.

[0057] The displacement-strain conversion module includes a cantilever beam 7, on which a fiber optic strain sensor 8 is attached. One end of the cantilever beam 7 is fixedly connected to the housing 1.

[0058] The housing 1 is provided with an optical cable through hole 9, and an optical cable 10 is inserted through the optical cable through hole 9. The optical cable 10 is connected to a plurality of fiber optic strain sensors 8 respectively, and the optical cable 10 is connected to a ground fiber optic demodulator.

[0059] It also includes a protective sleeve 11, and multiple stress monitoring units are equidistantly arranged inside the protective sleeve 11.

[0060] The displacement transmission rod 4 has a hemispherical groove at its bottom, and the free end of the cantilever beam 7 has a ball head 12 that matches the hemispherical groove. A movement gap is reserved between the bottom surface of the displacement transmission rod 4 and the upper surface of the cantilever beam 7.

[0061] The stress monitoring unit also includes a mechanical overload protection mechanism, which includes an adjustable limit screw 13. The adjustable limit screw 13 is located directly below the free end of the cantilever beam 7. The adjustable limit screw 13 is threadedly connected to the bottom surface of the housing 1. The length of the radial contact plate 2 on the side closer to the adjustable limit screw 13 is less than the length of the radial contact plate 2 away from the adjustable limit screw 13.

[0062] The housing 1 includes an upper housing and a lower housing, which are sealed and fixed together. The upper housing and the lower housing are combined to form a hollow cylindrical structure. The upper housing and the lower housing are mirror-shaped with annular through holes. The radial contact plate 2 is an arc-shaped plate that fits into the annular through holes.

[0063] The optical cable through-hole 9 is correspondingly opened on the upper and lower housings.

[0064] The housing 1, a cylindrical structure made of high-strength stainless steel, provides mechanical support and sealing protection for the entire stress monitoring unit. The radial contact plate 2 is an arc-shaped plate made of wear-resistant alloy. The stress monitoring unit can be inserted into the protective sleeve 11 using a push rod tool. The preload spring 5 provides an outward thrust to the radial contact plate 2, pushing the push rod 3 to make the radial contact plate 2 fit tightly against the protective sleeve 11.

[0065] Take three stress monitoring units, each with a different included angle to the axis, arranged in an arithmetic sequence (0°, 60°, and 120°). Repeat this process until all stress monitoring units are connected in series. Slowly and steadily place the entire protective sleeve 11 into the monitoring hole.

[0066] The working principle of the stress monitoring unit is as follows: when the monitoring hole deforms, the radial contact plate 2 and the displacement transmission rod 4 fixed thereto are pushed by the protective sleeve 11 to generate radial displacement. Through the ball head 12 fitted with a hemispherical groove, this radial displacement is linearly converted into the bending deformation of the cantilever beam 7. The fiber optic strain sensor 8 is stretched, causing its center wavelength ( Linear drift occurs ( ).

[0067] A monitoring method for a fiber optic grating-based geostress monitoring system based on radial-axial displacement conversion, comprising:

[0068] Step 1: Perform laboratory calibration on each stress monitoring unit and establish the center wavelength drift ( ) and radial displacement ( The precise relationship curve between the two units was recorded, and the calibration coefficients of each unit were recorded. The full-range cycle test and overload protection test were repeatedly performed to verify the smoothness, repeatability and effectiveness of the limit protection of the mechanical movement. Multiple stress monitoring units were connected to the optical cable 10 respectively and installed in the protective sleeve 11 according to the arithmetic sequence of the included angles to form a series of measuring points.

[0069] Step 2: Drill monitoring holes according to the design orientation and inclination. The diameter of the monitoring holes is 1-2 mm larger than the outer diameter of the protective casing 11. Grout between the protective casing 11 and the monitoring holes. The grouting material includes micro-expansion cement-based grout. The grouting process is continuous until the grout returns from the orifice of the monitoring hole. After the micro-expansion cement-based grout solidifies, it forms a continuous solid with matching elastic modulus between the rock mass and the protective casing 11.

[0070] The ground-based fiber optic grating demodulator scans all fiber optic strain sensors 8 at a preset frequency, and collects and records the center wavelength data of each fiber optic strain sensor 8.

[0071] Step 3: If the pressure changes caused by the deformation of the rock mass, the pressure is transmitted to multiple stress monitoring units through the grout and protective sleeve 11. The stress monitoring unit converts the displacement change into the wavelength signal of the fiber optic grating. The wavelength signal is transmitted to the fiber optic grating demodulator through the optical cable 10 for real-time acquisition, calculation and storage.

[0072] Step 4: Select stable monitoring data, convert the wavelength drift of each measuring point into radial displacement value according to the calibration relationship, and establish a set of equations relating displacement and stress components based on the stress concentration theory at the orifice of the monitoring hole in elasticity.

[0073] By solving a system of equations consisting of displacement values ​​in at least three different directions, the magnitude of the two-dimensional principal stress in the plane where the measuring point is located can be determined. ) and direction ( );

[0074] The three-dimensional stress field is calculated by combining the monitoring data from multiple stress monitoring units arranged at different angles.

[0075] Step 4 specifically includes:

[0076] Step 4.1: Read the wavelength signal and calculate the wavelength shift based on the wavelength data. ;

[0077] Step 4.2: Based on the calibration curve, Converted into radial displacement changes of the monitoring holes in various directions ;

[0078] Step 4.3: Substitute the displacement values ​​in at least three different directions into the system of equations using the Kirch solution;

[0079] For any direction θ, the relationship between the radial displacement of the hole wall and the far-field stress is shown in equations (1) and (2):

[0080] (1);

[0081] (2);

[0082] in, Here, 'a' represents the observed value, and 'a' represents the radius of the monitoring hole. The elastic modulus of the rock mass; Poisson's ratio of the rock mass These are the normal stress components in the x and y directions, defined in the measurement coordinate system. This represents the shear stress component;

[0083] Under the boundary condition of the borehole wall (r=a), a set of polar coordinate displacement observation equations is established. Each equation corresponds to a displacement-stress relationship at a specific azimuth angle. The matrix form of the polar coordinate observation system is constructed as shown in equation (3):

[0084] (3);

[0085] Step 4.4: Solve for the stress components and calculate the maximum and minimum principal stresses at the measuring points. ) and its direction angle ( As shown in equation (4):

[0086] (4);

[0087] in, These are the maximum and minimum principal stresses; The principal stress direction angle.

[0088] This invention provides a fiber optic grating geostress monitoring system and method based on radial-axial displacement conversion, offering an effective new technical means for long-term, real-time, and high-precision monitoring and disaster early warning of geostress under complex geological conditions such as soft rock and fault fracture zones.

[0089] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the claims of the present invention should be included within the scope of the present invention.

Claims

1. A fiber optic grating ground stress monitoring system based on radial-axial displacement conversion, characterized in that, It includes multiple stress monitoring units. The stress monitoring unit includes a housing (1), a contact and force transmission module, a pre-tightening and triggering mechanism, and a displacement strain conversion module. The contact and force transmission module includes radial contact plates (2) disposed on both sides of the housing (1) and a push rod (3) fixedly connected to the radial contact plates (2). A displacement transmission rod (4) is fixedly connected to the side of one of the radial contact plates (2) away from the push rod (3). A pre-tightening and triggering mechanism is provided between the two push rods (3). The pre-tightening and triggering mechanism includes a pre-tightening spring (5) and a limiting sleeve (6). The pre-tightening spring (5) is sleeved inside the limiting sleeve (6), and the two ends of the pre-tightening spring (5) are respectively fixed to the two push rods (3). The ends of the two push rods (3) are inside the limiting sleeve (6). The displacement-strain conversion module includes a cantilever beam (7), on which a fiber optic strain sensor (8) is attached, and one end of the cantilever beam (7) is fixedly connected to the housing (1); The housing (1) is provided with an optical cable through hole (9), and an optical cable (10) is inserted through the optical cable through hole (9). The optical cable (10) is connected to multiple fiber optic strain sensors (8) respectively, and the optical cable (10) is connected to a ground fiber optic demodulator. It also includes a protective sleeve (11), and multiple stress monitoring units are equidistantly arranged inside the protective sleeve (11).

2. The fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to claim 1, characterized in that, The displacement transmission rod (4) has a hemispherical groove at its bottom, and the free end of the cantilever beam (7) has a ball head (12) that is adapted to the hemispherical groove. A movement gap is reserved between the bottom surface of the displacement transmission rod (4) and the upper surface of the cantilever beam (7).

3. The fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to claim 1, characterized in that, The stress monitoring unit also includes a mechanical overload protection mechanism, which includes an adjustable limit screw (13). The adjustable limit screw (13) is located directly below the free end of the cantilever beam (7). The adjustable limit screw (13) is connected to the bottom surface of the housing (1) by a thread. The length of the radial contact plate (2) on the side closer to the adjustable limit screw (13) is less than the length of the radial contact plate (2) away from the adjustable limit screw (13).

4. The fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to claim 1, characterized in that, The housing (1) includes an upper housing and a lower housing. The upper housing and the lower housing are sealed and fixed. The upper housing and the lower housing are combined to form a hollow cylindrical structure. The upper housing and the lower housing are mirror-shaped with annular through holes. The radial contact plate (2) is an arc-shaped plate and fits against the annular through holes. The optical cable through-hole (9) is correspondingly opened on the upper and lower shells.

5. The fiber optic grating ground stress monitoring system based on radial-axial displacement conversion according to claim 1, characterized in that, Multiple stress monitoring units are positioned at different angles relative to the axis, and these angles are arranged in an arithmetic sequence.

6. A ground stress monitoring method based on a fiber optic grating ground stress monitoring system based on radial-axial displacement conversion as described in any one of claims 1-5, characterized in that, include: Step 1: Perform laboratory calibration on each stress monitoring unit, establish the relationship curve between wavelength drift and radial displacement, connect multiple stress monitoring units to optical cable (10) respectively, and install them into protective sleeve (11) according to the arithmetic sequence of the included angles to form a string of measuring points; Step 2: Lower the assembled measuring point string to the designed depth of the monitoring hole, with the radial contact plate (2) tightly attached to the wall of the protective sleeve (11), and grout between the protective sleeve (11) and the monitoring hole to solidify the measuring point string with the rock mass. Step 3: If the pressure changes caused by the deformation of the rock mass, the pressure is transmitted to multiple stress monitoring units through the grout and protective sleeve (11). The stress monitoring unit converts the displacement change into the wavelength signal of the fiber optic grating. The wavelength signal is transmitted to the fiber optic grating demodulator through the optical cable (10) for real-time acquisition, calculation and storage. Step 4: Select stable monitoring data, convert the wavelength drift of each measuring point into radial displacement value according to the calibration relationship, and establish a set of equations relating displacement and stress components based on the stress concentration theory at the orifice of the monitoring hole in elasticity. By solving a system of equations consisting of displacement values ​​in at least three different directions, the magnitude of the two-dimensional principal stress in the plane where the measuring point is located can be determined. ) and direction ( ); The three-dimensional stress field is calculated by combining the monitoring data from multiple stress monitoring units arranged at different angles.

7. The fiber optic grating ground stress monitoring method based on radial-axial displacement conversion according to claim 6, characterized in that, Step 2 specifically includes: Drill monitoring holes according to the design orientation and inclination. The diameter of the monitoring hole is 1-2 mm larger than the outer diameter of the protective casing (11). Grout is injected between the protective casing (11) and the monitoring hole. The grouting material includes micro-expansion cement-based grout. The grouting process is carried out continuously until the grout returns from the orifice of the monitoring hole. After the micro-expansion cement-based grout solidifies, it forms a continuous solid with a matching elastic modulus between the rock mass and the protective casing (11).

8. The fiber optic grating ground stress monitoring method based on radial-axial displacement conversion according to claim 6, characterized in that, Step 4 specifically includes: Step 4.1: Read the wavelength signal and calculate the wavelength shift based on the wavelength data. ; Step 4.2: Based on the calibration curve, Converted into radial displacement changes of the monitoring hole in each direction ; Step 4.3: Substitute the displacement values ​​in at least three different directions into the system of equations using the Kirch solution; For any direction θ, the relationship between the radial displacement of the hole wall and the far-field stress is shown in equations (1) and (2): （1）； （2）； in, Here, 'a' represents the observed value, and 'a' represents the radius of the monitoring hole. The elastic modulus of the rock mass; Poisson's ratio of the rock mass These are the normal stress components in the x and y directions, defined in the measurement coordinate system. This represents the shear stress component; Under the boundary condition of the borehole wall (r=a), a set of polar coordinate displacement observation equations is established. Each equation corresponds to a displacement-stress relationship at a specific azimuth angle. The matrix form of the polar coordinate observation system is constructed as shown in equation (3): （3）； Step 4.4: Solve for the stress components and calculate the maximum and minimum principal stresses at the measuring points. ) and its direction angle ( As shown in equation (4): （4）； in, These are the maximum and minimum principal stresses; The principal stress direction angle.

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