A composite fiber grating downhole steel pipe concrete pier column monitoring device and method

Abstract

This invention discloses a monitoring device and method for underground steel-concrete pier columns using composite fiber optic gratings, belonging to the field of underground engineering structural safety monitoring technology. The monitoring device includes embedded grating strain sensors and patch grating strain sensors. The embedded grating strain sensors are spaced along auxiliary reinforcing bars, while the patch grating strain sensors are spaced along the axial and circumferential directions of the steel pipe wall. Both embedded and patch grating strain sensors are connected to a fiber optic demodulator via optical fiber and an optical switch. This device and method can achieve pressure and bending monitoring. Combining embedded and patch sensors, it obtains multi-dimensional information such as axial stress and bending deformation. Automatic switching and demodulation via the optical switch improves real-time performance and reliability. Combined with a three-dimensional spatial morphology inversion method, it accurately calculates the bending direction and degree, providing high-resolution deformation analysis and comprehensively assessing the structural stress and deformation state.

Description

Technical Field

[0001] This invention relates to the field of underground engineering structure safety monitoring technology, specifically to a monitoring device and method for underground steel pipe concrete pier columns using composite fiber optic gratings. Background Technology

[0002] As mining in my country advances deeper into geological strata, deep wells reaching depths of over 1,000 meters have become commonplace. Deep surrounding rock is situated in a complex mechanical environment characterized by "three highs and one urgent condition" (high ground stress, high temperature, high karst water pressure, and typical dynamic disturbances). Concrete-steel tubular (CFST) piers, with their unique "hoop effect"—the lateral constraint of the outer steel tube on the inner core concrete—place the concrete under triaxial compression, significantly enhancing the structure's load-bearing capacity and ductility. This has made them a key support method for deep mines, long-span tunnels, and underground powerhouses.

[0003] However, the nonlinear large deformation characteristics of deep surrounding rock often result in extremely uneven load distribution for support structures. During long-term service, piers are subject to slow roof subsidence, lateral rock compression, or blasting impacts, making them highly susceptible to eccentric compression, asymmetric diameter reduction, or even overall instability and bending. If the health status of the piers cannot be accurately monitored throughout their entire life cycle, instability can trigger catastrophic safety accidents such as roof collapse.

[0004] Currently, the industry mainly relies on the following technical approaches for monitoring downhole support columns, but all of them have shown significant shortcomings in practical applications: (1) Electrical sensors (resistance strain gauges, vibrating wire sensors); Strain gauges are attached to the outer wall of the steel pipe, or vibrating wire strain gauges are pre-embedded inside the pier column to calculate strain by measuring changes in electrical parameters. However, this technology has the following drawbacks: Zero drift and extremely poor durability: The water in the well is acidic or alkaline and has extremely high humidity, which makes the insulation layer of the electrical sensor very easy to fail, leading to signal drift or even complete destruction.

[0005] Limited transmission distance: The weak electrical signal attenuates rapidly as the transmission cable grows longer, and the strong magnetic field generated by the underground high-voltage cable and transformer will create huge electromagnetic noise, which seriously interferes with the accuracy.

[0006] (2) Point-type mechanical sensing monitoring (hydraulic pillow, pressure box); A pressure cell is placed at the column head or bottom pad of the pier to measure the end force. However, this technique has the following drawbacks: Data dimension missing: This kind of "end measurement" is a typical black box model. It can only obtain the total load, but cannot know the stress transmission process along the column, let alone discover the local buckling that occurs in the middle section of the column or the evolution of voids inside the core concrete.

[0007] Uneven contact stress: Tiny gaps at the contact surface between the pressure box and the pier can cause stress concentration, resulting in a large deviation between the measured value and the actual bearing capacity.

[0008] (3) Measurement of traditional physical quantities (total station, convergence meter, inclinometer); This method relies on manual, periodic measurements of spatial coordinates using a total station, or on monitoring verticality using an inclinometer suspended from the surface. However, this technique has the following drawbacks: Non-real-time nature: The narrow working space and high dust concentration in the well often obstruct the laser path, making it impossible to achieve automated real-time early warning.

[0009] Poor bending reconstruction capability: Inclinometers can only measure the local inclination angle of the installation point, while the bending of the pier column often presents multiple curves. The true deflection curve and bending direction cannot be reconstructed based on limited point data. Summary of the Invention

[0010] Based on the above-mentioned technical problems, this invention proposes a monitoring device and method for underground steel pipe concrete pier columns using composite fiber optic gratings.

[0011] The technical solution adopted in this invention is: A monitoring device for underground steel-concrete piers with composite fiber optic gratings includes an embedded grating strain sensor and a patch grating strain sensor, both of which are installed on the steel-concrete pier. The steel-concrete composite pier includes a steel pipe and concrete poured inside the steel pipe, with auxiliary reinforcing bars provided at the center of the concrete and along the axial direction of the steel pipe. Multiple embedded grating strain sensors are provided and arranged at intervals along the auxiliary reinforcing bars to form an embedded measurement group; Multiple patch-type grating strain sensors are provided and are all arranged on the steel pipe wall; all patch-type grating strain sensors are divided into several patch-type measurement groups, and the patch-type grating strain sensors in each patch-type measurement group are arranged at intervals along the axial direction of the steel pipe wall, and all patch-type measurement groups are arranged at intervals along the circumference of the steel pipe wall. Both the embedded grating strain sensor and the patch grating strain sensor are connected to an optical switch via a conductive optical fiber, and the optical switch is connected to a fiber optic grating demodulator.

[0012] This invention also provides a method for monitoring downhole steel pipe concrete piers using composite fiber optic gratings. Employing the monitoring device described above, the pressure monitoring method includes the following steps: (1) Obtain the center wavelength shift of the embedded grating strain sensor using a fiber optic grating demodulator. ,in The labeling of the embedded grating strain sensor utilizes the wavelength shift-strain sensitivity coefficient of the embedded grating strain sensor itself. The strain distribution of auxiliary steel bars at the locations of each embedded grating strain sensor was obtained. ; (2) Establish the auxiliary reinforcement strain and its axial stress Linear relationship between them: ; in, It is the elastic modulus of the auxiliary reinforcement; through this linear relationship, the axial stress distribution at each location of the auxiliary reinforcement can be obtained; (3) Since the auxiliary reinforcement and concrete are in the same stress system and the bond between them can effectively transfer stress, the axial bearing pressure at each position of the steel-concrete composite pier can be obtained through the following relationship: ; in, It is the elastic modulus of concrete.

[0013] The bending monitoring method includes the following steps: (1) Wavelength shift of patch grating strain sensor obtained by demodulation using fiber optic grating demodulator ,in Each column of sensors is labeled, with n=1, 2, 3 representing different column numbers, i.e., axial sensor columns located at 0°, 120°, and 240° positions on the circumference of the steel pipe; the wavelength shift-strain sensitivity coefficient of the patch-type grating strain sensor itself is utilized. The strain distribution at the location of each patch grating strain sensor was obtained. ; (2) The strain values ​​measured by three sets of patch grating strain sensors at the same height constitute a calculation unit, which is composed of a total of [number] components along the steel-concrete composite pier. This set of computing units, through this The strain distribution of the calculation unit is used to invert the bending direction and degree of the steel-concrete composite pier. First, the curvature distribution of the steel-concrete composite pier is calculated. The distance from each row of patch-type optical grating strain sensors to the steel-concrete composite pier is... The relationship between the radius of curvature and strain can be calculated using the following formula: ; In the formula, Let be the radius of curvature. The circumferential direction angle of the steel pipe is for different column numbers n. When n=1, 2, and 3, the values ​​are 0°, 120°, and 240°, respectively. The bending direction is obtained by calculating the bending direction angle of the steel-concrete composite pier, and the calculation formula is as follows: ; Differentiating the obtained bending direction angle yields the functional expression for shape torsion: ; Define three mutually orthogonal space vectors , respectively, are the binormal vectors of the curves represented by the steel-concrete composite piers. tangent vector and normal vector These three spatial vectors are connected by curvature and torsion, and satisfy the following relationship: ; Assuming the top of the reinforced concrete pier column is the starting point The coordinates are (0,0,0), and the binormal vector is... Initial value [0,1,0], tangent vector Initial values ​​[1,0,0] and normal vector Initial value [0,0,1]; use the following iterative equations to calculate the coordinates of the bottommost point. The calculation of the three-dimensional shape is completed, and the direction and degree of curvature are obtained: ; Where m=1,2,3, … represents the label of the patch grating strain sensor counting from the top, and ds is the initial linear distance between two adjacent patch grating strain sensors.

[0014] The beneficial technical effects of the present invention are as follows: (1) The present invention uses a composite fiber grating sensor system, which combines an embedded grating strain sensor and a patch grating strain sensor to monitor axial stress and bending deformation. Compared with the traditional single sensor system, the composite fiber grating sensor can simultaneously obtain multi-dimensional information of the structure (such as axial stress and bending deformation). The present invention, together with a fiber grating demodulator and an automatic optical switch selection, can accurately demodulate the sensing signals at different locations, provide more detailed structural health data, and thus effectively improve the monitoring accuracy and reliability.

[0015] (2) This invention uses a fiber optic grating demodulator combined with an optical switch to achieve automatic switching and real-time demodulation of sensor signals. This design greatly improves the real-time performance of the monitoring system and reduces manual intervention and operational complexity. Compared with traditional single measurement methods, this invention can acquire stress and deformation data at various locations of steel-concrete composite piers in real time, provide timely warnings of structural anomalies, and improve the dynamic response capability of monitoring.

[0016] (3) By integrating axial stress and bending monitoring into the same device, this invention can comprehensively assess the stress and deformation state of steel-concrete composite piers. Compared with the prior art, this invention improves the depth and breadth of structural health monitoring, can simultaneously reflect stress changes in different directions, provides more comprehensive structural health data, provides a scientific basis for design and construction, and avoids the problem that a single monitoring method cannot comprehensively assess the structural state.

[0017] (4) In terms of structural design, the present invention, through the rational arrangement of embedded grating strain sensors and patch grating strain sensors, enables the monitoring device to be effectively integrated into the design and construction process of steel pipe concrete piers, simplifying the complex process that traditional monitoring equipment requires separate arrangement. This integrated design reduces additional procedures and material consumption, while improving the ease of installation and maintenance of the device.

[0018] (5) This invention uses a three-dimensional spatial morphology inversion method to accurately calculate and invert the bending direction and degree of steel-concrete composite piers, providing a new approach to the analysis of bending deformation. Compared with traditional bending monitoring methods, this invention provides higher spatial resolution and more accurate deformation analysis, especially with better monitoring performance under complex stress environments.

[0019] (6) This invention integrates fiber optic grating sensors, demodulators, optical switches, etc., resulting in high system integration and reducing the need for multiple devices and complex wiring required in traditional methods. Through automated signal demodulation and optical switching systems, the complexity and error probability of manual operation are greatly reduced, making operation simpler and easier to implement. Compared with traditional monitoring methods, this invention offers higher efficiency and lower maintenance costs in structural health monitoring.

[0020] (7) This invention utilizes fiber optic sensing technology, which effectively reduces power consumption during signal transmission. Compared to traditional electrically driven sensors and monitoring systems, this invention has a significant advantage in energy consumption. It reduces dependence on external power sources and energy consumption, making it suitable for long-term, low-energy monitoring needs, and particularly suitable for use in energy-constrained environments such as underground mines. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structural principle of the underground steel pipe concrete pier monitoring device with composite fiber optic grating of the present invention. Figure 2 for Figure 1 Top view of a steel-concrete composite pier column; Figure 3 for Figure 1 A schematic diagram of the arrangement of the three sets of patch grating strain sensors after unfolding.

[0022] In the figure: 1. Concrete; 2. Embedded grating strain sensor; 3. Auxiliary reinforcement; 4. Steel pipe; 5. Patch grating strain sensor; 6. Conducting optical fiber; 7. Optical switch; 8. Fiber grating demodulator; 9. Fiber guide hole. Detailed Implementation

[0023] Existing technologies for monitoring underground steel-concrete composite piers suffer from the following core bottlenecks: 1. Inability to achieve "integration of perception and structure": Sensors and support structures remain "separate," unable to deform synchronously with the structure; 2. Inability to achieve "full-scale stress analysis": There is a lack of a means to simultaneously perceive from the axial direction (top to bottom); 3. Inability to achieve "digital reconstruction of complex forms": For asymmetric bending induced by eccentric loads, there is a lack of a method to convert massive amounts of micro-strain data into three-dimensional bending vectors (magnitude + direction).

[0024] Based on this, the core objective of this invention is to overcome the core shortcomings of existing underground steel-concrete composite pier monitoring technologies, such as lack of sensing dimensions, sensitivity to electromagnetic interference, and inability to reconstruct complex bending morphologies in real time under extreme environments. Addressing the challenge that traditional point-based monitoring methods in deep mine and tunnel engineering struggle to capture the eccentric compression and multi-axis composite bending evolution of piers, this invention provides an integrated system and method for monitoring the bearing capacity, bending direction, and degree of underground piers with full-dimensional coverage, high stability, and strong resolution. By achieving deep integration of sensing units and support structures, it solves the problem of online assessment of structural health status and early warning of asymmetric instability disasters in deep-earth engineering.

[0025] This invention scientifically integrates a strain-sensing fiber optic grating array inside a steel-concrete composite pier and utilizes the multiplexing characteristics of fiber optic sensing to establish an internal mechanical sensing network that runs through the entire pier. The aim is to accurately invert and reconstruct the stress transmission characteristics and bearing state of the pier's core area under complex loads. Simultaneously, in conjunction with a patch-type strain-sensing array deployed according to a specific structure on the outer wall of the pier's steel tube, this invention aims to capture information on the non-uniform bending deformation field of the support structure's surface.

[0026] Building upon this foundation, this invention, through the deep fusion of quasi-distributed strain data from inner and outer sensor networks and combined with a unique three-dimensional morphological reconstruction method, aims to overcome the limitations of traditional "point-to-surface" monitoring and achieve real-time digital characterization of axial compression, radial expansion pressure, and three-dimensional spatial bending degree and azimuth angle throughout the entire life cycle of the support column. The ultimate goal is to provide an intelligent sensing method for safety assessment of support structures in complex underground environments (high dust, high humidity, strong electromagnetic interference, confined space), capable of resisting environmental noise interference and accurately reproducing the structural mechanical evolution trajectory.

[0027] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0028] Example 1 like Figure 1 As shown, a monitoring device for a downhole steel-concrete pier using a composite fiber optic grating includes an embedded grating strain sensor 2 and a patch grating strain sensor 5, both of which are mounted on the steel-concrete pier. The steel-concrete pier comprises a steel pipe 4 and concrete 1 poured inside the steel pipe 4. An auxiliary reinforcing bar 3 is also provided at the center of the concrete and along the axial direction of the steel pipe.

[0029] Multiple embedded grating strain sensors 2 are arranged at intervals along the auxiliary reinforcing bars 3 to form embedded measurement groups. Multiple patch grating strain sensors 5 are arranged on the outer or inner wall of the steel pipe. All patch grating strain sensors 5 are divided into several patch measurement groups. The patch grating strain sensors in each patch measurement group are arranged at intervals along the axial direction of the steel pipe, and all patch measurement groups are arranged at intervals along the circumference of the steel pipe.

[0030] Both the embedded grating strain sensor 2 and the patch grating strain sensor 5 are connected to the optical switch 7 via the optical fiber 6, and the optical switch 7 is connected to the fiber optic grating demodulator 8.

[0031] The aforementioned embedded grating strain sensor 2 is welded or tied to the auxiliary reinforcing steel bar 3. The patch-type grating strain sensor 5 is welded to the steel pipe wall.

[0032] like Figure 2 As shown, the patch-type grating strain sensor is divided into three patch-type measurement groups, with adjacent patch-type measurement groups at a 120° angle and evenly distributed. Of course, the number of patch-type measurement groups can also be adjusted accordingly, such as dividing it into four patch-type measurement groups, with adjacent patch-type measurement groups at a 90° angle.

[0033] To better illustrate the specific arrangement of the patch-type grating strain sensor, a side view of the outer wall of the steel pipe is shown below. Figure 3 As shown.

[0034] The assembly method of the composite fiber optic grating monitoring device for underground steel pipe concrete piers of the present invention includes the following steps: (1) Weld or tie the embedded grating strain sensor 2 to the auxiliary steel bar 3 and record the position (axial height) of each embedded grating strain sensor 2. Connect each embedded grating strain sensor with a conductive optical fiber to form an embedded measurement group or axial stress sensing module for axial stress sensing and monitoring.

[0035] (2) The patch-type grating strain sensor 5 is welded to the surface of the steel pipe. All patch-type grating strain sensors are divided into three patch-type measurement groups, which are respectively arranged at 0°, 120° and 240° of the circumference of the steel pipe, as follows: Figure 2 As shown. Furthermore, the patch-type grating strain sensors at the same position on each patch-type measurement group should be at the same horizontal height; that is, the sensors at each horizontal height must be strictly parallel, such as... Figure 3 As shown.

[0036] Each embedded grating strain sensor and patch grating strain sensor is connected to the conductive optical fiber 6. Specifically, after welding, the conductive optical fibers are connected one by one, with each column as a unit. Silicone sealant is then applied to the surfaces of the conductive optical fibers and patch grating strain sensors for protection and cushioning.

[0037] (3) Weld the auxiliary steel bars in step (1) to the bottom surface of the steel pipe, set a sealing base plate on the bottom surface of the steel pipe, and lead out the optical fiber connected to the embedded grating strain sensor from the optical fiber guide placement hole 9 preset on the bottom surface of the steel pipe. Then, use concrete to pour the steel pipe to form a composite fiber grating underground steel pipe concrete pier.

[0038] (4) Connect the optical fibers that are connected to the embedded grating strain sensor and the patch grating strain sensor to the optical switch (1×4 optical switch) to control the automatic selection and demodulation of the sensing links.

[0039] (5) Connect the optical switch in step (4) to the fiber optic grating demodulator 8 to realize quasi-distributed acquisition of grating signals.

[0040] Example 2 A method for monitoring underground steel pipe concrete piers using composite fiber optic gratings, employing the monitoring device described in Example 1, includes the following steps for pressure monitoring: (1) Obtain the center wavelength shift of the embedded grating strain sensor using a fiber optic grating demodulator. ,in The labeling of the embedded grating strain sensor utilizes the wavelength shift-strain sensitivity coefficient of the embedded grating strain sensor itself. ,Right now To determine the sensitivity coefficient of the embedded grating strain sensor, the axial auxiliary steel bar strain distribution at each embedded grating strain sensor location is obtained. .

[0041] (2) Establish the auxiliary reinforcement strain and its axial stress Linear relationship between them: ; in, It is the elastic modulus of the auxiliary reinforcement; through this linear relationship, the axial stress distribution at each position of the auxiliary reinforcement can be obtained.

[0042] (3) Since the auxiliary reinforcement and concrete are in the same stress system and are closely connected, and the bond between them can effectively transfer stress, the axial bearing pressure at each position of the steel-concrete composite pier can be obtained through the following relationship: ; in, It is the elastic modulus of concrete.

[0043] The elastic modulus of the aforementioned auxiliary reinforcing bars and the elastic modulus of the concrete were uniformly calibrated before the monitoring device was assembled.

[0044] Example 3 A method for monitoring downhole steel pipe concrete piers using composite fiber optic gratings, employing the monitoring device described above, includes the following steps for bending monitoring: (1) Wavelength shift of patch grating strain sensor obtained by demodulation using fiber optic grating demodulator ,in Each column of sensors is labeled, with n=1, 2, 3 representing different column numbers, i.e., axial sensor columns located at 0°, 120°, and 240° positions on the circumference of the steel pipe. The wavelength shift-strain sensitivity coefficient of the patch-type grating strain sensor is utilized. ,Right now The sensitivity coefficient of the patch grating strain sensor is used to obtain the strain distribution at each patch grating strain sensor location. .

[0045] (2) The strain values ​​measured by three sets of patch-type grating strain sensors at the same height constitute one calculation unit. A total of 6 calculation units are formed along the steel-concrete composite pier, such as... Figure 1 , Figure 3 As shown, the bending direction and degree of the steel-concrete composite pier are inverted by using the strain distribution of these 6 sets of calculation units.

[0046] First, the curvature distribution of the steel-concrete composite pier is calculated. The distance from each row of patch-type optical grating strain sensors to the central axis of the steel-concrete composite pier is... The relationship between the radius of curvature and strain can be calculated using the following formula: ; In the formula, Let be the radius of curvature. The circumferential direction angle of the steel pipe is for different number of columns n, i.e. different axial sensing arrays n. When n=1, 2, 3, the values ​​are 0°, 120°, and 240° respectively.

[0047] The bending direction is obtained by calculating the bending direction angle of the steel-concrete composite pier, and the calculation formula is as follows: ; Differentiating the obtained bending direction angle yields the functional expression for shape torsion: ; Define three mutually orthogonal space vectors , respectively, are the binormal vectors of the curves represented by the steel-concrete composite piers. tangent vector and normal vector These three spatial vectors are connected by curvature and torsion, and satisfy the following relationship: ; In the formula, , and These are the differential processing corresponding to T, N, and B, respectively.

[0048] Assuming the top of the reinforced concrete pier column is the starting point The coordinates are (0,0,0), and the binormal vector is... Initial value [0,1,0], tangent vector Initial values ​​[1,0,0] and normal vector Initial value [0,0,1]. Calculate the coordinates of the lowest point using the following iterative equations. (The integral of the tangent vector gives the three-dimensional shape of the curve), thus completing the calculation of the three-dimensional shape and obtaining the direction and degree of curvature: ; Where m=1,2,3, … represents the label of the patch grating strain sensor counting from the top, and ds is the initial linear distance between two adjacent patch grating strain sensors in each column.

[0049] In summary, this invention focuses on real-time health monitoring of the core load-bearing component, "concrete-steel tube (CFST) piers," widely used in coal mines, metal mine goaf areas, and transportation tunnels. It integrates a quasi-distributed sensing technology, fiber optic grating arrays, and physically fuses a composite sensor network with the CFST structure to achieve real-time sensing of the axial bearing capacity, bending direction, and degree of the entire pier. The aim is to solve the problem of real-time data acquisition and early risk warning for dynamic disasters such as instability, eccentric compression, and unstable bending of support structures caused by changes in surrounding rock pressure in deep underground engineering.

[0050] The innovations of this invention are mainly reflected in the following aspects: 1. Integrated application of composite fiber optic grating sensors: By combining embedded grating strain sensors with patch grating strain sensors, and using a fiber optic grating demodulator and optical switch system, real-time monitoring of the compressive and bending states of intelligent underground steel-concrete piers can be achieved.

[0051] 2. Dual function of axial stress and bending monitoring: The embedded grating strain sensor is used to monitor axial stress, and the patch grating strain sensor is used for bending monitoring. The dual function improves the accuracy and reliability of structural health monitoring.

[0052] 3. Automatic selection and demodulation of fiber Bragg grating demodulation system: Using a combination of 1×4 optical switches and fiber Bragg grating demodulator, the sensor signals are automatically selected and demodulated to achieve quasi-distributed data acquisition and dynamic monitoring.

[0053] 4. The stress transfer ratio between steel reinforcement and concrete; by calculating the stress transfer ratio between steel reinforcement and concrete, the axial stress of concrete is calculated, thus solving the problem of stress distribution between steel reinforcement and concrete in the stress system.

[0054] 5. Three-dimensional spatial shape inversion: Based on multiple sets of sensor data, the three-dimensional spatial shape of the steel-concrete composite pier is inverted using the Frenet frame method to accurately obtain the bending direction and degree, thereby realizing the bending deformation analysis of the steel-concrete composite pier.

[0055] 6. Environmental Change Compensation Mechanism: The sensor is protected with silicone sealant, and the effects of temperature and environmental factors on fiber Bragg grating measurements are considered to ensure monitoring accuracy in complex downhole environments. Specifically, this invention coats the surface of the fiber Bragg grating sensor with silicone sealant, providing effective protection and buffering, enabling stable operation even in harsh downhole environments (such as high humidity and high temperature). This effectively avoids monitoring errors caused by environmental factors and enhances the adaptability and stability of the equipment.

[0056] Of course, the above description is only a preferred embodiment of the present invention. The present invention is not limited to the above-described embodiments. It should be noted that any equivalent substitutions or obvious modifications made by those skilled in the art under the guidance of this specification fall within the scope of this specification and should be protected by the present invention.

Claims

1. A monitoring device for underground steel pipe concrete piers using composite fiber optic gratings, characterized in that: It includes embedded grating strain sensors and patch grating strain sensors, both of which are installed on steel-concrete composite piers. The steel-concrete composite pier includes a steel pipe and concrete poured inside the steel pipe, with auxiliary reinforcing bars provided at the center of the concrete and along the axial direction of the steel pipe. Multiple embedded grating strain sensors are provided and arranged at intervals along the auxiliary reinforcing bars to form an embedded measurement group; Multiple patch-type grating strain sensors are provided and are all arranged on the steel pipe wall; all patch-type grating strain sensors are divided into several patch-type measurement groups, and the patch-type grating strain sensors in each patch-type measurement group are arranged at intervals along the axial direction of the steel pipe wall, and all patch-type measurement groups are arranged at intervals along the circumference of the steel pipe wall. Both the embedded grating strain sensor and the patch grating strain sensor are connected to an optical switch via a conductive optical fiber, and the optical switch is connected to a fiber optic grating demodulator.

2. The monitoring device for underground steel pipe concrete piers with composite fiber optic gratings according to claim 1, characterized in that: The embedded grating strain sensor is welded or tied to the auxiliary reinforcing steel; the patch grating strain sensor is welded to the steel pipe wall.

3. The monitoring device for underground steel pipe concrete piers with composite fiber optic gratings according to claim 2, characterized in that: The patch-type grating strain sensor is divided into three patch-type measurement groups, with adjacent patch-type measurement groups at a 120° angle.

4. The monitoring device for underground steel pipe concrete piers with composite fiber optic gratings according to claim 3, characterized in that, The assembly of the monitoring device includes the following steps: (1) Weld or tie the embedded grating strain sensor to the auxiliary steel bar and record the axial height of each embedded grating strain sensor; connect each embedded grating strain sensor with a conductive optical fiber to form an embedded measurement group for axial stress sensing and monitoring. (2) Weld the patch grating strain sensor to the surface of the steel pipe wall. All patch grating strain sensors are divided into three patch measurement groups, which are arranged at 0°, 120° and 240° of the circumference of the steel pipe respectively; and the patch grating strain sensors of the same position on each patch measurement group are at the same horizontal height. All embedded grating strain sensors and patch grating strain sensors are connected to optical fibers; (3) Weld the auxiliary steel bars in step (1) to the bottom surface of the steel pipe, and lead out the conductive optical fiber connected to the embedded grating strain sensor from the pre-set placement hole on the bottom surface of the steel pipe. Then, use concrete to pour the steel pipe to form a composite fiber grating underground steel pipe concrete pier. (4) Connect the optical fibers that are respectively connected to the embedded grating strain sensor and the patch grating strain sensor to the optical switch to control the automatic selection and demodulation of the sensing links. (5) Connect the optical switch in step (4) to the fiber optic grating demodulator to realize quasi-distributed acquisition of grating signals.

5. A method for monitoring downhole steel pipe concrete piers using composite fiber optic gratings, employing the monitoring device as described in any one of claims 1-4, characterized in that... Pressure monitoring methods include the following steps: (1) Obtain the center wavelength shift of the embedded grating strain sensor using a fiber optic grating demodulator. ,in The labeling of the embedded grating strain sensor utilizes the wavelength shift-strain sensitivity coefficient of the embedded grating strain sensor itself. The strain distribution of auxiliary steel bars at the locations of each embedded grating strain sensor was obtained. ; (2) Establish the strain of auxiliary steel bars and their axial stress Linear relationship between them: ； in, It is the elastic modulus of the auxiliary reinforcement; through this linear relationship, the axial stress distribution at each position of the auxiliary reinforcement can be obtained; (3) Since the auxiliary reinforcement and concrete are in the same stress system and the bond between them can effectively transfer stress, the axial bearing pressure at each position of the steel-concrete composite pier can be obtained through the following relationship: ； in, It is the elastic modulus of concrete.

6. The method for monitoring underground steel-concrete pier columns using composite fiber optic gratings according to claim 5, characterized in that: The elastic modulus of the auxiliary reinforcing bars and the elastic modulus of the concrete are uniformly calibrated before the monitoring device is assembled.

7. A method for monitoring downhole steel pipe concrete piers using composite fiber optic gratings, employing the monitoring device as described in any one of claims 1-4, characterized in that... The bending monitoring method includes the following steps: (1) Wavelength shift of patch grating strain sensor obtained by demodulation using fiber optic grating demodulator ,in Each column of sensors is labeled, with n=1, 2, 3 representing different column numbers, i.e., axial sensor columns located at 0°, 120°, and 240° positions on the circumference of the steel pipe; the wavelength shift-strain sensitivity coefficient of the patch-type grating strain sensor itself is utilized. The strain distribution at the location of each patch grating strain sensor was obtained. ; (2) The strain values ​​measured by three sets of patch grating strain sensors at the same height constitute a calculation unit, which is composed of a total of [number] components along the steel-concrete composite pier. This set of computing units, through this The strain distribution of the calculation unit is used to invert the bending direction and degree of the steel-concrete composite pier. First, the curvature distribution of the steel-concrete composite pier is calculated. The distance from each row of patch-type optical grating strain sensors to the central axis of the steel-concrete composite pier is... The relationship between the radius of curvature and strain can be calculated using the following formula: ； In the formula, Let be the radius of curvature. The circumferential direction angle of the steel pipe is for different column numbers n. When n=1, 2, 3, the values ​​are 0°, 120°, and 240° respectively. The bending direction is obtained by calculating the bending direction angle of the steel-concrete composite pier, and the calculation formula is as follows: ； Differentiating the obtained bending direction angle yields the functional expression for shape torsion: ； Define three mutually orthogonal space vectors , respectively, are the binormal vectors of the curves represented by the steel-concrete composite piers. tangent vector and normal vector These three spatial vectors are connected by curvature and torsion, and satisfy the following relationship: ； Assuming the top of the reinforced concrete pier column is the starting point The coordinates are (0,0,0), and the binormal vector is... Initial value [0,1,0], tangent vector Initial values ​​[1,0,0] and normal vector Initial value [0,0,1]; use the following iterative equations to calculate the coordinates of the bottommost point. The calculation of the three-dimensional shape is completed, and the direction and degree of curvature are obtained: ； Where m=1,2,3, … represents the label of the patch grating strain sensor counting from the top, and ds is the initial linear distance between two adjacent patch grating strain sensors in each column.

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