A surrounding rock loose zone monitoring method and system based on fiber anchor rod strain

Abstract

The application relates to a surrounding rock loose zone monitoring method and system based on optical fiber anchor rod strain, which comprises the following steps: temperature compensation is performed on strain distribution information according to temperature distribution information, and real anchor rod strain distribution data are obtained; the axial force distribution and the shear force distribution of the anchor rod are analyzed according to the anchor rod strain distribution data, and a cross-section shear stress distribution model is constructed; the strain gradient distribution along the axial direction of the anchor rod is analyzed according to the anchor rod shear stress distribution model; a loose zone boundary identification model is constructed according to the axial force distribution and the strain gradient distribution, the strain mutation point position of the anchor rod is determined, the surrounding rock loose zone and the elastic zone are characterized by the strain mutation point, the strain mutation points of the anchor rods are connected, the boundary contour line of the surrounding rock loose zone is formed, and the range of the surrounding rock loose zone is determined. The scheme provided by the application can realize high-precision positioning, dynamic tracking of the surrounding rock loose zone boundary and dynamic optimization of a tunnel supporting structure design.

Description

Technical Field

[0001] This application relates to the field of surrounding rock loosening zone measurement technology, and in particular to a method and system for monitoring surrounding rock loosening zone based on fiber optic anchor strain. Background Technology

[0002] In tunnel engineering, accurate determination of the loosened rock zone is of great significance for support design, construction safety, and operational stability. The loosened rock zone refers to the non-load-bearing area formed after tunnel excavation due to stress redistribution, resulting in damage to the surrounding rock structure and deterioration of its strength. Its extent directly affects the assessment and determination of key indicators in tunnel construction.

[0003] Currently, commonly used methods for determining loosened zones include acoustic / ultrasonic testing, ground-penetrating radar (GPR), apparent resistivity, transient electromagnetic methods, and integrated analysis methods based on displacement monitoring and numerical inversion. However, in practical applications, acoustic methods are easily affected by borehole quality and coupling conditions, and have limited testing depth; GPR has poor penetration and low resolution in water-bearing or highly conductive surrounding rocks; apparent resistivity and transient electromagnetic methods are not sensitive enough to shallow loosened zones, and data interpretation is complex and susceptible to interference. In tunnel engineering under complex and variable lithological conditions, existing methods for determining loosened zones lack the support of mechanical models for collaborative analysis with the support structure, making it difficult to obtain continuous, full-section strain and stress distribution.

[0004] In recent years, distributed fiber optic sensing technology has been gradually applied to the field of geotechnical engineering safety monitoring due to its advantages such as long distance, high spatial resolution, resistance to electromagnetic interference, and ability to achieve simultaneous temperature-strain monitoring. However, existing fiber optic monitoring schemes are mostly focused on apparent strain acquisition and lack deep integration with the interaction mechanism between anchor bolts and surrounding rock. A systematic dynamic identification of loosened zone boundaries based on strain gradient and axial force evolution has not yet been formed, resulting in low utilization of monitoring data. It is impossible to achieve high-precision positioning, dynamic tracking, and visualization of loosened zone boundaries in surrounding rock, and it is also difficult to support dynamic optimization of tunnel support structure design and construction safety early warning.

[0005] Therefore, this application proposes a method and system for monitoring loosened rock zones based on fiber optic anchor strain. By deploying strain-sensitive and temperature-compensating optical fibers in the anchor and combining the mechanical behavior analysis of the anchor, high-precision and automatic identification of the loosened zone boundary can be achieved. Summary of the Invention

[0006] To address or partially address the problems existing in related technologies, this application provides a method and system for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain. The aim is to solve the problem of how distributed optical fibers can accurately capture the mechanical abrupt change characteristics of the interface between the loosened zone and the elastic zone in the construction of tunnels in complex geological conditions.

[0007] The first aspect of this application provides a method for monitoring the loosening zone of surrounding rock based on fiber optic anchor strain. This method acquires information on the strain and temperature distribution of the surrounding rock through strain-sensitive and temperature-compensating optical fibers deployed in the tunnel's surrounding rock anchor support structure. The method includes: Temperature compensation is performed on the strain distribution information based on the temperature distribution information to obtain the true strain distribution data of the anchor bolt. Based on the anchor strain distribution data, analyze the axial force distribution and shear force distribution of the anchor, and construct a cross-sectional shear stress distribution model; Based on the anchor bolt shear stress distribution model, the strain gradient distribution along the anchor bolt axis is analyzed; A loosening zone boundary identification model is constructed based on axial force distribution and strain gradient distribution to determine the location of strain abrupt change points in the anchor bolt. These strain abrupt change points characterize the interface between the loosening zone and the elastic zone of the surrounding rock. The loosening zone boundary identification model includes: constructing a strain gradient extremum calculation model based on the strain gradient distribution to identify the extreme points of the strain gradient distribution; constructing an axial force attenuation transition model based on the axial force distribution to identify the transition points in the axial force distribution from a growth mode to a attenuation mode; and verifying the consistency between the extreme points of the strain gradient distribution and the transition points of the axial force distribution to determine the final location of the strain abrupt change points. By connecting the strain abrupt change points of each anchor bolt, the boundary contour line of the loosened rock zone is formed, and the extent of the loosened rock zone is determined.

[0008] Optional strain gradient extremum calculation models include: Shear stress is proportional to strain gradient, strain gradient The reversal occurs at the interface between the loosened and elastic zones; near the reversal point... The absolute value will have a local maximum, resulting in the strain gradient extremum calculation model: (14) In the formula, denoted by , x represents the extreme point of the anchor bolt strain gradient, x represents the axial position of the anchor bolt, and L represents the length of the anchor bolt.

[0009] Optional axial force attenuation transition models include: Based on the mechanical relationship between axial force and anchor strain distribution data, at the interface between the loosened zone and the elastic zone, the relative displacement direction between the surrounding rock and the anchor is reversed; the tendency of the anchor to be stretched and the constraint of the surrounding rock reach an instantaneous equilibrium. Within the loosened zone, the surrounding rock deformation drives the anchor bolt, and the axial force increases towards the depth of the surrounding rock; within the elastic zone, the anchor bolt is anchored, and the axial force decreases towards the depth; at the interface, the axial force inevitably changes from increasing to decreasing, with the extreme point being the maximum point, resulting in the axial force attenuation transition model: (15) In the formula, x represents the axial position of the anchor bolt. , , These represent the loosening zone, the elastic zone, and the interface, respectively. Indicates the axial force gradient. This represents the rate of change of the axial force gradient, since it is directly found Points in discrete data are easily affected by noise, so the second derivative is used to transform the maximum point into a more stable and easily identifiable turning point of the trend.

[0010] Optionally, the strain gradient distribution along the anchor bolt axis is analyzed, including: strain gradient It represents the rate of change of the axial strain of the anchor bolt with position, and serves as a bridge between the shear stress and the axial force variation in the cross section. Strain gradient analysis based on cross-sectional shear stress calculation model Distribution: In the loosened zone, the surrounding rock deforms into the tunnel, causing the anchor bolts to be under tension. The shear stress direction points into the tunnel, and the strain of the anchor bolt is positive when under tension, increasing from the inside of the tunnel to the depth of the tunnel, with a strain gradient. ; In the elastic zone, the surrounding rock is basically stable, the deformation of the anchor bolt is constrained, the shear stress direction points towards the depth of the surrounding rock, and the anchor bolt strain decreases with depth, resulting in a strain gradient. ; strain gradient The reversal occurs at the interface between the loosened and elastic zones; near the reversal point... The absolute value of it will have a local maximum.

[0011] Optionally, analyze the axial force distribution of the anchor bolt, including: Based on the mechanical relationship between axial force and anchor strain distribution data, the axial force distribution is determined by taking a small segment of the anchor. Perform force analysis, The difference in axial force experienced by the segment and the effect of the surrounding rock The shear stress on the surface of the segment is in equilibrium; at the interface between the loosened and elastic zones, the following characteristics exist: The relative displacement direction between the surrounding rock and the anchor bolt reverses; The tendency of the anchor bolt to be stretched reaches an instantaneous equilibrium with the constraint of the surrounding rock, and the cross-sectional shear stress between the anchor bolt and the surrounding rock is zero; Based on the interface characteristics between the loosened zone and the elastic zone, the first derivative of the axial force at the anchor bolt junction is 0, which is a necessary condition for the axial force to reach an extreme value at the junction. Within the loosened zone, the deformation of the surrounding rock drives the anchor bolts, and the axial force increases deeper into the surrounding rock. Within the elastic zone, the anchor rod is anchored, and the axial force decreases as it goes deeper. At the interface, the axial force will inevitably change from increasing to decreasing, and the extreme point is the maximum point.

[0012] Optionally, a cross-sectional shear stress distribution model of the anchor bolt is constructed, including: Based on the mechanical relationship between axial force and anchor strain, and by Hooke's Law, the axial force of the anchor... The relationship between the anchor bolt strain and the anchor bolt strain can be expressed as: (9) In the formula, E is the elastic modulus of the material, and A is the cross-sectional area of ​​the anchor rod; For anchor bolt micro-segments Force analysis was performed, and the equilibrium equations for shear force and axial force are as follows: (11) In the formula, Indicates the shear stress distribution of the cross section. Indicates the diameter of the anchor bolt; Since the anchor bolts are made of the same material and have a uniform cross-section, their elastic modulus E and cross-sectional area A are both constants. Therefore, we can conclude that: (12) The shear stress of the surrounding rock on the anchor bolt section is obtained as follows: (13) In the formula, E represents the elastic modulus of the anchor material, and A represents the cross-sectional area of ​​the anchor.

[0013] Optionally, obtain the actual anchor bolt strain distribution data, including: Optical frequency domain reflectometry utilizes coherent detection and a swept laser to invert the strain and temperature distribution along an optical fiber by measuring the phase change of the backscattered Rayleigh light. For a small perturbation, the wavelength shift of the Rayleigh scattering center at a point in the optical fiber is... With strain Temperature changes A linear relationship exists: (1) In the formula, The center wavelength of the incident light The effective elastic modulus of the optical fiber; The coefficient of thermal expansion; Thermo-optic coefficient; The coefficient of optical path length variation caused by strain; This refers to the wavelength shift coefficient caused by temperature. Since optical frequency domain reflection obtains phase information at spatial location through frequency sweeping interference, the relationship between local phase change and strain and temperature is as follows: (2) In the formula, Let be the phase change of the backscattered Rayleigh light in the optical fiber, and n be the effective refractive index of the optical fiber. The length of the sensing segment in the optical fiber where Rayleigh scattering occurs; In distributed measurements in practical engineering, a phase-strain sensitivity coefficient is introduced through calibration. Then the equivalent strain expression under any state is: (3) strain It includes both real strain and pseudo-strain caused by temperature changes, namely: (4) Among them, temperature cross-sensitivity coefficient for: (5) In summary, the strain and temperature change data obtained from the fiber optic anchor are expressed as follows: (6) In the formula, The strain distribution measured by the strain-sensitive fiber. The phase change measured by the strain-sensitive fiber. The strain distribution measured by temperature-compensated fiber optic cable based solely on temperature changes. To compensate for phase changes in optical fibers due to temperature; The actual anchor bolt strain distribution data are as follows: (7).

[0014] A second aspect of this application provides a monitoring system for loosened rock zones based on fiber optic anchor strain, comprising: Fiber optic anchors, distributed fiber optic demodulators, and smart terminals; The fiber optic anchor is encapsulated with distributed optical fibers, including strain-sensitive fibers and temperature-compensating fibers, which are used to sense strain and temperature along the axial direction of the anchor. The fiber optic anchor is driven into the surrounding rock in the traditional anchor installation method and is laid out along the circumference of the surrounding rock. A distributed fiber optic demodulator connects to a distributed fiber optic cable and is used to demodulate and acquire strain and temperature data distributed along the fiber optic cable. The intelligent terminal, which communicates with the distributed fiber optic demodulator, is used to receive and analyze strain and temperature data. Based on the mechanical analysis algorithm that reflects the interaction between the anchor bolt and the surrounding rock, it processes the strain distribution data. By performing a dual criterion including strain gradient extreme value analysis and axial force distribution analysis, it identifies strain abrupt change points. Based on the strain abrupt change points, it determines the boundary location information of the loosened zone of the surrounding rock.

[0015] Optional, smart terminals include: The boundary identification module performs strain gradient analysis, axial force analysis, identification of extreme points in strain gradient distribution, identification of inflection points in axial force distribution, and consistency verification operations, and outputs the location of strain abrupt change points for each fiber anchor. The contour generation and display module connects various strain abrupt change points to generate and visualize two-dimensional or three-dimensional boundary contour maps of the loosened surrounding rock zone.

[0016] The technical solution provided in this application may include the following beneficial effects: By using distributed fiber optic sensing technology, strain and temperature data are collected in real time along the entire length of the anchor bolt. Based on the interaction mechanism between the anchor bolt and the surrounding rock, the strain data is converted into mechanical indicators such as shear stress and axial force. Combining axial force distribution analysis and strain gradient distribution analysis, a dual-criteria boundary identification model is established based on the extreme value of strain gradient and the inflection point of axial force attenuation. The loosening zone boundary is comprehensively identified from two dimensions: the strain change trend and the stress state of the anchor bolt. This enables high-precision positioning, dynamic tracking, and dynamic optimization of the tunnel support structure design for the loosening zone boundary.

[0017] By using distributed fiber optic sensing technology, strain and temperature data are collected in real time, enabling continuous perception and dynamic feedback of the mechanical state of the surrounding rock. The system integrates data acquisition, processing, analysis and visualization modules to automatically complete strain compensation, axial force inversion, gradient calculation, abrupt change point identification and loosening zone contour generation, reducing the cost of manual intervention.

[0018] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0019] The above and other objects, features and advantages of this application will become more apparent from the more detailed description of exemplary embodiments thereof in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments thereof.

[0020] Figure 1 This is a schematic flowchart illustrating the method for monitoring loosened areas of surrounding rock as shown in the embodiments of this application; Figure 2 This is a schematic diagram of the structure of the monitoring system for loosened surrounding rock shown in the embodiments of this application; Figure 3 This is a schematic diagram of the anchor bolt layout of the monitoring system for loosened surrounding rock, as shown in the embodiments of this application; Figure 4 This is a schematic diagram of the anchor shear force direction in the monitoring method for loosened surrounding rock zones shown in the embodiments of this application; Figure 5 This is a schematic diagram of the fiber optic anchor strain curve of the monitoring method for loosened surrounding rock zone shown in the embodiments of this application.

[0021] Reference numerals in the attached diagram: 1. Fiber optic anchor; 2. Fiber optic channel; 3. Distributed fiber optic cable; 4. Through hole; 5. Communication fiber optic cable; 6. Fiber optic demodulator; 7. Transmission network; 8. Smart terminal. Detailed Implementation

[0022] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. While embodiments of this application are shown in the drawings, it should be understood that this application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0023] It should be understood that in the description of this application, the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0024] Unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0025] The technical solutions of the embodiments of this application are described in detail below with reference to the accompanying drawings.

[0026] In some implementations, see Figure 2 A monitoring system for loosened rock zones based on fiber optic anchor strain, comprising: 1. Fiber optic anchor; 2. Fiber optic trough; 3. Distributed fiber optic cable; 4. Through hole; 5. Communication fiber optic cable; 6. Fiber optic demodulator; 7. Transmission network; and 8. Smart terminal. The fiber optic anchor 1 is a combination of fiber optic cable and traditional anchor, consisting of three parts: a head, a sensing area, and a tail. The anchor is 3m long, with the head and tail each measuring 0.5m. The sensing area is located between 0.5m and 2.5m along the anchor. Fiber optic grooves 2 are etched into the anchor within the sensing area, with a width × depth of 2mm × 3mm. A through hole 4 is provided at the tail end of the anchor. The dimensions of the internal through hole and surface groove of the anchor should be sufficient to allow the fiber optic cable to pass through or meet encapsulation requirements; the dimensions should not be too large, as this would significantly reduce the anchor's strength.

[0027] Distributed optical fibers are installed on the fiber optic slot 2 of the pole in the sensing area. The distributed optical fibers consist of two complete strain-sensitive fibers and a temperature-compensating fiber. To ensure good coupling between the optical fibers and the anchor rod, a custom-made polyimide (PI) coated fiber is selected as the strain-sensitive fiber, and a loose-tube fiber is used for the temperature-compensating fiber. The sensing fiber is encapsulated in the fiber optic slot of the pole sensing area, and the remaining fiber serves as the communication fiber 5, passing through the end through-hole 4 and connecting to the distributed optical fiber demodulator 6.

[0028] Fiber optic anchor bolt deployment types such as Figure 3 As shown, the anchor installation method is the same as the traditional full-length bonded anchor installation method. The anchors are distributed in a circle along the tunnel. Fiber optic anchors 1 and ordinary anchors are arranged in a combination at intervals. Fiber optic anchors 1 are installed at the arch crown, arch shoulder, arch waist, and arch foot to ensure real-time perception of the entire cross-section of the tunnel surrounding rock. The spacing between the fiber optic anchors 1 and ordinary anchors is 900×900mm. The ordinary anchors are made of HRB400 steel bars with a diameter of 22mm and a length of 3000mm. Each anchor uses 2 rolls of MSZ23 / 50 resin cartridges, with an anchoring force of not less than 50kN.

[0029] The distributed fiber optic demodulator employs optical frequency domain reflectance (OFDR) technology with four channels to ensure simultaneous monitoring of strain and temperature information. The demodulator host transmits data to the workstation and is configured with transmission network 7 to synchronize the data to the smart terminal 8. Transmission network 7 can be either wireless or fiber optic, depending on site conditions.

[0030] The intelligent terminal 8 includes a boundary recognition module and a contour generation and display module; the boundary recognition module performs strain gradient analysis, axial force analysis, identification of extreme points of strain gradient distribution, identification of inflection points of axial force distribution and consistency verification operations, and outputs the location of strain abrupt change points corresponding to each fiber anchor.

[0031] The contour generation and display module connects various strain abrupt change points to generate and visualize two-dimensional or three-dimensional boundary contour maps of the loosened surrounding rock zone.

[0032] In some implementations, corresponding to the aforementioned application function implementation system embodiments, this application also provides a method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain and corresponding embodiments.

[0033] See Figure 1 A method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain, comprising: S101. Perform temperature compensation on the strain distribution information based on the temperature distribution information to obtain the real anchor bolt strain distribution data; Specifically, optical frequency domain reflectometry uses coherent detection and a swept laser to invert the strain and temperature distribution along the optical fiber by measuring the phase change of the backscattered Rayleigh light. For a small perturbation, the wavelength shift of the Rayleigh scattering center at a point in the optical fiber is... With strain Temperature changes A linear relationship exists: (1) In the formula, The center wavelength of the incident light The effective elastic modulus of the optical fiber is 0.22 in this application; The coefficient of thermal expansion is taken as 0.55×10⁻⁶ / ℃ in this application; The thermo-optic coefficient is taken as 8.6×10⁻⁶ / ℃ in this application; The coefficient for the change in optical path length caused by strain is 0.78 in this application; The wavelength shift coefficient caused by temperature is approximately 9.15 × 10⁻⁶ / ℃; Since optical frequency domain reflection obtains phase information at spatial location through frequency sweeping interference, the relationship between local phase change and strain and temperature is as follows: (2) In the formula, Let be the phase change of the backscattered Rayleigh light in the optical fiber, and n be the effective refractive index of the optical fiber. The length of the sensing segment in the optical fiber where Rayleigh scattering occurs; In distributed measurements in practical engineering, a phase-strain sensitivity coefficient is introduced through calibration. ,unit Then the equivalent strain expression under any state is: (3) strain It includes both real strain and pseudo-strain caused by temperature changes, namely: (4) Among them, temperature cross-sensitivity coefficient for: (5) In summary, the strain and temperature change data obtained from the fiber optic anchor are expressed as follows: (6) In the formula, The strain distribution measured by the strain-sensitive fiber. The phase change measured by the strain-sensitive fiber. The strain distribution measured by temperature-compensated fiber optic cable based solely on temperature changes. To compensate for phase changes in optical fibers due to temperature; The actual anchor bolt strain distribution data are as follows: (7).

[0034] S102. Analyze the axial force distribution and shear force distribution of the anchor bolt based on the anchor bolt strain distribution data, and construct a cross-sectional shear stress distribution model; Based on the mechanical relationship between axial force and anchor strain, and by Hooke's law, the anchor strain... With axial force The relationship can be expressed by Hooke's Law as follows: (8) In the formula, E represents the elastic modulus of the anchor material, and x represents the axial position of the anchor. Anchor bolt axial force The relationship between the anchor bolt strain and the anchor bolt strain can be expressed as: (9) In the formula, A is the cross-sectional area of ​​the anchor rod; For anchor bolt micro-segments Force analysis was performed, and the equilibrium equations for shear force and axial force are as follows: (10) After sorting, we can obtain: (11) In the formula, Indicates the shear stress distribution of the cross section. Indicates the diameter of the anchor bolt; Since the anchor bolts are made of the same material and have a uniform cross-section, their elastic modulus E and cross-sectional area A are both constants. Therefore, we can conclude that: (12) The shear stress of the surrounding rock on the anchor bolt section is obtained as follows: (13) That is, the shear stress distribution model of the anchor bolt section.

[0035] Based on the mechanical relationship between axial force and anchor strain distribution data, a small segment of the anchor is taken. Perform force analysis, Axial force difference of segment Acting with the surrounding rock Shear stress on the surface of the segment Phase equilibrium; at the interface between the loose zone and the elastic zone It has the following characteristics: a. The relative displacement direction between the surrounding rock and the anchor bolt reverses; b. The tendency of the anchor bolt to be stretched reaches an instantaneous equilibrium with the constraint of the surrounding rock. There is no relative sliding tendency between the two, and the cross-sectional shear stress between the anchor bolt and the surrounding rock is... It is zero.

[0036] Based on the interface characteristics between the loosened and elastic zones, the first derivative of the axial force at the anchor bolt junction... , is axial force Necessary conditions for obtaining extreme values ​​(maximum or minimum values) at the boundary points; like Figure 4 As shown, within the loosened zone, the deformation of the surrounding rock drives the anchor bolt, and the axial force increases towards the depth of the surrounding rock; Within the elastic zone, the anchor rod is anchored, and the axial force decreases as it goes deeper. At the interface The axial force will inevitably change from increasing to decreasing, and the extreme point is the maximum point.

[0037] S103. Based on the anchor bolt shear stress distribution model, analyze the strain gradient distribution along the anchor bolt axis; Specifically, based on the cross-sectional shear stress Relationship with strain gradient Calculate the strain gradient distribution along the anchor bolt axis. Strain gradient It represents the rate of change of axial strain of the anchor bolt with position, and serves as a bridge between the shear stress and axial force changes in the cross section.

[0038] In the loosened zone, the surrounding rock deforms into the tunnel, causing the anchor bolts to be under tension. The shear stress is in the same direction as the deformation of the surrounding rock, resulting in the axial force of the anchor bolts increasing deeper into the tunnel. The direction of the shear stress points into the tunnel. At this point, the shear stress... Based on the physical process, the strain of the anchor rod is positive when it is under tension, and the strain gradient is... .

[0039] In the elastic zone, the surrounding rock is basically stable, the deformation of the anchor bolt is constrained, and the shear stress is... The shear stress direction points deep into the surrounding rock, and the anchor bolt strain decreases with depth, resulting in a strain gradient. The strain value decreases.

[0040] Due to shear stress With strain gradient Proportional to strain gradient At the interface between the loose zone and the elastic zone A reversal is inevitable at this point, that is... exist The sign changes at the point of reversal, near the inversion point. The absolute value of the strain curve will exhibit a local maximum, which is the point where the slope of the strain curve changes most drastically. The abrupt change in the mechanical state causes the strain rate to peak in this region, such as... Figure 5 As shown.

[0041] S104. Construct a boundary identification model for the loosened zone based on the axial force distribution and strain gradient distribution, determine the location of the strain abrupt change point of the anchor bolt, and characterize the interface between the loosened zone and the elastic zone of the surrounding rock through the strain abrupt change point; Specifically, the loosened zone boundary identification model includes: Based on the fact that shear stress is proportional to strain gradient, strain gradient The reversal occurs at the interface between the loosened and elastic zones; near the reversal point... The absolute value will have a local maximum, resulting in the strain gradient extremum calculation model: (14) In the formula, denoted by , x represents the extreme point of the anchor bolt strain gradient, x represents the axial position of the anchor bolt, and L represents the length of the anchor bolt.

[0042] Based on the strain gradient extremum calculation model, the extreme points of the strain gradient distribution are identified.

[0043] Based on the mechanical relationship between axial force and anchor strain distribution data, at the interface between the loosened and elastic zones, the relative displacement direction between the surrounding rock and the anchor reverses, and the tendency of the anchor to be stretched reaches an instantaneous equilibrium with the constraint of the surrounding rock. In the loosened zone, the deformation of the surrounding rock drives the anchor, and the axial force increases towards the depth of the surrounding rock; in the elastic zone, the anchor is anchored, and the axial force decreases towards the depth; at the interface, the axial force inevitably changes from increasing to decreasing, with the extreme point being the maximum point. Therefore, the axial force attenuation transition model is obtained as follows: (15) In the formula, x represents the axial position of the anchor bolt. , , These represent the loosening zone, the elastic zone, and the interface, respectively. Indicates the axial force gradient. This represents the rate of change of the axial force gradient, since it is directly found Points in discrete data are easily affected by noise, so the second derivative is used to transform the maximum point into a more stable and easily identifiable turning point of the trend.

[0044] Based on the axial force decay transition model, the inflection point in the axial force distribution that changes from a growth mode to a decay mode is identified.

[0045] The extreme points of the strain gradient distribution and the inflection points of the axial force distribution are verified to determine the final location of the strain abrupt change point.

[0046] S105. Connect the strain abrupt change points of each anchor bolt to form the boundary outline of the loosened rock zone and determine the extent of the loosened rock zone.

[0047] Specifically, the location of the strain abrupt change point of each fiber optic anchor is converted from local coordinates along the anchor axis to two-dimensional coordinates in a cross-sectional coordinate system based on the tunnel axis. Based on the two-dimensional coordinates of multiple strain abrupt change points, a continuous boundary contour line of the loosened surrounding rock is generated through a spatial interpolation algorithm. The boundary contour line is superimposed on the tunnel cross-sectional design drawing, and different colors or fill patterns are used to distinguish between the loosened area and the elastic area, generating and visualizing a two-dimensional map of the loosened surrounding rock area.

[0048] The two-dimensional boundary contours of each section are connected along the tunnel axis, and a three-dimensional spatial distribution model of the loosened surrounding rock zone is generated by surface fitting or volume rendering techniques.

[0049] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for monitoring loosened zones in surrounding rock based on fiber optic anchor strain, which acquires information on the strain and temperature distribution of the surrounding rock through strain-sensitive optical fibers and temperature-compensating optical fibers deployed in the anchor support structure of the tunnel surrounding rock, characterized in that... include: Temperature compensation is performed on the strain distribution information based on the temperature distribution information to obtain the actual anchor bolt strain distribution data; Based on the anchor strain distribution data, the axial force and shear force distribution of the anchor are analyzed, and a shear stress distribution model of the anchor section is constructed. This shear stress distribution model includes, based on the mechanical relationship between axial force and anchor strain, Hooke's law, the anchor axial force... The relationship between the anchor bolt strain and the anchor bolt strain can be expressed as: （9） In the formula, E is the elastic modulus of the material, and A is the cross-sectional area of ​​the anchor rod; For anchor bolt micro-segments Force analysis was performed, and the equilibrium equations for shear force and axial force are as follows: （11） In the formula, Indicates the shear stress distribution of the cross section. Indicates the diameter of the anchor bolt; Since the anchor bolts are made of the same material and have a uniform cross-section, their elastic modulus E and cross-sectional area A are both constants. Therefore, we can conclude that: （12） The shear stress of the surrounding rock on the anchor bolt section is obtained as follows: （13） In the formula, E represents the elastic modulus of the anchor material, and A represents the cross-sectional area of ​​the anchor. Based on the shear stress distribution model of the anchor section, the strain gradient distribution along the anchor axis is analyzed; A loosening zone boundary identification model is constructed based on the axial force distribution and strain gradient distribution to determine the location of the strain abrupt change point of the anchor bolt. The strain abrupt change point characterizes the interface between the loosening zone and the elastic zone of the surrounding rock. The loosening zone boundary identification model includes: constructing a strain gradient extreme value calculation model based on the strain gradient distribution to identify the extreme points of the strain gradient distribution. Based on the axial force distribution, an axial force attenuation transition model is constructed to identify the transition point in the axial force distribution from a growth mode to a attenuation mode; the extreme points of the strain gradient distribution are verified to match the transition points of the axial force distribution to determine the final location of the strain abrupt change point. By connecting the strain abrupt change points of each anchor bolt, the boundary contour line of the loosened rock zone is formed, and the extent of the loosened rock zone is determined.

2. The method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain according to claim 1, characterized in that, The strain gradient extremum calculation model includes: Shear stress is proportional to strain gradient, strain gradient The reversal occurs at the interface between the loosened and elastic zones; near the reversal point... The absolute value will have a local maximum, resulting in the strain gradient extremum calculation model: （14） In the formula, denoted by , x represents the extreme point of the anchor bolt strain gradient, x represents the axial position of the anchor bolt, and L represents the length of the anchor bolt.

3. The method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain according to claim 1, characterized in that, The axial force attenuation transition model includes: Based on the mechanical relationship between axial force and the strain distribution data of the anchor bolt, at the interface between the loosened zone and the elastic zone, the relative displacement direction between the surrounding rock and the anchor bolt reverses; the tendency of the anchor bolt to be stretched and the constraint of the surrounding rock reach an instantaneous equilibrium. Within the loosened zone, the surrounding rock deformation drives the anchor bolt, and the axial force increases towards the depth of the surrounding rock; within the elastic zone, the anchor bolt is anchored, and the axial force decreases towards the depth; at the interface, the axial force inevitably changes from increasing to decreasing, with the extreme point being the maximum point, resulting in the axial force attenuation transition model: （15） In the formula, x represents the axial position of the anchor bolt. , , These represent the loosening zone, the elastic zone, and the interface, respectively. Indicates the axial force gradient. This represents the rate of change of the axial force gradient, since it is directly found Points in discrete data are easily affected by noise, so the second derivative is used to transform the maximum point into a more stable and easily identifiable turning point of the trend.

4. The method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain according to claim 1, characterized in that, The analysis of the strain gradient distribution along the anchor bolt axis includes: strain gradient It represents the rate of change of the axial strain of the anchor bolt with position, and serves as a bridge between the shear stress and the axial force variation in the cross section. Strain gradient analysis based on cross-sectional shear stress calculation model Distribution: In the loosened zone, the surrounding rock deforms into the tunnel, causing the anchor bolts to be under tension. The shear stress direction points into the tunnel, and the strain of the anchor bolt is positive when under tension, increasing from the inside of the tunnel to the depth of the tunnel, with a strain gradient. ; In the elastic zone, the surrounding rock is basically stable, the deformation of the anchor bolt is constrained, the shear stress direction points towards the depth of the surrounding rock, and the anchor bolt strain decreases with depth, resulting in a strain gradient. ; strain gradient The reversal occurs at the interface between the loosened and elastic zones; near the reversal point... The absolute value of it will have a local maximum.

5. The method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain according to claim 1, characterized in that, Analysis of the axial force distribution in the anchor bolts includes: Based on the mechanical relationship between axial force and the strain distribution data of the anchor bolt, the axial force distribution is determined, and a small segment of the anchor bolt is taken. Perform force analysis, The difference in axial force experienced by the segment and the action of the surrounding rock The shear stress on the surface of the segment is in equilibrium; at the interface between the loosened and elastic zones, the following characteristics exist: The relative displacement direction between the surrounding rock and the anchor bolt reverses; The tendency of the anchor bolt to be stretched reaches an instantaneous equilibrium with the constraint of the surrounding rock, and the cross-sectional shear stress between the anchor bolt and the surrounding rock is zero; Based on the interface characteristics between the loosened zone and the elastic zone, the first derivative of the axial force at the anchor bolt junction is 0, which is a necessary condition for the axial force to reach an extreme value at the junction. Within the loosened zone, the deformation of the surrounding rock drives the anchor bolts, and the axial force increases deeper into the surrounding rock. Within the elastic zone, the anchor rod is anchored, and the axial force decreases as it goes deeper. At the interface, the axial force will inevitably change from increasing to decreasing, and the extreme point is the maximum point.

6. The method for monitoring the loosened zone of surrounding rock based on fiber optic anchor strain according to claim 1, characterized in that, The acquisition of actual anchor bolt strain distribution data includes: Optical frequency domain reflectometry utilizes coherent detection and a swept laser to invert the strain and temperature distribution along an optical fiber by measuring the phase change of the backscattered Rayleigh light. For a small perturbation, the wavelength shift of the Rayleigh scattering center at a point in the optical fiber is... With strain Temperature changes A linear relationship exists: （1） In the formula, The center wavelength of the incident light The effective elastic modulus of the optical fiber; The coefficient of thermal expansion; Thermo-optic coefficient; The coefficient of optical path length variation caused by strain; This refers to the wavelength shift coefficient caused by temperature. Since optical frequency domain reflection obtains phase information at spatial location through frequency sweeping interference, the relationship between local phase change and strain and temperature is as follows: （2） In the formula, Let be the phase change of the backscattered Rayleigh light in the optical fiber, and n be the effective refractive index of the optical fiber. The length of the sensing segment in the optical fiber where Rayleigh scattering occurs; In distributed measurements in practical engineering, a phase-strain sensitivity coefficient is introduced through calibration. Then the equivalent strain expression under any state is: （3） strain It includes both real strain and pseudo-strain caused by temperature changes, namely: （4） Among them, temperature cross-sensitivity coefficient for: （5） In summary, the strain and temperature change data obtained from the fiber optic anchor are expressed as follows: （6） In the formula, The strain distribution measured by the strain-sensitive fiber. The phase change measured by the strain-sensitive fiber. The strain distribution measured by temperature-compensated fiber optic cable based solely on temperature changes. To compensate for phase changes in optical fibers due to temperature; The actual anchor bolt strain distribution data are as follows: （7）。 7. A monitoring system for loosened rock zones based on fiber optic anchor strain, used to execute the monitoring method for loosened rock zones based on fiber optic anchor strain according to any one of claims 1-6, characterized in that, include: Fiber optic anchors, distributed fiber optic demodulators, and smart terminals; The fiber optic anchor is encapsulated with distributed optical fibers, including strain-sensitive fibers and temperature-compensating fibers, for sensing strain and temperature along the axial direction of the anchor. The fiber optic anchor is driven into the surrounding rock in the traditional anchor installation method and is arranged around the circumference of the surrounding rock. A distributed optical fiber demodulator, connected to the distributed optical fiber, is used to demodulate and acquire strain and temperature data distributed along the optical fiber. The intelligent terminal, which is communicatively connected to the distributed optical fiber demodulator, is used to receive and analyze the strain and temperature data, process the strain distribution data based on a mechanical analysis algorithm that reflects the interaction between the anchor bolt and the surrounding rock, identify strain abrupt change points by performing a dual-criteria criterion including strain gradient extreme value analysis and axial force distribution analysis, and determine the boundary location information of the loosened zone of the surrounding rock based on the strain abrupt change points.

8. The monitoring system for loosened surrounding rock based on fiber optic anchor strain according to claim 7, characterized in that, The smart terminal includes: The boundary identification module performs strain gradient analysis, axial force analysis, identification of extreme points in strain gradient distribution, identification of inflection points in axial force distribution, and consistency verification operations, and outputs the location of strain abrupt change points for each fiber anchor. The contour generation and display module connects various strain abrupt change points to generate and visualize two-dimensional or three-dimensional boundary contour maps of the loosened surrounding rock zone.

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