An LC resonance-based tunnel surrounding rock absolute displacement monitoring system and method

Abstract

The application discloses a kind of LC resonance-based tunnel surrounding rock absolute displacement monitoring system and method, will reference anchor rod deep anchoring in surrounding rock loose circle outside, establish absolute immovable space reference;Iron core is fixed to the end of reference anchor rod;Design an integrated protection installation box, pre-buried in primary support concrete, box bottom is inserted for iron core and reference anchor rod, box side wall is provided with clamping slot or screw hole, for installing replaceable sensor module (built-in inductance coil).When primary support moves with surrounding rock deformation, integrated protection box and replaceable sensor module (inductance coil) move with primary support, while iron core is fixed, thereby generating relative displacement, changing inductance of coil, causing LC resonance frequency change.Obtain the absolute displacement of surrounding rock accurately by detecting frequency change.The whole system is completely hidden in integrated protection box, no exposed components, completely avoid construction damage, while realizing the recycling use and full-automatic data acquisition of replaceable sensor module.

Description

Technical Field

[0001] This invention relates to the field of tunnel and underground engineering surrounding rock deformation monitoring technology, specifically to a tunnel surrounding rock absolute displacement monitoring system and method based on LC resonance. Background Technology

[0002] In the drill-and-blast method of railway tunnel construction, monitoring and measurement are core aspects of ensuring construction safety. Current specifications require full-section monitoring of items such as crown settlement and perimeter convergence, with monitoring points spaced at intervals (30m for Class III surrounding rock, 10m for Class IV, and 5m for Class V). However, on-site practice faces the following long-standing unresolved problems: (i) The survival rate of monitoring points is low, and the amount of repair work is huge. Traditionally, monitoring points are installed by directly exposing them with a reinforced steel base and reflector, at a depth of 50-70 cm. During drill-and-blast construction, flying rocks and construction vehicles frequently impact the exposed monitoring points. Statistics show that the damage rate of monitoring points generally exceeds 70%. Repair work requires the assistance of aerial work platforms or scaffolds; repairing a single point is time-consuming, and delays are often caused by untimely equipment scheduling, resulting in prolonged interruptions in monitoring data.

[0003] (ii) The use of sprayed concrete caused false deformation and failure of measuring points Frequent touch-up spraying is required due to the unevenness of the initial concrete surface. The thickness of the touch-up spray layer is usually more than 5cm, which can damage the crosshairs of the reflector and reduce its accuracy, or cause it to be completely covered by a thick layer of concrete.

[0004] (iii) Automated monitoring is difficult to implement. Existing automation methods have significant shortcomings: Total station + prism: Prisms are expensive to use and costly to repair if damaged; they require optical visibility and are severely affected by dust and moisture in tunnels. Machine vision: relies on targets and lighting; dust concentration > 10 mg / m³ 3 The recognition rate drops by more than 50%, and the target will also be damaged during construction. 3D laser scanning: The equipment is expensive (300,000 to 800,000 yuan / unit), the data post-processing is complicated, it cannot distinguish between the thickening of the sprayed concrete and the actual deformation, and it cannot achieve continuous real-time monitoring. Wired sensors (such as fiber optic gratings): The cables are easily damaged during the initial concrete spraying process, with a survival rate of less than 60%, and are susceptible to electromagnetic interference in the tunnel, causing data distortion.

[0005] (iv) There are prominent contradictions between the standard requirements and the actual situation on site. The standard requires that "the depth of the measuring point into the rock shall not be less than 30cm". In addition, considering the thickness of the initial support and the influence of over-excavation, the length of the measuring pile shall not be less than 55cm, making drilling and installation difficult.

[0006] Therefore, there is an urgent need for a new monitoring technology that does not rely on exposed components, can directly measure the absolute displacement of the surrounding rock, is not affected by construction, and can operate fully automatically. Summary of the Invention

[0007] In view of this, in order to solve the above-mentioned problems in the prior art, the present invention proposes a tunnel surrounding rock absolute displacement monitoring system and method based on LC resonance. The system uses a reference anchor to establish an absolutely stationary benchmark, and measures the surrounding rock displacement by changing the LC resonance frequency through the relative movement of the fixed iron core and the moving coil. The entire system is completely enclosed in an integrated protective box with no exposed parts, thus completely avoiding construction damage. At the same time, it enables the cyclic use of sensor modules and fully automatic data acquisition.

[0008] The present invention solves the above problems through the following technical means: On the one hand, the present invention provides a tunnel surrounding rock absolute displacement monitoring system based on LC resonance, including a reference anchor, an iron core, an integrated protection box, a replaceable sensor module and a data processing platform; The reference anchor is anchored in the surrounding rock borehole, and its anchoring depth exceeds the range of the loosening zone of the surrounding rock. The end is provided with an internal thread interface and an anti-rotation keyway. One end of the iron core is provided with an external threaded interface and an anti-rotation keyway, which is rigidly connected to the internal threaded interface at the end of the reference anchor rod. Its outer diameter matches the inner diameter of the inductor coil in the replaceable sensor module. The length of the iron core is determined according to the expected maximum displacement. After the iron core is connected to the reference anchor rod, the whole is fixed in place. The integrated protective box has an open bottom, allowing the iron core and reference anchor rod to be inserted into the box from the bottom. The top cover can be flipped up and locked and sealed. The cover surface is flush with the surface of the initial support. The side wall of the box is provided with a slot or internal thread hole for installing a replaceable sensor module. The entire box is pre-embedded in the concrete of the initial support. When spraying grout, the concrete wraps around the outer wall of the box, and the inside of the box remains a cavity. The replaceable sensor module includes a housing, an inductor coil, a resonant capacitor, a rechargeable battery, a wireless communication unit, an MCU control unit, and a temperature sensor. The outer casing sidewall is provided with a buckle or external thread, which matches the slot or internal thread hole of the integrated protective box sidewall. The front end of the outer casing has an opening for the iron core to be inserted. The inductor coil is wound axially along the inner wall of the outer casing, and the number of turns and wire diameter are determined according to the design frequency. The coil axis coincides with the outer casing axis. The resonant capacitor and the inductor are connected in series to form an LC resonant circuit; The temperature sensor is used to read the current temperature so that frequency temperature drift compensation can be performed subsequently. The MCU control unit is used to read the current frequency; The wireless communication unit is used to transmit the current frequency and current temperature; The data processing platform is used to receive the current frequency and current temperature, and convert the frequency offset into absolute displacement according to the pre-calibrated frequency-displacement curve and temperature compensation coefficient.

[0009] Preferably, the data processing platform is also used to generate absolute displacement-time curves and automatically alarm when exceeding limits.

[0010] Preferably, the data processing platform converts the frequency offset into an absolute displacement using the following method: Inductance change mechanism: Let the initial depth of the iron core inserted into the inductor coil be x0, corresponding to the initial inductance L0. When the initial support moves Δd towards the tunnel clearance direction, the replaceable sensor module moves Δd with the initial support, while the iron core remains stationary. Therefore, the depth of the iron core inserted into the inductor coil becomes x = x0 ± Δd, with the sign determined by the direction of displacement: when the initial support moves towards the interior of the surrounding rock, the insertion depth of the iron core increases; when it moves towards the tunnel clearance direction, the insertion depth decreases. There is a monotonic function relationship between the inductance L of the inductor coil and the insertion depth x of the iron core. Therefore, the initial support displacement Δd causes a change in the insertion depth of the iron core Δx, which in turn causes a change in the inductance ΔL, and ultimately causes a frequency offset Δf. LC resonant frequency variation: The basic formula for an LC resonant circuit is: C is the capacitance of the resonant capacitor. When the inductance L changes, the resonant frequency f0 shifts. There is a monotonic mapping relationship between the frequency shift Δf and the core displacement Δx, i.e., the initial support displacement Δd. The function Δd=Φ(Δf) is obtained through experimental calibration. When the core moves a distance of millimeters within the inductor coil, the resonant frequency changes by several megahertz, thus achieving high-resolution displacement measurement. Temperature compensation: The replaceable sensor module has a built-in temperature sensor that collects ambient temperature in real time and corrects the frequency according to a pre-calibrated temperature drift coefficient: f d =f t -α·(T t -T0), where f d For the corrected frequency, f t The current frequency is T, α is the temperature drift coefficient, and T is the current frequency. t T1 is the current temperature, and T0 is the initial temperature; Absolute displacement calculation: During initial installation, the system records the initial resonant frequency f0 and the initial temperature T0; at any monitoring time t, the data processing platform reads the current frequency f0. t and the current temperature T t The corrected frequency f is obtained after temperature correction. d Δf=f d- f0, substitute into the calibration function Δd=Φ(Δf) to obtain the current displacement, which is the absolute displacement of the initial support measuring point relative to the absolute stationary reference.

[0011] Preferably, the tunnel surrounding rock absolute displacement monitoring system based on LC resonance further includes a base station, which is used to forward the current frequency and current temperature sent by the wireless communication unit to the data processing platform.

[0012] Preferably, the tunnel surrounding rock absolute displacement monitoring system based on LC resonance further includes a PVC sleeve, which is sleeved on the reference anchor rod body and can slide relative to the reference anchor rod.

[0013] Preferably, the reference anchor is made of high-strength threaded steel, and its length is determined according to the depth of the loosened zone of the surrounding rock.

[0014] Preferably, the iron core is made of a cylindrical material with high magnetic permeability.

[0015] Preferably, the integrated protective box is made of high-strength metal or engineering plastic, and is in the shape of a cube or cuboid. The box dimensions are 100-150mm in length, width and height, and the wall thickness is 3-5mm. Cable or antenna outlets are reserved on other side walls of the box and sealed.

[0016] Preferably, the base station is located in the safety zone of the secondary lining of the tunnel or at the tunnel entrance.

[0017] On the other hand, the present invention provides a method for monitoring the absolute displacement of tunnel surrounding rock based on LC resonance, comprising the following steps: The reference anchor is deeply anchored outside the loosened zone of the surrounding rock to establish an absolutely immovable spatial reference; the iron core is fixed to the end of the reference anchor. Design an integrated protective box with a flip-up top cover that is embedded in the initial support concrete. The bottom of the box is for inserting an iron core and a reference anchor rod. The side wall of the box is provided with a slot or internal thread hole for installing a replaceable sensor module. The replaceable sensor module includes a housing, an inductor coil, a resonant capacitor, a rechargeable battery, a wireless communication unit, an MCU control unit, and a temperature sensor. The housing sidewall has snaps or external threads that match the slots or internal threaded holes on the sidewall of the integrated protective box. The front of the housing has an opening for inserting an iron core. The inductor coil is wound axially along the inner wall of the housing; the number of turns and wire diameter are determined according to the design frequency, and the coil axis coincides with the housing axis. The resonant capacitor is connected in series with the inductor coil to form an LC resonant circuit. The temperature sensor reads the current temperature for subsequent frequency-temperature drift compensation. The MCU control unit reads the current frequency. The wireless communication unit transmits the current frequency and current temperature. When the initial support moves with the deformation of the surrounding rock, the integrated protection box and replaceable sensor module move with the initial support, while the iron core remains stationary, thus generating relative displacement. The relative displacement between the inductor coil and the iron core changes the inductance of the coil, causing a change in the LC resonant frequency. The data processing platform receives the current frequency and current temperature, and converts the frequency offset into absolute displacement according to the pre-calibrated frequency-displacement curve and temperature compensation coefficient.

[0018] Compared with the prior art, the beneficial effects of the present invention include at least the following: 1. Absolute displacement measurement: By establishing an absolutely stationary benchmark through deep anchoring reference anchors, the absolute displacement of the initial support relative to the deep surrounding rock is directly output, solving the benchmark drift problem of existing methods.

[0019] 2. Completely concealed, resistant to construction damage: The integrated protective box is pre-embedded in the initial support concrete, and the sensor module is installed inside the box. The top cover is flush with the surface of the initial support, so flying rocks from blasting, construction vehicles, and sprayed concrete cannot damage the measuring points. The survival rate of the measuring points can reach over 95%.

[0020] 3. Simple structure and easy installation: The integrated protective box integrates the three functions of pre-drilled holes, base and protective cover into a prefabricated component. Only the box body needs to be pre-embedded on site, without the need for secondary positioning, which greatly reduces the complexity of construction.

[0021] 4. High precision: The LC resonant frequency detection accuracy can reach the kHz level, with a corresponding displacement resolution better than 0.1mm. The overall system accuracy is ±2~3mm, meeting the specifications.

[0022] 5. Low power consumption and long lifespan: Passive sensing, intermittent communication wake-up, and battery life of 3-5 years.

[0023] 6. Recyclable and low-cost: The sensor module can be recycled more than 20 times, reducing equipment costs by more than 95%.

[0024] 7. Fully automated operation: Automatic data collection, wireless transmission, cloud processing, and over-limit alarms require no manual intervention.

[0025] 8. Easy to operate: The top cover can be flipped open, allowing workers to disassemble and assemble the sensor module without additional tools, greatly improving recycling efficiency. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1This is a schematic diagram of the tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to the present invention; Figure 2 This is a cross-sectional view of the integrated protective box and replaceable sensor module assembly of the present invention; Figure 3 This is a cross-sectional view of the internal structure of the replaceable sensor module of the present invention; Figure 4 This is a system communication and data flow diagram of the present invention; Explanation of reference numerals in the attached figures: 1-Reference anchor bolt, 2-PVC sleeve, 3-Iron core, 4-Integrated protective box, 5-Replaceable sensor module, 6-Base station, 7-Interface between surrounding rock and primary support, 8-Inner surface of primary support; 4a-Top cover, 4b-Hinge, 4c-Slot or internal threaded hole; 5a-Outer shell, 5b-Inductor coil. Detailed Implementation

[0028] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 As shown, this invention provides a tunnel surrounding rock absolute displacement monitoring system based on LC resonance, comprising the following six parts: (a) Reference Anchor 1 Made of high-strength threaded steel, the length is determined based on the depth of the loosened rock zone (usually ≥2 times the tunnel diameter or determined by on-site testing), and the end is equipped with an internal threaded interface and an anti-rotation keyway. Reference anchor 1 is fully grouted and anchored within the surrounding rock borehole, with its anchoring depth exceeding the loosened rock zone to ensure no displacement at the end during tunnel construction. A PVC sleeve 2 is fitted onto the reference anchor 1, allowing relative sliding between the PVC sleeve 2 and the reference anchor 1 to prevent shotcrete from bonding the anchor to the initial support, ensuring that the reference anchor 1 deforms independently of the initial support.

[0030] (ii) Iron core 3 The core 3 is cylindrical, made of a high-permeability soft magnetic material (such as ferrite or silicon steel sheet), with an external threaded interface and anti-rotation keyway at one end, rigidly connected to the internal threaded interface at the end of the reference anchor rod 1. The length of the core 3 is determined according to the expected maximum displacement (usually 50-80 mm), and its outer diameter matches the inner diameter of the inductor coil 5b in the replaceable sensor module 5 (gap 0.5-1 mm). After the core 3 is connected to the reference anchor rod 1, the entire assembly remains fixed.

[0031] (III) Integrated protective box 4 like Figure 2 As shown, the integrated protective box 4 is made of high-strength metal or engineering plastic, and is cube or cuboid in shape with no bottom (i.e., the bottom is open). The top cover 4a can be flipped (connected by hinge 4b), and the cover can be locked and sealed. The cover surface is flush with the surface of the initial support. Box dimensions: approximately 100-150mm in length, width, and height, with a wall thickness of 3-5mm. The bottom of the box is open, allowing the iron core 3 and reference anchor rod 1 to be inserted into the box from below. One side wall (or opposite two side walls) of the box has a slot or internal threaded hole 4c for installing the replaceable sensor module 5. Other side walls of the box can have reserved cable or antenna outlets (sealed). The entire box is pre-embedded in the initial support concrete. During shotcreting, the concrete covers the outer wall of the box, and the interior of the box remains a cavity. The top cover 4a can be flipped open for workers to operate and disassemble the replaceable sensor module 5.

[0032] (iv) Replaceable sensor module 5 like Figure 3 As shown, the replaceable sensor module 5 is integrated and packaged within a high-strength cylindrical housing 5a, consisting of the following units (diameter 30–40 mm, length 60–80 mm, IP68 protection, pressure resistance ≥5 MPa): Inductor coil 5b: It is wound axially along the inner wall of the outer shell 5a. The number of turns and wire diameter are determined according to the design frequency. The coil axis coincides with the outer shell axis.

[0033] Resonant capacitor: It is connected in series with inductor 5b to form an LC resonant circuit, and the capacitance value is of high stability type (NP0 / C0G).

[0034] Miniature rechargeable battery: lithium-ion battery or lithium-ion battery with a capacity of ≥1500mAh.

[0035] Low-power wireless communication unit: LoRa module, operating frequency 433MHz / 470MHz / 868MHz.

[0036] MCU control unit: Ultra-low power microcontroller (such as STM32L0 series) with built-in real-time clock.

[0037] Temperature sensor: Used to read the current temperature for subsequent frequency temperature drift compensation.

[0038] Housing connection interface: The side wall of housing 5a is provided with a snap or external thread, which matches the slot or internal thread hole 4c on the side wall of the integrated protective box 4. The front end of housing 5a has an opening for the insertion of iron core 3.

[0039] After the replaceable sensor module 5 is installed, the inductor coil 5b and the iron core 3 are in a coaxial relative position, and the iron core 3 can move freely axially within the inductor coil 5b. When the initial support concrete shifts, the replaceable sensor module 5 (shell 5a and inductor coil 5b) moves with the initial support, while the iron core 3 remains stationary. Therefore, a relative displacement occurs between the inductor coil 5b and the iron core 3, changing the effective inductance of the inductor coil 5b.

[0040] (v) Base Station 6 Located in the safety zone of the secondary lining of the tunnel or at the tunnel entrance, it is equipped with a LoRa gateway, a high-gain antenna and a 4G / 5G router, and receives the current frequency and current temperature sent by the replaceable sensor module 5 and uploads them to the data processing platform.

[0041] (vi) Data processing platform like Figure 4 As shown, the data processing platform receives the current frequency and current temperature forwarded by base station 6. Based on the pre-calibrated frequency-displacement curve and temperature compensation coefficient, it converts the frequency offset into absolute displacement, generates an absolute displacement-time curve, and automatically alarms when the limit is exceeded.

[0042] Measurement Principle (Fixed Core - Moving Coil Type LC Resonant Displacement Sensor) The core measurement mechanism of this invention is as follows: the reference anchor rod 1, which is deeply anchored outside the loosening ring, is used as an absolutely stationary reference, and the iron core 3 is fixed at the end of the reference anchor rod 1; the replaceable sensor module 5 (including the inductor coil 5b) moves with the initial support; the relative displacement between the inductor coil 5b and the iron core 3 changes the inductance, causing the LC resonant frequency to shift, and the absolute displacement is inferred by frequency detection.

[0043] (I) Establishment of physical benchmarks The anchoring depth of reference anchor 1 exceeds the loosening zone of the surrounding rock, and its end remains in a fixed position throughout the entire tunnel construction cycle. Iron core 3 is rigidly connected to the end of reference anchor 1 via threads, forming a fixed assembly together with reference anchor 1. This assembly does not move with initial support deformation, providing an absolute zero displacement point.

[0044] (ii) Displacement transfer path The initial support concrete is sprayed onto the surface of the surrounding rock and adheres tightly to it. When the surrounding rock deforms (the arch sinks, the perimeter converges), the initial support concrete displaces in the same direction. The integrated protective box 4 is embedded in the initial support concrete and is fixed to the initial support. Therefore, the replaceable sensor module 5 (including the inductor coil 5b) installed on the side wall of the box moves synchronously with the initial support.

[0045] (III) Mechanism of Inductance Change Let the initial insertion depth of the iron core 3 into the inductor coil 5b be x0 (corresponding to the initial inductance L0). When the initial support moves Δd towards the tunnel clearance direction, the sensor module (inductor coil 5b) moves Δd with the initial support, while the iron core 3 remains stationary. Therefore, the insertion depth of the iron core 3 into the inductor coil 5b becomes x = x0 ± Δd (the sign is determined by the direction of displacement: when the initial support moves towards the interior of the surrounding rock, the insertion depth of the iron core 3 increases; when it moves towards the tunnel clearance direction, the insertion depth decreases). There is a monotonic functional relationship (usually approximately linear or exponential) between the inductance L of the inductor coil 5b and the insertion depth x of the iron core 3. Therefore, Δd (initial support displacement) → Δx (change in insertion depth of the iron core 3) → ΔL (change in inductance) → frequency offset Δf.

[0046] (iv) Changes in LC resonant frequency The basic formula for an LC resonant circuit is: C represents the capacitance of the resonant capacitor. When the inductance L changes, the resonant frequency f0 shifts. There is a monotonic mapping relationship between the frequency shift Δf and the displacement Δx of the core 3 (i.e., the initial support displacement Δd), which can be obtained through experimental calibration as the function Δd = Φ(Δf). This mapping relationship has been verified in existing research: when the core 3 moves a distance of millimeters within the inductor coil 5b, the resonant frequency can change by several megahertz, sufficient for high-resolution displacement measurement.

[0047] (v) Temperature compensation The replaceable sensor module 5 has a built-in temperature sensor that collects ambient temperature in real time and corrects the frequency based on a pre-calibrated temperature drift coefficient: f d =f t -α·(T t -T0), where f d For the corrected frequency, f t The current frequency is T, α is the temperature drift coefficient, and T is the current frequency. t T1 represents the current temperature, and T0 represents the initial temperature.

[0048] (vi) Calculation of absolute displacement During initial installation, the system records the initial resonant frequency f0 and the initial temperature T0. At any monitoring time t, the data processing platform reads the current frequency f0. t and temperature T t The corrected frequency f is obtained after temperature correction. d Δf=f d Substituting - f0 into the calibration function Δd=Φ(Δf) yields the current displacement. This current displacement is the absolute displacement of the initial support measuring point relative to the absolutely stationary datum.

[0049] Design advantages of the integrated protection box 4 (i) Simplified structure: Traditional solutions require separate construction of reserved holes, embedded bases, and installation of protective covers, which are complex and difficult to guarantee in terms of accuracy. The integrated protective box 4 of this invention integrates the functions of reserved holes, bases, and protective covers into a single prefabricated component. On-site, the box only needs to be embedded in the initial support concrete, which greatly simplifies the construction process.

[0050] (II) Improved Installation Accuracy: The housing can be precisely positioned based on the positions of the iron core 3 and the reference anchor rod 1 before spraying. The bottom of the housing is open for the insertion of the iron core 3, and the mating positions of the side wall slots / screw holes and the sensor module can be guaranteed during factory prefabrication, eliminating the need for secondary positioning on site. After the replaceable sensor module 5 is installed, the coaxiality and initial gap between the inductor coil 5b and the iron core 3 are guaranteed by the housing structure, avoiding on-site installation errors.

[0051] (III) Easy to use: The top cover 4a can be flipped open, allowing workers to open the protective cover without additional tools and directly operate the replacement sensor module 5 for assembly and disassembly. The cover can be locked and sealed to prevent sprayed slurry and dust from entering the box.

[0052] (iv) Stronger anti-interference: The box itself is a closed cavity (except for the bottom opening). During the shotcreting, the concrete completely covers the outer wall of the box, and the inside of the box remains clean. The iron core 3 is inserted from the bottom, and the replaceable sensor module 5 is installed from the side wall. The two are connected inside the box and are completely unaffected by the external environment.

[0053] (v) Recyclable and reusable: The replaceable sensor module 5 is connected to the side wall of the box via a snap or thread, allowing for quick disassembly. After recycling, it can be used in the next section of the box after charging and calibration.

[0054] Battery-assisted passive hybrid architecture The passive part of LC: The resonant circuit composed of inductor 5b, resonant capacitor and iron core 3 is completely passive, does not consume battery energy, and only passively responds to displacement changes.

[0055] Active component: The MCU control unit periodically wakes up (power consumption ≤ 5μA), reads the LC resonant frequency (through the resonant detection circuit) and temperature, initiates LoRa data transmission (peak current ≤ 120mA, duration < 0.5s), and then immediately goes into sleep mode. The overall average power consumption is extremely low, and the battery life can reach more than 3 years.

[0056] Communication mode: The replaceable sensor module 5 only transmits and does not receive (or only receives configuration commands), while the base station 6 is responsible for data collection and uploading.

[0057] Recyclable structure The replaceable sensor module 5 is connected to the side wall of the integrated protective box 4 by a snap-fit ​​or threaded connection, facilitating disassembly by hand or with simple tools. Recycling process: Open the top cover 4a → unscrew or pull out the replaceable sensor module 5 → replace the top cover 4a (temporary dust protection) → take the replaceable sensor module 5 back to the project site → charge → calibrate and verify → reinstall into the integrated protective box 4 of the next section. The reference anchor 1 and iron core 3 are permanently left in place (lower cost).

[0058] Construction process flow 1. Installation of reference anchor bolt 1 Drill a hole at the monitoring point (deep beyond the loosening ring), insert a reference anchor rod 1 fitted with a PVC sleeve 2, grout along the entire length, leave a threaded interface at the end and install a protective plug.

[0059] 2. Installation of iron core 3 Connect the iron core 3 to the end of the reference anchor rod 1 by threading, and tighten to prevent rotation. The exposed length of the iron core 3 is determined according to the design.

[0060] 3. Integrated protective box 4 pre-embedded Before applying the initial support shotcrete, place the integrated protective box 4 (bottomless, with a flip-up top cover 4a) over the iron core 3 and reference anchor 1, adjust its position so that the iron core 3 is in the center of the box, and weld or tie the box to the initial support mesh reinforcement. Apply the initial support shotcrete, with the concrete covering the outer wall of the box, maintaining a cavity inside the box, and close the top cover flush with the surface of the initial support.

[0061] 4. Replaceable sensor module 5 installation Open the top cover 4a, install the replaceable sensor module 5 into the box through the side wall slot / screw hole, insert the iron core 3 into the inductor coil 5b of the replaceable sensor module 5, adjust to the initial position, and close the top cover.

[0062] 5. Initial calibration When the system is powered on, the replaceable sensor module 5 records the initial frequency and temperature, and sends them to the base station 6 via LoRa. The base station 6 then uploads the data to the cloud to record the initial zero point.

[0063] 6. Automatic monitoring The replaceable sensor module 5 automatically wakes up, measures frequency / temperature, sends data, and goes into sleep mode according to a set cycle (which can be remotely configured).

[0064] 7. Recycling and reuse After the cross-section monitoring task is completed, open the top cover 4a, remove the replaceable sensor module 5, and use it for the next cross-section after charging and calibration. The reference anchor 1, iron core 3, and integrated protective box 4 remain in their original positions.

[0065] This invention also provides a method for monitoring the absolute displacement of tunnel surrounding rock based on LC resonance, comprising the following steps: The reference anchor is deeply anchored outside the loosened zone of the surrounding rock to establish an absolutely immovable spatial reference; the iron core is fixed to the end of the reference anchor. Design an integrated protective box with a flip-up top cover that is embedded in the initial support concrete. The bottom of the box is for inserting an iron core and a reference anchor rod. The side wall of the box is provided with a slot or internal thread hole for installing a replaceable sensor module. The replaceable sensor module includes a housing, an inductor coil, a resonant capacitor, a rechargeable battery, a wireless communication unit, an MCU control unit, and a temperature sensor. The housing sidewall has snaps or external threads that match the slots or internal threaded holes on the sidewall of the integrated protective box. The front of the housing has an opening for inserting an iron core. The inductor coil is wound axially along the inner wall of the housing; the number of turns and wire diameter are determined according to the design frequency, and the coil axis coincides with the housing axis. The resonant capacitor is connected in series with the inductor coil to form an LC resonant circuit. The temperature sensor reads the current temperature for subsequent frequency-temperature drift compensation. The MCU control unit reads the current frequency. The wireless communication unit transmits the current frequency and current temperature. When the initial support moves with the deformation of the surrounding rock, the integrated protection box and replaceable sensor module move with the initial support, while the iron core remains stationary, thus generating relative displacement. The relative displacement between the inductor coil and the iron core changes the inductance of the coil, causing a change in the LC resonant frequency. The data processing platform receives the current frequency and current temperature, and converts the frequency offset into absolute displacement according to the pre-calibrated frequency-displacement curve and temperature compensation coefficient.

[0066] Example 1: Differential Inductive LC Resonant Displacement Sensing At the arch of section DK12+345 of a tunnel, a 2.5m deep hole was drilled vertically (beyond the loosening zone). A reference anchor (25mm diameter, 2.5m long, with a PVC pipe casing) was installed and grouted along its entire length. A 60mm long, 10mm diameter iron core was threaded to the end of the reference anchor. An integrated protective box (100×100×100mm, bottomless, hinged top cover) was fitted over the iron core and welded to the initial support mesh reinforcement. Initial support concrete was sprayed, covering the outer wall of the box, and the top cover was closed flush with the initial support surface. The top cover was opened, and a sensor module (35mm diameter, 70mm long, with a built-in differential inductor coil) was installed inside the box through a side wall slot, inserting the iron core into the coil at an initial depth of 30mm. The system was powered on, and the initial frequency of 2.856MHz and temperature of 18.2℃ were recorded. The acquisition cycle was set to 30 minutes. After three months of operation, the cloud display showed a cumulative subsidence of 4.3mm, which is 0.2mm different from the total station's measured value of 4.1mm during the same period.

[0067] Example 2: Variable-pitch capacitive LC resonant displacement sensor (alternative solution) In the LC resonant circuit, the two plates of the capacitor are fixed to the inner wall of the sensor module housing and the end of the iron core, respectively, while the inductor coil remains stationary. When the sensor module moves with the initial support, the distance between the two plates changes, and the change in capacitance causes a shift in the resonant frequency. This scheme is suitable for scenarios with special requirements for non-contact measurement.

[0068] Example 3: Recyclable and Reusable Process The tunnel is 500m long with 100 cross-sections and 500 monitoring points. Only 20 sensor modules were produced. The first batch of modules was installed on cross-sections 1-20 (each cross-section is equipped with an integrated protective box and reference anchor). After monitoring was completed (after the secondary lining was poured), the modules were retrieved sequentially: the top cover was opened, and the sensor modules were disassembled using special tools. After being charged and calibrated at the project site (frequency drift <0.05% is considered acceptable), the modules were installed in the prefabricated protective boxes on cross-sections 21-40. Each module was reused an average of 25 times, and the total equipment cost was approximately 10,000 yuan, a 96% reduction compared to a one-time solution (250,000 yuan).

[0069] Example 4: Comparison and Verification with Total Station Accuracy verification was performed using a laboratory tunnel model. The model was subjected to a displacement of 1mm in increments of 0-30mm, and the system output and total station coordinates were recorded simultaneously. Results showed: average deviation 0.12mm, standard deviation 0.8mm, maximum deviation 1.9mm, and correlation coefficient R0. 2 =0.997. In a simulated high-dust environment (dust concentration 15 mg / m³)... 3 Under these conditions, the measurement deviation of the total station increases to 3.5mm, while the measurement deviation of this system remains within 1.5mm.

[0070] Beneficial effects 1. Absolute displacement measurement: By establishing an absolutely stationary benchmark through deep anchoring reference anchors, the absolute displacement of the initial support relative to the deep surrounding rock is directly output, solving the benchmark drift problem of existing methods.

[0071] 2. Completely concealed, resistant to construction damage: The integrated protective box is pre-embedded in the initial support concrete, and the sensor module is installed inside the box. The top cover is flush with the surface of the initial support, so flying rocks from blasting, construction vehicles, and sprayed concrete cannot damage the measuring points. The survival rate of the measuring points can reach over 95%.

[0072] 3. Simple structure and easy installation: The integrated protective box integrates the three functions of pre-drilled holes, base and protective cover into a prefabricated component. Only the box body needs to be pre-embedded on site, without the need for secondary positioning, which greatly reduces the complexity of construction.

[0073] 4. High precision: The LC resonant frequency detection accuracy can reach the kHz level, with a corresponding displacement resolution better than 0.1mm. The overall system accuracy is ±2~3mm, meeting the specifications.

[0074] 5. Low power consumption and long lifespan: Passive sensing, intermittent communication wake-up, and battery life of 3-5 years.

[0075] 6. Recyclable and low-cost: The sensor module can be recycled more than 20 times, reducing equipment costs by more than 95%.

[0076] 7. Fully automated operation: Automatic data collection, wireless transmission, cloud processing, and over-limit alarms require no manual intervention.

[0077] 8. Easy to operate: The top cover can be flipped open, allowing workers to disassemble and assemble the sensor module without additional tools, greatly improving recycling efficiency.

[0078] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A tunnel surrounding rock absolute displacement monitoring system based on LC resonance, characterized in that, Includes reference anchor bolts, iron cores, integrated protection boxes, replaceable sensor modules, and data processing platforms; The reference anchor is anchored in the surrounding rock borehole, and its anchoring depth exceeds the range of the loosening zone of the surrounding rock. The end is provided with an internal thread interface and an anti-rotation keyway. One end of the iron core is provided with an external threaded interface and an anti-rotation keyway, which is rigidly connected to the internal threaded interface at the end of the reference anchor rod. Its outer diameter matches the inner diameter of the inductor coil in the replaceable sensor module. The length of the iron core is determined according to the expected maximum displacement. After the iron core is connected to the reference anchor rod, the whole is fixed in place. The integrated protective box has an open bottom, allowing the iron core and reference anchor rod to be inserted into the box from the bottom. The top cover can be flipped up and locked and sealed. The cover surface is flush with the surface of the initial support. The side wall of the box is provided with a slot or internal thread hole for installing a replaceable sensor module. The entire box is pre-embedded in the concrete of the initial support. When spraying grout, the concrete wraps around the outer wall of the box, and the inside of the box remains a cavity. The replaceable sensor module includes a housing, an inductor coil, a resonant capacitor, a rechargeable battery, a wireless communication unit, an MCU control unit, and a temperature sensor. The outer casing sidewall is provided with a buckle or external thread, which matches the slot or internal thread hole of the integrated protective box sidewall. The front end of the outer casing has an opening for the iron core to be inserted. The inductor coil is wound axially along the inner wall of the outer casing, and the number of turns and wire diameter are determined according to the design frequency. The coil axis coincides with the outer casing axis. The resonant capacitor and the inductor are connected in series to form an LC resonant circuit; The temperature sensor is used to read the current temperature so that frequency temperature drift compensation can be performed subsequently. The MCU control unit is used to read the current frequency; The wireless communication unit is used to transmit the current frequency and current temperature; The data processing platform is used to receive the current frequency and current temperature, and convert the frequency offset into absolute displacement according to the pre-calibrated frequency-displacement curve and temperature compensation coefficient.

2. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The data processing platform is also used to generate absolute displacement-time curves and automatically alarm when limits are exceeded.

3. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The data processing platform converts the frequency offset into an absolute displacement using the following method: Inductance change mechanism: Let the initial depth of the iron core inserted into the inductor coil be x0, corresponding to the initial inductance L0. When the initial support moves Δd towards the tunnel clearance direction, the replaceable sensor module moves Δd with the initial support, while the iron core remains stationary. Therefore, the depth of the iron core inserted into the inductor coil becomes x = x0 ± Δd, with the sign determined by the direction of displacement: when the initial support moves towards the interior of the surrounding rock, the insertion depth of the iron core increases; when it moves towards the tunnel clearance direction, the insertion depth decreases. There is a monotonic function relationship between the inductance L of the inductor coil and the insertion depth x of the iron core. Therefore, the initial support displacement Δd causes a change in the insertion depth of the iron core Δx, which in turn causes a change in the inductance ΔL, and ultimately causes a frequency offset Δf. LC resonant frequency variation: The basic formula for an LC resonant circuit is: C is the capacitance of the resonant capacitor. When the inductance L changes, the resonant frequency f0 shifts. There is a monotonic mapping relationship between the frequency shift Δf and the core displacement Δx, i.e., the initial support displacement Δd. The function Δd=Φ(Δf) is obtained through experimental calibration. When the core moves a distance of millimeters within the inductor coil, the resonant frequency changes by several megahertz, thus achieving high-resolution displacement measurement. Temperature compensation: The replaceable sensor module has a built-in temperature sensor that collects ambient temperature in real time and corrects the frequency according to a pre-calibrated temperature drift coefficient: f d =f t -α·(T t -T0), where f d For the corrected frequency, f t The current frequency is T, α is the temperature drift coefficient, and T is the current frequency. t T1 is the current temperature, and T0 is the initial temperature; Absolute displacement calculation: During initial installation, the system records the initial resonant frequency f0 and the initial temperature T0; at any monitoring time t, the data processing platform reads the current frequency f0. t and the current temperature T t The corrected frequency f is obtained after temperature correction. d Δf=f d - f0, substitute into the calibration function Δd=Φ(Δf) to obtain the current displacement, which is the absolute displacement of the initial support measuring point relative to the absolute stationary reference.

4. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The tunnel surrounding rock absolute displacement monitoring system based on LC resonance also includes a base station, which is used to forward the current frequency and current temperature sent by the wireless communication unit to the data processing platform.

5. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The tunnel surrounding rock absolute displacement monitoring system based on LC resonance also includes a PVC sleeve, which is sleeved on the reference anchor rod body and can slide relative to the reference anchor rod.

6. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The reference anchor is made of high-strength threaded steel, and its length is determined according to the depth of the loosened zone of the surrounding rock.

7. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The iron core is made of a cylindrical material with high magnetic permeability.

8. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The integrated protection box is made of high-strength metal or engineering plastic, and is in the shape of a cube or cuboid. The box dimensions are 100-150mm in length, width and height, and the wall thickness is 3-5mm. The other side walls of the box are reserved with cable or antenna outlets and are sealed.

9. The tunnel surrounding rock absolute displacement monitoring system based on LC resonance according to claim 1, characterized in that, The base station is located in the safety zone of the secondary lining of the tunnel or at the tunnel entrance.

10. A method for monitoring absolute displacement of tunnel surrounding rock based on LC resonance, characterized in that, Includes the following steps: The reference anchor is deeply anchored outside the loosened zone of the surrounding rock to establish an absolutely immovable spatial reference; the iron core is fixed to the end of the reference anchor. Design an integrated protective box with a flip-up top cover that is embedded in the initial support concrete. The bottom of the box is for inserting an iron core and a reference anchor rod. The side wall of the box is provided with a slot or internal thread hole for installing a replaceable sensor module. The replaceable sensor module includes a housing, an inductor coil, a resonant capacitor, a rechargeable battery, a wireless communication unit, an MCU control unit, and a temperature sensor. The housing sidewall has a snap or external thread that matches the slot or internal thread hole on the sidewall of the integrated protective box. The front of the housing has an opening for the insertion of the iron core. The inductor coil is wound axially along the inner wall of the housing, and the number of turns and wire diameter are determined according to the design frequency. The coil axis coincides with the housing axis. A resonant capacitor and an inductor are connected in series to form an LC resonant circuit; a temperature sensor reads the current temperature for subsequent frequency-temperature drift compensation; the MCU control unit reads the current frequency; and the wireless communication unit transmits the current frequency and current temperature. When the initial support moves with the deformation of the surrounding rock, the integrated protection box and replaceable sensor module move with the initial support, while the iron core remains stationary, thus generating relative displacement. The relative displacement between the inductor coil and the iron core changes the inductance of the coil, causing a change in the LC resonant frequency. The data processing platform receives the current frequency and current temperature, and converts the frequency offset into absolute displacement according to the pre-calibrated frequency-displacement curve and temperature compensation coefficient.

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