A soil multi-directional stress monitoring device and method based on optical frequency domain reflection

Abstract

The present invention discloses a soil multi-directional stress monitoring device and method based on optical frequency domain reflection. The monitoring device includes a ring sensor, an optical frequency domain reflection demodulator and an optical fiber data processing module; the ring sensor is composed of a thin-walled ring, a strain sensing optical fiber and a temperature sensing optical fiber, wherein the thin-walled ring is made of a material whose elastic modulus is equal to the compression modulus of the soil, and the strain sensing optical fiber and the temperature sensing optical fiber are arranged in a groove opened by the thin-walled ring. During monitoring, the ring sensor is buried in the soil to be tested, and when the stress field of the soil changes, the thin-walled ring deforms with the soil; the optical frequency domain reflection demodulator continuously collects strain data of the strain sensing optical fiber and the temperature sensing optical fiber, and obtains the real-time annular strain of the ring sensor after temperature compensation of the strain data of the strain sensing optical fiber; the real-time annular strain data is transmitted to the optical fiber data processing module, and the change of the in-situ soil stress is obtained based on elastic mechanics theory calculation. The present invention realizes multi-directional real-time monitoring of soil stress, which makes up for the disadvantage of unidirectional monitoring of traditional soil pressure sensors.

Description

Technical Field

[0001] The invention relates to a soil stress monitoring device, and in particular to a soil multi-directional stress monitoring device and method based on optical frequency domain reflection. Background Art

[0002] In actual engineering, how to obtain the true soil stress has always been the focus and difficulty of geotechnical engineering monitoring. Traditional soil pressure sensors are generally made of metal materials with large deformation stiffness. Their installation has a great disturbance on the soil stress field, so they cannot accurately measure soil stress. In addition, traditional soil pressure sensors include vibrating string, inductive and differential resistance. The principle of the vibrating string soil pressure sensor is to measure the soil pressure by using the relationship between the natural frequency of the built-in steel string and the external tension on the steel string; the inductive soil pressure sensor is based on the principle of electromagnetic induction, using the change of coil inductance to achieve non-electrical measurement, and converting soil pressure into inductance change; the principle of the differential resistance soil pressure sensor is to use the elastic deformation of the elastic steel wire in the inner cavity under the change of force and temperature, and its deformation value is linearly related to the resistance of the steel wire, and the monitoring value is obtained through the change of resistance. These sensors have the disadvantages of poor long-term stability, poor durability, and susceptibility to electromagnetic interference, and cannot meet the requirements of information-based automated real-time monitoring.

[0003] With the rapid development of fiber optic sensing technology, fiber optic sensors are widely used in geotechnical engineering monitoring due to their many advantages such as fast response, corrosion resistance, anti-electromagnetic interference, compact size, and automated real-time monitoring. However, how to accurately monitor soil stress based on fiber optic sensing technology has become a technical problem that needs to be solved urgently.

[0004] At present, the fiber optic sensing technology used for soil stress monitoring is mainly fiber optic Bragg grating technology, and the fiber optic Bragg grating is usually attached to an elastic diaphragm, and the stress along the normal direction of the diaphragm is calculated by monitoring the strain of the diaphragm. Therefore, this technology is mostly quasi-distributed and unidirectional monitoring, and cannot achieve multi-directional stress monitoring. In recent years, although some scholars have invented multi-directional soil stress monitoring devices based on fiber optic Bragg grating technology, these multi-directional soil stress monitoring devices are often composed of multiple unidirectional soil pressure boxes, which have the disadvantages of large volume, complex wiring, and cumbersome installation. Some scholars have made improvements on this basis and developed an integrated monitoring sensor for fiber optic Bragg grating multi-directional soil stress. However, the housings of these sensors are made of metal materials with great rigidity, which are only suitable for monitoring ground stress deep in the rock mass, but are not sensitive to smaller stresses, so they are not suitable for soil stress monitoring. In addition, fiber optic Bragg grating soil stress sensors are prone to monitoring errors when the force is uneven or the temperature changes greatly. Summary of the invention

[0005] Purpose of the invention: In view of the shortcomings in the prior art, the present invention provides a soil multi-directional stress monitoring device and method based on optical frequency domain reflection. The monitoring device and the monitoring method realize all-round automatic monitoring of soil stress, and have low rigidity and high sensitivity, making up for the technical defects of traditional soil pressure sensors such as one-way monitoring, poor long-term stability, poor durability, susceptibility to electromagnetic interference, and large disturbance to the original soil stress field.

[0006] Technical solution: To achieve the above-mentioned purpose, the soil multi-directional stress monitoring device based on optical frequency domain reflection of the present invention comprises a ring sensor, an optical frequency domain reflection demodulator and an optical fiber data processing module;

[0007] The ring sensor includes a thin-walled ring, a strain sensing optical fiber, and a temperature sensing optical fiber;

[0008] The thin-walled ring is made of a material with an elastic modulus equal to the compression modulus of the soil;

[0009] The strain sensing optical fiber and the temperature sensing optical fiber are glued in the U-shaped groove opened in the thin-walled circular ring by using epoxy resin.

[0010] The soil multi-directional stress monitoring method based on optical frequency domain reflection of the present invention comprises the following steps:

[0011] (1) Two U-shaped grooves are opened in an annular direction along the thin-walled circular ring, and the strain sensing optical fiber and the temperature sensing optical fiber are fixed in the U-shaped grooves by using epoxy resin. The entire length of the strain sensing optical fiber is glued in the U-shaped groove of the thin-walled circular ring by using epoxy resin, and the temperature sensing optical fiber is fixed in the U-shaped groove of the thin-walled circular ring by using epoxy resin. The thin-walled circular ring, the strain sensing optical fiber and the temperature sensing optical fiber together constitute a ring sensor, and the ring sensor is buried in the soil body to be monitored for stress;

[0012] (2) connecting the strain sensing optical fiber and the temperature sensing optical fiber to the optical frequency domain reflectometry demodulator through an optical fiber jumper;

[0013] (3) Using an optical frequency domain reflectometry demodulator to collect strain data of the strain sensing optical fiber and the temperature sensing optical fiber in real time;

[0014] (4) Subtract the strain data of the temperature sensing optical fiber from the strain data of the strain sensing optical fiber to obtain the real-time annular strain ε of the ring sensor. θ ;

[0015] Among them, the real-time hoop strain contains many values. The curve formed by the real-time hoop strain includes a maximum value and a minimum value. The positions of the maximum value and the minimum value are related to the actual stress direction of the soil. The θ value represents the angle in the coordinate system.

[0016] (5) When the soil stress changes, the thin-walled ring and the strain sensing optical fiber deform along with the soil. The optical frequency domain reflectometry demodulator collects the annular strain data of the ring sensor in real time and transmits the collected real-time annular strain data to the optical fiber data processing module. Based on the theory of elastic mechanics, the spherical stress σ0 and deviatoric stress σ of the soil at the measured position are calculated. r , and then calculate the soil stress σ at the test location:

[0017] The process is:

[0018]

[0019] Where σ0 represents the spherical stress of the soil; E is the elastic modulus of the thin-walled ring; ν is the Poisson's ratio; r1 is the outer diameter of the thin-walled ring, r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value;

[0020] Assuming that the thin-walled ring (1) is subjected to pure bending, the cross section of the thin-walled ring changes from a circle to an ellipse after being subjected to deviatoric stress. Assuming that under small deformation, the inner and outer diameters of the thin-walled ring and the major and minor axes of the ellipse satisfy the following relationship:

[0021] ar m =r m -b

[0022] In the formula, a is the semi-major axis of the ellipse, b is the semi-minor axis of the ellipse, and r m is the average value of the inner and outer radius of the tube body;

[0023] Assume that the major axis of the ellipse is the x-axis and the minor axis is the y-axis; the intersection of the ellipse with the x-axis is point A, and the intersection with the y-axis is point B; the strain value monitored at point B is compressive strain, and the strain value is the smallest; the strain to be monitored at point A is tensile strain, and the strain value is the largest; there is the following relationship between the deviatoric strain of the soil and the annular strain of the ring sensor caused by it:

[0024]

[0025] In the formula, σ r represents the deviatoric stress of the soil; ν is Poisson's ratio; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value; θ represents Figure 3 The angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis is in the range of 0°-360°; k is a constant determined by calibration tests;

[0026] Conclusion

[0027]

[0028] In the formula, σ0 represents the spherical stress of the soil; σ r represents the deviatoric stress of the soil; E is the elastic modulus of the thin-walled ring; ν is the Poisson's ratio; r1 is the outer diameter of the thin-walled ring; r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value; θ represents Figure 3 The angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis ranges from 0° to 360°; k is a constant determined by calibration tests.

[0029] In step (1), the depth of the U-shaped groove engraved along the circumferential direction of the thin-walled circular ring is the same as the diameter of the strain sensing optical fiber or the temperature sensing optical fiber, so as to increase the contact area between the optical fiber and the thin-walled circular ring and ensure good coupling between the optical fiber and the thin-walled circular ring during deformation.

[0030] Apply friction-reducing vaseline to the sides of the thin-walled ring.

[0031] The temperature sensing optical fiber in step (1) is used for temperature compensation.

[0032] The top and bottom ends of the thin-walled ring in step (1) are sealed.

[0033] Working principle: Optical frequency domain reflectometry (OFDR) technology in the field of optical fiber is a strain monitoring technology based on the change of Rayleigh scattered light. The continuous light emitted by the light source is divided into two paths by the coupler, one of which is used as the reference light, and the other is emitted into the optical fiber to be tested as the detection light. When the detection light propagates forward in the optical fiber, it will continuously generate Rayleigh scattered signals. These signal lights and the reflected reference light pass through the coupler and interfere with each other, and are then detected by the photodetector. When the temperature or strain at a certain point inside the optical fiber changes, the refractive index inside the optical fiber will change, and the frequency of the corresponding Rayleigh scattered signal light will also change. By monitoring the frequency of the Rayleigh scattered signal back to the optical fiber, the changes in the external temperature field or strain field can be obtained.

[0034] When the soil stress field changes, the ring sensor deforms accordingly, and its hoop strain also changes accordingly. The optical fiber attached to the thin-walled ring can sense the change in the hoop strain of the ring sensor. The frequency of the backscattered Rayleigh scattering signal inside the optical fiber drifts, and the optical frequency drift is linearly related to the optical fiber strain. By monitoring the optical frequency drift, the hoop strain data of the ring sensor is obtained. The obtained hoop strain of the ring sensor is used to calculate the soil stress distribution at the measured position.

[0035] When monitoring is performed, the present invention buries the ring sensor in the soil body whose stress is to be monitored according to monitoring needs. When the stress field of the soil body changes, the thin-walled ring deforms along with the soil body, and its annular strain also changes accordingly. An optical frequency domain reflection demodulator continuously collects strain data of the strain sensing optical fiber and the temperature sensing optical fiber, and the temperature sensing optical fiber is used as a reference optical fiber. After temperature compensation is performed on the strain data of the strain sensing optical fiber, the real-time annular strain of the ring sensor is obtained. The real-time annular strain data is transmitted to an optical fiber data processing module, and the change of the in-situ soil stress is obtained based on elastic mechanics theory calculation.

[0036] In step (5), the soil stress is calculated using the hoop strain. No matter what stress state the soil is in, it can be decomposed into spherical stress and deviatoric stress.

[0037] The first part is to calculate the spherical stress on the soil. The relationship between the spherical stress on the soil and the annular strain of the ring sensor caused by it is as follows:

[0038]

[0039] Where σ0 represents the spherical stress of the soil; E is the elastic modulus of the thin-walled ring; ν is the Poisson's ratio; r1 is the outer diameter of the thin-walled ring, r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value.

[0040] The second part is to calculate the deviatoric stress. Assuming that the thin-walled ring is subjected to pure bending, the cross section of the thin-walled ring changes from a circle to an ellipse after the deviatoric stress is applied. Assuming that under small deformation, the inner and outer diameters of the thin-walled ring and the major and minor axes of the ellipse satisfy the following relationship:

[0041] ar m =r m -b

[0042] In the formula, a is the semi-major axis of the ellipse, b is the semi-minor axis of the ellipse, and r m is the average value of the inner and outer radius of the tube.

[0043] Assume that the major axis of the ellipse is the x-axis and the minor axis is the y-axis; the intersection of the ellipse with the x-axis is point A, and the intersection with the y-axis is point B. The strain value monitored at point B is the compressive strain, and the strain value is the smallest; the strain to be monitored at point A is the tensile strain, and the strain value is the largest. There is the following relationship between the deviatoric strain of the soil and the annular strain of the ring sensor caused by it:

[0044]

[0045] In the formula, σ r represents the deviatoric stress of the soil; ν is Poisson's ratio; ε max is the maximum hoop strain value; εmin is the minimum hoop strain value; θ represents Figure 3 The angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis ranges from 0° to 360°; k is a constant determined by calibration tests.

[0046] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0047] (1) The soil multi-directional stress monitoring device based on optical frequency domain reflection of the present invention realizes all-round real-time monitoring of soil stress, has the advantages of fast response, corrosion resistance, and anti-electromagnetic interference, which makes up for the disadvantage of one-way monitoring of traditional soil pressure sensors and meets the requirements of information-based and automated real-time monitoring.

[0048] (2) The thin-walled ring is made of a material with an elastic modulus equal to that of the soil. It is small in size, easy to install, and has little interference with the original stress field.

[0049] (3) The optical frequency domain reflectometry demodulator has high spatial resolution and can achieve high-precision and high-sensitivity monitoring.

[0050] (4) The strain sensing optical fiber and the thin-walled ring are directly bonded using epoxy resin. The two have good coupling properties and can dynamically monitor soil stress in real time.

[0051] (5) The temperature sensing optical fiber with a loose tube structure is used as a reference optical fiber for temperature compensation to eliminate the interference of temperature changes in the soil on strain monitoring.

[0052] (6) A layer of vaseline is applied on the surface of the thin-walled ring to effectively reduce the impact of the surface friction between the soil and the ring sensor on the monitoring results. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a structural schematic diagram of a soil multi-directional stress monitoring device based on optical frequency domain reflection according to the present invention;

[0054] Figure 2 Schematic diagram of a soil stress monitoring device under indoor displacement loading in Example 1 of the present invention;

[0055] Figure 3 It is a deformation analysis diagram of the ring sensor of the present invention;

[0056] Figure 4 is a strain curve diagram of an optical fiber in an embodiment of the present invention;

[0057] Figure 5 This is a stress fitting diagram calculated in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] In the present invention, loading, unloading, excavation and other measures will cause soil stress changes. In this embodiment, indoor displacement loading is used as an example for explanation.

[0059] like Figures 1 to 2 As shown, the soil stress monitoring device under indoor displacement loading in this embodiment includes a thin-walled ring 1, a strain sensing optical fiber 2, a temperature sensing optical fiber 3, an optical fiber jumper 4, an optical frequency domain reflection demodulator 5, an optical fiber data processing module 6, a test model box 7 and a loading device 8. A U-shaped groove is opened along the thin-walled ring 1, and the monitoring section optical fiber 2 and the temperature sensing optical fiber 3 are glued in the groove with epoxy resin; wherein, the strain sensing optical fiber 2 is a tight sleeve structure, and the entire length is glued in the U-shaped groove of the thin-walled ring 1 with epoxy resin. The temperature sensing optical fiber 3 is a loose-sleeve structure that is not affected by strain. It is fixed in the U-shaped groove of the thin-walled ring 1 by epoxy resin. The two are connected to the optical frequency domain reflection demodulator 5 through the optical fiber jumper 4. According to the monitoring needs, the ring sensor is buried in the soil body whose soil stress is to be measured. When the soil body is loaded, the thin-walled ring 1 deforms with the soil body, and the strain sensing optical fiber also generates strain accordingly. The optical frequency domain reflection demodulator 5 collects strain data in real time, transmits all the collected strain data to the optical fiber data processing module 6, and calculates based on the theory of elastic mechanics to obtain the change of in-situ soil stress.

[0060] In the embodiment of the present invention, the specific steps of the method for monitoring multi-directional stress of soil include:

[0061] (1) Two U-shaped grooves are opened along the thin-walled circular ring 1. The depth of the U-shaped groove is the same as the diameter of the strain sensing optical fiber or the temperature sensing optical fiber. The entire length of the strain sensing optical fiber 2 is glued to the U-shaped groove of the thin-walled circular ring 1 with epoxy resin, and the temperature sensing optical fiber 3 is fixed at a fixed point in the U-shaped groove of the thin-walled circular ring 1 with epoxy resin. The thin-walled circular ring 1, the strain sensing optical fiber 2, and the temperature sensing optical fiber 3 together constitute a ring sensor, and the ring sensor is buried in the soil body whose stress is to be monitored;

[0062] (2) According to the monitoring needs, the thin-walled ring 1 is arranged in the middle of the model box 7, and the falling sand method is used for filling;

[0063] (3) The strain sensing optical fiber 2 and the temperature sensing optical fiber 3 are connected to the optical fiber jumper 4 by fusion splicing, and the fusion splicing position is protected by a heat shrink tube, and then the optical fiber jumper 4 is connected to the optical frequency domain reflection demodulator 5;

[0064] (4) The strain data of the strain sensing optical fiber 2 and the temperature sensing optical fiber 3 are collected in real time by using the optical frequency domain reflection demodulator 5, and the strain data of the temperature sensing optical fiber 3 is subtracted from the strain data of the strain sensing optical fiber 2 to obtain the real-time annular strain ε of the annular sensor. θ ;

[0065] (5) Using the loading device 8 to apply lateral load to the soil in the model box, the loading is performed using a uniform displacement loading method, the loading level is set to five, and the lateral plate displacement of each loading level is 0.2 mm. After the displacement is stable, the loading value is read. The optical frequency domain reflection demodulator 5 collects the circumferential strain data in real time during the loading process, and transmits all the collected real-time circumferential strain data to the optical fiber data processing module 6;

[0066] (6) After obtaining the real-time circumferential strain data, the spherical stress σ0 and deviatoric stress σ of the soil at the measuring point are calculated based on the elastic mechanics theory. r ,

[0067] Among them, in step 5, there is the following relationship between the spherical stress σ0 of the soil and the annular strain of the ring sensor caused by it:

[0068]

[0069] Where σ0 is the spherical stress of the soil; E is the elastic modulus of the thin-walled ring, ν is the Poisson's ratio, r1 is the outer diameter of the thin-walled ring, and r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value.

[0070] In step 5, the soil deviator stress σ is calculated r When the ring sensor is subjected to pure bending, the cross section of the thin-walled ring changes from a circle to an ellipse after being subjected to deviatoric stress. The following relationship exists between the deviatoric stress on the soil and the annular strain of the ring sensor caused by it:

[0071]

[0072] In the formula, σ r represents the deviatoric stress on the soil; θ represents Figure 3 The angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis is in the range of 0°-360°; k is a constant that can be obtained from a calibration test; ν is the Poisson's ratio; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value.

[0073] Then, in step 6, the soil stress σ at the location to be measured is calculated:

[0074]

[0075] In the formula, σ0 represents the spherical stress of the soil, σ r represents the deviatoric stress of the soil, E is the elastic modulus of the thin-walled ring, ν is the Poisson's ratio, r1 is the outer diameter of the thin-walled ring, and r2 is the inner diameter of the thin-walled ring; ε max is the maximum strain value; ε minis the minimum strain; θ represents Figure 3 where k is the angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis. k is a constant determined by calibration tests.

[0076] In this example, if Figure 4 As shown in the strain data curve in , the optical fiber strain data is distributed in a cosine function, and the strain distribution curve trends under different loading levels are basically the same. With the increase of loading levels, the monitored strain peak value is also gradually increasing.

[0077] According to the hoop strain data of the ring sensor, the multi-directional stress distribution of the soil at the test location is obtained. Figure 5 It can be seen that the ring sensor can measure the magnitude of soil stress in all directions, and determine the maximum and minimum values ​​of soil stress, i.e., the maximum principal stress and the minimum principal stress, according to the stress distribution. The direction of the minimum principal stress is perpendicular to the loading direction. As the loading level increases, the maximum principal stress, i.e., the magnitude of the soil stress in the loading direction, increases step by step, while the magnitude of the minimum principal stress remains unchanged.

Claims

1. A soil multi-directional stress monitoring method based on optical frequency domain reflection, characterized in that: The method is implemented by a soil multi-directional stress monitoring device based on optical frequency domain reflection, wherein the monitoring device comprises a ring sensor, an optical frequency domain reflection demodulator (5) and an optical fiber data processing module (6); The ring sensor comprises a thin-walled ring (1), a strain sensing optical fiber (2) and a temperature sensing optical fiber (3); the thin-walled ring (1) is made of a material having an elastic modulus equal to a soil compression modulus; The strain sensing optical fiber (2) and the temperature sensing optical fiber (3) are glued in the U-shaped groove opened in the thin-walled circular ring (1) using epoxy resin; The method comprises the following steps: 1) Two U-shaped grooves are formed in a circular direction along a thin-walled circular ring (1), and a strain sensing optical fiber (2) and a temperature sensing optical fiber (3) are pasted into the U-shaped grooves by using epoxy resin. The thin-walled circular ring (1), the strain sensing optical fiber (2) and the temperature sensing optical fiber (3) form a ring sensor, and the ring sensor is buried in the soil body whose stress is to be monitored; 2) connecting the strain sensing optical fiber (2) and the temperature sensing optical fiber (3) to an optical frequency domain reflection demodulator (5) via an optical fiber jumper (4); 3) using an optical frequency domain reflection demodulator (5) to collect strain data of the strain sensing optical fiber (2) and the temperature sensing optical fiber (3) in real time; 4) subtracting the strain data of the temperature sensing optical fiber (3) from the strain data of the strain sensing optical fiber (2) to obtain the real-time annular strain of the annular sensor; 5) When the soil stress changes, the thin-walled ring (1) and the strain sensing optical fiber (2) deform along with the soil, and the optical frequency domain reflection demodulator collects the annular strain data of the ring sensor in real time, transmits the collected real-time annular strain data to the optical fiber data processing module (6), and calculates the spherical stress σ0 and the deviatoric stress σ of the soil at the measured position based on the elastic mechanics theory. r , and then calculate the soil stress σ at the measured location. The process is: Where σ0 represents the spherical stress of the soil; E is the elastic modulus of the thin-walled ring; ν is the Poisson's ratio; r1 is the outer diameter of the thin-walled ring, r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value; Assuming that the thin-walled ring (1) is subjected to pure bending, the cross section of the thin-walled ring changes from a circle to an ellipse after being subjected to deviatoric stress. Assuming that under small deformation, the inner and outer diameters of the thin-walled ring and the major and minor axes of the ellipse satisfy the following relationship: a-r m =r m -b In the formula, a is the semi-major axis of the ellipse, b is the semi-minor axis of the ellipse, and r m is the average value of the inner and outer radius of the tube body; Assume that the major axis of the ellipse is the x-axis and the minor axis is the y-axis; the intersection of the ellipse with the x-axis is point A, and the intersection with the y-axis is point B; the strain value monitored at point B is compressive strain, and the strain value is the smallest; the strain to be monitored at point A is tensile strain, and the strain value is the largest; there is the following relationship between the deviatoric strain of the soil and the annular strain of the ring sensor caused by it: In the formula, σ r represents the deviatoric stress of the soil; ν is Poisson's ratio; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value; θ represents the angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis, and the value range is 0°-360°; k is a constant determined by calibration test; Conclusion In the formula, σ0 represents the spherical stress of the soil; σ r represents the deviatoric stress of the soil; E is the elastic modulus of the thin-walled ring; ν is the Poisson's ratio; r1 is the outer diameter of the thin-walled ring; r2 is the inner diameter of the thin-walled ring; ε max is the maximum hoop strain value; ε min is the minimum hoop strain value; θ represents the angle of rotation from the positive direction of the x-axis along the first quadrant to the positive direction of the y-axis in Figure 3, and its value range is 0°-360°; k is a constant determined by calibration test.

2. The soil multi-directional stress monitoring method based on optical frequency domain reflection according to claim 1 is characterized in that: In step 1), the depth of the U-shaped groove formed along the circumferential direction of the thin-walled circular ring (1) is the same as the diameter of the strain sensing optical fiber (2) or the temperature sensing optical fiber (3).

3. The soil multi-directional stress monitoring method based on optical frequency domain reflection according to claim 1 is characterized in that: The side surface of the thin-walled circular ring (1) is coated with vaseline to reduce friction.

4. The soil multi-directional stress monitoring method based on optical frequency domain reflection according to claim 1 is characterized in that: The temperature sensing optical fiber (3) in step 1) is used for temperature compensation.

5. The soil multi-directional stress monitoring method based on optical frequency domain reflection according to claim 1 is characterized in that: The top and bottom ends of the thin-walled ring (1) in step 1) are sealed.

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