Lateral stress monitoring method under one-dimensional consolidation condition of soil body

Abstract

The invention provides a lateral stress monitoring method under a soil body one-dimensional consolidation condition, which comprises the following steps: manufacturing FBG (Fiber Bragg Grating) limiting rings: fixedly arranging a grating fiber on the periphery of a ring body along the circumferential direction of the ring body to obtain the FBG limiting ring for testing and the FBG limiting ring for temperature compensation; preparing a compacted soil sample: compacting the soil sample in the ring body of the FBG limiting ring for the test, and arranging a load above the soil sample; and lateral strain monitoring: respectively connecting grating fiber joints of the FBG limiting ring for testing and the FBG limiting ring for temperature compensation into a demodulator and connecting the demodulator with a computer system, respectively measuring the current wavelength lambda1 of the FBG limiting ring for testing and the current wavelength lambda2 of the FBG limiting ring for temperature compensation, and obtaining a dependent variable epsilon 1 according to a formula (1), and calculating the lateral stress P of the soil body acting on the inner wall of the ring body of the FBG limiting ring for the test according to a formula (2).

Description

Technical Field

[0001] This invention relates to the field of soil physical and mechanical property monitoring technology, specifically to a method for monitoring lateral stress under one-dimensional consolidation conditions of soil. Background Technology

[0002] Lateral stress is a critical factor in geotechnical engineering design and analysis. Accurate estimation of lateral stress is essential for ensuring the stability and safety of geotechnical structures. In tunnel engineering, increased lateral stress promotes the propagation of tensile cracks, severely jeopardizing tunnel stability. In retaining structure design, elevated lateral stress can lead to excessive soil displacement, resulting in over-range deformation and ultimately structural failure. Furthermore, excessive lateral stress acting on pile foundations may cause shear failure of the piles, potentially leading to the collapse of the superstructure.

[0003] Existing methods for measuring lateral stress can be divided into two categories: direct methods and indirect methods. Direct methods measure the lateral stress of soil under vertical loads directly using instruments such as stress gauges. In contrast, indirect methods estimate lateral stress by converting other measured mechanical parameters. A common technique is to use resistance strain gauges (ESGs) around a mold. These strain gauges calculate the lateral stress by detecting the circumferential strain of the mold. While these methods generally provide reliable lateral stress measurements, they are limited by inherent problems such as installation complexity, electromagnetic interference, and lateral sensitivity effects, hindering their widespread application. Furthermore, the durability and availability of these monitoring devices are significantly reduced in harsh laboratory environments (high temperatures, strong acids, etc.). In addition, methods such as stress gauges and strain gauges often only provide measurements at a single point or a limited number of locations in many cases. Improvements to multi-point measurements are also constrained by equipment size, wiring complexity, and other limiting factors. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for monitoring lateral stress under one-dimensional soil consolidation conditions.

[0005] To solve the above-mentioned technical problems, the technical solution adopted in the first aspect of the present invention is as follows:

[0006] A method for monitoring lateral stress in soil under one-dimensional consolidation conditions includes the following steps:

[0007] Fabrication of FBG confinement ring: The grating fiber is fixedly set around the outer periphery of the ring body along the circumference of the ring body to obtain the experimental FBG confinement ring and the temperature compensation FBG confinement ring.

[0008] Preparation of compacted soil samples: The soil sample is compacted and placed inside the ring body of the FBG confinement ring for the test, and a load is placed on top of the soil sample;

[0009] Lateral strain monitoring: Connect the fiber optic connectors of the test FBG confinement ring and the temperature compensation FBG confinement ring to the demodulator and the computer system respectively, and measure the current wavelength λ1 of the test FBG confinement ring and the current wavelength λ2 of the temperature compensation FBG confinement ring respectively. Obtain the strain ε1 according to the following formula (1):

[0010]

[0011] Where λ B1 λ is the center wavelength of the FBG confinement ring used in the experiment. B2 c is the center wavelength of the temperature compensation FBG confinement ring. ε1 The strain coefficient;

[0012] Lateral stress conversion: The lateral stress P exerted by the soil on the inner wall of the FBG confinement ring used in the test is calculated according to the following formula (2):

[0013]

[0014] Where E is the elastic modulus of the ring body of the FBG confinement ring used in the experiment, and R in R is the inner radius of the ring body of the FBG confinement ring used in the experiment. out The outer radius of the ring body of the FBG confinement ring used in the experiment.

[0015] In one embodiment, during the FBG confinement ring fabrication step, a single grating fiber is arranged in a single loop around the periphery of the ring body.

[0016] In one embodiment, during the FBG confinement ring fabrication step, the grating fiber is closed at both ends to form the single loop and a grating fiber connector is reserved.

[0017] In one embodiment, during the FBG confinement ring fabrication step, the grating fiber is arranged to surround the centerline of the ring body in the horizontal direction.

[0018] In one embodiment, during the FBG confinement ring fabrication step, the grating fiber is fixed to the ring body using epoxy resin adhesive.

[0019] In one embodiment, the ring body is made of one of acrylic, PVC, or steel.

[0020] In one embodiment, during the compacted soil sample preparation step, the upper surface of the soil sample is flush with the upper surface of the ring body.

[0021] In one embodiment, during the compacted soil sample preparation step, when the working condition is a soil consolidation test and it is a drained consolidation test, the first permeable stone needs to be placed at the bottom of the ring body and a first filter paper is placed on top. Then, the soil sample is compacted and placed on top of the first filter paper. A second filter paper and a second permeable stone are sequentially placed on the upper surface of the soil sample. A load is then placed on top of the second permeable stone. The diameters of the first permeable stone, the second permeable stone, the first filter paper, and the second filter paper are all the same as the inner diameter of the ring body.

[0022] In one embodiment, in the lateral strain monitoring step, the acquisition of the current wavelength λ1 of the test FBG confinement ring and the current wavelength λ2 of the temperature compensation FBG confinement ring is set to be acquired simultaneously once at fixed intervals within a time period.

[0023] In one embodiment, the height of the ring body is 1.5 to 2.5 cm, and / or the difference between the outer radius and the inner radius of the ring body is 0.5 to 1 cm.

[0024] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0025] a. High sensitivity and high-quality data acquisition: The fiber optic sensing technology in this invention is based on changes in optical signals and has extremely high sensitivity; the accuracy of the FBG (fiber Bragg grating) confinement loop can reach 10. -12 Meter-level precision enables the accurate detection of minute deformations, such as strain and temperature changes. Compared to traditional strain gauges and stress cells that rely on electrical signals, fiber Bragg gratings are immune to electromagnetic interference, can operate stably in complex environments, and acquire high-quality data.

[0026] b. High-density measurement: The diameter of the fiber grating in the vertical direction of the present invention is at the micrometer level, so multiple fiber gratings can be set in the vertical direction of a single ring body according to the actual situation to achieve high-density, quasi-distributed measurement in the deep soil direction. Traditional displacement gauges and stress cells are difficult to achieve such high-density measurement due to their large size and complex wiring.

[0027] c. Environmentally friendly and suitable for on-site monitoring: Traditional testing methods, such as strain gauges and stress cells, are mainly made of copper, which is prone to corrosion during long-term use in real-world environments, leading to equipment failure. Furthermore, metal corrosion can pollute soil and groundwater. In contrast, the fiber optic grating used in this invention is less prone to degradation or failure and causes less environmental pollution. The optical fiber itself possesses strong resistance to electromagnetic interference and acid rain corrosion, greatly improving its long-term adaptability for use in real-world environments.

[0028] d. Through formula design, not only the influence of ambient temperature on lateral stress P is considered, but also the influence of factors such as the material and thickness of the ring body on lateral stress P, making the final lateral stress P more accurate. Attached Figure Description

[0029] Figure 1 A schematic diagram showing a soil sample placed on a test FBG confinement ring according to an embodiment of the present invention;

[0030] Figure 2 This is a schematic diagram of the experimental FBG limiting ring being set in operating conditions according to an embodiment of the present invention;

[0031] Numbering on the map:

[0032] 1-Ring body; 2-Fiber optic grating; 3-Dial gauge; 4-FBG confinement ring for testing; 5-Loading disk; Y-Vertical load; Z-Lateral load. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0034] like Figure 1-2 As shown, the method for monitoring lateral stress in soil under one-dimensional consolidation conditions according to the present invention includes the following steps:

[0035] Fabrication of FBG confinement rings: A grating fiber is fixedly positioned around the periphery of the ring body to obtain an experimental FBG confinement ring and a temperature compensation FBG confinement ring. The grating fiber refers to an optical fiber with a section of Bragg grating. In one specific embodiment, in both the experimental and temperature compensation FBG confinement rings, a single grating fiber is arranged in a single loop on the ring body, with the ends of the single grating fiber closed. A grating fiber connector is provided, and the grating fiber is horizontally arranged around the centerline of the ring body in the horizontal direction. That is, the grating fiber is positioned within the ring body. At the upper and lower equidistant points, the single grating fiber is closed at both ends to prevent lateral strain concentration around the grating fiber, which would affect the accuracy of subsequent measurements. In other examples, the single grating can be only provided with an open section around the circumference of the ring body to save fiber grating raw materials. The grating fiber can be fixed to the ring body with epoxy resin. Preferably, the part of the ring body surrounding the fiber grating is coated with epoxy resin to ensure that the fiber grating is firmly bonded to the ring body, so that the ring body and the fiber grating can deform synchronously and ensure the accuracy of subsequent measurements.

[0036] Compacted soil sample preparation: The compacted soil sample is placed inside the FBG confinement ring of the test, and a load is placed on top of the soil sample. In one specific embodiment, the upper surface of the soil sample is flush with the upper surface of the ring body. When the working condition is a soil consolidation test, and it is a drained consolidation test, the first permeable stone needs to be placed at the bottom of the ring body, and the first filter paper is placed on top. Then the compacted soil sample is placed on top of the first filter paper. The second filter paper and the second permeable stone are placed on the upper surface of the soil sample in sequence, and then a load is placed on top of the second permeable stone. The diameters of the first permeable stone, the second permeable stone, the first filter paper, and the second filter paper are all the same as the inner diameter of the ring body. Figure 2 As shown, the load can be set by a consolidation apparatus, which has a loading disk to adjust the load weight. The consolidation apparatus can also be set with a dial gauge to ensure test accuracy. In other embodiments of soil consolidation tests, the working condition can also be undrained compression.

[0037] Lateral strain monitoring: The fiber optic connectors of the test FBG confinement ring and the temperature compensation FBG confinement ring are connected to the demodulator and then to the computer system. The current wavelength λ1 (in nm) of the test FBG confinement ring and the current wavelength λ2 (in nm) of the temperature compensation FBG confinement ring are measured respectively. In a specific implementation, the acquisition of the current wavelength λ1 of the test FBG confinement ring and the current wavelength λ2 of the temperature compensation FBG confinement ring is set to be acquired simultaneously at fixed intervals within a time period, such as acquiring λ1 and λ2 simultaneously every 5 seconds. The strain ε1 (in με) is obtained according to the following formula (1). -1 This refers to the change in length per unit length of the grating.

[0038]

[0039] Where λ B1 λ is the center wavelength (in nm) of the FBG confinement ring used in the experiment. B2 The center wavelength (in nm) of the FBG confinement ring used for temperature compensation. Wherein, the center wavelength λ... B1 and λ B2 The wavelengths λ1 and λ2 are obtained from the fiber's factory parameters, which depend on the grating etching technology and fiber materials; the current wavelengths λ1 and λ2 are obtained from the demodulator readings, which depend on the current fiber deformation state. ε1 The strain coefficient (unit: με) is used here. The purpose of setting up the FBG confinement ring for temperature compensation is to eliminate the influence of ambient temperature. The strain ε1 obtained here is the deformation of the grating fiber caused solely by the strain of the sample.

[0040] The meaning and calculation process of the above formula (1) are as follows:

[0041] The grating fiber with FBG confinement ring will exhibit wavelength change when affected by external strain ε or temperature T. The wavelength λ will differ from the center wavelength λ. B The difference between them is called the wavelength difference, and their relationship can be expressed as follows:

[0042]

[0043] Where λ represents the measured current wavelength, ΔT represents the temperature change, and c ε With c T These refer to the strain coefficient (unit / με) and the temperature coefficient (unit / ℃), respectively, where the strain coefficient c ε This refers to the amplitude of the grating wavelength change caused by unit strain. This value is determined by the photoelastic constant of the grating, the change in grating period caused by strain, and the temperature deformation coefficient c. T This refers to the change in grating wavelength caused by a unit temperature change, a value determined by both the grating's coefficient of thermal expansion and its thermo-optical coefficient. In one embodiment, c ε1 It is 0.78*10 -6 / με,c T It is 6.67*10 -6 / ℃, both of these coefficients can be determined by indoor calibration tests, and the same fiber grating can be considered a constant during the test.

[0044] During the experiment, to eliminate the influence of ambient temperature, a temperature compensation FBG limiting ring was also installed. Both the experimental FBG limiting ring and the temperature compensation FBG limiting ring follow the relationship above, that is, the experimental FBG limiting ring satisfies the following formula:

[0045]

[0046] That is, for the experimental FBG confinement ring, λ B1 The center wavelength (in nm) of the FBG confinement ring used in the experiment is given, λ1 represents the measured current wavelength (in nm), ΔT1 represents the temperature change, and c ε1 With c T1 These represent the strain coefficient (unit / με) and temperature coefficient (unit / ℃), respectively, while ε1 represents the strain (unit / με). -1 ).

[0047] For temperature compensation FBG confinement rings, the following equation must be satisfied:

[0048]

[0049] That is, for temperature compensation using FBG confinement loops, λ B2The center wavelength (in nm) of the FBG confinement ring used for temperature compensation, where λ2 represents the measured current wavelength (in nm), ΔT2 represents the temperature change, and c ε2 With c T2 ε1 and ε2 respectively represent the strain coefficient (unit / με) and temperature coefficient (unit / ℃), and the strain (unit / με) is the variable amount of strain. -1 ).

[0050] In the actual test, since the FBG confinement ring used for the test and the FBG confinement ring used for temperature compensation are made under the same conditions, it can be assumed that the strain deformation coefficients and the temperature deformation coefficients in the two are equal and are both constants.

[0051] For temperature-compensated FBG confinement rings, since no strain is applied to them, i.e., the strain ε2 is zero, it is assumed that the temperature-compensated FBG confinement rings are only affected by temperature changes.

[0052] For the experimental FBG confinement ring, in order to eliminate the influence of temperature changes (c T1 Since both the experimental FBG confinement ring and the temperature compensation FBG confinement ring are placed at the same ambient temperature, it can be assumed that ΔT1 is equal to ΔT2.

[0053] In summary, for the experimental FBG confinement ring, its formula can be rewritten as:

[0054]

[0055] Transforming the above equation, we obtain equation (1):

[0056]

[0057] Lateral stress conversion: The lateral stress P (unit kPa) of the soil acting on the inner wall of the FBG confinement ring used in the test is calculated according to the following formula (2):

[0058]

[0059] Where E is the elastic modulus of the ring body of the FBG confinement ring used in the experiment. In one embodiment, E is 3.2 GPa. The ring body is made of acrylic material. In other embodiments, the ring body material can also be steel or PVC. in To test the inner radius of the FBG-constrained ring body, R out The outer radius of the ring body is used to constrain the FBG for the experiment.

[0060] The present invention will be further described below with reference to embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific applications, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0061] Example 1:

[0062] The method for monitoring lateral stress in soil under one-dimensional consolidation conditions of the present invention includes the following steps:

[0063] Fabrication of the FBG confinement ring: Align the section of the fiber grating containing the Bragg grating with the center line of the ring body vertically and horizontally wrap it around the ring body. Quickly apply epoxy resin adhesive, manually maintaining close contact between the fiber grating and the ring body during this process. After the epoxy resin cures, arrange the fiber gratings around the center line of the ring body one by one and adhere them firmly with ring resin, ensuring synchronous deformation of the FBG confinement ring and the fiber grating. This yields the experimental FBG confinement ring and the temperature compensation FBG confinement ring. The ring body is 2cm high, 5cm inner diameter, and 6cm outer diameter, made of acrylic, using SMF-28e fiber optic cable with a grating length of 10mm. The center wavelength λ of the experimental FBG confinement ring is... B1 The center wavelength λ of the temperature-compensated FBG confinement ring is 1542.851 nm. B2 The center wavelength λ is 1549.369 nm. B1 and λ B2 The optical fiber specifications are obtained from the manufacturer's specifications: bandwidth < 0.3nm, side-mode rejection ratio > 15dB, reflectivity > 90%, and the grating coating is polyimide.

[0064] Preparation of compacted soil samples: Naturally dried soil is ground and dried using a crusher. In this embodiment, expansive soil from Pi-Shi-Hang, Anhui Province is used. An appropriate amount of deionized water is added to the dry soil to achieve the desired specific moisture content. The soil is then sealed and stored for a period of time to allow the moisture in the soil to gradually distribute evenly. The cured soil is statically compacted into the body of the FBG confinement ring used in the test using a press to obtain a soil sample with the designed moisture content and designed dry density. The upper surface of the soil sample is flush with the upper surface of the ring body. This embodiment is a soil consolidation test, specifically a drained consolidation test. The first permeable stone is placed at the bottom of the ring body, and a first filter paper is placed underneath. The compacted soil sample is then placed on top of the first filter paper. A second filter paper and a second permeable stone are sequentially placed on the upper surface of the soil sample. A load is then placed on top of the second permeable stone. The diameters of the first permeable stone, the second permeable stone, the first filter paper, and the second filter paper are all the same as the inner diameter of the ring body. Figure 2As shown, the load is specifically set by the consolidation apparatus. The pressure head is installed above the second permeable stone and placed at the bottom of the loading end of the consolidation apparatus lever. The lever is adjusted to ensure good contact between the pressure head and the pressure head.

[0065] Lateral strain monitoring: The fiber optic connectors of the experimental FBG confinement ring and the temperature compensation FBG confinement ring were connected to the demodulator and then to the computer system. The demodulator model was NZS-A01. The current wavelength λ1 (in nm) of the experimental FBG confinement ring and the current wavelength λ2 (in nm) of the temperature compensation FBG confinement ring were measured. The acquisition of the current wavelengths λ1 and λ2 of the experimental FBG confinement ring and the temperature compensation FBG confinement ring was set to acquire λ1 and λ2 simultaneously every 5 seconds. The strain ε1 (in με) was obtained according to the following formula (1). -1 ):

[0066]

[0067] Among them, c ε1 It is 0.78*10 -6 / με,

[0068] Lateral stress conversion: The lateral stress P (unit kPa) of the soil acting on the inner wall of the FBG confinement ring used in the test is calculated according to the following formula (2):

[0069]

[0070] The elastic modulus E of the FBG confinement ring used in the experiment was 3.2 GPa, and the inner radius R of the FBG confinement ring used in the experiment was... in The outer radius R of the FBG-constrained ring body is 2.5cm. out It is 3cm.

[0071] The ten sets of data obtained by measurement and calculation according to the method in Example 1 are shown in Table 1:

[0072] Table 1

[0073]

[0074] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for monitoring lateral stress under one-dimensional soil consolidation conditions, characterized in that, Includes the following steps: Fabrication of FBG confinement ring: The grating fiber is fixedly set around the outer periphery of the ring body along the circumference of the ring body to obtain the experimental FBG confinement ring and the temperature compensation FBG confinement ring. Preparation of compacted soil samples: The soil sample is compacted and placed inside the ring body of the FBG confinement ring for the test, and a load is placed on top of the soil sample; Lateral strain monitoring: Connect the fiber optic connectors of the test FBG confinement ring and the temperature compensation FBG confinement ring to the demodulator and the computer system respectively, and measure the current wavelength λ1 of the test FBG confinement ring and the current wavelength λ2 of the temperature compensation FBG confinement ring respectively. Obtain the strain ε1 according to the following formula (1): Where λ B1 λ is the center wavelength of the FBG confinement ring used in the experiment. B2 c is the center wavelength of the temperature compensation FBG confinement ring. ε1 The strain coefficient; Lateral stress conversion: The lateral stress P exerted by the soil on the inner wall of the FBG confinement ring used in the test is calculated according to the following formula (2): Where E is the elastic modulus of the ring body of the FBG confinement ring used in the experiment, and R in R is the inner radius of the ring body of the FBG confinement ring used in the experiment. out The outer radius of the ring body of the FBG confinement ring used in the experiment.

2. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: In the FBG confinement ring fabrication step, a single grating fiber is arranged in a single loop around the periphery of the ring body.

3. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 2, characterized in that: In the FBG confinement ring fabrication step, the grating fiber is closed at both ends to form the single loop and a grating fiber connector is reserved.

4. The method for monitoring lateral stress under one-dimensional soil consolidation conditions according to claim 2, characterized in that: In the FBG confinement ring fabrication step, the grating fiber is arranged around the center line of the ring body in the horizontal direction.

5. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 2, characterized in that: In the FBG confinement ring fabrication step, the grating fiber is fixed to the ring body using epoxy resin adhesive.

6. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: The ring body is made of one of the following materials: acrylic, PVC, or steel.

7. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: In the compacted soil sample preparation step, the upper surface of the soil sample is flush with the upper surface of the ring body.

8. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: In the compacted soil sample preparation step, when the working condition is a soil consolidation test and it is a drained consolidation, the first permeable stone needs to be placed at the bottom of the ring body and the first filter paper is placed on it. Then the soil sample is compacted and placed on top of the first filter paper. The second filter paper and the second permeable stone are placed on the upper surface of the soil sample in sequence. Then a load is placed on top of the second permeable stone. The diameters of the first permeable stone, the second permeable stone, the first filter paper, and the second filter paper are all the same as the inner diameter of the ring body.

9. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: In the lateral strain monitoring step, the acquisition of the current wavelength λ1 of the test FBG confinement ring and the current wavelength λ2 of the temperature compensation FBG confinement ring is set to be acquired simultaneously once at fixed intervals within a time period.

10. The method for monitoring lateral stress under one-dimensional consolidation conditions of soil according to claim 1, characterized in that: The height of the ring body is 1.5 to 2.5 cm, and / or the difference between the outer radius and the inner radius of the ring body is 0.5 to 1 cm.

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