Three-dimensional membrane type earth pressure cell calibration method

Abstract

The invention discloses a three-dimensional membrane type earth pressure cell calibration method. The method comprises the following steps: assembling a calibration device; preparing calibration soil; soil for calibration is filled in an inner box of the calibration box, and a three-dimensional membrane type soil pressure box to be calibrated is buried in the middle of the soil; mounting axial force meters and displacement meters in x, y and z directions; determining two boundary loads according to an actual to-be-tested soil body load test curve so as to determine a graded loading value; carrying out vertical loading step by step, and carrying out faceted and segmented calibration on the soil pressure cell in x, y and z directions according to two boundary loads; and replacing the calibration soil, and repeating the calibration process to obtain proper calibration soil and faceted and segmented calibration coefficients. According to the three-dimensional membrane type soil pressure cell calibration method, segmented and faceted calibration can be carried out according to the deformation characteristics of the to-be-measured soil body, and accurate measurement of the three-dimensional stress state in the soil body is achieved.

Description

Technical Field

[0001] The invention relates to the calibration of a membrane type earth pressure cell, in particular to a calibration method of a three-dimensional membrane type earth pressure cell. Background Art

[0002] The membrane earth pressure cell is mainly used to measure the size of the earth pressure in the soil and its changing process. It is widely used in engineering construction in the fields of water conservancy, civil engineering, transportation, etc. and indoor model tests to measure the internal stress of the soil and the earth pressure at the contact surface between the soil and the structure. Due to the difference in stiffness between the membrane earth pressure cell material buried in the soil and the surrounding soil, as well as the structural characteristics of the earth pressure cell itself, such as the difference in stiffness between the earth pressure cell diaphragm and shell, the stress on the earth pressure cell diaphragm and shell is different after the earth pressure cell is buried in the soil. Therefore, there is an error between the earth pressure value measured by the earth pressure cell and the actual earth pressure value in the soil, that is, a matching error, and it needs to be calibrated before use.

[0003] At present, the common calibration method of membrane earth pressure cell is mainly for one-dimensional membrane earth pressure cell, that is, the earth pressure cell can only measure the earth pressure value in one direction at a certain part of the soil body, but cannot measure the earth pressure in three orthogonal directions, and cannot obtain the three-dimensional stress state of the soil body to be tested.

[0004] The three-dimensional membrane earth pressure cell can measure the earth pressure values ​​in three orthogonal directions at a certain part of the soil, and can also measure the horizontal and vertical deflection angles of the earth pressure cell in the soil as the surrounding soil moves. The force modes of the membranes in the three orthogonal directions of the three-dimensional membrane earth pressure cell are not exactly the same during the soil loading and deformation process, and the soil's own settlement-load relationship is not linear during the soil loading process. The traditional overall calibration uses a calibration coefficient to establish the relationship between the measured value and the load value, which has a large error. Only by establishing calibration coefficients for the earth pressure measurement surfaces in the three orthogonal directions according to the load stage can the three-dimensional membrane earth pressure cell be accurately measured. How to accurately calibrate the three-dimensional membrane earth pressure cell, realize the accurate measurement of the three-dimensional stress inside the soil, and monitor the internal stress state of the soil in real time to ensure the safety and quality of the project is crucial. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a three-dimensional membrane earth pressure cell calibration method, which can calibrate the three-dimensional membrane earth pressure cell according to the earth pressure measurement surfaces in three orthogonal directions in sections according to the load, thereby realizing accurate measurement of the three-dimensional stress state inside the soil.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A three-dimensional membrane earth pressure cell calibration method comprises the following steps:

[0008] S1. Assemble the three-dimensional membrane earth pressure box calibration device to form a calibration box-in-box;

[0009] S2. According to the size of the diaphragm of the three-dimensional membrane earth pressure box to be calibrated, sand is configured as the calibration soil;

[0010] S3. Fill the calibration box with soil for calibration and bury the three-dimensional membrane earth pressure box to be calibrated in the middle of the soil;

[0011] S4. Install the top surface loading device, and install the axial force gauge and displacement gauge in the x, y, and z directions;

[0012] S5. Determine the temporary plastic load and the ultimate load according to the load test curve of the soil to be tested in the actual project, and then determine the maximum value and graded loading value of the calibration load of the three-dimensional membrane earth pressure cell to be calibrated;

[0013] S6. Carry out vertical graded loading according to the graded loading values, and record the readings of each axial force gauge and displacement gauge under each grade of load;

[0014] S7. The three-dimensional membrane earth pressure cell to be calibrated is calibrated in sections and planes according to the three directions of x, y and z and the three load sections divided by two limit loads;

[0015] S8. Replace the calibration soil, change its particle size distribution, and repeat steps S2-S7;

[0016] S9. Obtain suitable calibration soil and surface and segment calibration coefficients for the three-dimensional membrane earth pressure cell to be calibrated.

[0017] In S1, the three-dimensional membrane earth pressure box calibration device includes a calibration box, in which a pair of transverse retaining plates are arranged on the left and right sides, and a pair of longitudinal retaining plates are arranged on the front and back sides. The outer sides of the transverse retaining plates and the longitudinal retaining plates are fixed by gaskets, bolts, inner nuts and outer nuts; the bottom plate, transverse retaining plates, longitudinal retaining plates and top loading plates of the calibration box constitute the inner box of the calibration box; a loading device is arranged on the upper part of the top loading plate, and the loading device includes a jack installed on the top loading plate, and the jack is counteracted by the reaction frame on the top of the calibration box through a pad.

[0018] The axial force gauges in the x, y and z directions in S4 include a first axial force gauge and a second axial force gauge, wherein the first axial force gauge is installed between the top loading plate and the jack, and the second axial force gauge is installed at the end of the gasket and the other end is in contact with the side wall of the transverse retaining plate or the longitudinal retaining plate; the displacement gauges in the x, y and z directions include a first displacement gauge and a second displacement gauge, wherein the first displacement gauge is installed between the reaction frame and the top loading plate, and one end of the second displacement gauge is against the inner wall of the calibration box and the other end is in contact with the side wall of the transverse retaining plate or the longitudinal retaining plate.

[0019] In S2, the most suitable calibration soil is determined by performing multiple calibration tests by changing the particle gradation of the sand; the particle gradation of the sand includes the maximum particle size of the sand, the distribution range of the particle size and the content of each particle group.

[0020] In S3, the calibration soil is filled in layers in the inner box of the calibration box to ensure uniform filling, and the three-dimensional membrane earth pressure box to be calibrated is buried in the middle of the soil. The bottom surface of the three-dimensional membrane earth pressure box to be calibrated is horizontal, and the six surfaces of the three-dimensional membrane earth pressure box to be calibrated are respectively parallel to the six surfaces of the inner box of the calibration box.

[0021] In S4, the top loading plate is installed to ensure that its bottom surface is horizontal, and the axial force gauge and displacement gauge are installed and left to stand for 24 hours.

[0022] In S5, soil deformation is divided into linear elastic deformation stage, plastic deformation stage and failure stage by using the pre-plastic load and ultimate load. The maximum value and graded loading value of the calibration load of the three-dimensional membrane earth pressure box are determined according to the load imposed on the soil in the actual project and the deformation stage of the soil.

[0023] In S8, the calibration soil is replaced, its particle gradation is changed, that is, its maximum particle size, the distribution range of particle size and the content of each particle group are changed, and steps S2-S7 are repeated to explore the size of the soil arch effect of different soils in the three-dimensional earth pressure box calibration and its influence on the earth pressure box test results in the x, y and z directions.

[0024] The present invention provides a three-dimensional membrane earth pressure cell calibration method, which has the following technical effects:

[0025] 1) By calibrating the three-dimensional membrane earth pressure cell in sections and planes according to the deformation characteristics of the soil to be tested, the influence of the discreteness of the soil in the calibration of the three-dimensional membrane earth pressure cell and the non-uniformity and nonlinear characteristics of its deformation on the test results of the earth pressure cell is revealed. That is, due to the aforementioned characteristics of the soil, the embedding of the earth pressure cell will cause uneven deformation inside the soil, and then produce a soil arch effect. The strength of the soil arch effect varies greatly with the relationship between the direction of the earth pressure cell diaphragm and the direction of the major principal stress in the soil, and changes with the change of the load stage. Therefore, the proportional relationship between the measured values ​​and the true values ​​on the three orthogonal surfaces of the three-dimensional membrane earth pressure cell is not the same, and it shows a large difference with the change of the load stage.

[0026] 2) By calibrating the three-dimensional membrane earth pressure box in sections and planes according to the deformation characteristics of the soil to be measured, it has obvious advantages over the current liquid calibration and other calibration methods. It can accurately calibrate the three-dimensional membrane earth pressure box and realize the accurate measurement of the three-dimensional stress inside the soil. When the three-dimensional membrane earth pressure is buried in the soil, the upper and lower surfaces are in the horizontal direction. Then, during the loading process, the upper and lower surfaces are perpendicular to the direction of the major principal stress, and the four sides are perpendicular to the other two principal stress directions. During the loading process, the loading values ​​on the three orthogonal surfaces of the three-dimensional membrane earth pressure box (the loading values ​​are measured by the axial force gauges in the three directions in the calibration device) and the measured values ​​on the three orthogonal surfaces (the measured values ​​are directly measured by the three-dimensional membrane earth pressure box) are simultaneously measured, and the three proportional relationships between the measured values ​​and the loading values ​​in this load stage can be established, that is, the calibration coefficients on the three surfaces of the three-dimensional membrane earth pressure box. After the load stage changes, it is necessary to establish three new calibration coefficients, that is, it is necessary to establish the calibration coefficients on the three surfaces of each stage in the linear elastic deformation stage, the plastic deformation stage and the failure stage.

[0027] 3) By calibrating the three-dimensional membrane earth pressure box in sections and planes according to the deformation characteristics of the soil to be tested, combined with actual projects, some calibration coefficients can be simplified in specific cases. For example, in common foundation projects, if only vertical loading is applied (such as building houses on the ground), the vertical direction in the soil is the major principal stress, and the two principal stresses in the two orthogonal directions on the horizontal plane are the same. At this time, for each load stage, only two calibration coefficients need to be established for the major principal stress direction (vertical) and the minor principal stress direction (lateral). In addition, if the load is small (for example, the building built on the ground is not high), and the final loading value in the soil does not reach the destruction stage, it is only necessary to establish the calibration coefficients on three surfaces of the linear elastic deformation stage and the plastic deformation stage during the soil loading process. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0029] Figure 1 It is a flow chart of the present invention.

[0030] Figure 2 It is the front view of the present invention.

[0031] Figure 3 It is a structural schematic diagram of the calibration box in the present invention.

[0032] Figure 4 It is a top view of the calibration box in the present invention.

[0033] Figure 5 It is a test curve diagram of the vertical direction of the three-dimensional membrane earth pressure box under loading in the present invention.

[0034] Figure 6It is a test curve diagram of the horizontal direction of the three-dimensional membrane earth pressure box under loading in the present invention.

[0035] In the figure: calibration box 1, cushion block 2, reaction frame 3, first displacement meter 4, jack 5, first axial force meter 6, top loading plate 7, data acquisition instrument 8, computer 9, three-dimensional membrane earth pressure box to be calibrated 10, transverse retaining plate 11, longitudinal retaining plate 12, inner nut 13, bolt 14, gasket 15, second axial force meter 16, outer nut 17, wire hole 18, bottom plate 19, second displacement meter 20. DETAILED DESCRIPTION

[0036] like Figure 1-2 As shown, a three-dimensional membrane earth pressure cell calibration method includes the following steps:

[0037] S1. Assemble the three-dimensional membrane earth pressure box calibration device, install four side retaining plates, namely, a pair of transverse retaining plates 11 in the x direction and a pair of longitudinal retaining plates 12 in the y direction, and together with the bottom plate 19 and the top loading plate 7 installed at the back, form the inner box of the calibration box 1. The four side retaining plates are respectively installed and fixed by gaskets 15, bolts 14, inner nuts 13 and outer nuts 17.

[0038] S2. According to the size of the diaphragm of the three-dimensional membrane earth pressure box to be calibrated, the sand with particle gradation is designed as the calibration soil. In order to reduce the soil arch effect on the diaphragm of the earth pressure box, the maximum particle size of the sand is less than 1 / 10 of the diameter of the diaphragm. In this embodiment, a sand group with a particle size of 0.5mm-1mm is used to prepare the soil to be calibrated with a porosity of 0.7 for calibration.

[0039] S3. Fill the inner box of the calibration box 1 with soil for calibration, and bury the three-dimensional membrane earth pressure box 10 to be calibrated in the middle of the soil; in this embodiment, after filling with 10 cm thickness of soil, it is evenly compacted to ensure that the inner box soil of the calibration box 1 is as evenly distributed as possible, and pay attention to the amount of filling to maintain the porosity of the soil in the box; bury the three-dimensional membrane earth pressure box 10 to be calibrated in the middle of the soil, pay attention to the x, y, z directions of the three-dimensional membrane earth pressure box 10 to be calibrated and the x, y, z directions of the inner box of the calibration box 1 to be consistent, and pay attention to the wiring of the three-dimensional membrane earth pressure box 10 to be calibrated. The wire of the three-dimensional membrane earth pressure box 10 to be calibrated is led out from the wire hole 18 to avoid calibration errors caused by improper wiring. After the soil in the box is filled, it needs to be left to stand for 24 hours to release the internal stress of the soil.

[0040] S4. Install the top loading plate 7, install the first axial force meter 6, jack 5, pad 2, reaction frame 3 and first displacement meter 4 on the top loading plate 7, install the second axial force meter 16 on the four side retaining plates and the second displacement meter 20 on the side that allows outward movement, that is, install the axial force meters and displacement meters in the x, y and z directions. Connect all data cables to the data acquisition instrument 8, and the data acquisition instrument 8 is connected to the computer 9.

[0041] Figure 4 There is only one second displacement meter 20 in the top view. In most cases, the stresses in the two orthogonal directions on the horizontal plane are the same, so only one displacement meter and one axial force meter need to be installed in the vertical direction and horizontal direction. In special cases, that is, the stresses in the two orthogonal directions on the horizontal plane are different, such as excavation of a foundation pit on one side of the soil body to be tested, in this case, one displacement meter and one axial force meter need to be installed in both orthogonal directions on the horizontal plane.

[0042] When there is no horizontal excavation or other side unloading, the displacement of the four side retaining plates is locked; when there is horizontal excavation or other side unloading, the retaining plate on the excavation side is allowed to move slightly outward by rotating the inner nut on that side to reflect the stress environment when the soil on that side is deformed due to soil excavation.

[0043] S5. Determine its two limit loads (critical plastic load and ultimate load) according to the load test curve of the earth pressure soil body to be tested in the actual project, and then determine the maximum value and graded loading value of the calibration load of the three-dimensional membrane earth pressure box.

[0044] Since the earth pressure cell is composed of a rigid shell and a flexible diaphragm, the stress at the buried location of the earth pressure cell is determined by measuring the deformation of the diaphragm or the stress transmitted by the diaphragm. However, the deformation and stress of the soil on site are not linearly related, so the deformation law of the soil after being stressed will affect the stress transmitted to the diaphragm of the earth pressure cell.

[0045] According to the on-site soil load test, the load-settlement PS curve is clearly divided into three stages: linear elastic deformation stage, plastic deformation stage and failure stage, divided by two limit loads (plastic load and ultimate load). The load-settlement laws of these three stages are obviously different, so they should be calibrated in sections to reduce errors. The failure load determined by the load test is used as the maximum load value for calibration, and the average of 5 load values ​​in each section of the three stages is taken as the load values ​​for each grade.

[0046] S6. Perform vertical graded loading according to the graded loading values, and record the readings of each axial force gauge and displacement gauge under each load level. Read the readings of the first axial force gauge 6 and the first displacement gauge 4 on the top loading plate 7, the readings of the second axial force gauge 16 and the second displacement gauge 20 on the side retaining plate, and the stress readings of the three-dimensional membrane earth pressure cell 10 to be calibrated on the x, y, and z directions.

[0047] S7. The three-dimensional membrane earth pressure cell 10 to be calibrated is calibrated in sections and planes according to the three directions of x, y and z and the three load sections divided by two limit loads. That is, in addition to the section calibration, the three-dimensional membrane earth pressure cell 10 to be calibrated also needs to be calibrated in sections.

[0048] Since the stress states of the vertical and horizontal surfaces on the three-dimensional membrane earth pressure box are not the same when the soil is vertically loaded, the deformation of the top diaphragm is symmetrical on the left and right, but the deformation of the lower part of the side diaphragm is greater than that of the upper part of the diaphragm due to the influence of the movement direction of the soil particles. The stress and deformation processes in the vertical and horizontal directions around the three-dimensional membrane earth pressure box 10 to be calibrated are very different, so it is necessary to calibrate each surface separately.

[0049] S8. Replace the calibration soil, change its particle size distribution, i.e. change its maximum particle size, particle size distribution range and the content of each particle group, repeat steps S2-S7, and explore the soil arching effect of different soils in the three-dimensional earth pressure box calibration and its influence on the earth pressure box test results in the x, y and z directions. Since the soil particle size distribution affects the soil arching effect on the earth pressure box diaphragm, the best calibration soil suitable for the current earth pressure box size is selected by calibrating the three-dimensional earth pressure box in soils with different particle size distributions.

[0050] S9. Obtain the maximum particle size of the soil suitable for calibration of the three-dimensional membrane earth pressure cell, the distribution range of the particle size, the composition of each particle group, and the segmented calibration coefficients corresponding to the load stage in the elastic deformation stage, plastic deformation stage and failure stage of the surface line in the x, y and z directions.

[0051] Since the earth pressure cell is composed of a rigid shell and a flexible diaphragm, the stress at the buried location of the earth pressure cell is determined by measuring the deformation of the diaphragm or the stress transmitted by the diaphragm. The stiffness of the shell, diaphragm and soil are quite different. During the test, the earth pressure cell has a significant impact on the stress distribution and deformation of the surrounding soil. When the load is small, stress concentration first appears on the shell of the earth pressure cell until a soil arch effect is formed. As the load increases, the soil arch effect in the soil undergoes a process of weakening and destruction, and the stress concentration of the soil on the shell of the earth pressure cell is weakened. The stress and deformation processes in the vertical and horizontal directions around the earth pressure cell are very different and affect each other. There is a large difference between the value measured by the earth pressure cell and the true value, and it needs to be used after error analysis and calibration tests. The ratio of the average stress on the shell to the average stress on the diaphragm is the shell-to-membrane stress ratio n sd ,Right now

[0052]

[0053] Where: s1 , σ s2 , σ d are the average stresses on the shell and diaphragm on both sides of the earth pressure.

[0054] The matching error of the earth pressure box is defined as follows:

[0055]

[0056] Where: σ0 is the true stress value of the soil at the test point of the earth pressure box (here it is equal to the load value applied in this direction of the soil); α is the matching coefficient of the earth pressure box, that is, the ratio of the membrane stress on the earth pressure box to the true stress:

[0057]

[0058] When the three-dimensional membrane earth pressure is buried in the soil, the upper and lower surfaces are in the horizontal direction. In the loading process, the upper and lower surfaces are perpendicular to the direction of the major principal stress (z direction), and the four side surfaces are perpendicular to the other two principal stress directions (x and y directions). During the loading process, the loading values ​​on the three-dimensional membrane earth pressure box in the x, y, and z directions (the loading values ​​are measured by the axial force gauges in the three directions in the calibration device) and the measured values ​​on the three orthogonal surfaces (the measured values ​​are directly measured by the three-dimensional membrane earth pressure box) are measured simultaneously, and three proportional relationships between the measured values ​​and the loading values ​​in this load stage are established, that is, the calibration coefficients on the three surfaces of the three-dimensional membrane earth pressure box. After the load stage changes, three new calibration coefficients need to be established, that is, the calibration coefficients on the three surfaces of each stage in the linear elastic deformation stage, the plastic deformation stage, and the failure stage need to be established respectively.

[0059] Combined with actual projects, some calibration coefficients can be simplified in certain circumstances. For example, in common foundation projects, if only vertical loading is applied (such as building houses on the ground), the vertical direction in the soil is the major principal stress (z direction), and the two principal stresses in the two orthogonal directions (x and y directions) on the horizontal plane are of the same magnitude. At this time, only two calibration coefficients need to be established for the major principal stress direction (z direction) and the minor principal stress direction (x direction) in each load stage. In addition, if the load is small (for example, the building built on the ground is not high), and the final loading value in the soil does not reach the destruction stage, it is only necessary to establish the calibration coefficients for the linear elastic deformation stage and the plastic deformation stage during the soil loading process.

[0060] Figure 5-6 For a calibration test data curve, the stress and deformation processes in the vertical and horizontal directions around the earth pressure box are very different and affect each other, and the calibration coefficients are completely different. Therefore, the earth pressure measurement unit in each direction needs to be calibrated separately.

[0061] In conventional calibration tests, overall calibration is generally used, that is, a calibration coefficient is used to establish the relationship between the measured value and the load value. However, after the measured value is corrected with a unified calibration coefficient, there is a problem of large errors in different load stages. In many engineering projects, such as soft soil roadbed filling, in the early filling process, the soil has no time to consolidate and the strength is very low. A small load change will have a great impact on the stability of the roadbed, and it is necessary to monitor the stress changes in the soil more accurately. If the large matching error is superimposed with the instrument error and human error, it will seriously affect the stress monitoring effect during the construction process and have an important impact on the safety of the project. Therefore, it is necessary to calibrate in sections according to the linear elastic deformation stage, plastic deformation stage and destruction stage of the load.

[0062] The present invention proposes a three-dimensional membrane earth pressure cell calibration method for the first time. The three-dimensional membrane earth pressure cell is calibrated in three load segments divided by three directional surfaces of x, y, and z and two limit loads. The mechanism of interaction between the three-dimensional membrane earth pressure cell and the soil during the test process is revealed, the accuracy of the three-dimensional stress measurement inside the soil is improved, the research on the error of the earth pressure cell test is enriched, and the existing theories related to the calibration of the earth pressure cell are supplemented.

Claims

1. A three-dimensional membrane earth pressure cell calibration method, comprising the following steps: S1. Assemble the three-dimensional membrane earth pressure box calibration device to form a calibration box-in-box; S2. According to the size of the diaphragm of the three-dimensional membrane earth pressure box to be calibrated, sand is configured as the calibration soil; S3. Fill the calibration box with soil for calibration and bury the three-dimensional membrane earth pressure box to be calibrated in the middle of the soil; S4. Install the top surface loading device, and install the axial force gauge and displacement gauge in the x, y, and z directions; S5. Determine the temporary plastic load and the ultimate load according to the load test curve of the soil to be tested in the actual project, and then determine the maximum value and graded loading value of the calibration load of the three-dimensional membrane earth pressure cell to be calibrated; S6. Carry out vertical graded loading according to the graded loading values, and record the readings of each axial force gauge and displacement gauge under each grade of load; S7. The three-dimensional membrane earth pressure cell to be calibrated is calibrated in sections and planes according to the three directions of x, y and z and the three load sections divided by two limit loads; S8. Replace the calibration soil, change its particle size distribution, and repeat steps S2-S7; S9. Obtain suitable calibration soil and surface and segment calibration coefficients for the three-dimensional membrane earth pressure cell to be calibrated.

2. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S1, a three-dimensional membrane earth pressure box calibration device comprises a calibration box (1), wherein a pair of transverse retaining plates (11) are arranged on the left and right sides of the calibration box (1), and a pair of longitudinal retaining plates (12) are arranged on the front and rear sides, and the outer sides of the transverse retaining plates (11) and the longitudinal retaining plates (12) are fixed by means of gaskets (15), bolts (14), inner nuts (13) and outer nuts (17); the bottom plate of the calibration box (1), the transverse retaining plates (11), the longitudinal retaining plates (12) and the top surface loading plate (7) constitute the inner box of the calibration box; a loading device is arranged on the top of the top surface loading plate (7), and the loading device comprises a jack (5) installed on the top surface loading plate (7), and the jack (5) is abutted against a reaction frame (3) on the top of the calibration box (1) through a cushion block (2).

3. A three-dimensional membrane earth pressure cell calibration method according to claim 2, characterized in that: The axial force gauges in the x, y and z directions in S4 include a first axial force gauge (6) and a second axial force gauge (16), wherein the first axial force gauge (6) is installed between the top surface loading plate (7) and the jack (5), and the second axial force gauge (16) is installed at the end of the gasket (15) and the other end is in contact with the side wall of the transverse retaining plate (11) or the longitudinal retaining plate (12); the displacement gauges in the x, y and z directions include a first displacement gauge (4) and a second displacement gauge (20), wherein the first displacement gauge (4) is installed between the reaction frame (3) and the top surface loading plate (7), and one end of the second displacement gauge (20) is against the inner wall of the calibration box (1) and the other end is in contact with the side wall of the transverse retaining plate (11) or the longitudinal retaining plate (12).

4. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S2, the most suitable calibration soil is determined by performing multiple calibration tests by changing the particle gradation of the sand; the particle gradation of the sand includes the maximum particle size of the sand, the distribution range of the particle size and the content of each particle group.

5. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S3, the calibration soil is filled in layers in the inner box of the calibration box to ensure uniform filling, and a three-dimensional membrane earth pressure box (10) to be calibrated is buried in the middle of the soil. The bottom surface of the three-dimensional membrane earth pressure box (10) to be calibrated is horizontal, and the six surfaces of the three-dimensional membrane earth pressure box (10) to be calibrated are respectively parallel to the six surfaces of the inner box of the calibration box.

6. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S4, when installing the top loading plate (7), ensure that its bottom surface is horizontal, and after installing the axial force meter and displacement meter, let it stand for 24 hours.

7. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S5, soil deformation is divided into linear elastic deformation stage, plastic deformation stage and failure stage by using the pre-plastic load and ultimate load. The maximum value and graded loading value of the calibration load of the three-dimensional membrane earth pressure box are determined according to the load imposed on the soil in the actual project and the deformation stage of the soil.

8. A three-dimensional membrane earth pressure cell calibration method according to claim 1, characterized in that: In S8, the calibration soil is replaced, its particle gradation is changed, that is, its maximum particle size, the distribution range of particle size and the content of each particle group are changed, and steps S2-S7 are repeated to explore the size of the soil arch effect of different soils in the three-dimensional earth pressure box calibration and its influence on the earth pressure box test results in the x, y and z directions.

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