Device and method for testing mechanical properties and shear modulus of soil body through combination of bending element and true triaxial

Abstract

The invention relates to the technical field of soil tests, in particular to a device and a method for testing mechanical properties and shear modulus of a soil body through combination of a bending element and true triaxial. The device comprises a true triaxial test system and a bending element test system, the true triaxial system has independent three-way loading capability, and the bending element system is integrated in the loading cap. During testing, the cubic sample is loaded in the true triaxial system according to a set path, after each stage of load is stable, the bending element system is started to test the shear wave velocity, and deformation, load and pore pressure data of the sample are synchronously collected. The stress-strain relationship and the maximum shear modulus of the soil body in different stress states can be synchronously obtained through one test, and integrated test of static and dynamic parameters is realized. According to the method, the error caused by the difference between the duplicated sample and the test condition is avoided, and the test efficiency and the data consistency are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of geotechnical testing technology, specifically to a device and method for combined bending element and true triaxial testing of soil mechanical properties and shear modulus. Background Technology

[0002] Currently, the true triaxial apparatus is a geotechnical testing instrument that realistically simulates the triaxial stress state of soil. The true triaxial apparatus uses a cubic specimen and loads it independently from the three principal stress directions, so as to generate uniform stress and strain in the soil. Compared with the conventional triaxial test, the true triaxial test can more comprehensively and realistically reflect the deformation law of soil elements under triaxial stress state. It can be used to study the stress-strain characteristics of soil and to verify and develop soil constitutive models.

[0003] The bending element is a simple and quick indoor test method for determining the shear wave velocity of soil. The shear modulus of soil can be calculated from the shear wave velocity. Due to its simple principle and convenient testing, the bending element has been frequently installed on geotechnical testing instruments such as consolidation apparatus, dynamic triaxial apparatus, and resonant column in recent years.

[0004] The inventors of this application discovered through research that true triaxial testing can obtain the mechanical properties of soil, and bending element system can obtain the shear modulus of soil. Combining the two can simultaneously obtain the static and dynamic parameters of soil in a set of tests. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a device for testing the mechanical properties and shear modulus of soil using a bending element and a true triaxial method, which can simultaneously meet the requirements for testing the mechanical properties and shear modulus of soil.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A device for combined bending element and true triaxial testing of soil mechanical properties and shear modulus, characterized in that it comprises:

[0008] A true triaxial testing system includes a pressure chamber, a cavity disposed within the pressure chamber for holding a cubic specimen, a loading mechanism for independently applying loads to the cubic specimen in three orthogonal directions, a servo control module for controlling the loading mechanism, and a measurement and data acquisition module for acquiring test data.

[0009] The bending element testing system includes at least two pairs of bending elements, one pair of bending elements being embedded in loading caps located at the vertical ends of a cubic specimen, and the other pair of bending elements being embedded in loading caps located at the horizontal ends of the cubic specimen.

[0010] The loading cap is connected to the loading mechanism, and the bending element is electrically connected to the measurement and data acquisition module.

[0011] Furthermore, the pressure chamber is provided with a bottom water inlet for water saturation and providing back pressure, and a gas source interface for applying confining pressure at the top.

[0012] Furthermore, the loading cap or base located at the bottom of the cubic sample is provided with an independent pore pressure inlet and a saturation inlet. The pore pressure inlet is connected to the pore pressure sensor through a pipeline, and the saturation inlet is connected to an external water source control valve through a pipeline.

[0013] Furthermore, the loading cap located on top of the cubic specimen is equipped with a back pressure inlet and outlet for draining water or applying back pressure during specimen saturation and testing.

[0014] Furthermore, square permeable stones are provided between the top and bottom of the cubic specimen and the loading cap, and the cubic specimen is wrapped with a latex film.

[0015] Furthermore, the loading mechanism includes vertical and horizontal loading rods. The vertical loading rod is connected to the top loading cap via a load sensor, and the horizontal loading rod is connected to the horizontal loading cap via a load sensor. A compressible force transmission plate is provided between the horizontal loading cap and the cubic specimen.

[0016] A method for testing the mechanical properties and shear modulus of soil using the apparatus described above includes the following steps:

[0017] S1: Sample preparation and system assembly

[0018] Prepare a cubic sample, wrap it with a latex film, place it on the base in the pressure chamber, install the top and horizontal loading caps, and seal the latex film between the cubic sample and the loading caps.

[0019] S2: Pressure chamber closure and system initialization

[0020] A closed pressure chamber is filled with water through the bottom inlet and air is expelled. Once full, a set confining pressure is applied through the air source interface at the top. The cubic sample is saturated through the saturation inlet and the back pressure inlet and outlet, and the pore pressure change is monitored using a pore pressure sensor.

[0021] S3: Combined Loading and Data Acquisition

[0022] The loading mechanism is driven by a servo control module to apply loads to the cubic specimen in three directions in stages according to the preset stress path. After the specimen deformation stabilizes under each load, the bending element test system is started to excite the bending element to generate shear waves and receive signals. At the same time, the displacement, load and pore pressure data of the specimen are collected through the measurement module.

[0023] S4: Parameter Calculation and Analysis

[0024] Based on the displacement and load data collected in step S3, the stress-strain relationship of the cubic specimen under different stress states is calculated to obtain the soil mechanical property parameters; based on the shear wave propagation time received by the bending element, the shear wave velocity is calculated, and then the maximum shear modulus of the cubic specimen under different stress states is obtained.

[0025] S5: Continuous testing until sample failure.

[0026] Repeat steps S3 and S4 until the cubic specimen reaches the failure criterion or completes the preset loading path, then end the test.

[0027] Furthermore, in step S2, the sample is saturated using either head saturation or back pressure saturation methods. The standard for saturation completion is that the B value measured by the pore pressure sensor is not less than 0.95.

[0028] Furthermore, in step S3, the criterion for determining the stability of the specimen deformation under each load level is: the axial deformation increment of the specimen is less than 0.001 mm within 10 minutes.

[0029] Furthermore, in step S4, the maximum shear modulus G max Using the formula Gmax=ρV s 2 Calculate, where ρ is the sample density and V s The shear wave velocity is calculated based on the arrival time difference and propagation distance of the first wave of the bending element signal.

[0030] This invention uses a true triaxial test combined with a bending element to test the mechanical properties and shear modulus parameters of soil. It can obtain multiple soil parameters in a single test. The soil samples are tested under the same environmental conditions, which can effectively avoid the influence of sample preparation and the differences in the soil samples themselves on the test results. At the same time, it avoids repeated tests and improves work efficiency. Attached Figure Description

[0031] Figure 1 This is a cross-sectional view of a device for combined bending element and true triaxial testing of soil mechanical properties and shear modulus according to an embodiment of the present invention.

[0032] Figure 2 This is a planar schematic diagram of the top loading cap according to an embodiment of the present invention;

[0033] Figure 3 This is a planar schematic diagram of the bottom loading cap according to an embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram of the external connection of a device for testing the mechanical properties and shear modulus of soil using a bending element and a true triaxial method, according to an embodiment of the present invention.

[0035] The reference numerals in the figure are described below:

[0036] 1-Loading rod, 2-Vibrator, 3-Servo valve, 4-Pressure chamber, 5-Displacement sensor, 6-Air source interface, 7-Load sensor, 8-Loading cap, 9-Compressible force transmission plate, 10-Bending element, 11-Bottom inlet, 12-Cube sample, 13-Upper loading cap inlet / outlet, 14-First valve, 15-Second valve, 16-Base inlet / outlet, 17-Pore pressure outlet, 18-Pore pressure sensor, 19-Third valve, 20-Back pressure inlet / outlet, 21-Top signal line hole, 22-Saturation inlet, 23-Bottom signal line hole, 24-Pore pressure inlet. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] like Figure 1-4 As shown, this embodiment of the invention provides a device for joint testing of soil mechanical properties and shear modulus using a bending element and a rigid-flexible composite loading true triaxial method, including a true triaxial testing system and a bending element testing system.

[0039] The true triaxial testing system includes a pressure chamber 4, a cavity disposed within the pressure chamber 4 for housing a cubic specimen 12, a loading mechanism for independently applying loads to the cubic specimen 12 in three orthogonal directions, a servo control module for controlling the loading mechanism, and a measurement and data acquisition module for collecting test data.

[0040] The loading mechanism includes a true triaxial apparatus, a vertical loading rod 1, a vibrator 2, a servo valve 3, and loading caps 8 located at both ends of the cube sample 12 in the vertical and horizontal directions. The true triaxial apparatus applies three-dimensional loading to the cube sample 12 through the loading rod 1 acting on the loading caps 8. The servo valve 3 controls the vibrator 2 to drive the loading rod 1 to perform loading.

[0041] The bending element testing system includes at least two pairs of bending elements 10, one pair of bending elements 10 being embedded in the loading caps 8 located at the vertical ends of the cubic specimen 12, and the other pair of bending elements 10 being embedded in the loading caps 8 located at the horizontal ends of the cubic specimen 12.

[0042] The loading cap 8 and the loading rod 1 are connected via a load sensor 7 to control the magnitude of the loading force during the test. The cubic specimen 12 is loaded in both left and right directions via a compressible force transmission plate 9 to ensure uniform load application.

[0043] After the cubic specimen 12 is installed, the pressure chamber 4 is fixed to the base with bolts. Water is filled into the pressure chamber 4 through the bottom water inlet 11. The confining pressure required for the test is applied to the water-filled pressure chamber 4 through the air source interface 6. At the same time, the second valve 15 connected to the base water inlet 16 can be opened to achieve water head saturation through the base water inlet 16, or the first valve 14 connected to the upper loading cap water inlet 13 can be opened to achieve back pressure saturation through the upper loading cap water inlet 13. During the saturation process and the test, the water in the cubic specimen 12 can flow out through the pore pressure outlet 17. The pore pressure sensor 18 can obtain the pore water pressure change of the specimen during the test. During the test, the third valve 19 connected to the pore pressure outlet 17 is in a closed state throughout. The deformation of the cubic specimen 12 during the test is monitored by the displacement sensor 5.

[0044] Each of the 8 loading caps is equipped with an inlet / outlet and a signal cable hole. Figure 2 The back pressure inlet / outlet 20 and the top signal line hole 21 are located in the middle. Figure 3 The saturation inlet 22 and bottom signal wire hole 23 are located in the middle. The signal wire of the bending element 10 passes through the top signal wire hole 21 and the bottom signal wire hole 23 and is led to the external connection. Figure 4 The data acquisition system in the middle.

[0045] like Figure 2 As shown, the loading cap 8 is provided with a back pressure inlet / outlet 20. The inlet / outlet 13 of the upper loading cap is connected to the first valve 14 on the pressure chamber 4 via a water pipe. This is used to apply back pressure and to discharge water from the soil during the saturation process of the cubic sample 12.

[0046] like Figure 3 As shown, the base loading cap 8 is provided with a back pressure inlet / outlet 22 for saturation, a bottom signal line hole 23 and a pore pressure inlet 24. The two inlets are connected to the pore pressure sensor 18 and the base inlet / outlet 16, and are connected to the external valves (third valve 19 and second valve 15) of the pressure chamber 4 via water pipes.

[0047] like Figure 4 As shown, after the cubic specimen 12 is loaded, water is injected to apply confining pressure and back pressure saturation. Then, loads in three directions are applied to the specimen through the hydraulic system and servo control system. The deformation and pore pressure changes of the specimen during the test are automatically collected by the data acquisition system. The data during the test can be displayed on the terminal screen throughout the entire process.

[0048] like Figure 1As shown, the soil sample 12 is covered with a latex film, which tightly wraps the soil sample. The latex film is tied to the loading cap 8 with rubber bands to prevent water in the pressure chamber from entering the soil sample 12. The soil deformation can be measured during the back pressure saturation and test.

[0049] During the test, loading rods 1 in two directions apply forces of different magnitudes to the soil sample under the action of the servo control system. The cubic specimen 12 deforms under the force, and the maximum shear modulus of the soil under different conditions can be tested using the bending element 10. The load is continued to be applied, and the soil deforms further. The bending element 10 is used to test again, and this process is repeated until the soil fails, ending the test. Therefore, the soil strain and maximum shear modulus can be tested simultaneously in one test, which can effectively avoid the influence of the sample preparation process on the test results and improve the test efficiency.

[0050] This invention also provides a method for testing the mechanical properties and shear modulus of soil using the device described above, comprising the following steps:

[0051] S1: Sample preparation and system assembly

[0052] Prepare a cubic sample 12, wrap it with a latex film, place it on the base inside the pressure chamber 4, install the top and horizontal loading caps 8, and seal the latex film between the cubic sample 12 and the loading caps 8.

[0053] S2: Pressure chamber closure and system initialization

[0054] The pressure chamber 4 is sealed, water is injected into the pressure chamber 4 through the bottom water inlet 11 and air is discharged. After it is full, the set confining pressure is applied through the air source interface 6 at the top. The cubic sample 12 is saturated through the saturation water inlet 22 and the back pressure water inlet and outlet 20, and the pore pressure change is monitored by the pore pressure sensor 18.

[0055] Among them, the sample saturation is carried out by water head saturation or back pressure saturation method, and the standard for saturation completion is that the B value measured by the pore pressure sensor (18) is not less than 0.95.

[0056] S3: Combined Loading and Data Acquisition

[0057] The loading mechanism is driven by the servo control module to apply loads to the cubic specimen 12 in three directions according to the preset stress path. After the specimen deformation stabilizes under each load (for example, the axial deformation increment of the specimen is less than 0.001 mm within 10 minutes), the bending element test system is started to excite the bending element 10 to generate shear waves and receive signals. At the same time, the displacement, load and pore pressure data of the specimen are collected through the measurement module.

[0058] S4: Parameter Calculation and Analysis

[0059] Based on the displacement and load data collected in step S3, the stress-strain relationship of the cubic specimen 12 under different stress states is calculated to obtain the soil mechanical property parameters; based on the shear wave propagation time received by the bending element, the shear wave velocity is calculated, and then the maximum shear modulus of the cubic specimen 12 under different stress states is obtained.

[0060] Specifically, the maximum shear modulus G max Using the formula Gmax=ρV s 2 Calculate, where ρ is the sample density and V s The shear wave velocity is calculated based on the arrival time difference and propagation distance of the first wave of the bending element signal.

[0061] S5: Continuous testing until sample failure.

[0062] Repeat steps S3 and S4 until the cubic specimen 12 reaches the failure criterion or completes the preset loading path, then end the test.

[0063] This invention has the following features and effects:

[0064] 1. It achieves the integrated testing of true triaxial static force and bending element dynamics, breaking through the limitations of traditional separate testing.

[0065] This invention is the first to integrate a true triaxial loading system and a bending element testing system into the same pressure chamber, which can simultaneously perform triaxial static loading and shear wave velocity testing in a single test. This fundamentally avoids parameter mismatch problems caused by sample differences and environmental changes, and ensures the inherent consistency of static and dynamic parameters.

[0066] 2. Significantly improves experimental efficiency and data reliability, and reduces experimental costs.

[0067] Traditional methods require separate true triaxial tests and bending element tests, involving repetitive sample preparation, saturation, and loading processes, which are time-consuming and material-intensive. This invention, however, obtains the mechanical properties and shear modulus under the entire stress path through a single sample loading, single saturation, and continuous testing, improving testing efficiency by approximately 50% while reducing systematic errors introduced by multiple sample preparations.

[0068] 3. Supports simultaneous research on soil mechanics and dynamic properties under complex stress paths, expanding experimental capabilities.

[0069] The device has independent three-way servo control loading capability, which can simulate complex stress states in real foundations; combined with the bending element system, it can monitor the changes in soil shear modulus at different stress stages in real time, providing more comprehensive and continuous data support for soil constitutive model development and dynamic response analysis.

[0070] 4. The system has a high degree of integration, standardized operation procedures, and strong applicability.

[0071] The device features a rational structural design, with coordinated operation of the loading, measurement, control, and data acquisition modules. The entire process, from sample saturation and confining pressure application to data acquisition, is controllable and measurable. This method is applicable to the static and dynamic property testing of various cohesive soils, sandy soils, and soft rock materials, and has broad engineering and scientific research application value.

[0072] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A device for combined bending element and true triaxial testing of soil mechanical properties and shear modulus, characterized in that, include: The true triaxial testing system includes a pressure chamber (4), a cavity set in the pressure chamber (4) for holding a cubic specimen (12), a loading mechanism for independently applying loads to the cubic specimen (12) in three orthogonal directions, a servo control module for controlling the loading mechanism, and a measurement and data acquisition module for collecting test data. The bending element test system includes at least two pairs of bending elements (10), one pair of bending elements (10) being embedded in the loading caps (8) located at the vertical ends of the cubic specimen (12), and the other pair of bending elements (10) being embedded in the loading caps (8) located at the horizontal ends of the cubic specimen (12). The loading cap (8) is connected to the loading mechanism, and the bending element (10) is electrically connected to the measurement and data acquisition module.

2. The apparatus according to claim 1, characterized in that: The pressure chamber (4) has a bottom water inlet (11) for water saturation and back pressure, and a gas source interface (6) for applying confining pressure at the top.

3. The apparatus according to claim 2, characterized in that: The loading cap (8) or base located at the bottom of the cubic sample (12) is provided with an independent pore pressure inlet (24) and a saturation inlet (22). The pore pressure inlet (24) is connected to the pore pressure sensor (18) through a pipeline, and the saturation inlet (22) is connected to an external water source control valve through a pipeline.

4. The apparatus according to claim 3, characterized in that: The loading cap (8) located on top of the cubic specimen (12) is provided with a back pressure inlet / outlet (20) for draining water or applying back pressure during specimen saturation and testing.

5. The apparatus according to claim 1, characterized in that: The cube sample (12) is provided with square permeable stones between its top and bottom and the loading cap (8), and the cube sample (12) is wrapped with a latex film.

6. The apparatus according to claim 1, characterized in that: The loading mechanism includes vertical and horizontal loading rods. The vertical loading rod (1) is connected to the top loading cap (8) through a load sensor (7). The horizontal loading rod (1) is connected to the horizontal loading cap (8) through a load sensor (7). A compressible force transmission plate (9) is provided between the horizontal loading cap (8) and the cubic specimen (12).

7. A method for testing the mechanical properties and shear modulus of soil using the apparatus as described in any one of claims 1-6, characterized in that, Includes the following steps: S1: Sample preparation and system assembly Prepare a cubic sample (12), wrap it with a latex film, place it on the base inside the pressure chamber (4), install the top and horizontal loading caps (8), and seal the latex film between the cubic sample (12) and the loading caps (8). S2: Pressure chamber closure and system initialization The pressure chamber (4) is sealed, water is injected into the pressure chamber (4) through the bottom water inlet (11) and air is discharged. After it is full, the set confining pressure is applied through the air source interface (6) at the top. The cubic sample (12) is saturated through the saturation water inlet (22) and the back pressure water inlet and outlet (20), and the pore pressure change is monitored by the pore pressure sensor (18). S3: Combined Loading and Data Acquisition The loading mechanism is driven by the servo control module to apply loads to the cubic specimen (12) in three directions according to the preset stress path. After the specimen deformation stabilizes under each load, the bending element test system is started to excite the bending element (10) to generate shear waves and receive signals. At the same time, the displacement, load and pore pressure data of the specimen are collected through the measurement module. S4: Parameter Calculation and Analysis Based on the displacement and load data collected in step S3, the stress-strain relationship of the cubic specimen (12) under different stress states is calculated to obtain the soil mechanical property parameters; based on the shear wave propagation time received by the bending element, the shear wave velocity is calculated, and then the maximum shear modulus of the cubic specimen (12) under different stress states is obtained. S5: Continuous testing until sample failure. Repeat steps S3 and S4 until the cubic specimen (12) reaches the failure standard or completes the preset loading path, and then end the test.

8. The method according to claim 7, characterized in that: In step S2, the sample is saturated by water head saturation or back pressure saturation. The standard for saturation completion is that the B value measured by the pore pressure sensor (18) is not less than 0.

95.

9. The method according to claim 7, characterized in that: In step S3, the criterion for determining the stability of the specimen deformation under each load level is: the axial deformation increment of the specimen is less than 0.001 mm within 10 minutes.

10. The method according to claim 7, characterized in that: In step S4, the maximum shear modulus G max Using the formula Gmax=ρV s 2 Calculate, where ρ is the sample density and V s The shear wave velocity is calculated based on the arrival time difference and propagation distance of the first wave of the bending element signal.

Images