Stress path test method and system for foundation pit excavation process

Abstract

The invention discloses a stress path test method and system for a foundation pit excavation process, and the method comprises the steps: building a soil body sample model through an improved triaxial test system, and setting initial consolidation stress states sigma1 and sigma3 (the ratio is 1.0-3.0); calculating the lateral unloading amount; lateral grading unloading is carried out under the drainage condition, a stress-strain curve is obtained, and pore water pressure is recorded; deformation is monitored through a digital image technology, and parameters are updated; introducing a shear test to carry out an undrained shear test; and judging the trend of the shear strength index according to the data, adjusting the confining pressure, completing test analysis and obtaining a final index. The problems that in the prior art, obvious defects exist in the aspects of foundation pit excavation stress path simulation and shear strength parameter measurement, and the requirement of foundation pit engineering design for high-precision parameters is difficult to meet are solved.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering technology, and in particular to a stress path testing method and system for a foundation pit excavation process. Background Art

[0002] Currently, in geotechnical engineering, the accuracy of numerical analysis and calculations is highly dependent on the correct selection of constitutive models and parameters, and determining the calculation parameters of geotechnical structures has become a key issue. In particular, in foundation pit excavation design, with increasing environmental constraints, the design has gradually shifted from traditional strength control to deformation control. Accurately calculating the deformation characteristics of the soil surrounding the foundation pit is crucial for support structure design and deformation control. The proper selection of soil shear strength parameters (such as cohesion c and internal friction angle ψ) directly impacts the success of the design. However, shear strength parameters are significantly affected by soil properties, test methods, and data processing methods, and results under different test conditions can vary significantly. Therefore, selecting the appropriate test method based on the actual stress state, drainage conditions, and stress path of the soil, and improving data processing accuracy through modern mathematical theory, are technical challenges that urgently need to be addressed.

[0003] At the same time, the mechanical properties of soil are not only related to the current stress state, but also affected by the past and recent stress history, that is, the role of stress path. The traditional stress path method provides a reasonable way to study soil properties, but due to the significant differences in the stress paths of soil in different parts during foundation pit excavation, traditional triaxial tests are difficult to truly reflect the stress conditions in actual engineering. For example, the outer area of ​​the foundation pit exhibits lateral unloading, the bottom area exhibits axial unloading, and the transition area involves deflection of the principal stress direction. In contrast, conventional triaxial tests usually adopt the method of isobaric consolidation followed by loading, and its stress path is completely different from the unloading path during foundation pit excavation. As a result, the shear strength index obtained by traditional tests cannot accurately reflect the changes in soil stress state caused by foundation pit excavation and its impact on deformation characteristics.

[0004] In addition, the deformation of the soil during foundation pit excavation usually occurs under unloading conditions, exhibiting mechanical properties different from those under loading conditions. Existing studies have shown that under the lateral unloading stress path, the volume change of the soil is small, and it can even be regarded as an undrained state for a short period of time. However, traditional soil mechanics theory is mostly based on conventional triaxial test parameters of undisturbed soil, ignoring the impact of the unloading path on the mechanical properties of the soil. This deviation may lead to an overestimation of the shear strength index in the design, which in turn affects the safety and economy of the foundation pit support structure. Therefore, developing a test method that can simulate the actual stress path during foundation pit excavation is of great significance for revealing the impact of unloading on the shear strength index of the soil.

[0005] Existing experimental studies on stress paths during foundation pit excavation still have numerous shortcomings. Firstly, due to the influence of soil disturbance, the undrained strength parameters measured in indoor tests are highly discrete, making it difficult to ensure parameter reliability. Secondly, existing testing equipment and techniques have limitations in accurately measuring soil deformation, making it difficult to fully capture the mechanical response of soil under complex stress paths. Therefore, there is an urgent need to improve experimental methods and testing techniques for foundation pit excavation processes to more realistically simulate the stress paths during excavation and obtain shear strength parameters that meet actual engineering conditions. Summary of the Invention

[0006] To this end, the present invention provides a stress path test method and system for the foundation pit excavation process to solve the obvious deficiencies in the existing technology in foundation pit excavation stress path simulation and shear strength parameter determination, and the technical problem that it is difficult to meet the high-precision parameter requirements of foundation pit engineering design.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A stress path test method for foundation pit excavation process, which is a test analysis method combining unloading stress path and digital image measurement technology, specifically includes the following steps:

[0009] S1: Use the improved triaxial test system to establish a soil sample model;

[0010] S2: Set the initial consolidation stress states σ1 and σ3 based on the soil specimen model, where σ1 is the axial stress, σ3 is the confining pressure, and the ratio of σ1 to σ3 ranges from 1.0 to 3.0;

[0011] S3: Let the lateral unloading amount Δσ3 = (σ3max-σ3min) / N, where σ3max is the maximum confining pressure, σ3min is the minimum confining pressure, and N is the total number of unloading stages;

[0012] S4: Perform lateral graded unloading under drainage conditions, obtain the stress-strain curve of the soil sample, and record the changes in pore water pressure after each level of unloading;

[0013] S5: Implement a deformation monitoring strategy based on digital image measurement technology to update the radial and axial deformation parameters in the soil specimen model;

[0014] S6: Introduce shear test, apply different confining pressures to the unloaded soil sample to perform undrained shear test, and record the principal stress difference;

[0015] S7: Based on the convergence of the test data, determine the trend of the soil shear strength index. If the principal stress difference decreases significantly, it indicates that the soil shear capacity has decreased, and the confining pressure is adjusted to the next test condition. Otherwise, maintain the current confining pressure and continue the test.

[0016] S8: Repeat steps S3 to S7 until all levels of unloading conditions are completed, thereby completing the stress path test analysis during the foundation pit excavation process and obtaining the final shear strength index.

[0017] In a specific embodiment, in step S2, the initial consolidation stress states σ1 and σ3 are 160 kPa and 120 kPa, respectively.

[0018] In a specific embodiment, in step S5, digital image measurement technology is used to monitor the radial and axial deformations of the soil sample in real time during the lateral unloading process, and the position changes of the white marking lines are extracted through image processing algorithms to update the deformation parameters.

[0019] In a specific embodiment, in step S5, if the monitored radial deformation exceeds a preset threshold, the lighting device angle is adjusted to improve image clarity, and image acquisition and processing are performed again.

[0020] In a specific embodiment, in step S6, the loading rate of the shear test is 0.125% axial strain per minute, and the test termination condition is that the soil sample is destroyed or the strain reaches 15% of the soil sample height.

[0021] In a specific embodiment, an improved triaxial test system is used in step S1, including a square tempered glass pressure chamber, a black rubber membrane, and auxiliary lighting equipment.

[0022] In a specific embodiment, in step S7, the test data convergence condition is set as the pore water pressure change rate is less than 0.01% / min. If the pore water pressure change rate fails to meet the condition, the test is considered not to have converged; otherwise, the test is considered to have converged.

[0023] A stress path testing system used in a foundation pit excavation process can execute the stress path testing method.

[0024] The present invention has the following beneficial effects:

[0025] This method utilizes an improved triaxial testing system, combined with digital image measurement technology and unloading stress path simulation, to establish a dynamic monitoring and analysis method for soil mechanical properties during foundation pit excavation. This method achieves accurate determination of soil shear strength indicators, resolving the technical problem that traditional testing methods cannot truly reflect the impact of the foundation pit unloading process on soil mechanical properties, thereby providing a more reliable experimental basis for foundation pit engineering design. This method has the advantages of convenient operation, accurate data, and strong applicability, and has opened up a new technical path for stress path experimental research during foundation pit excavation under complex conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read. Any structural modification, change in proportional relationship or adjustment of size should still fall within the scope of the technical content disclosed in the present invention without affecting the efficacy and purpose that can be achieved by the present invention.

[0027] Figure 1 This is a flowchart of the overall implementation of the stress path test method during foundation pit excavation of the present invention.

[0028] Figure 2 This is a structural diagram of the improved triaxial test system of the present invention.

[0029] Figure 3 Schematic diagram of the stress path change monitoring principle during the lateral graded unloading process of the present invention.

[0030] Figure 4 This is a flow chart of soil sample deformation monitoring based on digital image measurement technology in the present invention.

[0031] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0032] 1. Square tempered glass pressure chamber; 2. Black rubber membrane; 3. Auxiliary lighting equipment; 4. Soil sample; 5. Initial consolidation stress state. DETAILED DESCRIPTION

[0033] The following describes the implementation of the present invention using specific embodiments. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. Obviously, the embodiments described are only a portion of the present invention, not all of it. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0034] The terms "upper", "lower", "left", "right", "middle", etc. used in this specification are only for the convenience of description and are not intended to limit the scope of the present invention. Changes or adjustments to their relative relationships should be regarded as within the scope of the present invention without substantially changing the technical content.

[0035] like Figure 1 As shown, embodiments of the present invention provide a stress path testing method for foundation pit excavation and a stress path testing system for executing the stress path testing method. By combining an improved triaxial testing system with digital image measurement technology, dynamic monitoring and precise analysis of soil mechanical properties are achieved. The technical solutions of the present invention are described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] In the specific implementation process, first of all, it is necessary to build an improved triaxial test system according to the requirements of step S1. Figure 2 As shown, the improved triaxial test system includes a square tempered glass pressure chamber 1, a black rubber membrane 2, and auxiliary lighting equipment 3. The square tempered glass pressure chamber 1 has good transparency and pressure resistance, which can ensure that the state changes of the soil sample 4 during the test are clearly visible. The black rubber membrane 2 is wrapped around the outside of the soil sample 4 to isolate the sample from the external environment and maintain the confining pressure conditions. The auxiliary lighting equipment 3 provides stable light source support for digital image measurement technology to ensure image acquisition quality. The above device establishes a highly controllable test environment to simulate the stress path changes during foundation pit excavation.

[0037] After completing the construction of the test system, the soil sample model is further established, and step S2 is entered to set the initial consolidation stress states σ1 and σ3.

[0038] Figure 3 The principle of monitoring the changes in stress paths during lateral graded unloading is demonstrated, where the initial consolidation stress state 5 corresponds to the stress condition at the beginning of the test.

[0039] In this example, the initial consolidation stress state σ1 is selected to be 160 kPa, and σ3 is selected to be 120 kPa. The ratio is set to 1.33, within the preset range of 1.0 to 3.0. This initial stress state is intended to simulate the natural consolidation conditions of the soil before excavation, providing a reference for the subsequent unloading process.

[0040] Next, step S3 is executed to determine the lateral unloading amount Δσ3. The lateral unloading amount is calculated according to the formula Δσ3 = (σ3max-σ3min) / N, where σ3max is the maximum confining pressure, σ3min is the minimum confining pressure, and N is the total number of unloading levels. In this embodiment, assuming that σ3max is 120kPa, σ3min is 40kPa, and N is 8, the unloading amount Δσ3 each time is 10kPa. The setting of this parameter enables the unloading process to gradually approach the actual stress release mode of foundation pit excavation, thereby more realistically reflecting the mechanical response of the soil under unloading conditions.

[0041] In step S4, a lateral, staged unloading operation is performed. Drainage conditions are maintained throughout the test, allowing pore water to drain to simulate the drainage effect experienced during actual foundation pit excavation. As each stage of unloading progresses, the stress-strain curve of the soil sample and the changes in pore water pressure after each stage of unloading are recorded.

[0042] In step S5, digital image measurement technology is introduced to monitor the radial and axial deformation of the soil sample in real time. Figure 4 The soil sample deformation monitoring flow chart shown uses a uniform light source provided by auxiliary lighting equipment 3 to illuminate the surface of the soil sample 4, and a high-resolution camera captures images. An image processing algorithm extracts the position changes of the white marker lines, thereby updating the data of the radial and axial deformation monitoring points in the soil sample model. In this embodiment, if the monitored radial deformation exceeds a preset threshold, the angle of the auxiliary lighting equipment 3 is adjusted to improve image clarity, and image acquisition and processing are repeated. This ensures the accuracy of the deformation monitoring results and provides a foundation for subsequent experimental analysis.

[0043] Then, step S6 is entered to apply different confining pressures to the unloaded soil sample for an undrained shear test. The loading rate of the shear test is 0.125% axial strain per minute, and the test termination condition is that the soil sample is destroyed or the strain reaches 15% of the soil sample height. By recording the principal stress difference, the shear capacity of the soil under different confining pressure conditions is evaluated. In this embodiment, if the principal stress difference decreases significantly, it indicates that the shear capacity of the soil has decreased, and the confining pressure is adjusted to the next test condition; otherwise, the current confining pressure is maintained and the test continues.

[0044] In step S7, the trend of the soil shear strength index is determined based on the convergence of the test data. The convergence condition for the test data is set to a pore water pressure change rate of less than 0.01% / min. If the pore water pressure change rate fails to meet this condition, the test is considered unconverged; otherwise, the test is considered converged. By analyzing the test data results, the changes in the mechanical properties of the soil under different unloading conditions can be accurately determined.

[0045] Finally, in step S8, steps S3 through S7 are repeated until all unloading conditions are complete. During this process, test parameters are continuously adjusted and relevant data is recorded, ultimately yielding the stress path analysis results from the excavation process. The entire test method, from setting the initial consolidation stress state to determining the final shear strength index, is seamlessly integrated, forming a complete test and analysis system.

[0046] In summary, the present invention combines an improved triaxial test system with digital image measurement technology to construct a stress path test method suitable for foundation pit excavation. This method can not only accurately measure the shear strength index of the soil, but also solve the technical problem that traditional test methods cannot truly reflect the impact of the foundation pit unloading process on the mechanical properties of the soil, providing a more reliable test basis for foundation pit engineering design.

[0047] In practical applications, this method can be widely used in foundation pit excavation projects under complex geological conditions, providing strong guarantees for project safety and economy.

[0048] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A stress path test method for foundation pit excavation, characterized in that: The method is an experimental analysis method combining unloading stress path and digital image measurement technology, and includes the following steps: S1: Use the improved triaxial test system to establish a soil sample model; S2: Set the initial consolidation stress states σ1 and σ3 based on the soil specimen model, where σ1 is the axial stress, σ3 is the confining pressure, and the ratio of σ1 to σ3 ranges from 1.0 to 3.0; S3: Let the lateral unloading amount Δσ3 = (σ3max-σ3min) / N, where σ3max is the maximum confining pressure, σ3min is the minimum confining pressure, and N is the total number of unloading stages; S4: Perform lateral graded unloading under drainage conditions, obtain the stress-strain curve of the soil sample, and record the changes in pore water pressure after each level of unloading; S5: Implement a deformation monitoring strategy based on digital image measurement technology to update the radial and axial deformation parameters in the soil specimen model; S6: Introduce shear test, apply different confining pressures to the unloaded soil sample to perform undrained shear test, and record the principal stress difference; S7: Based on the convergence of the test data, determine the changing trend of the soil shear strength index; if the principal stress difference decreases significantly, adjust the confining pressure to the next test condition; otherwise, maintain the current confining pressure and continue the test; S8: Repeat steps S3 to S7 until all levels of unloading conditions are completed, thereby completing the stress path test analysis during the foundation pit excavation process and obtaining the final shear strength index.

2. The stress path test method for foundation pit excavation according to claim 1, characterized in that: In step S2, the initial consolidation stress states σ1 and σ3 are set to 160 kPa and 120 kPa, respectively.

3. The stress path test method for foundation pit excavation according to claim 1, characterized in that: In step S5, digital image measurement technology is used to monitor the radial and axial deformations of the soil sample in real time during the lateral unloading process, and the position changes of the white marking lines are extracted through image processing algorithms to update the deformation parameters.

4. The stress path test method for foundation pit excavation according to claim 3, characterized in that: In step S5, if the monitored radial deformation exceeds a preset threshold, the angle of the auxiliary lighting device (3) is adjusted to improve the image clarity, and image acquisition and processing are performed again.

5. The stress path test method for foundation pit excavation according to claim 1, characterized in that: In step S6, the loading rate of the shear test is 0.125% axial strain per minute, and the test termination condition is that the soil sample is destroyed or the strain reaches 15% of the soil sample height.

6. The stress path test method for foundation pit excavation according to claim 1, characterized in that: The improved triaxial test system used in step S1 includes a square tempered glass pressure chamber (1), a black rubber membrane (2) and auxiliary lighting equipment (3).

7. The stress path test method for foundation pit excavation according to claim 1, characterized in that: In step S7, the test data convergence condition is set as the pore water pressure change rate is less than 0.01% / min. If the pore water pressure change rate fails to meet the condition, the test is considered not to have converged; otherwise, the test is considered to have converged.

8. The stress path test method for foundation pit excavation according to any one of claims 1 to 7, characterized in that: In step S3, σ3max is 120 kPa, σ3min is 40 kPa, and N is 8; The single unloading amount Δσ3 is 10kPa.

9. The stress path test method for foundation pit excavation according to claim 1, characterized in that: In step S5, the position changes of the radial deformation monitoring points and the axial deformation monitoring points monitored in real time by the digital image measurement technology are extracted and recorded by an image processing algorithm.

10. A stress path test system for use in foundation pit excavation, characterized in that: The stress path test method according to any one of claims 1 to 9 can be performed.

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