Subway foundation pit supporting structure optimization design method based on three-dimensional finite element simulation

Abstract

The invention belongs to the technical field of subway deep foundation pit construction in a complex environment, and particularly relates to a subway foundation pit supporting structure optimization design method based on three-dimensional finite element simulation. Comprising the steps that S1, the deformation conditions of a subway deep foundation pit adjacent bridge and the surrounding environment in the excavation stage are monitored; s2, establishing a numerical simulation model of the whole foundation pit excavation process; s3, comparing and verifying a numerical simulation result and an actual measurement result; s4, analyzing the influence of the change of the comprehensive rigidity caused by the change of each parameter on the ground surface settlement; and S5, the supporting structure combination of the excavated subway deep foundation pit is optimized. According to the method, the numerical model of the whole foundation pit excavation process is established and compared with the actually measured data result for verification, and the numerical model can well reflect the influence of subway foundation pit excavation on the surrounding environment and the adjacent bridge in the whole foundation pit excavation process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of subway deep foundation pit construction in complex environment, and particularly relates to a subway foundation pit supporting structure optimization design method based on three-dimensional finite element simulation. BACKGROUND

[0002] At present, subway construction is booming, and subway deep foundation pit excavation is in a complex environment, especially adjacent to existing roads and bridges, which faces severe challenges.

[0003] At present, although there are certain achievements in the research on the influence of foundation pit excavation on surrounding existing roads and bridges, there are deficiencies in the research on the influence of existing roads and bridges in operation and the optimization of the overall supporting combination form. The existing supporting structure design method is difficult to accurately simulate the deformation of the surrounding soil and existing roads and bridges during the foundation pit excavation process, resulting in poor accuracy of the supporting structure design. In the construction process, it is easy to cause large deformation disturbance to the existing roads and bridges, affecting their normal use and safety, and may also cause unnecessary increase in engineering cost. When determining the optimization scheme of the supporting structure, only a single target is often pursued, such as simply pursuing the lowest cost or the smallest deformation. The balance between construction safety, influence on the surrounding environment, and economic cost and other factors is not fully considered, which may cause some problems.

[0004] Therefore, a more accurate and effective supporting structure optimization design method is needed to ensure construction safety, reduce interference to the surrounding environment, and achieve the economy of the project. SUMMARY

[0005] The application provides a subway foundation pit supporting structure optimization design method based on three-dimensional finite element simulation to solve the above problems.

[0006] The application adopts the following technical scheme: a subway foundation pit supporting structure optimization design method based on three-dimensional finite element simulation, comprising: S1: monitoring the deformation of the bridge adjacent to the deep foundation pit and the surrounding environment during the excavation stage; S2: establishing a numerical simulation model of the whole foundation pit excavation process; S3: comparing and verifying the numerical simulation results and the measured results; S4: analyzing the influence of the change of each parameter on the comprehensive stiffness change on the ground settlement; S5: optimizing the supporting structure combination of the deep foundation pit.

[0007] In some embodiments, step S1 comprises: S11: arranging displacement monitoring points on the surrounding ground surface outside the deep foundation pit according to the shape characteristics of the deep foundation pit; The displacement monitoring points are asymmetrically arranged on the side close to the bridge and the opposite side, and the ground surface settlement monitoring points are arranged in the middle of the span away from the existing bridge on one side and in the middle of the span close to the existing bridge and the starting well / receiving well on the other side; S12: Displacement monitoring points are arranged near the bridge piers, and according to the distribution of the bridge piers and the shortest distance from the bridge piers to the outer edge of the foundation pit, 1-3 bridge piers are selected on each side for monitoring. Each bridge pier monitoring should arrange one monitoring point on the south and north sides of the monitored bridge pier.

[0008] Step S2 includes: S21: A three-dimensional finite element model of the subway deep foundation pit excavation process is established based on Midas GTS NX; S22: Soil parameters are selected, and through detailed engineering geological investigation, the mechanical parameters of the model soil are obtained. The modified Mohr-Coulomb constitutive model is used, and the parameters include soil thickness, specific gravity, Poisson's ratio, internal friction angle, cohesion, triaxial secant stiffness, tangent stiffness and unloading elastic modulus; S23: Component parameter selection, according to the design drawing, accurately set the material, cross-sectional shape and size parameters of the diaphragm wall, socketed pile, bridge pier cast-in-place pile, internal support, lattice column, uplift pile, bridge body and bridge pier, and corbel / waist beam.

[0009] In some embodiments, in step S21, The overall size of the model is required to ensure that the minimum distance from the outer edge of the foundation pit to the model boundary is not less than 5 times the excavation depth, so as to reduce the influence of boundary effect; A fixed constraint is applied to the bottom surface of the model, the corresponding surface normal direction constraint is set around, and the top surface is set as a free surface; The soil layer, excavated soil, socketed pile, bridge body and bridge pier in the model are 3D solid elements, the diaphragm wall is 2D plate element, the internal support, corbel / waist beam, angle brace, lattice column and uplift pile are 1D beam element, and the bridge pier cast-in-place pile is simulated by 1D embedded beam element.

[0010] In some embodiments, step S3 includes: The on-site ground surface settlement and the adjacent bridge settlement results are obtained, and compared with the time-history analysis results of the three-dimensional finite element simulation to verify whether the ground monitoring results are consistent; If the overall simulation results and the monitoring point measured values are consistent in change trend and consistent in value, then the subsequent steps are performed; If the overall simulation results and the monitoring point measured values are inconsistent in change trend or have large difference in value, then the model is adjusted and compared and verified again.

[0011] In some embodiments, step S4 includes: S41: The MVSS comprehensive stiffness is used as the structure optimization index; The MVSS comprehensive stiffness calculation formula is as follows: In the formula, E is the elastic modulus of the enclosure material, I is the unit length section moment of inertia of the enclosure, k t is a comprehensive adjustment coefficient of the space-time effect, insertion ratio, etc., k j is a foundation reinforcement influence factor, k s is a support stiffness influence coefficient, s is the average horizontal spacing of the internal support, N is the number of internal support lanes, H is the excavation depth, gamma w is the specific weight of water; S42: Select different enclosure sizes, select different internal support lane numbers, select different internal support horizontal spacings, and combine all the changed parameters; S43: Perform an analysis of the influence of changes in the comprehensive stiffness caused by changes in each parameter on the environment.

[0012] In some embodiments, in step S41, the support stiffness influence coefficient k s is represented by the following formula: m is the number of steel support lanes, n is the number of concrete support lanes, k 1 is the steel support stiffness, k 2 is the concrete support stiffness.

[0013] In some embodiments, step S43 includes: S431: By the control variable method, each time the simulation analysis only changes one parameter, the simulation results of the changed combinations are compared, the influence of changes in this parameter on the excavation pit surrounding ground surface settlement caused by changes in the comprehensive stiffness is analyzed, and the change law provides a basis for subsequent optimal combination structure; fix the changed parameter, select the unchanged parameter for combination, simulation and result analysis, and repeat the above operation until all parameters are changed and simulated. S432: Through the control variable method, only one parameter is changed each time, the simulation results of the changed combinations are compared, the influence of the change of the comprehensive stiffness caused by the change of the parameter on the deformation of the adjacent bridge is analyzed, and the change law is analyzed to provide a basis for subsequent optimal combination structure.

[0014] In some embodiments, step S5 comprises: S51: Combine the comprehensive stiffness in each combination with the ratio of the maximum vertical settlement to the excavation depth, and draw a scatter plot; S52: Perform scatter fitting analysis; S53: Select the optimal combination mode according to the fitting result.

[0015] In some embodiments, in step S52, a power function is used for scatter fitting analysis, and the fitting function is as follows, is the ratio of the maximum vertical settlement to the excavation depth, a 、 b is the fitting formula parameter.

[0016] Compared with the prior art, the present application has the following beneficial effects: 1. The present application establishes a numerical model of the whole foundation pit excavation process, and compares and verifies the measured data results, so that the numerical model of the whole foundation pit excavation process can better reflect the influence of the subway foundation pit excavation on the surrounding environment and the adjacent bridge.

[0017] 2. The present application analyzes the influence of the change of the comprehensive stiffness caused by the change of each subway foundation pit supporting structure parameter on the ground settlement, and can determine the optimal supporting structure combination mode.

[0018] 3. The optimal supporting combination mode proposed by the present application fully guarantees the construction safety and the stability of the existing structure, ingeniously takes into account the economy, reasonably reduces the engineering cost input, improves the resource utilization efficiency, maximizes the engineering benefit, and has important reference significance and practical value for similar subway deep foundation pit engineering. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is the step flow chart of the embodiment of the present application; Figure 2 is the monitoring point layout schematic diagram of the embodiment of the present application; Figure 3Monitoring and finite element simulation analysis results comparison diagram of deformation of bridge and surrounding environment near deep foundation pit of subway in excavation stage of embodiment of the present application Figure 1 (ground settlement); Figure 4 Monitoring and finite element simulation analysis results comparison diagram of deformation of bridge and surrounding environment near deep foundation pit of subway in excavation stage of embodiment of the present application Figure 2 (bridge displacement); Figure 5 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on ground settlement near excavation foundation pit Figure 1 (change of diameter of bite pile); Figure 6 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on ground settlement near excavation foundation pit Figure 2 (change of number of support lanes); Figure 7 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on ground settlement near excavation foundation pit Figure 3 (change of horizontal spacing of support); Figure 8 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on deformation of bridge near excavation foundation pit Figure 1 (change of diameter of bite pile); Figure 9 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on deformation of bridge near excavation foundation pit Figure 2 (change of number of support lanes); Figure 10 Results comparison diagram of influence of change of comprehensive stiffness caused by change of each parameter in embodiment of the present application on deformation of bridge near excavation foundation pit Figure 3 (change of horizontal spacing of support); Figure 11 Scatter plot and fitting curve of relationship between comprehensive stiffness and maximum settlement value of each combined support form in embodiment of the present application and ratio of excavation depth. DETAILED DESCRIPTION

[0020] The present application will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made. These all belong to the protection scope of the present application.

[0021] The present application will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made. These all belong to the protection scope of the present application.

[0022] Referring to Figure 1 A subway foundation pit support structure optimization design method based on three-dimensional finite element simulation, comprising the following steps: S1: monitoring the deformation of the adjacent bridge and the surrounding environment of the excavated subway deep foundation pit; S2: establishing a numerical simulation model of the whole foundation pit excavation process; S3: comparing and verifying the numerical simulation results with the measured results; S4: analyzing the influence of the change of each parameter on the ground settlement caused by the change of the comprehensive stiffness; S5: carrying out excavation of the deep foundation pit support structure optimization.

[0023] In this embodiment, the numerical simulation model of the whole foundation pit is compared and verified with the measured results. Based on the fact that the numerical simulation finite element model of the whole foundation pit can well reflect the influence on the surrounding ground and the adjacent bridge during the foundation pit excavation process, the support structure optimization design suitable for the subway deep foundation pit adjacent to the bridge is carried out, and the accuracy and reliability of the support structure optimization design method for the subway deep foundation pit adjacent to the bridge are improved.

[0024] In a specific example, the step S1 of monitoring the deformation of the adjacent bridge and the surrounding environment of the excavated subway deep foundation pit comprises the following steps: S11: According to the shape characteristics of the excavated subway deep foundation pit, displacement monitoring points are arranged on the surrounding ground of the excavated foundation pit. Referring to Figure 2 , the ground displacement monitoring points should be arranged asymmetrically on both sides near the bridge and on the opposite side. On one side, ground settlement monitoring points DBC1-1, DBC1-2, DBC1-3 are arranged in the middle of the span away from the existing bridge, and on the other side, DBC2-1, DBC2-2, DBC2-3 are arranged in the middle of the span near the existing bridge, the middle of the starting well / receiving well; DBC3-1, DBC3-2, DBC3-3; DBC4-1, DBC4-2, DBC4-3; S12: Referring to Figure 2 , displacement monitoring points are arranged at the piers adjacent to the bridge body. According to the distribution of the piers and the shortest distance from the piers to the outer edge of the foundation pit, 1-3 piers are selected on each side for monitoring. On each side of the monitored pier, one monitoring point QDC3-1, QDC3-2; QDC4-1, QDC4-2; QDC7-1, QDC7-2; QDC11-1, QDC11-2 is arranged.

[0025] In a specific example, the step S2 of establishing a numerical simulation model of the whole foundation pit and comparing and verifying it with the measured results comprises the following steps: S21: Referring to Figure 3, a spatial finite element model of the subway deep foundation pit excavation process is established based on the finite element calculation software Midas GTS NX. The overall size requirement of the model ensures that the minimum distance from the outer edge of the foundation pit to the model boundary is not less than 5 times the excavation depth. Fixed constraints are applied to the bottom surface of the model, corresponding surface normal direction constraints are set around, and the top surface is set as a free surface. In the model, the soil layer, excavated soil, interlocking pile, bridge body, and bridge pier are 3D solid elements, the diaphragm wall is a 2D plate element, the internal support, corbel beam / waist beam, angle brace, lattice column, and uplift pile are 1D beam elements, and the bridge pier cast-in-place pile is simulated by using a 1D embedded beam element.

[0026] S22: Soil parameter selection, through detailed engineering geological investigation, the mechanical parameters of the model soil are obtained, the modified Mohr-Coulomb constitutive model is adopted, and the parameters cover soil thickness, specific gravity, Poisson's ratio, internal friction angle, cohesion, triaxial secant stiffness, tangent stiffness, unloading elastic modulus, etc., see Table 1.

[0027] Table 1 Mechanical parameters of soil S23: Component parameter selection, according to the design drawings, the material, cross-sectional shape, size, etc. of the diaphragm wall, interlocking pile, bridge pier cast-in-place pile, internal support, lattice column, uplift pile, bridge body and bridge pier, corbel beam / waist beam, etc. are accurately set, see Table 2.

[0028] Table 2 Structural parameters S24: Reference Figures 4-5 , the results of the field surface settlement and the adjacent bridge settlement are obtained, and the time history analysis results of the finite element simulation are compared, and it is verified that the ground monitoring results are in good agreement, thereby the research on the optimization of the supporting structure combination is carried out.

[0029] In a specific example, step S3 comprises: obtaining the results of the field surface settlement and the adjacent bridge settlement, and comparing the time history analysis results of the three-dimensional finite element simulation, to verify whether the ground monitoring results are consistent; if the overall simulation results and the measured values of the monitoring points are consistent in change trend and in value, then the subsequent steps are carried out; if the overall simulation results and the measured values of the monitoring points are inconsistent in change trend or differ greatly in value, then the model needs to be adjusted and compared and verified again.

[0030] In a specific example, the step S4 of analyzing the influence of the change of each parameter on the comprehensive stiffness on the ground settlement comprises the following steps: S41: using the MVSS comprehensive stiffness calculation method as the structure optimization index; The MVSS comprehensive stiffness calculation formula is as follows: (1) wherein: E E is the elastic modulus of the envelope material, I I is the unit length section moment of inertia of the envelope, k t is a comprehensive adjustment coefficient of the space-time effect, the insertion ratio, etc. k j is a foundation reinforcement influence factor, k s is a support stiffness influence coefficient, s is the average horizontal spacing of the internal support, N is the number of internal support lanes, H is the excavation depth, gamma w is the water density.

[0031] wherein the support stiffness influence coefficient k s may be represented by the following formula (2), m is the number of steel support lanes, n is the number of concrete support lanes, k 1 is the steel support stiffness, k 2 is the concrete support stiffness: (2) S42: Select different envelope sizes (thickness of diaphragm wall, diameter of bored pile, etc.), select different numbers of internal support lanes, select different horizontal spacings of internal support, and combine all the changed parameters, see Table 3.

[0032] Table 3 Structure parameters S43: Perform analysis of the influence of changes in comprehensive stiffness on the environment.

[0033] In a specific example, the step S33 of performing analysis of the influence of changes in comprehensive stiffness on the environment includes the following steps: S431: Referring to Figures 6-8 , by the control variable method, each time the simulation analysis only changes one parameter, the simulation results of the changed combinations are compared, the influence of the change in this parameter on the ground surface settlement around the excavation foundation pit is analyzed, and the change law is used to provide a basis for subsequent optimal combination structure; fixing the changed parameter, selecting the unchanged parameter for combination, simulation and result analysis, repeating the above operation until all parameters are changed and simulated.

[0034] S432: Referring to Figures 9-11By controlling variable method, each time simulation analysis only changes one parameter, the simulation results of the changed combination are compared, the influence of the change of the comprehensive stiffness on the deformation of the adjacent bridge is analyzed, and the change law provides basis for subsequent optimal combination structure; the changed parameter is fixed, and the unchanged parameter is selected for combination, simulation and result analysis, and the above operation is repeated until all parameters are changed and simulated.

[0035] In a specific example, the step S5 of carrying out the excavation of the subway deep foundation pit supporting structure combination optimization includes the following steps: S51: the comprehensive stiffness under each combination form is combined with the corresponding maximum vertical settlement and the ratio percentage of the excavation depth, and a scatter plot is drawn.

[0036] S52: power function is used for scatter fitting analysis, and the fitting function is shown in the following formula (3), is the ratio of the maximum vertical settlement value and the excavation depth, a 、 b is a fitting formula parameter: (3) S53: the optimal combination mode is selected according to the fitting result. In the example, after comprehensive trade-off, it is determined that the 1.2m diameter of the interlocking pile, 4 internal supports and 6m horizontal spacing of the internal support are the optimal combination mode.

[0037] To sum up, the present application develops the optimization design method of the subway deep foundation pit supporting structure near the bridge in the civil engineering technical field, provides a monitoring scheme and model verification method of the subway deep foundation pit near the bridge, enriches the consideration factors of the optimization design of the subway deep foundation pit supporting structure near the bridge, so that the design personnel can more comprehensively consider the influence parameters of the supporting structure design of the subway deep foundation pit; through the parameter comparison and analysis of the subway deep foundation pit supporting structure, the influence law of each supporting structure parameter on the surrounding ground surface environment of the subway deep foundation pit and the adjacent bridge can be obtained, and the optimal combination scheme of the supporting structure of the subway deep foundation pit near the bridge can be more systematically proposed. Therefore, the method provided in the example can maximize the safety operation of the existing road and bridge in the subway foundation pit construction process, and reduce the influence on the surrounding traffic infrastructure.

[0038] The above only describes the preferred embodiments of the present application and should not be used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation, characterized in that: include: S1: Monitor the deformation of the adjacent bridges and surrounding environment of the subway deep foundation pit during the excavation phase; S2: Establish a numerical simulation model for the entire foundation pit excavation process; S3: Comparison and verification of numerical simulation results with measured results; S4: Analyze the impact of changes in comprehensive stiffness caused by changes in various parameters on surface settlement; S5: Optimization of support structure combination for deep subway excavation pit.

2. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 1 is characterized in that: The step S1 comprises: S11: According to the shape characteristics of the deep foundation pit for subway excavation, displacement monitoring points are arranged on the surface around the excavation pit; Displacement monitoring points are arranged asymmetrically on the side close to the bridge and on the opposite side. On one side, surface settlement monitoring points are arranged at the mid-span away from the existing bridge, and on the other side, monitoring points are arranged at the mid-span close to the existing bridge and at the mid-span of the starting well / receiving well. S12: Arrange displacement monitoring points near the bridge piers. Select 1-3 piers for monitoring on each side based on the distribution of the piers and their shortest distance from the outer edge of the foundation pit. For each pier, one monitoring point should be arranged on the north and south sides of the monitored pier.

3. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 1 is characterized in that: The step S2 comprises: S21: Establish a 3D finite element model of the subway deep foundation pit excavation process based on Midas GTS NX; S22: Soil parameter selection: Through detailed engineering geological survey, various mechanical parameters of the model soil are obtained. The modified Mohr-Coulomb constitutive model is adopted, and its parameters include soil thickness, density, Poisson's ratio, internal friction angle, cohesion, triaxial secant stiffness, tangent stiffness, and unloading elastic modulus; S23: Component parameter selection: Based on the design drawings, accurately set the material, cross-sectional shape, and size parameters of the ground-connected walls, interlocking piles, pier cast-in-place piles, internal supports, lattice columns, pull-out piles, bridge body and piers, and crown / waist beams.

4. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 3 is characterized in that: In the step S21, The overall size of the model must ensure that the minimum distance from the outer edge of the foundation pit to the model boundary is no less than 5 times the excavation depth to reduce the impact of boundary effects; Apply fixed constraints to the bottom surface of the model, set normal direction constraints on the corresponding surfaces around it, and set the top surface as a free surface; In the model, the soil layer, excavated soil, interlocking piles, bridge body and piers are simulated as 3D solid elements, the ground-connected walls are simulated as 2D plate elements, the internal supports, crown beams / waist beams, angle braces, lattice columns and pull-out piles are simulated as 1D beam elements, and the pier cast-in-place piles are simulated using 1D implanted beam elements.

5. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 3 is characterized in that: The step S3 comprises: Obtain the results of on-site surface settlement and adjacent bridge settlement, compare them with the time-history analysis results of 3D finite element simulation, and verify whether the ground monitoring results are consistent; If the overall simulation results are consistent with the trend of the measured values ​​at the monitoring points and the values ​​are consistent, proceed to the next steps; If the overall simulation results are inconsistent with the changing trends of the actual measured values ​​at the monitoring points or the numerical values ​​differ greatly, the model should be adjusted and re-compared and verified.

6. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 1 is characterized in that: The step S4 comprises: S41: MVSS comprehensive stiffness is used as the structural optimization index; The calculation formula of MVSS comprehensive stiffness is as follows: Where: E is the elastic modulus of the enclosure material, I is the moment of inertia per unit length of the enclosure structure, k t is the comprehensive adjustment coefficient of influencing factors such as time-space effect and insertion ratio, k j is the influencing factor of foundation reinforcement, k s is the support stiffness influence coefficient, s is the average horizontal spacing of the inner supports, N is the number of internal supports, H is the excavation depth, γ w is the weight of water; S42: Select different enclosure structure sizes, select different numbers of internal support paths, select different internal support horizontal spacings, and combine all the changing parameters; S43: Conduct analysis on the impact of changes in comprehensive stiffness caused by changes in various parameters on the environment.

7. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 6 is characterized in that: In the step S41, Support stiffness influence coefficient k s It is expressed by the following formula: m is the number of steel support tracks, n is the number of concrete support tracks, k 1 is the steel support stiffness, k 2 is the concrete support stiffness.

8. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 6 is characterized in that: The step S43 includes: S431: Using the control variable method, only one parameter is changed in each simulation analysis. The simulation results of each combination after the change are compared to analyze the impact of the change in comprehensive stiffness caused by the parameter change on the surface settlement around the excavation pit. The change pattern provides a basis for the subsequent proposal of the optimal combination structure; the changed parameter is fixed, and the unchanged parameter is selected for combination, simulation and result analysis. The above steps are repeated until all parameters are simulated and analyzed; S432: By using the control variable method, only one parameter is changed in each simulation analysis, and the simulation results of each combination after the change are compared. The impact of the change in comprehensive stiffness caused by this parameter change on the deformation of the adjacent bridge is analyzed, and the change pattern provides a basis for the subsequent proposal of the optimal combination structure; fix the changed parameters, and then select the unchanged parameters for combination, simulation and result analysis, and repeat the above operations until all parameters are changed and simulated.

9. The method for optimizing the design of subway foundation pit support structures based on three-dimensional finite element simulation according to claim 6, characterized in that: The step S5 comprises: S51: Combine the comprehensive stiffness of each combination with the corresponding percentage of the maximum vertical settlement to the excavation depth, and draw a scatter plot; S52: perform scatter fitting analysis; S53: Select the optimal combination method according to the fitting results.

10. The method for optimizing design of subway foundation pit support structure based on three-dimensional finite element simulation according to claim 9, characterized in that: In step S52, a power function is used to perform scatter point fitting analysis, and the fitting function is as follows: is the ratio of the maximum vertical settlement to the excavation depth, a 、 b are the fitting formula parameters.

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