Comprehensive rigidity determination method for tunnel composite support system

Abstract

The invention discloses a comprehensive rigidity determination method for a tunnel composite support system, which comprises the following steps of: 1, establishing a large three-dimensional model, and simulating construction to obtain a curve of convergence displacement of a feature point along with excavation and tunneling; 2, a tunnel in the model is excavated through at a time, initial virtual supporting force is applied to the wall of the tunnel, and a relation curve of convergence displacement of feature points and the virtual supporting force is obtained; 3, establishing a corresponding curve graph of the distance between the tunnel face and the monitoring face and the virtual supporting force of the tunnel wall; 4, drawing a relation curve of the distance between the tunnel face and the monitoring face and the displacement completion rate of the monitoring point under the supporting effect; 5, the relation curve of the displacement completion rate and the virtual supporting force and the relation curve of the distance between the tunnel face and the monitoring face under the supporting effect and the displacement completion rate of the feature points are combined and processed, and the actual comprehensive rigidity value of the composite supporting system is calculated; the method has the advantages that the support rigidity is quantitatively evaluated, the simulation effect is good, and construction is convenient.

Description

Technical Field

[0001] The invention belongs to the technical field of tunnel simulation construction, and in particular relates to a method for determining the comprehensive stiffness of a tunnel composite support system. Background Art

[0002] LDP (Longitudinal deformation curve of surrounding rock) and GRC (Displacement characteristic curve) together constitute the basis of the tunnel convergence constraint method. They can be used to analyze the deformation of surrounding rock and the response of support structure caused by tunnel excavation, providing important theoretical basis and practical guidance for tunnel support design. ‌‌‌Under the background of rapid development of rock theory and numerical simulation methods, numerical simulation calculation methods have become an important method for calculation analysis and design of tunnels. At the same time, with the innovation of support structure forms and the continuous improvement of the monitoring level of New Austrian Tunneling Method construction, in order to intuitively express the correlation between surrounding rock and support in the development of support design, the convergence constraint method came into being. Although the convergence constraint method can make a simple estimate of the load borne by the simple support structure installed in the tunnel, for complex tunnel support systems containing multiple support structures and different layout forms, it is also necessary to consider the influence of time effects caused by different construction timings of each support. It cannot reflect the interaction mechanism between surrounding rock and structure based on the excavation process, the collaborative working characteristics of the tunnel-support system and the role of the support structure in the system. However, this deficiency is indeed what designers urgently need to understand. In order to answer the above questions, experts and scholars use theoretical analysis and numerical calculation methods to study the confining pressure-support system, and extract support characteristic curves to characterize the support stiffness, because the support stiffness can directly reflect the support capacity of the support structure.

[0003] Existing research on the collaborative working characteristics and support stiffness of support systems has many shortcomings: (1) Theoretical studies often assume that the support structure is laid uniformly in the entire cross-section. However, in actual projects, sprayed concrete, anchor rods, and steel arch frames are often only applied within a certain range in the tunnel circumference. At the same time, the theoretical assumption cannot fully consider the uneven stress conditions of the support structure. (2) Numerical methods can better make up for the shortcomings of the above theoretical methods, but the stiffness of composite supports is less studied in numerical studies than that of single supports. At the same time, the actual stiffness evolution process of the composite support system after considering the time effect of support application is unclear, and the actual stiffness contribution value of the composite support system is not easy to calculate. No effective and feasible calculation process and method have been proposed. Summary of the invention

[0004] The purpose of the present invention is to provide a method for determining the comprehensive stiffness of a tunnel composite support system in view of the problems existing in the prior art. The method has the advantages of quantitatively calculating the support stiffness, having good simulation effect and convenient construction.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a method for determining the comprehensive stiffness of a tunnel composite support system, comprising the following steps:

[0006] Step 1: Establish a large three-dimensional model including the tunnel, select a monitoring surface, select feature points on the monitoring surface, excavate the model step by step without support measures, monitor the selected feature points, and simulate the construction process to obtain the evolution curve of the convergence displacement of the feature points as the excavation progresses;

[0007] Step 2: Based on the principle of virtual support force, first obtain the support force of the rock mass of the excavated tunnel on the surrounding rock of the tunnel, then dig through the tunnel in the model once, apply the initial virtual support force on the tunnel wall, and then gradually reduce the virtual support force in proportion, monitor the selected feature points, and obtain the corresponding mathematical relationship curve between the convergence displacement of the feature points and the virtual support force;

[0008] Step 3, normalizing the convergence displacement of the characteristic points in Step 1 and Step 2 to calculate the displacement completion rate, and then obtaining a relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate, and a relationship curve between the displacement completion rate and the virtual support force, and establishing a corresponding curve diagram between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall based on the two relationship curves;

[0009] Step 4, determine the distance between the construction section of each support structure and the tunnel face in the composite support system, obtain the virtual support force provided by the tunnel face at each position through the corresponding graphical relationship between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall, simulate the tunnel face advancement process through stress release, apply support, set monitoring points on the support section, calculate the displacement completion rate of the monitoring points at each support section, and finally draw the relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate of the monitoring points under the support action;

[0010] Step 5, using the displacement completion rate as the horizontal coordinate, the relationship curve between the displacement completion rate and the virtual support force and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the characteristic point under the support action are combined to form an upper and lower distribution, and the intersection of the relationship curve between the displacement completion rate and the virtual support force and the displacement asymptote and the foot point of the support starting point on the horizontal axis are connected. The product of the slope of the line segment and the final convergent displacement of the characteristic point without support is the actual comprehensive stiffness value of the composite support system.

[0011] In the above scheme, in step 1, a large three-dimensional model is established to simulate the construction process, and the evolution curve of the convergence displacement at the feature point with the excavation progress is monitored, that is, the relationship curve between the distance between the face and the monitoring surface and the convergence displacement at the feature point; in step 2, the tunnel is excavated once, and the initial virtual support force is applied, and then the virtual support force is reduced proportionally, and the convergence displacement of the selected feature points is monitored to obtain the relationship curve between the convergence displacement of the feature point and the virtual support force; in step 3, the convergence displacement of the feature point is normalized to obtain the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate, and the relationship curve between the displacement completion rate and the virtual support force. The two formulas are combined by the displacement completion rate to obtain the relationship between the distance between the face and the monitoring surface and the virtual support force of the tunnel wall. The corresponding curve diagram of the pseudo support force; in step 4, according to the position of the support structure in the composite support system, combined with the corresponding graphical relationship between the distance between the face and the monitoring surface and the virtual support force of the tunnel wall, the virtual support force at each face is obtained, the advancement process is simulated, the displacement completion rate of the monitoring point of each support section is calculated, and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the monitoring point under the support effect is obtained; in step 5, the displacement completion rate is used as the horizontal coordinate to combine the relationship curve between the displacement completion rate and the virtual support force, and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the characteristic point under the support effect, and the actual comprehensive stiffness value of the composite support system is obtained by multiplying the slope of the line segment with the final convergence displacement of the characteristic point in the absence of support through the graphical relationship.

[0012] Furthermore, in step 3, the convergence displacement of the characteristic points in the step-by-step excavation condition of the model is converted into the displacement completion rate using formula (1):

[0013] (1)

[0014] Where: The distance from the tunnel face to the monitoring surface; is the convergence displacement of the feature point when the tunnel face reaches the monitoring surface.

[0015] The convergence displacement at the feature point when the tunnel face reaches the monitoring surface is calculated Convergence displacement of the characteristic point when the full longitudinal excavation of the model is completed By comparison, we can get the displacement completion rate and perform conversion calculations.

[0016] Furthermore, in step 3, the fitting process of the displacement completion rate in the step-by-step excavation condition of the model adopts formula (2):

[0017] (2)

[0018] Where: is the displacement completion rate of the feature point when the tunnel face coincides with the monitoring surface; x is the distance from the tunnel face to the monitoring surface, and X is a constant to be determined.

[0019] The LDP curve fitting calculation of the displacement completion rate is performed by formula (2), which facilitates intuitive understanding of the changing trend of the displacement completion rate.

[0020] Furthermore, in step 2, the virtual support force is calculated using formula (3):

[0021] (3)

[0022] Where: is the stress release coefficient. In this calculation, it increases evenly from 0 to 100% with a step size of 10%. F0 is the initial virtual support force.

[0023] The virtual support force is gradually reduced by adjusting the stress release coefficient, so that it is convenient to obtain the corresponding relationship curve between the convergence displacement of the characteristic point and the virtual support force.

[0024] Furthermore, in step 3, the displacement completion rate of the feature point is calculated using formula (4):

[0025] (4)

[0026] Where: The stress release coefficient of the characteristic point P is Convergence displacement when ; For feature points Converged displacement at completion of load release.

[0027] The calculation formulas for displacement completion rate and virtual support force under various stress release coefficients are obtained through formula (4).

[0028] Furthermore, in step 4, the spacing between the sections of each support structure in the composite support system is used to determine the distance between the sections of each support structure and the tunnel face.

[0029] According to the actual construction spacing of the support structures in the composite support system, the positions of the support structures and the distances from the tunnel face are simulated to improve the simulation effect and provide more accurate data for actual construction.

[0030] Furthermore, the cross section of the tunnel is horseshoe-shaped or circular.

[0031] The horseshoe-shaped tunnel has a horseshoe-shaped or circular cross-section, which is convenient for construction.

[0032] Furthermore, the monitoring point is selected from the vertex of the support section.

[0033] The monitoring point is selected at the top of the support section, where deformation is significant and detection is convenient.

[0034] Furthermore, the monitoring surface is located at the axial middle position of the tunnel, and the characteristic point is located at the vertex of the monitoring surface.

[0035] The monitoring surface is set in the middle of the tunnel axis. The force and convergence displacement there are representative and can be used to simulate the overall situation of the tunnel. The characteristic points are selected from the vertices of the monitoring surface, where the deformation is significant and convenient for detection.

[0036] Furthermore, the stress release in step 4 is calculated using formula (3). Increase evenly from 0 to 100% in steps of 10%.

[0037] In step 4, stress release calculation is performed to gradually strengthen the support as the stress is released until the stress disappears and the final support strength is reached.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. By establishing a large three-dimensional model, the convergence displacement curve of the characteristic points in the unsupported construction process is simulated and calculated, and the characteristic point convergence displacement and virtual support force relationship curve is combined with the construction situation of the simulated composite support system to obtain the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the monitoring point under the support action. After data processing, the actual comprehensive stiffness value of the composite support system is finally obtained, and the support stiffness value is quantitatively calculated, which is convenient for intuitively understanding the stiffness of the composite support system and quantitatively evaluating the ability of the support system to maintain the stability of the surrounding rock;

[0040] 2. The unevenness of the support structure is taken into account in the comprehensive stiffness evaluation of the support system. The characteristic points and monitoring points are selected at the top of the tunnel section, which can reflect the actual working characteristics of the support system and achieve good simulation results.

[0041] 3. By calculating the comprehensive stiffness value of the support system, it provides a reference for the design process of the support system, facilitating design and construction;

[0042] 4. The present invention can demonstrate the convergence and deformation process of the surrounding rock and the stiffness evolution process of the support system during the construction of the composite support system, and the calculation principle is clear and easy to accept. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a corresponding curve diagram of the distance between the tunnel face and the monitoring surface and the virtual supporting force of the tunnel wall in Example 1 of the present invention;

[0044] Figure 2 It is a combined graph of the relationship curve between the displacement completion rate and the virtual support force in Example 1 of the present invention and the relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate of the characteristic point under the support action. DETAILED DESCRIPTION

[0045] The technical solution of the present invention will be clearly and completely described below in conjunction with the drawings in the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. In the description of the present invention, it should be noted that the terms front, back, left, right, etc. indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings, or are the directions or positional relationships in which the invented product is usually placed when used. They are only for the convenience of describing the present invention or simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed or operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0046] Example 1

[0047] like Figure 1-2 As shown, a method for determining the comprehensive stiffness of a tunnel composite support system comprises the following steps:

[0048] Step 1: Establish a large three-dimensional model including the tunnel, select a monitoring surface, select feature points on the monitoring surface, excavate the model step by step without support measures, monitor the selected feature points, and simulate the construction process to obtain the evolution curve of the convergence displacement of the feature points as the excavation progresses;

[0049] Step 2: Based on the principle of virtual support force, first obtain the support force of the rock mass of the excavated tunnel on the surrounding rock of the tunnel, then dig through the tunnel in the model once, apply the initial virtual support force on the tunnel wall, and then gradually reduce the virtual support force in proportion, monitor the selected feature points, and obtain the corresponding mathematical relationship curve between the convergence displacement of the feature points and the virtual support force;

[0050] Step 3, normalizing the convergence displacement of the characteristic points in Step 1 and Step 2 to calculate the displacement completion rate, and then obtaining a relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate, and a relationship curve between the displacement completion rate and the virtual support force, and establishing a corresponding curve diagram between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall based on the two relationship curves;

[0051] Step 4, determine the distance between the construction section of each support structure and the tunnel face in the composite support system, obtain the virtual support force provided by the tunnel face at each position through the corresponding graphical relationship between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall, simulate the tunnel face advancement process through stress release, apply support, set monitoring points on the support section, calculate the displacement completion rate of the monitoring points at each support section, and finally draw the relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate of the monitoring points under the support action;

[0052] Step 5, using the displacement completion rate as the horizontal coordinate, the relationship curve between the displacement completion rate and the virtual support force and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the characteristic point under the support action are combined to form an upper and lower distribution, and the intersection of the relationship curve between the displacement completion rate and the virtual support force and the displacement asymptote and the foot point of the support starting point on the horizontal axis are connected. The product of the slope of the line segment and the final convergent displacement of the characteristic point without support is the actual comprehensive stiffness value of the composite support system.

[0053] In the above scheme, in step 1, a large three-dimensional model is established to simulate the construction process, and the evolution curve of the convergence displacement at the feature point with the excavation progress is monitored, that is, the relationship curve between the distance between the face and the monitoring surface and the convergence displacement at the feature point; in step 2, the tunnel is excavated once, and the initial virtual support force is applied, and then the virtual support force is reduced proportionally, and the convergence displacement of the selected feature points is monitored to obtain the relationship curve between the convergence displacement of the feature point and the virtual support force; in step 3, the convergence displacement of the feature point is normalized to obtain the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate, and the relationship curve between the displacement completion rate and the virtual support force. The two formulas are combined by the displacement completion rate to obtain the relationship between the distance between the face and the monitoring surface and the virtual support force of the tunnel wall. The corresponding curve diagram of the pseudo support force; in step 4, according to the position of the support structure in the composite support system, combined with the corresponding graphical relationship between the distance between the face and the monitoring surface and the virtual support force of the tunnel wall, the virtual support force at each face is obtained, the advancement process is simulated, the displacement completion rate of the monitoring point of each support section is calculated, and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the monitoring point under the support effect is obtained; in step 5, the displacement completion rate is used as the horizontal coordinate to combine the relationship curve between the displacement completion rate and the virtual support force, and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the characteristic point under the support effect, and the actual comprehensive stiffness value of the composite support system is obtained by multiplying the slope of the line segment with the final convergence displacement of the characteristic point in the absence of support through the graphical relationship.

[0054] In step 1, a large three-dimensional model including the tunnel is established by considering the boundary effect of the numerical model. The distance between the center of the tunnel and the upper and lower and left and right boundaries of the model is not less than 7 times the tunnel diameter, and the longitudinal length of the model is not less than 10 times the tunnel diameter. Feature points are selected, and the model is excavated step by step without support measures. The selected feature points are monitored. Get the convergence displacement of the feature points , convergence displacement of feature points Normalized and converted into displacement completion rate , and then the displacement completion rate The longitudinal deformation curve (LDP) of the surrounding rock is obtained by fitting, as shown in line ① in the figure.

[0055] In step 2, based on the principle of virtual support force, the support force of the rock mass of the excavated tunnel on the surrounding rock is first obtained. Then, the tunnel in the model is excavated once and an initial virtual support force is applied to the tunnel wall. , select feature points in the model , and then gradually reduce the virtual support force in proportion , get the feature points Convergence displacement and virtual support The corresponding relationship between the feature points and the convergence displacement Normalized and converted into displacement completion rate Post-drawing displacement completion rate With virtual support Relationship curve②.

[0056] Step 3, line ① in the figure represents the displacement completion rate Distance from tunnel face and monitoring surface The relationship curve of displacement completion rate is shown in line ②. With virtual support The relationship curve of displacement completion rate For bridges, the distance between the tunnel face and the monitoring face can be established Virtual support force of cave wall The corresponding relationship is as follows Figure 1 shown.

[0057] Step 4: Determine the distance between the construction section of each support structure and the tunnel face according to the spacing between the construction positions of each support structure in the composite support system. , , ; The virtual support force provided by the face at each position can be obtained through the corresponding relationship in step 3 , the process of tunnel face advancement is simulated by stress release, support is applied, the convergence displacement of the tunnel wall at each support application point is calculated, and the displacement completion coefficient is calculated to obtain the point , , , and finally draw the surrounding rock deformation curve under the support effect③.

[0058] Step 5, see Figure 2 , connect the support starting point The corresponding horizontal axis (displacement completion rate ) and the intersection J of the displacement asymptote of the surrounding rock deformation curve ③ and the GRC curve, as shown in the figure, the slope of the line segment ④IJ The final convergent displacement of the characteristic point in the case of no support The product of is the actual comprehensive stiffness value of the composite support system.

[0059] Furthermore, in step 3, the convergence displacement of the characteristic points in the step-by-step excavation condition of the model is converted into the displacement completion rate using formula (1):

[0060] (1)

[0061] Where: The distance from the tunnel face to the monitoring surface; is the convergence displacement of the feature point when the tunnel face reaches the monitoring surface.

[0062] The convergence displacement at the feature point when the tunnel face reaches the monitoring surface is calculated Convergence displacement of the characteristic point when the full longitudinal excavation of the model is completed By comparison, we can get the displacement completion rate and perform conversion calculations.

[0063] Furthermore, in step 3, the fitting process of the displacement completion rate in the step-by-step excavation condition of the model adopts formula (2):

[0064] (2)

[0065] Where: is the displacement completion rate of the feature point when the tunnel face coincides with the monitoring surface; x is the distance from the tunnel face to the monitoring surface, and X is a constant to be determined.

[0066] The LDP curve fitting calculation of the displacement completion rate is performed by formula (2), which facilitates intuitive understanding of the changing trend of the displacement completion rate. The positive and negative values ​​of x represent the direction relative to the detection surface. Formula (2) corresponds to line ① in the figure.

[0067] Furthermore, in step 2, the virtual support force is calculated using formula (3):

[0068] (3)

[0069] Where: is the stress release coefficient. In this calculation, it increases evenly from 0 to 100% with a step size of 10%. F0 is the initial virtual support force.

[0070] The virtual support force is gradually reduced by adjusting the stress release coefficient, so that it is convenient to obtain the corresponding relationship curve between the convergence displacement of the characteristic point and the virtual support force.

[0071] Furthermore, in step 3, the displacement completion rate of the feature point is calculated using formula (4):

[0072] (4)

[0073] Where: The stress release coefficient of the characteristic point P is Convergence displacement when ; For feature points Converged displacement at completion of load release.

[0074] The calculation formulas for displacement completion rate and virtual support force under various stress release coefficients are obtained through formula (4).

[0075] Furthermore, in step 4, the spacing between the sections of each support structure in the composite support system is used to determine the distance between the sections of each support structure and the tunnel face.

[0076] According to the actual construction spacing of the support structures in the composite support system, the positions of the support structures and the distances from the tunnel face are simulated to improve the simulation effect and provide more accurate data for actual construction.

[0077] Furthermore, the cross section of the tunnel is horseshoe-shaped or circular.

[0078] The horseshoe-shaped tunnel has a horseshoe-shaped or circular cross-section, which is convenient for construction.

[0079] Furthermore, the monitoring point is selected from the vertex of the support section.

[0080] The monitoring point is selected at the top of the support section, where deformation is significant and detection is convenient.

[0081] Furthermore, the monitoring surface is located at the axial middle position of the tunnel, and the characteristic point is located at the vertex of the monitoring surface.

[0082] The monitoring surface is set in the middle of the tunnel axis. The force and convergence displacement there are representative and can be used to simulate the overall situation of the tunnel. The characteristic points are selected from the vertices of the monitoring surface, where the deformation is significant and convenient for detection.

[0083] Furthermore, the stress release in step 4 is calculated using formula (3). Increase evenly from 0 to 100% in steps of 10%.

[0084] In step 4, stress release calculation is performed to gradually strengthen the support as the stress is released until the stress disappears and the final support strength is reached.

[0085] You can reduce the step size as needed to get more accurate data.

[0086] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the comprehensive stiffness of a tunnel composite support system, characterized in that: The steps include: Step 1: Establish a large three-dimensional model including the tunnel, select a monitoring surface, select feature points on the monitoring surface, excavate the model step by step without support measures, monitor the selected feature points, and simulate the construction process to obtain the evolution curve of the convergence displacement of the feature points as the excavation progresses; Step 2: Based on the principle of virtual support force, first obtain the support force of the rock mass of the excavated tunnel on the surrounding rock of the tunnel, then dig through the tunnel in the model once, apply the initial virtual support force on the tunnel wall, and then gradually reduce the virtual support force in proportion, monitor the selected feature points, and obtain the corresponding mathematical relationship curve between the convergence displacement of the feature points and the virtual support force; Step 3, normalizing the convergence displacement of the characteristic points in Step 1 and Step 2 to calculate the displacement completion rate, and then obtaining a relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate, and a relationship curve between the displacement completion rate and the virtual support force, and establishing a corresponding curve diagram between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall based on the two relationship curves; Step 4, determine the distance between the construction section of each support structure and the tunnel face in the composite support system, obtain the virtual support force provided by the tunnel face at each position through the corresponding graphical relationship between the distance between the tunnel face and the monitoring surface and the virtual support force of the tunnel wall, simulate the tunnel face advancement process through stress release, apply support, set monitoring points on the support section, calculate the displacement completion rate of the monitoring points at each support section, and finally draw the relationship curve between the distance between the tunnel face and the monitoring surface and the displacement completion rate of the monitoring points under the support action; Step 5, using the displacement completion rate as the horizontal coordinate, the relationship curve between the displacement completion rate and the virtual support force and the relationship curve between the distance between the face and the monitoring surface and the displacement completion rate of the characteristic point under the support action are combined to form an upper and lower distribution, and the intersection of the relationship curve between the displacement completion rate and the virtual support force and the displacement asymptote and the foot point of the support starting point on the horizontal axis are connected. The product of the slope of the line segment and the final convergent displacement of the characteristic point without support is the actual comprehensive stiffness value of the composite support system.

2. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that: In step 3, the convergence displacement of the characteristic points in the model step-by-step excavation condition is converted into the displacement completion rate using formula (1): （1） Where: The distance from the tunnel face to the monitoring surface; is the convergence displacement of the feature point when the tunnel face reaches the monitoring surface; It is the convergence displacement of the feature point when the full longitudinal excavation of the model is completed.

3. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that: In step 3, the fitting process of the displacement completion rate in the step-by-step excavation condition of the model adopts formula (2): （2） Where: is the displacement completion rate of the feature point when the tunnel face coincides with the monitoring surface; x is the distance from the tunnel face to the monitoring surface, and X is a constant to be determined.

4. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that: In step 2, the virtual support force is calculated using formula (3): （3） Where: is the stress release coefficient.

5. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 4, characterized in that: In step 3, the displacement completion rate of the feature point is calculated using formula (4): （4） Where: For feature points The stress release factor is Convergence displacement when ; For feature points Converged displacement at completion of load release.

6. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that In step 4, the spacing distance between the construction sections of each support structure in the composite support system is used to determine the distance between the construction section of each support structure and the tunnel face.

7. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that , the cross section of the tunnel is horseshoe-shaped or circular.

8. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that , the monitoring point is selected from the vertex of the support section.

9. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 1, characterized in that ,The monitoring surface is located in the axial middle position of the tunnel, and the characteristic point is located at the vertex of the monitoring surface.

10. The method for determining the comprehensive stiffness of a tunnel composite support system according to claim 4, characterized in that , the stress release in step 4 is calculated using formula (3). Increase evenly from 0 to 100% in steps of 10%.

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