Design method for combined support structure of pipe roof and freezing curtain in extra-large section tunnel

Abstract

The present application provides a design method for the combined support structure of a super-large cross-section tunnel pipe roof and a freezing curtain, including: before the secondary lining is constructed after the excavation and support of a single cross-section, the first combined support composed of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure bears the top load; after the freezing curtain thaws, the second combined support structure composed of the primary support structure and the lining structure bears the top load; wherein, before the secondary lining is constructed after the excavation and support of a single cross-section, the combined support composed of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure bears the top load, and the proportion of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure in bearing the top load is 6:3:1, which can at least accurately design the support structure, ensure that the tensile and compressive strengths of the pipe roof freezing curtain meet the design requirements, and guarantee the safety of tunnel construction.

Description

Technical Field

[0001] The present application relates to the field of tunnel design, and more specifically, to a design method for a combined support structure of a pipe curtain and a freezing curtain in a tunnel of an extra-large section. Background Art

[0002] The design and construction of extra-large cross-section water-rich tunnels is a major technical challenge in tunnel engineering, which requires comprehensive response to issues such as large span, high water pressure, surrounding rock stability and sudden water inrush risks.

[0003] In the existing technology, extra-large cross-section water-rich tunnels are usually constructed using the pipe curtain freezing method + step method. At different construction stages, the tunnel support combination forms are different. How to accurately design the support structure has always been a difficult problem that has troubled engineers. Summary of the invention

[0004] In view of this, the purpose of this application is to provide a design method for a combined support structure of a pipe curtain and a freezing curtain for a tunnel with an extra-large section. The first combined support composed of a pipe curtain freezing curtain structure, an initial support structure, and a temporary steel support structure bears the top load, and the second combined support structure composed of the initial support structure and a lining structure bears the top load. The load is accurately calculated to accurately design the support structure, ensure that the tensile and compressive strength and coordinated bearing capacity of the pipe curtain freezing curtain meet the design requirements, and ensure the safety of tunnel construction.

[0005] The embodiment of the present application provides a design method for a combined support structure of a pipe curtain and a frozen curtain in a large-section tunnel, which is applied to the construction of a combined support structure of a pipe curtain and a frozen curtain in a large-section tunnel. The method comprises: before excavation and support of a single cross section to the construction of a secondary lining, a first combined support consisting of a pipe curtain frozen curtain structure, an initial support structure, and a temporary steel support structure bears the top load; after the frozen curtain is thawed, a second combined support structure consisting of the initial support structure and a lining structure bears the top load; wherein, before excavation and support of a single cross section to the construction of a secondary lining, a combined support consisting of a pipe curtain frozen curtain structure, an initial support structure, and a temporary steel support structure bears the top load, and the ratio of the pipe curtain frozen curtain structure, the initial support structure, and the temporary steel support structure to bear the top load is 6:3:1; wherein, the top load is calculated by the following steps:

[0006] ;

[0007] in, is the total pressure on the tunnel top; is the earth pressure on the tunnel top; is the water pressure on the top of the tunnel; It is the weight of soil; is the height of the soil layer; is the unit weight of water; is the height of water.

[0008] Optionally, the pipe roof freezing circle consists of the pipe roof freezing curtain structure and the freezing circle around the pipe roof freezing curtain structure. Among them, the functional relationship between the thickness of the freezing circle and the maximum tensile stress of the freezing circle is calculated by the following formula;

[0009] ;

[0010] where t is the thickness of the freezing circle, is the maximum tensile stress of the freezing circle.

[0011] Optionally, the pipe roof freezing curtain structure is designed through the following steps: Determine the diameter of the pipe roof freezing curtain structure according to the top load of the pipe roof freezing curtain structure and the ratio of the diameter of the pipe roof freezing curtain structure to the thickness of the freezing circle to meet the requirement of coordinated bearing of the structure; Among them, the diameter of the steel pipe of the pipe roof freezing curtain is determined by the following formula:

[0012] ;

[0013] where t is the thickness of the freezing circle, is the total pressure received at the top of the tunnel, is the maximum tensile stress of the freezing circle, and D is the diameter of the steel pipe of the pipe roof freezing curtain structure.

[0014] Optionally, the method includes: Determine whether the pipe roof freezing curtain structure meets the bearing requirement according to the critical value of the ratio of the diameter of the steel pipe of the pipe roof freezing curtain structure to the thickness of the freezing circle, the total pressure received at the top of the tunnel, and the thickness of the freezing circle; Among them, the following formula is used to determine whether the pipe roof freezing curtain structure meets the bearing requirement:

[0015] ;

[0016] Among them, the critical value of the ratio of the diameter of the steel pipe of the pipe roof freezing curtain structure to the thickness of the freezing circle is calculated by the following formula:

[0017] ;

[0018] where, is the unit weight of soil; is the height of the soil layer; is the unit weight of water; is the height of water, is the critical value of the ratio of the diameter of the steel pipe of the pipe roof freezing curtain structure to the thickness of the freezing circle.

[0019] Optionally, the temporary steel support structure is designed through the following steps: determine the support positions of the temporary steel support structure, where the support positions include the first position, the second position, and the third position corresponding to 12 o'clock and 15° on each side of the pipe curtain freezing curtain structure, the fourth position and the fifth position corresponding to 9 o'clock and 30° counterclockwise, the sixth position and the seventh position corresponding to 3 o'clock and 30° counterclockwise, and the eighth position and the ninth position corresponding to 15° on each side of 6 o'clock. The steel supports are connected by cross braces and vertical braces to limit the deformation of the freezing circle; according to the critical condition of the stability of the steel support compression bar, combined with the performance data of various steel grades, the cross-sectional areas of various steels, and the cross-sectional moment of inertia of various steels, determine the target steel grade.

[0020] Optionally, the initial support structure is composed of shotcrete and steel arch. Among them, the initial support structure is designed through the following steps: determine the shape of the shotcrete according to the weak position of the combined structure of the pipe curtain and the freezing circle; according to the elastic modulus of the steel arch, the moment of inertia of the steel arch section, the elastic modulus of the concrete, and the equivalent moment of inertia of the section, equivalent the steel arch to an equivalent concrete thickness; determine the thickness of the initial support structure according to the equivalent concrete thickness and the thickness of the shotcrete; determine the maximum stress of the initial support structure according to the tunnel top pressure borne by the initial support structure and the thick-walled cylinder theory; determine the initial support structure according to the compressive strength of the shotcrete, the preset safety factor, and the maximum stress of the initial support structure.

[0021] Optionally, the steel arch is equivalent to an equivalent concrete thickness through the following formula:

[0022] ;

[0023] where b is the excavation step distance of the bench method; n is the number of steel arches within the distance b, is the elastic modulus of the steel arch, is the moment of inertia of the steel arch section, is the elastic modulus of the concrete, is the equivalent concrete thickness; among them, the thickness of the initial support structure is calculated through the following formula:

[0024] ;

[0025] where t3 is the thickness of the shotcrete and t1 is the thickness of the initial support structure.

[0026] Optionally, the method further includes: determining the secondary lining structure according to the initial support design and the thick-walled thin cylinder theory, and the secondary lining structure adopts a C30 shotcrete structure.

[0027] Optionally, the method further includes the construction process of the combined support structure of the extra-large cross-section tunnel pipe roof and the freezing curtain, including: carrying out pipe roof jacking construction, which adopts a bottom-up and symmetric jacking method; carrying out freezing construction; and carrying out tunnel excavation construction.

[0028] Optionally, the freezing construction is carried out through the following steps: controlling the brine temperature to drop below the first target temperature to accelerate the freezing of the soil; controlling the brine temperature to be stable between the first target temperature and the second target temperature to ensure the uniform expansion of the freezing curtain; and controlling the brine temperature to the second target temperature and maintaining it for a preset time.

[0029] The design method of the combined support structure of the extra-large cross-section tunnel pipe roof and the freezing curtain provided by the embodiments of the present application accurately designs the support structure by means of the first combined support composed of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure to bear the top load, the second combined support structure composed of the primary support structure and the lining structure to bear the top load, and accurately calculating the load, ensuring that the tensile and compressive strengths of the pipe roof freezing curtain meet the design requirements and guaranteeing the safety of tunnel construction.

[0030] To make the above objects, features, and advantages of the present application more obvious and understandable, the following specifically enumerates preferred embodiments and, in conjunction with the accompanying drawings, makes the following detailed description. Description of the Drawings

[0031] To more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0032] Figure 1 It is a schematic flow chart of a design method for a combined support structure of an extra-large cross-section tunnel pipe roof and a freezing curtain provided by an embodiment of the present application;

[0033] Figure 2 It is a schematic diagram of the calculation results provided by an embodiment of the present application;

[0034] Figure 3 It is a schematic diagram of a dangerous cross-section provided by an embodiment of the present application Figure 1 ;

[0035] Figure 4 It is a schematic diagram of a dangerous cross-section provided by an embodiment of the present application Figure 2 ;

[0036] Figure 5Schematic diagram of the maximum tensile stress of the freezing circle in different forms under pd = 100 kPa provided by the embodiments of the present application;

[0037] Figure 6 Schematic diagram of the maximum tensile stress of the freezing circle in different forms under pd = 130 kPa provided by the embodiments of the present application;

[0038] Figure 7 Schematic diagram of the maximum tensile stress of the freezing circle in different forms under pd = 160 kPa provided by the embodiments of the present application;

[0039] Figure 8 Schematic diagram of fitting the thickness of the freezing circle and the maximum tensile stress of the freezing circle provided by the embodiments of the present application;

[0040] Figure 9 Schematic diagram of the position of the steel support structure provided by the embodiments of the present application;

[0041] Figure 10 Schematic diagram of the selection of steel grades provided by the embodiments of the present application;

[0042] Figure 11 Schematic diagram of the combined support of shotcrete and steel arch provided by the embodiments of the present application;

[0043] Figure 12 Schematic diagram of the pipe roof construction provided by the embodiments of the present application;

[0044] Figure 13 Schematic diagram of the four-bench and fourteen-step excavation provided by the embodiments of the present application. Detailed implementation manners

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Apparently, the described embodiments are only a part rather than all of the embodiments of the present application. Usually, the components of the embodiments of the present application described and illustrated herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed present application, but merely represents selected embodiments of the present application. Based on the embodiments of the present application, every other embodiment obtained by those skilled in the art without creative efforts falls within the scope of protection of the present application.

[0046] First, the applicable application scenarios of the present application are introduced. The present application can be applied to the technical field of tunnel design.

[0047] Research has found that the design and construction of extra-large cross-section water-rich tunnels is a major technical challenge in tunnel engineering, which requires comprehensive response to issues such as large span, high water pressure, surrounding rock stability and sudden water inrush risks.

[0048] In the existing technology, extra-large cross-section water-rich tunnels are usually constructed using the pipe curtain freezing method + step method. At different construction stages, the tunnel support combination forms are different. How to accurately design the support structure has always been a difficult problem that has troubled engineers.

[0049] Based on this, an embodiment of the present application provides a design method for a combined support structure of a pipe curtain and a freezing curtain for a tunnel of extra-large cross-section. The top load is borne by a first combined support composed of a pipe curtain freezing curtain structure, an initial support structure, and a temporary steel support structure. The top load is borne by a second combined support structure composed of the initial support structure and a lining structure. The load is accurately calculated to accurately design the support structure, thereby ensuring that the tensile and compressive strength of the pipe curtain freezing curtain meets the design requirements and ensuring the safety of tunnel construction.

[0050] See also Figure 1 , Figure 1 A schematic diagram of a design method for a combined support structure of a large-section tunnel pipe curtain and a freezing curtain provided in an embodiment of the present application. Figure 1 As shown in the embodiment of the present application, the design method of the combined support structure of the extra-large-section tunnel pipe curtain and the freezing curtain includes:

[0051] S101. Before the excavation and support of a single cross section and the construction of the secondary lining, the first combined support consisting of the pipe-curtain frozen curtain structure, the initial support structure and the temporary steel support structure bears the top load.

[0052] S102. After the freezing curtain thaws, the second combined support structure consisting of the initial support structure and the lining structure bears the top load.

[0053] It should be noted that the design method of the combined support structure of the pipe curtain and freezing curtain for a super-large cross-section tunnel proposed in this application is applied to the construction of the combined support structure of the pipe curtain and freezing curtain for a super-large cross-section tunnel.

[0054] Specifically, the construction method includes: performing pipe-roof jacking construction, wherein the pipe-roof jacking construction adopts a bottom-up, symmetrical jacking method; performing freezing construction; and performing tunnel excavation construction.

[0055] As an example, see Figure 12 , from bottom to top, symmetrical jacking method, is the Figure 12 The I and II areas are pushed forward symmetrically first, and then the III and IV areas are pushed forward symmetrically until the arch top pipe is pushed forward.

[0056] Specifically, the freezing construction is carried out through the following steps: controlling the brine temperature to drop below the first target temperature to accelerate the freezing of the soil; controlling the brine temperature to be stable between the first target temperature and the second target temperature to ensure the uniform expansion of the freezing curtain; controlling the brine temperature to the second target temperature and maintaining it for a preset time.

[0057] Among them, the freezing construction adopts a pipe layout method of circular freezing pipes + limiting pipes + special-shaped freezing pipes. Solid steel pipes are arranged with circular freezing pipes + limiting pipes, and hollow steel pipes are arranged with special-shaped freezing pipes. The freezing pipes circulate low-temperature brine to reduce the temperature of the formation around the pipe curtain. The groundwater freezes to form a freezing wall, which together with the pipe curtain forms a curtain to jointly bear the upper soil and water pressure.

[0058] Please continue to refer to Figure 12 , it should be noted that there are 5 weak positions in the pipe curtain freezing curtain. A total of 16 pipes numbered 1, 2, 3, 7, 8, 9, 10, 16, 17, 18, 19, 26, 27, 28, 35, 35 are the positions where the freezing circle is prone to break away from the pipe curtain. To address this, the brine flow rate and brine temperature are controlled to ensure that the freezing circle is always connected to the pipe curtain.

[0059] As an example, the first target temperature can be -30 °C. The brine temperature is initially reduced below -30 °C to accelerate the freezing of the soil, increased to 120% - 150% of the designed flow rate to improve the heat exchange efficiency, ensure the rapid formation of an ice crystal framework in the curtain area, and shorten the freezing time.

[0060] As an example, the second target temperature can be -25 °C. The brine temperature is stabilized between -25 °C and -30 °C, the temperature field in the area between the freezing pipe spacings is monitored, the flow rate of the special-shaped freezing pipes is adjusted, and the temperature gradient is maintained to ensure the uniform expansion of the freezing curtain and avoid local weak areas.

[0061] As an example, the preset time can be 15 days. The brine temperature can slightly rise to -20 °C, and the freezing time is extended by at least 15 days to ensure the full development of strength.

[0062] Specifically, please refer to Figure 13 , for the steps of tunnel excavation construction, as shown in Table 1 below.

[0063] Table 1: Construction Steps of the New Five-Step Fourteen-Method Excavation

[0064]

[0065] Among them, before the construction of the secondary lining for the excavation and support of a single cross-section, the combined support composed of the pipe curtain freezing curtain structure, the primary support structure, and the temporary steel support structure bears the top load. The proportions of the pipe curtain freezing curtain structure, the primary support structure, and the temporary steel support structure in bearing the top load are 6:3:1.

[0066] Among them, the top load is calculated through the following steps:

[0067] ;

[0068] Among them, is the total pressure on the top of the tunnel; is the earth pressure on the top of the tunnel; is the water pressure on the top of the tunnel; is the unit weight of soil; is the height of the soil layer; is the unit weight of water; is the height of water.

[0069] Specifically, the pipe curtain freezing circle is composed of the pipe jacking pipe curtain freezing curtain structure and the freezing circle around the pipe jacking pipe curtain freezing curtain structure.

[0070] Among them, in some projects, although the overall bearing capacity of the pipe jacking pipe curtain freezing curtain steel pipe and the freezing circle around the pipe jacking pipe curtain freezing curtain structure meets the design requirements, they cannot bear jointly and coordinately, resulting in problems such as structural instability and excessive settlement in the project. Therefore, this application establishes 27 groups of abaqus working conditions. On the basis of analyzing their overall bearing and coordinated bearing, a combined structure design method for the pipe curtain freezing circle is proposed to make it meet the standards in both overall bearing and coordinated bearing, ensuring construction safety.

[0071] As an example, please refer to Figures 2 - 4 , the typical calculation working conditions are as follows: the tunnel section is circular with a radius of 9.8m, the buried depth is 4.3m, the diameter of the pipe curtain steel pipe D = 1.6m, which is uniformly arranged along the tunnel, with a total of 36 pipe curtains, the thickness of the freezing circle t = 2m, the elastic modulus of concrete in the pipe curtain E2 = 3×107kPa, and the elastic modulus of frozen soil is taken as E1 = 3×105kPa. The top load p of the tunnel is calculated by the above formula d = 109kPa. The pipe curtain and the freezing circle, and the freezing circle and the surrounding rock are all subjected to bonded constraints. The steel support is simplified to vertical and horizontal displacement constraints. The structure is subjected to the action of upper, two-side water and earth pressures. The model and results are shown in the figure.

[0072] It can be seen from the model results that the freezing circles in several ranges, namely, 15° on each side at 12 o'clock of the pipe curtain, 30° counterclockwise at 9 o'clock, 30° counterclockwise at 3 o'clock, and 15° on each side at 6 o'clock, are in tension along the tunnel radial direction and compression along the circumferential direction, and there is a tendency to detach along the tunnel axis and away from the pipe curtain. Whether they bear jointly and coordinately needs to be considered at the above positions. At the same time, the primary support needs to be strengthened and steel supports need to be set. The structure as a whole obtains the maximum tensile and compressive stresses at 6 o'clock and 9 o'clock directions respectively. The stress of the cross-sections at the two positions is shown in the figure. The maximum tensile stress, = 0.95MPa; the maximum compressive stress = 1.14 MPa. Therefore, the freezing parameters need to be strictly controlled at these two positions to ensure that the frozen circle meets the bearing requirements.

[0073] Please refer to Figures 5 - 7 , and calculate the maximum stress borne by the frozen soil between the pipe curtains under different Pd, t, and D. The shallow-buried tunnel has a buried depth of 4 - 6 m and the groundwater depth is 2 m. From Equation (1), the load Pd on the upper part of the tunnel ranges from 100 kPa to 160 kPa. Taking 100, 130, and 160, three working conditions are considered. The diameter D of the steel pipe is restricted by the number of pipe curtains and the size of the excavation section, and its value ranges from 1.4 m to 1.8 m. Taking 1.4, 1.6, and 1.8, three working conditions are designed; the thickness t of the frozen circle ranges from 2 m to 3 m. Taking 2 m, 2.5 m, and 3.0 m, three working conditions are considered. The calculation results of the specific working conditions are shown in Table 2 below:

[0074] Table 2: Calculation Working Condition Table

[0075]

[0076] Among them, the functional relationship between the thickness of the frozen circle and the maximum tensile stress of the frozen circle is calculated through the following formula;

[0077] ;

[0078] where t is the thickness of the frozen circle, is the maximum tensile stress of the frozen circle.

[0079] Here, in this application, the thickness t of the frozen circle and the maximum tensile stress of the frozen circle are fitted to obtain the functional relationship between the two. The growth rate of the maximum tensile stress of the frozen circle decreases with the increase in the thickness of the frozen circle, that is, increasing the thickness of the frozen circle reduces the improvement of the structural stress efficiency and also increases the construction cost. Therefore, the thickness of the frozen circle should be small rather than large on the basis of meeting the overall bearing capacity of the structure. As Figure 8 shown, the value corresponding to the tensile strength value of the frozen soil is preferably used as the thickness of the frozen circle.

[0080] Design the pipe curtain freezing curtain structure through the following steps: Determine the diameter of the steel pipe of the pipe curtain freezing curtain structure according to the top load of the pipe curtain freezing curtain structure and the ratio of the diameter of the pipe curtain steel pipe to the thickness of the frozen circle to meet the requirements of coordinated bearing of the structure.

[0081] Among them, the diameter of the steel pipe of the pipe curtain structure is determined through the following formula:

[0082] ;

[0083] where t is the thickness of the frozen circle, is the total pressure received at the top of the tunnel, is the maximum tensile stress of the frozen circle, and D is the diameter of the steel pipe of the pipe curtain freezing curtain structure.

[0084] Specifically, the method includes: determining whether the pipe-roof freezing curtain structure meets the bearing requirements according to the critical value of the ratio of the diameter of the steel pipe of the pipe-roof freezing curtain steel pipe structure to the freezing circle thickness, the total pressure received at the top of the tunnel, and the freezing circle thickness

[0085] Among them, it is determined whether the pipe-roof freezing curtain structure meets the bearing requirements through the following formula:

[0086] ;

[0087] Among them, the critical value of the ratio of the diameter of the steel pipe of the pipe-roof freezing curtain structure to the freezing circle thickness is calculated through the following formula:

[0088] ;

[0089] Among them, is the unit weight of soil; is the soil layer height; is the unit weight of water; is the height of water, is the critical value of the ratio of the diameter of the steel pipe of the pipe-roof freezing curtain structure to the freezing circle thickness.

[0090] Please refer to Figure 8 , and by calculating the diameter of the steel pipe of the designed pipe-roof structure, the coordinated bearing requirements of the structure can be met.

[0091] Specifically, the temporary steel support structure is designed through the following steps: determining the support positions of the temporary steel support structure, where the support positions include the first position, the second position, and the third position corresponding to 12 o'clock of the pipe-roof freezing curtain structure and 15° on each side, the fourth position and the fifth position at 9 o'clock and 30° counterclockwise, the sixth position and the seventh position at 3 o'clock and 30° counterclockwise, and the eighth position and the ninth position at 6 o'clock and 15° on each side. The steel supports are connected by cross braces and vertical braces to limit the deformation of the freezing circle; according to the critical condition of the stability of the steel support compression bar, combined with the performance data of various steel grades, the cross-sectional areas of various steels, and the cross-sectional moment of inertia of various steels, the target steel grade is determined.

[0092] As an example, the positions of the steel support structure are as shown in Figure 9 .

[0093] Specifically, the temporary support uses a steel support with an I-shaped cross-section. The form of its force failure is the instability of the compression bar. For the i-th steel support, its axial compressive stress σ i , the critical pressure of the compression bar stability σ cri are obtained by:

[0094] ;

[0095] Among them, Z i The axial pressure borne by the steel support can be obtained by simulation with abaqus; E i is the elastic modulus of the steel; L i is the length of the steel support; A i is the cross-sectional area of the steel; r i is the radius of gyration of the steel cross-section; μ i is a coefficient related to the connection form at both ends of the steel support.

[0096] Its axial pressure needs to be less than the critical buckling pressure of the compression bar:

[0097] ;

[0098] Simplifying the above formula gives:

[0099] ;

[0100] Furthermore, it can be assumed that the I-beam model is , the left side of the above formula is related to the steel model x i , let = , the right side of the above formula is a constant, and it is replaced by G to get:

[0101] ;

[0102] Substituting into the above formula gives:

[0103] ;

[0104] Please refer to Figure 10 , according to the steel model standard specification, draw the function image of f(x i ), and take the minimum value of r i A i A i on the right side of the intersection point of y = G and y = f(x

[0105] Among them, the initial support structure is composed of shotcrete and steel arch.

[0106] Among them, the initial support structure is designed through the following steps: determine the shape of the shotcrete according to the weak positions of the combined structure of the pipe roof and the freezing circle; equivalent the steel arch frame to an equivalent concrete thickness according to the elastic modulus of the steel arch frame, the moment of inertia of the steel arch frame section, the elastic modulus of the concrete, and the equivalent section moment of inertia; determine the thickness of the initial support structure according to the equivalent concrete thickness and the thickness of the shotcrete; determine the maximum stress of the initial support structure according to the tunnel top pressure borne by the initial support structure and the thick-walled cylinder theory; determine the initial support structure according to the compressive strength of the shotcrete, the preset safety factor, and the maximum stress of the initial support structure.

[0107] Specifically, the steel arch frame is equivalent to an equivalent concrete thickness through the following formula:

[0108] ;

[0109] wherein, b is the excavation step distance of the bench method; n is the number of steel arch frames within the distance b, is the elastic modulus of the steel arch frame, is the moment of inertia of the steel arch frame section, is the elastic modulus of the concrete, is the equivalent concrete thickness.

[0110] Among them, the thickness of the initial support structure is obtained through the following formula:

[0111] ;

[0112] wherein, t3 is the thickness of the shotcrete, and t1 is the thickness of the initial support structure.

[0113] Specifically, please refer to Figure 11 , the initial support structure bears 30% of the top load. According to the thick-walled cylinder theory, the maximum circumferential stress of the initial support can be calculated, and it needs to be less than the concrete compressive strength to obtain:

[0114] ;

[0115] wherein, is the compressive strength of the shotcrete; K is the safety factor, and the value range is 1.5 - 2.0.

[0116] Substituting the above formula, we can get:

[0117] ;

[0118] Thus, the stiffness of the initial support structure is calculated according to this formula.

[0119] Specifically, the method further includes: determining the secondary lining structure according to the initial support design and the thick-walled thin cylinder theory, and the secondary lining structure adopts a C30 shotcrete structure.

[0120] In the design method of the combined support structure of the extra-large section tunnel pipe roof and the freezing curtain provided by the embodiments of the present application, in the first stage, when the tunnel structure is borne by the pipe roof freezing circle, the primary support, and the steel support, the combined support of the pipe roof freezing circle plays a major bearing role and bears 60% of the upper load. In the prior art, the overall bearing capacity of the two meets the design requirements, but the coordination between the two is not considered. Therefore, a design method for the combined support structure with coordinated bearing of the pipe roof and the freezing circle is proposed. Secondly, the primary support bears 30% of the upper load and adopts two support forms of shotcrete and steel arch. A structural form of uneven shotcrete for the weak positions of the pipe roof freezing curtain is proposed, and a design method for the support structure in which the steel arch is equivalent to concrete is proposed to design the overall form of the structure. Then, combined with the coordinated bearing state of the pipe roof freezing circle, shotcrete is strengthened at specific parts. Finally, the temporary steel support bears 10% of the upper load, and the position of the steel support is determined by the coordinated bearing state of the pipe roof freezing circle. In the second stage, the tunnel structure is borne by the primary support and the secondary lining for the upper load of the tunnel. Since the primary support structure is known, the thick-walled cylinder method is used to design the secondary lining structure.

[0121] On this basis, relevant parameters in the pipe roof construction process, such as pipe roof spacing, pipe roof quantity, and pipe roof jacking sequence, are designed; a freezing scheme for pipe roof freezing is designed, and a specific pipe layout method is adopted, which is divided into the initial rapid cooling stage, the intermediate stable cooling stage, and the later heat preservation maintenance stage to control the brine temperature and flow rate to ensure the strength of the freezing curtain.

[0122] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in the present application.

[0123] The above-described embodiments only represent several implementation manners of the present application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the patent scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application.

Claims

1. Design method for combined support structure of pipe roof and freezing curtain in extra-large cross-section tunnel, which is applied to the construction of combined support structure of pipe roof and freezing curtain in extra-large cross-section tunnel, characterized in that The method includes: Before the secondary lining is constructed after the excavation and support of a single cross-section, the first combined support composed of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure bears the top load; After the freezing curtain thaws, the second combined support structure composed of the primary support structure and the lining structure bears the top load; Among them, before the secondary lining is constructed after the excavation and support of a single cross-section, the combined support composed of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure bears the top load, and the proportion of the pipe roof freezing curtain structure, the primary support structure, and the temporary steel support structure in bearing the top load is 6:3:1; Among them, the top load is calculated through the following steps: ； Among them, is the total pressure on the top of the tunnel; is the earth pressure on the top of the tunnel; is the water pressure on the top of the tunnel; is the unit weight of soil; is the height of the soil layer; is the unit weight of water; is the height of water; The pipe roof freezing circle consists of the pipe roof freezing curtain structure of the pipe jacking and the freezing circle around the pipe roof freezing curtain structure of the pipe jacking, Among them, the functional relationship between the freezing circle thickness and the maximum tensile stress of the freezing circle is calculated through the following formula; ； where t is the thickness of the frozen wall, is the maximum tensile stress of the frozen wall; The pipe roof freezing curtain structure is designed through the following steps: According to the top load of the pipe roof freezing curtain structure and the ratio of the diameter of the pipe roof freezing curtain steel pipe to the freezing circle thickness, determine the diameter of the pipe roof freezing curtain steel pipe to meet the requirements of coordinated bearing of the structure; Among them, the diameter of the steel pipe of the pipe roof freezing curtain structure is determined through the following formula: ； where t is the thickness of the frozen circle, is the total pressure on the top of the tunnel, is the maximum tensile stress of the frozen circle, and D is the diameter of the steel pipe of the pipe-roof frozen curtain; The method includes: According to the critical value of the ratio of the diameter of the steel pipe of the pipe roof freezing curtain structure to the freezing circle thickness, the total pressure received at the top of the tunnel, and the freezing circle thickness, determine whether the pipe roof freezing curtain structure meets the bearing requirements; Among them, whether the pipe roof freezing curtain structure meets the bearing requirements is determined through the following formula: ； Among them, the critical value of the ratio of the diameter of the pipe roof freezing curtain steel pipe to the freezing circle thickness is calculated through the following formula to determine whether the pipe roof freezing curtain structure meets the requirements of coordinated deformation: ； Among them, is the unit weight of soil; is the height of soil layer; is the unit weight of water; is the height of water, is the critical value of the ratio of the steel pipe diameter of the pipe roof freezing curtain to the freezing circle thickness.

2. The method according to claim 1, characterized in that, The temporary steel support structure is designed through the following steps: Determine the support positions of the temporary steel support structure. The support positions include the first position, the second position, and the third position corresponding to 12 o'clock of the pipe roof freezing curtain structure and 15° on both sides, the fourth position and the fifth position at 9 o'clock and 30° counterclockwise, the sixth position and the seventh position at 3 o'clock and 30° counterclockwise, and the eighth position and the ninth position at 6 o'clock and 15° on both sides. Each steel support is connected by cross braces and vertical braces to limit the deformation of the freezing circle; According to the critical condition of the stability of the steel support compression bar, combined with the performance data of various steel grades, the cross-sectional areas of various steels, and the moment of inertia of the cross-section of various steels, determine the target steel grade.

3. The method according to claim 1, wherein The primary support structure is composed of shotcrete and steel arch frames; Among them, the primary support structure is designed through the following steps: According to the weak position of the combined structure of the pipe roof and the freezing circle, determine the shape of the shotcrete; According to the elastic modulus of the steel arch frame, the moment of inertia of the cross-section of the steel arch frame, the elastic modulus of the concrete, and the equivalent moment of inertia of the cross-section, the steel arch frame is equivalent to an equivalent concrete thickness; According to the equivalent concrete thickness and the thickness of the shotcrete, determine the thickness of the primary support structure; According to the tunnel top pressure borne by the primary support structure and the thick-walled cylinder theory, determine the maximum stress of the primary support structure; Determine the primary support structure according to the compressive strength of the shotcrete, a preset safety factor, and the maximum stress of the primary support structure.

4. The method according to claim 3, characterized in that, Equivalent the steel arch frame to an equivalent concrete thickness through the following formula: ； Among them, b is the excavation step distance of the bench method; n is the number of steel arch frames within the distance b, is the elastic modulus of the steel arch frame, is the moment of inertia of the steel arch frame section, is the elastic modulus of concrete, is the equivalent concrete thickness; Among them, the thickness of the primary support structure is calculated by the following formula: ； Among them, t3 is the thickness of the shotcrete, and t1 is the thickness of the primary support structure.

5. The method according to claim 1, wherein The method further includes: Determine the secondary lining structure according to the primary support design and the thick-walled thin cylinder theory. The secondary lining structure adopts a C30 shotcrete structure.

6. The method according to claim 1, wherein The method further includes the construction process of the combined support structure of the pipe roof and the freezing curtain for extra-large section tunnels, including: Carry out pipe roof jacking construction, and the pipe roof jacking construction adopts a bottom-up and symmetric jacking method; Carry out freezing construction; Carry out tunnel excavation construction.

7. The method according to claim 6, characterized in that, Carry out freezing construction through the following steps: Control the brine temperature to drop below the first target temperature to accelerate soil freezing; Control the brine temperature to be stable between the first target temperature and the second target temperature to ensure uniform expansion of the freezing curtain; Control the brine temperature to the second target temperature and maintain it for a preset time.

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