Tunnel primary spraying concrete structure design method based on bearing characteristics of anchoring layer

Abstract

The invention belongs to the field of primary shotcrete structure design, and particularly discloses a tunnel primary shotcrete structure design method based on the bearing characteristic of an anchoring layer, and the method comprises the following steps: step 1, analyzing the stress characteristics and mechanical action mechanism of primary shotcrete based on the surface protection effect of the primary shotcrete; 2, according to the stress characteristics and the mechanical action mechanism of the primarily-sprayed concrete, a primarily-sprayed concrete load-structure calculation model is established; the initial spraying concrete load-structure calculation model comprises a load, a structure and a spring; wherein the load represents the overlying gravel load borne by the initial spraying concrete, the structure is the structure of the initial spraying concrete, and the spring represents the bonding effect between the initial spraying concrete and the anchoring layer; step 3, establishing an initial spraying concrete load calculation method; step 4, determining initial spraying concrete strength values, including structural strength and interfacial strength; and step 5, determining a safety evaluation method of the primarily-sprayed concrete, and realizing design of the primarily-sprayed concrete structure.

Description

Technical Field

[0001] The invention relates to the technical field of mountain tunnel engineering, and in particular to a tunnel initial shotcrete structure design method based on the bearing characteristics of an anchoring layer. Background Art

[0002] The initial shotcrete spraying of tunnel is a relatively important link in the construction of tunnel support and an indispensable part of the support system. Its control over the stability of the tunnel is mainly reflected in the following aspects: it can quickly seal the surrounding rock to prevent it from loosening or collapsing; effectively inhibit the initial deformation of the surrounding rock to avoid further instability; isolate air and water to slow down the weathering and erosion of the surrounding rock; seal cracks to reduce groundwater leakage and improve the construction environment; after combining with the surrounding rock, it forms a bearing structure to enhance the overall stability; evenly distribute the stress of the surrounding rock to prevent stress concentration; timely support to reduce the risk of landslides and roof falls; after sealing the surrounding rock, it reduces dust and falling rocks to improve safety.

[0003] However, in the current tunnel support construction process, the design of the tunnel primary shotcrete structure is basically empirical, that is, it is designed according to the specified values ​​in the design specifications (railway, highway and other industry specifications), and the mechanical properties of the primary shotcrete used are also consistent with those of the secondary shotcrete, and the rationality of the primary shotcrete structure has not been mechanically verified.

[0004] The above design method has the following problems in terms of tunnel structure stability:

[0005] First, the initial shotcrete design has poor flexibility under different surrounding rock environments and cannot effectively carry out quantitative design for various surrounding rock environments, which makes it difficult to control the stability of the surrounding rock.

[0006] Secondly, the initial shotcrete has a great influence on the formation of the anchoring layer structure. The instability of the initial shotcrete affects the bearing effect of the overall anchoring structure, which in turn affects the design of the anchor support components. Summary of the invention

[0007] In order to solve the technical problems existing in the above-mentioned background technology, the patent of the present invention provides a tunnel initial shotcrete structure design method based on the bearing characteristics of the anchor layer.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] The design method of tunnel initial shotcrete structure based on the bearing characteristics of the anchor layer is as follows:

[0010] Step 1: Based on the surface protection function of primary shotcrete, analyze its stress characteristics and mechanical action mechanism;

[0011] Step 2: Establishing a load-structure calculation model of primary sprayed concrete according to the stress characteristics and mechanical action mechanism of primary sprayed concrete; the load-structure calculation model of primary sprayed concrete includes load, structure and spring; wherein the load represents the overlying gravel load borne by the primary sprayed concrete, the structure is the structure of the primary sprayed concrete itself, and the spring represents the bonding effect between the primary sprayed concrete and the anchoring layer;

[0012] Step 3: Establish the initial shotcrete load calculation method;

[0013] Step 4: Determine the strength value of the initial shotcrete, including structural strength and interface strength;

[0014] Step 5: Determine the safety evaluation method for primary shotcrete and implement the design of the primary shotcrete structure.

[0015] As a further technical solution, the mechanical action mechanism of the primary shotcrete is to improve the support strength of the anchoring layer while ensuring the integrity of the anchoring layer; however, due to the effect of the gravel between the anchoring layers, the primary shotcrete is deformed, which plays a reverse support role for the collapsed gravel. Therefore, its stress characteristic is to bear the gravel load between the overlying anchoring layers;

[0016] As a further technical solution, the analysis method in step 1 is as follows: the analysis is performed through a numerical calculation model, and it is assumed that the anchor layer is an elastic continuous medium material with low mechanical parameters; the liner structural unit is used in the numerical simulation to simulate the initial sprayed concrete, and its mechanical properties include the liner structural unit itself, as well as the interaction between the liner unit and the zone unit interface. The interface normal mechanical properties are affected by the interface normal spring stiffness k n , tensile strength f t The shear properties of the interface are determined by the shear spring stiffness k s , interface cohesion c, interface residual cohesion c r , interface friction angle φ and normal stress magnitude σ n Control; by changing the interface shear strength parameters, the protective effect of initial shotcrete on the anchoring layer under different shear strengths is analyzed;

[0017] As a further technical solution, the spring includes a horizontal spring and a vertical spring, and the horizontal spring and the vertical spring are used to characterize the shear bonding and tensile bonding effects between interfaces.

[0018] As a further technical solution, in step 3, the calculation method of the initial sprayed concrete load is as follows:

[0019] The arch collapse area is simplified into a triangular prism, and according to the geometric relationship, the crushed stone load q in the collapse area is obtained: s The calculation formula is as follows:

[0020] q s =0.5γh (7)

[0021] h=0.5S Tb tanα (8)

[0022] In the formula, γ is the weight of surrounding rock, h is the height of rock collapse, S Tb is the anchor spacing, and α is the angle between the collapse boundary and the horizontal direction.

[0023] As a further technical solution, the interface strength in step 4 includes the tensile bond strength of the initial sprayed concrete interface and the shear bond strength of the initial sprayed concrete; and the structural strength is the compressive strength.

[0024] As a further technical solution, the safety evaluation method for the initial shotcrete in step 5 includes an interface safety evaluation method and a structural safety evaluation method.

[0025] As a further technical solution, the interface safety evaluation method is as follows: through model calculation, the horizontal spring force σ is extracted. x and the vertical spring force σ y , and analyze the interface security as follows:

[0026] Among them, the interface tensile failure determination formula is as follows:

[0027] k ic σ x ≤[σ] (9)

[0028] The determination formula of interface shear failure is as follows:

[0029] k ic σ y ≤[τ] (10)

[0030] Where, [σ] is the control value of the interface tensile bond strength; [τ] is the control value of the interface shear bond strength; k ic is the interface safety factor value, which can be selected by referring to the structural safety factor determination criterion; when k ic ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. ic ≥[K]; [K] is the safety control value of the initial shotcrete structure, which can be selected by referring to the structural safety factor determination benchmark.

[0031] As a further technical solution, the plain shotcrete structure adopts a safety factor for safety evaluation. There are two cases according to the eccentricity of the axial force e0=M / N, and its cross-sectional strength is calculated according to the following formula.

[0032] Compressive strength control (when e0≤0.20d)

[0033]

[0034] Tensile strength control (when e0>0.20d)

[0035]

[0036] Where N is the actual axial force of the cross section, N; k c Strength safety factor value; R a is the ultimate compressive strength of concrete, MPa; α is the axial force eccentricity influence coefficient; φ is the longitudinal bending coefficient of the member; b is the longitudinal calculation width of the structure (taken as 1m); d is the section thickness, m; h is the section height, m; R1 is the ultimate tensile strength of concrete, MPa; when k c ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. c ≥[K]; [K] is the safety control value of the initial shotcrete structure, which can be selected by referring to the structural safety factor determination benchmark.

[0037] Beneficial effects of the present invention:

[0038] This invention firstly realizes the quantitative design of the structural parameters of the initial sprayed concrete, and provides a mechanical reference method for the rationality of the initial sprayed concrete structure from the perspective of mechanical design. Secondly, it effectively assists the stability of the anchor layer structure and improves the support effect of the anchor rod. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A schematic flow chart of a tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer proposed in the present invention;

[0040] Figure 2(a) is a schematic diagram of prestressed anchor support, and Figure 2(b) is an enlarged schematic diagram of prestressed anchor support;

[0041] Figure 3 This is a schematic diagram of the surface protection function of the initial shotcrete;

[0042] Figure 4(a), Figure 4(b), Figure 4(c), Figure 4(b), and Figure 4(e) are the model diagrams for analyzing the surface protection effect of initial shotcrete;

[0043] Figure 5(a), Figure 5(b), Figure 5(c), and Figure 5(d) are the deformation cloud diagrams of the anchoring layer at different initial shotcrete cohesion;

[0044] Figure 6 This is a schematic diagram of the calculation model for the initial shotcrete structure;

[0045] Figure 7This is a simplified schematic diagram of the initial shotcrete load;

[0046] Figure 8 The influence of different lithology and interface roughness on tensile bond strength; DETAILED DESCRIPTION

[0047] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0048] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless otherwise explicitly stated in the present invention, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "include" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof;

[0049] Based on the technical problems existing in the prior art, this embodiment proposes a tunnel initial shotcrete structure design method based on the bearing characteristics of the anchor layer, which mainly includes four steps:

[0050] Step 1: Based on the surface protection function of primary shotcrete, analyze its stress characteristics and mechanical action mechanism;

[0051] Step 2: Establishing a load-structure calculation model of primary sprayed concrete according to the stress characteristics and mechanical action mechanism of primary sprayed concrete; the load-structure calculation model of primary sprayed concrete includes load, structure and spring; wherein the load represents the overlying gravel load borne by the primary sprayed concrete, the structure is the structure of the primary sprayed concrete itself, and the spring represents the bonding effect between the primary sprayed concrete and the anchoring layer;

[0052] Step 3: Establish the initial shotcrete load calculation method;

[0053] Step 4: Determine the strength value of the initial shotcrete, including structural strength and interface strength;

[0054] Step 5: Determine the safety evaluation method for primary shotcrete and implement the design of the primary shotcrete structure.

[0055] The specific solution proposed in this embodiment is described in detail below in conjunction with the specific drawings:

[0056] (1) Analysis of the protective effect in step 1

[0057] The mechanical properties of the anchor layer are related to the surface protection strength. Existing studies believe that the surface protection strength is determined by the size of the anchor tray. The larger the tray size, the greater the surface protection strength and the better the bearing capacity of the anchor layer. However, in actual tunnel construction, the size of the anchor tray is relatively small relative to the actual size of the tunnel, and it cannot play a strong surface protection role. As shown in Figure 2, the circumferential spacing of the anchors is 100 cm, and the tray width is 15 cm. The tray width is relatively small relative to the spacing of the anchors, and it cannot bear the load generated by the arch collapse area, that is, it cannot play an effective surface protection role.

[0058] However, according to the actual tunnel construction process, before the prestressed anchor support is applied, the surrounding rock will first be sprayed with concrete to form a preliminary sprayed concrete layer of about 5 cm thick (determined by the surrounding rock environment) to seal the loose rock blocks on the surface of the surrounding rock, as shown in Figure 2. The anchor layer forms a potential arch collapse area during the load-bearing process, and the preliminary sprayed concrete that plays a protective role can support the gravel within the arch collapse area. That is, under the load of the collapse area, the preliminary sprayed concrete is deformed and generates a reaction force to resist the collapse of the gravel, thereby achieving a supporting effect on the collapsed gravel and ensuring the stability of the anchor layer. Figure 3 As shown in Figure 2. Among them, the load q s It is the gravel load on the initial shotcrete.

[0059] It can be seen from this that the primary shotcrete on the surrounding rock should be the real surface protection in the actual tunnel. In order to prove the surface protection effect of the primary shotcrete on the broken surrounding rock of the anchor layer, a numerical calculation model is used for analysis, and it is assumed that the anchor layer is an elastic continuous medium material with low mechanical parameters. In practice, after hardening, the shotcrete can withstand both bending moment and axial force, and nonlinear interface mechanical behavior will occur between the shotcrete and the surrounding rock. Therefore, the liner structural unit is used in the numerical simulation to simulate the primary shotcrete. Its mechanical properties include the liner structural unit itself, as well as the interaction between the liner unit and the zone unit interface. The normal mechanical properties of the interface are affected by the normal spring stiffness k of the interface. n , tensile strength f t The shear properties of the interface are determined by the shear spring stiffness k s , interface cohesion c, interface residual cohesion c r , interface friction angle φ and normal stress magnitude σ n control.

[0060] According to a large number of engineering practices, shotcrete is mainly damaged by shear and adhesion, and crushing damage rarely occurs. Therefore, by changing the interface shear strength parameters, the protective effect of the initial shotcrete on the anchor layer under different shear strengths is analyzed. As shown in Figure 4(a), Figure 4(b), Figure 4(c), Figure 4(b), and Figure 4(e), it is a simplified analysis model of the initial shotcrete protective effect, in which the anchor layer is assumed to be an elastic continuous medium material (E = 1GPa, v = 0.4), and the radial and tangential spring stiffness k n / k s is 10GPa / m, and the interface tensile strength f t is 1MPa, the interface friction angle φ is 30°, and the interface cohesion c is 0.01~0.2Mpa. Specifically, Figure 4(a) is a simplified mechanical model diagram; Figure 4(b) is a finite element solid numerical model; Figure 4(c) is the anchoring layer part in the numerical model; Figure 4(d) is the initial sprayed concrete part in the numerical model; Figure 4(e) is a schematic diagram of the numerical model calculation results (deformation cloud diagram).

[0061] As shown in Figures 5(a), 5(b), 5(c) and 5(d), they are deformation cloud diagrams of the anchoring layer under different interface cohesion under the same load (0.1MPa). In Figure 5(a), when the interface cohesion c=0.00Mpa, the maximum deformation of the anchoring layer is 3.32mm; in Figure 5(b), when the interface cohesion c=0.05Mpa, the maximum deformation of the anchoring layer is 1.58mm; in Figure 5(c), when the interface cohesion c=0.15Mpa, the maximum deformation of the anchoring layer is 1.34mm; in Figure 5(d), when the interface cohesion c=0.20Mpa, the maximum deformation of the anchoring layer is 1.23mm;

[0062] As shown in Figures 5(a), 5(b), 5(c) and 5(d), as the cohesion increases, the deformation of the anchoring layer gradually decreases. When the cohesion changes from 0.00MPa to 0.20MPa, the maximum deformation of the anchoring layer decreases from 3.32mm to 1.23mm, which is 51% to 63% less than that without initial spraying. Combined with the failure mechanism of the anchoring layer, it can be seen that as the deformation of the anchoring layer increases, the rock mass gradually loosens, resulting in a decrease in the mutual bite force, a weakening of the force chain effect between the rock mass, and a collapse of the rock mass, which reduces the stability of the anchoring layer. This shows that initial spraying concrete can effectively protect the surface and ensure the stability of the anchoring layer. Therefore, the bonding effect between the sprayed concrete and the surrounding rock should be taken seriously when designing initial spraying concrete.

[0063] (2) Calculation of initial shotcrete structure

[0064] Under the action of crushed stone load, the primary shotcrete is deformed, generating a reaction force to resist the collapse of crushed stone, thereby supporting the collapsed crushed stone. Therefore, in order to quantify the calculation of the parameters of the primary shotcrete, a spring calculation model for the primary shotcrete beam is established, such as Figure 6 As shown in the figure, since the initial shotcrete may cause shear failure and tensile failure between interfaces under the action of crushed stone, horizontal springs and vertical springs are set between the initial shotcrete and the surrounding rock to characterize the shear bond and tensile bond between interfaces.

[0065] Therefore, when using the initial shotcrete structural calculation model to analyze the surface protection strength, it is necessary to analyze the load, structural strength, interface strength and safety evaluation method of the initial shotcrete. The details are as follows:

[0066] 1) Calculation of initial shotcrete load

[0067] In order to simplify the analysis of the load generated by the arch collapse area, the arch collapse area is simplified into a triangular prism, such as Figure 7 As shown. Where h is the height of the gravel collapse (m), S Tb is the anchor spacing (m), α is the angle between the collapse boundary and the horizontal direction, which can be approximately 45°.

[0068] According to the geometric relationship, the crushed stone load q in the collapse area is obtained s The calculation formula can be expressed as:

[0069] q s =0.5γh (13)

[0070] h=0.5S Tb tanα (14)

[0071] Where γ is the weight of surrounding rock, kN / m 3 According to the current Railway Tunnel Design Code (TB

[0072] 10003-2016) for selection.

[0073] 2) Calculation of compressive strength of initial shotcrete,

[0074] The structural strength of initial shotcrete generally refers to the initial shotcrete compressive strength. According to data survey, there is no unified requirement for the early compressive strength index of shotcrete. The domestic requirement for the early compressive strength of shotcrete is generally based on the 1d age, mostly 5-10MPa. The highest index abroad is greater than that in China, but the early compressive strength is less than that in China. The early compressive strength of shotcrete stipulated in various specifications is shown in Table 1. In the absence of tests, the data in Table 1 can be referenced for selection.

[0075] Table 1 Early compressive strength requirements for shotcrete

[0076]

[0077]

[0078] Note: The European standard gives a continuous curve of strength changing with time. For the sake of comparison, this article only takes two time points, 1h and 1d.

[0079] 3) The method for determining the tensile bond strength of the initial shotcrete interface is as follows:

[0080] The bond strength of shotcrete generally refers to the interface strength between the surrounding rock and shotcrete, including the tensile bond strength and shear bond strength of the interface. The specific value is obtained as follows:

[0081] 3.1 As the tensile bond strength has an important influence on the supporting effect of shotcrete, the tensile bond strength index is currently regulated both at home and abroad, as shown in Table 2. However, due to differences in industry requirements, structural types and construction environments, there are differences in the regulations on the tensile strength of shotcrete bond in various industries. In the absence of tests, the data in Table 2 can be used for reference.

[0082] Table 2 Requirements for tensile bond strength between shotcrete and surrounding rock

[0083]

[0084]

[0085] In addition, many scholars have also studied the tensile bond strength of shotcrete. For example, Dong Chunyuan et al. found through experiments that the tensile bond strength between high-performance wet shotcrete and rock is 21.5% higher than that between ordinary wet shotcrete and rock, and the 28-day strength reaches 1.47MPa. Li Binbin compared the bond strengths of ordinary shotcrete, early high-strength shotcrete and surrounding rock at different ages based on experiments, and found that the bond strength of early high-strength shotcrete within 14 days is more than 50% higher than that of ordinary shotcrete, reaching 0.9MPa in 1 day and 2.1MPa in 28 days. L. Malmgren et al. found through the study of the tensile bond strength of shotcrete under different rock types and interface roughness that the tensile bond strength of shotcrete is related to factors such as the cleanliness of the interface, the roughness of the rock surface, the mineral composition of the rock, the mechanical properties of the shotcrete, and the construction method. They also summarized the interface tensile strengths between different types of rocks and shotcrete, such as Figure 8 shown.

[0086] 3.2 The method for determining the shear bond strength of initial shotcrete is as follows:

[0087] The shear bond strength between shotcrete and surrounding rock also has a great influence on the mechanical properties of shotcrete. However, the existing specifications rarely stipulate the shear bond strength between shotcrete and surrounding rock. However, many scholars at home and abroad have studied the shear bond strength characteristics of shotcrete and rock. The interface shear strength parameters of shotcrete and rock measured by direct shear tests are shown in Table 3. It can be seen from Table 3 that the shear strength is not only related to the roughness of the interface, but also closely related to the curing conditions, rock types, etc. In the absence of tests, the data in Table 3 can be referenced for selection.

[0088] Table 3 Study on shear strength parameters of shotcrete and rock

[0089]

[0090]

[0091] 5) Safety evaluation method

[0092] 5) Safety evaluation method

[0093] The safety evaluation of primary shotcrete structure can be mainly divided into two types: interface safety evaluation and structural safety evaluation.

[0094] ①Interface safety evaluation

[0095] Through model calculation, extract the horizontal spring force σ x and the vertical spring force σ y , and analyze the interface security according to the following formula.

[0096] Determination of interface tensile failure

[0097] k ic σ x ≤[σ] (15)

[0098] Determination of interface shear failure

[0099] k ic σ y ≤[τ] (16)

[0100] Where, [σ] is the control value of the interface tensile bond strength; [τ] is the control value of the interface shear bond strength; k ic is the interface safety factor value, when k ic ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. ic ≥[K]. [K] is the safety control value of the initial shotcrete structure, which can be selected with reference to the structural safety factor determination criteria, as shown in Table 4 and Table 5;

[0101] ②Structural safety evaluation

[0102] The safety factor of plain shotcrete structure is used for safety evaluation. There are two cases according to the eccentricity of the axial force e0=M / N, and the section strength is calculated according to the following formula.

[0103] Compressive strength control (when e0≤0.20d)

[0104]

[0105] Tensile strength control (when e0>0.20d)

[0106]

[0107] Where N is the actual axial force of the cross section, N; k c Strength safety factor value; R a is the ultimate compressive strength of concrete, MPa; α is the axial force eccentricity influence coefficient; φ is the longitudinal bending coefficient of the member; b is the longitudinal calculation width of the structure (taken as 1m); d is the section thickness, m; h is the section height, m; R1 is the ultimate tensile strength of concrete, MPa;

[0108] The criteria for determining the safety factor of structural strength are shown in Tables 4 and 5. When calculating the strength during the construction phase, the safety factor benchmark value is the value in the "main load + additional load" column multiplied by the reduction factor of 0.9. c ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. c ≥[K].

[0109] Table 4 Criteria for determining strength safety factor of plain concrete structures

[0110]

[0111] Table 5 Determination criteria for strength safety factor of reinforced concrete structures

[0112]

[0113] In view of the shortcomings of the existing non-quantitative calculation method for tunnel primary shotcrete, a tunnel primary shotcrete structure design method based on the bearing characteristics of the anchor layer was established considering the surface protection characteristics of the primary shotcrete.

[0114] The calculation model of the primary shotcrete structure, the load calculation method and the safety evaluation method proposed in the present invention overcome the defects of the existing empirical values ​​of the primary shotcrete parameters and enrich the relevant theories of the refined design of the tunnel engineering support structure.

Claims

1. A tunnel initial shotcrete structure design method based on the bearing characteristics of the anchor layer, characterized in that: as follows: Step 1: Based on the surface protection function of primary shotcrete, analyze its stress characteristics and mechanical action mechanism; Step 2: Establishing a load-structure calculation model of primary sprayed concrete according to the stress characteristics and mechanical action mechanism of primary sprayed concrete; the load-structure calculation model of primary sprayed concrete includes load, structure and spring; wherein the load represents the overlying gravel load borne by the primary sprayed concrete, the structure is the structure of the primary sprayed concrete itself, and the spring represents the bonding effect between the primary sprayed concrete and the anchoring layer; Step 3: Establish the initial shotcrete load calculation method; Step 4: Determine the strength value of the initial shotcrete, including structural strength and interface strength; Step 5: Determine the safety evaluation method for primary shotcrete and implement the design of the primary shotcrete structure.

2. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The mechanical action mechanism of the primary shotcrete is: to improve the support strength of the anchor layer while ensuring the integrity of the anchor layer.

3. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The stress-bearing characteristic of the primary shotcrete is that it bears the gravel load between the overlying anchoring layers.

4. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The analysis method in step 1 is as follows: the analysis is performed through a numerical calculation model, and it is assumed that the anchor layer is an elastic continuous medium material with low mechanical parameters; the liner structural unit is used in the numerical simulation to simulate the initial sprayed concrete, and its mechanical properties include the liner structural unit itself, as well as the interaction between the liner unit and the zone unit interface. The interface normal mechanical properties are affected by the interface normal spring stiffness k n , tensile strength f t The shear properties of the interface are determined by the shear spring stiffness k s , interface cohesion c, interface residual cohesion c r , interface friction angle φ and normal stress magnitude σ n Control; by changing the interface shear strength parameters, the surface protection effect of initial shotcrete on the anchoring layer under different shear strengths is analyzed.

5. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The springs include horizontal springs and vertical springs, and the horizontal springs and vertical springs are used to characterize the shear bonding and tensile bonding effects between interfaces.

6. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: In step 3, the calculation method of the initial sprayed concrete load is as follows: the arched collapse area is simplified into a triangular prism, and the crushed stone load q of the collapse area is obtained according to the geometric relationship: s The calculation formula is as follows: q s =0.5γh (1) h=0.5S Tb tanα (2) In the formula, γ is the weight of surrounding rock, h is the height of rock collapse, S Tb is the anchor spacing, and α is the angle between the collapse boundary and the horizontal direction.

7. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The interface strength includes the tensile bond strength of the initial shotcrete interface and the shear bond strength of the initial shotcrete interface.

8. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1 is characterized in that: The safety evaluation method for the initial shotcrete in step 5 includes an interface safety evaluation method and a structural safety evaluation method.

9. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1, characterized in that: The interface safety evaluation method described is as follows: Through model calculation, the horizontal spring force σ is extracted x and the vertical spring force σ y , and analyze the interface security as follows: Among them, the determination formula of interface tensile failure is as follows: k ic s x ≤[σ] (3) The determination formula of interface shear failure is as follows: k ic s y ≤[τ] (4) Where, [σ] is the control value of the interface tensile bond strength; [τ] is the control value of the interface shear bond strength; k ic is the interface safety factor value, which can be selected by referring to the structural safety factor determination criterion; when k ic ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. ic ≥[K]; [K] is the safety control value of the initial shotcrete structure, which can be selected by referring to the structural safety factor determination benchmark.

10. The tunnel initial shotcrete structure design method based on the bearing characteristics of the anchoring layer according to claim 1, characterized in that: The plain shotcrete structure described above adopts a safety factor for safety evaluation. There are two cases according to the eccentricity of the axial force e0=M / N. Its cross-sectional strength is calculated as follows: When e0≤0.20d, the compressive strength control is calculated according to formula (5); When e0>0.20d, the tensile strength is calculated according to formula (6): Where N is the actual axial force of the cross section, N; k c Strength safety factor value; R a is the ultimate compressive strength of concrete, MPa; α is the axial force eccentricity influence coefficient; φ is the longitudinal bending coefficient of the member; b is the longitudinal calculation width of the structure (taken as 1m); d is the section thickness, m; h is the section height, m; R1 is the ultimate tensile strength of concrete, MPa; when k c ≥[K], the initial shotcrete is in a safe state and the structural design is reasonable. Otherwise, redesign is carried out until k is satisfied. c ≥[K]; [K] is the safety control value of the initial shotcrete structure, which can be selected by referring to the structural safety factor determination benchmark.

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