Multi-layer yielding supporting structure adopting spraying type polyurethane buffer layer and design method

Abstract

The invention discloses a multi-layer yielding support structure adopting a spraying type polyurethane buffer layer and a design method. The multi-layer yielding support structure comprises a toughening double-layer primary support, the spraying type polyurethane buffer layer and a secondary lining which are sequentially arranged from outside to inside. The design method comprises the following steps: step 1, constructing a first layer of toughening primary support; 2, calculating the radial displacement of the tunnel wall after the first layer of toughening primary support is constructed; 3, calculating the construction starting time of a second layer of toughening primary support; 4, a second layer of toughening primary support is constructed; 5, calculating the radial displacement of the tunnel wall after the second layer of toughening primary support is constructed; step 6, calculating the construction starting time of the polyurethane buffer layer; step 7, spraying a polyurethane buffer layer; and step 8, calculating the total contact pressure of the lining structure and the total radial displacement of the tunnel wall at any moment after the secondary lining construction is completed. According to the method, input parameters are simple, the structure can be designed without relying on monitoring and test data, and dynamic design adjustment of actual construction is facilitated.

Description

Technical Field

[0001] The invention relates to the field of disaster prevention of large deformation tunnels, in particular to a multi-layer pressure-yielding support structure and a design method using a spray-coated polyurethane buffer layer. Background Art

[0002] The essence of tunnel engineering is to dig a space in the stratum below the ground to meet various functional requirements. As a result, the stratum that has been in a state of equilibrium for a long time will undergo stress adjustment and deformation due to excavation disturbance, and try to form a new state of equilibrium as soon as possible. However, due to the differences in stratum conditions, the surrounding rock will have different stable states. Some surrounding rocks are difficult to achieve self-balance during the stress adjustment process. As the deformation develops, they will be destroyed and unstable, which will endanger the safety of the project. Therefore, timely external intervention of the surrounding rock is required. According to different surrounding rock conditions, three methods can be adopted: prior intervention, process intervention and post-intervention, corresponding to advance support, initial support and secondary lining respectively. However, for surrounding rock conditions with poor stability, in the actual support process, various factors act simultaneously and the relationship is extremely complex, which brings challenges to engineering design and construction.

[0003] The soft rock mass of deep buried tunnels often has obvious rheological characteristics and time-weakening effects. After tunnel excavation, the surrounding rock parameters gradually decrease with the increase of time, resulting in increased rheological deformation of the surrounding rock, which is easy to cause cracking of the initial support and secondary lining of the tunnel, seriously affecting the safety of the tunnel during construction and operation. In order to control the rapid development of soft rock deformation under high ground stress and ensure the self-bearing capacity of the surrounding rock, a multi-layer lining structure of "double-layer primary support + secondary lining" is often used in the tunnel construction process. Although multi-layer lining has achieved many successful cases in the treatment of large deformation tunnels, this strong support measure has mostly ended in failure in practice when faced with large deformation of strong extrusion surrounding rock. Large deformation problems such as cracking, falling blocks, and even local collapse of the secondary lining frequently occur during the tunnel operation stage.

[0004] In order to ensure the safety of the tunnel, a buffer layer that can absorb the rheological deformation of the surrounding rock is set between the secondary lining and the surrounding rock. Its special mechanical properties enable it to achieve deformation coordination with the surrounding rock, absorb the long-term deformation energy of the surrounding rock, and achieve long-term stability of the weak tunnel surrounding rock. At present, foam materials are often used as filling materials for the compression support buffer layer due to their high compressibility, such as foam concrete and polymer foam. Compared with foam concrete, polymer foam (polyethylene, polystyrene, polyurethane, etc.) has broad application prospects due to its higher compressibility, light weight and low cost. However, the existing polymer foam buffer layer mostly adopts prefabricated structure, which is manually laid, with complex construction procedures and low efficiency. In contrast, spray-type polyurethane has the advantages of adjustable density, high compression strain, moderate strength and rapid spray forming, which is more in line with the requirements of "high compressibility, medium strength and rapid forming" that an ideal buffer layer material should have. However, at present, there is little research on polyurethane buffer layer compression support, and there is a lack of stress and deformation calculation methods for multi-layer compression support using polyurethane buffer layers, which makes it impossible to effectively carry out structural design, thus greatly limiting its application in practical engineering.

[0005] Patent 202311579466.7 proposes a model test method for a multi-layer support structure of a tunnel, and provides a feasible mechanical analysis method, but it is only for traditional multi-layer strong support structures, and does not consider the timeliness of deformation. Although Patent 202111572047.1 "Large deformation tunnel buffer layer support design method" and Patent 202310976187.8 "A tunnel construction buffer layer parameter design method based on deformation monitoring data" propose a buffer layer parameter design method based on a combination of field monitoring and material testing, but only for the yield structure with a single-layer primary support, it cannot meet the dual requirements of bearing capacity and deformation capacity of strong extrusion and large deformation tunnels; the buffer layer construction still adopts a prefabricated laying method, and the construction efficiency is low; in addition, the structural design depends on the results of the early monitoring and indoor tests of the tunnel, and the flexibility of the calculation method is poor. Summary of the invention

[0006] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and to provide a multi-layer pressure-yielding support structure and design method using a sprayed polyurethane buffer layer. The multi-layer pressure-yielding support structure and design method using a sprayed polyurethane buffer layer adopts a multi-layer pressure-yielding support deformation calculation process and method of a sprayed polyurethane buffer layer. The input parameters are simple and the structure can be designed without relying on monitoring and test data, which is convenient for dynamic design adjustment in actual construction.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] A multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer comprises a toughened double-layer initial support, a polyurethane buffer layer and a secondary lining which are arranged in sequence from the outside to the inside.

[0009] The toughened double-layer initial support includes a first layer of toughened initial support and a second layer of toughened initial support which are arranged sequentially from the outside to the inside.

[0010] The spray-coated polyurethane buffer layer is a spray-coated polyurethane material.

[0011] The pre-densification strain of the sprayed polyurethane material is not less than 50%, and the strength of the sprayed polyurethane material is 0.2-3MPa.

[0012] The toughened double-layer primary support adopts steel-polypropylene hybrid fiber concrete.

[0013] A design method for a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer comprises the following steps.

[0014] Step 1: Apply the first layer of toughening initial support and record the start time t1.

[0015] Step 2: Calculation is the radial displacement of the cave wall after the first layer of toughening initial support is applied, which is a function of time t.

[0016] Step 3, calculate t2: t2 is the start time of the second layer of toughening initial support, that is, The time to reach the set elastic deformation limit threshold.

[0017] Step 4: Apply the second layer of toughened primary support.

[0018] Step 5: Calculation is the radial displacement of the cave wall after the second layer of toughening initial support is applied, which is a function of time t.

[0019] Step 6, calculate t3: t3 is the start time of the spray-coated polyurethane buffer layer, that is, The time when the set elastic deformation limit threshold value 2 is reached.

[0020] Step 7: Spray the polyurethane buffer layer, and after the polyurethane buffer layer hardens, apply the secondary lining.

[0021] Step 8: Calculate the total contact pressure p3(t) of the lining structure and the total radial displacement of the tunnel wall at any time after the secondary lining is completed.

[0022] In step 2, The calculation formula is:

[0023]

[0024] in:

[0025]

[0026] Where p0 is the initial ground stress of surrounding rock; R0 is the tunnel excavation radius.

[0027] J(t) is the creep compliance at time t; J(t1) is the creep compliance at time t1.

[0028] K1 is the support stiffness of the first layer of toughened primary support.

[0029] G M and G K They are the shear moduli in the Maxwell model and the Kelvin model, respectively, and both are known quantities.

[0030] η M , η K They are the Newtonian viscosity coefficients in the Maxwell model and the Kelvin model, respectively, and are both known quantities.

[0031] In step 5, The calculation method includes the following steps:

[0032] Step 5-1. Calculation is the radial displacement of the cave wall at time t2, and the specific calculation formula is:

[0033]

[0034] in:

[0035]

[0036] Step 5-2, calculate p1(t2): p1(t2) is the lining pressure at time t2, and the specific calculation formula is:

[0037]

[0038] Step 5-3, Calculation The specific calculation formula is:

[0039]

[0040] Where R1 is the inner diameter of the first layer of toughened primary support; K2 is the support stiffness of the second layer of toughened primary support.

[0041] In step 5, the calculation formulas for the support stiffness K1 of the first layer of toughened primary support and the support stiffness K2 of the second layer of toughened primary support are:

[0042]

[0043] Where E1 and υ1 are the elastic modulus and Poisson’s ratio of the first toughened support layer, respectively.

[0044] E2 and υ2 are the elastic modulus and Poisson's ratio of the second layer toughened primary support, respectively.

[0045] R2 is the inner diameter of the second layer of toughened primary support.

[0046] In step 7, the spraying method of the polyurethane buffer layer comprises the following steps:

[0047] Step 7-1, cleaning the inner surface of the second layer of toughened primary support: Clean the inner surface of the second layer of toughened primary support to increase the bonding force between the polyurethane buffer layer and the second layer of toughened primary support.

[0048] Step 7-2, temperature and humidity control of the second layer of toughened primary support: test the temperature and humidity of the second layer of toughened primary support, and make the moisture content of the second layer of toughened primary support less than 8%, and control the temperature at 10-35°C.

[0049] Step 7-3, layered spraying of polyurethane: spray the debugged polyurethane evenly on the inner surface of the second toughened primary support layer in layers until the set spraying thickness is reached; the first spraying layer is the primer spraying layer with a thickness of h0; the thickness of each subsequent spraying layer is equal, which is h1, then h0

[0050] Step 7-4, surface smoothing: clean and smooth the sprayed polyurethane surface to form a polyurethane buffer layer.

[0051] In step 8, The calculation method includes the following steps:

[0052] Step 8-1. Calculation The specific calculation formula is:

[0053]

[0054] in:

[0055]

[0056] Step 8-2, calculate p2(t3): p2(t3) is the lining pressure at time t3, and the specific calculation formula is:

[0057]

[0058] Step 8-3, Calculation The specific calculation formula is:

[0059] ​

[0060] Where K4 is the support stiffness of the secondary lining; R3 is the outer diameter of the secondary lining; E3 is the elastic modulus of the polyurethane buffer layer.

[0061] In step 8, the calculation formula for the secondary lining support stiffness K4 is:

[0062]

[0063] Where E4 and υ4 are the elastic modulus and Poisson's ratio of the secondary lining respectively; R4 is the inner diameter of the secondary lining.

[0064] The present invention has the following beneficial effects:

[0065] 1. Existing polymer foam buffer layers mostly adopt prefabricated structures and are manually laid, with complex construction procedures and low efficiency. In contrast, the present invention proposes to use spray-coated polyurethane as a buffer layer, which has the advantages of adjustable density, high compression strain, moderate strength and rapid spray forming, and is convenient for practical engineering promotion and application.

[0066] 2. The traditional yielding support is only a single-layer primary support, and conventional shotcrete is used. The ultimate bearing capacity and early crack toughness of the structure are insufficient. Taking into account the convenience of actual engineering, the present invention proposes to use high-toughness fiber shotcrete and sprayed polyurethane as toughened primary support and buffer layer respectively, to construct a multi-layer yielding support structure consisting of a double-layer toughened primary support-buffer layer-secondary lining. It has the advantages of high overall ultimate bearing capacity and strong early crack toughness, and can meet the long-term disaster prevention needs of strong extrusion and large deformation tunnels.

[0067] 3. Compared with the existing structural design method that relies on preliminary tunnel monitoring and indoor tests, the present invention takes into account the timeliness of the successive application of multi-layer compression support, and proposes a multi-layer compression support deformation calculation process and method using a sprayed polyurethane buffer layer. The input parameters are simple, and the structure can be designed without relying on monitoring and test data, which is convenient for dynamic design adjustment in actual construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 A schematic structural diagram of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer is shown in the present invention.

[0069] Figure 2 The interaction mechanical model of the multi-layer yielding support structure in the present invention is shown; wherein, (a) is the stress analysis diagram of the surrounding rock; (b) is the stress analysis diagram of the first layer of toughened primary support; (c) is the stress analysis diagram of the second layer of toughened primary support; (d) is the stress analysis diagram of the polyurethane buffer layer; and (e) is the stress analysis diagram of the secondary lining.

[0070] Among them are: 10. The first layer of toughened primary support; 20. The second layer of toughened primary support; 30. Polyurethane buffer layer; 40. Secondary lining.

[0071] Figure 3 Schematic diagram showing the comparison between theoretical and numerical solutions for tunnel displacement and support pressure. DETAILED DESCRIPTION

[0072] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

[0073] In the description of the present invention, it should be understood that the terms "left side", "right side", "upper part", "lower part" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second" and the like do not indicate the importance of the components, and therefore cannot be understood as limiting the present invention. The specific dimensions used in this embodiment are only for illustrating the technical solution, and do not limit the scope of protection of the present invention.

[0074] In this embodiment, a multi-layer yielding support is used to deeply bury a circular multi-layer lining tunnel under the action of hydrostatic pressure.

[0075] like Figure 1 As shown, a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer includes a toughened double-layer initial support, a polyurethane buffer layer 30 and a secondary lining 40 (also called a second lining) arranged in sequence from the outside to the inside.

[0076] The toughened double-layer initial support includes a first layer of toughened initial support 10 (also called a first layer of toughened initial support) and a second layer of toughened initial support 20 (also called a second layer of toughened initial support) arranged sequentially from the outside to the inside.

[0077] The spray-coated polyurethane buffer layer is a spray-coated polyurethane material, and its pre-densification strain is not less than 50%. The strength of the spray-coated polyurethane material is preferably 0.2-3MPa, so it has strong deformation ability, high redundancy, wide strength range and wide adaptability.

[0078] The above-mentioned toughened double-layer initial support preferably adopts steel-polypropylene hybrid fiber concrete.

[0079] A design method for a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer comprises the following steps.

[0080] Step 1: Apply the first layer of toughening initial support and record the start time t1.

[0081] Step 2: Calculation Preferably, the method comprises the following steps.

[0082] Step 2-1: According to the elastic-plastic theory, when there is no lining effect, the stress field of the circular tunnel satisfies:

[0083]

[0084] In the formula, σ r and σ θ are radial and tangential stresses respectively; p0 represents the initial geostress of the tunnel; R0 and r represent the tunnel excavation radius and the distance from any point in the surrounding rock to the tunnel center respectively.

[0085] Step 2-2: Under the conditions of zero plane strain and volumetric strain, the axial stress σ z and hoop stress σ θ , radial stress σ r There are the following relationships between them:

[0086]

[0087] Therefore, the mean stress σ mean for:

[0088]

[0089] According to the above formulas (2-1a), (2-1b) and (2-1d), the radial stress of the surrounding rock can be obtained: for:

[0090]

[0091] Step 2-3: Radial stress σ of surrounding rock r and radial strain σ r Satisfies the following relationship:

[0092]

[0093] Among them, G * (t) is the creep modulus of the surrounding rock, satisfying:

[0094] That is:

[0095]

[0096] Where J(t) is the creep flexibility of the surrounding rock at time t; J(t1) is the creep flexibility of the surrounding rock at time t1.

[0097] G M and G K They are the shear moduli in the Maxwell model and the Kelvin model, respectively, and both are known quantities.

[0098] ηM , η K They are the Newtonian viscosity coefficients in the Maxwell model and the Kelvin model, respectively, and are both known quantities.

[0099] Step 2-4: Substitute formula (2-1e) into formula (2-1f) to obtain:

[0100]

[0101] Step 2-5: By integrating the radial stress of the surrounding rock, the radial displacement of the surrounding rock (u r (t)) is:

[0102]

[0103] Step 2-6: When r = R0, the radial displacement of the tunnel wall can be obtained: The expression is:

[0104]

[0105] Step 2-7: For a structure that is only in the initial support stage, its mechanical model can be regarded as an axisymmetric ring that bears a uniform load on the outer surface. According to elastic mechanics, the support pressure p1(t) of a layer of toughened initial support is proportional to its displacement Satisfies the following relationship:

[0106]

[0107] The above K1 is the support stiffness of one layer of toughened primary support, and the specific calculation formula is:

[0108]

[0109] Where E1 and υ1 are the elastic modulus and Poisson’s ratio of the first toughened support layer, respectively.

[0110] Step 2-8: Due to the resistance of the support to the surrounding rock, after the first layer of toughened support is applied, the radial deviator stress of the surrounding rock becomes:

[0111]

[0112] Similarly, the radial displacement of the tunnel wall after the first layer of toughened support is applied can be obtained. for:

[0113]

[0114] Step 2-9: Combine formula (2-7) and formula (2-8) to obtain the radial displacement of the cave wall after the first layer of toughening initial support is applied: The specific calculation formula is preferably:

[0115]

[0116] Step 3, calculate t2: t2 is the start time of the second layer of toughening initial support, that is, The time to reach the set elastic deformation limit threshold.

[0117] Step 4: Apply the second layer of toughened primary support.

[0118] Step 5: Calculate the radial displacement of the tunnel wall after the second layer of toughening primary support is applied The specific steps include:

[0119] Step 5-1. Calculation is the radial displacement of the cave wall at time t2, and the specific calculation formula is:

[0120]

[0121] in:

[0122]

[0123] Step 5-2, calculate p1(t2): p1(t2) is the lining pressure at time t2, which is obtained by combining formula (2-7) and formula (2-8). The specific calculation formula is:

[0124]

[0125] Step 5-3, Calculation The specific calculation formula is:

[0126]

[0127] Where R1 is the inner diameter of the first layer of toughening support.

[0128] K2 is the support stiffness of the second layer of toughened primary support, and the calculation formula is:

[0129]

[0130] Where E2 and υ2 are the elastic modulus and Poisson’s ratio of the second layer toughened primary support, respectively.

[0131] R2 is the inner diameter of the second layer of toughened primary support.

[0132] Step 6, calculate t3: t3 is the start time of the spray-coated polyurethane buffer layer, that is, The time when the set elastic deformation limit threshold value 2 is reached.

[0133] Step 7: Spray the polyurethane buffer layer, and after the polyurethane buffer layer hardens, apply the secondary lining.

[0134] The spraying method of the polyurethane buffer layer preferably comprises the following steps:

[0135] Step 7-1, cleaning the inner surface of the second layer of toughened primary support: Clean the inner surface of the second layer of toughened primary support, such as using a high-pressure water gun, a hair dryer or other measures to remove dust and sand on the surface of the base layer to increase the bonding strength between the polyurethane buffer layer and the second layer of toughened primary support.

[0136] Step 7-2, temperature and humidity control of the second layer of toughened primary support: test the temperature and humidity of the second layer of toughened primary support, and make the moisture content of the second layer of toughened primary support less than 8%, and control the temperature at 10-35°C.

[0137] Step 7-3: Polyurethane layered spraying

[0138] Polyurethane spray debugging: According to the spraying process, each system and compressed air in the polymer rigid foam spraying equipment system shall be correctly installed and connected on site in sequence from the raw material storage, transportation and processing system, the raw material delivery system, the metering pumping and heating system, the feeding pipe system and the spray gun. After checking and confirming again, turn on the power to check the no-load status of the equipment, ensure that all spraying equipment systems can operate normally, and then conduct trial spraying. When the effect of the trial spraying meets the requirements, formal spraying can be carried out.

[0139] The debugged polyurethane is sprayed evenly on the inner surface of the second toughened primary support in layers until the set spraying thickness is reached; the first spraying layer is the primer spraying layer with a thickness of h0; the thickness of each subsequent spraying layer is equal, which is h1, then h0

[0140] During construction, the density and thickness of the rigid foam polymer buffer layer should be sampled and tested in time. In this embodiment, the density of the sprayed polyurethane buffer layer is preferably 100kg / m 3 .

[0141] Step 7-4, surface smoothing: Clean and smooth the surface of the sprayed polyurethane, such as the obstruction and thickness protruding areas, to form a polyurethane buffer layer.

[0142] Step 8: Calculate the total contact pressure p3(t) of the lining structure and the total radial displacement of the tunnel wall at any time after the secondary lining is completed.

[0143] Above ​The calculation method preferably includes the following steps.

[0144] Step 8-1. Calculation The specific calculation formula is:

[0145]

[0146] in:

[0147]

[0148] Step 8-2, calculate p2(t3): p2(t3) is the lining pressure at time t3, and the specific calculation formula is:

[0149]

[0150] Step 8-3, Calculation The specific calculation formula is:

[0151]

[0152] Where K4 is the support stiffness of the secondary lining; R3 is the outer diameter of the secondary lining; E3 is the elastic modulus of the polyurethane buffer layer.

[0153] The calculation formula for the secondary lining support stiffness K4 is preferably:

[0154]

[0155] Where E4 and υ4 are the elastic modulus and Poisson's ratio of the secondary lining respectively; R4 is the inner diameter of the secondary lining.

[0156] Step 9: According to the engineering design and geological survey parameters, combined with the above steps, determine the reasonable support timing and structural stiffness of multi-layer yielding support.

[0157] Based on the cross-section of Gaoligongshan Tunnel used in the previous study, the proposed time-effect solution is compared with the PD numerical simulation calculation results. The installation time of the first layer of toughened primary support, the second lining toughened primary support, the polyurethane buffer layer and the second lining are: t1 = 0, t2 = 7 days, t3 = 30 days. The supports are all linear elastic constitutive, and their damage is not considered. The density of the sprayed polyurethane buffer layer is 100kg / m 3 , Referring to the existing literature, the surrounding rock and support calculation parameters are determined as shown in Table 1.

[0158] Table 1

[0159]

[0160] The results of theoretical and numerical solutions of tunnel displacement and support pressure, such as Figure 3As shown. It can be seen that when the maximum calculation time is 10 years, the theoretical solution and numerical simulation of the radial displacement of the tunnel wall, surrounding rock pressure, radial displacement of the secondary lining and support pressure are consistent. Both show the creep deformation characteristics of soft rock, and the theoretical solution better reflects the initial creep characteristics of soft rock at t = 0 (the numerical solution is time-integrated, so the deformation starts from 0). In addition, it can be seen that the stress and deformation of the secondary lining are significantly smaller than the surrounding rock pressure and extrusion deformation, indicating that the presence of the toughened primary support and the buffer layer effectively coordinates and absorbs the surrounding rock deformation, keeping the stress and deformation of the secondary lining at a low level.

[0161] In conclusion, this example verifies the accuracy of the derived failure solution for the interaction between surrounding rock and multi-layer toughened support.

[0162] The preferred embodiments of the present invention are described in detail above; however, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all belong to the protection scope of the present invention.

Claims

1. A multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer, characterized in that: It includes a toughened double-layer initial support, a polyurethane buffer layer and a secondary lining arranged in sequence from the outside to the inside; The toughened double-layer initial support includes a first layer of toughened initial support and a second layer of toughened initial support arranged from the outside to the inside; The spray-coated polyurethane buffer layer is a spray-coated polyurethane material.

2. The multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 1 is characterized in that: The pre-densification strain of the sprayed polyurethane material is not less than 50%, and the strength of the sprayed polyurethane material is 0.2-3MPa.

3. The multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 1 is characterized in that: The toughened double-layer primary support adopts steel-polypropylene hybrid fiber concrete.

4. A design method for a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer, characterized in that: The steps include: Step 1, apply the first layer of toughening initial support and record the start time t1; Step 2: Calculation is the radial displacement of the cave wall after the first layer of toughening is applied, which is a function of time t; Step 3, calculate t2: t2 is the start time of the second layer of toughening initial support, that is, The time to reach the set elastic deformation limit threshold; Step 4, applying the second layer of toughened primary support; Step 5: Calculation is the radial displacement of the cave wall after the second layer of toughening primary support is applied, which is a function of time t; Step 6, calculate t3: t3 is the start time of the spray-coated polyurethane buffer layer, that is, The time when the set elastic deformation limit threshold value 2 is reached; Step 7, spraying the polyurethane buffer layer, and applying the secondary lining after the polyurethane buffer layer hardens; Step 8: Calculate the total contact pressure p3(t) of the lining structure and the total radial displacement of the tunnel wall at any time after the secondary lining is completed.

5. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 4 is characterized in that: In step 2, The calculation formula is: in: Where, p0 is the initial ground stress of surrounding rock; R0 is the tunnel excavation radius; f(t) is the creep compliance at time t; J(t1) is the creep compliance at time t1; K1 is the support stiffness of the first layer of toughened primary support; G M and G K They are the shear moduli in the Maxwell model and the Kelvin model, respectively, both of which are known quantities; η M , η K They are the Newtonian viscosity coefficients in the Maxwell model and the Kelvin model, respectively, and are both known quantities.

6. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 5 is characterized in that: In step 5, The calculation method includes the following steps: Step 5-1. Calculation is the radial displacement of the cave wall at time t2, and the specific calculation formula is: in: Step 5-2, calculate p1(t2): p1(t2) is the lining pressure at time t2, and the specific calculation formula is: Step 5-3, Calculation The specific calculation formula is: Where R1 is the inner diameter of the first layer of toughened primary support; K2 is the support stiffness of the second layer of toughened primary support.

7. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 6, characterized in that: In step 5, the calculation formulas for the support stiffness K1 of the first layer of toughened primary support and the support stiffness K2 of the second layer of toughened primary support are: In the formula, E1 and v1 are the elastic modulus and Poisson’s ratio of the first layer of toughening, respectively; E2 and υ2 are the elastic modulus and Poisson’s ratio of the second layer toughened primary support, respectively; R2 is the inner diameter of the second layer of toughened primary support.

8. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 4, characterized in that: In step 7, the spraying method of the polyurethane buffer layer comprises the following steps: Step 7-1, cleaning the inner surface of the second layer of toughened primary support: cleaning the inner surface of the second layer of toughened primary support to increase the bonding force between the polyurethane buffer layer and the second layer of toughened primary support; Step 7-2, temperature and humidity control of the second layer of toughened primary support: test the temperature and humidity of the second layer of toughened primary support, and make the moisture content of the second layer of toughened primary support less than 8%, and control the temperature at 10-35°C; Step 7-3, polyurethane layer spraying: spray the debugged polyurethane evenly on the inner surface of the second toughened primary support layer in layers until the set spraying thickness is reached; the first spraying layer is the primer spraying layer with a thickness of h0; the thickness of each subsequent spraying layer is equal, which is h1, then h0<h1≤2cm; Step 7-4, surface smoothing: clean and smooth the sprayed polyurethane surface to form a polyurethane buffer layer.

9. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 6, characterized in that: In step 8, The calculation method includes the following steps: Step 8-1. Calculation The specific calculation formula is: in: Step 8-2, calculate p2(t3): p2(t3) is the lining pressure at time t3, and the specific calculation formula is: Step 8-3, Calculation The specific calculation formula is: Where K4 is the support stiffness of the secondary lining; R3 is the outer diameter of the secondary lining; E3 is the elastic modulus of the polyurethane buffer layer.

10. The design method of a multi-layer pressure-yielding support structure using a spray-coated polyurethane buffer layer according to claim 9, characterized in that: In step 8, the calculation formula for the secondary lining support stiffness K4 is: Where E4 and υ4 are the elastic modulus and Poisson's ratio of the secondary lining respectively; R4 is the inner diameter of the secondary lining.

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