Double-layer combined steel arch structure of large deformation tunnel, design method and construction method

Abstract

The present application relates to a double-layer combined steel arch structure of a large deformation tunnel, a design method and a construction method. The combined steel arch structure, the first layer steel arch adopts a profile steel structure, which can ensure the steel arch and the surrounding rock support area, and control the initial deformation of the surrounding rock; the second layer steel arch adopts a steel pipe concrete arch, which fully utilizes the three-way compression characteristics of the steel pipe concrete to improve the rigidity and bearing capacity of the initial support. The steel pipe is provided with an expansion type hose, so that the steel pipe and the fine stone mortar concrete can be closely attached and have high completeness. The elastic device is used to connect the two layers of steel arches, which can realize the connection of the two layers of steel arches as a whole, allow the first layer steel arch to deform and buffer to a certain extent, release the surrounding rock pressure, slow down the support pressure of the second layer steel arch, and let the anti-combination fully play the self-stability of the surrounding rock to ensure the overall stability of the initial support; when the deformation is too large, the second layer steel pipe concrete arch can quickly exert its bearing capacity to ensure the stability of the surrounding rock.

Description

Technical Field

[0001] This invention relates to the field of double-layer initial support technology for large deformation soft rock, and in particular to the double-layer combined steel arch frame structure, design method and construction method of large deformation tunnel. Background Technology

[0002] During tunnel construction, it is inevitable to encounter a large amount of soft rock strata, characterized by low rock strength, fractured rock mass, and poor surrounding environment. Simultaneously, with the continuous expansion of my country's construction scale, the construction of three-lane and four-lane tunnels is becoming more widespread, leading to a continuous increase in tunnel excavation cross-sections. Some ultra-large cross-section tunnels traverse soft rock strata, requiring high initial support strength, posing significant challenges to the design and construction of tunnel support structures. Domestic and international scholars are actively exploring new initial support systems to meet the load-bearing capacity requirements of support structures for ultra-large cross-section tunnels under soft rock strata conditions; steel-concrete composite tunnels offer advantages such as high load-bearing capacity, good plasticity and toughness, and good economic benefits. Currently, the initial support system for ultra-large cross-section tunnels under conditions of large deformation in soft rock faces the following problems:

[0003] 1. The support structure has high load-bearing capacity requirements. The existing double-layer initial support cannot meet the corresponding requirements, often resulting in large deformation and severe twisting of the steel arch frame structure. Therefore, it is necessary to seek a steel arch frame structure with superior load-bearing capacity.

[0004] 2. Under the condition of a large tunnel cross section, the construction of the second layer of initial support is difficult and the control of the support time is challenging.

[0005] 3. Under large deformation of soft rock, the initial support of the tunnel needs to be implemented in a timely manner. However, the current construction procedures for the initial support of the tunnel are relatively complicated, and the timing of the support is greatly affected by the construction capacity of the construction unit. It is necessary to provide a rapid construction method for large deformation of soft rock to ensure the timeliness of the support.

[0006] 4. Currently, steel-concrete composite structures are widely used in port engineering, bridges, and high-rise buildings, and their structural calculations are relatively mature. However, in the field of tunnels, their structural calculations lack standardized support, especially for this type of composite support structure, where there is no relevant technical support. Summary of the Invention

[0007] The purpose of this invention is to address the problem that existing double-layer initial support technologies cannot meet the corresponding bearing capacity requirements and thus cannot satisfy the stability of initial support for large deformation soft rock, and to provide a double-layer combined steel arch frame structure, design method, and construction method for large deformation tunnels.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A double-layer composite steel arch frame structure for a large deformation tunnel includes a steel arch frame, a steel-concrete composite arch frame, and elastic connecting devices. The steel arch frame is located outside the steel-concrete composite arch frame, and the steel arch frame and the steel-concrete composite arch frame are connected by several elastic connecting devices. The steel-concrete composite arch includes a steel pipe, an expandable hose, and fine aggregate mortar concrete. The expandable hose is sleeved inside the steel pipe, and the fine aggregate mortar concrete fills the expandable hose and squeezes the expandable hose against the inner wall of the steel pipe.

[0009] The double-layer composite steel arch frame structure for large deformation tunnels described in this invention comprises two layers. The first layer uses a steel profile structure to ensure the supporting area between the steel arch frame and the surrounding rock, thus controlling the deformation of the surrounding rock. The second layer uses a steel-concrete composite arch frame, which possesses advantages such as high load-bearing capacity, good plasticity and toughness, and good economic benefits. Applying steel-concrete composite to the initial support of tunnels can fully utilize the triaxial compression characteristics of steel-concrete composite, improving the stiffness and load-bearing capacity of the initial support. The steel pipe is a hollow circular steel structure with an internal expansion hose. During construction, after erection, fine aggregate mortar concrete is poured in, and the internal expansion hose expands outward to fit the inner wall of the steel pipe, allowing for a tight fit between the steel pipe and the fine aggregate mortar concrete with high integrity. Furthermore, the steel-concrete composite arch frame is lighter and stronger than the steel profile arch frame. Meanwhile, depending on the geological conditions, the interior of the steel pipe can be filled with fine aggregate mortar concrete, which has a higher strength than shotcrete, to maintain the stability of the initial support. The two layers of steel arch frames are connected by an elastic device, thus connecting the two layers of steel arch frames into a whole. At the same time, it can provide a reserved deformation amount for the first layer of steel arch frames, allowing the first layer of steel arch frames to undergo a certain deformation, releasing the pressure of the surrounding rock and allowing the anti-bonding. Under large deformation of soft rock, the anti-bonding method is conducive to resisting the deformation of the surrounding rock while giving full play to the self-stabilizing ability of the surrounding rock and ensuring the stability of the initial support. When the deformation is too large, the second layer of steel pipe concrete arch frame can quickly exert its load-bearing capacity, and a second layer of shotcrete can be sprayed to ensure the stability of the surrounding rock.

[0010] Preferably, the elastic connection device includes a high-performance spring and a round steel connector. One end of the high-performance spring is connected to the steel arch frame, and the other end is connected to the round steel connector. The round steel connector has an arc surface adapted to connect the steel pipe concrete arch frame.

[0011] The round steel connector can guarantee the welding area with the steel pipe concrete arch frame, and the high-performance spring can provide the reserved deformation of the first layer of steel arch frame and the longitudinal displacement of the two layers of arch frames to release the surrounding rock pressure; at the same time, when the surrounding rock pressure is too high, the first and second layers of arch frames can quickly form a load-bearing body to ensure the stability of the surrounding rock and the initial support.

[0012] Preferably, the steel arch frame and the steel-concrete composite arch frame are each divided into several segments along their arch direction. Adjacent segments of the steel arch frame are connected by steel connectors, and adjacent segments of the steel-concrete composite arch frame are connected by round steel connectors.

[0013] The steel arch frame and the steel-concrete composite arch frame are each divided into several segments along their arch direction, facilitating processing and installation. Each step of the steel-concrete composite arch frame can be connected into a single unit via expansion hoses, effectively reducing the detrimental effect of segmental connections on the steel-concrete composite arch frame.

[0014] Preferably, both the profile steel connector and the round steel connector are quick-connect devices. The quick-connect device includes an insertion end and a fixing end, which are correspondingly arranged. Adjacent ends of the insertion end and the fixing end are connected to connecting steel plates. The end of the insertion end away from the fixing end is provided with a compression spring device. The front end of the insertion end is also kept in a compressed state by the fixing device. After the fixing device releases the compression of the spring device, the spring device springs the front end device to lock with the fixing end.

[0015] The quick-connection device consists of two steel plates: one welded to the insertion end and the other welded to the fixing end. The two plates are positioned by a device at the front of the insertion end, and then the fixing device releases the pressure on the spring device. The spring device at the rear of the insertion end pushes the front end device to quickly anchor to the fixing end. This rapid connection effectively avoids the difficulties of connecting segments during the simultaneous construction of double-layer steel arches. The quick-connection device is easy to operate and can further improve construction efficiency.

[0016] A design method for a double-layer composite steel arch structure for a large deformation tunnel, comprising the following design steps: S1. Calculation and determination of the sharing ratio of the surrounding rock pressure: Calculate the surrounding rock pressure of the tunnel based on the tunnel outline, burial depth, topography and geological conditions, and determine the load sharing ratio. S2. Determination of initial support structure parameters for composite steel arch frame: Through data research and engineering comparison, determine the steel type of the double-layer composite steel arch frame structure for large deformation tunnels, the diameter and wall thickness of the steel pipes of the steel pipe concrete arch frame, the spacing of the steel arch frames, the thickness of the first and second layer initial support and the concrete grade, and the reserved deformation amount. S3. Based on the load sharing ratio, surrounding rock pressure, and initial support structure parameters of the composite steel arch frame, calculate the bending moment and axial force of the first initial support before the second layer of shotcrete and the force borne by the elastic connection device, as well as the bending moment and axial force borne by the initial support structure of the composite steel arch frame after the second layer of shotcrete, using the load structure method; then determine the stiffness of the high-performance spring of the elastic connection device. S4. Safety factor calculation of the first layer of initial support: The safety factor of the first layer of initial support formed by the combined load-bearing structure of the first layer of shotcrete and steel arch frame is calculated. S5. Determine whether the initial support of the first layer meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. S6. Determine the standard and design values ​​of the compressive strength and tensile strength of the steel-concrete composite arch frame; Then, the safety factor of the steel-concrete arch frame before the second layer of shotcrete is verified. S7. Determine whether the steel-concrete arch frame before the second layer of shotcrete meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. S8. After the second layer of shotcrete, the first layer of shotcrete, the steel arch frame, the elastic connection device, the steel pipe concrete arch frame and the second layer of shotcrete are used as the initial support structure of the composite steel arch frame for safety factor verification. S9. Determine whether the initial support structure of the combined steel arch frame meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. If it meets the safety factor requirements, determine the parameters of the initial support structure of the combined steel arch frame.

[0017] The design method for the double-layer combined steel arch frame structure of the large deformation tunnel described in this invention adopts the calculation method of the combined initial support structure. It can scientifically select relevant parameters of the combined initial support structure and perform accurate calculations to ensure the structural safety of the double-layer combined steel arch frame structure as the initial support steel arch frame of the large deformation tunnel during construction and after construction and operation.

[0018] Preferably, in step S3, the formula for determining the stiffness coefficient of the high-performance spring of the elastic connection device is as follows:

[0019] Where: k—stiffness coefficient of high-performance spring; F max —The maximum force that the high-performance spring can withstand before the second layer of shotcrete; — Deformation allowance reserved between the first and second initial support layers; In step S6, the formula for determining the standard value and design value of the compressive strength of the steel-concrete composite arch is as follows: ; ; ; ; ; ; ; ; ; In the formula: , —Standard and design values ​​of compressive strength for steel-concrete composite arch frames; θ — Hoop index; A s A c —Cross-sectional area of ​​steel pipe and fine aggregate mortar concrete; f sk f s —Standard and design values ​​of yield strength for steel pipes; f ck f c —Standard and design values ​​of axial compressive strength of fine aggregate mortar concrete; B, C —Influence coefficients of hoop index, B=0.000826*fs+1.01, C=-0.00722*fc+0.12; k1 —Influence coefficient of slenderness ratio of steel-concrete composite arch frame; L e — Segment length of the steel-concrete composite arch frame; D — Outer diameter of the steel-concrete composite arch frame; k2 — Reliability correction factor; α sc —Steel content of steel-concrete composite arch frame; k e —Eccentricity influence coefficient; e0—Eccentricity, calculated before and after the second layer of shotcrete; r c —The radius of the cross-section of fine aggregate mortar concrete; The formulas for determining the standard and design values ​​of the tensile strength of steel-concrete composite arch frames are as follows: ; In the formula: —Standard value of tensile strength for steel-concrete composite arch frames; C1—The coefficient for increasing the tensile strength of the steel pipe, taken as 1.1; A s —Cross-sectional area of ​​the steel pipe; A sc —The cross-sectional area of ​​the steel-concrete composite arch frame; In step S6, the steel-concrete composite arch frame mainly bears the reaction force of the elastic connection device, and its structural safety factor is calculated as follows: ; Where: K—safety factor, taken from the standard; S—Spacing between steel-concrete composite arch frames; R—Calculation radius of the steel-concrete composite arch frame; p——The steel-concrete composite arch frame bears the surrounding rock pressure. Considering the transmission effect of the first layer of initial support, p=0.4p1, where p1 is the surrounding rock pressure borne by the surrounding rock and the first layer of initial support. Nu —Ultimate bearing capacity of steel-concrete composite arch members under compression and bending, N u =f sc A sc ; In step S8, the reinforcing effect of the combined steel arch frame on the shotcrete is simulated by means of equivalent reinforcement. The first layer of steel arch frame is regarded as the outer reinforcement, and the second layer of steel pipe concrete arch frame is regarded as the inner reinforcement. The safety factor calculation formula for the initial support structure of the combined steel arch frame is as follows: ①When , ; ②When , ; ③When , ; In the formula, K is the safety factor, which is taken according to the specification; N is the axial force. —Ultimate flexural compressive strength of concrete; —The standard value of tensile strength of the steel reinforcement in the tension zone is calculated based on the internal forces of the structure to determine whether it is under tension on the inside or the outside. If it is under tension on the inside, the cross-sectional area of ​​the steel reinforcement in the tension zone is taken as the area of ​​the steel-concrete composite arch frame, and the cross-sectional area of ​​the steel reinforcement in the compression zone is taken as the area of ​​the first layer of steel arch frame, otherwise the opposite is true. —Standard value of compressive strength of steel reinforcement in the compression zone; , —Cross-sectional area of ​​the reinforcement in the tension and compression zones; a, a' —Distance from the center of the reinforcement in the tension and compression zones to the nearest edge of the section; h —Initial support height of the composite steel arch; h0 —Effective height of the section. =ha; x——height of the concrete compression zone; b——width of the rectangular section, taken as the longitudinal spacing between adjacent combined steel arch frames; e——axial force eccentricity.

[0020] The safety factor of the steel-concrete arch frame before the second layer of shotcrete and the safety factor of the initial support structure of the composite steel arch frame were verified using the above method. The results are accurate and can ensure the safety of construction process and operation after construction.

[0021] Preferably, when performing safety factor verification on the initial support structure of the composite steel arch frame, it is also necessary to multiply the axial force eccentricity e by the eccentricity amplification factor. Eccentricity amplification factor The calculation formula is as follows: ； ； In the formula, —Eccentricity amplification factor; K —Safety factor; —Compressive elastic modulus of shotcrete; —Moment of inertia of the shotcrete section; —The compressive modulus of elasticity of the first layer of steel arch frame; —Moment of inertia of the first layer of steel arch frame section; k —Stiffness coefficient of high-performance spring; —The compressive elastic modulus of fine aggregate mortar concrete in steel-concrete composite arch frames; —Moment of inertia of the fine aggregate mortar concrete section of the steel-concrete composite arch frame; —The compressive modulus of elasticity of the steel tubes in a steel-concrete composite arch; —Moment of inertia of the steel pipe section in a steel-concrete composite arch frame; —Eccentricity coefficient; h —Initial support structure height of the composite steel arch frame.

[0022] Due to the presence of deflection, the surrounding rock pressure will cause the initial support structure of the composite steel arch frame to deflect. This deflection will increase the axial force eccentricity, generating additional force, i.e., the second-order effect of the eccentrically compressed structure. At the same time, the two layers of steel arch frames in the initial support structure of the composite steel arch frame are elastically connected, which has an inhibitory effect on deflection deformation. Therefore, by multiplying the axial force eccentricity by the eccentricity amplification factor, we can obtain a higher safety of the initial support structure of the composite steel arch frame.

[0023] Preferably, when performing safety factor verification of the combined initial support structure, the equivalent elastic modulus and unit weight of the combined initial support structure are first calculated; then, the bending moment M and axial force N of the initial support are calculated using the load structure method; and then the axial force eccentricity e is obtained through the bending moment M and axial force N of the initial support. The formulas for calculating the equivalent elastic modulus and unit weight of the combined initial support structure are as follows: ; ; In the formula, E is the equivalent elastic modulus, I is the equivalent moment of inertia relative to the centroid, γ is the equivalent unit weight, and A is the total area of ​​the initial support structure of the composite steel arch frame. , , , The elastic moduli of the shotcrete, the first layer of steel arch frame, the second layer of steel-concrete composite arch frame, and the second layer of steel pipe are respectively. , , , The moments of inertia are respectively those of shotcrete, the first layer of steel arch frame, the second layer of steel-concrete composite arch frame, and the second layer of steel pipe. , , , The weights of the following components are respectively: shotcrete, first-layer steel arch frame, second-layer steel-concrete composite arch frame, and second-layer steel pipe. , , , These represent the areas of the shotcrete, the first layer of steel arch frame, the second layer of steel-concrete composite arch frame, and the second layer of steel pipe, respectively.

[0024] Using the above method, the axial force eccentricity e can be obtained quickly and accurately.

[0025] Preferably, in step S1, the load sharing ratio is as shown in the table below: Surrounding rock level Surrounding rock + first layer of initial support Initial support structure of composite steel arch frame Secondary lining V 0.25 0.25 0.5 IV 0.4 0.4 0.2 .

[0026] By adopting the above load-sharing ratio, the double-layer composite steel arch structure of the tunnel designed for large deformation is made safer. The load-sharing ratio can be adjusted according to different engineering examples.

[0027] A construction method for a combined initial support structure, using the double-layer combined steel arch frame structure of the large deformation tunnel as the steel arch frame of the combined initial support structure, the construction method of the combined initial support structure starts from the upper bench for excavation and support, and the excavation of each bench of each segment includes the following steps: Step 1: Excavate the current step; Step 2: Assemble and install the steel pipes and expansion hoses of the outer steel arch frame, elastic connection device and inner steel pipe concrete arch frame corresponding to the current step; Step 3: Inject grout into the expansion hose to form a steel-concrete structure; Step 4: Spray the first layer of concrete; When the first layer of sprayed concrete deforms to the reserved deformation position between the steel pipe concrete arch frame and the steel arch frame, the corresponding second layer of concrete is sprayed.

[0028] The construction method of the combined initial support structure described in this invention is simple in construction process and safer in construction.

[0029] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. The double-layer combined steel arch frame structure for large deformation tunnels described in this invention comprises two layers. The first layer uses a steel profile structure to ensure the supporting area between the steel arch frame and the surrounding rock, controlling the initial deformation of the surrounding rock. The second layer uses a steel-concrete composite arch frame, which has advantages such as high load-bearing capacity, good plasticity and toughness, and good economic benefits. Applying steel-concrete composite to the initial support of tunnels can fully utilize the triaxial compression characteristics of steel-concrete composite, improving the stiffness and load-bearing capacity of the initial support. The steel pipe is a hollow circular steel structure with an internal expansion hose. During construction, after erection, fine aggregate mortar concrete is poured in, and the internal expansion hose expands outward to fit the inner wall of the steel pipe, allowing the steel pipe and the fine aggregate mortar concrete to achieve a tight fit with high integrity. Furthermore, the steel-concrete composite arch frame is lighter, stronger, and has consistent resistance in all directions compared to the steel profile arch frame. Meanwhile, depending on the geological conditions, the interior of the steel pipe can be filled with fine aggregate mortar concrete, which has a higher strength than shotcrete, to maintain the stability of the initial support. An elastic device connects the two layers of steel arches, enabling them to be connected as a whole while also allowing for some deformation of the first layer, thus buffering the pressure on the surrounding rock and reducing the support pressure on the second layer. This allows for better bonding between the support and the rock. Under large deformations in soft rock, this bonding method helps resist rock deformation while fully utilizing the rock's self-stabilizing capacity, ensuring the overall stability of the initial support. When the deformation is too large, the second layer of steel-concrete arches can quickly exert its load-bearing capacity, and a second layer of shotcrete can be applied to ensure the stability of the surrounding rock. With the same amount of steel used, this invention provides a support with greater load-bearing capacity and a safer, more reliable structure.

[0030] 2. The double-layer combined steel arch frame structure for large deformation tunnels described in this invention includes a high-performance spring and a round steel connector. The round steel connector can ensure the welding area with the steel pipe concrete arch frame. The high-performance spring can provide the reserved deformation amount of the first layer of steel arch frame and the longitudinal displacement of the two layers of arch frames, releasing the surrounding rock pressure. At the same time, when the surrounding rock pressure is too high, the first and second layers of arch frames can quickly form a load-bearing body to ensure the stability of the surrounding rock and the initial support.

[0031] 3. The double-layer combined steel arch frame structure for large deformation tunnels described in this invention employs a quick-connect device to connect the segments of the steel arch frame and the steel-concrete composite arch frame. One connecting steel plate of the quick-connect device has an inserted end welded to it, and the other connecting steel plate has a fixed end welded to it. The two connecting steel plates are positioned by a device at the front end of the inserted end, and then the spring device is released from pressure by a fixing device. The spring device at the tail end of the inserted end pushes the front end device to quickly anchor to the fixed end. This rapid connection effectively avoids problems such as the difficulty of connecting segments during simultaneous construction of double-layer steel arch frames. The quick-connect device is simple to operate, further improving construction efficiency and enhancing construction safety. Traditional double-layer support steel frame units have joints arranged in the same location, creating weak points. In this invention, the joints of the inner and outer steel frames are staggered, further improving the overall support's load-bearing capacity and stability.

[0032] 4. The design method of the double-layer combined steel arch frame structure for large deformation tunnels described in this invention adopts the calculation method of combined initial support structure, which can scientifically select relevant parameters of combined initial support structure and perform accurate calculations to ensure the structural safety of the double-layer combined steel arch frame structure as the initial support steel arch frame for large deformation tunnels during construction and after construction.

[0033] 5. The construction method of the combined initial support structure described in this invention has a simple construction process and is safer during construction. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the double-layer combined steel arch frame structure of the large deformation tunnel of the present invention.

[0035] Figure 2 This is a construction schematic diagram of the double-layer combined steel arch frame structure of the large deformation tunnel of the present invention; Figure 3 This is a schematic diagram of the arrangement of the double-layer combined steel arch frame structure of the large deformation tunnel of the present invention; Figure 4 This is a schematic diagram of the structure of the flexible connection device; Figure 5 This is a schematic diagram showing the state of the steel pipe before fine aggregate mortar concrete was poured inside. Figure 6 This is a schematic diagram showing the state of a steel pipe filled with fine aggregate mortar concrete, which is used to compress and expand the hose. Figure 7 This is a schematic diagram showing how fine aggregate mortar concrete is poured into a steel pipe to squeeze the expansion hose against the inner wall of the steel pipe. Figure 8 This is a structural diagram of a section steel connector and a round steel connector; Figure 9 This is a schematic diagram of a steel section connector connecting steel sections; Figure 10This is a schematic diagram of a round steel connector connecting round steel segments; Figure 11 This is a structural schematic diagram of the second connecting plate; Figure 12 This is a schematic diagram showing the state of the quick-connect device before connection; Figure 13 This is a schematic diagram showing the alignment of the quick-connect device; Figure 14 This is a schematic diagram showing the state after the quick-connect device is connected; Figure 15 This is a schematic diagram illustrating the design process of the double-layer combined steel arch frame structure for the large deformation tunnel of the present invention.

[0036] Icons: 1-Steel arch frame; 2-Steel pipe concrete arch frame; 21-Steel pipe; 22-Expansion hose; 23-Fine aggregate mortar concrete; 3-Elastic connection device; 31-High-performance spring; 32-Round steel connector; 33-Combined steel plate; 41-Connecting steel plate; 42-Spring device; 43-Response device; 44-Insertion end; 441-Front end device; 45-Fixed end; 46-Fixed device; 5-Steel connector; 51-First connecting plate; 52-First connecting bolt; 53-Unequal angle steel; 6-Round steel connector; 61-Second connecting plate; 62-Round steel through hole; 63-Second connecting bolt; 7-Pump; 71-Grouting pipe. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings.

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0039] Example 1 This embodiment provides a double-layer composite steel arch frame structure for a large deformation tunnel. See [link to relevant documentation]. Figures 1-3 It includes a steel arch frame 1, a steel pipe concrete arch frame 2 and elastic connection devices 3. The steel arch frame 1 is located outside the steel pipe concrete arch frame 2, and the steel arch frame 1 and the steel pipe concrete arch frame 2 are connected by a number of elastic connection devices 3. The steel-concrete composite arch frame 2 includes a steel pipe 21, an expansion hose 22, and fine aggregate mortar concrete 23. The expansion hose 22 is sleeved inside the steel pipe 21, and the fine aggregate mortar concrete 23 fills the expansion hose 22 and squeezes the expansion hose 22 against the inner wall of the steel pipe 21.

[0040] In this scheme, the steel arch frame 1 can be an I-beam or H-beam, etc., and its overall shape is an arch in the tunnel transverse direction. The steel arch frame 1 is located on the side closest to the surrounding rock, directly contacting the surrounding rock, serving as the first layer of initial support. The steel-concrete composite arch frame 2 uses a steel-concrete composite structure. The steel pipe 21 has an arch shape in the tunnel transverse direction, and the steel pipe 21 has an expansion hose 22 inside. When fine aggregate mortar concrete is pumped into the steel pipe 21, the fine aggregate mortar concrete will squeeze the expansion hose 22 to fit tightly against the inner wall of the steel pipe 21. Figures 5-6 .

[0041] The double-layer combined steel arch frame structure for the large deformation tunnel described in this scheme has two layers. The first layer uses a steel profile structure to ensure the support area between the steel arch frame and the surrounding rock, controlling the deformation of the surrounding rock. The second layer uses a steel-concrete composite arch frame 2. Steel-concrete composite has advantages such as high load-bearing capacity, good plasticity and toughness, and good economic benefits. Applying steel-concrete composite to the initial support of the tunnel can fully utilize the triaxial compression characteristics of steel-concrete composite, improving the stiffness and load-bearing capacity of the initial support. The steel pipe is a hollow circular steel structure with an internal expansion hose. During construction, after erection, fine aggregate mortar concrete is poured in. The internal expansion hose expands outward to fit the inner wall of the steel pipe, allowing for a tight fit between the steel pipe and the fine aggregate mortar concrete, resulting in high integrity. Furthermore, the steel-concrete composite arch frame is lighter and stronger than the steel profile arch frame. The expansion hose can be made of expandable materials.

[0042] Meanwhile, depending on the geological conditions, the interior of the steel pipe can be filled with fine aggregate mortar concrete, which has a higher strength than shotcrete, to maintain the stability of the initial support. The two layers of steel arch frames are connected by an elastic device, thus connecting the two layers of steel arch frames into a whole. At the same time, it can provide a reserved deformation amount for the first layer of steel arch frames, allowing the first layer of steel arch frames to undergo a certain deformation, releasing the pressure of the surrounding rock and allowing the anti-bonding. Under large deformation of soft rock, the anti-bonding method is conducive to resisting the deformation of the surrounding rock while giving full play to the self-stabilizing ability of the surrounding rock and ensuring the stability of the initial support. When the deformation is too large, the second layer of steel pipe concrete arch frame can quickly exert its load-bearing capacity, and a second layer of shotcrete can be sprayed to ensure the stability of the surrounding rock.

[0043] In this embodiment, the elastic connection device only needs to provide the allowable deformation for the first layer of steel arch frame. As a preferred real-time method, such as... Figures 4-5As shown, the elastic connection device includes a high-performance spring 31 and a round steel connector 32. One end of the high-performance spring 31 is connected to the steel arch frame 1, and the other end is connected to the round steel connector 32. The round steel connector 32 has an arc surface adapted to connect to the steel-concrete composite arch frame 2. The round steel connector can ensure the welding area with the steel-concrete composite arch frame 2. The high-performance spring can provide the reserved deformation amount of the first layer of steel arch frame and the longitudinal displacement of the two layers of arch frames, releasing the surrounding rock pressure. At the same time, when the surrounding rock pressure is too high, the first layer of arch frame and the second layer of arch frame can quickly form a load-bearing body to ensure the stability of the surrounding rock and the initial support. Wherein, as... Figure 4 As shown, the first layer of steel arch frame uses I-beams, while several high-performance springs 31 are arranged in an array. The upper end of the high-performance springs 31 needs to be connected to the steel arch frame 1. In order to improve the integrity of the high-performance springs 31, a combined steel plate 33 is connected to the upper end of the high-performance springs 31, and then the bottom of the steel arch frame 1 is connected through the combined steel plate 33, so that the connection area is larger and the integrity of all the high-performance springs 31 of each elastic connection device is stronger.

[0044] In this embodiment, for ease of processing and installation, the steel arch frame 1 and the steel-concrete composite arch frame 2 can be divided into several segments along their arch direction. Each segment of the steel-concrete composite arch frame 2 can be connected into a single unit via expansion hoses, effectively reducing the reduction effect of segmental connections on the steel-concrete composite arch frame 2.

[0045] In this embodiment, adjacent segments of the steel arch frame 1 are connected by steel connectors 5, such as... Figure 9 As shown, the steel connector 5 includes a first connecting plate 51 connected to the end of a segment of the steel arch frame 1. Two first connecting plates 51 at the ends of adjacent segments of the steel arch frame 1 are connected together by first connecting bolts 52 to form a whole. In order to strengthen the connection between the end of the segment of the steel arch frame 1 and the first connecting plate 51, unequal angle steel 53 is also welded to the sides of the first connecting plate 51 and the sides of the steel arch frame 1 on both sides of the steel arch frame 1.

[0046] In this embodiment, adjacent segments of the steel-concrete composite arch frame 2 are connected by round steel connectors 6. For example... Figure 10 As shown, the round steel connector 6 includes a second connecting plate 61, and the second connecting plate 61 has a round steel through hole 62, so that the adjacent ends of the adjacent segments of the steel pipe concrete arch frame 2 extend into the round steel through hole 62 and are welded to the second connecting plate 61, avoiding the cutting of the steel pipes between the adjacent segments of the steel pipe concrete arch frame 2 and increasing the connection strength. The second connecting plates 61 at the adjacent ends of the adjacent segments of the steel pipe concrete arch frame 2 are connected and fixed by the second connecting bolts 63 to form a whole.

[0047] As another preferred embodiment, both the profile steel connector 5 and the round steel connector 6 are quick-connect devices, such as... Figures 12-14 The quick-connect device includes an insertion end 44 and a fixed end 45, which are correspondingly arranged. A connecting steel plate 41 is connected to each adjacent end of the insertion end 44 and the fixed end 45. A compression spring device 42 is provided at the end of the insertion end 44 away from the fixed end 45. Figure 12 As shown, the front end device 441 of the insertion end 44 is also kept in a compressed state by the fixing device 46; as Figures 13-14 As shown, after the fixing device 46 releases the compression of the spring device 42, the spring device 42 springs the front end device 441 to lock with the fixing end 45.

[0048] The aforementioned quick-connection device consists of one welded steel plate at the insertion end and another welded steel plate at the fixed end. The two steel plates are positioned by a device at the front end of the insertion end, and then the fixing device 46 releases the pressure on the spring device 42. The spring device at the rear of the insertion end pushes the front end device to quickly anchor to the fixed end. This rapid connection effectively avoids the problems of simultaneous construction of double-layer steel arches and the difficulty of connecting segments. The quick-connection device is simple to operate and can further improve construction efficiency. Figures 12-14 As shown, the insertion end includes an insertion sleeve, with the tail end of the insertion sleeve sealed and the front end open. The tail end of the insertion sleeve is equipped with a compression spring device 42, such as... Figure 12 As shown, the insertion sleeve has a first pin as a front end device 441, with the tip of the first pin extending out of the insertion sleeve. The fixing device 46 is a second pin, radially inserted into the insertion sleeve, which can limit the first pin and ensure the compression of the spring device. The fixing end 45 is a fixing sleeve, with its front end corresponding to the tip of the first pin. The tail end of the fixing sleeve has a response device 43, which can be a response light. After the second pin is radially withdrawn, as... Figure 13 As shown, the front end device of the insertion end pops out into the fixed end. When the front end device of the insertion end contacts the response device, the response light of the response device lights up, indicating that the connection is in place, realizing a quick and accurate connection between the arch frame segments. Simultaneously, rubber gaskets are provided between the two connecting steel plates at the ends of the two segments to reduce wear between the two connecting steel plates. Furthermore, the connection points between the two layers of steel arch frames are staggered by a certain distance, such as... Figure 1 As shown, the first layer of steel arch frame includes steel frame 11, steel frame 12, steel frame 13, steel frame 14 and steel frame 15. Correspondingly, the steel pipe concrete arch frame 2 includes steel frame 21, steel frame 22, steel frame 23, steel frame 24 and steel frame 25. The connection points at both ends of steel frame 11 and steel frame 22 are not corresponding and are staggered to ensure the stability of weak stress points.

[0049] Example 2 Currently, steel-concrete composite tunnels are widely used in port engineering, bridges, and high-rise buildings, and their structural calculations are relatively mature. However, in the field of tunnels, their structural calculations lack standardized support, especially for the calculation of such composite initial support structures, where there is no relevant technical support. Therefore, it is necessary to provide a design method for a double-layer composite steel arch frame structure for tunnels with large deformation to ensure the construction safety of soft rock tunnels and ultra-large cross-section tunnels.

[0050] This embodiment provides a design method for a double-layer composite steel arch structure for a large deformation tunnel. The method involves designing the double-layer composite steel arch structure for the large deformation tunnel described in Embodiment 1, determining reasonable dimensions for the steel-concrete composite arch, such as diameter, wall thickness, segment length, and the allowable deformation between the two layers of steel arches. Considering the tunnel's internal contour, burial depth, topography, and geological conditions, the bending moment and axial force borne by the combined initial support structure are calculated. Then, it is determined whether the initial support safety factor requirements are met. If met, the method is adopted; if not, adjustments are made and verification is repeated until the requirements are met.

[0051] The design method of double-layer composite steel arch frame structure for large deformation tunnels is as follows: Figure 15 As shown, the design steps include the following: S1. Calculation and determination of the sharing ratio of the surrounding rock pressure: Calculate the surrounding rock pressure of the tunnel based on the tunnel outline, burial depth, topography and geological conditions, and determine the load sharing ratio. In step S1, to make the double-layer composite steel arch frame structure of the tunnel with large deformation design safer, the load distribution can be planned and selected. As a better option, the load distribution ratio is shown in the table below: Surrounding rock level Surrounding rock + first layer of initial support Initial support structure of composite steel arch frame Secondary lining V 0.25 0.25 0.5 IV 0.4 0.4 0.2 The load-sharing ratio can be adjusted based on the specific example.

[0052] S2. Determination of Initial Support Structure Parameters for Composite Steel Arch Frames: Through data research and engineering analogies, the initial support parameters for the double-layer composite steel arch frame structure of large deformation tunnels are determined, including the steel type of steel arch frame 1, the diameter and wall thickness of steel pipe 21 of steel-concrete composite arch frame 2, the spacing of steel arch frames, the thickness of the first and second layer initial support, the concrete grade, and the allowable deformation. The spacing between steel arch frames is related to the width b of the rectangular section in the safety factor, the initial support height h of the composite steel arch frame, the type of steel, and the moment of inertia.

[0053] S3. Based on the load sharing ratio, surrounding rock pressure, and initial support structure parameters of the composite steel arch frame, the bending moment and axial force of the first-layer initial support before the second layer of shotcrete and the force borne by the elastic connection device 3 are calculated using the load structure method, as well as the bending moment and axial force borne by the initial support structure of the composite steel arch frame after the second layer of shotcrete. The bending moment and axial force of the first-layer initial support before the second layer of shotcrete and the bending moment and axial force borne by the initial support structure of the composite steel arch frame after the second layer of shotcrete are used to verify the safety factor for the corresponding time period. Then determine the stiffness of the high-performance spring 31 of the elastic connection device 3; In step S3, the formula for determining the stiffness coefficient of the high-performance spring 31 of the elastic connection device 3 is as follows:

[0054] Where: k——stiffness coefficient of high-performance spring 31; F max —The maximum force that the high-performance spring 31 can withstand before the second layer of shotcrete; —The allowance for deformation between the first layer of initial support and the second layer of initial support; the first layer of initial support refers to the initial support after the first shotcrete application, and the second layer of initial support refers to the initial support after the second shotcrete application.

[0055] Then, the structural safety factor calculation for the second layer of shotcrete is performed. At this point, the first layer of steel arch frame structure and steel-concrete composite structure are considered as independent structures, and their safety factors are calculated separately. The combined load-bearing structure of the first layer of shotcrete + steel arch frame is calculated according to the relevant requirements of the specifications, and the reaction force provided by the connected high-performance spring 31 is regarded as the safety reserve of the first layer of initial support.

[0056] S4. Safety factor verification of the first layer of initial support: The safety factor of the first layer of initial support formed by the combined load-bearing structure of the first layer of shotcrete and steel arch frame 1 is verified; the safety verification method of the first layer of initial support is conventional technical means.

[0057] S5. Determine whether the initial support of the first layer meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2, that is, adjust the relevant parameters. The adjustment method of the parameters is based on experience and relevant specifications. S6. Determine the standard and design values ​​of the compressive strength and tensile strength of the steel-concrete composite arch frame 2. Then, the safety factor of the steel-concrete arch frame 2 before the second layer of shotcrete is verified. In step S6, combining the survey results and the applicability of the tunnel steel arch structure, and comprehensively considering the influence of factors such as steel pipe strength, fine stone mortar concrete strength, steel pipe segment length, and eccentricity on the bearing capacity of the steel-concrete composite arch, the formulas for determining the standard value and design value of the compressive strength of the steel-concrete composite arch 2 are as follows: ; ; ; ; ; ; ; ; ; In the formula: , —Standard and design values ​​of compressive strength of steel-concrete composite arch frame 2; θ — Hoop index; A s A c —Cross-sectional area of ​​steel pipe 21 and fine aggregate mortar concrete 23; f sk f s —Standard and design values ​​of yield strength for steel pipe 21; f ck f c —Standard and design values ​​of axial compressive strength of fine aggregate mortar concrete; B, C —Influence coefficients of hoop index, B=0.000826*fs+1.01, C=-0.00722*fc+0.12; k1 —Influence coefficient of slenderness ratio of steel-concrete composite arch frame; L e — Segment length of steel-concrete composite arch frame 2; D — Outer diameter of steel-concrete composite arch frame 2; k2 — Reliability correction factor; α sc —Steel content of steel-concrete composite arch frame 2; k e —Eccentricity influence coefficient; e0—Eccentricity, calculated separately for the eccentricity before and after the second layer of shotcrete; r c —The radius of the cross section of the fine aggregate mortar concrete; The formula for determining the standard value and design value of the tensile strength of the steel-concrete composite arch frame is as follows: ; In the formula: —Standard value of tensile strength for steel-concrete composite arch frame 2; C1—The tensile strength enhancement factor of steel pipe 21, taken as 1.1; A s —Cross-sectional area of ​​steel pipe 21; A sc —The cross-sectional area of ​​steel-concrete composite arch frame 2; In step S6, the steel-concrete composite arch frame 2 mainly bears the reaction force of the elastic connection device 3, and its structural safety factor is calculated as follows: ; Where: K—safety factor, taken from the standard; S—Spacing of steel-concrete composite arch frame 2; R—Calculation radius of the steel-concrete composite arch frame; p——The steel-concrete composite arch frame 2 bears the surrounding rock pressure. Considering the transmission effect of the first layer of initial support, p=0.4p1, where p1 is the surrounding rock pressure borne by the surrounding rock and the first layer of initial support. N u —Ultimate bearing capacity of steel-concrete composite arch members under compression and bending, N u =f sc A sc ; The safety factor of the steel-concrete arch frame 2 before the second layer of shotcrete was verified using the above method. The result was accurate and could guarantee the safety of the construction process and the operation after construction.

[0058] S7. Determine whether the steel-concrete arch frame 2 before the second layer of shotcrete meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. S8. After the second layer of shotcrete, the first layer of shotcrete, steel arch frame 1, elastic connection device 3, steel pipe concrete arch frame 2 and the second layer of shotcrete are used as the initial support structure of the combined steel arch frame for safety factor verification. In step S8, the reinforcing effect of the combined steel arch frame on the shotcrete is simulated by means of equivalent reinforcement. The first layer of steel arch frame 1 is regarded as the outer reinforcement, and the second layer of steel pipe concrete arch frame 2 is regarded as the inner reinforcement. The safety factor calculation formula for the initial support structure of the combined steel arch frame is as follows: ①When , ; ②When , ; ③When , ; In the formula, K is the safety factor, which is taken according to the specification; N is the axial force. —Ultimate flexural compressive strength of concrete; —The standard value of tensile strength of the steel reinforcement in the tension zone is calculated based on the internal forces of the structure to determine whether it is under tension on the inside or the outside. If it is under tension on the inside, the cross-sectional area of ​​the steel reinforcement in the tension zone is taken as the area of ​​the steel-concrete composite arch frame 2, and the cross-sectional area of ​​the steel reinforcement in the compression zone is taken as the area of ​​the first layer of steel arch frame 1. Otherwise, the opposite applies. —Standard value of compressive strength of steel reinforcement in the compression zone; , —Cross-sectional area of ​​the reinforcement in the tension and compression zones; a, a' —Distance from the center of the reinforcement in the tension and compression zones to the nearest edge of the section; h —Initial support height of the composite steel arch; h0 —Effective height of the section. =ha; x——height of the concrete compression zone; b——width of the rectangular section, taken as the longitudinal spacing between adjacent combined steel arch frames; e——axial force eccentricity.

[0059] The safety factor of the initial support structure of the composite steel arch frame was verified using the above method. The result was accurate and could guarantee the safety of the construction process and the operation after construction.

[0060] When verifying the safety factor of the initial support structure of the composite steel arch frame, it can be seen that the axial force eccentricity e needs to be obtained. In order to obtain the axial force eccentricity e quickly and accurately, this embodiment provides a preferred implementation method. When verifying the safety factor of the composite initial support structure, the equivalent elastic modulus and unit weight of the composite initial support structure are first calculated; then, the bending moment M and axial force N of the initial support are calculated using the load structure method; and finally, the axial force eccentricity e is obtained through the bending moment M and axial force N of the initial support. The formulas for calculating the equivalent elastic modulus and unit weight of the combined initial support structure are as follows: ; ; In the formula, E is the equivalent elastic modulus, I is the equivalent moment of inertia relative to the centroid, the centroid refers to the center of the cross section of the combined initial support structure, γ is the equivalent unit weight, and A is the total area of ​​the combined steel arch frame initial support structure. , , , The elastic moduli of the shotcrete, the first layer of steel arch frame 1, the second layer of steel-concrete composite arch frame 2, and the second layer of steel pipe 21 are respectively. , , , The moments of inertia are respectively those of the shotcrete, the first layer of steel arch frame 1, the second layer of steel-concrete composite arch frame 2, and the second layer of steel pipe 21. , , , The weights of the shotcrete, the first layer of steel arch frame 1, the second layer of steel-concrete composite arch frame 2, and the second layer of steel pipe 21 are respectively. , , , The areas are respectively the area of ​​the shotcrete, the first layer of steel arch frame 1, the second layer of steel pipe concrete arch frame 2, and the second layer of steel pipe 21.

[0061] Furthermore, due to the presence of deflection, the surrounding rock pressure will cause deflection in the initial support structure of the composite steel arch frame. This deflection will increase the axial eccentricity of the eccentric force, generating additional force, i.e., the second-order effect of eccentrically compressed structure. Simultaneously, the two layers of steel arch frames in the initial support structure of the composite steel arch frame are elastically connected, which has a suppressive effect on deflection deformation. Therefore, multiplying the axial eccentricity by the eccentricity amplification factor yields a higher safety for the initial support structure of the composite steel arch frame. As a superior real-time method, when performing safety factor verification on the initial support structure of the composite steel arch frame, it is also necessary to multiply the axial eccentricity e by the eccentricity amplification factor. Eccentricity amplification factor The calculation formula is as follows: ; ; In the formula, —Eccentricity amplification factor; K —Safety factor; —Compressive elastic modulus of shotcrete; —Moment of inertia of the shotcrete section; —The compressive modulus of elasticity of the first layer of steel arch frame 1; —Moment of inertia of section 1 of the first layer of steel arch frame; k —Stiffness coefficient of high-performance spring 31; —The compressive elastic modulus of the fine aggregate mortar concrete 23 of the steel-concrete composite arch frame 2; —Moment of inertia of the section of fine aggregate mortar concrete 23 of steel pipe concrete arch frame 2; —The compressive modulus of elasticity of the steel pipe 21 of the steel-concrete composite arch frame 2; —Moment of inertia of the cross section of steel pipe 21 of steel pipe concrete arch frame 2; —Eccentricity coefficient; h —Initial support structure height of the composite steel arch frame.

[0062] S9. Determine whether the initial support structure of the combined steel arch frame meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. If it meets the safety factor requirements, determine the parameters of the initial support structure of the combined steel arch frame.

[0063] The design method of the double-layer combined steel arch frame structure for large deformation tunnels described in this embodiment, along with the calculation method for the combined initial support structure, allows for the scientific selection of relevant parameters for the combined initial support structure and precise calculations. This ensures the structural safety of the double-layer combined steel arch frame structure as the initial support steel arch frame for large deformation tunnels during construction and during operation after construction.

[0064] Example 3 This embodiment provides a construction method for a combined initial support structure. The double-layer combined steel arch structure of the large deformation tunnel described in Embodiment 1 is used as the steel arch frame of the combined initial support structure. The construction method for the combined initial support structure starts excavation and support from the upper bench. For tunnels with large cross-sections and soft rock with large deformation, a longitudinal segmented cyclic construction method is generally adopted. The excavation of each bench in each segment includes the following steps: Step 1: Excavate the current step; Step 2: Assemble and install the steel pipe 21 and expansion hose 22 of the outer steel arch frame 1, elastic connection device 3 and inner steel pipe concrete arch frame 2 corresponding to the current step; Step 3: Inject grout into the expansion hose 22 to form a steel-concrete structure; Step 4: Spray the first layer of concrete; When the first layer of sprayed concrete deforms to the reserved deformation position between the steel pipe concrete arch frame 2 and the steel arch frame 1, the corresponding second layer of concrete is sprayed.

[0065] The construction method of the combined initial support structure described in this embodiment is simple and safer.

[0066] Specifically, based on the reserved deformation between the two layers of steel arch frames, the elastic modulus of the high-performance spring 31 in the elastic connection device between the steel arch frames is calculated using Hooke's Law, that is, to ensure that the maximum tensile amount of the high-performance spring 31 is equal to the reserved deformation between the two layers of steel arch frames.

[0067] Step 2: Assemble the outer steel arch frame of the upper step. (For example...) Figure 1 As shown, the stepped steel arch frame is divided into 5 sections. Steel frame 11 is erected, and then steel frames 12 and 13 are installed on both sides of steel frame 11 using a quick-connect device. Pulling out the second pin of the quick-connect device will illuminate the response device, indicating that the steel frame connection is complete. Finally, similar to the above steps, steel frames 14 and 1 are connected to steel frames 12 and 13.

[0068] Then, install the elastic connection device between the two layers of arch frames at the bottom of the first layer of steel arch frame, with a circumferential spacing of 100~150cm, and weld the elastic connection device.

[0069] Then, assemble the steel pipe concrete arch frame on the inner side of the upper step. Clamp steel frame 21 to the designated position and weld it to the bottom arc surface of the round steel connector 21 of the elastic connection device; then, steel frames 22 and 23 are fixed to both sides of steel frame 21 through a quick connection device and welded to the bottom arc surface of the round steel connector 21 of the elastic connection device to achieve the connection between the two layers of steel arch frames; finally, install steel frames 24 and 25 to form the double-layer steel frame structure of the upper step.

[0070] Step 3: Grouting inside the steel pipes of the steel-concrete composite arch frame, such as... Figure 2 As shown, a steel-concrete composite structure is formed. After the double-layer arch frame structure is erected, the grouting pipe 71 is inserted into the upper step steel pipe structure through the grouting holes reserved in the steel pipe. Fine stone mortar concrete is injected using the pump 7. Finally, the grouting pipe 71 is pulled out and sealed to form a steel arch frame + steel-concrete composite steel arch frame structure.

[0071] Step 4: First Layer of Shotcrete. The first layer of shotcrete is applied to form the initial support system of shotcrete + combined steel arch frame structure. Once the shotcrete and the fine aggregate mortar concrete inside the steel pipes have fully set, it can effectively control the deformation of the surrounding rock and ensure construction safety. Steps 2, 3, and 4 correspond to the construction of the upper steps.

[0072] Subsequent excavation of the middle and lower benches follows similar steps to the upper bench excavation, enabling the soft rock tunnel to advance. Once the first layer of shotcrete deforms to the pre-deformation allowance of the steel-concrete composite arch frame, a second layer of concrete is sprayed to further enhance the initial support's bearing capacity, forming a double-layer initial support + composite steel arch frame load-bearing structure. Furthermore, with the assistance of large machinery, step 2 can be simplified to: connecting the steel arch frame segments and the steel-concrete composite arch frame segments outside the tunnel using elastic connection devices; inside the tunnel, only the connections between the composite arch frame segments need to be made, resulting in faster construction.

[0073] In this embodiment, the simultaneous construction of two layers of initial support effectively saves construction time. With the assistance of large machinery, the steel frame composite structure can be fabricated outside the tunnel, and construction inside the tunnel only requires completing the connection between segments, effectively ensuring construction efficiency. Meanwhile, when the tunnel excavation cross-section is large, if the second layer of initial support is constructed separately, it requires the assistance of large machinery and is greatly affected by the construction conditions of the construction unit. This invention effectively solves this problem.

[0074] The double-layer combined steel arch frame structure for large deformation tunnels of the present invention has a simple construction process, easy component processing, and reasonable design process. While giving full play to the self-stabilizing ability of the surrounding rock, it can provide sufficient bearing capacity to resist the deformation of the surrounding rock and ensure construction safety.

[0075] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A double-layer composite steel arch frame structure for a large deformation tunnel, characterized in that, It includes a steel arch frame (1), a steel pipe concrete arch frame (2) and elastic connecting devices (3). The steel arch frame (1) is located outside the steel pipe concrete arch frame (2). The steel arch frame (1) and the steel pipe concrete arch frame (2) are connected by several elastic connecting devices (3). The steel-concrete composite arch frame (2) includes a steel pipe (21), an expansion hose (22), and fine stone mortar concrete (23). The expansion hose (22) is sleeved inside the steel pipe (21), and the fine stone mortar concrete (23) fills the expansion hose (22) and squeezes the expansion hose (22) against the inner wall of the steel pipe (21). The steel arch frame (1) and the steel pipe concrete arch frame (2) are divided into several segments along their arch direction. The adjacent segments of the steel arch frame (1) are connected by steel connectors (5), and the adjacent segments of the steel pipe concrete arch frame (2) are connected by round steel connectors (6). Both the steel profile connector (5) and the round steel connector (6) are quick-connect devices. The quick-connect device includes an insertion end (44) and a fixed end (45). The insertion end (44) and the fixed end (45) are correspondingly arranged. The adjacent ends of the insertion end (44) and the fixed end (45) are connected to a connecting steel plate (41). The end of the insertion end (44) away from the fixed end (45) is provided with a compression spring device (42). The front end device (441) of the insertion end (44) is also kept in a compressed state by the fixing device (46). After the fixing device (46) releases the compression of the spring device (42), the spring device (42) springs the front end device (441) to lock with the fixed end (45).

2. The double-layer composite steel arch frame structure for a large deformation tunnel according to claim 1, characterized in that, The elastic connection device includes a high-performance spring (31) and a round steel connector (32). One end of the high-performance spring (31) is connected to the steel arch frame (1), and the other end is connected to the round steel connector (32). The round steel connector (32) has an arc surface adapted to connect the steel pipe concrete arch frame (2).

3. A design method for a double-layer composite steel arch frame structure for a large deformation tunnel, characterized in that, The design of a double-layer composite steel arch frame structure for a large deformation tunnel as described in any one of claims 1-2 includes the following design steps: S1. Calculation of surrounding rock pressure and determination of load sharing ratio: Calculate the surrounding rock pressure of the tunnel based on the tunnel's internal profile, burial depth, topography, and geological conditions, and determine the load sharing ratio. S2. Parameters of the initial support structure of the combined steel arch frame: Through data research and engineering comparison, the steel type of the double-layer combined steel arch frame (1), the diameter and wall thickness of the steel pipe (21) of the steel pipe concrete arch frame (2), the spacing of the steel arch frames, the thickness of the first layer initial support and the second layer initial support, the concrete grade, and the reserved deformation amount of the double-layer combined steel arch frame structure of the large deformation tunnel are determined. S3. Based on the load sharing ratio, surrounding rock pressure and initial support structure parameters of the composite steel arch frame, calculate the bending moment and axial force of the first initial support before the second layer of shotcrete and the force borne by the elastic connection device (3) respectively, as well as the bending moment and axial force borne by the initial support structure of the composite steel arch frame after the second layer of shotcrete, using the load structure method; then determine the stiffness of the high-performance spring (31) of the elastic connection device (3). S4. Safety factor verification of the first layer initial support: The safety factor of the first layer initial support formed by the combined load-bearing structure of the first layer shotcrete and steel arch frame (1) is verified. S5. Determine whether the initial support of the first layer meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. S6. Determine the standard and design values ​​of compressive strength and tensile strength of the steel-concrete composite arch frame (2); Then, the safety factor of the steel-concrete arch frame (2) before the second layer of shotcrete is checked; S7. Determine whether the steel pipe concrete arch frame (2) before the second layer of shotcrete meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. S8. After the second layer of shotcrete, the first layer of shotcrete, the steel arch frame (1), the elastic connection device (3), the steel pipe concrete arch frame (2) and the second layer of shotcrete are used as the initial support structure of the combined steel arch frame for safety factor verification. S9. Determine whether the initial support structure of the combined steel arch frame meets the safety factor requirements. If it does not meet the safety factor requirements, return to step S2. If it meets the safety factor requirements, determine the parameters of the initial support structure of the combined steel arch frame.

4. The design method for the double-layer composite steel arch frame structure of a large deformation tunnel according to claim 3, characterized in that, In step S3, the formula for determining the stiffness coefficient of the high-performance spring (31) of the elastic connection device (3) is as follows: ； Where: k——stiffness coefficient of high-performance spring (31); F max —The maximum force borne by the high-performance spring (31) before the second layer of shotcrete; — Deformation allowance reserved between the first and second initial support layers; In step S6, the formula for determining the standard value and design value of the compressive strength of the steel-concrete composite arch frame (2) is as follows: ； ； ； ； ； ； ； ； ； In the formula: , —Standard and design values ​​of compressive strength of steel-concrete composite arch frame (2); θ — Hoop index; A s A c —Cross-sectional areas of steel pipe (21) and fine aggregate mortar concrete (23); f sk f s —Standard and design values ​​of yield strength for steel pipe (21); f ck f c —Standard and design values ​​of axial compressive strength of fine aggregate mortar concrete (23); B, C —influence coefficients of hoop index, B=0.000826*fs+1.01, C=-0.00722*fc+0.12; k1 —influence coefficient of slenderness ratio of steel pipe concrete arch frame (2); L e — Segment length of steel-concrete composite arch frame (2); D — Outer diameter of steel-concrete composite arch frame (2); k2 — Reliability correction coefficient; α sc —Steel pipe concrete arch frame (2) steel content; k e —Eccentricity influence coefficient; e0 —Eccentricity; r c — The radius of the cross section of fine aggregate mortar concrete (23); The formulas for determining the standard value and design value of the tensile strength of the steel-concrete composite arch (2) are as follows: ； In the formula: —Standard value of tensile strength of steel-concrete composite arch frame (2); C1—The tensile strength enhancement factor of steel pipe (21) is taken as 1.1; A s —Cross-sectional area of ​​the steel pipe (21); A sc —The cross-sectional area of ​​the steel-concrete composite arch frame (2); In step S6, the steel-concrete composite arch frame (2) mainly bears the reaction force of the elastic connection device (3), and its structural safety factor is calculated as follows: ； Where: K—safety factor, taken from the standard; S—Spacing of steel-concrete composite arch frame (2); R—Calculation radius of steel-concrete composite arch frame (2); p——The steel pipe concrete arch frame (2) bears the surrounding rock pressure. Considering the transmission effect of the first layer of initial support, p=0.4p1, where p1 is the surrounding rock pressure borne by the surrounding rock and the first layer of initial support. N u —Ultimate bearing capacity of steel-concrete composite arch members under compression and bending (2), N u =f sc A sc ; In step S8, the reinforcing effect of the combined steel arch frame on the shotcrete is simulated by the equivalent reinforcement method. The first layer of steel arch frame (1) is regarded as the outer reinforcement, and the second layer of steel pipe concrete arch frame (2) is regarded as the inner reinforcement. The safety factor calculation formula for the initial support structure of the combined steel arch frame is as follows: ①When , ； ②When , ； ③When , ； In the formula, K is the safety factor, which is taken according to the specification; N is the axial force. —Ultimate flexural compressive strength of concrete; —Standard value of tensile strength of steel bars in tension zone. Calculate the structure based on the internal forces of the structure and determine whether the inner side is under tension or the outer side is under tension. If the inner side is under tension, the cross-sectional area of ​​the steel bars in tension zone is taken as the area of ​​the steel pipe concrete arch frame (2), and the cross-sectional area of ​​the steel bars in compression zone is taken as the area of ​​the first layer of steel arch frame (1). Otherwise, the opposite applies. —Standard value of compressive strength of steel reinforcement in the compression zone; , —Cross-sectional area of ​​the reinforcement in the tension and compression zones; a, a' —Distance from the center of the reinforcement in the tension and compression zones to the nearest edge of the section; h —Initial support height of the composite steel arch; h0 —Effective height of the section. =ha; x——height of the concrete compression zone; b——width of the rectangular section, taken as the longitudinal spacing between adjacent combined steel arch frames; e——axial force eccentricity.

5. The design method for the double-layer composite steel arch frame structure of a large deformation tunnel according to claim 4, characterized in that, When performing safety factor calculations on the initial support structure of the composite steel arch frame, it is also necessary to multiply the axial force eccentricity e by the eccentricity amplification factor. Eccentricity amplification factor The calculation formula is as follows: ； ； In the formula, —Eccentricity amplification factor; K —Safety factor; —Compressive elastic modulus of shotcrete; —Moment of inertia of the shotcrete section; —The first layer of steel arch frame (1) compressive elastic modulus; —Moment of inertia of the first layer of steel arch frame (1); k—Stiffness coefficient of high-performance spring (31); —The compressive elastic modulus of fine aggregate mortar concrete (23) of steel pipe concrete arch frame (2); — Moment of inertia of the section of fine stone mortar concrete (23) of steel pipe concrete arch frame (2); —The compressive modulus of elasticity of the steel pipe (21) of the steel pipe concrete arch frame (2); —Moment of inertia of the cross section of the steel pipe (21) of the steel pipe concrete arch frame (2); —Eccentricity coefficient; h —Initial support structure height of the composite steel arch frame.

6. The design method for the double-layer composite steel arch frame structure of a large deformation tunnel according to claim 5, characterized in that, When performing safety factor verification of the combined initial support structure, first calculate the equivalent elastic modulus and unit weight of the combined initial support structure; then use the load structure method to calculate the bending moment M and axial force N of the initial support; and then obtain the axial force eccentricity e through the bending moment M and axial force N of the initial support. The formulas for calculating the equivalent elastic modulus and unit weight of the combined initial support structure are as follows: ； ； In the formula, E is the equivalent elastic modulus, I is the equivalent moment of inertia relative to the centroid, γ is the equivalent unit weight, and A is the total area of ​​the initial support structure of the composite steel arch frame. , , , The elastic modulus of the shotcrete, the first layer of steel arch frame (1), the second layer of steel-concrete composite arch frame (2), and the second layer of steel pipe (21) are respectively. , , , The moments of inertia of the shotcrete, the first layer of steel arch frame (1), the second layer of steel-concrete composite arch frame (2), and the second layer of steel pipe (21) are respectively. , , , The weights of the shotcrete, the first layer of steel arch frame (1), the second layer of steel-concrete composite arch frame (2), and the second layer of steel pipe (21) are respectively. , , , The areas are respectively the area of ​​the shotcrete, the first layer of steel arch frame (1), the second layer of steel pipe concrete arch frame (2), and the second layer of steel pipe (21).

7. The design method for a double-layer composite steel arch frame structure for a large deformation tunnel according to any one of claims 3-6, characterized in that, In step S1, the load sharing ratio is shown in the table below: 。 8. A construction method for a combined initial support structure, characterized in that, The double-layer composite steel arch frame structure of the large deformation tunnel as described in any one of claims 1-2 is used as the steel arch frame of the composite initial support structure. The construction method of the composite initial support structure starts from the upper bench for excavation and support. The excavation of each bench of each segment includes the following steps: Step 1: Excavate the current step; Step 2: Assemble and install the steel pipe (21) and expansion hose (22) of the outer steel arch frame (1), elastic connection device (3) and inner steel pipe concrete arch frame (2) corresponding to the current step. Step 3: Grout into the expansion hose (22) to form a steel-concrete structure; Step 4: Spray the first layer of concrete; When the first layer of concrete is deformed to the reserved deformation position between the steel pipe concrete arch frame (2) and the steel arch frame (1), the corresponding second layer of concrete is sprayed.

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