A design and construction method for a precast invert arch to prevent tunnel floor heave deformation

Abstract

This invention discloses a design and construction method for a precast invert arch to prevent tunnel floor heave deformation. The construction method for the precast invert arch includes the following steps: 1) precast invert arch dimension design; 2) precast invert arch shape design; 3) precast invert arch fabrication; 4) on-site installation of the precast invert arch; and 5) road paving. The precast invert arch includes a left precast block, a right precast block, and a central precast block. The left precast block is laid on the left side of the tunnel foundation, the right precast block is laid on the right side of the tunnel foundation, and the central precast block is laid between the left and right precast blocks. The sum of the lengths of the top surfaces of the left, right, and central precast blocks is equal to the width of the tunnel's horizontal plane. Precast prefabricated joints are provided on the contact surfaces between the left, right, and central precast blocks. Each precast block is positioned and connected by grouting after being joined through the precast prefabricated joints.

Description

Technical Field

[0001] This invention belongs to the field of tunnel technology, specifically relating to a design and construction method for a precast invert arch to prevent tunnel floor bulging deformation. Background Technology

[0002] A tunnel invert is a reverse arch structure installed at the bottom of a tunnel to improve the stress conditions of the superstructure. It is one of the main components of the tunnel structure. On the one hand, it effectively transfers the ground pressure above the tunnel to the underground through the tunnel sidewall structure or the load on the road surface, and on the other hand, it effectively resists the reaction force from the strata below the tunnel. If an invert can be installed in the tunnel, the overall stability of the tunnel can be greatly improved. When the tunnel passes through high ground stress zones, extremely soft rock, or water-rich expansive soil strata, invert bottom heave deformation is very likely to occur. Taking expansive soil as an example, under the influence of factors such as tunnel excavation, groundwater fluctuations, and construction disturbances, the weakly expansive and contractile tunnel surrounding rock will inevitably undergo expansion and contraction. The expansion and contraction forces acting on the tunnel invert may cause invert bottom heave, which is particularly prominent during the operation phase.

[0003] Invert arch bottom bulging is a major type of invert arch defect. This deformation damages the overall tunnel support structure formed by the invert arch and secondary lining, posing significant risks to both tunnel construction and operation. The main hazards of invert arch bottom bulging during tunnel construction include: it is often accompanied by severe tunnel deformation or damage, such as secondary lining cracking and spalling, seriously endangering personnel and property safety, affecting construction vehicle traffic, threatening construction safety, and causing project delays. The main hazards of invert arch bottom bulging during tunnel operation include: road surface cracking and damage, encroachment on clearance limits, changes in cross slope angle, and providing channels for tunnel water leakage, directly affecting tunnel safety and service life. To address the invert arch bottom bulging problem, those skilled in the art have conducted extensive research and improvements on tunnel design methods and construction techniques.

[0004] Chinese patent CN202111011339.8 discloses a prefabricated block structure for tunnel invert arches, designed for tunnel construction using a combination of loosening blasting and manual excavation. When encountering water inrush or concentrated water outlets, drainage is first carried out, followed by spraying steel fiber reinforced concrete and polypropylene fiber reinforced concrete. Then, the steel arch frame is installed, and a pre-cast, segmented, full-width concrete pouring method using a trestle bridge is employed to complete the concrete pouring and curing. This construction method is limited by the fabrication and installation of the steel arch frame and the pouring and curing of the concrete, thus lengthening the overall invert arch construction cycle and failing to achieve the goal of rapid tunnel loop formation, resulting in slow construction progress and quality issues. This construction method suffers from drawbacks such as delayed progress, difficulty in guaranteeing quality, and site limitations, making traditional cast-in-place invert arch types increasingly unable to meet the demands of modern tunnel construction.

[0005] Prefabricated structures have gradually become a new development direction. Compared with cast-in-place concrete structures, prefabricated structures have many advantages: prefabricated components are manufactured in factories, and the quality of prefabricated components is well controlled through mechanization; the components are highly standardized, which can realize the universality of components; the construction is highly mechanized, which can reduce construction procedures, shorten construction time, improve efficiency, and thus reduce project costs; the construction site occupies less space and has less impact on the surrounding environment, which is beneficial to environmental protection. Therefore, the prefabrication of tunnel support structures is the main trend of future tunnel lining construction.

[0006] The current method has the following problems:

[0007] (1) Tunnel construction in areas with high ground stress, extremely soft rock and expansive rock presents serious problems of large deformation and bulging cracks at the invert arch;

[0008] (2) The lining structure adopts the cast-in-place construction method, which has a long cycle and cannot meet the goal of rapid ring formation, resulting in slow construction progress, large pollution and poor construction quality.

[0009] (3) Once the tunnel invert (including the invert filling layer) cracks under the expansion force, its development will be difficult to control, forming a larger area of ​​bottom heave in a short period of time, and maintenance will be difficult and costly. Summary of the Invention

[0010] This invention provides a design and construction method for a prefabricated inverted arch to prevent tunnel floor bulging deformation. The purpose is to provide a novel prefabricated inverted arch structure, improve the construction speed of tunnel inverted arches, and reduce the risk of tunnel floor bulging deformation.

[0011] Therefore, the present invention adopts the following technical solution:

[0012] A precast invert arch for preventing tunnel floor bulging deformation, the construction method of the precast invert arch includes the following steps:

[0013] 1) Design of the shape of the precast invert arch

[0014] The cross-sections of the left and right precast blocks are right-angled triangles, with the hypotenuse of the triangles being an arc shape that fits into the tunnel; the cross-section of the central precast block is rectangular, with the bottom surface of the central precast block being an arc shape that fits into the tunnel; the left, right, and central precast blocks are all designed as hollow cavities, with the shape of the hollow portion similar to that of each precast block, and after assembly, the hollow portions of each precast block should be continuous along the longitudinal direction of the tunnel; precast prefabricated joints are provided on the contact surfaces between the left, right, and central precast blocks, and each precast block is connected by grouting after being joined together through the precast prefabricated joints;

[0015] The upper left side of the left precast block is provided with an outwardly protruding embedded connecting steel bar for connecting the lining; the upper left side of the right precast block is provided with a corresponding embedded connecting steel bar.

[0016] 2) Design of precast invert arch dimensions

[0017] Based on the tunnel radius and the thickness of the initial support and secondary lining concrete, the dimensions of the precast invert arch are determined. The circumferential width of the left and right precast blocks is 180–250 cm, and the circumferential width of the central precast block is 130–190 cm. The hollow portions of each precast block divide the invert arch thickness into top and bottom thicknesses. The bottom thickness should be greater than the top thickness to ensure structural safety and deformation coordination at the bottom of the invert arch. The top thickness is 20–40 cm, and the bottom thickness is 30 cm. ~40cm, the sum of the top and bottom thicknesses is the design invert arch thickness of 50~80cm; the left precast block is laid on the left side of the tunnel foundation, the right precast block is laid on the right side of the tunnel foundation, and the center precast block is laid between the left and right precast blocks. The sum of the lengths of the top surfaces of the left, right and center precast blocks is the width of the tunnel horizontal plane of 490~690cm. The widths of the left, right and center precast blocks along the tunnel excavation direction are equal, at 120~200cm.

[0018] 3) Fabrication of precast invert arches

[0019] Based on the shape and size determined in steps 1) and 2), precast invert arches are produced; the left and right precast blocks are produced by casting fiber concrete material with a steel skeleton according to the design specifications, and concrete repair particles are added to the fiber concrete material in the center precast block; the precast invert arch block mold and the manufacturing process of the precast invert arch block can be implemented with reference to existing technologies, such as Chinese patent CN 109676749 A.

[0020] 4) Installation of precast invert arches at the construction site

[0021] Based on the tunnel excavation method and initial support construction, after the invert arch excavation is completed, a crushed stone concrete cushion layer is laid under the invert arch and leveled. The first group of three prefabricated invert arch blocks are positioned and placed in the designed location. The specific installation steps are as follows: the left prefabricated block is assembled and connected with the center prefabricated block, and the center prefabricated block is assembled and connected with the right prefabricated block. Then, the second group of three prefabricated invert arch blocks are installed, with the left prefabricated block assembled and connected with the center prefabricated block and the left prefabricated block of the first group, the center prefabricated block assembled and connected with the right prefabricated block and the center prefabricated block of the first group, and the right prefabricated block assembled and connected with the right prefabricated block of the first group.

[0022] The pre-embedded connecting steel bars at both ends of the left and right precast blocks are respectively connected to the cast-in-place secondary lining above;

[0023] 5) Road paving

[0024] Construction joints at the splicing points of precast blocks are filled with concrete grout; the left, right, and central precast blocks are connected to form a box-shaped precast inverted arch; after the tunnel deformation stabilizes, the cast-in-place concrete cushion layer, drainage ditch inspection hole cover plate, cable trench, track bed, or other components are constructed.

[0025] Furthermore, the outer ends of the top of the left and right precast blocks are provided with protruding concrete blocks along the width direction, and pre-embedded connecting steel bars are poured and fixed on the protruding concrete blocks; the lower end of the secondary lining is a corresponding concave concrete block, and pre-embedded connecting steel bars are poured and fixed on the concave concrete block.

[0026] Furthermore, the left precast block has several positioning holes on its rear end face, which are perpendicular to the rear end face. Several outwardly extending positioning bars are fixed at the bottom of the positioning holes. The left precast block has several outwardly protruding positioning posts on its front end face, which correspond one-to-one with the positioning holes. A forward-extending positioning flange is fixed at the front end of the positioning post, and holes for connecting the positioning bars are provided on the edge of the positioning flange. The left precast block also has positioning holes and positioning bars on its right side face.

[0027] The right precast block is equipped with positioning holes, positioning steel bars, positioning columns and positioning flanges corresponding to those on the left precast block.

[0028] Furthermore, the rear end face of the central precast block is provided with a plurality of positioning holes, and positioning steel bars are provided in the positioning holes; the front end face of the central precast block is provided with corresponding positioning posts and positioning flanges; the left and right sides of the central precast block are respectively provided with positioning posts and positioning flanges, the positioning posts on the left side of the central precast block correspond to the positioning holes on the right side of the left precast block, and the positioning posts on the right side of the central precast block correspond to the positioning holes on the left side of the right precast block.

[0029] Furthermore, the center of the left precast block, the right precast block, and the central precast block is hollow, and the cavity extends through the front and rear surfaces of the precast block along the width direction.

[0030] Furthermore, the concrete material composition and proportions of the left and right precast blocks are as follows: 550-580 parts of silicate cement, 230-260 parts of fly ash, 1000-1200 parts of sand, 40-60 parts of silica fume, 50-83 parts of polyvinyl alcohol fiber, and 150-180 parts of water are mixed in proportion.

[0031] A design method for a precast invert arch to prevent tunnel floor heave deformation, the design method steps are as follows:

[0032] 1) Determine the burial depth, radius, surrounding rock grade, and thickness and grade parameters of the primary and secondary lining concrete for the tunnel project;

[0033] 2) Take samples of expansive rock and soil from the tunnel construction site and obtain the elastic modulus E, cohesion c, and internal friction angle of the expansive rock through testing. Determine the elastic modulus of expansive rock after plastic softening under time effects. Cohesion internal friction angle

[0034]

[0035] In Equation ①: α, β, and ξ are the weakening coefficients of the elastic modulus, cohesion, and internal friction angle of the surrounding rock, respectively, which are determined according to the properties of the expansive soil at the engineering site and the design specifications;

[0036] 3) Calculate the concrete support force P1 of the lining and the concrete support force P2 of the secondary lining;

[0037]

[0038] In equation ②: σ s1 σ s2 t1 and t2 are the yield stresses of the initial and secondary lining concrete, respectively; t1 and t2 are the thicknesses of the initial and secondary lining concrete, respectively; R0 is the radius of the tunnel cavities; R1 is the radius of the outer boundary of the tunnel after the initial lining is applied.

[0039] 4) Design of invert arch stress parameters

[0040] The yield stress of the invert arch should not exceed the yield stress of the concrete in the upper structure of the invert arch; the mix proportions and types of the invert arch materials should be adjusted according to the lining concrete and the secondary lining concrete to meet the yield stress requirements.

[0041] 5) Design of displacement and deformation parameters at the bottom of the invert arch

[0042] Based on the surrounding rock grade, tunnel cross-sectional dimensions, and support parameters in the design specifications, the ideal elastic-plastic displacement u is calculated respectively. elas-plas Rheological displacement u caused by weakening of surrounding rock parameters under long-term rheological action rheo The difference between the two, Δu, is absorbed by the deformation coordination characteristics at the bottom of the invert arch; therefore, Δu is the displacement deformation parameter at the bottom of the invert arch.

[0043]

[0044] In equation ③, v is the Poisson's ratio of the surrounding rock; P0 is the original rock stress; R0 is the radius of the tunnel cavities; P s For the resistance of surrounding rock support, P s =P1 + P2;

[0045] 6) Design of the thickness at the bottom of the invert arch

[0046] Since the compression thickness of the invert arch material is between 40% and 70%, in order to ensure that the bottom can exert its deformation effect while ensuring redundancy and safety, the optimal compression thickness is 55% to 65%. That is, when the deformation coordination material at the bottom of the invert arch is compressed to 55% to 65% of its original thickness, it is the thickness of the bottom of the invert arch.

[0047] The design principle of this invention is as follows:

[0048] This invention includes a box-type prefabricated invert arch, wherein the prefabricated invert arch comprises three blocks transversely: a left prefabricated block, a central prefabricated block, and a right prefabricated block (the left and right prefabricated blocks are axially symmetrical), which are connected to the secondary lining of the tunnel through prefabrication and assembly. The left, central, and right prefabricated blocks are mainly prefabricated from fiber-reinforced concrete with a reinforced steel frame. The concrete material composition and proportions are as follows: 550-580 parts silicate cement, 230-260 parts fly ash, 1000-1200 parts sand, 40-60 parts silica fume, 50-83 parts polyvinyl alcohol fiber, and 150-180 parts water, etc., mixed in proportion. The sand is natural river sand, the cement is PO42.5R ordinary Portland cement, the fly ash is Grade I, the volume content of polyvinyl alcohol fiber (PVA) is 2%, and the mechanical properties of the fiber are shown in Table 1. The fiber-reinforced concrete in the central precast block is specially mixed with a kind of concrete repair particle microparticle, the volume content of the particle is less than 2%, the concrete repair particle is composed of the outer wall of the particle and the particle liquid wrapped inside the outer wall of the particle, the outer wall of the particle is made of urea-formaldehyde resin, and the particle liquid is sodium silicate solution.

[0049] Table 1 - Performance Indicators of Polyvinyl Alcohol Fiber

[0050]

[0051] The specific connection method of the invert arch components is as follows: the left precast block and the center precast block are assembled and connected in the circumferential direction; the center precast block and the right precast block are assembled and connected in the circumferential direction; the left and right precast blocks are respectively connected to the cast-in-place secondary lining above them; during the circumferential connection, each component is connected to the precast components of the previous ring through reserved interfaces; a hole is reserved at the splice point of the left precast block and the center precast block, and a positioning steel bar is embedded at the bottom of the hole; a concrete positioning column is precast at the splice point of the center precast block and the left precast block, and a positioning flange is embedded at the end of the column; the left precast block and the center precast block are connected at the corresponding hole column positions, and the positioning steel bar in the hole of the left precast block is connected to the hole of the positioning flange of the center precast block; the connection method of the center precast block and the right precast block is the same as above; the longitudinal connection method is consistent with the circumferential connection.

[0052] Construction joints at component splicing points are filled with concrete grout. The left, center, and right precast blocks of the invert arch are connected to form a box-shaped precast invert arch, with convex steel reinforcement joints reserved at both ends of the invert arch. Reinforcing steel frames are embedded at the joints, and after aligning with the pre-designed concave joints of the cast-in-place secondary lining above them, a cast-in-place concrete connection is used. Longitudinal connections can be performed after circumferential connections are completed, or the precast components can be longitudinally spliced ​​first, depending on the surrounding rock conditions.

[0053] The central area on the upper surface of the precast block at the center of the tunnel invert is a region where structural bottom drum cracks are highly likely to develop. When cracks appear on the upper surface of the component, they trigger the self-healing particles randomly distributed in the concrete. After the outer wall of the particles breaks under stress, the granular liquid inside flows out along the crack due to capillary action and fills the crack. It reacts chemically with the unhydrated substances in the concrete to generate chemical substances that fill the crack densely. On the basis of effectively inhibiting the development of cracks, the self-repair of the invert cracks is achieved.

[0054] The beneficial effects of this invention are as follows:

[0055] 1. A prefabricated inverted arch structure that can effectively prevent tunnel floor heave damage. The inverted arch structure adopts a prefabricated construction hoisting and assembly method, which can effectively shorten the construction period, achieve the goal of rapid ring formation of tunnel cross section, and has low pollution and high construction quality.

[0056] 2. The invert arch is made of fiber-reinforced concrete, which has good deformation coordination characteristics. It can effectively absorb adverse deformations caused by the expansion and contraction of weak surrounding rock and expanding rock, thereby controlling the deformation and crack development of the invert arch.

[0057] 3. The micro-repair particles incorporated into the precast concrete of the prefabricated invert arch center of this invention can break under stress after cracks appear at the center of the invert arch. The internal liquid seeps out and fills the cracks, which can effectively inhibit and repair the development of cracks at the center of the invert arch. It can also limit and repair the damage to the tunnel floor heave within a certain range, so as to achieve the purpose of limiting the development of cracks at the center of the invert arch in a short time. To a certain extent, it can avoid replacing the invert arch during tunnel operation and reduce maintenance costs.

[0058] 4. The three invert arch components are fixed by "mortise and tenon joint + flange joint + concrete grouting" process, which can ensure the overall rigidity performance of the invert arch structure, improve the construction efficiency of the invert arch, achieve the purpose of rapid tunnel ring formation, make the surrounding rock load quickly and evenly distributed, limit the shrinkage deformation of the surrounding rock, and improve the construction quality and structural strength. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the installation cross-section of the prefabricated inverted arch of the present invention;

[0060] Figure 2 This is a schematic diagram of the structure of the left prefabricated block of the present invention;

[0061] Figure 3 This is a schematic diagram of the structure of the central prefabricated block of the present invention;

[0062] Figure 4 This is a schematic diagram of the prefabricated assembled joint of the present invention;

[0063] Figure 5 This is a schematic diagram illustrating the function of the precast concrete repair particles in the center of the invention.

[0064] Figure 6 This is a schematic diagram of the concrete thickness h at the bottom of the inverted arch of the present invention;

[0065] Figure 7 This is a flowchart of the prefabricated invert arch manufacturing process of the present invention;

[0066] In the diagram: 1-Left precast block, 2-Right precast block, 3-Center precast block, 4-Positioning hole, 5-Positioning column, 6-Precast connecting steel bar, 7-Positioning steel bar, 8-Positioning flange, 9-Hole. Detailed Implementation

[0067] The present invention will be further described below with reference to the accompanying drawings:

[0068] A precast invert arch for preventing tunnel floor heave deformation, the precast invert arch includes the following steps:

[0069] 1) Design of the shape of the precast invert arch

[0070] The cross-sections of the left precast block 1 and the right precast block 2 are right-angled triangles, with the hypotenuse of the triangles being an arc shape that fits into the tunnel; the cross-section of the central precast block 3 is rectangular, with the bottom surface of the central precast block 3 being an arc shape that fits into the tunnel; the left precast block 1, the right precast block 2, and the central precast block 3 are all designed as hollow cavities, with the shape of the hollow part being similar to the shape of each precast block. After assembly, the hollow part of each precast block is longitudinally connected along the tunnel; precast assembly joints are provided on the contact surfaces between the left precast block 1, the right precast block 2, and the central precast block 3, and each precast block is connected through the precast assembly joints;

[0071] The upper left side of the left precast block 1 is provided with an outwardly protruding pre-embedded connecting steel bar 6 for connecting the lining; the upper left side of the right precast block 2 is provided with a corresponding pre-embedded connecting steel bar 6.

[0072] 2) Design of precast invert arch dimensions

[0073] like Figure 1As shown, the dimensions of the precast invert arch are determined based on the tunnel radius and the thickness of the primary and secondary lining concrete. The precast invert arch includes a left precast block 1, a right precast block 2, and a central precast block 3. The circumferential width of the left and right precast blocks 1 and 2 is 180–250 cm, and the circumferential width of the central precast block 3 is 130–190 cm. The hollow portion of each precast block divides the thickness of the invert arch into a top thickness and a bottom thickness. The bottom thickness should be greater than the top thickness to ensure structural safety and deformation coordination at the bottom of the invert arch. The thickness is 20-40cm; the bottom thickness is 30-40cm, and the sum of the top and bottom thicknesses is the design invert arch thickness of 50-80cm; the left precast block 1 is laid on the left side of the tunnel, the right precast block 2 is laid on the right side of the tunnel, and the central precast block 3 is laid between the left precast block 1 and the right precast block 2. The sum of the lengths of the top surfaces of the left precast block 1, the right precast block 2, and the central precast block 3 is the width of the tunnel horizontal plane, which is 490-690cm. The widths of the left precast block 1, the right precast block 2, and the central precast block 3 are equal, which is 120-200cm. Figure 2 and 3 As shown, the left precast block 1 has several positioning holes 4 on its rear end face, the positioning holes 4 are perpendicular to the rear end face, and several outwardly extending positioning steel bars 7 are fixed at the bottom of the positioning holes 4; the left precast block 1 has several outwardly protruding positioning posts 5 on its front end face, the positioning posts 5 correspond one-to-one with the positioning holes 4, the front end of the positioning posts 5 is fixed with a forward-extending positioning flange 8, and the edge of the positioning flange 8 is provided with holes 9 for connecting the positioning steel bars 7; the left precast block 1 also has positioning holes 4 and positioning steel bars 7 on its right side face.

[0074] The right precast block 2 is provided with positioning holes 4, positioning steel bars 7, positioning columns 5 and positioning flanges 8 corresponding to the left precast block 1.

[0075] The rear end face of the central precast block 3 is provided with several positioning holes 4, and positioning steel bars 7 are provided in the positioning holes 4; the front end face of the central precast block 3 is provided with corresponding positioning posts 5 and positioning flanges 8; the left side and right side face of the central precast block 3 are respectively provided with positioning posts 5 and positioning flanges 8, the positioning posts 5 on the left side face of the central precast block 3 correspond to the positioning holes 4 on the right side face of the left precast block 1, and the positioning posts 5 on the right side face of the central precast block 3 correspond to the positioning holes 4 on the left side face of the right precast block 2.

[0076] The center of the left precast block 1, the right precast block 2, and the center precast block 3 is hollow, and the cavity extends through the front and back surfaces of the precast block along the width direction.

[0077] The concrete material composition and mix proportions of the left precast block 1 and the right precast block 2 are as follows: 550-580 parts of silicate cement, 230-260 parts of fly ash, 1000-1200 parts of sand, 40-60 parts of silica fume, 50-83 parts of polyvinyl alcohol fiber, and 150-180 parts of water, mixed in proportion.

[0078] 3) Fabrication of precast invert arches

[0079] Based on the dimensions and shape determined in steps 1) and 2), precast invert arches are produced. Left precast block 1 and right precast block 2 are produced using reinforced concrete with a steel frame, cast according to design specifications. The central precast block 3 incorporates concrete repair particles into the fiber-reinforced concrete material. The mold for the precast invert arch blocks and the manufacturing process of the precast invert arch blocks can be implemented with reference to existing technologies, such as Chinese Patent CN 109676749 A. The preparation method of the self-healing concrete particles of this invention can be implemented using existing technologies, such as Chinese Patent CN 103396652 B - A self-healing microcapsule for asphalt concrete cracks and its preparation method.

[0080] 4) Installation of precast invert arches at the construction site

[0081] Based on the tunnel excavation method and initial support construction, after the invert arch excavation is completed, a crushed stone concrete cushion layer is laid under the invert arch and leveled. The three prefabricated invert arch blocks of the first group are positioned and placed in the designed location. The specific installation steps are as follows: the left prefabricated block 1 is assembled and connected with the center prefabricated block 3, and the center prefabricated block 3 is assembled and connected with the right prefabricated block 2. Then, the three prefabricated invert arch blocks of the second group are installed. The left prefabricated block 1 is assembled and connected with the center prefabricated block 3 and the left prefabricated block 1 of the first group. The center prefabricated block 3 is assembled and connected with the right prefabricated block 2 and the center prefabricated block 3 of the first group. The right prefabricated block 2 is assembled and connected with the right prefabricated block 2 of the first group.

[0082] The precast connecting steel bars 6 at both ends of the left precast block 1 and the right precast block 2 are respectively connected to the cast-in-place secondary lining above. The outer ends of the top of the left precast block 1 and the right precast block 2 are provided with outwardly protruding concrete blocks along the width direction, and the precast connecting steel bars are cast and fixed on the outwardly protruding concrete blocks; the lower end of the secondary lining is a corresponding concave concrete block, and the precast connecting steel bars 6 are cast and fixed on the concave concrete block.

[0083] 5) Road paving

[0084] Construction joints at the splicing points of precast blocks are filled with concrete grout; left precast block 1, right precast block 2 and central precast block 3 are connected to form a box-shaped precast inverted arch; after the tunnel deformation stabilizes, the construction of cast-in-place concrete cushion, drainage ditch inspection hole cover plate, cable trench, track bed or other components is carried out.

[0085] A design method for a precast invert arch to prevent tunnel floor heave deformation, comprising the following steps:

[0086] 1) Determine the burial depth, radius, surrounding rock grade, and thickness and grade parameters of the primary and secondary lining concrete for the tunnel project;

[0087] 2) Take samples of expansive rock and soil from the tunnel construction site and obtain the elastic modulus E, cohesion c, and internal friction angle of the expansive rock through testing. Determine the elastic modulus of expansive rock after plastic softening under time effects. Cohesion internal friction angle

[0088]

[0089] In Equation ①: α, β, and ξ are the weakening coefficients of the elastic modulus, cohesion, and internal friction angle of the surrounding rock, respectively, which are determined according to the properties of the expansive soil at the engineering site and the design specifications;

[0090] 3) Calculate the concrete support force P1 of the lining and the concrete support force P2 of the secondary lining;

[0091]

[0092] In equation ②: σ s1 σ s2 t1 and t2 are the yield stresses of the initial and secondary lining concrete, respectively; t1 and t2 are the thicknesses of the initial and secondary lining concrete, respectively; R0 is the radius of the tunnel cavities; R1 is the radius of the outer boundary of the tunnel after the initial lining is applied.

[0093] 4) Design of invert arch stress parameters

[0094] The yield stress of the invert arch should not exceed the yield stress of the concrete in the upper structure of the invert arch; the mix proportions and types of the invert arch materials should be adjusted according to the lining concrete and the secondary lining concrete to meet the yield stress requirements.

[0095] 5) Design of displacement and deformation parameters at the bottom of the invert arch

[0096] Based on the surrounding rock grade, tunnel cross-sectional dimensions, and support parameters in the design specifications, the ideal elastic-plastic displacement u is calculated respectively. elas-plas Rheological displacement u caused by weakening of surrounding rock parameters under long-term rheological action rheo The difference between the two, Δu, is absorbed by the deformation coordination characteristics at the bottom of the invert arch; therefore, Δu is the displacement deformation parameter at the bottom of the invert arch.

[0097]

[0098] In equation ③, v is the Poisson's ratio of the surrounding rock; P0 is the original rock stress; R0 is the radius of the tunnel cavities; P s For the resistance of surrounding rock support, P s =P1 + P2;

[0099] 6) Design of the thickness at the bottom of the invert arch

[0100] like Figure 6As shown, since the compression thickness of the invert arch material is between 40% and 70%, in order to ensure the redundancy safety while allowing the bottom to exert its deformation effect, the optimal compression thickness is taken as 55% to 65%. That is, when the deformation coordination material at the bottom of the invert arch is compressed to 55% to 65% of its original thickness, it is the thickness of the bottom of the invert arch.

Claims

1. A precast invert arch for preventing tunnel floor bulging deformation, characterized in that, The construction method for precast invert arches includes the following steps: 1) Design of the shape of the precast invert arch The cross-sections of the left precast block (1) and the right precast block (2) are right-angled triangles, and the hypotenuse of the triangle is an arc that fits the tunnel. The cross-section of the central precast block (3) is rectangular, and the bottom surface of the central precast block (3) is an arc that fits the tunnel. The left precast block (1), the right precast block (2) and the central precast block (3) are all designed as cavities. The shape of the cavity is similar to that of each precast block. After assembly, the cavity of each precast block should be connected along the longitudinal direction of the tunnel. The contact surfaces between the left precast block (1), the right precast block (2) and the central precast block (3) are respectively provided with precast assembly joints. Each precast block is connected by grouting after positioning and docking through the precast assembly joints. The left precast block (1) has an outwardly protruding embedded connecting steel bar (6) at the upper left side for connecting the lining; the right precast block (2) has a corresponding embedded connecting steel bar (6) at the upper left side. 2) Design of precast invert arch dimensions Based on the tunnel radius and the thickness of the primary support and secondary lining concrete, the dimensions of the precast invert are determined. The circumferential width of the left precast block (1) and right precast block (2) of the precast invert is 180-250cm, and the circumferential width of the central precast block (3) is 130-190cm. The hollow portion of each precast block divides the thickness of the invert into a top thickness and a bottom thickness. The bottom thickness should be greater than the top thickness to ensure structural safety and deformation coordination at the bottom of the invert. The top thickness is 20-40cm, and the bottom thickness is 30-40cm. The sum of the lengths is the design arch thickness of 50-80cm; the left precast block (1) is laid on the left side of the tunnel foundation, the right precast block (2) is laid on the right side of the tunnel foundation, and the center precast block (3) is laid between the left precast block (1) and the right precast block (2). The sum of the lengths of the top surfaces of the left precast block (1), the right precast block (2) and the center precast block (3) is the width of the tunnel horizontal plane of 490-690cm. The widths of the left precast block (1), the right precast block (2) and the center precast block (3) along the tunnel excavation direction are equal to 120-200cm. 3) Fabrication of precast invert arches Based on the dimensions and shape determined in steps 1) and 2), precast invert arches are produced; the left precast block (1) and the right precast block (2) are produced by casting fiber concrete material with steel reinforcement skeleton according to the design specifications, and the center precast block (3) has concrete repair particles added to the fiber concrete material. 4) Installation of precast invert arches at the construction site According to the tunnel excavation method and the initial support construction, after the invert arch is excavated, a crushed stone concrete cushion layer is laid under the invert arch and leveled; the three prefabricated invert arch blocks of the first group are positioned and placed in the design position; the specific installation steps are as follows: the left prefabricated block (1) is assembled and connected with the center prefabricated block (3), and the center prefabricated block (3) is assembled and connected with the right prefabricated block (2); then the three prefabricated invert arch blocks of the second group are installed, the left prefabricated block (1) is assembled and connected with the center prefabricated block (3) and the left prefabricated block (1) of the first group, the center prefabricated block (3) is assembled and connected with the right prefabricated block (2) and the center prefabricated block (3) of the first group, and the right prefabricated block (2) is assembled and connected with the right prefabricated block (2) of the first group; The pre-embedded connecting steel bars at both ends of the left precast block (1) and the right precast block (2) are respectively connected to the cast-in-place secondary lining above; 5) Road paving Construction joints at the splicing points of precast blocks are filled with concrete grout; the left precast block (1), the right precast block (2) and the central precast block (3) are connected to form a box-shaped precast inverted arch; after the tunnel deformation stabilizes, the construction of cast-in-place concrete cushion, drainage ditch inspection hole cover plate, cable trench, track bed or other components is carried out.

2. The precast invert arch for preventing tunnel floor bulging deformation according to claim 1, characterized in that, The top outer ends of the left precast block (1) and the right precast block (2) are provided with outwardly protruding concrete blocks along the width direction, and pre-embedded connecting steel bars are poured and fixed on the outwardly protruding concrete blocks; the lower end of the secondary lining is a corresponding concave concrete block, and pre-embedded connecting steel bars are poured and fixed on the concave concrete block.

3. The precast invert arch for preventing tunnel floor bulging deformation according to claim 1, characterized in that, The left precast block (1) has several positioning holes (4) on its rear end face. The positioning holes (4) are perpendicular to the rear end face. Several outwardly extending positioning steel bars (7) are fixed at the bottom of the positioning holes (4). The left precast block (1) has several outwardly protruding positioning posts (5) on its front end face. The positioning posts (5) correspond one-to-one with the positioning holes (4). The front end of the positioning posts (5) is fixed with a forward-extending positioning flange (8). The edge of the positioning flange (8) is provided with holes (9) for connecting the positioning steel bars (7). The right side of the left precast block (1) is also provided with positioning holes (4) and positioning steel bars (7). The right precast block (2) is provided with positioning holes (4), positioning steel bars (7), positioning columns (5) and positioning flanges (8) corresponding to the left precast block (1).

4. The precast invert arch for preventing tunnel floor bulging deformation according to claim 3, characterized in that, The rear end face of the central precast block (3) is provided with several positioning holes (4), and positioning steel bars (7) are provided in the positioning holes (4); the front end face of the central precast block (3) is provided with corresponding positioning posts (5) and positioning flanges (8); the left side and right side of the central precast block (3) are respectively provided with positioning posts (5) and positioning flanges (8), the positioning posts (5) on the left side of the central precast block (3) correspond to the positioning holes (4) on the right side of the left precast block (1), and the positioning posts (5) on the right side of the central precast block (3) correspond to the positioning holes (4) on the left side of the right precast block (2).

5. The precast invert arch for preventing tunnel floor bulging deformation according to claim 4, characterized in that, The center of the left precast block (1), right precast block (2), and center precast block (3) is hollow, and the cavity extends through the front and rear surfaces of the precast block along the width direction.

6. The precast invert arch for preventing tunnel floor bulging deformation according to claim 5, characterized in that, The concrete material composition and proportion of the left precast block (1) and right precast block (2) are as follows: 550-580 parts of silicate cement, 230-260 parts of fly ash, 1000-1200 parts of sand, 40-60 parts of silica fume, 50-83 parts of polyvinyl alcohol fiber, and 150-180 parts of water are mixed in proportion.

7. A design method for a precast invert arch for preventing tunnel floor heave deformation as described in any one of claims 1-6, characterized in that, The design method steps are as follows: 1) Determine the burial depth, radius, surrounding rock grade, and thickness and grade parameters of the primary and secondary lining concrete for the tunnel project; 2) Take samples of expansive rock and soil from the tunnel construction site and obtain the elastic modulus E, cohesion c, and internal friction angle of the expansive rock through testing. Determine the elastic modulus of expansive rock after plastic softening under time effects. Cohesion internal friction angle In Equation ①: α, β, and ξ are the weakening coefficients of the elastic modulus, cohesion, and internal friction angle of the surrounding rock, respectively, which are determined according to the properties of the expansive soil at the engineering site and the design specifications; 3) Calculate the concrete support force P1 of the lining and the concrete support force P2 of the secondary lining; In equation ②: σ s1 σ s2 t1 and t2 are the yield stresses of the initial and secondary lining concrete, respectively; t1 and t2 are the thicknesses of the initial and secondary lining concrete, respectively; R0 is the radius of the tunnel cavities; R1 is the radius of the outer boundary of the tunnel after the initial lining is applied. 4) Design of invert arch stress parameters The yield stress of the invert arch is not greater than the yield stress of the concrete in the upper structure of the invert arch. The mix proportions and types of materials for the invert arch are adjusted according to the lining concrete and secondary lining concrete to meet the yield stress requirements. 5) Design of displacement and deformation parameters at the bottom of the invert arch Based on the surrounding rock grade, tunnel cross-sectional dimensions, and support parameters in the design specifications, the ideal elastic-plastic displacement u is calculated respectively. elas-plas Rheological displacement u caused by weakening of surrounding rock parameters under long-term rheological action rheo The difference between the two, Δu, is absorbed by the deformation coordination characteristics at the bottom of the invert arch; therefore, Δu is the displacement deformation parameter at the bottom of the invert arch. Δu=u rheo -u elas-plas ; In equation ③, v is the Poisson's ratio of the surrounding rock; P0 is the original rock stress; R0 is the radius of the tunnel cavities; P s For the resistance of surrounding rock support, P s =P1 + P2; 6) Design of the thickness at the bottom of the invert arch Since the compression thickness of the invert arch material is between 40% and 70%, in order to ensure that the bottom can exert its deformation effect while ensuring redundancy and safety, the optimal compression thickness is 55% to 65%. That is, when the deformation coordination material at the bottom of the invert arch is compressed to 55% to 65% of its original thickness, it is the thickness of the bottom of the invert arch.

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