Support system and construction method of hard rock long-span flat cavern

Abstract

The present application relates to a kind of hard rock giant span flat cavern supporting system and its construction method, including primary support, secondary lining and lattice beam system from outside to inside in sequence;Secondary lining includes arch part giant span variable arch cover structure, arch corner longitudinal hidden beam and side wall secondary lining;Side wall secondary lining is located at the bottom of the transverse end of giant span variable arch cover structure;Arch corner longitudinal hidden beam is connected between the transverse end of giant span variable arch cover structure and the top of side wall secondary lining, is arranged longitudinally and is located on hard surrounding rock;Lattice beam system is located in the inside of secondary lining.The design of arch part variable arch cover of the present application is in line with cavern stress mode, which enhances the integrity and stability of arch cover, the hinge joint design of side wall and arch part is beneficial to the stability of side wall structure, lattice beam system can make stress transmission more dispersed, and the overall structure is good, suitable for the construction of hard rock giant span flat cavern.

Description

Technical Field

[0001] This invention relates to the field of underground engineering construction technology, specifically to a support system and construction method for a large-span, flat cavern in hard rock. Background Technology

[0002] With the rapid development of energy development, transportation construction, and the development and utilization of large underground spaces in cities, the demand for hard rock mega-span flat caverns is increasing, such as underground powerhouses of pumped storage power stations, underground railway stations, and large underground storage facilities. These caverns are characterized by large spatial spans, flat cross-sections, and complex stress distribution in the surrounding rock. Especially in hard rock strata, although the rock mass itself has high strength, stress concentration zones are easily generated at the junction of the cavern arch walls due to excavation unloading, stress release, and tectonic fissures, making it difficult for traditional support systems to effectively control the deformation and stability of the surrounding rock.

[0003] At present, the initial support in hard rock cavern engineering is generally constructed by combining shotcrete and anchor support with steel arch support. The secondary lining is made of cast-in-place concrete for the arch and sidewalls. However, its application in the support of large-span flat caverns has certain limitations: (1) The shotcrete structure has poor crack resistance and is prone to radial cracks due to bending tensile stress in the high stress environment of hard rock, which weakens the integrity and waterproof performance; (2) The cavern has a large span and a large radius of curvature, and the steel arch frame is difficult to fit tightly against the surrounding rock. After installation, "void behind the frame" is likely to appear, resulting in uneven stress transmission and failure to form effective support; (3) A large stress concentration occurs at the junction of the secondary lining sidewall and the arch, which can easily lead to concrete cracking and failure, and ultimately the overall structure fails.

[0004] Therefore, it is necessary to propose new support methods and measures for mega-span flat caverns in hard rock to ensure the safety of the cavern support structure. Summary of the Invention

[0005] The purpose of this invention is to provide a support system and construction method for a large-span flat cavern in hard rock, so as to solve the limitations of existing support methods in the construction of large-span flat caverns.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A support system for a mega-span flat cavern in hard rock is provided, the support system comprising, from the outside to the inside, initial support, secondary lining and lattice beam system;

[0008] The secondary lining includes a giant-span gradually changing arch structure, longitudinal hidden beams at the arch corners, and secondary lining of the side walls; the secondary lining of the side walls is located at the bottom of both ends of the giant-span gradually changing arch structure and is arranged vertically in the longitudinal direction; the longitudinal hidden beams at the arch corners are enlarged arch corners of the giant-span gradually changing arch structure, connecting the transverse ends of the giant-span gradually changing arch structure and the top of the secondary lining of the side walls, and are arranged longitudinally and rest on the hard surrounding rock;

[0009] The lattice beam system is located inside the secondary lining.

[0010] Furthermore, the initial support includes an anchor system and shotcrete with steel fiber reinforced concrete;

[0011] The scope of the sprayed steel fiber concrete includes the top of the mega-span gradient arch structure, the outer side of the longitudinal hidden beam at the arch corner, and the outer side of the secondary lining of the sidewall;

[0012] The anchor bolts are installed in hard surrounding rock and include prestressed long anchor bolts with large spacing and short anchor bolts with small spacing.

[0013] Furthermore, the top of the giant span gradient arch structure and the top of the longitudinal hidden beam at the arch corner are provided with prestressed long anchor rods with large spacing, which are evenly arranged in the circumferential direction, and multiple short anchor rods with small spacing are provided between adjacent prestressed long anchor rods in the circumferential direction.

[0014] The small-spaced short anchor rods are installed on the outer side of the secondary lining of the side wall.

[0015] Furthermore, the lattice beam system includes circumferential lattice beams and longitudinal lattice beams;

[0016] The circumferential lattice beam extends to the bottom of the mega-span gradually changing arch structure and the inner side of the secondary lining of the sidewall;

[0017] The longitudinal lattice beam is located on the inner side of the secondary lining of the side wall and intersects with the circumferential lattice beam.

[0018] Furthermore, the lattice beam system is a cast-in-place reinforced concrete structure, and the circumferential lattice beam and the longitudinal lattice beam are integrally cast.

[0019] Furthermore, the giant-span gradually changing arch structure is a cast-in-place reinforced concrete structure with a thickness that gradually increases from the arch crown to the arch corner.

[0020] Furthermore, the transverse end of the mega-span gradient arch structure is connected to the top of the secondary lining of the side wall through a hinged joint, which is located inside the longitudinal hidden beam at the arch corner.

[0021] Furthermore, the hinged joint includes an upper support plate and a lower support plate. The bottom surface of the upper support plate is a convex cylindrical surface, and the top surface of the lower support plate is a concave cylindrical surface. The convex cylindrical surface is embedded downward into the concave cylindrical surface, and a cylindrical sliding plate is sandwiched between the two.

[0022] The upper support plate is connected to the upper giant span gradual arch structure by anchor bolts, and the lower support plate is connected to the lower side wall secondary lining by anchor bolts.

[0023] A flexible sealing material is also provided on the contact interface between the upper support plate and the lower support plate.

[0024] Furthermore, a horizontal base plate is provided at the bottom of the cavern, and the base plate is connected to the secondary lining of the side walls on both sides of the lateral direction and is circumferentially closed.

[0025] On the other hand, a construction method for a support system for a hard rock mega-span flat cavern is provided, the construction method comprising:

[0026] The cross-section of the cavern is divided into the central guide tunnel of the arch, the upper side guide tunnel, the reserved rock pillar of the arch, the rock mass of the lower guide tunnel, and the reserved rock platform of the lower section. The upper side guide tunnel is located on both sides of the central guide tunnel of the arch, the reserved rock pillar of the arch is located between the central guide tunnel of the arch and the upper side guide tunnel, the rock mass of the lower guide tunnel is located below the upper side guide tunnel, and the reserved rock platform of the lower section is located below the central guide tunnel of the arch and between the upper side guide tunnel and the rock mass of the lower guide tunnel on both sides of the arch.

[0027] Excavate the central guide tunnel and the upper side guide tunnel of the arch. After the excavation is completed, install prestressed long anchor rods with large spacing and short anchor rods with small spacing, and then construct sprayed steel fiber concrete.

[0028] Rock pillars were reserved in the excavation arch. After the excavation was completed, prestressed long anchor rods with large spacing and short anchor rods with small spacing were installed, and shotcrete was applied.

[0029] A large-span gradually changing arch cover structure and longitudinal hidden beams at the arch corners are constructed in the arch of the cave to form a stable arch-shaped load-bearing structure.

[0030] Continue excavating the rock mass of the lower pilot tunnel, install short anchor bolts with small spacing, and construct shotcrete.

[0031] Excavate the lower reserved rock platform, construct the secondary lining of the side wall, and connect the secondary lining of the side wall to the giant span gradual arch structure with a hinged joint.

[0032] Circumferential and longitudinal lattice beams are constructed on the inner side of the secondary lining to form a stable lattice beam system;

[0033] Construct the base slab and complete the construction.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention provides a support system and construction method for a large-span flat cavern in hard rock, which is suitable for the support of large-span flat caverns with a span of more than 35m. It has strong bearing capacity, good stability, strong deformation resistance, and takes into account both safety and economy. This method involves installing anchor bolts of varying lengths at different intervals within the cavern. Longer anchor bolts secure the deep, stable rock mass, while shorter anchor bolts reinforce the shallow, fractured zone. Combined with differential prestressing design, this creates a three-dimensional load-bearing structure that combines deep and shallow rock layers and controls tension and shear stress. The use of sprayed steel fiber reinforced concrete effectively controls radial cracks caused by bending tensile stress, ensuring the concrete's strength and waterproofing performance. The arch adopts a gradually changing arch structure, conforming to the cavern's arch stress pattern. The enlarged angle design at the arch corners effectively controls structural damage caused by stress concentration, ensuring both safety and economy. Longitudinal hidden beams at the arch corners provide a solid foundation for the arch cap, enhancing its overall integrity and distributing stress. The hinged connection between the sidewalls and the arch prevents excessive bending moments at the arch corners from being transferred to the sidewalls, while also constraining sidewall displacement, thus contributing to the stability of the sidewall structure. The inclusion of circumferential and longitudinal grid beams within the secondary lining further enhances the overall structural integrity, effectively preventing excessive localized deformation and subsequent structural damage. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained from these drawings without creative effort.

[0037] Figure 1 A cross-sectional view of the support system for a mega-span flat cavern in hard rock provided in an embodiment of the present invention.

[0038] Figure 2 This is a longitudinal section view of the support system for a large-span, flat cavern in hard rock, provided in an embodiment of the present invention.

[0039] Figure 3 A three-dimensional rendering of the support system for a mega-span flat cavern in hard rock provided in an embodiment of the present invention.

[0040] Figure 4 The diagram shows the method steps S2 provided in the embodiment of the present invention.

[0041] Figure 5 The diagram shows the method step S3 provided in the embodiment of the present invention.

[0042] Figure 6 The diagram shows the method step S4 provided in the embodiment of the present invention.

[0043] Figure 7 The diagram shows the method step S5 provided in the embodiment of the present invention.

[0044] Figure 8 The diagram shows the method step S6 provided in the embodiment of the present invention.

[0045] Figure 9 The diagram shows the method step S7 provided in the embodiment of the present invention.

[0046] Figure 10 This is a schematic diagram showing the connection between the giant-span gradually changing arch structure and the secondary lining of the side wall provided in an embodiment of the present invention.

[0047] The diagram is labeled as follows:

[0048] 1-Prestressed long anchor bolts with large spacing, 2-Short anchor bolts with small spacing, 3-Shotcrete steel fiber concrete, 4-Giant span gradually changing arch structure, 5-Longitudinal hidden beams at the arch corners, 6-Secondary lining of side walls, 7-Hinged joints, 8-Circular lattice beams, 9-Longitudinal lattice beams, 10-Base plate.

[0049] A - Middle guide tunnel of the arch, B - Upper guide tunnel on the side, C - Reserved rock pillar in the arch, D - Lower guide tunnel on the side, E - Reserved rock platform in the lower part;

[0050] 71-Upper support plate, 72-Lower support plate, 73-Cylindrical sliding plate, 74-Anchor bolt, 75-Flexible filling material. Detailed Implementation

[0051] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0052] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0053] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connection," "setting," etc., should be interpreted broadly. For example, they can refer to a fixed connection or setting, a detachable connection or setting, or an integral connection or setting. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0054] It should also be noted that although the order of steps is mentioned in the method description, in some cases, steps may be performed in a different order than that described here, and this should not be interpreted as a restriction on the order of steps.

[0055] In a specific implementation, the length direction of the cavern is defined as longitudinal, and the direction perpendicular to it is defined as transverse (i.e., Figure 1 (From left to right) The side closer to the surrounding rock is defined as the outer side, and the side closer to the center of the cave is defined as the inner side.

[0056] Traditional support construction methods for hard rock caverns cannot be directly applied to large-span flat caverns. They have limitations in terms of structural crack resistance and support capacity, and stress concentration areas cannot be avoided in the structure, which can easily lead to defects such as concrete cracking.

[0057] Therefore, this invention provides a support system and construction method for a large-span flat cavern in hard rock. In view of the characteristics of large-span flat caverns such as large spatial span, flat cross-section, and complex stress distribution in the surrounding rock, the construction structure and construction steps have been optimized and improved. It can be used for the support construction of large-span flat caverns with a span of more than 35m and a height of about 10m.

[0058] like Figure 1-3 The support system for a mega-span flat cavern in hard rock according to the present invention comprises, from the outside in, initial support, secondary lining, and a lattice beam system. The initial support includes an anchor system and shotcrete 3. The secondary lining is located inside the initial support and includes the mega-span gradually changing arch structure 4 of the arch section, longitudinal hidden beams 5 at the arch corners on both sides, and secondary lining of the sidewalls 6. The lattice beam system is located inside the secondary lining.

[0059] Specifically:

[0060] In the secondary lining, the secondary lining 6 of the sidewall is located at the bottom of both transverse ends of the mega-span gradually changing arch structure 4 and is arranged vertically in the longitudinal direction. The secondary lining 6 of the sidewall is cast-in-place reinforced concrete with a thickness of 500mm and a concrete strength grade of not less than C40. The mega-span gradually changing arch structure 4 adopts an enlarged arch angle design. The longitudinal hidden beam 5 at the arch angle is the enlarged arch angle of the mega-span gradually changing arch structure 4, connecting the transverse end of the mega-span gradually changing arch structure 4 and the top of the secondary lining 6 of the sidewall, arranged longitudinally and directly resting on the hard surrounding rock. The longitudinal hidden beam at the arch angle provides a solid foundation for the arch and enhances the integrity of the arch, allowing stress to be distributed and transferred to the surrounding rock.

[0061] In the initial support, the sprayed steel fiber reinforced concrete 3 extends to the top of the mega-span gradually changing arch structure 4, the outer side of the longitudinal hidden beam 5 at the arch corner, and the outer side of the secondary lining 6 of the sidewall. The use of sprayed steel fiber reinforced concrete effectively controls radial cracks caused by bending tensile stress, ensuring the strength and waterproofing performance of the concrete. The anchor system is installed within the hard surrounding rock, including prestressed long anchors 1 with large spacing and short anchors 2 with small spacing. In this invention, the construction scheme for the anchor system in different areas may vary.

[0062] (1) Arch assembly scheme:

[0063] The top of the giant span gradual arch structure 4 and the top of the longitudinal hidden beam 5 at the arch corner are equipped with prestressed long anchor rods 1 with large spacing, which are evenly arranged in the circumference. Multiple short anchor rods 2 with small spacing are set between adjacent prestressed long anchor rods 1 in the circumference. The prestressed long anchor rods 1 with large spacing and short anchor rods 2 with small spacing can be arranged in a quincunx pattern.

[0064] (2) Single sidewall option:

[0065] Short anchor rods with small spacing are installed on the outer side of the secondary lining 6 of the side wall.

[0066] This invention involves installing anchor bolts of varying lengths at different intervals within the tunnel. Longer anchor bolts secure the deep, stable rock mass, while shorter anchor bolts reinforce the shallow, fractured zone. Combined with differential prestressing design, this creates a three-dimensional load-bearing structure that combines deep and shallow rock formations with separate tension and shear control. Furthermore, the specific anchor bolt system construction plan can be flexibly adjusted according to the geological conditions and support requirements of the specific construction site, and is not limited to the two plans mentioned above. The anchor bolt length and spacing can also be determined based on specific geological conditions.

[0067] The lattice beam system includes circumferential lattice beams 8 and longitudinal lattice beams 9. The beams have rectangular cross-sections, with widths ranging from 250 to 400 mm and heights from 400 to 500 mm. The circumferential lattice beams 8 extend from the bottom of the mega-span gradually changing arch structure 4 and the inner side of the secondary lining 6 of the side walls, circumferentially extending upwards from the bottom of the left side wall to the bottom of the right side wall. The longitudinal lattice beams 9 are located on the inner side of the secondary lining 6 of the side walls and intersect with the circumferential lattice beams 8. Therefore, the lattice beam system of the arch includes longitudinal multi-ring arch beams, while the lattice beam system of the side walls is a grid-like planar beam network. The lattice beam system is a cast-in-place reinforced concrete structure. The circumferential lattice beams 8 and longitudinal lattice beams 9 are integrally cast and poured simultaneously to form a single, unified structure. The lattice beam system further enhances the overall structural integrity and prevents excessive localized deformation due to stress, which could lead to localized structural damage.

[0068] The large-span gradually changing arch structure 4 is a cast-in-place reinforced concrete structure with a thickness that gradually increases from the arch crown to the arch corners, ranging from 900 to 1800 mm, and the concrete strength grade is not lower than C40. This invention adopts a gradually changing arch structure, which conforms to the stress mode of the cavern arch. The enlarged angle design at the arch corners can effectively control structural damage caused by stress concentration, ensuring safety while also taking into account economy.

[0069] The transverse end of the mega-span gradually changing arch structure 4 is connected to the top of the secondary lining 6 of the sidewall via a hinged joint 7, which is located inside the longitudinal hidden beam 5 at the arch corner. The hinged joint 7 is a hinged support, comprising an upper support plate 71 and a lower support plate 72. The bottom surface of the upper support plate 71 is a convex cylindrical surface, and the top surface of the lower support plate 72 is a concave cylindrical surface. The convex cylindrical surface is embedded downwards into the concave cylindrical surface, and a sliding plate 73 is sandwiched between them to reduce contact friction. The upper support plate 71 is connected to the upper mega-span gradually changing arch structure 4 via anchor bolts 74, and the lower support plate 72 is connected to the lower secondary lining 6 of the sidewall via anchor bolts 74. Additionally, a flexible sealing material 75 is provided on the contact interface between the upper support plate 71 and the lower support plate 72. The hinged connection between the sidewall and the arch avoids excessive bending moment at the arch corner from being transmitted to the sidewall, while also constraining sidewall displacement, thus contributing to the stability of the sidewall structure.

[0070] The bottom of the cavern is provided with a horizontal base plate 10, which is connected to the secondary lining 6 of the side walls on both sides and is circumferentially closed.

[0071] like Figure 4-9 The construction method for the support system of the aforementioned hard rock mega-span flat cavern specifically includes the following steps:

[0072] S1: Divide the cross-section of the cavern into sections.

[0073] Specifically, the cavern cross-section is divided into four parts: the central arch guide tunnel A, the upper side guide tunnel B, the arch-reserved rock pillar C, the lower guide tunnel rock mass D, and the lower reserved rock platform E. The upper side guide tunnel B is located on both sides of the central arch guide tunnel A laterally; the arch-reserved rock pillar C is located between the central arch guide tunnel A and the upper side guide tunnel B; the lower guide tunnel rock mass D is located below the upper side guide tunnel B; and the lower reserved rock platform E is located below the central arch guide tunnel A and between the upper side guide tunnel B and the lower guide tunnel rock mass D laterally. The bottom of the upper side guide tunnel B is lower than the bottoms of the central arch guide tunnel A and the arch-reserved rock pillar C, forming a U-shaped structure with an opening facing downwards. This ensures that the construction space on both sides and in the middle is roughly the same size, and that the construction efficiency and progress of the different guide tunnels are consistent, allowing for a steady advancement of cavern construction.

[0074] S2: Excavate the intermediate guide tunnel A and the upper side guide tunnel B of the arch. After excavation, promptly apply initial support to the exposed face, including installing prestressed long anchor bolts with large spacing 1 and short anchor bolts with small spacing 2, and applying shotcrete 3. Depending on the rock mass conditions, temporary support may be applied to the exposed face of the reserved rock pillar.

[0075] S3: Excavate the arch section and reserve rock pillar C. After excavation, promptly implement initial support for the exposed face, including installing prestressed long anchor bolts with large spacing 1 and short anchor bolts with small spacing 2, and constructing shotcrete 3. Depending on the rock mass conditions, temporary support may be implemented for the exposed face of the intermediate reserved rock platform. The area containing the intermediate guide tunnel A, the upper side guide tunnel B, and the reserved rock pillar C of the arch section is connected into a single unit located in the upper part of the tunnel chamber, forming a U-shaped space with an opening facing downwards.

[0076] S4: Construct a large-span gradually changing arch cover structure 4 and longitudinal hidden beams 5 at the arch corners of the cavern to form a stable arch-shaped load-bearing structure.

[0077] S5: Continue excavating the lower pilot tunnel rock mass D, and promptly implement initial support for the excavated free face, including installing short anchor bolts with small spacing 2 and constructing shotcrete 3. Depending on the rock mass conditions, temporary support may be implemented for the free face of the reserved intermediate rock platform. At this point, the two sides of the U-shaped space expand downwards to the bottom of the tunnel.

[0078] S6: Excavate the lower reserved rock platform E, construct the secondary lining of the side wall 6, and connect the secondary lining of the side wall 6 with the giant span gradual arch structure 4 using a hinged node 7.

[0079] The process of installing hinge node 7 is as follows:

[0080] First, the upper support plate 71 is anchored to the giant span gradual arch structure 4 using anchor bolts 74.

[0081] Then, the lower support plate 72 is connected to the secondary lining reinforcement of the side wall through anchor bolts 74, the lower support plate 72 is fixed, and the secondary lining 6 of the side wall is poured.

[0082] Finally, fill the gaps between the upper and lower support plates with flexible sealing material (rubber, silicone, etc.) to wrap the joints and prevent dust and moisture from entering the friction surface.

[0083] The hinged node 7 can be set along the entire longitudinal length or at intervals. The length of each segment and the longitudinal spacing need to be determined based on the actual load-bearing capacity calculation results.

[0084] At this point, the spaces containing the central guide tunnel A, the upper side guide tunnel B, the reserved rock pillar C in the arch, the rock mass D in the lower guide tunnel, and the reserved rock platform E in the lower part are all connected, and the entire interior space of the tunnel has been excavated.

[0085] S7: Construct circumferential lattice beams 8 and longitudinal lattice beams 9 on the inner side of the secondary lining to form a stable lattice beam system.

[0086] The circumferential lattice beams 8 and longitudinal lattice beams 9 are constructed simultaneously, from bottom to top. First, the circumferential and longitudinal lattice beams of the side walls are constructed, followed by the circumferential lattice beams of the arch. Connecting steel bars can be pre-reserved in the arch and side walls, and the formwork can be erected directly for integral casting.

[0087] S8: Construct base plate 10 to complete construction.

[0088] Alternatively, the tunnel can be divided longitudinally into multiple segments, and each segment can be constructed step by step according to a specified advance. The pilot tunnels can also be constructed longitudinally in a staggered manner, with proper temporary supports and reinforcements to ensure the safety of construction inside the tunnel.

[0089] In summary, the gradient arch design of the arch section of this invention conforms to the stress pattern of the cavern, enhancing the integrity and stability of the arch section while balancing safety and economy. The hinged connection design between the sidewalls and the arch section is beneficial to the stability of the sidewall structure. The design of the lattice beam system can further disperse stress transmission and improve the overall structural integrity. The combination of long and short anchor bolts, combined with differential prestressing design, can organically integrate shallow fractured rock mass with deep stable rock mass, forming a three-dimensional load-bearing structure with "deep-shallow combination and tension-shear separation control".

[0090] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. A support system for a large-span, flat cavern in hard rock, characterized by: The support system, from the outside to the inside, includes initial support, secondary lining, and grid beam system. The secondary lining includes a large-span gradually changing arch cover structure (4) of the arch, a longitudinal hidden beam at the arch corner (5) and a secondary lining of the side wall (6); the secondary lining of the side wall (6) is located at the bottom of both ends of the large-span gradually changing arch cover structure (4) and is arranged vertically in the longitudinal direction; the longitudinal hidden beam at the arch corner (5) is an enlarged arch corner of the large-span gradually changing arch cover structure (4), connecting the transverse end of the large-span gradually changing arch cover structure (4) and the top of the secondary lining of the side wall (6), and is arranged longitudinally and rests on the hard surrounding rock; The lattice beam system is located inside the secondary lining; The lattice beam system includes circumferential lattice beams (8) and longitudinal lattice beams (9); The circumferential lattice beam (8) extends to the bottom of the mega-span gradient arch structure (4) and the inner side of the secondary lining (6) of the sidewall; The longitudinal lattice beam (9) is located on the inner side of the secondary lining (6) of the side wall and intersects with the circumferential lattice beam (8); The lattice beam system is a cast-in-place reinforced concrete structure, and the circumferential lattice beam (8) and the longitudinal lattice beam (9) are integrally cast. The giant span gradually changing arch structure (4) is a cast-in-place reinforced concrete structure whose thickness gradually increases from the arch top to the arch corner; The transverse end of the giant span gradient arch structure (4) is connected to the top of the secondary lining (6) of the side wall through a hinged node (7), which is located inside the longitudinal hidden beam (5) of the arch corner. The hinge node (7) includes an upper support plate (71) and a lower support plate (72). The bottom surface of the upper support plate (71) is a convex cylindrical surface, and the top surface of the lower support plate (72) is a concave cylindrical surface. The convex cylindrical surface is embedded downward into the concave cylindrical surface, and a cylindrical sliding plate (73) is sandwiched between the two. The upper support plate (71) is connected to the upper giant span gradual arch structure (4) by anchor bolts (74), and the lower support plate (72) is connected to the lower side wall secondary lining (6) by anchor bolts (74). A flexible sealing material (75) is also provided on the contact interface between the upper support plate (71) and the lower support plate (72).

2. The support system for a hard rock mega-span flat cavern according to claim 1, characterized in that: The initial support includes an anchor system and shotcrete (3). The scope of the sprayed steel fiber concrete (3) extends to the top of the mega-span gradient arch structure (4), the outer side of the longitudinal hidden beam (5) at the arch corner, and the outer side of the secondary lining (6) of the side wall. The anchor system is installed in hard surrounding rock and includes prestressed long anchors with large spacing (1) and short anchors with small spacing (2).

3. The support system for a mega-span flat cavern in hard rock according to claim 2, characterized in that: The top of the giant span gradual arch structure (4) and the top of the longitudinal hidden beam (5) at the arch corner are provided with prestressed large-spacing long anchor rods (1), which are evenly arranged in the circumferential direction. Multiple small-spacing short anchor rods (2) are provided between adjacent prestressed large-spacing long anchor rods (1) in the circumferential direction. The small-pitch short anchor rods (2) are installed on the outer side of the secondary lining (6) of the side wall.

4. The support system for a mega-span flat cavern in hard rock according to claim 1, characterized in that: The bottom of the cavern is provided with a horizontal base plate (10), which is connected to the secondary lining (6) of the side walls on both sides and is circumferentially closed.

5. A construction method for a support system for a hard rock mega-span flat cavern as described in claim 1, characterized in that: The construction method includes: The cross-section of the cavern is divided into the central arch tunnel (A), the upper side tunnel (B), the arch reserved rock pillar (C), the lower tunnel rock mass (D), and the lower reserved rock platform (E). The upper side tunnel (B) is located on both sides of the central arch tunnel (A). The arch reserved rock pillar (C) is located between the central arch tunnel (A) and the upper side tunnel (B). The lower tunnel rock mass (D) is located below the upper side tunnel (B). The lower reserved rock platform (E) is located below the central arch tunnel (A) and between the upper side tunnel (B) and the lower tunnel rock mass (D) on both sides. Excavate the middle guide tunnel (A) and the upper side guide tunnel (B) of the arch. After the excavation is completed, install prestressed long anchor rods with large spacing (1) and short anchor rods with small spacing (2), and construct sprayed steel fiber concrete (3). Excavate the arch and reserve rock pillars (C). After the excavation is completed, install prestressed long anchor rods with large spacing (1) and short anchor rods with small spacing (2). Then, spray steel fiber concrete (3). A large-span gradually changing arch cover structure (4) and longitudinal hidden beams at the arch corners (5) are constructed in the arch of the cave to form a stable arch-shaped load-bearing structure; Continue excavating the lower guide tunnel rock mass (D), install short anchor bolts with small spacing (2), and construct shotcrete (3). Excavate the lower reserved rock platform (E), construct the secondary lining of the side wall (6), and connect the secondary lining of the side wall (6) with the giant span gradual arch structure (4) using a hinged node (7). A circumferential lattice beam (8) and a longitudinal lattice beam (9) are constructed on the inner side of the secondary lining to form a stable lattice beam system; Construct the base plate (10) to complete the construction.

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