A prefabricated paving structure without central support column for semi-cover excavation pits and its construction method

Abstract

This invention relates to the field of semi-cover excavation pit construction equipment technology, specifically a prefabricated cover structure and construction method for semi-cover excavation pits without central support columns. The prefabricated cover structure of this invention includes a retaining structure installed around the perimeter of the pit. The top of the retaining structure is divided into a prefabricated cover side and an uncovered side along the transverse direction of the pit. Prefabricated cover plates are horizontally laid on the prefabricated cover side, with one end of the cover plate forming a fixed end fixed to the top of the retaining structure, and the other end forming a suspended end above the working area within the pit. The suspended end of the prefabricated cover plate and the retaining structure are connected by a diagonal bracing system, which does not interfere with the working area within the pit. This invention provides support for the prefabricated cover plate through the diagonal bracing system, thereby replacing the support columns, increasing the effective space for large equipment construction within the pit, and eliminating space constraints.

Description

Technical Field

[0001] This invention relates to the field of semi-cover excavation foundation pit construction equipment technology, specifically a semi-cover excavation foundation pit prefabricated cover structure without central support column and construction method. Background Technology

[0002] The semi-cut-and-cover method, as a deep foundation pit construction method that integrates the advantages of open-cut and cut-and-cover techniques, sets up a partial road surface cover system along the longitudinal direction of the foundation pit according to the road traffic diversion needs (forming a temporary traffic surface), and uses the uncovered area as an open-cut construction transportation channel. It adopts the process of "top-down" excavation and "bottom-up" construction of the main structure, which effectively expands the construction space in urban restricted sites, alleviates the contradiction of land shortage, and significantly improves the foundation pit excavation efficiency and construction function realization.

[0003] Currently, in the process of semi-cut-and-cover construction, central columns are typically used to support the cover structure in order to help create a certain working space inside the foundation pit. For example, patent CN109356196B provides a steel support and erection structure for a semi-cut-and-cover subway station foundation pit, which uses multiple temporary central columns to support the crossbeams, longitudinal connecting beams, and cover slabs to improve the stability of the support and create a corresponding working space.

[0004] While the central column support system used in the current semi-cut-and-cover method improves support stability, it also significantly restricts the working space inside the excavation pit. These vertically extending central columns are arranged relatively densely within the pit, with their columns and foundation structures occupying a large amount of working space, severely impacting construction activities. The presence of the central columns creates spatial obstacles, forcing large construction machinery to frequently adjust its operating routes and methods, significantly reducing equipment efficiency. Simultaneously, the arrangement of the central columns compresses the effective width of transport channels, forcing originally designed two-way transport routes to become one-way traffic, greatly affecting the efficiency of earthwork and material transportation. Furthermore, the local over-excavation for setting up the column foundations creates irregular elevation changes at the bottom of the pit, which not only increases construction difficulty but also poses a potential risk to the movement safety of heavy equipment. This spatial constraint also affects other auxiliary operations such as material stacking and temporary support installation, ultimately forming a systemic obstacle restricting the overall construction progress. Currently, in the field of semi-cut-and-cover excavation pit construction technology, there are various solutions to address the issues of support and space utilization. Some technologies focus on improving traditional central column support systems, such as optimizing column arrangement and using new column materials to enhance support stability, but they have consistently failed to overcome the limitation of working space imposed by the central column. Patent CN201810241549.8 uses a triangular truss to replace the temporary support columns within the cap beam span, which solves some temporary support problems, but faces new challenges such as the complex structure of the triangular truss itself, cumbersome installation, and stress concentration at the nodes, threatening the stability of the entire paving structure. Other technologies focus on optimizing the steel support structure, such as adjustable steel support structures that improve installation efficiency and safety by precisely controlling the erection angle, but they do not address the improvement of the space limitation imposed by the central column.

[0005] It is evident that neither the centrally supported cover structure nor the traditional truss alternative has completely resolved the core contradiction between space constraints and structural efficiency. Therefore, to eliminate the encroachment of the central column on the working space and avoid the drawbacks of complex truss construction and easy node failure, there is an urgent need for a prefabricated cover structure and construction method for semi-cover excavation pits without a central column, based on the core concept of "central column-free construction," to provide an efficient solution for semi-cover excavation pit projects. Summary of the Invention

[0006] To avoid and overcome the technical problems existing in the prior art, this invention provides a prefabricated cover structure for semi-excavated foundation pits without central columns. This invention provides support for the prefabricated cover plate through a diagonal bracing system, thereby replacing columns, increasing the effective space for the construction of large equipment within the foundation pit, and eliminating space constraints.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A prefabricated cover structure for a semi-excavated foundation pit without a central column includes a retaining structure installed around the perimeter of the foundation pit. The top of the retaining structure is divided into a prefabricated cover side and an uncovered side along the transverse direction of the foundation pit. A prefabricated cover plate is horizontally laid on the prefabricated cover side, with one end of the prefabricated cover plate forming a fixed end fixedly installed on the top of the retaining structure, and the other end forming a suspended end suspended above the working area inside the foundation pit. The suspended end of the prefabricated cover plate and the retaining structure are connected to each other by a diagonal bracing system, and the diagonal bracing system does not interfere with the working area inside the foundation pit.

[0009] As a further aspect of the present invention: the bracing system includes multiple sets of bracing groups arranged sequentially along the longitudinal direction of the foundation pit. Each set of bracing groups contains multiple bracing members located in the same inclined plane, and the bracing members in the same set of bracing groups are parallel to each other.

[0010] As a further embodiment of the present invention: the diagonal bracing group includes a secondary diagonal bracing group and a main diagonal bracing group arranged sequentially from the inside out; the main diagonal bracing group includes multiple vertical main diagonal braces, and the secondary diagonal bracing group includes multiple vertical secondary diagonal braces, and each vertical main diagonal brace and each vertical secondary diagonal brace is parallel to each other; the two ends of the vertical main diagonal brace are respectively provided with a reserved slot at the top and a reserved slot at the bottom of the vertical main diagonal brace, and the two ends of the vertical secondary diagonal brace are respectively provided with a reserved slot at the top and a reserved slot at the bottom of the vertical secondary diagonal brace; the top connector of the vertical main diagonal brace and the top connector of the vertical secondary diagonal brace are fixedly installed on the lower surface of the prefabricated cover plate, and the bottom connector of the vertical main diagonal brace and the bottom connector of the vertical secondary diagonal brace are installed on the inner side of the corresponding retaining pile in the retaining structure; each connector is inserted into the corresponding reserved slot and is fixedly installed in the reserved slot by a pin passing through the connection.

[0011] As a further embodiment of the present invention: the diagonal bracing system also includes vertical reinforcing square steel, each of which is installed sequentially along the longitudinal direction of the foundation pit on the corresponding retaining pile; the bottom connector of the vertical secondary diagonal bracing is installed on the vertical reinforcing square steel; the bottom of each vertical reinforcing square steel is connected to the same horizontal reinforcing angle steel, and the bottom connector of the vertical main diagonal bracing is sequentially installed on the horizontal reinforcing angle steel.

[0012] As a further aspect of the present invention: the prefabricated cover plate is installed on the retaining structure through a prefabricated cover system; the prefabricated cover system includes multiple horizontal beams that span the foundation pit and are erected on the retaining structure at both ends, and each horizontal beam is arranged horizontally in sequence along the longitudinal direction of the foundation pit; each horizontal beam has multiple horizontally arranged longitudinal beams installed on the beam body located on the side of the prefabricated cover, and each longitudinal beam is arranged horizontally in sequence along the transverse direction of the foundation pit; each horizontal beam and each longitudinal beam intersects with each other to form a mesh support frame, and each prefabricated cover plate is laid flat on the mesh support frame in sequence.

[0013] As a further embodiment of the present invention: the top of the retaining piles on both sides of the foundation pit is equipped with a cap beam extending longitudinally along the foundation pit, and each cap beam is connected to the end of the corresponding cross beam. The cap beam located on the prefabricated paving side acts as a longitudinal beam and cooperates in forming the mesh support frame. The retaining structure also includes steel walers installed on the inner side of the retaining piles on both sides, and multiple steel supports are connected between two opposite steel walers, and each steel support is arranged sequentially along the longitudinal direction of the foundation pit.

[0014] As a further aspect of the present invention: T-shaped steel is installed on the beams inside the mesh support frame, and angle steel is installed on the beams located at the edges. The angle steel and the T-shaped steel are staggered to form the mounting groove for installing the assembled cover plate.

[0015] As a further aspect of the invention: In the same diagonal brace group, an auxiliary brace is arranged between each pair of adjacent diagonal braces, and the auxiliary braces are connected end-to-end, with each auxiliary brace cooperating to form a sawtooth-wave distribution; the diagonal braces and auxiliary braces cooperate to ensure that the ultimate axial force that a single diagonal brace can withstand is... The calculation formula is as follows:

[0016] ;

[0017] In the formula, K 1 represents the load combination factor; n The number of auxiliary supports connected to a single diagonal brace; η To support the synergy coefficient; f This represents the design value of the compressive strength of the bracing material. A The cross-sectional area of ​​a single diagonal brace; α The angle between the axial direction of the diagonal brace and the vertical direction; L The lateral span of the prefabricated paving side; H This refers to the depth of the foundation pit; K 2 represents the span-depth influence coefficient; ξ This is the correction factor for formation reinforcement; R D The ultimate bearing capacity of the foundation; R S This is the design value for the bearing capacity of the foundation.

[0018] A method for constructing a semi-cover excavation pit, which utilizes the aforementioned prefabricated cover structure without a central support column for semi-cover excavation pits, includes the following construction steps:

[0019] S1. Determine the dimensions and location of the semi-covered excavation pit;

[0020] S2. Install retaining piles and reinforced bored piles on the outside of the retaining piles;

[0021] S3. Install crossbeams, longitudinal beams, steel walers and matching components;

[0022] S4. Excavate the first layer of soil down to the bottom of the steel waler. After the excavation is completed, install and erect the steel supports and steel walers.

[0023] S5. Install horizontal reinforcing angle steel and vertical reinforcing square steel;

[0024] S6. Complete the installation of the prefabricated cover system, and then lay two rows of prefabricated cover plates along the longitudinal direction of the foundation pit on the prefabricated cover system. The inner row serves as the motor vehicle lane, and the outer row serves as the non-motor vehicle lane.

[0025] As a further solution to the method, the load calculation process for the prefabricated cover plate used in non-motorized vehicle lanes is as follows:

[0026] First, perform the dead load calculation for the prefabricated cover plate, that is, calculate the self-weight of the prefabricated cover plate and the weight of its ancillary structures. The specific calculation formula is as follows:

[0027] ;

[0028] In the formula, Indicates the dead load of the assembled cover plate; Indicates the self-weight of the assembled cover plate; Indicates the weight of the assembled cover plate's auxiliary structures; Indicates the weight of the assembled cover plate material; This indicates the area of ​​the upper surface of the assembled cover plate; Indicates the thickness of the assembled cover plate; This indicates the unit area self-weight of the prefabricated cover plate (attached structure);

[0029] Next, the live load calculation for the prefabricated cover plate is performed, which involves calculating the non-motorized vehicle load and pedestrian load on the prefabricated cover plate. The specific calculation formula is as follows:

[0030] ;

[0031] In the formula, Indicates the live load on the assembled cover plate; This indicates the non-motorized vehicle load on the assembled cover plate; This indicates the crowd load on the assembled cover plate; This indicates the standard value of uniformly distributed load on the non-motorized vehicle lane; Indicates the area of ​​the influence line; Indicates the standard value of crowd load; Indicates the area affected by the load on the crowd;

[0032] Next, wind load calculations are performed on the prefabricated cover plate. The specific calculation formula is as follows:

[0033] ;

[0034] In the formula, This indicates the wind load on the assembled cover plate; Indicates the wind vibration coefficient; Indicates the wind load shape coefficient; Indicates the coefficient of variation of wind pressure at height; Indicates the basic wind pressure; Indicates the area affected by the wind load;

[0035] Finally, calculate the design load values ​​that can be applied to the prefabricated cover plate. The calculation formula is as follows:

[0036] ;

[0037] ;

[0038] In the formula, Indicates the design value of structural resistance;

[0039] When wind load plays a controlling role, at this time .

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. This invention employs a column-free design, effectively solving the problem of limited working space caused by the presence of a central column in traditional semi-cut-and-cover construction methods. By directly transferring the load of the cover system to the pit retaining structure and foundation, it not only simplifies the structural stress path and reduces the number of central column support links, thus lowering the complexity of structural stress, but also significantly improves the overall stability and reliability of the structure. The prefabricated cover system, through its pre-designed lifting holes and standardized connection structure, achieves rapid and efficient modular installation, greatly improving construction efficiency while reducing on-site work intensity. This structural form is particularly suitable for implementation in complex urban environments, effectively reducing interference with the surrounding environment.

[0042] 2. The coordinated arrangement of main and auxiliary diagonal braces constructs a spatial truss-like force-bearing system, significantly improving the overall structural stiffness. This arrangement not only ensures effective load transfer but also endows the structure with better deformation coordination capabilities. The design feature that allows the length of the diagonal braces to be selected according to the actual construction scenario enables this scheme to flexibly adapt to the needs of foundation pit projects with different spans and depths.

[0043] 3. The slot-pin dual connection mechanism ensures reliable force transmission at the nodes while also facilitating construction. The use of standardized connectors significantly improves construction accuracy, reliably guaranteeing the quality of component installation. Particularly noteworthy is that this connection design fully considers future dismantling needs, employing a detachable bolt connection method, creating favorable conditions for the main structure construction and embodying the advanced concept of full-cycle construction.

[0044] 4. The reinforcing structural system, through its ingenious design, achieves continuity and uniformity of stress distribution. The synergistic work of the vertical reinforcing square steel and the horizontal reinforcing angle steel effectively improves the structural performance. This integrated design not only improves structural efficiency but also makes full use of materials, demonstrating excellent economic efficiency.

[0045] 5. The grid-based support system, through the rational arrangement of longitudinal and transverse beams, forms an efficient spatial force-bearing system, ensuring not only uniform load distribution but also significantly improving overall stiffness. The modular design concept gives this solution excellent adaptability, meeting the needs of projects of varying scales. Simultaneously, the standardized component design creates favorable conditions for factory prefabrication and rapid on-site installation.

[0046] 6. The capping beam and longitudinal beams function as one integrated design, which simplifies the structural construction and improves construction efficiency. The precise positioning of the embedded parts ensures reliable connections between systems, reflecting the concept of meticulous construction. The continuous capping beam structure not only enhances the overall structural integrity but also forms an effective waterproofing system, significantly improving the structure's durability.

[0047] 7. The coordinated work of steel supports and steel walers ensures the overall stability of the foundation pit, and the arrangement of multiple steel supports effectively controls the concave deformation of the foundation pit. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the overall structure of the cover structure in this invention.

[0049] Figure 2 This is a schematic diagram of the prefabricated cover side in this invention.

[0050] Figure 3 This is a bottom view of the structure of the diagonal bracing system in this invention.

[0051] Figure 4 This is a schematic diagram of the diagonal bracing system in this invention.

[0052] Figure 5 This is a schematic diagram of the assembled cover plate with nested cover plates in this invention.

[0053] Figure 6This is a schematic diagram of the assembled cover plate for removing the nested cover plate in this invention.

[0054] Figure 7 This is a schematic diagram of the assembly structure of the crossbeam and longitudinal beam in this invention.

[0055] Figure 8 This is a schematic diagram of the structure of the bottom connector of the diagonal brace in this invention.

[0056] Figure 9 This is a schematic diagram of the vertical main diagonal brace in this invention.

[0057] Figure 10 This is a schematic diagram of the vertical secondary diagonal brace in this invention.

[0058] Figure 11 This is a schematic diagram of the cross-shaped lifting frame in this invention.

[0059] Figure 12 This is a schematic diagram of the assembly structure of the main and auxiliary diagonal braces in this invention.

[0060] In the diagram: 10. Enclosure structure; 10a. Left enclosure structure; 10b. Right enclosure structure; 11. Enclosure piles; 12. Cast-in-place piles; 13. Steel walers; 14. Steel supports; 20. Prefabricated cover system; 21. Left cap beam; 22. Right cap beam; 23. Crossbeam; 231. Laterally fixed T-beams; 24. Longitudinal beams; 24a. Longitudinal beam No. 1; 24b. Longitudinal beam No. 2; 241. Longitudinal fixed angle steel; 242. Longitudinal fixed T-beams; 25. Prefabricated cover plate; 251. Lifting hole; 252. Cross lifting frame; 253. Nested cover plate; 3 0. Diagonal bracing system; 31. Main vertical diagonal bracing; 311. Top connector of main vertical diagonal bracing; 312. Bottom connector of main vertical diagonal bracing; 313. Top slot of main vertical diagonal bracing; 314. Bottom slot of main vertical diagonal bracing; 315. Auxiliary main diagonal bracing; 32. Secondary vertical diagonal bracing; 321. Top connector of secondary vertical diagonal bracing; 322. Bottom connector of secondary vertical diagonal bracing; 323. Top slot of secondary vertical diagonal bracing; 324. Bottom slot of secondary vertical diagonal bracing; 325. Auxiliary secondary diagonal bracing; 33. Vertical reinforcing square steel; 34. Horizontal reinforcing angle steel. Detailed Implementation

[0061] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0062] Please see Figures 1-12In this embodiment of the invention, a prefabricated paving structure for a semi-covered excavation pit without a central column includes a retaining structure 10, a prefabricated paving system 20, a diagonal bracing system 30, and a ground reinforcement structure.

[0063] I. Enclosure Structure

[0064] The retaining structure 10 is the core support system in the foundation pit construction, used to withstand soil pressure, water pressure, and other external loads on the outside of the foundation pit, ensuring the stability of the foundation pit during excavation. The retaining structure 10 of this invention mainly includes a left retaining structure 10a and a right retaining structure 10b that provide protection for the transverse sidewalls of the foundation pit. The left retaining structure 10a is located on the uncovered side, and the right retaining structure 10b is located on the prefabricated covered side. Both the left retaining structure 10a and the right retaining structure 10b adopt the form of diaphragm walls and deep piles. Diaphragm walls are suitable for soft soil strata or areas with strict deformation control requirements, while deep piles are suitable for general strata, offering greater construction flexibility.

[0065] A left cap beam 21 and a right cap beam 22 are respectively installed at the top of the left retaining structure 10a and the right retaining structure 10b. Both the left cap beam 21 and the right cap beam 22 are made of reinforced concrete with a rectangular cross-section of 1000mm × 1200mm. Based on the requirements for the installation of the prefabricated cover plate 25 on site, a longitudinal fixing angle steel 241 is pre-embedded at the top of the right cap beam 22, and its position is flush with the side of the right cap beam 22.

[0066] Steel walers 13 are installed on the inner sides of the retaining piles 11 on both sides of the foundation pit, and are supported and abutted against each other by multiple steel supports 14. To further improve the strength of the support, multiple steel supports 14 are arranged at equal intervals along the length of the steel walers 13. At the same time, to further improve the strength of the support, multiple sets of steel walers 13 facing each other are arranged from bottom to top between the retaining piles 11 on both sides, and are equipped with corresponding steel supports 14. Through the synergistic effect of the steel supports 14 and steel walers 13, the retaining structure 10 effectively controls the deformation of the foundation pit and provides reliable support conditions for the prefabricated paving system without central support columns.

[0067] II. Prefabricated paving system

[0068] The prefabricated cover system 20 mainly consists of prefabricated prefabricated cover plates 25, longitudinal beams 24, transverse beams 23, longitudinal fixed angle steel 241, longitudinal fixed T-shaped steel 242, and transverse fixed T-shaped steel 231.

[0069] A transverse beam 23 is longitudinally spaced between the left cap beam 21 and the right cap beam 22. The transverse beam 23 horizontally spans the prefabricated tarpaulin side and the uncovered side, with both ends supported by the retaining structure 10. On the prefabricated tarpaulin side, longitudinal beams 1 (24a) and 2 (24b) are transversely spaced. These beams and beams form a horizontal mesh support frame. All beams are reinforced concrete and interconnected to form a stable frame structure supporting the prefabricated cover plate 25. The cross-sectional dimensions of all beams are 1000mm × 1000mm, but can be adjusted according to the size of the pit, with a reference range of 800~1400mm. The longitudinal beam 24 is generally located at the mid-span of the transverse beam 23, and the spacing of the transverse beam 23 is 3~9m.

[0070] A longitudinal fixing angle steel 241, with dimensions greater than 100mm × 100mm, is pre-embedded at the top of longitudinal beam 24a, and its embedding position is flush with the side of longitudinal beam 24. Longitudinal fixing T-shaped steel 242 and transverse fixing T-shaped steel 231 are pre-embedded on longitudinal beam 24b and transverse beam 23, respectively, and their positions should be consistent with the central axis of longitudinal beam 24 and transverse beam 23. The dimensions of both longitudinal fixing T-shaped steel 242 and transverse fixing T-shaped steel 231 are greater than or equal to 100mm × 200mm × 8mm × 12mm. At the bottom of the intersection of longitudinal beam 24a, longitudinal beam 24b, and transverse beam 23, a vertical main diagonal brace top connector 311 and a vertical secondary diagonal brace top connector 321 are pre-embedded.

[0071] The angle steel and T-shaped steel are staggered to form mounting grooves for the prefabricated cover plates 25, which are then sequentially installed in these grooves. The prefabricated cover plates 25 are laid on the prefabricated paving side of the semi-excavated pit, extending transversely along the pit to include N sections, where N is typically 2-3; in this embodiment, N is 2. The prefabricated cover plates 25 are sequentially divided into prefabricated paving for the No. 1 motor vehicle lane and the No. 2 non-motor vehicle lane from the left retaining structure 10a to the right retaining structure 10b.

[0072] The shape and size of the prefabricated cover plate 25 should be determined based on the net spacing between the longitudinal fixing angle steel 241, the longitudinal fixing T-shaped steel 242, and the transverse fixing T-shaped steel 231 to ensure that they can be securely embedded within them. The prefabricated cover plate 25 is made of prestressed concrete slab or steel-concrete composite slab, etc., which has high strength and rigidity and can withstand the upper load of the cover plate. The upper surface of the prefabricated cover plate 25 has four reserved lifting holes 251, located on two diagonal lines of the prefabricated cover plate 25. A cross-shaped lifting frame 252 is provided in the lifting holes 251, and a nested cover plate 253 is provided at the opening of the lifting holes 251. Before lifting, the nested cover plate 253 is removed, and after lifting, the nested cover plate 253 is installed to maintain the integrity of the surface of the prefabricated cover plate 25 and avoid the appearance of pits.

[0073] III. Diagonal Bracing System

[0074] The diagonal bracing system 30 includes a main vertical diagonal bracing group, a secondary vertical diagonal bracing group, a transverse reinforcing angle steel 34, and a vertical reinforcing square steel 33. The diagonal bracing system 30 is installed in the semi-excavated foundation pit. One end of the diagonal bracing system 30 is connected to the right-side vertical reinforcing square steel 33 and transverse reinforcing angle steel 34, and the other end is connected to the top connector 311 of the main vertical diagonal bracing and the top connector 321 of the secondary vertical diagonal bracing of the prefabricated paving system 20. This is the main connection and installation method of the diagonal bracing system 30.

[0075] Within the foundation pit, vertical secondary diagonal bracing groups are installed sequentially from top to bottom as the first level of support, and vertical main diagonal bracing groups as the second level of support. The overall size of the vertical main diagonal bracing groups is larger than that of the vertical secondary diagonal bracing groups. Since the cavity formed by the clamping of the crossbeam 23 and the retaining pile 11 is at a right angle, the vertical secondary diagonal bracing groups and the vertical main diagonal bracing groups correspond to the hypotenuses of the right triangle. The vertical secondary diagonal bracing groups are extended proportionally to become the vertical main diagonal bracing groups. Furthermore, during actual construction, when the width of the prefabricated paving side increases or the load increases, several more sets of corresponding vertical secondary diagonal bracing groups and vertical main diagonal bracing groups can be added in the same way.

[0076] Each brace in the vertical secondary diagonal brace group and the vertical main diagonal brace group is made of steel pipe with a diameter greater than 600mm. Furthermore, the diagonal braces can be made into structures capable of applying axial force as needed. By adjusting the length of the diagonal braces, the stress state of the braces can be ensured to meet design requirements. Monitoring units can also be added for load monitoring to prevent instability of the support system caused by excessively heavy vehicles. The top and bottom ends of the vertical secondary diagonal brace 32 and the vertical main diagonal brace 31 are equipped with slots and bolt holes. The top ends are connected to the top connectors 311 and 321 of the vertical main diagonal brace and the vertical secondary diagonal brace respectively, using bolts as pins. The bottom ends are also connected to the vertical reinforcing square steel 33 and the horizontal reinforcing angle steel 34 on the retaining pile 11 via bolts. The top connector 311 of the vertical main diagonal brace and the top connector 321 of the vertical secondary diagonal brace have the same planar dimensions as the intersection of the longitudinal beam 24 and the transverse beam 23, and their thickness should be greater than 50mm. The top of each connector is provided with a cross structure for fixing to the beam, and the bottom is provided with bolt holes for fixing the corresponding diagonal brace.

[0077] The tops of the vertical secondary diagonal brace 32 and the vertical main diagonal brace 31 are respectively fitted together at the junctions of the corresponding top connectors 321 and 311 of the vertical secondary diagonal brace. The bottom of the vertical secondary diagonal brace 32 should be fitted together at the junction of the vertical reinforcing square steel 33, and the bottom of the vertical main diagonal brace 31 should be fitted together at the junction of the horizontal reinforcing angle steel 34. The vertical angle between the vertical secondary diagonal brace 32 and the vertical main diagonal brace 31 and the right-side enclosure structure 10b is... αBetween 45° and 60°, the central axis of each diagonal brace and the central axis of its corresponding crossbeam 23 are kept in the same vertical plane, and the spacing between adjacent vertical planes is the same. This ensures that the diagonal brace system 30 can effectively and evenly transfer the prefabricated paving system 20 and the loads of vehicles and pedestrians above it to the enclosure structure 10. To further improve the support strength of the diagonal braces, secondary diagonal braces are also provided. A secondary diagonal brace auxiliary support 325 is installed between two adjacent vertical secondary diagonal braces 32, connecting the two vertical secondary diagonal braces 32 end to end, so that the entire first set of vertical secondary diagonal braces 32 presents a sawtooth wave distribution. Similarly, a main diagonal brace auxiliary support 315 is installed between two adjacent vertical main diagonal braces 31, connecting the two vertical main diagonal braces 31 end to end, so that the entire second set of vertical main diagonal braces 31 also presents a sawtooth wave distribution.

[0078] The bottom end of the vertical reinforcing square steel 33 is welded to the top end of the horizontal reinforcing angle steel 34. The top end of the vertical reinforcing square steel 33 is connected to the capping beam. The vertical reinforcing square steel 33 and the retaining pile 11 should fit together. Its axis in the vertical plane should be consistent with the central axis of the horizontal beam 23 and the spacing should be the same as the arrangement spacing of the horizontal beam 23. The width of the vertical reinforcing square steel 33 should be greater than 200mm and the thickness should be greater than 30mm. The vertical reinforcing square steel 33 is provided with a vertical secondary diagonal brace bottom connector 322 in the middle. This connector is provided with bolt holes for connection with the corresponding reserved slots.

[0079] The bottom end of the transverse reinforcing angle steel 34 can be welded to the steel waler 13. The longitudinal length should be the same as that of the steel waler 13. The angle steel size is greater than or equal to 100mm×100mm. The bottom connector 312 of the vertical main diagonal brace is set according to the spacing of the crossbeam 23 in the longitudinal direction. The connector is provided with bolt holes for connecting with the corresponding reserved slots.

[0080] Based on the structural design and construction concept of this invention, steel support 14 can be selectively used as diagonal bracing, or steel support 14 can be used in combination with diagonal bracing to maximize the working range of the semi-covered excavation pit.

[0081] IV. Ground Reinforcement Structure

[0082] The ground reinforcement structure is located in the stratum outside the foundation pit near the contact point between the inclined bracing system 30 and the right-side retaining structure 10b. The ground reinforcement structure employs bored piles 12, anchor bolts, and anchor cables for reinforcement. The diameter of the bored piles 12 is determined based on the ground conditions, generally greater than or equal to 1000mm. They are arranged along the contact line between the inclined bracing and the retaining structure 10 and are close to the retaining piles 11. The top of the piles should be flush with the top of the retaining piles 11. The pile length is twice the length of the vertical reinforcing square steel 33, generally 5-10m. The anchor bolts or anchor cables are made of high-strength steel strands, with a length of 8-15m and an inclination angle of 15°-30°. The anchor bolts or anchor cables are implanted into the ground through drilling, followed by grouting for anchoring, thereby improving the bearing capacity and stability of the ground.

[0083] V. Construction Method

[0084] The construction method mainly includes construction steps, calculation of the width of the prefabricated cover plate 25, and load calculation of the prefabricated cover plate 25, as detailed below:

[0085] 1. Construction steps:

[0086] The first construction method: adopting the "excavation-support-paving-through" construction method, specifically including the following steps:

[0087] (1) Determine the size of the semi-covered excavation pit and set out the positioning lines.

[0088] (2) Construction of retaining piles 11 and reinforced bored piles 12.

[0089] (3) Installation and construction of embedded parts for cap beam, cross beam 23, and longitudinal tie beam.

[0090] (4) Excavate the first layer of soil down to the bottom of the steel waler 13. After the excavation is completed, install and erect the steel support 14 and the steel waler 13.

[0091] (5) Install the horizontal reinforcing angle steel 34 and the vertical reinforcing square steel 33.

[0092] (6) Erect vertical secondary diagonal bracing group, hoist and install the outer prefabricated cover plate 25, and restore part of the traffic. At this time, the prefabricated cover plate 25 is used as a non-motorized vehicle lane.

[0093] (7) Erect the vertical main diagonal bracing group, hoist and install the inner prefabricated cover plate 25, and restore road traffic. At this time, the prefabricated cover plate 25 is used as a motor vehicle lane. At this time, a prefabricated paving with the No. 1 motor vehicle lane on the inner side and the No. 2 non-motor vehicle lane on the outer side is formed.

[0094] The second construction method: adopting the "excavation-paving-through-support-paving-through" construction method, specifically including the following steps:

[0095] (1) Determine the size of the semi-covered excavation pit and set out the positioning lines.

[0096] (2) Construction of retaining piles 11 and reinforced bored piles 12.

[0097] (3) Installation and construction of embedded parts for cap beam, cross beam 23, and longitudinal tie beam.

[0098] (4) The prefabricated cover plate 25 on the outside is hoisted and installed to restore some traffic. At this time, the prefabricated cover plate 25 is used as a non-motorized vehicle lane.

[0099] (5) Excavate the first layer of soil down to the bottom of the steel waler 13. After the excavation is completed, install and erect the steel support 14 and the steel waler 13.

[0100] (6) Install the horizontal reinforcing angle steel 34 and the vertical reinforcing square steel 33.

[0101] (7) Install vertical secondary diagonal bracing.

[0102] (8) Erect the vertical main diagonal bracing group, hoist and install the inner prefabricated cover plate 25, and restore road traffic. At this time, the prefabricated cover plate 25 is used as a motor vehicle lane. At this time, a prefabricated paving with the No. 1 motor vehicle lane on the inner side and the No. 2 non-motor vehicle lane on the outer side is formed.

[0103] 2. Load calculation for prefabricated cover plates

[0104] Before the vertical secondary bracing 32 is applied, the prefabricated cover plate 25 on lane 1 is used as a non-motorized vehicle lane, and the allowable load on the prefabricated cover plate 25 is determined as follows:

[0105] First, perform the dead load calculation for the prefabricated cover plate 25, that is, calculate the self-weight of the prefabricated cover plate 25 and the weight of its auxiliary structures. The specific calculation formula is as follows:

[0106] ;

[0107] In the formula, This indicates the dead load of the prefabricated cover plate 25; This indicates the self-weight of the assembled cover plate 25; Indicates the weight of the auxiliary structure of the assembled cover plate 25; This indicates the density of the prefabricated cover plate material 25, in kN / m³. 3 ; This indicates the area of ​​the prefabricated cover plate 25, in meters. 2 ; The thickness of the assembled cover plate 25 is indicated in meters (m). This indicates the unit area self-weight of the auxiliary structure of the prefabricated cover plate 25, in kN / m². 2 .

[0108] Next, the live load calculation of the prefabricated cover plate 25 is performed, that is, the non-motorized vehicle load and pedestrian load on the prefabricated cover plate 25 are calculated. The specific calculation formula is as follows:

[0109] ;

[0110] In the formula, Indicates the live load on the assembled cover plate 25; This indicates the non-motorized vehicle load on the prefabricated cover plate 25; This indicates the crowd load on the assembled cover plate 25; This represents the standard value of the uniformly distributed load on the non-motorized vehicle lane, in kN / m. 2 ; Indicates the area of ​​the influence line; This represents the standard value of the load on the human body, in kN / m. 2 ; The area of ​​the load on the crowd is represented in meters. 2 .

[0111] Next, the wind load calculation for the prefabricated cover plate 25 is performed. The specific calculation formula is as follows:

[0112] ;

[0113] In the formula, Indicates the wind load on the prefabricated cover plate 25; Indicates the wind vibration coefficient; Indicates the wind load shape coefficient; Indicates the coefficient of variation of wind pressure at height; This indicates the basic wind pressure, in kN / m. 2 ; This indicates the area affected by the wind load.

[0114] Finally, the design load values ​​that can be applied to the prefabricated cover plate 25 are calculated. The calculation formula is as follows:

[0115] ;

[0116] ;

[0117] In the formula, This indicates the design value of structural resistance, such as flexural capacity or shear capacity;

[0118] When wind load plays a controlling role, at this time .

[0119] When calculating the allowable load per unit area, use the following formula:

[0120] ;

[0121] In the formula, This indicates the allowable load per unit area of ​​the prefabricated cover plate 25, in kN / m. 2 .

[0122] 3. Calculation of the width of the prefabricated cover plate

[0123] After calculating the load on the prefabricated cover plate 25, the width of the prefabricated cover plate 25 is calculated. The calculation steps are as follows:

[0124] (1) Load calculation: Consider road traffic load, crowd load, construction load, seismic load, etc., and determine the load values ​​according to relevant specifications and standards.

[0125] (2) Structural analysis: The prefabricated paving structure is subjected to stress analysis using the finite element analysis method or a simplified calculation method, and the internal forces and deformations of the structure are calculated.

[0126] (3) Component design: Based on the structural analysis results, design components such as cover plate, longitudinal beam 24, transverse beam 23 and diagonal brace, and determine the size, reinforcement and materials of the components.

[0127] (4) Connection node design: Design the connection nodes of the diagonal bracing group to ensure the strength, stiffness and ductility of the nodes and meet the stress requirements of the structure.

[0128] (5) Calculation of cover plate width: Based on factors such as the width of the pit, the spacing of the retaining structures 10, and the distribution of vehicle loads, the cover plate width is calculated using the following formula. B :

[0129] ;

[0130] in, L The net width of the foundation pit. a The thickness of the enclosure structure 10, b To account for the vehicle load distribution and structural stress safety, generally b Take 1~2m.

[0131] 4. Road surface optimization

[0132] After the complete structure was built, the road surface was optimized based on actual usage. The optimization process is as follows:

[0133] (1) Lane division: The prefabricated paving side is set as a non-motorized vehicle lane and a motorized vehicle lane. At the junction of the non-motorized vehicle lane and the motorized vehicle lane, isolation facilities, such as isolation piers or isolation barriers, are set up to separate the non-motorized vehicle lane from the motorized vehicle lane and ensure the driving safety of non-motorized vehicles.

[0134] (2) Signage: Set up obvious traffic signs on the road, such as lane lines, directional arrows, speed limit signs, etc., to guide non-motorized vehicles and motorized vehicles to drive correctly.

[0135] (3) Entrance and exit settings: Non-motorized vehicle entrances and exits shall be set at both ends of the foundation pit or at suitable locations. The slope and width of the entrances and exits shall meet the requirements for non-motorized vehicle passage.

[0136] (4) Set up tidal lanes: When the traffic flow is low during nighttime construction, lane 2 (non-motorized vehicle lane) can be closed, leaving only lane 1 (motorized vehicle lane) open. Lane 1 (motorized vehicle lane) can be temporarily converted into a non-motorized vehicle lane, and motorized vehicles are prohibited from passing through. The prefabricated paving of lane 2 (non-motorized vehicle lane) can be opened to facilitate excavation and hoisting. When the traffic flow is high during the day, lane 2 (non-motorized vehicle lane) can be reopened to reduce traffic pressure.

[0137] 5. Applicable conditions and design methods for semi-covered excavation pits without central columns.

[0138] The core of the column-free structure is to directly transfer the load of the paving to the enclosure structure 10 through the diagonal bracing system 30, replacing the supporting function of the traditional central column. Its applicability needs to be determined through load transfer feasibility calculations and verification of the bearing capacity of the enclosure structure 10.

[0139] Load transfer feasibility verification

[0140] The test verified whether the load-bearing chain composed of the diagonal bracing system 30 and the enclosure structure 10 could withstand the total load and whether the deformation was controlled within the allowable range.

[0141] First, the bearing capacity of the diagonal bracing system was verified using a 30mm diameter test: (Through...) , For the load borne by a single diagonal brace, The angle between the diagonal brace and the vertical direction. For stability coefficient, Let the cross-sectional area of ​​the diagonal brace be denoted as . f This refers to the design value of the compressive strength of the steel used to construct the diagonal brace. Here, a single brace refers to either a main vertical diagonal brace or a secondary vertical diagonal brace.

[0142] Then, the bearing capacity of the retaining structure 10 is checked: After the load is transferred to the retaining structure 10 through the diagonal bracing, the vertical bearing capacity and horizontal resistance of the retaining piles 11 or the underground continuous wall need to be checked.

[0143] The structure without a central column must ensure that the working space inside the foundation pit is not disturbed by the diagonal bracing system. That is, the arrangement of the diagonal bracing should not affect the passage and operation of large equipment (such as excavators and cranes). It must meet the following requirements: the net height from the bottom of the diagonal bracing to the bottom of the foundation pit is ≥ the working height of the large equipment, and the spacing of the diagonal bracing (longitudinal) is ≥ 6m (to avoid dense arrangement that obstructs passage).

[0144] A column-free structure can be used when the following conditions are met: ① The axial force of a single diagonal brace ( ① Less than the design bearing capacity; ② Vertical bearing capacity of the enclosure structure and horizontal resistance All are greater than the transmitted load; ③ The net height of the working space in the foundation pit and the spacing of the diagonal braces meet the construction requirements.

[0145] 6. Criteria for determining the addition of main and auxiliary diagonal braces

[0146] The main diagonal brace 315 and the secondary diagonal brace 325 are installed between adjacent diagonal braces, creating a sawtooth-like wave distribution in the brace group. The conditions for their installation are directly related to the load magnitude and the stability of the diagonal brace; the specific determination method is as follows:

[0147] Based on the determination of the axial force of the diagonal brace, when the axial force of a single diagonal brace... N When the load exceeds 80% of its stable bearing capacity (safety reserve threshold), auxiliary supports must be added to distribute the load; auxiliary support installation... n After that, the load borne by a single main / secondary diagonal brace N’ Reduce using the following formula: N'=N / (1+n).

[0148] Based on the determination of the span and depth of the foundation pit, when the width of the prefabricated paving side... B Greater than or equal to 12m (lateral span) or foundation pit depth H When the length is greater than or equal to 10m, additional auxiliary supports are required to enhance the overall integrity of the diagonal bracing system.

[0149] Based on the judgment of load increase, when the live load (such as the temporary load of construction vehicles) exceeds 1.2 times the design value, it is necessary to temporarily add auxiliary supports to resist the additional load.

[0150] Based on the core design concept and method of "replacing the central column with a diagonal bracing system", combined with relevant structural mechanics calculations and engineering practice parameters, the safety and applicability of the column-free structure can be ensured.

[0151] Calculation method for ultimate bearing capacity of diagonal bracing system

[0152] Based on the "no central column + diagonal bracing system + prefabricated paving" structure, and considering the load transfer path, the synergistic force distribution of the diagonal bracing, and the ground reinforcement effect, a system was constructed that takes into account the angle of the diagonal bracing. Quantity of auxiliary supports Foundation pit span L , formation stiffness ξ Comprehensive calculation formula for the ultimate bearing capacity of a multi-parameter bracing system:

[0153] ;

[0154] In the formula, This represents the ultimate axial force that a single diagonal brace can withstand, expressed in kN, reflecting the overall load-bearing capacity. K 1 represents the load combination factor, which is taken as 1.2 to 1.5; n The number of auxiliary supports connected to a single diagonal brace ( n ≥0; without auxiliary support, n =0). η The auxiliary support coordination coefficient reflects the load distribution effect of the auxiliary support on the diagonal brace (taken as 0.3~0.5 when the auxiliary support is arranged in a sawtooth wave pattern). f This represents the design value for the compressive strength of the diagonal bracing material. A This represents the cross-sectional area of ​​a single diagonal brace. α It is the angle between the diagonal brace and the vertical direction. L The lateral span of the prefabricated paving side. H For the depth of the foundation pit ( H For spans of 10m or more, additional auxiliary supports are required. K 2 represents the span-depth influence coefficient ( L / H If the value is greater than or equal to 1.2, take 1.1; otherwise, take 0.8. ξ This is the correction factor for formation reinforcement. R D The ultimate bearing capacity of the foundation; R S The bearing capacity of the foundation must be greater than or equal to 1.5 times the design value after soil reinforcement.

[0155] The above formula employs multi-factor coupling to reflect the collaborative force characteristics of "diagonal bracing-layout-stratum"; through 1+ η · n The quantitative support effect reflects the mechanical gain of the asymmetric arrangement. (Introduction) This item reflects the improvement of the bearing capacity of the diagonal bracing by the "soil reinforcement structure (drilled piles / anchors)". When the foundation strength is insufficient, this coefficient automatically reduces the total bearing capacity to ensure safety. The adverse effects of large spans / deep foundation pits are reflected, and the rationality of the structure is constrained by formulas.

[0156] A subway station's semi-excavated foundation pit project utilized a prefabricated cover slab structure without a central support column. During construction, no diagonal bracing was applied; only the outer prefabricated cover slab 25 was laid as a non-motorized vehicle lane. The prefabricated cover slab 25 was made of prestressed concrete. The prefabricated cover slab 25 was 6m long, 3m wide, and 0.2m thick, with a prestressed concrete unit weight... =25kN / m 3 Flexural bearing capacity design value =400kN·m (structural resistance). The design calculations only consider non-motorized vehicle loads, pedestrian loads, and self-weight; wind load has a negligible impact. The prefabricated cover plate has a self-weight of 90kN, and the auxiliary structures (connectors, lifting frames, etc.) have a unit self-weight of 9kN. Total dead load... G =99kN; Non-motorized vehicle load =67.5kN, crowd load =63kN, total live load =130.5kN; wind load is 0.

[0157] Substitute the design load values Calculation formula, to obtain =301.5kN.

[0158] Design value of mid-span bending moment for prefabricated cover plate M =226.1kN·m≤ =400kN·m, which meets the safety requirements.

[0159] Further calculations yielded the allowable load per unit area. =16.75kN / m 2 The actual standard live load value is 8.5 kN / m. 2 The design value is 11.9 kN / m 2 It is much smaller than the allowable value, meeting the safety requirements.

[0160] Without the diagonal bracing applied, the total design load that the non-motorized vehicle lane can withstand is 301.5 kN, and the allowable load per unit area is 16.75 kN / m². 2 It meets the traffic needs of non-motorized vehicles (including pedestrians) and is structurally safe.

[0161] In another embodiment, a column-free prefabricated cover plate structure is used, taking the main diagonal brace as an example. The diameter of the vertical main diagonal brace 31 is 600mm. A =0.283m 2 , α =45°, n =1 (1 auxiliary support, at this time the vertical main diagonal brace 31 is located at the end of the foundation pit, so only one auxiliary support is set at this time; when it is located in the middle of the foundation pit, two auxiliary supports will be set on each vertical main diagonal brace 31. n =2), η =0.4, L =10m, H =8m, L / H =1.25, K 2=1.1, after soil reinforcement R D =300kPa,R S =200kPa, ξ =1.4, K 1 = 1.3 f =295MPa, obtained through calculation. =44.8×10 3 The results show that the total axial force that the diagonal brace can withstand is approximately 44.8 × 10 kN. 3 kN, meeting the design requirement of "transferring the load of the paving system to the enclosure structure without a central column".

[0162] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A prefabricated paving structure for a semi-covered excavation pit without a central column, characterized in that, The structure includes a retaining structure (10) installed around the perimeter of the pit. The top of the retaining structure (10) is divided into a prefabricated cover side and an uncovered side along the transverse direction of the pit. A prefabricated cover plate (25) is laid horizontally on the prefabricated cover side. One end of the prefabricated cover plate (25) is fixedly installed on the top of the retaining structure (10), and the other end is suspended above the working area inside the pit. The suspended end of the prefabricated cover plate (25) and the retaining structure (10) are connected to each other by a diagonal bracing system (30), and the diagonal bracing system (30) does not interfere with the working area inside the pit. The diagonal bracing system (30) includes a structure along the perimeter of the pit. Multiple sets of diagonal bracing groups are arranged longitudinally in the pit. Each set of diagonal bracing groups contains multiple diagonal bracings located in the same inclined plane, and the diagonal bracings in the same set are parallel to each other. The diagonal bracing group includes secondary diagonal bracing groups and primary diagonal bracing groups arranged sequentially from the inside to the outside. The primary diagonal bracing group includes multiple vertical primary diagonal bracings (31), and the secondary diagonal bracing group includes multiple vertical secondary diagonal bracings (32), and each vertical primary diagonal bracing (31) and vertical secondary diagonal bracing (32) is parallel to each other. The two ends of the vertical primary diagonal bracing (31) are respectively provided with a pre-reserved slot (313) at the top of the vertical primary diagonal bracing and a pre-reserved slot (314) at the bottom of the vertical primary diagonal bracing. The vertical secondary diagonal bracing (313) is provided with a pre-reserved slot (314) at the top of the vertical primary diagonal bracing and a pre-reserved slot (314) at the bottom of the vertical primary diagonal bracing. 2) The two ends of the vertical secondary diagonal brace are respectively provided with a reserved slot (323) at the top and a reserved slot (324) at the bottom; the top connector (311) of the vertical main diagonal brace and the top connector (321) of the vertical secondary diagonal brace are fixedly installed on the lower plate surface of the prefabricated cover plate (25), and the bottom connector (312) of the vertical main diagonal brace and the bottom connector (322) of the vertical secondary diagonal brace are installed on the inner side of the corresponding retaining pile (11) in the retaining structure (10); each connector is inserted into the corresponding reserved slot and is fixedly installed in the reserved slot by a pin through the connection; the prefabricated cover plate (25) Installed on the retaining structure (10) by a prefabricated cover system (20); the prefabricated cover system (20) includes multiple horizontal beams (23) that span the foundation pit and are erected on the retaining structure (10) at both ends, and each horizontal beam (23) is arranged horizontally along the longitudinal direction of the foundation pit; each horizontal beam (23) has multiple horizontally arranged longitudinal beams (24) installed on the beam body located on the side of the prefabricated cover, and each longitudinal beam (24) is arranged horizontally along the transverse direction of the foundation pit; each horizontal beam (23) and each longitudinal beam (24) intersects to form a mesh support frame, and each prefabricated cover plate (25) is laid flat on the mesh support frame in sequence.

2. The prefabricated cover structure for a semi-covered excavation pit without a central column as described in claim 1, characterized in that, The diagonal bracing system (30) also includes vertical reinforcing square steel (33), each vertical reinforcing square steel (33) is installed sequentially on the corresponding retaining pile (11) along the longitudinal direction of the foundation pit; the bottom connector (322) of the vertical secondary diagonal bracing is installed on the vertical reinforcing square steel (33); the bottom of each vertical reinforcing square steel (33) is connected to the same horizontal reinforcing angle steel (34), and the bottom connector (312) of the vertical main diagonal bracing is installed sequentially on the horizontal reinforcing angle steel (34).

3. The prefabricated cover structure for a semi-covered excavation pit without a central column as described in claim 2, characterized in that, The top of the retaining piles (11) on both sides of the foundation pit is equipped with a crown beam extending longitudinally along the foundation pit. Each crown beam is connected to the end of the corresponding crossbeam (23). The crown beam located on the prefabricated paving side acts as a longitudinal beam (24) and cooperates to form the mesh support frame. The retaining structure (10) also includes steel walers (13) installed on the inner side of the retaining piles (11) on both sides. Multiple steel supports (14) are connected between the two steel walers (13) and each steel support (14) is arranged sequentially along the longitudinal direction of the foundation pit.

4. The prefabricated cover structure for a semi-covered excavation pit without a central column as described in claim 3, characterized in that, T-shaped steel is installed on the beams inside the mesh support frame, and angle steel is installed on the beams at the edges. The angle steel and the T-shaped steel are interlocked to form the mounting groove for installing the assembled cover plate (25).

5. The prefabricated cover structure for a semi-covered excavation pit without a central column as described in claim 4, characterized in that, In the same bracing group, an auxiliary brace is arranged between each pair of adjacent braces, and they are connected end-to-end through the auxiliary braces. The auxiliary braces cooperate with each other to form a sawtooth wave distribution; the braces and auxiliary braces cooperate with each other to ensure that the ultimate axial force that a single brace can withstand is... The calculation formula is as follows: ； In the formula, K 1 represents the load combination factor; n The number of auxiliary supports connected to a single diagonal brace; η To support the synergy coefficient; f This represents the design value of the compressive strength of the bracing material. A The cross-sectional area of ​​a single diagonal brace; α The angle between the axial direction of the diagonal brace and the vertical direction; L The lateral span of the prefabricated paving side; H The depth of the foundation pit; K 2 represents the span-depth influence coefficient; ξ This is the correction factor for formation reinforcement; R D The ultimate bearing capacity of the foundation; R S This is the design value for the bearing capacity of the foundation.

6. A method for constructing a semi-cover excavation pit, wherein the method applies the prefabricated cover structure without central support column described in claim 5, characterized in that, The construction steps include the following: S1. Determine the dimensions and location of the semi-covered excavation pit; S2. Install retaining piles (11) and reinforced bored piles (12) on the outside of retaining piles (11). S3. Install the crossbeam (23), longitudinal beam (24), steel waler (13) and matching components; S4. Excavate the first layer of soil to the bottom of the steel waler (13). After the excavation is completed, install and erect the steel support (14) and the steel waler (13). S5. Install horizontal reinforcing angle steel (34) and vertical reinforcing square steel (33); S6. Complete the installation of the prefabricated cover system (20), and then lay two rows of prefabricated cover plates (25) along the longitudinal direction of the pit on the prefabricated cover system (20). The inner row serves as the motor vehicle lane, and the outer row serves as the non-motor vehicle lane.

7. A method for constructing a semi-covered excavation foundation pit according to claim 6, characterized in that, The load calculation process for the prefabricated cover plate (25) used for non-motorized vehicle lanes is as follows: First, perform the dead load calculation of the prefabricated cover plate (25), that is, calculate the self-weight of the prefabricated cover plate (25) and the weight of its auxiliary structures. The specific calculation formula is as follows: ； In the formula, Indicates the dead load of the assembled cover plate (25); Indicates the self-weight of the assembled cover plate (25); Indicates the weight of the attached structure of the assembled cover plate (25); Indicates the weight of the assembled cover plate (25) material; Indicates the area of ​​the upper surface of the assembled cover plate (25); Indicates the thickness of the assembled cover plate (25); This indicates the unit area self-weight of the prefabricated cover plate (25) auxiliary structure; Next, the live load calculation of the prefabricated cover plate (25) is performed, that is, the non-motorized vehicle load and pedestrian load on the prefabricated cover plate (25) are calculated. The specific calculation formula is as follows: ； In the formula, Indicates the live load on the assembled cover plate (25); Indicates the non-motorized vehicle load on the assembled cover plate (25); Indicates the crowd load on the assembled cover plate (25); This indicates the standard value of uniformly distributed load on the non-motorized vehicle lane; Indicates the area of ​​the influence line; Indicates the standard value of crowd load; Indicates the area affected by the load on the crowd; Next, the wind load calculation for the prefabricated cover plate (25) is performed. The specific calculation formula is as follows: ； In the formula, Indicates the wind load on the assembled cover plate (25); Indicates the wind vibration coefficient; Indicates the wind load shape coefficient; Indicates the coefficient of variation of wind pressure at height; Indicates the basic wind pressure; Indicates the area affected by the wind load; Finally, the design load values ​​that can be applied to the prefabricated cover plate (25) are calculated. The calculation formula is as follows: ； ； In the formula, Indicates the design value of structural resistance; When wind load plays a controlling role, at this time .

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