An underground structure system and a construction method thereof

Abstract

The application discloses an underground structure system and a construction method thereof, and belongs to the field of building construction. The underground structure system comprises a foundation pit permanent support pile wall and an underground space frame structure. The foundation pit permanent support pile wall comprises a surrounding structure formed by alternately arranging support piles and cement soil mixing walls. The underground space frame structure comprises vertical supporting piles and multilayer horizontal components. A plurality of vertical supporting piles are arranged according to the design positions to form a column net. The support piles and the vertical supporting piles are reinforced concrete bored piles, which comprise upper pile bodies and lower pile bodies. The upper pile bodies are cuboid column bodies, and the lower pile bodies are cylindrical bodies. The upper pile bodies and the lower pile bodies are provided with expansion sections. The cross-sectional dimension of the expansion sections is larger than that of the upper pile bodies and the lower pile bodies. The application adopts a top-down reverse construction sequence, and the vertical components are constructed at one time. The horizontal components are constructed by supporting a formwork with foundation soil, and no scaffold is needed. The resource investment is greatly reduced, the construction period is shortened, and the safety is improved.

Description

Technical Field

[0001] This invention relates to the field of building construction, and in particular to an underground structural system and its construction method. Background Technology

[0002] Currently, underground building structures such as underground parking garages generally adopt a frame structure, independent foundations, and waterproof slabs. The construction method typically involves excavating earth to the base with a foundation pit support, then constructing a subbase, independent foundations, and waterproof slabs, followed by the construction of columns and beams until the roof is completed and backfilled.

[0003] For two-story underground structures with an excavation depth exceeding 10 meters, temporary support structures, such as pile anchors or pile braces, are typically used for the foundation pit. During the excavation phase, the foundation pit support structure bears the external horizontal loads, providing safety assurance for the construction of the main underground structure. After the main underground structure is completed, the outer walls of the main underground structure bear the external horizontal loads.

[0004] The existing support piles are circular reinforced concrete bored piles (i.e., with a circular cross-section). Reinforced concrete bored piles are constructed by drilling a hole directly at the pile location on-site, then pouring concrete into the hole or placing a reinforcing cage before pouring concrete. Reinforced concrete bored piles are a common type, typically involving drilling a hole at the pile location on-site using a drilling rig, lowering the reinforcing cage, and then pouring concrete.

[0005] Existing circular reinforced concrete bored piles are significantly affected by the geological strata. When used as permanent foundation pit support pile walls, their poor verticality and flatness, along with the presence of grooves in the circular pile walls, necessitate numerous auxiliary measures to ensure the verticality and flatness of the support pile walls, thus hindering the use and widespread adoption of permanent support structures. Furthermore, during subsequent beam structure construction, the cylindrical surface of the circular reinforced concrete bored piles needs to be flattened to achieve connection with the beam structure, leading to cumbersome subsequent construction processes.

[0006] The existing frame structure of underground spaces generally consists of rectangular columns, independent foundations, and a waterproof slab. Column construction is completed after the independent foundations and waterproof slab are finished, through rebar tying, formwork erection, and layer-by-layer pouring.

[0007] Traditional underground space frame structures require the following steps after the foundation pit is excavated: first, the foundation layer, waterproofing, and waterproofing liner are constructed; then, independent foundations are built; next, column formwork is erected and reinforcement is tied; full-span scaffolding is erected to support the horizontal beams and slab formwork and reinforcement; and finally, concrete is poured in layers for columns, beams, and slabs. This process is complex, inefficient, involves numerous steps, is cumbersome to erect, has high rental costs for scaffolding, results in significant material waste, and has many safety vulnerabilities. Summary of the Invention

[0008] This invention provides an underground structural system and its construction method, which adopts a top-down reverse construction sequence. The vertical components of the underground space are constructed in one go without a foundation pit, and the horizontal components are constructed with the foundation soil supporting the formwork without scaffolding. This greatly reduces resource input, shortens the construction period, and improves safety.

[0009] The technical solution provided by this invention is as follows:

[0010] An underground structural system includes a permanent retaining pile wall for an excavation pit and an underground space frame structure connected to the permanent retaining pile wall for the excavation pit, wherein:

[0011] The permanent support pile wall of the foundation pit includes a retaining structure formed by alternating support piles and cement-soil mixing walls. The underground space frame structure includes vertical support piles and multi-layer horizontal members connected to the vertical support piles. Multiple vertical support piles are arranged in their designed positions to form a column grid.

[0012] The support piles and vertical bearing piles are reinforced concrete bored piles, including a vertically arranged pile body located in a pile hole formed before the excavation of the foundation pit. The pile body is formed by a steel cage and concrete located in the pile hole. The pile body includes an upper pile body and a lower pile body. The upper pile body is a cuboid prism, and the lower pile body is a cylinder. There is an enlarged section between the upper pile body and the lower pile body. The cross-sectional dimension of the enlarged section is larger than the cross-sectional dimensions of the upper pile body and the lower pile body. The upper pile body, the enlarged section, and the lower pile body are arranged sequentially from top to bottom in the pile hole with their central axes coincident and are integrally formed.

[0013] The interior of the retaining structure is used for excavating the foundation pit. The multi-layered horizontal members are constructed layer by layer from top to bottom and connected to the plane of the upper pile side of the vertical support pile during the excavation of the foundation pit. Each layer of horizontal members is connected to the inner side of the permanent support pile wall of the foundation pit.

[0014] Furthermore, the multi-layer horizontal component includes a waterproof membrane and at least one layer of horizontal beams and floor slabs. The waterproof membrane is located on the bottom surface of the excavated foundation pit. The top surface of the expanded section is flush with the top surface of the waterproof membrane. The expanded section and the lower pile body are located below the top surface of the waterproof membrane, and the upper pile body is located above the top surface of the waterproof membrane.

[0015] The at least one layer of horizontal beams and floor slabs are located on the plane of the upper pile side of the vertical support pile above the waterproofing slab.

[0016] Furthermore, a capping beam is provided at the top of the permanent support pile wall of the foundation pit, and several layers of waist beams are provided at the designed height of the inner side of the permanent support pile wall of the foundation pit. During the excavation of the foundation pit, the capping beam / waist beam is constructed layer by layer from top to bottom. The capping beam / waist beam is connected to the horizontal beam of the same layer, and the floor slab is set on the horizontal beam of the same layer.

[0017] Furthermore, the span of the vertical support pile is an integer multiple of the span of the retaining pile.

[0018] Furthermore, the expanded section includes an upper frustum and a lower cylinder, the central axes of the upper frustum and the lower cylinder coincide, the diameter of the upper frustum gradually increases from top to bottom, and the bottom diameter of the upper frustum is equal to the top diameter of the lower cylinder.

[0019] Furthermore, the rectangular cross-section of the upper pile body is inscribed within the rectangle formed by the projection of the top surface of the upper frustum, which is within the circle of the top surface of the upper frustum. The circular cross-section of the lower pile body is inscribed within the rectangle formed by the projection of the top surface of the upper frustum, which is within the rectangle formed by the projection of the upper pile body.

[0020] Furthermore, the reinforcing cage includes longitudinal reinforcing bars and transverse stirrups, wherein the longitudinal reinforcing bars are arranged vertically and run along the length of the pile body.

[0021] Furthermore, the longitudinal reinforcing bars are arranged in a rectangular shape within the upper pile body according to the rectangular cross-section of the upper pile body, and the longitudinal reinforcing bars are arranged in a circular shape within the lower pile body according to the circular cross-section of the lower pile body. The portion of the longitudinal reinforcing bars within the expanded section is an inwardly bent transition zone.

[0022] Furthermore, taking the center point of the rectangular cross-section of the upper pile body as the center, the longitudinal reinforcement closer to the outside bends inward to a greater degree, while the longitudinal reinforcement at the point where the circular cross-section of the lower pile body is tangent to the rectangular cross-section of the upper pile body does not bend.

[0023] Furthermore, the longitudinal reinforcement bars on the stress-bearing side of the pile body and on the opposite side of the stress-bearing side are reinforced.

[0024] Furthermore, the transverse stirrups are densely arranged in the bottom area of ​​the upper pile body, the top area of ​​the lower pile body, and the entire area of ​​the expanded section.

[0025] A construction method for an underground structural system, the method comprising:

[0026] S1: Before the foundation pit is excavated, the cement-soil mixing wall is constructed at the designed location of the cement-soil mixing wall.

[0027] S2: After the cement-soil mixing wall is completed and before the cement-soil has solidified, the pile holes are constructed by drilling at the designed positions of the support piles and the vertical support piles.

[0028] S3: After the pile hole construction is completed, the tied steel cage is lowered into the pile hole and concrete is poured into the pile hole to realize the construction of the support pile and the vertical support pile;

[0029] The retaining piles and the cement-soil mixing wall are arranged alternately to form a retaining structure, and multiple vertical support piles form a column grid;

[0030] S4: After the concrete has cured, excavate the foundation pit inside the retaining structure;

[0031] S5: During the excavation of the foundation pit, construction is carried out layer by layer from top to bottom;

[0032] During the construction of each floor, a capping beam / waist beam is constructed on the enclosure structure, a horizontal beam is constructed on the upper pile body of the vertical support pile, the horizontal beam is connected to the capping beam / waist beam of the same floor, a floor slab is constructed on the horizontal beam, and a construction access passage is reserved on the floor slab.

[0033] S6: After the foundation pit is excavated, a waterproof membrane is constructed on the bottom surface of the foundation pit, and the top surface of the expanded section is flush with the top surface of the waterproof membrane. The expanded section and the lower pile body are located below the top surface of the waterproof membrane, and the upper pile body is located above the top surface of the waterproof membrane.

[0034] S7: Block off the construction access passages reserved on each floor slab.

[0035] The present invention has the following beneficial effects:

[0036] This invention utilizes the advantages of the rectangular column shape of the upper part of the support pile to improve the verticality and flatness of the permanent support wall of the foundation pit, facilitating subsequent treatment of local defects. No flatness finishing is required during subsequent construction; it can be directly connected to the waterproof structure, reducing the workload of subsequent unevenness treatment of the permanent support wall and simplifying waterproofing construction. In the permanent support wall of the foundation pit, it can be directly connected to beam and slab structures without the need for flatness treatment, eliminating unnecessary steps and reducing the flatness and verticality deviations of existing cylindrical piles, thus improving the flatness level of the permanent support pile wall. Furthermore, the upper part of the pile body experiences a larger bending moment, and the bending resistance coefficient of a rectangular section is greater than that of a circular section for the same cross-sectional area. Therefore, the upper part of the pile body with a rectangular cross-section can withstand a larger bending moment.

[0037] This invention utilizes an expanded section to enlarge the pile cross-section for a certain length near the bottom of the foundation pit, thereby correspondingly and specifically increasing the maximum bending moment of the support pile at the bottom of the foundation pit. It also optimizes the cross-section and embedment depth of the embedded end, thus optimizing the pile cross-section and its stiffness from the perspective of stress. This is beneficial for bearing the maximum bending moment of the pile, achieving a targeted, applicable, and scientific approach, and improving the horizontal support capacity of the pile. On the one hand, this invention further enhances the horizontal bending resistance and settlement resistance of the support pile compared to existing support piles. On the other hand, it improves the efficiency of pile material utilization, adapts to the stress law of the support pile, and saves materials. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of an underground structural system according to the present invention (vertical support piles and floor slabs are omitted).

[0039] Figure 2 This is a schematic diagram of the permanent support pile wall for the foundation pit;

[0040] Figure 3 This is a schematic diagram of the arrangement of vertical support piles;

[0041] Figure 4 This is a partial structural diagram of an underground structural system according to the present invention;

[0042] Figure 5 This is a schematic diagram of the arrangement of the support piles;

[0043] Figure 6 This is a schematic diagram of a support pile / vertical bearing pile;

[0044] Figure 7 This is a schematic diagram showing the connection between the waterproof membrane and the vertical support piles;

[0045] Figure 8 This is a cross-sectional view of the reinforcement of the upper pile body;

[0046] Figure 9 This is a cross-sectional view of the reinforcement of the lower pile body;

[0047] Figure 10 This is a transition section diagram of the longitudinal reinforcement;

[0048] Figure 11 This is a schematic diagram showing the projection of the upper and lower pile bodies onto the top surface of the expanded section;

[0049] Figure 12 This is a schematic diagram showing the transition of longitudinal reinforcement and the densification of transverse stirrups. Detailed Implementation

[0050] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0051] This invention provides an underground structural system, such as... Figure 1-12 As shown, the structure includes a permanent support pile wall 100 for the foundation pit and an underground space frame structure 200 connected to the permanent support pile wall 100 for the foundation pit, wherein:

[0052] The permanent support pile wall 100 for the foundation pit includes a retaining structure 500 (e.g., a square, rectangular, or other shaped retaining structure) formed by alternating arrangement of support piles 300 and cement-soil mixing walls 400. The underground space frame structure 200 includes vertical support piles 600 and multi-layered horizontal members 700 connected to the vertical support piles 600, with multiple vertical support piles 600 arranged in their designed positions to form a column grid.

[0053] The support piles 300 and vertical support piles 600 are reinforced concrete bored piles, which include a pile body 1 located in a pile hole that has been formed before the excavation of the foundation pit 13 and is vertically arranged. The pile body 1 is formed by a steel cage and concrete located in the pile hole.

[0054] The pile body 1 includes an upper pile body 2 and a lower pile body 3. The upper pile body 2 is a cuboid prism with a rectangular cross-section, preferably a square cross-section. The lower pile body 3 is a cylinder with a circular cross-section.

[0055] There is an enlarged section 4 between the upper pile body 2 and the lower pile body 3. The cross-sectional dimensions of the enlarged section 4 are larger than the cross-sectional dimensions of the upper pile body 2 and the lower pile body 3.

[0056] The upper pile body 2, the expanded section 4, and the lower pile body 3 are arranged sequentially from top to bottom in the pile hole with their central axes coinciding, and are formed as a whole.

[0057] The aforementioned retaining structure 500 is used for excavating the foundation pit 13. During the excavation of the foundation pit, the aforementioned multi-layer horizontal components 700 are constructed layer by layer from top to bottom and connected to the upper pile body 2 of the vertical support pile 600, that is, connected to the plane on the side of the upper pile body 2. At the same time, each layer of horizontal components 700 is connected to the inner side of the permanent support pile wall 100 of the foundation pit to form an underground structure system.

[0058] This invention configures the reinforced concrete bored piles of the vertical support pile 600 and the retaining pile 300 into three parts, forming a pile body with a variable cross-section. For the retaining pile 300, the upper pile body 2 has a rectangular cross-section, which facilitates connection with subsequent underground structures (such as beams, slabs, etc.) and serves as part of the structure, bearing horizontal and vertical forces. The expanded section 4 in the middle and lower part can effectively cope with the maximum bending moment in the pile body. Below the expanded section 4 is the lower pile body 3 with a circular cross-section, which is convenient and fast to construct, effectively saving materials and construction time.

[0059] For the vertical support piles 600, the upper pile body 2 has a rectangular cross-section, serving as a column in the underground space. It facilitates connection with horizontal components to form a unified structure, acting as a column within the underground structure and bearing vertical forces. The lower expansion section 4 functions as a pile cap, and the lower pile body 3 below it has a circular cross-section, allowing for convenient and rapid construction, effectively saving materials and time. The expansion section 4 and the lower pile body 3 are embedded into the foundation of the excavated pit 13, functioning as a pile foundation instead of an independent foundation. The horizontal components 700 at each level are effectively connected to the vertical support piles 600, forming a novel underground space frame structure system. Furthermore, the horizontal components 700 at each level are connected to the permanent support pile wall 100 of the pit, forming the underground structural system of this invention.

[0060] This invention can be used for deep foundation pit projects with a depth greater than 10m that employ permanent support pile walls. Before excavation, the support piles are arranged at intervals to form a row of piles; between the support piles 300 is a cement-soil mixing wall 400 (or simply a cement-soil mixing wall), which serves to prevent water ingress. This invention combines the support piles 300 and the cement-soil mixing wall 400 to form a permanent support pile wall 100 for deep foundation pits, which facilitates verticality and flatness control, integrating water retention and support functions. Simultaneously, before excavation, multiple vertical support piles 600 with rectangular upper and circular lower cross-sections are formed as load-bearing components through ground piling. Then, excavation is carried out layer by layer, with horizontal components such as beams and slabs constructed from top to bottom to connect with the vertical support piles 600 and the permanent support pile wall 100.

[0061] This invention changes the traditional cylindrical support pile structure, adopting a variable cross-section form with an upper cuboid column and a lower cylindrical column, connected by an expanded section to form the variable cross-section pile body. After construction, the upper cuboid column protrudes from the foundation pit 13 and is integrated with the upper frame structure as a load-bearing component. The expanded section and the lower cylindrical column are embedded in the lower part of the foundation pit 13.

[0062] This invention utilizes the advantages of the rectangular column shape of the upper part of the support pile to improve the verticality and flatness of the permanent support wall of the foundation pit, facilitating subsequent treatment of local defects. No flatness finishing is required during subsequent construction; it can be directly connected to the waterproof structure, reducing the workload of subsequent unevenness treatment of the permanent support wall and simplifying waterproofing construction. In the permanent support wall of the foundation pit, it can be directly connected to beam and slab structures without the need for flatness treatment, eliminating unnecessary steps and reducing the flatness and verticality deviations of existing cylindrical piles, thus improving the flatness level of the permanent support pile wall. Furthermore, the upper part of the pile body experiences a larger bending moment, and the bending resistance coefficient of a rectangular section is greater than that of a circular section for the same cross-sectional area. Therefore, the upper part of the pile body with a rectangular cross-section can withstand a larger bending moment.

[0063] This invention utilizes an expanded section to enlarge the pile cross-section for a certain length near the bottom of the foundation pit, thereby correspondingly and specifically increasing the maximum bending moment of the support pile at the bottom of the foundation pit. It also optimizes the cross-section and embedment depth of the embedded end, thus optimizing the pile cross-section and its stiffness from the perspective of stress. This is beneficial for bearing the maximum bending moment of the pile, achieving a targeted, applicable, and scientific approach, and improving the horizontal support capacity of the pile. On the one hand, this invention further enhances the horizontal bending resistance and settlement resistance of the support pile compared to existing support piles. On the other hand, it improves the efficiency of pile material utilization, adapts to the stress law of the support pile, and saves materials.

[0064] This invention replaces independent foundations with expanded sections and lower pile bodies of vertical support piles. Through ground-driven, integrated, and efficient construction, it directly forms a load-bearing component integrating an upper rectangular and lower circular foundation with columns. This invention achieves integrated, mechanized, and rapid construction of vertical load-bearing components, providing a new vertical structure construction technology for the prefabricated and industrialized development of underground structures. Furthermore, unlike traditional underground parking garage construction, this invention changes the traditional bottom-up construction method for underground spaces, adopting a top-down reverse construction sequence. Through effective connection of horizontal components at each level, it forms an underground space frame structure. Underground space construction requires no foundation pit or support, saving significant investment in beam, slab, column scaffolding, formwork, and other measures compared to traditional top-down construction, greatly improving efficiency and avoiding safety hazards associated with elevated formwork.

[0065] In summary, this invention provides an underground structural system based on upper rectangular and lower circular variable cross-section support piles and vertical support piles. The support piles serve as lateral permanent retaining walls, combined with cement-soil mixing walls to form a permanent retaining pile wall structure for the foundation pit, facilitating verticality and flatness control—essentially, the outer wall of the underground structure. This improves the verticality and flatness of the permanent retaining wall, facilitates waterproofing construction, and allows direct connection to beams and slabs without requiring flatness treatment, eliminating unnecessary steps. The vertical support piles serve as vertical load-bearing components, constructed directly from the ground, combined with reverse excavation of the foundation pit and construction of horizontal beams and floor slabs, connecting with the permanent retaining pile wall to form an integrated pile-column underground structural system. This saves on the significant investment in beam, slab, and column scaffolding and formwork required in traditional forward construction, avoiding safety hazards associated with elevated formwork. It overcomes the drawbacks of complex structural nodes and difficulty in controlling the flatness and verticality of intermediate support columns in traditional reverse construction methods. It achieves integration of the foundation pit retaining wall with the outer structure of the underground space and permanent foundation pit support.

[0066] The aforementioned multi-layered horizontal structure 700 may include a waterproof membrane 701 and at least one layer of horizontal beams 702 and floor slabs. The waterproof membrane 701 is located on the bottom surface 14 of the excavated foundation pit 13. The top surface of the expanded section 4 is flush with the top surface of the waterproof membrane 701. The expanded section 4 and the lower pile body 3 are located below the top surface of the waterproof membrane 701, and the upper pile body 2 is located above the top surface of the waterproof membrane 701. At least one layer of horizontal beams 702 and floor slabs are sequentially located on the plane of the side of the upper pile body 2 of the vertical support pile 600 above the waterproof membrane 701.

[0067] In this invention, the expanded section 4 functions as a support platform and is easily connected to the waterproof membrane 701. It utilizes the rigidity of the waterproof membrane 701, saves on formwork and other construction measures, and greatly improves efficiency.

[0068] Correspondingly, a crown beam 101 is set on the top of the permanent support pile wall 100 of the foundation pit, and several layers of waist beams 102 are set on the inner side of the permanent support pile wall 100 of the foundation pit at the designed height. During the excavation of the foundation pit 13, the crown beam 101 / waist beam 102 are constructed layer by layer from top to bottom, and the crown beam 101 / waist beam 102 are connected to the horizontal beam 702 of the same layer. The floor slab of the same layer is set on the horizontal beam 702 of the same layer.

[0069] Preferably, the span of the vertical support pile 600 is an integer multiple of the span of the retaining pile 300, so that the horizontal beam 702 is connected to the position of the retaining pile 300, rather than to the position of the cement-soil mixing wall 400, thereby improving the reliability of the connection.

[0070] As an example, the expanded segment 4 may include an upper frustum 5 and a lower cylinder 6, with the central axes of the upper frustum 5 and the lower cylinder 6 coinciding. The diameter of the upper frustum 5 gradually increases from top to bottom, transitioning the diameter, and the bottom diameter of the upper frustum 5 is equal to the top diameter of the lower cylinder 6.

[0071] Preferably, the thickness of the waterproof membrane 701 can be the same as the thickness of the upper frustum 5, so that the upper frustum 5 is entirely located inside the waterproof membrane 701, and the lower cylindrical body 6 is located below the waterproof membrane 701.

[0072] Specifically, the rectangular cross-section of the upper pile body 2 is projected onto the top surface of the upper frustum 5, forming a rectangle 7 which is inscribed within the circle 8 on the top surface of the upper frustum 5. The circular cross-section of the lower pile body 3 is projected onto the top surface of the upper frustum 5, forming a circle 9 which is inscribed within the rectangle 7 formed by the projection of the upper pile body 2.

[0073] In this invention, the reinforcing cage includes longitudinal reinforcing bars 10 and transverse stirrups 11. The longitudinal reinforcing bars 10 are arranged vertically and run continuously along the length of the pile body 1, that is, the number of longitudinal reinforcing bars 10 is the same in the upper pile body 2, the lower pile body 3, and the expanded section 4. Furthermore, the number of longitudinal reinforcing bars 10 and transverse stirrups 11 is reasonably arranged according to the results of mechanical calculations at the engineering site.

[0074] Multiple longitudinal reinforcing bars 10 are arranged in a rectangular shape within the upper pile body 2 according to the rectangular cross-section of the upper pile body 2, such as... Figure 7 As shown; multiple longitudinal reinforcing bars 10 are arranged in a circular pattern within the lower pile body 3 according to the circular cross-section of the lower pile body 3, as shown. Figure 8 As shown; the part of the longitudinal steel bar 10 within the expanded section 4 is a transition zone 12 that bends inward, changing the shape of the longitudinal steel bar 10 to achieve the transition from the upper pile body 2 to the lower pile body 3.

[0075] Taking the center point of the rectangular cross-section of the upper pile body 2 as the center, the longitudinal steel bars 10 that are closer to the outside bend inward to a greater degree. The longitudinal steel bars 10-1 at the point where the circular cross-section of the lower pile body 3 is tangent to the rectangular cross-section of the upper pile body 2 do not bend. The longitudinal steel bars at the four corners of the rectangular cross-section of the upper pile body 2 bend to the greatest degree.

[0076] The longitudinal reinforcement 10-2 on the stress-bearing side of pile 1 and on the opposite side of the stress-bearing side is reinforced, for example, as shown in the figure. Figure 8 , 9 As shown, the upper side of pile 1 is the stress-bearing side (the side that bears the bending moment). Therefore, the longitudinal reinforcement 10-2 is reinforced on the upper and lower sides of the upper pile 2 and the lower pile 3 in the figure. The reinforcement can be achieved by increasing the diameter of the longitudinal reinforcement 10-2 or by increasing the density of the longitudinal reinforcement 10-2.

[0077] In the bottom area of ​​the upper pile body 2, the top area of ​​the lower pile body 3, and the entire area of ​​the expanded section 4, the transverse stirrups 11 are densely arranged. For the support pile 300, they are used to cope with the maximum bending moment of the pile body. For the vertical support pile 600, they are used to enhance the stress performance.

[0078] In this invention, based on the actual engineering foundation pit depth, soil type, and pile penetration depth, the expanded section 4 of the support pile 300 is positioned at the location where the pile body 1 bears the maximum bending moment. Specifically, the expanded section 4 and the lower pile body 3 are located below the excavated foundation pit 13, with the expanded section 4 situated at the bottom of the foundation pit, its upper surface flush with the waterproof membrane on the bottom surface of the foundation pit. The upper pile body 2 is located above the excavated foundation pit 13.

[0079] This invention also provides a construction method for the aforementioned underground structural system, which includes the following steps:

[0080] S1: Before excavating the foundation pit 13, construct the cement-soil mixing wall 400 at the designed location.

[0081] Cement-soil mixing walls are a method for reinforcing saturated soft clay foundations. They utilize cement as a hardening agent, and through specialized mixing machinery, the soft soil and hardening agent are forcibly mixed deep within the foundation. A series of physicochemical reactions between the hardening agent and the soft soil cause the soft soil to harden into a high-quality foundation with integrity, water stability, and a certain strength. For example, rotating mixing blades force cement slurry injected into the soft soil to mix with the surrounding soft soil, forming a mud-reinforced solid. The specific construction process of cement-soil mixing walls is existing technology and will not be elaborated upon in this invention.

[0082] S2: After the cement-soil mixing wall 400 is completed and before the cement-soil has solidified, the pile holes are constructed at the designed positions of the support piles 300 and the vertical support piles 600 using a drilling rig.

[0083] During pile hole construction, according to design requirements, there are different cross sections at different depths. The upper cross section is rectangular, which is used to form the upper pile body 2, and the lower cross section is circular, which is used to form the lower pile body 3. The section in between is the enlarged part of the pile hole cross section (referred to as the enlarged part), which is used to form the enlarged body section 4.

[0084] In this invention, the length and side length / diameter of the upper pile body 2 and the lower pile body 3 can be adjusted according to engineering needs, and the diameter and height of the expanded section 4 can be adjusted according to engineering needs.

[0085] S3: After the pile hole construction is completed, the tied steel cage is lowered into the pile hole and concrete is poured into the pile hole to realize the construction of the support pile 300 and the vertical support pile 600.

[0086] Among them, the retaining piles 300 and the cement-soil mixing wall 400 are arranged alternately to form the retaining structure 500, and multiple vertical support piles 600 form a column grid.

[0087] S4: After the concrete has cured to the preset strength, excavate the foundation pit inside the retaining structure;

[0088] S5: During the excavation of foundation pit 13, construction was carried out layer by layer from top to bottom.

[0089] During the construction of each floor, a capping beam 101 / waist beam 102 is constructed at the designed position of the retaining structure 500, and a horizontal beam 702 is constructed on the side plane of the upper pile body of the vertical support pile 600. The horizontal beam 702 is connected to the capping beam 101 / waist beam 102 of the same floor, and a floor slab is constructed on the horizontal beam 702, with a construction access passage reserved on the floor slab.

[0090] When reserving access routes for construction, a floor slab can be left uninstalled at a certain location to serve as the access route.

[0091] S6: After the excavation of the foundation pit 13 is completed, a waterproof membrane 701 is constructed on the bottom surface 14 of the foundation pit 13, so that the top surface of the expanded section 4 of the support pile 300 and the vertical support pile 600 is flush with the top surface of the waterproof membrane 701. The expanded section 4 and the lower pile body 2 are located below the top surface of the waterproof membrane 701, and the upper pile body 2 is located above the top surface of the waterproof membrane 701.

[0092] S7: Seal off the construction access passages reserved on each floor slab to enable the construction of the underground structural system.

[0093] This invention involves ground construction before excavation of the foundation pit, including the construction of cement-soil mixing walls and the drilling of holes for the upper and lower circular piles. A reinforcing cage is then lowered, concrete is poured, and support piles and vertical bearing piles are constructed to form the retaining structure and column grid. After the concrete reaches the preset strength, the capping beam and garage roof slab are constructed. The horizontal beams and floor slabs of the garage roof slab are connected to the vertical bearing piles and capping beam. After the horizontal beams and floor slabs of the garage roof slab reach the predetermined strength, the foundation pit excavation continues. Once the predetermined depth is reached, the next layer of lintel beams, horizontal beams, and floor slabs are constructed. After reaching the bottom layer, the foundation layer and waterproofing membrane are constructed.

[0094] This invention contrasts with traditional underground structure construction, overcoming the drawbacks of complex, inefficient, and resource-intensive traditional underground space construction methods. By adopting a reverse construction method, construction begins from the ground, improving construction efficiency and saving significant investment in beams, slabs, columns, scaffolding, formwork, and other measures. It also avoids safety hazards associated with elevated formwork, thereby efficiently forming an underground space structure system, significantly reducing resource investment, shortening the construction period, and improving safety.

[0095] This invention achieves integrated, rapid construction and lateral connection of vertical load-bearing components and horizontal support components, providing a new construction technology for the prefabricated and industrialized development of underground structures. It changes the traditional bottom-up construction method for underground spaces, utilizing the rigidity of waterproof membranes and saving on formwork, thus greatly improving efficiency. It reduces the technical requirements of traditional reverse construction methods, minimizes foundation pit deformation during construction, is beneficial to the surrounding environment, and allows for simultaneous construction above and below ground, shortening the construction period.

[0096] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An underground structural system, characterized in that, This includes a permanent retaining pile wall for the foundation pit and an underground space frame structure connected to the permanent retaining pile wall for the foundation pit, wherein: The permanent support pile wall of the foundation pit includes a retaining structure formed by alternating support piles and cement-soil mixing walls. The underground space frame structure includes vertical support piles and multi-layer horizontal members connected to the vertical support piles. Multiple vertical support piles are arranged in their designed positions to form a column grid. The support piles and vertical bearing piles are reinforced concrete bored piles, including a vertically arranged pile body located in a pile hole formed before the excavation of the foundation pit. The pile body is formed by a steel cage and concrete located in the pile hole. The pile body includes an upper pile body and a lower pile body. The upper pile body is a cuboid prism, and the lower pile body is a cylinder. There is an enlarged section between the upper pile body and the lower pile body. The cross-sectional dimension of the enlarged section is larger than the cross-sectional dimensions of the upper pile body and the lower pile body. The upper pile body, the enlarged section, and the lower pile body are arranged sequentially from top to bottom in the pile hole with their central axes coincident and are integrally formed. The interior of the retaining structure is used for excavating the foundation pit. The multi-layered horizontal members are constructed layer by layer from top to bottom and connected to the plane of the upper pile side of the vertical support pile during the excavation of the foundation pit. Each layer of horizontal members is connected to the inner side of the permanent support pile wall of the foundation pit.

2. The underground structural system according to claim 1, characterized in that, The multi-layer horizontal structure includes a waterproof slab and at least one layer of horizontal beams and floor slabs. The waterproof slab is located on the bottom surface of the excavated foundation pit. The top surface of the expanded section is flush with the top surface of the waterproof slab. The expanded section and the lower pile body are located below the top surface of the waterproof slab, and the upper pile body is located above the top surface of the waterproof slab. The at least one layer of horizontal beams and floor slabs are located on the plane of the upper pile side of the vertical support pile above the waterproofing slab.

3. The underground structural system according to claim 2, characterized in that, A capping beam is installed at the top of the permanent support pile wall of the foundation pit. Several layers of waist beams are installed on the inner side of the permanent support pile wall of the foundation pit at the designed height. During the excavation of the foundation pit, the capping beam / waist beam is constructed layer by layer from top to bottom. The capping beam / waist beam is connected to the horizontal beam of the same layer. The floor slab is set on the horizontal beam of the same layer.

4. The underground structural system according to claim 3, characterized in that, The span of the vertical support pile is an integer multiple of the span of the retaining pile.

5. An underground structural system according to claim 4, characterized in that, The expanded section includes an upper frustum and a lower cylinder. The central axes of the upper frustum and the lower cylinder coincide. The diameter of the upper frustum gradually increases from top to bottom, and the bottom diameter of the upper frustum is equal to the top diameter of the lower cylinder.

6. An underground structural system according to claim 5, characterized in that, The rectangular cross-section of the upper pile body is inscribed within the rectangle formed by the projection of the top surface of the upper frustum, which is within the circle of the top surface of the upper frustum. The circular cross-section of the lower pile body is inscribed within the rectangle formed by the projection of the top surface of the upper frustum, which is within the rectangle formed by the projection of the upper pile body.

7. An underground structural system according to claim 6, characterized in that, The steel cage includes longitudinal steel bars and transverse stirrups, wherein the longitudinal steel bars are arranged vertically and run along the length of the pile body.

8. An underground structural system according to claim 7, characterized in that, The longitudinal reinforcing bars are arranged in a rectangular shape within the upper pile body according to the rectangular cross-section of the upper pile body, and the longitudinal reinforcing bars are arranged in a circular shape within the lower pile body according to the circular cross-section of the lower pile body. The portion of the longitudinal reinforcing bars within the expanded section is an inwardly bent transition zone.

9. An underground structural system according to claim 8, characterized in that, With the center point of the rectangular cross-section of the upper pile body as the center, the longitudinal steel bars closer to the outside bend inward to a greater degree, and the longitudinal steel bars at the point where the circular cross-section of the lower pile body is tangent to the rectangular cross-section of the upper pile body do not bend.

10. An underground structural system according to claim 8, characterized in that, The longitudinal reinforcement on the stress-bearing side of the pile and on the opposite side of the stress-bearing side is reinforced.

11. An underground structural system according to claim 10, characterized in that, The transverse stirrups are densely arranged in the bottom area of ​​the upper pile body, the top area of ​​the lower pile body, and the entire area of ​​the expanded section.

12. A construction method for an underground structural system according to any one of claims 3-11, characterized in that, The method includes: S1: Before the foundation pit is excavated, the cement-soil mixing wall is constructed at the designed location of the cement-soil mixing wall. S2: After the cement-soil mixing wall is completed and before the cement-soil has solidified, the pile holes are constructed by drilling at the designed positions of the support piles and the vertical support piles. S3: After the pile hole construction is completed, the tied steel cage is lowered into the pile hole and concrete is poured into the pile hole to realize the construction of the support pile and the vertical support pile; The retaining piles and the cement-soil mixing wall are arranged alternately to form a retaining structure, and multiple vertical support piles form a column grid; S4: After the concrete has cured, excavate the foundation pit inside the retaining structure; S5: During the excavation of the foundation pit, construction is carried out layer by layer from top to bottom; During the construction of each floor, a capping beam / waist beam is constructed on the enclosure structure, a horizontal beam is constructed on the upper pile body of the vertical support pile, the horizontal beam is connected to the capping beam / waist beam of the same floor, a floor slab is constructed on the horizontal beam, and a construction access passage is reserved on the floor slab. S6: After the foundation pit is excavated, a waterproof membrane is constructed on the bottom surface of the foundation pit, and the top surface of the expanded section is flush with the top surface of the waterproof membrane. The expanded section and the lower pile body are located below the top surface of the waterproof membrane, and the upper pile body is located above the top surface of the waterproof membrane. S7: Block off the construction access passages reserved on each floor slab.

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