Composite foundation pit supporting method

Abstract

The invention provides a construction method of a composite foundation pit supporting structure, which adopts a cement-soil mixing pile wall, a plurality of H-shaped steels, a reinforced concrete coping plate, a cement-soil wall top beam, a cast-in-situ bored pile row, a plurality of pre-stressed anchor rods, a pile row coping beam and a plurality of connecting beams, a cast-in-situ bored pile row is arranged on a construction working face, close to the foundation pit side, of a cement soil mixing pile wall, the upper end of a pre-stressed anchor rod is arranged on a pile row capping beam, the pre-stressed anchor rod obliquely penetrates through the cement soil mixing pile wall through a first channel, and a cement soil wall top beam and the pile row capping beam are in rigid connection through a connecting beam. The cement-soil mixing pile wall serves as an initial cantilever soil retaining main body, and is combined with the cast-in-situ bored piles to form an integral frame for connecting the cement-soil wall top beam and the pile-in-row capping beam, so that support-free excavation of the foundation pit can be realized, the construction operation is convenient, the construction period is effectively shortened, the manufacturing cost is saved, and meanwhile, the safety and the stability of the whole foundation pit in the excavation period are ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of foundation pit support in geotechnical engineering, and particularly relates to a composite foundation pit support method, and relates to a retaining type for foundation pit excavation, which is suitable for foundation pit projects in soft soil, sandy soil and high groundwater level areas. Background Art

[0002] With the acceleration of urbanization, the depth and scale of underground space development are also increasing. Foundation pit excavation projects are affected by many factors such as geological conditions, surrounding environment, project cost and construction period. How to adopt more economical support structures while ensuring the safety and stability of foundation pit projects is a research focus of current scientific researchers. Traditional deep foundation pit projects mostly use internal support or anchor systems, which have problems such as long construction period, high cost and large environmental impact. Although cement soil mixing pile walls can provide water stopping and shallow support, their flexural rigidity is insufficient and it is difficult to use them alone in deep foundation pits. Although pile support has strong anti-lateral displacement ability, it needs to be combined with a water-stop curtain, and the support system occupies foundation pit space, affecting earth excavation efficiency.

[0003] Therefore, for deep foundation pit projects with special requirements on construction period and cost, proposing a retaining structure type that can not only ensure the safety of the foundation pit, but also meet economic efficiency and shorten the construction period is an urgent problem that technical personnel in this engineering field need to solve. Summary of the Invention

[0004] The present invention aims to provide a composite foundation pit support method, which uses connecting beams, prestressed anchor rods, and H-shaped steel to form a stable support structure with cement-soil mixing pile walls and bored cast-in-place pile rows, thereby realizing unsupported excavation of the foundation pit, facilitating construction operations, effectively shortening the construction period and saving costs, and ensuring the overall safety and stability of the foundation pit during excavation.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: A construction method for a composite foundation pit support structure, comprising: Process 1: After leveling the site and constructing the hard floor, cement-soil mixing pile wall is constructed around the foundation pit; Step 2: Insert H-shaped steel before the cement of the cement-soil mixing pile wall solidifies and hardens. The H-shaped steel is vertically arranged on the side of the cement-soil mixing pile wall away from the foundation pit and is evenly spaced along the foundation pit wall. The exposed height of the H-shaped steel is not less than 500mm. A plurality of first channels for the rods of the prestressed anchor rods to pass through and a plurality of second channels for the installation of connecting beams are reserved in the cement-soil mixing pile wall. Step 3: Tie and cast the reinforced concrete top plate and the cement-soil wall cap beam. The cement-soil wall cap beam is set at the top of the cement-soil mixing pile wall on the side away from the foundation pit. The reinforced concrete top plate is set on the top of the cement-soil mixing pile wall except for the cement-soil cap beam. The cement-soil wall cap beam is connected to each H-shaped steel. The top surface of the cement-soil wall cap beam is flush with the top surface of the reinforced concrete top plate. When casting the reinforced concrete top plate and the cement-soil wall cap beam, a notch for the subsequent construction of the connecting beam is reserved on the reinforced concrete top plate and the cement-soil wall cap beam at the corresponding connecting beam. The notch is connected to the second channel. Step 4: Drill holes on the cement-soil mixing pile wall near the foundation pit side, hoist the steel cage and pour concrete to form bored and cast-in-place piles. Use compacted grouting to fill the space between the cement-soil mixing pile wall and the bored and cast-in-place piles. Step 5: After the foundation pit is pre-dewatered, excavate to the bottom of the pile top beam to be constructed, and construct prestressed anchor rods. The upper ends of the prestressed anchor rods are set on the pile top beams. The prestressed anchor rods obliquely pass through the cement-soil mixing pile wall via the first channel. The horizontal height of the lower ends of the prestressed anchor rods is lower than the horizontal height of the bottom ends of the bored and cast-in-place piles. The pile top beams are tied and cast on the tops of the bored and cast-in-place piles. The horizontal height of the pile top beams is lower than the horizontal height of the cement-soil wall crown beams. The prestressed anchor rods are tensioned and locked. Process 6: Construct the connecting beam. The two ends of the connecting beam are rigidly connected to the cement soil wall crown beam and the pile pressure beam, and the groove is built back simultaneously, so that the reinforced concrete top plate, cement soil wall crown beam, connecting beam and pile pressure beam are connected into one to form an overall force system.

[0006] Preferably, the construction method of the composite foundation pit support structure further comprises: Step 7: After the cement-soil mixing pile wall, reinforced concrete capping slab, cement-soil wall cap beam, bored cast-in-place pile row, and pile capping beam meet the strength requirements, use the basin excavation method to first excavate the soil in the middle of the foundation pit to the designed depth, reserve a trapezoidal counter-pressure soil mass around it, and promptly pour the cushion layer and foundation slab in the middle of the foundation pit; Step 8: After the concrete of the cushion layer and foundation slab in the middle of the foundation pit reaches the designed strength, excavate the surrounding trapezoidal counter-pressure soil and complete the construction of the remaining cushion layers and foundation slab in a timely manner; Process 9: Continue to construct the basement structure to the basement top plate, densely backfill the gap between the basement structure and the foundation pit wall, remove the inner H-shaped steel, and grouting backfill the gap left by the H-shaped steel.

[0007] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 6, the connecting beam adopts a steel connecting beam, the steel connecting beam is inserted from the lower port of the second channel, the upper end of the steel connecting beam is fixedly connected to the cement soil wall crown beam, and the lower end of the steel connecting beam is fixedly connected to the pile pressure top beam.

[0008] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 6, the connecting beam adopts a reinforced concrete connecting beam, the steel cage of the reinforced concrete connecting beam is inserted into the second channel from the lower port of the second channel, the upper end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the cement soil wall crown beam, the lower end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the pile pressure top beam, and the reinforced concrete connecting beam is cast.

[0009] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, the H-shaped steel is arranged in one-to-one correspondence with the bored cast-in-place piles inside the row of piles, the connecting line between the H-shaped steel and the corresponding cast-in-place piles is perpendicular to the foundation pit wall, the prestressed anchor rod is located between adjacent H-shaped steels and between adjacent cast-in-place piles, the distance between the prestressed anchor rod and adjacent H-shaped steels is equal, and the distance between the prestressed anchor rod and adjacent cast-in-place piles is equal.

[0010] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, two connecting beams are arranged between adjacent prestressed anchor rods, the two connecting beams are parallel to each other, the plumb plane where the central axis of the connecting beam is located is perpendicular to the foundation pit wall, and the distances between the two connecting beams and their respective adjacent prestressed anchor rods are equal and the distance between the two connecting beams is equal.

[0011] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 3, when tying and casting the cement soil wall crown beam, the first embedded parts for connecting the connecting beam are respectively arranged on the cement soil wall crown beam, and in process 5, the pile pressure beam is tied and cast on the top of the bored cast-in-place pile row, and the second embedded parts for connecting the connecting beam are respectively arranged on the pile pressure beam.

[0012] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 6, the connecting beam adopts a steel connecting beam, the first embedded part adopts a first embedded steel plate, the second embedded part adopts a second embedded steel plate, the steel connecting beam passes through the second channel from the lower port of the second channel, the upper end of the steel connecting beam is fixedly connected to the first embedded steel plate on the cement soil wall crown beam by bolts or welding, and the lower end of the steel connecting beam is fixedly connected to the second embedded steel plate on the pile pressure beam by bolts or welding.

[0013] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 6, the connecting beam adopts a reinforced concrete connecting beam, the first embedded part adopts a first connecting beam anchoring steel bar, the second embedded part adopts a second connecting beam anchoring steel bar, the steel cage of the reinforced concrete connecting beam is inserted into the second channel from the lower port of the second channel, the upper end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the first connecting beam anchoring steel bar on the cement soil wall crown beam by binding or welding, the lower end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the second connecting beam anchoring steel bar on the pile pressure top beam by binding or welding, and the reinforced concrete connecting beam is cast.

[0014] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, the top surface of the trapezoidal counter-pressure soil body is flush with the top surface of the pile pressure beam, the distance h1 between the natural ground elevation and the bottom surface of the pile pressure beam, the distance h2 between the bottom surface of the pile pressure beam and the bottom surface of the foundation pit, and the distance h2 between the bottom surface of the pile pressure beam and the bottom surface of the foundation pit are greater than the distance h2 between the bottom surface of the pile pressure beam and the bottom surface of the foundation pit, the width b of the cement soil mixing pile wall is 0.6~0.8 times the distance h1 between the natural ground elevation and the bottom surface of the pile pressure beam, and one or two rows of cement soil mixing piles at the front toe of the cement soil mixing wall are lengthened and inserted 7m below the bottom of the pit to ensure the water-stopping requirement at the bottom of the pit.

[0015] It can be seen from the technical solutions disclosed above that, compared with the prior art, the present invention has the following beneficial effects: The present invention provides a construction method for a composite foundation pit support structure, which comprises the following steps: arranging a cement-soil mixing pile wall around the foundation pit, arranging bored and cast-in-place piles on the construction working surface of the cement-soil mixing pile wall close to the foundation pit, the cement-soil wall cap beam being arranged on the top of the cement-soil mixing pile wall away from the foundation pit, the upper end of the prestressed anchor rod being arranged on the pile capping beam, the prestressed anchor rod obliquely passing through the cement-soil mixing pile wall via a first channel, the lower end of the prestressed anchor rod having a lower horizontal height than the bottom end of the bored and cast-in-place pile, tying and casting the pile capping beam on the top of the bored and cast-in-place pile, the pile capping beam having a lower horizontal height than the cement-soil wall cap beam, and tensioning and locking the prestressed anchor rod. The two ends of the connecting beam are rigidly connected with the crown beam of the cement soil wall and the top beam of the pile row respectively. The connecting beam and prestressed anchor rods can be used to make the cement soil mixing pile wall and the bored cast-in-place pile row form a stable foundation pit support structure. The cement soil mixing pile wall is used as the initial cantilever retaining body and water-stop curtain. Combined with the deep bending and shearing capacity of the bored cast-in-place pile row, the prestressed anchor rods work together to effectively control the deformation of the support system, realize unsupported excavation of the foundation pit, facilitate foundation pit construction operations, effectively shorten construction period and save costs, and at the same time ensure the overall safety and stability of the foundation pit during excavation. Compared with the traditional internal support scheme, it eliminates the support installation and removal processes, saves costs, speeds up construction period, reduces construction noise and carbon emissions, and realizes green construction. In addition, by inserting H-shaped steel into the cement-soil mixing pile wall 1 before the cement solidifies and hardens, the H-shaped steels are vertically arranged on the side of the cement-soil mixing pile wall away from the foundation pit and are arranged at equal intervals along the wall of the foundation pit. The exposed height of the H-shaped steel is not less than 500 mm. The cement-soil wall crown beam connects each H-shaped steel, which can improve the bending, shear strength and anti-overturning stability of the cement-soil mixing wall 1. The connecting beam 8 connects the cement-soil wall crown beam 4 and the pile pressure beam 7 into one, ensuring that the cement-soil mixing wall 1 and the bored cast-in-place pile row 5 form a "rigid and flexible combination" composite lateral force resistance system, thereby improving the stability and safety of the foundation pit support structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram (sectional view) of a composite foundation pit supporting method of the present invention.

[0017] Figure 2 It is a structural schematic diagram (top view) of a composite foundation pit supporting method of the present invention.

[0018] In the figure: 1-cement soil mixing pile wall, 2-H-steel, 3-reinforced concrete top plate, 4-cement soil wall cap beam, 5-bored cast-in-place pile row, 51-cast-in-place pile, 6-prestressed anchor rod, 61-anchor section, 62-free section, 63-anchor head, 7-pile top beam, 8-connecting beam, 9-compression grouting, 10-foundation bottom plate, 11-trapezoidal counter-pressure soil, 12-basement top plate. DETAILED DESCRIPTION

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following examples, combined with the accompanying drawings, will provide a detailed description of the technical content and features of the present invention. It should be noted that the drawings are all in a very simplified form and are not precisely proportioned, and are only used to conveniently and clearly assist in illustrating the purpose of the embodiments of the present invention. For ease of description, the "upper" and "lower" described below are consistent with the upper and lower directions in the accompanying drawings, but this does not constitute a limitation of the technical solution of the present invention.

[0020] See also Figures 1 to 2 , a construction method of a composite foundation pit support structure, comprising: Process 1: After the site is leveled and the hard floor is constructed, the cement-soil mixing pile wall 1 is constructed around the foundation pit.

[0021] Step 2: Insert H-shaped steel 2 before the cement of the cement-soil mixing pile wall 1 solidifies and hardens. The H-shaped steel 2 is vertically arranged on the side of the cement-soil mixing pile wall 1 away from the foundation pit and is arranged at equal intervals along the wall of the foundation pit. The exposed height of the H-shaped steel 2 is not less than 500mm to facilitate the subsequent extraction and recovery of the H-shaped steel. A plurality of first channels for the rod body of the prestressed anchor rod 6 to pass through and a plurality of second channels for setting the connecting beam 8 are reserved in the cement-soil mixing pile wall 1.

[0022] Step 3: Tie and cast the reinforced concrete top plate 3 and the cement soil wall crown beam 4. The cement soil wall crown beam 4 is arranged on the top of the cement soil mixing pile wall 1 away from the foundation pit. The top of the cement soil mixing pile wall 1 is provided with the reinforced concrete top plate except for the cement soil crown beam. The cement soil wall crown beam 4 is connected to each H-shaped steel 2. The top surface of the cement soil wall crown beam 4 is flush with the top surface of the reinforced concrete top plate 3. When casting the reinforced concrete top plate 3 and the cement soil wall crown beam 4, a notch for the subsequent construction of the connecting beam 8 is reserved at the corresponding connecting beams on the reinforced concrete top plate 3 and the cement soil wall crown beam 4. The notch is connected to the second channel. By reserving a notch for the subsequent construction of the connecting beam 8 at the corresponding connecting beams 8 on the reinforced concrete top plate 3 and the cement soil wall crown beam 4, the subsequent construction of the connecting beam 8 can be facilitated.

[0023] Step 4: Drill holes on the cement-soil mixing pile wall 1 near the foundation pit side construction work surface, hoist the steel cage and pour concrete to form bored cast-in-place piles 5, and use compaction grouting 9 to fill the space between the cement-soil mixing pile wall 1 and the bored cast-in-place piles 5.

[0024] Step 5: After the foundation pit is pre-dewatered, excavate the earth to the bottom of the pile pressure beam 7 to be constructed, and construct the prestressed anchor rod 6. The upper end of the prestressed anchor rod 6 is set on the pile pressure beam 7. The prestressed anchor rod obliquely passes through the cement-soil mixing pile wall 1 through the first channel. The horizontal height of the lower end of the prestressed anchor rod 6 is lower than the horizontal height of the bottom end of the bored cast-in-place pile row 5. The pile pressure beam 7 is tied and cast on the top of the bored cast-in-place pile row 5. The horizontal height of the pile pressure beam 7 is lower than the horizontal height of the cement-soil wall crown beam 4. The prestressed anchor rod is tensioned and locked.

[0025] Step 6: Construct the connecting beam 8. The two ends of the connecting beam 8 are rigidly connected to the cement soil wall crown beam 4 and the pile pressure beam 7 respectively, and the groove is built back simultaneously, so that the reinforced concrete top plate 3, the cement soil wall crown beam 4, the connecting beam 8 and the pile pressure beam 7 are connected into one, forming an overall force system.

[0026] The present invention is characterized by arranging a cement-soil mixing pile wall 1 around the foundation pit, arranging a bored cast-in-place row pile 5 on the construction working surface of the cement-soil mixing pile wall 1 close to the foundation pit side, the cement-soil wall crown beam 4 being arranged on the top of the cement-soil mixing pile wall 1 away from the foundation pit side, the upper end of the prestressed anchor rod 6 being arranged on the row pile pressure beam 7, the prestressed anchor rod obliquely passing through the cement-soil mixing pile wall 1 via a first channel, the horizontal height of the lower end of the prestressed anchor rod 6 being lower than the horizontal height of the bottom end of the bored cast-in-place row pile 5, tying and casting the row pile pressure beam 7 on the top of the bored cast-in-place row pile 5, the horizontal height of the row pile pressure beam 7 being lower than the horizontal height of the cement-soil wall crown beam 4, tensioning and locking the prestressed anchor rod 6, The two ends of the connecting beam 8 are rigidly connected with the cement soil wall crown beam 4 and the pile pressure beam 7 respectively. The connecting beam 8 and the prestressed anchor rod 6 can be used to make the cement soil mixing pile wall 1 and the bored cast pile row 5 form a stable foundation pit support structure. The cement soil mixing pile wall 1 is used as the initial cantilever retaining body and water-stop curtain. Combined with the deep bending and shearing capabilities of the bored cast pile row 5 and the coordinated support work of the prestressed anchor rod 6, the deformation of the support system can be effectively controlled, and the unsupported excavation of the foundation pit can be realized, which is convenient for the foundation pit construction operation, effectively shortens the construction period and saves the cost, and at the same time ensures the overall safety and stability of the foundation pit during the excavation. Compared with the traditional internal support scheme, the support installation and removal process is eliminated, the cost is saved, the construction period is accelerated, the construction noise and carbon emissions are reduced, and green construction is achieved. In addition, by inserting H-shaped steel 2 before the cement of the cement-soil mixing pile wall 1 solidifies and hardens, the several H-shaped steels 2 are vertically arranged on the side of the cement-soil mixing pile wall 1 away from the foundation pit and are arranged at equal intervals along the wall of the foundation pit. The exposed height of the H-shaped steel 2 is not less than 500 mm. The cement-soil wall crown beam 4 connects each H-shaped steel 2, which can improve the bending, shear strength and anti-overturning stability of the cement-soil mixing wall 1, and the connecting beam 8 connects the cement-soil wall crown beam 4 and the pile pressure beam 7 into one, ensuring that the cement-soil mixing wall 1 and the bored cast-in-place pile row 5 form a "rigid and flexible combination" composite lateral force resistance system, thereby improving the stability and safety of the foundation pit support structure.

[0027] Preferably, the construction method of the composite foundation pit support structure further comprises: Step 7: After the cement-soil mixing pile wall 1, reinforced concrete top plate 3, cement-soil wall cap beam 4, bored cast-in-place pile row 5, and pile top beam 7 meet the strength requirements, a basin excavation method is used to first excavate the soil in the middle of the foundation pit to the designed depth, and reserve a trapezoidal counter-pressure soil body 11 around it. The trapezoidal counter-pressure soil body 11 is a right-angled trapezoid, and the right-angled sides are attached to the inner wall of the bored cast-in-place pile row 5. The cushion layer and foundation bottom plate 10 in the middle of the foundation pit are poured in time; Step 8: After the concrete of the cushion layer and foundation slab 10 in the middle of the foundation pit reaches the designed strength, excavate the surrounding trapezoidal counter-pressure soil 11, and complete the construction of the remaining cushion layers and foundation slab 10 in a timely manner; Step 9: Continue constructing the basement structure to the basement top plate 12. After densely backfilling the gap between the basement structure and the foundation pit wall, remove the inner H-shaped steel 2 and grouting backfill the gap left by the H-shaped steel 2.

[0028] The present invention reserves a trapezoidal counter-pressure soil body 11 during excavation to balance the soil pressure. Combined with the support effect of the overall structure composed of the cement-soil mixing pile wall 1, bored pile rows 5, connecting beams 8 and prestressed anchor rods 6, it can further control the deformation of the support system, facilitate foundation pit construction operations, effectively shorten the construction period and save costs, and at the same time ensure the overall safety and stability of the foundation pit during excavation.

[0029] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, the H-shaped steel 2 and the bored cast-in-place piles 51 inside the bored cast-in-place pile row 5 are arranged in a one-to-one correspondence, and the connecting line between the H-shaped steel 2 and the corresponding bored cast-in-place pile 51 is perpendicular to the foundation pit wall. The prestressed anchor rod 6 is located between adjacent H-shaped steels 2 and between adjacent bored cast-in-place piles 51. The distance between the prestressed anchor rod 6 and the adjacent H-shaped steel 2 is equal, that is, the prestressed anchor rod 6 is located in the middle position of the adjacent H-shaped steel 2. The distance between the prestressed anchor rod 6 and the adjacent bored cast-in-place pile 51 is equal, that is, the prestressed anchor rod 6 is located in the middle position of the adjacent H-shaped steel 2, so that the overall structure composed of the cement-soil mixing pile wall 1 and the bored cast-in-place pile row 5 is stable and the force transmission is smooth.

[0030] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, two connecting beams 8 are arranged between adjacent prestressed anchor rods 6, and the distance between the two connecting beams 8 and their respective adjacent prestressed anchor rods 6 is equal and the distance between the two connecting beams 8 is equal, which can further improve the stability of the overall structure composed of the cement-soil mixing pile wall 1 and the bored cast-in-place pile row 5, and the smooth force transmission effect achieved between the cement-soil mixing pile wall 1 and the bored cast-in-place pile row 5.

[0031] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, in process 3, when tying and casting the cement soil wall crown beam 4, a first embedded part is reserved on the cement soil wall crown beam 4 to facilitate a firm connection between the cement soil wall crown beam 4 and the connecting beam 8, and in process 5, the pile pressure beam 7 is tied and cast on the top of the bored cast-in-place pile row 5, and a second embedded part is reserved on the pile pressure beam 7 to facilitate a firm connection between the pile pressure beam 7 and the connecting beam 8.

[0032] Preferably, in the construction method of the composite foundation pit support structure described above, in step 6, the connecting beam 8 is a reinforced concrete connecting beam, the first embedded part is a first connecting beam anchoring steel bar, and the second embedded part is a second connecting beam anchoring steel bar. The steel cage of the reinforced concrete connecting beam is inserted into the second channel from the lower end of the second channel, the upper end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the first connecting beam anchoring steel bar on the cement soil wall crown beam 4 by tying or welding, and the lower end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the second connecting beam anchoring steel bar on the pile capping beam 7 by tying or welding, and the reinforced concrete connecting beam is cast. Using a reinforced concrete connecting beam as the connecting beam 8 can improve the connection strength between the cement soil mixing pile wall 1 and the bored cast-in-place pile row 5.

[0033] Of course, in step 6, the connecting beam 8 can also be a steel connecting beam, with the first embedded part being a first embedded steel plate and the second embedded part being a second embedded steel plate. The steel connecting beam passes through the second channel from the lower end thereof. The upper end of the steel connecting beam is fixedly connected to the first embedded steel plate on the cement-soil wall crown beam 4 by bolts or welding, and the lower end of the steel connecting beam is fixedly connected to the second embedded steel plate on the pile-pressing cap beam 7 by bolts or welding. Using a steel connecting beam as the connecting beam 8 facilitates construction and improves efficiency.

[0034] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, the top surface of the trapezoidal counter-pressure soil body 11 is flush with the top surface of the pile pressure beam 7, the distance h1 between the natural ground elevation and the bottom surface of the pile pressure beam 7, the excavation depth of h1 is generally 3~5m, the distance h2 between the bottom surface of the pile pressure beam 7 and the bottom surface of the foundation pit, the distance h2 between the bottom surface of the pile pressure beam 7 and the bottom surface of the foundation pit is greater than the distance h2 between the bottom surface of the pile pressure beam 7 and the bottom surface of the foundation pit, the width b of the cement soil mixing pile wall 1 is 0.6~0.8 times the distance h1 between the natural ground elevation and the bottom surface of the pile pressure beam 7, and the front toe of the cement soil mixing wall is lengthened by one or two rows of cement soil mixing piles and inserted 7m below the pit bottom to ensure the water-stopping requirement at the pit bottom. Therefore, when the shallow h1 depth soil is excavated, the cement soil mixing pile wall 1 serves as the initial cantilever retaining body and water-stop curtain, and then the deep h2 soil is excavated using the cement soil mixing pile wall 1, bored cast-in-place piles 5, connecting beams 8 and prestressed anchor rods to form a self-stabilizing foundation pit retaining structure, which serves as the retaining body and water-stop curtain. There is no need to set up an additional support system, thus realizing unsupported excavation of the foundation pit.

[0035] In this embodiment, the prestressed anchor rod 6 includes an anchoring section 61, a free section 62 and a 63-anchor head 63. The upper end of the free section 62 is anchored to the pile top beam 7 through the anchor head 63. The upper and lower rod sections of the free section 62 are coaxially arranged on the anchoring section 61. The upper end of the anchoring section 61 is located at the cement soil mixing pile wall 1. The lower end of the anchoring section 61, i.e., the lower end of the prestressed anchor rod 6, is located in the soil. The vertical distance from the lower end of the anchoring section 61 to the inner wall of the cement soil mixing pile wall 1 is greater than the width b of the cement soil mixing pile wall 1. The distance from the lower end of the anchoring section 61 to the natural ground elevation is greater than twice the sum of the distance h1 between the natural ground elevation and the bottom surface of the pile top beam 7 and the distance h2 between the bottom surface of the pile top beam 7 and the bottom surface of the foundation pit, so that the prestressed anchor rod has a better supporting effect on the overall structure composed of the cement soil mixing pile wall 1 and the bored cast-in-place pile row 5.

[0036] Preferably, in the above-mentioned construction method of the composite foundation pit support structure, the slope rate of the trapezoidal counter-pressure soil body 11 is 1:1.5~1:2, and the slope is protected by cast-in-place reinforced concrete surface layer or wire mesh cement mortar or wire mesh sprayed concrete, so as to improve the stability of the trapezoidal counter-pressure soil body 11 and ensure the soil counter-pressure effect.

[0037] The above description is only a description of the preferred embodiments of the present invention and does not limit the scope of the present invention. Any changes and modifications made by ordinary technicians in the field of the present invention based on the above disclosure shall fall within the scope of protection of the claims.

Claims

1. A construction method of a composite foundation pit support structure, characterized in that: include: Process 1: After leveling the site and constructing the hard floor, cement-soil mixing pile wall is constructed around the foundation pit; Step 2: Insert H-shaped steel before the cement in the cement-soil mixing pile wall solidifies. The H-shaped steel is vertically arranged on the side of the cement-soil mixing pile wall away from the foundation pit and is evenly spaced along the foundation pit wall. A plurality of first channels for the rods of prestressed anchor rods to pass through and a plurality of second channels for arranging connecting beams are reserved in the cement-soil mixing pile wall. Step 3: Tie and cast the reinforced concrete top plate and cement soil wall crown beam. The cement soil wall crown beam is set on the top of the cement soil mixing pile wall away from the foundation pit. The reinforced concrete top plate cement soil wall crown beam is set on the top of the cement soil mixing pile wall except for the cement soil crown beam to connect each H-shaped steel. The top surface of the cement soil wall crown beam is flush with the top surface of the reinforced concrete top plate. When casting the reinforced concrete top plate and cement soil wall crown beam, a notch for subsequent construction of the connecting beam is reserved at the corresponding connecting beam on the reinforced concrete top plate and the cement soil wall crown beam, and the notch is connected to the second channel; Step 4: Drill holes on the cement-soil mixing pile wall near the foundation pit side, hoist the steel cage and pour concrete to form bored and cast-in-place piles. Use compacted grouting to fill the space between the cement-soil mixing pile wall and the bored and cast-in-place piles. Step 5: After the foundation pit is pre-dewatered, excavate to the bottom of the pile top beam to be constructed, and construct prestressed anchor rods. The upper ends of the prestressed anchor rods are set on the pile top beam. The prestressed anchor rods obliquely pass through the cement-soil mixing pile wall through the first channel. The horizontal height of the lower ends of the prestressed anchor rods is lower than the horizontal height of the bottom ends of the bored and cast-in-place piles. The pile top beam is tied and cast on the top of the bored and cast-in-place piles. The horizontal height of the pile top beam is lower than the horizontal height of the cement-soil wall crown beam. The prestressed anchor rods are tensioned and locked. Process 6: Construct the connecting beam. The two ends of the connecting beam are rigidly connected to the cement soil wall crown beam and the pile pressure beam, and the groove is built back simultaneously, so that the reinforced concrete top plate, the cement soil wall crown beam connecting beam and the pile pressure beam are connected into one, forming an overall force system.

2. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: Also includes: Step 7: After the cement-soil mixing pile wall, reinforced concrete capping slab, cement-soil wall cap beam, bored cast-in-place pile row, and pile capping beam meet the strength requirements, use the basin excavation method to first excavate the soil in the middle of the foundation pit to the designed depth, reserve a trapezoidal counter-pressure soil mass around it, and promptly pour the cushion layer and foundation slab in the middle of the foundation pit; Step 8: After the concrete of the cushion layer and foundation slab in the middle of the foundation pit reaches the designed strength, excavate the surrounding trapezoidal counter-pressure soil and complete the construction of the remaining cushion layers and foundation slab in a timely manner; Process 9: Continue to construct the basement structure to the basement top plate, densely backfill the gap between the basement structure and the foundation pit wall, remove the inner H-shaped steel, and grouting backfill the gap left by the H-shaped steel.

3. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: In step 6, the connecting beam adopts a reinforced concrete connecting beam, the steel cage of the reinforced concrete connecting beam is inserted into the second channel from the lower port of the second channel, the upper end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the cement soil wall crown beam, the lower end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the pile pressure beam, and the reinforced concrete connecting beam is cast.

4. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: In step 6, the connecting beam adopts a steel connecting beam, which passes through the second channel from the lower port of the second channel. The upper end of the steel connecting beam is fixedly connected to the cement soil wall crown beam, and the lower end of the steel connecting beam is fixedly connected to the pile pressure top beam.

5. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: The H-shaped steels are arranged in a one-to-one correspondence with the bored piles inside the row of bored piles. The connecting line between the H-shaped steels and the corresponding bored piles is perpendicular to the wall of the foundation pit. The prestressed anchor rods are located between adjacent H-shaped steels and between adjacent bored piles. The distances between the prestressed anchor rods and adjacent H-shaped steels are equal, and the distances between the prestressed anchor rods and adjacent bored piles are equal.

6. The construction method of the composite foundation pit support structure according to claim 5, characterized in that: Two connecting beams are set between adjacent prestressed anchor rods. The two connecting beams are parallel to each other. The plumb plane where the central axis of the connecting beam is located is perpendicular to the foundation pit wall. The distances between the two connecting beams and their respective adjacent prestressed anchor rods are equal and the distance between the two connecting beams is equal.

7. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: In process 3, when tying and casting the cement soil wall crown beam, the first embedded parts for connecting the connecting beam are respectively set on the cement soil wall crown beam. In process 5, the pile pressure beam is tied and cast on the top of the bored cast-in-place pile row, and the second embedded parts for connecting the connecting beam are respectively set on the pile pressure beam.

8. The construction method of the composite foundation pit supporting structure according to claim 7, characterized in that: In process 6, the connecting beam adopts a reinforced concrete connecting beam, the first embedded part adopts the first connecting beam anchoring steel bar, and the second embedded part adopts the second connecting beam anchoring steel bar. The steel cage of the reinforced concrete connecting beam is inserted into the second channel from the lower port of the second channel, the upper end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the first connecting beam anchoring steel bar on the cement soil wall crown beam by binding or welding, the lower end of the steel cage of the reinforced concrete connecting beam is fixedly connected to the second connecting beam anchoring steel bar on the pile top beam by binding or welding, and the reinforced concrete connecting beam is cast.

9. The construction method of the composite foundation pit supporting structure according to claim 7, characterized in that: In process 6, the connecting beam adopts a steel connecting beam, the first embedded part adopts a first embedded steel plate, the second embedded part adopts a second embedded steel plate, the upper end of the steel connecting beam passes through the second channel from the lower port of the second channel, the upper end of the steel connecting beam is fixedly connected to the first embedded steel plate on the cement soil wall crown beam by bolts or welding, and the lower end of the steel connecting beam is fixedly connected to the second embedded steel plate on the pile pressure beam by bolts or welding.

10. The construction method of the composite foundation pit support structure according to claim 1, characterized in that: The top surface of the trapezoidal counter-pressure soil body is flush with the top surface of the pile-row pressure beam, the distance h1 from the natural ground elevation to the bottom surface of the pile-row pressure beam, the distance h2 from the bottom surface of the pile-row pressure beam to the bottom surface of the foundation pit, the distance h2 from the bottom surface of the pile-row pressure beam to the bottom surface of the foundation pit is greater than the distance h2 from the bottom surface of the pile-row pressure beam to the bottom surface of the foundation pit, the width b of the cement-soil mixing pile wall is 0.6 to 0.8 times the distance h1 from the natural ground elevation to the bottom surface of the pile-row pressure beam, and one or two rows of cement-soil mixing piles at the front toe of the cement-soil mixing wall are lengthened and inserted 7m below the pit bottom to ensure the water-stopping requirements at the pit bottom.

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