Construction method of large-area foundation pit support

Abstract

The invention relates to the technical field of foundation pit supporting, and discloses a large-area foundation pit supporting construction method.A reinforcing mesh is arranged on a slope on one side of a drainage ditch on the ground, soil nails are arranged on the reinforcing mesh, the reinforcing mesh is fixed through the soil nails, a concrete layer is sprayed to form shotcrete-anchor supporting, and a waterproof curtain is arranged on the surface of the lower end of the reinforcing mesh; row piles are arranged on the inner side of the waterproof curtain and adopt cast-in-situ bored piles as foundation pit fender posts, a soil body below a foundation pit excavation face on the inner sides of the cast-in-situ bored piles is reinforced to achieve passive area soil body reinforcement, a top beam is arranged at the upper ends of the cast-in-situ bored piles, a reinforcing pier is arranged on the inner side of the top beam, and a steel lattice inclined pile is arranged on the side, away from the top beam, of the reinforcing pier. According to the construction method of the large-area foundation pit support, after the steel lattice inclined piles are put down, the reinforcing pier construction reinforcing bars and the steel lattice columns at the tops of the steel lattice inclined piles are firmly fixed, and the top beams and the reinforcing piers are poured, so that the top beams, the reinforcing piers and the steel lattice inclined piles form an integral structure.

Description

Technical Field

[0001] The invention relates to the technical field of foundation pit support, and in particular to a construction method for large-area foundation pit support. Background Art

[0002] With the construction of cities, large foundation pit projects adjacent to existing buildings are becoming more and more common. Since the external construction has been completed, the available space outside the foundation pit is limited, and the deformation control requirements of existing buildings are strict. For such large-scale deep foundation pits, the traditional pile row plus internal support support method is often difficult to construct, and the cost is also high. Therefore, a new support structure form and construction method are needed to better meet the support requirements of this type of foundation pit. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a construction method for large-area foundation pit support, which solves the problems mentioned in the above background.

[0004] The present invention provides the following technical solutions: a construction method for large-area foundation pit support, a large-area foundation pit support structure, comprising: a drainage ditch opened on the ground, a steel mesh is arranged on a slope on one side of the drainage ditch, soil nails are arranged on the steel mesh, the steel mesh is fixed by the soil nails and a concrete layer is sprayed to form a sprayed anchor support, a drainage hole is opened on the sprayed concrete panel of the sprayed anchor support, and a water-stop curtain is arranged on the lower end surface of the steel mesh; A row of piles is arranged inside the water-stop curtain, and the row of piles adopts Φ700 bored piles as foundation pit retaining piles, and drainage ditches are also arranged around the bottom of the foundation pit; The soil below the excavation surface of the foundation pit inside the bored pile is reinforced to achieve passive zone soil reinforcement, a cap beam is arranged at the upper end of the bored pile, a reinforcement pier is arranged inside the cap beam, and a steel lattice inclined pile is arranged on the side of the reinforcement pier away from the cap beam; Construction methods, including: Step S1, construction preparation; Step S2, measurement positioning, elevation control and review; Step S3, pile construction; Step S4, construction of water-stop curtain and passive zone soil reinforcement; Step S5, steel lattice inclined pile construction; Step S6, excavation of crown beam and construction of crown beam; Step S7, earth excavation; Step S8: spray anchor support.

[0005] Preferably, the drainage ditch is arranged around the top and bottom of the foundation pit. The drainage ditch includes a U-shaped concrete trough and a reinforcing rib mesh arranged inside the U-shaped concrete trough. The cross-sectional size of the drainage ditch is 500x500mm. C20 concrete is poured and a certain number of sump pits are arranged to promptly drain the water in the pit. A sump pit is arranged every 25m along the bottom of the pit. The water in the sump pit needs to be directly pumped into the peripheral drainage system by a water pump connected to a fire hose. Sprayed anchor support is used for the inner gap of the pile row. M12 150mm long steel bars are used at a vertical spacing of 2m. The φ6.5@250mm steel mesh is fixed to the pile row steel bars, and 80mm thick C20 concrete is sprayed for reinforcement.

[0006] Preferably, the steel lattice inclined piles are formed by combining steel lattice columns and cement mixing piles. The steel lattice columns are composed of angle steels and tie plates. Four L140mm*12mm angle steels are used to form a square frame. Two adjacent angle steels are connected by tie plates. The tie plates are divided into two types. The tie plates perpendicular to the direction of the angle steels adopt specifications of 440mm*200mm*8mm, and the tie plates inclined to the direction of the angle steels adopt specifications of 550mm*120mm*8mm. The spacing is 80mm and they are evenly arranged from top to bottom along the length of the angle steels. The connection between the steel lattice columns uses 440mm*440mm*8mm tie plates to reinforce and cover them on all sides, and all tie plates are fully welded.

[0007] Preferably, the water-stop curtain adopts the double-axis mixing pile technology, with a construction diameter of 700mm, a length of 8m, and a cement content of not less than 20%. A single row of cement mixing piles reinforces the soil outside the row of piles, and they bite each other 200mm to form a water-stop curtain. The concrete strength grade of the bored pile body is C35, and the main reinforcement adopts 18 φ22mm steel bars. The pile length varies from 13 to 23m. The reverse circulation method is adopted during construction, and the one-jump-two method is adopted. The crown beam is 1500mm wide and 1000mm high. The steel lattice columns at the top of the row of piles and the steel lattice inclined piles are inserted into the crown beam for reinforcement. Inside the pier, it is tied and fixed with the reinforcing pier steel bars to make the three of them tightly combined to reduce the deformation after the later excavation. For the underground garage in the foundation pit, when the steel lattice inclined piles pass through the bottom plate and the side wall, water stop plates are set to improve the anti-seepage performance of the structure, and angle braces are set in the corners of the foundation pit where there are no steel lattice inclined piles for construction. Support columns are set for support, and lattice column supports are set at the lower part of the long suspended section. The lattice column is inserted into the cast-in-place pile for not less than 3 meters. A concrete support beam is set on the top of the lattice column, and the top of the lattice column extends into the concrete support beam and is firmly fixed to the support beam steel bars to form a whole.

[0008] Preferably, in step S2, before the pile construction, after the site is leveled, measurement and layout are first carried out, the pile position layout must be reviewed, the position of the pile hole is accurately located using a total station, and four guard piles are symmetrically set on the periphery, and four control piles are placed by pulling crosshairs according to the pile hole positioning points. The four control guard piles are used as a reference to control the buried position of the casing and the accurate positioning of the drilling rig. The guard piles must be well protected to prevent them from being disturbed during the construction process, so that the pile position can be reviewed at any time during the construction process to maintain the accuracy of the pile position, and the drilling construction can only be carried out after the measurement and supervision engineer has re-measured and confirmed that there are no errors.

[0009] Preferably, in step S3: Step S31, step S31, burying the guard pile and the steel casing, the guard pile adopts wooden pile or steel pile, the burial depth into the soil is 30cm-40cm, and it cannot be loose. After the guard pile is positioned, the loose soil around it is removed, and concrete is filled around it for protection; after the positioning is completed, the steel casing is buried. The steel casing is formed by rolling 10mm steel plate, and its inner diameter should be at least 20cm larger than the pile diameter. The casing length is 3 meters, and the casing is buried by excavation and backfilling. The top of the casing is 0.3m higher than the original ground, so that the drill bit can be positioned and the pile hole can be protected. The deviation of the plane position of the casing does not exceed 5cm, and the deviation of the verticality is controlled at 1.0%. The mud is intended to be made of bentonite, water, soda ash, etc. in proportion to make a non-dispersed, low-solid phase, high-viscosity mud. The relative density of the mud meets 1.20-1.45 and the viscosity meets 19-28S; Step S32, installing the drilling rig and conducting a test run. Before installing the drilling rig, check whether the drilling rig platform meets the design requirements and construction requirements of the platform. The platform should be flat and stable. The drilling rig installation mainly controls the drilling rig and the drilling frame to be stable and reliable, and the position should be accurate. After the drilling rig is installed, a test run should be conducted; Step S33, drilling construction and hole formation detection, the drilling adopts a positive circulation drilling process, the mud pump pipe is connected to the upper part of the drill pipe, the mud is pressed to the bottom of the drill bit through the center of the drill pipe, and the mud is flushed and cut while rotating, and the mud blocks are stirred into mud and overflowed out of the hole, and this cycle is continuous; in the hole formation construction, the drilling speed is controlled by adjusting the parameters such as drilling pressure, rotation speed, and mud index to prevent the occurrence of hole tilting, shrinkage, etc., and at the same time, the pressure of the mud in the hole is used to balance the hole wall pressure and water pressure, so as to play a wall protection role; After the drilling rig is accurately aligned with the axis pile position, the drilling rig frame must be horizontal and stable, and the drill rod must be vertical; the drill tip hanging line must be aligned with the cross pile line to ensure that the error between the drill bit and the center of the hole diameter is within 1 cm. The hole diameter must not be less than the design requirement, and it must be guaranteed by the drill bit diameter. The hole depth must not be less than the design requirement. The drilling tool must be measured accurately. At the same time, according to the measured elevation of the hole mouth, it is supplemented by measuring rope measurement to ensure that the depth meets the requirements. The thickness of the sediment at the bottom of the hole must be less than 20CM after cleaning the hole and before pouring. Step S34, pouring concrete. After the hole measurement is completed, the steel cage is lowered. Before pouring concrete, the air lift reverse circulation process should be used for secondary hole cleaning and the thickness of the sediment at the bottom of the hole should be tested. After the sediment thickness at the bottom of the hole is tested to be qualified, concrete is poured.

[0010] Preferably, in step S4: Step S41, indoor test of cement-reinforced soil. Before construction, indoor test of cement-reinforced soil should be carried out. The cement mixing ratio is determined according to the properties of the reinforced soil and the bearing capacity requirements of the single pile. Cement slurry is prepared according to the ratio determined by the test pile and stored in the aggregate. Step S42, spraying and mixing, the water-cement ratio of cement slurry is preferably 0.45-0.55, the motor working time shall not exceed 60A when the mixing head is pre-stirring and sinking, if the sinking speed is too slow and the resistance is too large, cement can be appropriately delivered through the slurry pipe to dilute the soil to facilitate drilling, and a flow pump should be used to control the spraying speed during construction, and the outlet pressure of the grouting pump should be maintained at 0.2-0.4MPa. The equipment must be leveled after it is in place to ensure that it does not tilt or move during construction. The verticality deviation of the frame and the drill rod is not greater than 1%. A hanging hammer is used to observe the verticality of the drill rod during construction. If the deviation is too large, it must be adjusted in time; the centering deviation of the drilling rig pile position shall not be greater than 5cm, and the prepared cement slurry shall not have segregation. The stagnation time shall not exceed 2 hours. If the stagnation time is too long, it should be reduced. Grade use, strictly control the spraying amount and mixing speed according to the parameters determined by the test pile, to ensure construction quality, improve work efficiency, reduce cement waste, try to carry out continuous construction; during the slurry delivery stage, sufficient slurry delivery pressure must be guaranteed, and slurry must be supplied continuously. If the slurry is stopped for a short time for some reason, the slurry mixing head should be sunk to 0.5m below the slurry stop point, and slurry spraying and mixing should be resumed after the slurry supply is restored. If the slurry is stopped for more than 10 minutes, the slurry delivery pipeline must be thoroughly cleaned to prevent the cement slurry from condensing in the pipeline and affecting the construction. The sinking and lifting speed of the mixer should be strictly controlled. During construction, if the drill rod cannot be drilled due to underground obstacles, the supervisor and design personnel should be notified in time so that timely measures can be taken to supplement the piles. In addition, the size of the mixing blades should be checked regularly. If the blade diameter is smaller than the designed pile diameter due to wear, the blades should be replaced.

[0011] Preferably, in step S5: Step S51, before construction, carefully familiarize yourself with the steel lattice design drawings, prepare the steel lattice processing plan and installation plan, and do a good job of briefing the construction personnel; do a good job of understanding the construction site environment, purchase and maintain materials and equipment, connect temporary electricity and water, and plan construction access roads and travel plans; Step S52, before drilling the cement mixing body, the positioning is carried out according to the drawing, the drilling positioning error is less than 50mm, the hole inclination error is less than 3°, and the pile diameter deviation is not more than 20mm. In the excavated trench, according to the requirements of the design drawings, the position, elevation and positioning of the lattice column are accurately measured and positioned using RTK. After the positioning is accepted by the supervisor and the owner, the drilling rig is moved to the construction hole position, the drilling rig is adjusted to make the base level, the main tower inclination angle is adjusted to meet the design requirements, the drilling tool and the steel lattice are hoisted on the main tower, the inclination of the main tower is determined again, and the drill bit is aligned with the construction pile position. If the alignment exceeds the allowable deviation, it is readjusted to keep the drilling tool, the drill bit and the construction pile position on the same axis; Step S53, after alignment, start the power, wait until the current is stable, start the winch to lower the power head, pump cement slurry in the background at the same time, drive the drill tool by the rotation of the power head, rotate the drill bit, cut the soil by the deadweight of the power head and the drill tool, and make holes while stirring. If the soil layer is complex and it is difficult to make holes, start the pressurized winch to pressurize, the power head continues to drill into the soil until the drill rod reaches the limit depth, stop drilling, stop pumping slurry, disconnect the power head from the drill rod, lift the power head, hoist the drill tool and the steel lattice on the main tower, connect the first section of the drill rod, continue to pump cement slurry in the background, and continue drilling; The designed hole depth is met and the bottom expansion is completed. The bottom expansion is mainly to improve the bearing capacity of the pile end, as well as the pull-out and displacement resistance. Finally, the steel lattice column is inserted and sent to the pile top elevation. The power head is lifted and the drill rod and drill bit are gradually removed. The selection of the oblique forced cement mixing body drilling machine should meet the requirements of the design parameters. The diameter of the anchor body: the enlarged end is φ1500mm, and the ordinary section is φ1200mm. The drilling and lifting speed of the mixing drill rod (0.5, 0.8m / min) has an error of no more than ±10cm / min. The oblique forced cement mixing body grouting material uses PO 42.5 cement, the cement addition amount of solid piles is 35%, the empty pile part is 7%, and the water-cement ratio is 0.5-0.7. The cement slurry should be mixed evenly and used as soon as it is mixed. The cement slurry mixed once should be used up before the initial setting. The construction and installation of the steel lattice diagonal brace should strictly control the plane position, angle and other parameters, and engineering piles should be avoided.

[0012] Preferably, in step S6, crown beam steel bars and reinforcing pier structural reinforcement are constructed on the top of the retaining piles, and the crown beam steel bars and reinforcing pier structural reinforcement are tied or welded to fix, and after the steel lattice inclined piles are lowered, the reinforcing pier structural reinforcement is firmly fixed to the steel lattice columns on the top of the steel lattice inclined piles, and the crown beam and reinforcing piers are cast, so that the crown beam, reinforcing piers and steel lattice inclined piles form an integral structure.

[0013] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention constructs crown beam steel bars and reinforced pier structural reinforcement on the top of the retaining piles, ties or welds the crown beam steel bars and reinforced pier structural reinforcement to fix them, and after the steel lattice inclined piles are lowered, the reinforced pier structural reinforcement is firmly fixed to the steel lattice columns on the top of the steel lattice inclined piles, and the crown beam and reinforced piers are cast, so that the crown beam, reinforced piers and steel lattice inclined piles form an integral structure, which is convenient for construction and ensures stability.

[0014] 2. The present invention increases the support for the upper part of the pile row by arranging inclined piles, reduces the displacement and deformation of the upper part of the foundation pit, and reinforces the passive zone at the lower part of the pile row to increase the shear strength, rigidity and lateral resistance of the soil, reduces the displacement and deformation of the lower part of the pile row. A water-stop curtain is arranged on the outer side of the gap between the pile rows, and sprayed anchor concrete is used on the inner side to isolate the foundation pit and reinforce the soil between the piles. For the corners where inclined piles cannot be installed, reinforced concrete angle braces are used for reinforcement to form a complete foundation pit support system.

[0015] 3. In the present invention, for the underground garage in the foundation pit, when the steel lattice inclined piles pass through the bottom plate and the side wall, water stop plates are arranged to improve the anti-seepage performance of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the steel lattice inclined pile support structure of the present invention; Figure 2 This is a general drawing of the drainage ditch of the present invention; Figure 3 It is a schematic diagram of the drainage hole and the sprayed anchor support structure of the present invention; Figure 4 It is a top view of the passive zone soil reinforcement, bored piles and water-stop curtain of the present invention; Figure 5 It is a structural schematic diagram of the steel lattice structure and its diagonal bracing nodes of the present invention; Figure 6 It is a structural schematic diagram of the steel lattice water stop plate of the present invention; Figure 7 This is a structural schematic diagram of the steel lattice inclined piles of the present invention with the reinforcing piers inserted into the upper part; Figure 8 It is a schematic diagram of the support column of the present invention and its connection structure with the concrete support; Fig. 9 This is one of the construction working condition diagrams of foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.10 The second construction condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.11 The third working condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.12 The fourth construction condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.13 This is the fifth working condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.14 This is the sixth construction condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.15 This is the seventh working condition diagram of the foundation pit excavation after the inclined pile construction of the present invention is completed; Fig.16 The present invention is a construction flow chart of large-area foundation pit support.

[0017] In the figure: 1. Ground; 2. Drainage ditch; 201. Reinforcement mesh; 202. U-shaped concrete trough; 3. Reinforcement mesh; 4. Soil nails; 5. Drainage hole; 6. Water-stop curtain; 7. Bored piles; 8. Steel lattice inclined piles; 801. Steel lattice column; 8011. Angle steel; 8012. Tie plate; 802. Cement mixing pile; 9. Water-stop plate; 10. Crown beam; 11. Reinforced pier; 12. Concrete support beam; 13. Support column; 14. Passive zone soil reinforcement. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] See also Figure 1-Figure 16 A construction method for large-area foundation pit support, a large-area foundation pit support structure, comprising: a drainage ditch 2 opened on the ground 1, a steel mesh 3 is arranged on a slope on one side of the drainage ditch 2, a soil nail 4 is arranged on the steel mesh 3, the steel mesh 3 is fixed by the soil nail 4 and a concrete layer is sprayed to form a spray anchor support, a drainage hole 5 is opened on the sprayed concrete panel of the spray anchor support, and a water-stop curtain 6 is arranged on the lower end surface of the steel mesh 3; A row of piles is arranged inside the water-stop curtain 6, and the row of piles adopts Φ700 bored piles 7 as foundation pit retaining piles, and drainage ditches 2 are also arranged around the bottom of the foundation pit; The soil reinforcement below the excavation surface of the foundation pit inside the bored pile 7 realizes the passive zone soil reinforcement 14. A cap beam 10 is arranged on the upper end of the bored pile 7, a reinforcing pier 11 is arranged inside the cap beam 10, and a steel lattice inclined pile 8 is arranged on the side of the reinforcing pier 11 away from the cap beam 10; Construction methods, including: Step S1, construction preparation; Step S2, measurement positioning, elevation control and review; Step S3, pile construction; Step S4, construction of water-stop curtain 6 and passive zone soil reinforcement 14; Step S5, construction of steel lattice inclined piles 8; Step S6, earth excavation and construction of the crown beam 10; Step S7, earth excavation; Step S8: spray anchor support.

[0020] The drainage ditch 2 is arranged around the top and bottom of the foundation pit. The drainage ditch 2 includes a U-shaped concrete trough 202 and a reinforcing rib mesh 201 arranged inside the U-shaped concrete trough 202. The cross-sectional size of the drainage ditch 2 is 500x500mm. C20 concrete is poured and a certain number of sump pits are arranged to promptly drain the water in the pit. A sump pit is arranged every 25m along the bottom of the pit. The water in the sump pit needs to be directly pumped into the peripheral drainage system by a water pump connected to a fire hose. Sprayed anchor support is used for the inner gap of the pile row. M12 150mm long steel bars are used at a vertical spacing of 2m. The φ6.5@250mm steel mesh 3 is fixed to the pile row steel bars, and 80mm thick C20 concrete is sprayed for reinforcement.

[0021] The steel lattice inclined pile 8 is a combination of a steel lattice column 801 and a cement mixing pile 802. The steel lattice column 801 is composed of an angle steel 8011 and a gusset plate 8012. Four L140mm*12mm angle steels 8011 are used to form a square frame. Two adjacent angle steels 8011 are connected by gusset plates 8012. The gusset plates 8012 are divided into two types. The gusset plates 8012 perpendicular to the direction of the angle steel 8011 are 440mm*200mm*8mm in size, and the gusset plates 8012 inclined to the direction of the angle steel 8011 are 550mm*120mm*8mm in size. The spacing is 80mm. They are evenly arranged from top to bottom along the length of the angle steel 8011. The connection between the steel lattice columns 801 and the steel lattice columns 801 is reinforced with gusset plates 8012 of 440mm*440mm*8mm on all sides. Cover, all the gussets 8012 are fully welded. After the welding of the steel lattice column 801 is completed, the drill rod needs to pass through the middle. When the steel lattice column 801 is connected, the 440mm*440mm*8mm gussets 8012 are used to reinforce and cover on all sides, and the full welding method is used. The cement mixing pile 802 is realized by cutting the soil and injecting 35% cement. The cement mixing pile 802 is a φ1200mm cylindrical pile with an enlarged end at the bottom. The length is 1.5m and it is a φ1500mm cylindrical pile. The steel lattice column 801 is embedded in the cement mixing pile 802 and bears the force together. The cutter head for cutting the soil is placed at the front end of the steel lattice column 801, and the soil is cut by the transmission rod placed on the inner side of the steel lattice column 801, and is pressed into the steel lattice column 801. The synchronous grouting forms a force-bearing whole, and the enlarged pile head is formed at the end by reversing the cutter head.

[0022] The water-stop curtain 6 adopts the double-axis mixing pile technology, with a construction diameter of 700mm, a length of 8m, and a cement content of not less than 20%. The single-row cement mixing piles reinforce the soil outside the row of piles, and bite each other by 200mm to form a water-stop curtain 6. The concrete strength grade of the bored pile 7 is C35, and the main reinforcement adopts 18 φ22mm steel bars. The pile length varies from 13 to 23m. The reverse circulation method is used during construction, and the one-jump-two method is adopted. The crown beam 10 is 1500mm wide and 1000mm high. The row of piles and the steel lattice oblique piles 8 are inserted into the reinforcing pier of the crown beam 10. 11, and tied and fixed with the reinforcement pier 11 steel bars so that the three are tightly combined to reduce the deformation after the later excavation. For the underground garage in the foundation pit, when the steel lattice inclined piles 8 pass through the bottom plate and the side wall, water stop plates 9 are set to improve the anti-seepage performance of the structure, and angle braces are set in the corners of the foundation pit where there are no steel lattice inclined piles 8 for construction, and supporting columns 13 are set for support. Lattice column support is set at the lower part of the long suspended section, and the lattice column is inserted into the cast-in-place pile for not less than 3 meters. A concrete support beam 12 is set on the top of the lattice column, and the top of the lattice column extends into the concrete support beam 12 and is firmly fixed to the support beam steel bars to form a whole.

[0023] In step S2, before the pile construction, after the site is leveled, measurement and layout are first carried out. The pile position layout must be reviewed. The position of the pile hole is accurately located using a total station, and four guard piles are symmetrically set on the periphery. Four control piles are placed by pulling crosshairs according to the pile hole positioning points. The four control guard piles are used as a reference to control the buried position of the casing and the accurate positioning of the drilling rig. The guard piles must be well protected to prevent them from being disturbed during the construction process, so that the pile position can be reviewed at any time during the construction process to maintain the accuracy of the pile position. The drilling construction can only be carried out after the measurement and supervision engineer has confirmed that the measurement is correct.

[0024] In step S3: Step S31, burying the guard pile and the steel casing. The guard pile adopts wooden pile or steel pile, and the burial depth is 30cm-40cm. It cannot be loose. After the guard pile is positioned, the loose soil around it is removed, and concrete is filled around it for protection; after the positioning is completed, the steel casing is buried. The steel casing is formed by rolling 10mm steel plate. Its inner diameter should be at least 20cm larger than the pile diameter. The length of the casing is 3 meters. The casing is buried by excavation and backfilling. The top of the casing is 0.3m higher than the original ground to facilitate drill bit positioning and pile hole protection. The deviation of the plane position of the casing does not exceed 5cm, and the deviation of the verticality is controlled at 1.0%. The mud is prepared by using bentonite, water, soda ash, etc. in proportion to make non-dispersible, low solid phase, high viscosity mud. The relative density of the mud meets 1.20-1.45 and the viscosity meets 19-28S; Step S32, installing the drilling rig and conducting a test run. Before installing the drilling rig, check whether the drilling rig platform meets the design requirements and construction requirements of the platform. The platform should be flat and stable. The drilling rig installation mainly controls the drilling rig and the drilling frame to be stable and reliable, and the position should be accurate. After the drilling rig is installed, a test run should be conducted; Step S33, drilling construction and hole formation detection, the drilling adopts a positive circulation drilling process, the mud pump pipe is connected to the upper part of the drill pipe, the mud is pressed to the bottom of the drill bit through the center of the drill pipe, and the mud is flushed and cut while rotating, and the mud blocks are stirred into mud and overflowed out of the hole, and this cycle is continuous; in the hole formation construction, the drilling speed is controlled by adjusting the parameters such as drilling pressure, rotation speed, and mud index to prevent the occurrence of hole tilting, shrinkage, etc., and at the same time, the pressure of the mud in the hole is used to balance the hole wall pressure and water pressure, so as to play a wall protection role; After the drilling rig is accurately aligned with the axis pile position, the drilling rig frame must be horizontal and stable, and the drill rod must be vertical; the drill tip hanging line must be aligned with the cross pile line to ensure that the error between the drill bit and the center of the hole diameter is within 1 cm. The hole diameter must not be less than the design requirement, and it must be guaranteed by the drill bit diameter. The hole depth must not be less than the design requirement. The drilling tool must be measured accurately. At the same time, according to the measured elevation of the hole mouth, it is supplemented by measuring rope measurement to ensure that the depth meets the requirements. The thickness of the sediment at the bottom of the hole must be less than 20CM after cleaning the hole and before pouring. Step S34, pouring concrete. After the hole measurement is completed, the steel cage is lowered. Before pouring concrete, the air lift reverse circulation process should be used for secondary hole cleaning and the thickness of the sediment at the bottom of the hole should be tested. After the sediment thickness at the bottom of the hole is tested to be qualified, concrete is poured.

[0025] In step S4: Step S41, indoor test of cement-reinforced soil. Before construction, indoor test of cement-reinforced soil should be carried out. The cement mixing ratio is determined according to the properties of the reinforced soil and the bearing capacity requirements of the single pile. Cement slurry is prepared according to the ratio determined by the test pile and stored in the aggregate. Step S42, spraying and mixing, the water-cement ratio of cement slurry is preferably 0.45-0.55, the motor working time shall not exceed 60A when the mixing head is pre-stirring and sinking, if the sinking speed is too slow and the resistance is too large, cement can be appropriately delivered through the slurry pipe to dilute the soil to facilitate drilling, and a flow pump should be used to control the spraying speed during construction, and the outlet pressure of the grouting pump should be maintained at 0.2-0.4MPa. The equipment must be leveled after it is in place to ensure that it does not tilt or move during construction. The verticality deviation of the frame and the drill rod is not greater than 1%. A hanging hammer is used to observe the verticality of the drill rod during construction. If the deviation is too large, it must be adjusted in time; the centering deviation of the drilling rig pile position shall not be greater than 5cm, and the prepared cement slurry shall not have segregation. The stagnation time shall not exceed 2 hours. If the stagnation time is too long, it should be reduced. Grade use, strictly control the spraying amount and mixing speed according to the parameters determined by the test pile, to ensure construction quality, improve work efficiency, reduce cement waste, try to carry out continuous construction; during the slurry delivery stage, sufficient slurry delivery pressure must be guaranteed, and slurry must be supplied continuously. If the slurry is stopped for a short time for some reason, the slurry mixing head should be sunk to 0.5m below the slurry stop point, and slurry spraying and mixing should be resumed after the slurry supply is restored. If the slurry is stopped for more than 10 minutes, the slurry delivery pipeline must be thoroughly cleaned to prevent the cement slurry from condensing in the pipeline and affecting the construction. The sinking and lifting speed of the mixer should be strictly controlled. During construction, if the drill rod cannot be drilled due to underground obstacles, the supervisor and design personnel should be notified in time so that timely measures can be taken to supplement the piles. In addition, the size of the mixing blades should be checked regularly. If the blade diameter is smaller than the designed pile diameter due to wear, the blades should be replaced.

[0026] In step S5: Step S51, before construction, carefully familiarize yourself with the steel lattice design drawings, prepare the steel lattice processing plan and installation plan, and do a good job of briefing the construction personnel; do a good job of understanding the construction site environment, purchase and maintain materials and equipment, connect temporary electricity and water, and plan construction access roads and travel plans; Step S52, before drilling the cement mixing body, the positioning is carried out according to the drawing, the drilling positioning error is less than 50mm, the hole inclination error is less than 3°, and the pile diameter deviation is not more than 20mm. In the excavated trench, according to the requirements of the design drawings, the position, elevation and positioning of the lattice column are accurately measured and positioned using RTK. After the positioning is accepted by the supervisor and the owner, the drilling rig is moved to the construction hole position, the drilling rig is adjusted to make the base level, the main tower inclination angle is adjusted to meet the design requirements, the drilling tool and the steel lattice are hoisted on the main tower, the inclination of the main tower is determined again, and the drill bit is aligned with the construction pile position. If the alignment exceeds the allowable deviation, it is readjusted to keep the drilling tool, the drill bit and the construction pile position on the same axis; Step S53, after alignment, start the power, wait until the current is stable, start the winch to lower the power head, and pump cement slurry in the background at the same time, the power head rotates to drive the drill tool, the drill bit rotates, and the soil is cut by the weight of the power head and the drill tool, and the drilling depth is formed while stirring. If the soil layer is complex and the drilling depth is difficult, start the pressurized winch to pressurize, the power head continues to drill into the soil until the drill rod is drilled to the limit depth (about 2 meters from the bottom of the power head to the ground), stop drilling, stop slurry delivery, disconnect the power head from the drill rod, lift the power head, and hoist the drill tool and the steel lattice on the main tower, connect the first section of the drill rod, and continue to pump cement slurry in the background and continue drilling; (if the length of the drill tool still cannot meet the designed hole depth, repeat this step to add drill rods until the length of the drill tool meets the construction depth) to meet the designed hole depth and complete the hole bottom expansion (positive drilling to form a hole, reverse drilling to expand the hole), the hole bottom expansion is mainly to increase the pile end Bearing capacity, as well as pull-out and displacement resistance, finally insert the steel lattice column 801, send the steel lattice column 801 to the pile top elevation, lift the power head and gradually remove the drill rod and drill bit; the selection of oblique forced cement mixing body drilling machinery should meet the requirements of design parameters, anchor body diameter: expanded end φ1500mm (length 1.5m), ordinary section φ1200mm; the drilling and lifting speed of the mixing drill rod (0.5, 0.8m / min), the error is not more than ±10cm / min; the oblique forced cement mixing body grouting material uses P.O42.5 cement, the cement addition amount of solid piles is 35%, the empty pile empty mixing part is 7%, the water-cement ratio is 0.5~0.7, the cement slurry should be mixed evenly and used as soon as it is mixed. The cement slurry mixed at one time should be used up before initial setting. The construction and installation of steel lattice diagonal braces should strictly control the plane position, angle and other parameters, and engineering piles should be avoided.

[0027] In step S6, the crown beam 10 steel bars and the reinforcing pier 11 structural reinforcement are constructed on the top of the retaining piles, and the crown beam 10 steel bars and the reinforcing pier 11 structural reinforcement are tied or welded to fix, and after the steel lattice inclined piles 8 are lowered, the reinforcing pier 11 structural reinforcement is firmly fixed to the steel lattice column 801 on the top of the steel lattice inclined piles 8, and the crown beam 10 and the reinforcing pier 11 are cast, so that the crown beam 10, the reinforcing pier 11 and the steel lattice inclined piles 8 form an integral structure.

[0028] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A construction method for large-area foundation pit support, characterized in that: A large-area foundation pit support structure, comprising: a drainage ditch (2) opened on the ground (1), a steel mesh (3) arranged on a slope on one side of the drainage ditch (2), soil nails (4) arranged on the steel mesh (3), the steel mesh (3) being fixed by the soil nails (4) and sprayed with a concrete layer to form a sprayed anchor support, a drainage hole (5) being opened on the sprayed concrete panel of the sprayed anchor support, and a water-stop curtain (6) being arranged on the lower end surface of the steel mesh (3); A row of piles is arranged inside the water-stop curtain (6), and the row of piles adopts Φ700 bored cast-in-place piles (7) as foundation pit retaining piles, and drainage ditches (2) are also arranged around the bottom of the foundation pit; The soil below the excavation surface of the foundation pit inside the bored cast-in-place pile (7) is reinforced to achieve passive zone soil reinforcement (14); a crown beam (10) is arranged at the upper end of the bored cast-in-place pile (7); a reinforcement pier (11) is arranged inside the crown beam (10); and a steel lattice inclined pile (8) is arranged on the side of the reinforcement pier (11) facing away from the crown beam (10); Construction methods, including: Step S1, construction preparation; Step S2, measurement positioning, elevation control and review; Step S3, pile construction; Step S4, construction of the water-stop curtain (6) and passive zone soil reinforcement (14); Step S5, construction of steel lattice inclined piles (8); Step S6, excavation of the crown beam (10) and construction of the crown beam (10); Step S7, earth excavation; Step S8: spray anchor support.

2. A construction method for large-area foundation pit support according to claim 1, characterized in that: The drainage ditch (2) is arranged around the top and bottom of the foundation pit. The drainage ditch (2) includes a U-shaped concrete trough (202) and a reinforcing rib mesh (201) arranged inside the U-shaped concrete trough (202). The cross-sectional size of the drainage ditch (2) is 500x500mm. C20 concrete is poured and a certain number of sump pits are arranged to timely drain the water in the pit. A sump pit is arranged every 25m along the bottom of the pit. The water in the sump pit needs to be directly pumped into the peripheral drainage system by a water pump connected to a fire hose. Spray anchor support is used for the inner gap of the pile row. M12 150mm long steel bars are used at a vertical interval of 2m. The steel mesh (3) is fixed on the pile row steel bars, and 80mm thick C20 concrete is sprayed for reinforcement.

3. A construction method for large-area foundation pit support according to claim 1, characterized in that: The steel lattice inclined pile (8) is formed by combining a steel lattice column (801) and a cement mixing pile (802). The steel lattice column (801) is composed of an angle steel (8011) and a gusset plate (8012). Four angle steels (8011) are used to form a square frame. Two adjacent angle steels (8011) are connected by a gusset plate (8012). The gusset plate (8012) is divided into two types. The gusset plate (8012) perpendicular to the direction of the angle steel (8011) is 440 mm thick. The specifications of the steel lattice columns (8011) are 550mm*120mm*8mm, and the gusset plates (8012) inclined in the direction of the angle steel (8011) are 80mm apart and are evenly arranged from top to bottom along the length of the angle steel (8011). The connection between the steel lattice columns (801) and the steel lattice columns (801) is reinforced and covered on all sides with 440mm*440mm*8mm gusset plates (8012), and all gusset plates (8012) are fully welded.

4. A construction method for large-area foundation pit support according to claim 1, characterized in that: The water-stop curtain (6) is constructed by a double-axis mixing pile process, with a diameter of 700 mm, a length of 8 m, and a cement content of not less than 20%. A single row of cement mixing piles is used to reinforce the soil outside the row of piles, and the piles are interlocked with each other by 200 mm to form a water-stop curtain (6). The concrete strength grade of the bored pile (7) is C35, and the main reinforcement is 18 steel bars with a diameter of 22 mm. The reverse circulation method is used during construction, and a one-jump-two method is used. The crown beam (10) is 1500 mm wide and 1000 mm high. The row of piles and the steel lattice inclined piles (8) are inserted into the reinforcing pier of the crown beam (10). (11), and tied and fixed with the reinforcement pier (11) steel bars so that the three are tightly combined. For the underground garage in the foundation pit, when the steel lattice inclined piles (8) pass through the bottom plate and the side wall, a water stop plate (9) is set to improve the anti-seepage performance of the structure. Angle braces are set in the corners of the foundation pit where there is no steel lattice inclined pile (8) for construction, and supporting columns (13) are set for support. A lattice column support is set at the lower part of the long suspended section. The lattice column is inserted into the cast-in-place pile for not less than 3 meters. A concrete support beam (12) is set on the top of the lattice column. The top of the lattice column extends into the concrete support beam (12) and is firmly fixed to the support beam steel bars to form a whole.

5. The construction method for large-area foundation pit support according to claim 1 is characterized in that: In step S2, before the pile construction, after the site is leveled, measurement and layout are first carried out, and the pile position layout must be reviewed. The position of the pile hole is accurately located using a total station, and four guard piles are symmetrically set on the periphery. Four control piles are placed by pulling crosshairs according to the pile hole positioning points. The four control guard piles are used as a reference to control the buried position of the casing and the accurate positioning of the drilling rig.

6. A construction method for large-area foundation pit support according to claim 1, characterized in that: In step S3: Step S31, burying the guard pile and the steel casing. The guard pile adopts wooden pile or steel pile, and the burial depth into the soil is 30cm-40cm. It cannot be loose. After the guard pile is positioned, the loose soil around it is removed, and concrete is filled around it for protection; after the positioning is completed, the steel casing is buried. The steel casing is formed by rolling 10mm steel plate, and its inner diameter is at least 20cm larger than the pile diameter. The length of the casing is 3 meters. The casing is buried by excavation and backfilling. The top of the casing is 0.3m higher than the original ground to facilitate the positioning of the drill bit and the protection of the pile hole. The deviation of the plane position of the casing does not exceed 5cm, and the deviation of the verticality is controlled at 1.0%. The mud is prepared by using bentonite, water, soda ash, etc. in proportion to make a non-dispersed, low-solid-phase, high-viscosity mud. The relative density of the mud meets 1.20-1.45 and the viscosity meets 19-28S; Step S32, installing the drilling rig and conducting a test run. The drilling rig installation mainly controls that the drilling rig and the drilling frame should be stable and reliable, and the position should be accurate. After the drilling rig is installed, a test run is conducted; Step S33, drilling construction and hole detection, the drilling adopts positive circulation drilling technology, the mud pump pipe is connected to the upper part of the drill pipe, the mud is pressed to the bottom of the drill bit through the center of the drill pipe, and the mud is flushed and cut while rotating, and the mud blocks are stirred into mud and overflowed out of the hole, and this cycle continues; in the hole construction, the drilling speed is controlled by adjusting the drilling pressure, rotation speed, and mud index parameters to prevent the occurrence of hole tilting and shrinkage, and the pressure of the mud in the hole is used to balance the hole wall pressure and water pressure; the drilling rig is adjusted according to the axis After the pile position is accurately aligned, the drilling rig frame must be horizontal and stable, and the drill rod must be vertical; the drill tip hanging line must be aligned with the cross pile protection line to ensure that the error between the drill bit and the center of the hole diameter is within 1 cm. The hole diameter must not be less than the design requirement, and it must be guaranteed by the drill bit diameter. The hole depth must not be less than the design requirement. The drilling tool must be measured accurately. At the same time, according to the measured elevation of the hole mouth, it is supplemented by measuring rope measurement to ensure that the depth meets the requirements. The thickness of the sediment at the bottom of the hole must be less than 20CM after cleaning the hole and before pouring. Step S34, pouring concrete. After the hole measurement is completed, the steel cage is lowered. Before pouring concrete, the air lift reverse circulation process is used to perform secondary hole cleaning and detect the thickness of the sediment at the bottom of the hole. After the sediment thickness at the bottom of the hole is tested to be qualified, concrete is poured.

7. A construction method for large-area foundation pit support according to claim 1, characterized in that: In step S4: Step S41, indoor test of cement-reinforced soil: before construction, indoor test of cement-reinforced soil is carried out, cement mixing ratio is determined according to the properties of the reinforced soil and the bearing capacity requirements of the single pile, cement slurry is prepared according to the ratio determined by the test pile, and stored in the aggregate; Step S42, spraying and mixing, the water-cement ratio of cement slurry is 0.45-0.55, the motor working time shall not exceed 60A when the mixing head is pre-stirring and sinking, if the sinking speed is too slow and the resistance is too large, cement can be appropriately delivered through the slurry pipe to dilute the soil to facilitate drilling, and a flow pump is used to control the spraying speed during construction, and the outlet pressure of the grouting pump is maintained at 0.2-0.4MPa. The equipment must be leveled after it is in place to ensure that it does not tilt or move during construction. The verticality deviation of the frame and the drill rod is not more than 1%. A hanging hammer is used to observe the verticality of the drill rod during construction. If the deviation is too large, it must be adjusted in time; the centering deviation of the drilling rig pile position shall not be greater than 5cm, and the prepared cement slurry There shall be no segregation phenomenon, and the stagnation time shall not exceed 2 hours. If the stagnation time is too long, the grade shall be reduced. The grouting volume and mixing speed shall be strictly controlled according to the parameters determined by the test piles, and continuous construction shall be carried out as much as possible. Sufficient grouting pressure must be ensured during the slurry delivery stage, and slurry shall be supplied continuously. If the slurry is stopped for a short time for some reason, the slurry mixing head shall sink to 0.5m below the slurry stopping point, and slurry spraying and mixing shall be resumed after the slurry supply is restored. If the slurry is stopped for more than 10 minutes, the slurry delivery pipeline shall be thoroughly cleaned to prevent the cement slurry from condensing in the pipeline and affecting the construction. The sinking and lifting speed of the mixer shall be strictly controlled. If the drill rod cannot be drilled due to underground obstacles during construction, the supervisor and design personnel shall be notified in time, and pile supplementation measures shall be taken in time.

8. The construction method for large-area foundation pit support according to claim 1, characterized in that: In step S5: Step S51, before construction, carefully familiarize yourself with the steel lattice design drawings, prepare the steel lattice processing plan and installation plan, and do a good job of briefing the construction personnel; do a good job of understanding the construction site environment, purchase and maintain materials and equipment, connect temporary electricity and water, and plan construction access roads and travel plans; Step S52, before drilling the cement mixing body, the positioning is carried out according to the drawing, the drilling positioning error is less than 50mm, the hole inclination error is less than 3°, and the pile diameter deviation is not more than 20mm. In the excavated trench, according to the requirements of the design drawings, the position, elevation and positioning of the lattice column are accurately measured and positioned using RTK. After the positioning is accepted and qualified, the drilling rig is moved to the construction hole position, the drilling rig is adjusted to make the base level, the main tower inclination angle is adjusted to meet the design requirements, the drilling tool and the steel lattice are hoisted on the main tower, the inclination of the main tower is determined again, and the drill bit is aligned with the construction pile position. If the alignment exceeds the allowable deviation, it is readjusted to keep the drilling tool, the drill bit and the construction pile position on the same axis; Step S53, after alignment, the power is started, and after the current is stable, the winch is turned on to lower the power head, and cement slurry is pumped in the background at the same time. The power head rotates to drive the drilling tool, the drill bit rotates, and the soil is cut by the deadweight of the power head and the drilling tool, and the drilling is progressed while stirring; if the soil layer is complex and the drilling is difficult, the pressure winch is turned on to increase the pressure, and the power head continues to drill into the soil until the drill rod is drilled to the limit depth, and the drilling is stopped, the slurry is stopped, the connection between the power head and the drill rod is disconnected, the power head is lifted, and the drilling tool and the steel lattice are hoisted on the main tower, and the first section of the drill rod is connected. The background continues to pump cement slurry and drilling continues; the designed hole depth is met and the bottom hole expansion is completed, and finally the steel lattice column (801) is inserted, and the steel lattice column (801) is sent to the pile top elevation, the power head is lifted, and the drill rod and the drill bit are gradually removed; The selection of oblique forced cement mixing body drilling machinery meets the requirements of design parameters, anchor body diameter: expanded end φ1500mm, ordinary section φ1200mm; the drilling and lifting speed of the mixing drill rod, the error is not more than ±10cm / min; the oblique forced cement mixing body grouting material uses PO 42.5 cement, the solid pile cement addition amount is 35%, the empty pile empty mixing part is 7%, the water-cement ratio is 0.5~0.7, the cement slurry is mixed evenly and used as soon as it is mixed. The cement slurry mixed at one time should be used up before initial setting. The plane position and angle parameters should be strictly controlled during the construction and installation of steel lattice diagonal braces, and engineering piles should be avoided.

9. A construction method for large-area foundation pit support according to claim 1, characterized in that: In the step S6, the crown beam (10) steel bars and the reinforcing pier (11) structural reinforcement are constructed on the top of the retaining pile, and the crown beam (10) steel bars and the reinforcing pier (11) structural reinforcement are tied or welded to fix, and after the steel lattice inclined pile (8) is lowered, the reinforcing pier (11) structural reinforcement is firmly fixed to the steel lattice column (801) at the top of the steel lattice inclined pile (8), and the crown beam (10) and the reinforcing pier (11) are cast, so that the crown beam (10), the reinforcing pier (11) and the steel lattice inclined pile (8) form an integral structure.

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