Construction method of foundation pit support in soft soil area

Abstract

The present invention relates to the technical field of foundation pit support, and discloses a foundation pit support construction method in a soft soil area, comprising the following construction steps: 1) excavating a pit, with the construction surface at the bottom of the pit being inclined; 2) hardening the construction surface to form a hard layer; a construction position is formed on the hard layer; 3) providing a rotary pile driver, with a drill bit provided with a plurality of grouting holes, and moving the rotary pile driver to the hard layer; 4) sinking the drill bit to the center of the construction position and drilling to a set depth, and then lifting the drill rod until the drill bit is pulled out of the construction position; in this process, slurry is rotary-sprayed through the grouting holes, and after the slurry solidifies, a mixing pile is formed; 5) a plurality of mixing piles are formed to form a support structure; 6) digging downward on the construction surface to a set depth to form a foundation pit; by arranging multiple rows of mixing piles in an inclined manner, the bottoms of the mixing piles are supported within the pit bottom range, and the soil pressure of the soil outside the foundation pit on the mixing pile support structure will be greatly reduced, effectively avoiding the phenomenon of the pile body being sheared off.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation pit support, and in particular to a foundation pit support construction method in soft soil areas. Background Art

[0002] In a soft soil area containing silt or silty soil layers, the soft soil area can be divided from top to bottom into a fill layer 1 composed of fill soil, a silt layer 2 containing silt, and a clay layer 3 composed of clay;

[0003] In the prior art, in soft soil foundation pits, the traditional foundation pit support method generally adopts multiple rows of grid-type cement mixing piles 4 for support. Multiple cement mixing piles 4 bite each other to form a grid-like reinforcement body, referring to Figure 1-2 shown.

[0004] This support structure is a combined support and reinforcement system, similar to a gravity retaining wall that simultaneously reinforces soft soil and supports the soil, preventing foundation pit landslides or collapses. However, this support method has many disadvantages, such as susceptibility to cracking and deformation and cracking under soil pressure, especially for silt or silty soft soil with poor mechanical properties. Furthermore, this type of support method generally results in significant pit deformation, typically ranging from 80 to 500 mm at the top, far exceeding the pit deformation warning value specified in industry standards.

[0005] Since the failure mode of foundation pits in general soft soil areas is basically deep sliding failure, that is, the fill layer 1 slides or fails in an inclined surface, but the lower silt layer 2 slides or fails in a nearly horizontal surface. Figure 2 The potential sliding surface in the middle is 5.

[0006] Therefore, a very large earth pressure is generated on the supporting structure. Since the fill in the fill layer 1 generates a very large earth pressure on the foundation pit supporting structure, the traditional supporting structure is similar to a cantilever pile. Not only is the deformation of the top of the mixing pile 4 very large, but the pile body is often sheared. Summary of the Invention

[0007] The purpose of the present invention is to provide a foundation pit support construction method in soft soil areas, aiming to solve the problem in the prior art that the support structure in soft soil areas is not stable enough and the mixing piles are often sheared off.

[0008] The present invention is implemented in this way: the foundation pit support construction method in soft soil area includes the following construction steps:

[0009] 1) A pit of set size is excavated in a soft soil area, wherein the bottom of the pit has an inclined construction surface, and the angle between the construction surface and the horizontal plane is between 5° and 20°;

[0010] 2) After the construction surface is leveled, the construction surface is hardened with concrete to form a hard layer; before the construction surface is hardened, a plurality of circular areas that are not hardened are reserved on the construction surface, and after the hardening, the circular areas form construction sites on the hard layer;

[0011] 3) Providing a rotary pile driver, the rotary pile driver having a drill rod, a drill bit connected to the bottom of the drill rod, a plurality of grouting holes provided on the drill bit, the plurality of grouting holes being arranged at intervals along the circumference of the drill bit; moving the rotary pile driver to a set position on the hard layer, the rotary pile driver being arranged in an inclined manner, and the drill rod being arranged perpendicular to the hard layer;

[0012] 4) The drill bit is aligned with the construction site and drilled downward until it reaches the set depth, and then the drill rod is lifted upward until the drill bit is pulled out of the construction site. During the process of sinking and drilling and lifting the drill bit upward, the drill bit sprays grout outward through the grouting hole, and the slurry mixes with the soil in the construction site to form a mixing column. After the slurry solidifies, the mixing column forms an inclined mixing pile.

[0013] 5) Repeat step 4) to form a plurality of the mixing piles, which are arranged in an array to form a support structure, with adjacent mixing piles interlocking with each other;

[0014] 6) Digging downwards to a set depth on the construction surface to form a foundation pit. The support structure is arranged around the periphery of the foundation pit, and the lower part of the mixing pile extends to below the bottom of the foundation pit in a fixed arrangement.

[0015] Optionally, the mixing piles are arranged obliquely away from the foundation pit in a bottom-up direction; the foundation pit has an inclined pit wall, and the mixing piles include outer mixing piles adjacent to the pit wall and inner mixing piles away from the pit wall; the top of the outer mixing pile is higher than the top of the inner mixing pile.

[0016] Optionally, when construction is being carried out at a construction position corresponding to the outer mixing pile, in step 5), before the slurry solidifies, an inclined guide frame is placed outside the pile position of the unformed outer mixing pile; the guide frame has a chute, which extends along an extension direction of the guide frame, and the extension direction of the chute coincides with the extension direction of the corresponding mixing column;

[0017] The reinforcement is hoisted onto the slideway by a truck crane, and the reinforcement slides downward along the slideway to be inserted into the pile position. Under the action of gravity, the reinforcement is completely immersed in the pile position.

[0018] Optionally, the truck crane is provided with a vibratory hammer, which is used to press the reinforcement member down into the pile position along the extension direction of the chute until the reinforcement member is completely immersed in the pile position.

[0019] Optionally, a rock and soil curing agent is used to mix the slurry, wherein the main components of the rock and soil curing agent include P.O42.5R cement, L.SAC42.5 low alkalinity sulphoaluminate cement, S95 slag powder, sodium sulfate, and slaked lime;

[0020] The content of the P.O42.5R cement is between 10-40%, the content of the L.SAC42.5 low-alkalinity sulphoaluminate cement is between 5-25%, the content of the S95 slag powder is between 40-80%, the content of the sodium sulfate is between 1-10%, and the content of the slaked lime is between 2-15%.

[0021] Optionally, in step 4), the slurry spraying rate through the grouting hole is not less than 200 L / min; the sinking drilling speed of the drill bit is less than or equal to 0.5 m / min, and the lifting speed of the drill rod is less than or equal to 0.4 m / min.

[0022] Optionally, in step 2), before using concrete to harden the construction surface, multiple annular placement pieces are fixedly placed on the construction surface, and then the construction surface is hardened with concrete; after the concrete solidifies, the placement pieces are removed from the construction surface, and multiple circular construction positions are formed on the hard layer.

[0023] Optionally, before moving the rotary pile driver onto the hard layer in step 3), the hard layer is compacted and leveled.

[0024] Optionally, in step 4), the slurry is delivered to a drill rod through a grouting pipe, the drill rod is provided with a longitudinally arranged upper channel for slurry circulation; the drill bit is provided with a lower channel connected to the upper channel, the bottom of the lower channel is connected to a plurality of side channels, and the plurality of side channels extend outward and penetrate the outer surface of the drill bit to form the grouting holes;

[0025] The outer circumference of the drill rod is provided with a plurality of mixing heads extending horizontally outward, and the plurality of mixing heads are staggered and spaced apart along the direction from top to bottom; a horizontally arranged connecting channel is provided inside the mixing head, the rear end of the connecting channel is connected to the upper channel, and the front portion of the connecting channel has a spiral section, and the spiral section is arranged to extend spirally outward along the extension direction of the mixing head, and the front end of the spiral section passes through the middle of the outer end surface of the mixing head to form a rotary spray hole;

[0026] The outer periphery of the drill rod is provided with a stirring ring for stirring, and the stirring ring is spirally arranged along the extending direction of the drill rod, and the plurality of stirring heads are staggered with the stirring ring.

[0027] Optionally, a track is provided on the outer circumference of the stirring head, and the track is arranged around the circumference of the stirring head. A rotating ring that rotates longitudinally is sleeved on the outer circumference of the stirring head, and the rotating ring has a movable segment extending toward the track, the movable segment is annular and movably embedded in the track, and a rack is provided on the movable segment, and the rack is arranged around the circumference of the movable segment;

[0028] A transverse bevel gear meshing with the rack is provided in the track, the transverse bevel gear meshing with the longitudinal bevel gear, and the lower part of the longitudinal bevel gear is exposed in the connecting channel; a plurality of peripheral knives are provided on the outer periphery of the rotating ring, the peripheral knives are arranged at intervals along the circumference of the mixing head, a plurality of outer end knives are provided on the outer end surface of the mixing head, the outer end knives are extended outward away from the outer end surface, and the outer end knives are arranged outwardly away from the rotary spray hole;

[0029] When the drill bit sinks for drilling and is lifted upward, the slurry is sprayed outward toward the spray hole through the connecting channel, and drives the longitudinal bevel gear to rotate. Under the action of the gear transmission and the force of the soil, the movable section rotates along the orbital direction, and the rotating ring rotates; the mixing head, peripheral knife and outer end knife evenly mix the soil and slurry in the construction site.

[0030] Compared with the existing technology, the foundation pit support construction method in soft soil areas provided by the present invention first digs out an inclined construction surface, reserves a construction position while hardening the construction surface, and then moves the rotary drilling pile driver to the hard layer and aligns the construction position for construction, thereby forming inclined mixing piles; by forming multiple rows of inclined mixing piles, the bottom of the mixing piles is supported at the bottom of the pit, and the soil pressure of the soil outside the foundation pit on the mixing pile support structure will be greatly reduced, effectively avoiding the phenomenon of the pile body being sheared. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a plan view of a cement mixing pile of a foundation pit support structure in a soft soil area according to the prior art provided by the present invention;

[0032] Figure 2 It is a front view schematic diagram of the foundation pit support structure in soft soil area provided by the present invention;

[0033] Figure 3 This is a front view schematic diagram of the foundation pit support structure in soft soil area provided by the present invention;

[0034] Figure 4 It is a schematic plan view of the mixing piles of the foundation pit support structure in soft soil areas provided by the present invention;

[0035] Figure 5 is a schematic cross-sectional view of the drill rod provided by the present invention;

[0036] Figure 6 The present invention provides Figure 5 Enlarged schematic diagram of point A in the middle. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] The implementation of the present invention is described in detail below with reference to specific embodiments.

[0039] The same or similar numbers in the drawings of this embodiment correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "up", "down", "left", "right", etc. indicate directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. This is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0040] Reference Figure 3-6 The figure shows a preferred embodiment of the present invention.

[0041] The invention provided by the present invention is implemented as follows: the foundation pit support construction method in soft soil area includes the following construction steps:

[0042] 1) Excavate a pit of set size in the soft soil area. The bottom of the pit has an inclined construction surface, and the angle between the construction surface and the horizontal plane is between 5° and 20°;

[0043] 2) After the construction surface is leveled, it is hardened with concrete to form a hard layer. Before the construction surface is hardened, a plurality of circular areas that are not hardened are reserved on the construction surface. After the hardening treatment, the circular areas form construction positions on the hard layer.

[0044] 3) Provide a rotary pile driver, comprising a drill rod 200, a drill bit 210 connected to the bottom of the drill rod 200, and a plurality of grouting holes provided on the drill bit 210, the plurality of grouting holes being arranged around the circumference of the drill bit 210 at intervals; move the rotary pile driver to a set position on the hard layer, the rotary pile driver being arranged in an inclined manner, and the drill rod 200 being arranged perpendicular to the hard layer;

[0045] 4) The drill bit 210 is aligned with the construction site and drilled downward until the drill bit 210 reaches the set depth, and then the drill rod 200 is lifted upward until the drill bit 210 is pulled out of the construction site. During the process of sinking and drilling and lifting the drill bit 210 upward, the drill bit 210 sprays grout outward through the grouting hole. The slurry mixes with the soil in the construction site to form a mixed column. After the slurry solidifies, the mixed column forms an inclined mixing pile 100.

[0046] 5) Repeat the above step 4) to form a plurality of mixing piles 100, which are arranged in an array to form a support structure, with adjacent mixing piles 100 interlocking with each other;

[0047] 6) Dig down to a set depth on the construction surface to form a foundation pit. The supporting structure is arranged around the periphery of the foundation pit, and the lower part of the mixing pile 100 extends to below the bottom of the foundation pit in a fixed arrangement.

[0048] The above-mentioned method for foundation pit support construction in soft soil areas is to first dig out an inclined construction surface, reserve a construction position while hardening the construction surface, and then move the rotary drilling pile driver to the hard layer and align the construction position for construction, thereby forming an inclined arrangement of mixing piles 100; by forming multiple rows of inclined arrangement of mixing piles 100, the bottom of the mixing piles 100 is supported at the bottom of the pit, and the soil pressure of the soil outside the foundation pit on the retaining structure of the mixing piles 100 will be greatly reduced, effectively avoiding the phenomenon of the pile body being sheared.

[0049] Specifically, the soil layers in the soft soil area can be divided from top to bottom into: a fill layer composed of fill, a silt layer containing silt, i.e., silty soil, and a clay layer composed of clay. The bottom of the foundation pit is generally excavated to the lower part of the silt layer. In specific implementation, a pit is first excavated in the fill layer, the pit wall is inclined, the pit bottom is inclined, and the construction surface is hardened to ensure that the rotary drilling pile driver is arranged stably and obliquely on the construction surface. Two rows of piles are constructed on the outer side of the construction surface. After the construction of the two rows of piles on the outer side is completed, dig down a certain distance, and similarly, multiple rows of piles are constructed on the inner side. After the construction is completed, the pit is excavated to the set depth to form a foundation pit with a flat bottom.

[0050] Specific principles:

[0051] Multiple rows of mixing piles 100 are arranged at an angle, with their bases supported at the bottom of the pit, acting like diagonal braces. This exerts a reaction force on the retaining structure, acting from the inside of the pit to the outside, offsetting the soil pressure outside the foundation pit. Assuming the angle β between the mixing piles 100 and the vertical is β, and the bearing capacity of the inclined mixing piles 100 is R, the reaction force exerted by the mixing piles 100 on the soil pressure outside the foundation pit is Rsinβ, which partially offsets the soil pressure.

[0052] At the same time, the geotechnical solidifying agent is used instead of traditional cement to construct the mixing pile 100, so that the mixing pile 100 has higher pile body strength than the traditional cement mixing pile 100. The geotechnical solidifying agent mixing pile 100 is more than 2-3 times stronger than the cement mixing pile 100 with the same dosage.

[0053] From bottom to top, the mixing piles 100 are arranged obliquely away from the foundation pit; the foundation pit has an inclined pit wall, and the mixing piles 100 include outer mixing piles 100 adjacent to the pit wall and inner mixing piles 100 away from the pit wall; the top of the outer mixing pile 100 is higher than the top of the inner mixing pile 100.

[0054] Specifically, the outer mixing piles 100 in the two outermost rows extend upward to near the top of the foundation pit, which not only plays a role in supporting the part above the pile top platform, but also the counter-pressure soil 10 on the rear side of the outer mixing piles 100 acts on the pile top of the outer mixing piles 100, applying a downward oblique load to the outer mixing piles 100, playing a role of counter-pressure, acting on the outer mixing piles 100 in the two outermost rows, and can offset part of the soil pressure.

[0055] When construction is carried out at the construction position corresponding to the outer mixing pile 100, in step 5), before the slurry solidifies, an inclined guide frame is placed outside the pile position of the unformed outer mixing pile 100; the guide frame has a slide groove, which extends along the extension direction of the guide frame, and the extension direction of the slide groove coincides with the extension direction of the corresponding mixing column.

[0056] The reinforcement 110 is lifted onto the chute by a truck crane, and the reinforcement 110 slides downward along the chute until it is inserted into the pile position. Under the action of gravity, the reinforcement 110 is completely immersed in the pile position.

[0057] By fixing the reinforcement member 110 in the pile position before the slurry solidifies, the reinforcement member 110 can be a steel section, a steel pipe, etc., which greatly enhances the strength of the pile body after solidification. Specifically, the insertion direction of the reinforcement member 110 is achieved by a guide frame.

[0058] The truck crane is equipped with a vibrating hammer, which is used to press the reinforcement member 110 down into the pile position along the extension direction of the chute until the reinforcement member 110 is completely immersed in the pile position. In this way, when encountering too much resistance, the vibrating hammer can be used to press down in the original direction.

[0059] Specifically, a geotechnical solidifying agent is used to mix the slurry, and the main components of the geotechnical solidifying agent include P.O42.5R cement, L.SAC42.5 low-alkalinity sulphoaluminate cement, S95 slag powder, sodium sulfate, and slaked lime.

[0060] The content of P.O42.5R cement is between 10-40%, the content of L.SAC42.5 low alkalinity sulphoaluminate cement is between 5-25%, the content of S95 slag powder is between 40-80%, the content of glauber salt is between 1-10%, and the content of slaked lime is between 2-15%.

[0061] The water-cement ratio is 0.6-0.8, and the slurry specific gravity is 1.4-1.9. The specific data of each material are as follows:

[0062] The cement used is P.O42.5R cement with a specific surface area of ​​≥360 m2 / kg; the low-alkalinity sulphoaluminate cement used is L.SAC42.5, with a calcium oxide (CaO) content of greater than 60%, an aluminum oxide (Al2O3) content between 2-5%, a specific surface area of ​​≥420 m2 / kg, an initial setting time of no earlier than 20 minutes, and a final setting time of no later than 60 minutes; the slag powder used is S95 grade, with a specific surface area of ​​≥450 m2 / kg and a 7-day activity index of greater than 65%; the Na2SO4 content in the sodium sulfate is not less than 95%; the Ca(OH)2 content in the slaked lime used is not less than 85%, and the 90μm sieve residue is not more than 5%;

[0063] According to the test results of multiple specimens in the laboratory, after the geotechnical solidifier and silt configured in the above proportions are evenly mixed, at a dosage of 10% and a water-cement ratio of 0.7, the strength of the geotechnical solidifier and silt mixed together is more than 2-3 times the strength of the cement and silt mixed together.

[0064] In step 4), the slurry is jetted through the grouting holes at a rate of no less than 200 L / min. The drill bit 210 is lowered and drilled at a speed of less than or equal to 0.5 m / min, and the drill rod 200 is raised and lowered at a speed of less than or equal to 0.4 m / min. This ensures that the slurry is evenly mixed in the surrounding soil layers.

[0065] In step 2), before hardening the construction surface with concrete, multiple annular placement pieces are fixed on the construction surface, and then the concrete is hardened. After the concrete solidifies, the placement pieces are removed from the construction surface, forming multiple circular construction sites on the hard layer. This allows for accurate positioning of the pile holes, with the specific construction site diameter being 80-100mm larger than the diameter of the 100-ton mixing pile.

[0066] Before moving the pile driver onto the hard surface in step 3), compact the hard surface to ensure it is level and prevent the pile driver from shaking.

[0067] Specifically, in step 3), since the construction surface is an inclined plane, the rotary drilling pile driver used in the construction is a small rotary drilling pile driver with a height not exceeding 10m and a base length not less than 7m, so as to ensure that the rotary drilling pile driver remains stable in an inclined state and will not fall over.

[0068] In step 4), the slurry is transported to the drill rod 200 through the grouting pipe. The drill rod 200 is provided with a longitudinally arranged upper channel 201 for the circulation of the slurry; the drill bit 210 is provided with a lower channel 211 connected to the upper channel 201, and the bottom of the lower channel 211 is connected to multiple side channels 212. The multiple side channels 212 extend outward and penetrate the outer surface of the drill bit 210 to form grouting holes.

[0069] The outer periphery of the drill rod 200 is provided with a plurality of stirring heads 230 extending horizontally outward, and the plurality of stirring heads 230 are staggered and spaced apart along the top-down direction; the stirring head 230 is provided with a horizontally arranged connecting channel 231, the rear end of the connecting channel 231 is connected to the upper channel 201, and the front part of the connecting channel 231 is provided with a spiral section 2310, and the spiral section 2310 is arranged to extend spirally outward along the extension direction of the stirring head 230, and the front end of the spiral section 2310 passes through the middle of the outer end face of the stirring head 230 to form a rotary spray hole.

[0070] The outer periphery of the drill rod 200 is provided with a stirring ring 220 for stirring. The stirring ring 220 is arranged in a spiral pattern along the extension direction of the drill rod 200, and a plurality of stirring heads 230 are arranged in an offset manner relative to the stirring ring 220. The design of the stirring heads 230 and the stirring ring 220 ensures that the slurry and soil are evenly mixed, ensuring the quality of the pile. The design of the spray holes also ensures that the slurry is evenly mixed with the soil.

[0071] A track is provided on the outer circumference of the stirring head 230, and the track is arranged around the circumference of the stirring head 230. A longitudinally rotating rotating ring 240 is provided on the outer circumference of the stirring head 230. The rotating ring 240 has a movable segment 2400 extending toward the track. The movable segment 2400 is annular and movably embedded in the track. A rack is provided on the movable segment 2400, and the rack is arranged around the circumference of the movable segment 2400.

[0072] A transverse bevel gear 243 meshing with the rack is provided in the track, and the transverse bevel gear 243 meshes with the longitudinal bevel gear 242, and the lower portion of the longitudinal bevel gear 242 is exposed in the connecting channel 231; a plurality of peripheral knives 241 are provided on the outer periphery of the rotating ring 240, and the peripheral knives 241 are arranged at intervals along the circumference of the mixing head 230, and a plurality of outer end knives 232 are provided on the outer end surface of the mixing head 230, and the outer end knives 232 are arranged to extend outward away from the outer end surface, and the outer end knives 232 are arranged to be inclined outward away from the rotary spray hole;

[0073] As the drill bit 210 sinks and drills and is lifted upward, the slurry is sprayed outward through the connecting channel 231 toward the spray hole, driving the longitudinal bevel gear 242 to rotate. Under the gear transmission and the force of the soil, the movable section 2400 rotates along the orbital direction, and the rotating ring 240 rotates. The mixing head 230, the peripheral blade 241, and the outer end blade 232 evenly mix the soil and slurry at the construction site. When the slurry is sprayed outward, it drives the longitudinal bevel gear 242 to rotate, which in turn drives the transverse bevel gear 243 to rotate, and then drives the rotating ring 240 to rotate. The peripheral blade 241 on the rotating ring 240 and the outer end blade 232 on the mixing head 230 can break up large pieces of soil and rocks, promoting uniform mixing and ensuring the strength of the pile body.

[0074] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. The foundation pit support construction method in soft soil area is characterized by: The construction steps include: 1) A pit of set size is excavated in the soft soil area, the bottom of the pit has an inclined construction surface, and the angle between the construction surface and the horizontal plane is between 5° and 20°; 2) After the construction surface is leveled, the construction surface is hardened with concrete to form a hard layer; before the construction surface is hardened, a plurality of circular areas that are not hardened are reserved on the construction surface, and after the hardening, the circular areas form construction sites on the hard layer; 3) Provide a rotary pile driver, the rotary pile driver having a drill rod, a drill bit connected to the bottom of the drill rod, and a plurality of grouting holes arranged at intervals along the circumference of the drill bit; move the rotary pile driver to a set position on the hard layer, the rotary pile driver being arranged in an inclined position, and the drill rod being arranged perpendicular to the hard layer; 4) The drill bit is aligned with the construction site and drilled downward until it reaches the set depth, and then the drill rod is lifted upward until the drill bit is pulled out of the construction site. During the process of sinking and drilling and lifting the drill bit upward, the drill bit sprays grout outward through the grouting hole. The grout mixes with the soil in the construction site to form a mixing column. After the slurry solidifies, the mixing column forms an inclined mixing pile. 5) Repeat step 4) to form a plurality of the mixing piles, wherein the plurality of the mixing piles are arranged in an array to form a support structure, and adjacent mixing piles are engaged with each other; 6) Digging downwards on the construction surface to a set depth to form a foundation pit, the support structure is arranged around the periphery of the foundation pit, and the lower part of the mixing pile extends to below the bottom of the foundation pit in a fixed arrangement; In step 4), the slurry is delivered to the drill rod through a grouting pipe. The drill rod is provided with a longitudinally arranged upper channel for slurry circulation. The drill bit is provided with a lower channel connected to the upper channel. The bottom of the lower channel is connected to a plurality of side channels. The plurality of side channels extend outward and penetrate the outer surface of the drill bit to form the grouting holes. The outer circumference of the drill rod is provided with a plurality of mixing heads extending horizontally outward, and the plurality of mixing heads are staggered and spaced apart along the direction from top to bottom; a horizontally arranged connecting channel is provided inside the mixing head, the rear end of the connecting channel is connected to the upper channel, and the front portion of the connecting channel has a spiral section, and the spiral section is arranged to extend spirally outward along the extension direction of the mixing head, and the front end of the spiral section passes through the middle of the outer end surface of the mixing head to form a rotary spray hole; The outer periphery of the drill rod is provided with a stirring ring for stirring, and the stirring ring is spirally arranged along the extending direction of the drill rod, and the plurality of stirring heads are staggered with the stirring ring.

2. The soft soil area foundation pit support construction method according to claim 1, characterized in that: From bottom to top, the mixing piles are arranged obliquely away from the foundation pit; the foundation pit has an inclined pit wall, and the mixing piles include outer mixing piles adjacent to the pit wall and inner mixing piles away from the pit wall; the top of the outer mixing pile is higher than the top of the inner mixing pile.

3. The soft soil area foundation pit support construction method according to claim 2, characterized in that: When construction is being carried out at the construction position corresponding to the outer mixing pile, in step 5), before the slurry solidifies, an inclined guide frame is placed outside the pile position of the unformed outer mixing pile; the guide frame has a chute extending along the extension direction of the guide frame, and the extension direction of the chute coincides with the extension direction of the corresponding mixing column; The reinforcement is hoisted onto the slideway by a truck crane, and the reinforcement slides downward along the slideway to be inserted into the pile position. Under the action of gravity, the reinforcement is completely immersed in the pile position.

4. The soft soil area foundation pit support construction method according to claim 3, characterized in that: The truck crane is provided with a vibrating hammer, which is used to press the reinforcement member down into the pile position along the extension direction of the chute until the reinforcement member is completely immersed in the pile position.

5. The soft soil area foundation pit support construction method according to claim 1, characterized in that: The slurry is mixed with a rock and soil curing agent, wherein the main components of the rock and soil curing agent include P.O42.5R cement, L.SAC42.5 low-alkalinity sulphoaluminate cement, S95 slag powder, sodium sulfate, and slaked lime; The P.O42.5R cement content is between 10-40%, the L.SAC42.5 low-alkalinity sulphoaluminate cement content is between 5-25%, the S95 slag powder content is between 40-80%, the sodium sulfate content is between 1-10%, and the slaked lime content is between 2-15%.

6. The soft soil area foundation pit support construction method according to claim 1, characterized in that: In step 4), the slurry is jetted through the grouting holes at a rate of not less than 200 L / min; the drill bit sinking speed is less than or equal to 0.5 m / min, and the drill rod lifting speed is less than or equal to 0.4 m / min.

7. The soft soil area foundation pit support construction method according to claim 1, characterized in that: In step 2), before hardening the construction surface with concrete, a plurality of annular placement pieces are fixedly placed on the construction surface, and then hardening the construction surface with concrete; After the concrete solidifies, the placement piece is removed from the construction surface, and a plurality of circular construction positions are formed on the hard layer.

8. The soft soil area foundation pit support construction method according to claim 1, characterized in that: Before moving the rotary pile driver onto the hard layer in step 3), the hard layer is compacted and leveled.

9. The soft soil area foundation pit support construction method according to any one of claims 1 to 8, characterized in that: A track is provided on the outer circumference of the mixing head, and the track is arranged around the circumference of the mixing head. A rotating ring is provided on the outer circumference of the mixing head for longitudinal rotation, and the rotating ring has a movable section extending toward the track. The movable section is annular and movably embedded in the track. A rack is provided on the movable section, and the rack is arranged around the circumference of the movable section. A transverse bevel gear meshing with the rack is provided in the track, the transverse bevel gear meshing with the longitudinal bevel gear, and the lower part of the longitudinal bevel gear is exposed in the connecting channel; a plurality of peripheral knives are provided on the outer periphery of the rotating ring, the peripheral knives are arranged at intervals along the circumference of the mixing head, a plurality of outer end knives are provided on the outer end surface of the mixing head, the outer end knives are extended outward away from the outer end surface, and the outer end knives are arranged outwardly away from the rotary spray hole; When the drill bit sinks for drilling and is lifted upward, the slurry is sprayed outward toward the spray hole through the connecting channel, and drives the longitudinal bevel gear to rotate. Under the action of the gear transmission and the force of the soil, the movable section rotates along the orbital direction, and the rotating ring rotates; the mixing head, peripheral knife and outer end knife evenly mix the soil and slurry in the construction site.

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