Method for reinforcing ultra-deep covering layer foundation in targeted mode through ultrahigh-pressure jet grouting piles

Abstract

The invention discloses a method for reinforcing an ultra-deep covering layer foundation in a targeted mode through ultrahigh-pressure jet grouting piles, relates to the field of pile foundations, and solves the problems that when the ultra-deep covering layer foundation is reinforced through high-pressure jet grouting, the construction difficulty is large, and the reinforcing effect is poor. According to the technical scheme, the method for reinforcing the ultra-deep covering layer foundation in the targeted mode through the ultrahigh-pressure jet grouting piles comprises the steps that firstly, a drilling construction point is determined, construction equipment is arranged, then drilling is conducted to obtain a guide hole, and the lower end of the guide hole is located in a soft layer or located on the bottom face of the soft layer; the diameter of the guide hole is the minimum value meeting the follow-up water-gas-slurry sectional type cutting rotary jet grouting construction requirement, then the high-pressure rotary jet pipe is lowered to the bottom of the guide hole, water-gas-slurry sectional type cutting rotary jet grouting is conducted on a soft layer, anti-liquefaction reinforcement is conducted on an unconsolidated stratum on the surface layer of a foundation, and the water-gas-slurry sectional type cutting rotary jet grouting construction requirement is met. And maintaining the constructed high-pressure jet grouting pile, finally, determining a next drilling construction point, and repeating the construction. The method is used for targeted reinforcement of the ultra-deep covering layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of pile foundation, in particular to a method for targeted reinforcement of a foundation with super-deep overburden layer by using super-high pressure jet grouting pile. BACKGROUND

[0002] With the increasing perfection of eastern infrastructure, large-scale infrastructure construction in China is advancing to the west. The western region is located in a strong earthquake zone in China, with complex geological conditions, significant dynamic disasters, and especially in the valley region. In the field of water conservancy construction, heavy load structures have high requirements for the deformation and stability of the foundation, but the thickness of the overburden layer at many construction sites exceeds 100m, and the overburden layer is a super-deep overburden layer, which contains soft soil, silt, silt, sand and other soft layers. These overburden layers have the following characteristics: (1) the soil is stratified, each layer of soil has different properties and strengths, and there are soft layers in the deep part of the foundation, which need to be targeted for reinforcement; (2) the upper load is large, and the stiffness of the entire foundation needs to be improved to control the long-term deformation of the foundation under the action of the upper load; (3) the surface of the foundation in the valley region often has liquefiable soil layers, such as saturated sand and gravel soil, which are prone to liquefaction instability under strong earthquakes.

[0003] To reinforce the super-deep overburden layer foundation, the problems of targeted reinforcement of soft layers, overall improvement of foundation stiffness and surface foundation liquefaction need to be solved. High-pressure jet grouting pile is a foundation reinforcement technology that injects cement slurry into the stratum through high pressure, and the cement slurry is fully mixed with the soil, and then hardened to form a high-strength pile. The existing high-pressure jet grouting pile adopts a "full-section grouting" mode, and a large amount of returned slurry is not effectively utilized during construction, usually directly discharged or abandoned, resulting in a large amount of waste of cement slurry and significantly increasing the material cost. For super-deep overburden layer foundation, there are not only soft layers but also hard soil layers in the overburden layer, and the hole of the high-pressure jet grouting pile is cylindrical, which has the problem of construction difficulty when penetrating through the hard soil layer. In addition, the effective reinforcement depth of the high-pressure jet grouting pile is shallow, and it cannot target the reinforcement of the deep soft layer. Combined with insufficient jet pressure and unstable jet state, it is prone to problems such as small pile diameter, large slurry consumption and low slurry utilization rate, resulting in ineffective consumption of slurry in the hard soil layer section and insufficient reinforcement effect of the soft layer, thereby causing insufficient strength of the pile or unsatisfactory reinforcement effect. SUMMARY

[0004] The present application provides a method for targeted reinforcement of a super-deep overburden layer foundation by using a super-high pressure jet grouting pile, which solves the problems of large construction difficulty and poor reinforcement effect of high-pressure jet grouting reinforcement of super-deep overburden layer foundation.

[0005] The technical scheme adopted by the present application is: a method for targeted reinforcement of a super-deep overburden layer foundation by using a super-high pressure jet grouting pile, comprising the following steps: S1, determine the drilling construction point and arrange the construction equipment around the drilling construction point, including positioning the high-pressure jet grouting pile host to the drilling construction point, and arranging the high-pressure slurry pump, high-pressure water pump and air compressor.

[0006] In order to facilitate subsequent construction, further: step S1 also arranges the electric control cabinet, slurry storage tank, slurry mixing tank, stock bin and water tank around the high-pressure jet grouting pile host, and also configures a crane around the high-pressure jet grouting pile host.

[0007] In order to reduce the length of the pipeline of the high-pressure slurry pump and the high-pressure water pump, further: in step S1, the distance between the high-pressure slurry pump and the high-pressure jet grouting pile host is 3-5m, and the distance between the high-pressure water pump and the high-pressure jet grouting pile host is 3-5m.

[0008] S2, drill a hole to the design elevation at the drilling construction point, and the resulting pilot hole is vertically arranged and penetrates the loose stratum of the ground surface layer, the lower end of the pilot hole is located in the soft layer or at the bottom surface of the soft layer, and the diameter of the pilot hole is the minimum value that meets the construction requirements of step S3.

[0009] In order to ensure the hole forming quality of the pilot hole and avoid hole collapse, further: during drilling in step S2, a casing is used to protect the wall in the loose stratum of the ground surface layer, and the pipe wall of the casing is provided with holes. For example, in step S2, the holes of the casing are distributed in a quincunx shape, the hole diameter is 150-200mm, and the hole spacing is 2-5m.

[0010] S3, lower the high-pressure jet grouting pipe to the bottom of the pilot hole, and then perform water-air-slurry segmented cutting and jet grouting and grouting on the soft layer from bottom to top, wherein: a first jet hole and a second jet hole are provided around the lower end of the high-pressure jet grouting pipe in a vertically downward direction, and when performing water-air-slurry segmented cutting and jet grouting and grouting, high-pressure water is first jet grouted to the soft layer through the first jet hole, and then slurry and compressed air are simultaneously jet grouted to the soft layer through the second jet hole to form a gas-liquid mixture, part of the gas-liquid mixture mixes with the soil of the soft layer and forms an expanded pile, the remaining gas-liquid mixture naturally returns to the slurry by gas lifting effect, part of the returned slurry fills the pilot hole and forms a pilot pile, part of the returned slurry enters the loose stratum around the pilot hole, and the remaining returned slurry is discharged from the hole of the pilot hole.

[0011] S3-1, use the high-pressure jet grouting pipe to perform water-air-slurry segmented cutting and jet grouting and grouting on the soft layer, and raise the high-pressure jet grouting pipe during the jet grouting and grouting process, and when the second jet hole of the high-pressure jet grouting pipe is raised to the top surface of the soft layer being processed, stop the jet grouting and grouting.

[0012] S3-2, raise the high-pressure jet grouting pipe until the first jet hole enters the bottom surface of the last layer of untreated soft layer, and then repeat step S3-1 until the treatment of all soft layers is completed.

[0013] S3-3, stop the jet grouting and grouting, and pull out the high-pressure jet grouting pipe from the pilot hole.

[0014] Step S3 carries out water-gas-slurry segmented cutting rotary jet grouting on the weak layer to form a diameter-expanded pile in the weak layer, and realizes targeted reinforcement of the weak layer. In order to sufficiently reinforce the weak layer, specifically: in step S3, the diameter-expanded pile has a diameter not less than 2.0 m, the high-pressure water has a pressure ≥20 MPa, the slurry has a pressure ≥40 MPa, and the compressed air has a pressure of 1-2 MPa.

[0015] S4, carries out anti-liquefaction reinforcement on the loose stratum of the ground surface layer.

[0016] In order to sufficiently utilize the back slurry discharged from the hole of the guide hole, further: step S1 also constructs a slurry collecting ditch and a waste slurry pool around the drilling construction point, in step S3, the back slurry discharged from the guide hole through the ground is collected in the waste slurry pool through the slurry collecting ditch to form waste slurry, and step S4 injects the waste slurry back to the loose stratum of the ground surface layer. For example, step S4 excavates a grid-shaped grouting groove on the ground, or lays a grid-shaped grouting pipe on the ground, and injects the waste slurry back to the loose stratum.

[0017] S5, carries out maintenance on the completed high-pressure rotary jet pile.

[0018] S6, determines the next drilling construction point, and repeats steps S1-S5 until all the high-pressure rotary jet piles are completed.

[0019] The drilling construction points are generally multiple and arranged in rows and columns, and the distance between two adjacent drilling construction points is close. In order to avoid the problem of slurry stringing caused by the water-gas-slurry segmented cutting rotary jet grouting of the adjacent drilling construction points in sequence, further: when step S6 determines the next drilling construction point, a hole separation construction method is adopted.

[0020] The beneficial effects of the present application are: the present application adopts the ultra-high pressure jet grouting pile to target reinforce the ultra-deep and thick overburden layer foundation, the soft layer is reinforced by the water-gas-slurry segmented cutting jet grouting, the ultra-deep and thick overburden layer foundation can be reinforced, the reinforcement depth reaches 50-100m, and even can reach more than 100m. The present application drills a hole at the drilling construction point, the diameter of the hole only needs to meet the water-gas-slurry segmented cutting jet grouting construction, the diameter of the hole is small, and the construction difficulty of drilling in the ultra-deep and thick overburden layer foundation is reduced. The present application forms a diameter expansion pile in the soft layer, and does not jet grout in the hard soil layer which does not need to be reinforced, so that the effect of "targeted reinforcement" is achieved, and the material utilization rate is high. During the water-gas-slurry segmented cutting jet grouting, part of the returned slurry fills the pilot hole and enters the loose stratum around the pilot hole, not only the loose stratum around the pilot hole is reinforced, but also a small-diameter pile body is formed in the pilot hole, which is equivalent to forming a small-diameter pile composite foundation, the stiffness of the loose stratum on the foundation surface is improved, and the deformation resistance of the loose stratum is improved. The present application also reinforces the liquefaction resistance of the loose stratum on the foundation surface, and solves the problem of insufficient liquefaction resistance of the loose stratum on the foundation surface. The present application solves the problems of deep soft layer soil reinforcement, upper layer soil stiffness improvement and shallow layer soil liquefaction resistance at the same time, has a triple reinforcement effect, has high material utilization rate, and has superior economy. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a schematic diagram of the planar arrangement of the construction equipment and construction facilities in step S1 of the present application around the drilling construction point.

[0022] Figure 2 is a schematic diagram of the water-gas-slurry segmented cutting jet grouting in step S3 of the present application.

[0023] Figure 3 is a schematic diagram of the structure of the high-pressure jet grouting pile in the vertical section obtained by the construction of the present application and the schematic diagram of the return of the waste slurry to the loose stratum on the foundation surface.

[0024] Figure 4 is a schematic diagram of the collection of the returned slurry in the waste slurry pool and the return of the waste slurry to the loose stratum by the slurry collecting trench of the present application.

[0025] The drawings show that: the drilling construction point 1, the high-pressure jet grouting pile host 2-1, the high-pressure slurry pump 2-2, the high-pressure water pump 2-3, the air compressor 2-4, the electric control cabinet 2-5, the slurry storage tank 2-6, the slurry stirring tank 2-7, the stock bin 2-8, the water tank 2-9, the crane 2-10, the slurry collecting trench 2-11, the waste slurry pool 2-12, the loose stratum 3-1, the soft layer 3-2, the gravel layer 3-3, the high-pressure jet grouting pipe 4, the first jet hole 4-1, the second jet hole 4-2, the diameter expansion pile 5-1, the pilot hole pile 5-2, the casing 6, the grouting pipe 7. DETAILED DESCRIPTION

[0026] The application will be further described below with reference to the accompanying drawings.

[0027] The application is used for treating foundation with complex geological conditions, deep soil layer and difficult to be directly treated, aiming to improve the stability and bearing capacity of the foundation. The method for targeted reinforcement of super-deep overburden foundation by super-high pressure jet grouting pile comprises the following steps S1-S6.

[0028] S1, determine the drilling construction point 1 and arrange the construction equipment around the drilling construction point 1, including positioning the high-pressure jet grouting pile host 2-1 to the drilling construction point 1, and arranging the high-pressure slurry pump 2-2, the high-pressure water pump 2-3 and the air compressor 2-4.

[0029] Step S1 precisely positions the high-pressure jet grouting pile host 2-1 to the corresponding position of the drilling construction point 1, which is the starting point and basis of the entire construction process. The high-pressure jet grouting pile host 2-1 is the core equipment of the construction, and the selection of its position directly affects the accuracy and efficiency of the subsequent construction. In order to ensure the stability and efficiency of the slurry supply, the high-pressure slurry pump 2-2 and the high-pressure water pump 2-3 are generally arranged at a distance of 3-5 meters from the high-pressure jet grouting pile host 2-1. This distance makes the length of the pipeline moderate, reduces the pressure loss, and at the same time ensures that the slurry can be supplied to the construction area in time and in sufficient quantity. The air compressor 2-4 is used to provide compressed air, and in order to ensure the stability and efficiency of the compressed air supply, the distance between the air compressor 2-4 and the high-pressure jet grouting pile host 2-1 is generally 10-15m.

[0030] In order to facilitate subsequent construction, step S1 generally also arranges an electric control cabinet 2-5, a slurry storage tank 2-6, a slurry stirring tank 2-7, a material bin 2-8 and a water tank 2-9 around the high-pressure jet grouting pile host 2-1, as shown in Figure 1 The material bin 2-8 and the water tank 2-9 are used to supply materials to the slurry stirring tank 2-7, and the stirred slurry in the slurry stirring tank 2-7 is temporarily stored in the slurry storage tank 2-6. In addition, step S1 also configures a crane 2-10 around the high-pressure jet grouting pile host 2-1, which is used to hoist and move various equipment and materials such as drill pipes, cement slurry barrels, etc. during the construction process, improving the flexibility and efficiency of the construction.

[0031] In order to timely discharge the mud and waste soil generated during the construction process and avoid pollution and obstruction to the construction area, a mud discharge ditch is also set up at the corresponding position of the high-pressure jet grouting pile host 2-1. The mud discharge ditch is generally set up at the downwind side of the construction area and ensures the shortest path of the ditch.

[0032] S2, drill a hole to the design elevation at the drilling site 1, the hole obtained is vertically arranged and penetrates the loose stratum 3-1 of the ground surface layer, the lower end of the hole is located in the soft layer 3-2 or at the bottom surface of the soft layer 3-2, and the diameter of the hole is the minimum value meeting the construction requirements of step S3.

[0033] During the drilling operation, the hole position deviation is generally controlled to be less than 5 cm, and the hole bottom deflection rate is generally controlled to be within 1%, so as to ensure the accuracy and stability of the drilling. If hole collapse occurs during drilling, a casing 6 is used for wall protection, see Figure 2 The casing 6 is generally a PVC pipe, and the pipe wall of the casing 6 is provided with holes. The casing 6 is generally arranged in the loose stratum 3-1 of the ground surface layer, and the holes of the casing 6 are used for the slurry to enter the loose stratum around the hole. For example, the holes of the casing 6 are distributed in a quincunx shape, the hole diameter is 150-200 mm, and the hole spacing is 2-5 m. The lower end of the hole is located in the soft layer 3-2 or at the bottom surface of the soft layer 3-2. When the thickness of the soft layer 3-2 is too large, the hole cannot reach the bottom surface of the soft layer 3-2, and the lower end of the hole is located in the soft layer 3-2 after the hole is drilled to the design elevation. For example, see Figure 3 , the hole penetrates the loose stratum 3-1 and the soft layer 3-2 from top to bottom, the lower end of the hole is located at the bottom surface of the soft layer 3-2, and the lower end of the hole is also located at the top surface of the hard stratum, for example, the hard stratum is a gravel layer 3-3.

[0034] The drill rod used for drilling is a hollow circular tube structure, the inner cavity of the drill rod is a mud discharge pipe, the end face of the drill rod is provided with a connecting hole, for example, four connecting holes with equal central angles, a plurality of through holes are arranged in the rod body along the axial direction, each through hole forms a high-pressure slurry pipeline, a compressed air pipeline, a high-pressure water pipeline, a pressure sensor line pipe, and a reverse suction cement pipeline, a reverse suction air pipeline, a control mud valve oil pressure pipeline, and a pressure sensor line pipe.

[0035] S3, lower the high-pressure jet pipe 4 to the bottom of the hole, and then perform water-gas-slurry segmented cutting and jet grouting on the soft layer 3-2 from bottom to top. Step S3 includes steps S3-1 to S3-3.

[0036] S3-1, use the high-pressure jet pipe 4 to perform water-gas-slurry segmented cutting and jet grouting on the soft layer 3-2.

[0037] The lower end of the high-pressure jet pipe 4 is provided with a first jet hole 4-1 and a second jet hole 4-2 in the vertically downward direction, see Figure 2The first jet hole 4-1 and the second jet hole 4-2 are arranged along the circumferential direction of the high-pressure rotary jet pipe 4. During the water-gas-slurry segmented cutting rotary jet grouting, high-pressure water is first jetted to the soft layer 3-2 through the first jet hole 4-1, and then slurry and compressed air are simultaneously jetted to the soft layer 3-2 through the second jet hole 4-2. Generally, the slurry and the compressed air can be sent in 30 seconds later than the high-pressure water. The high-pressure water forms a high-pressure water jet to cut and crush the soil of the soft layer 3-2. The pressure of the high-pressure water is generally not less than 20 MPa, so as to ensure the effective cutting and crushing of the water jet to the soil. In order to sufficiently reinforce the soft layer 3-2, the pressure of the slurry is generally greater than or equal to 40 MPa, and the pressure of the compressed air is generally 1-2 MPa, so as to ensure the fullness and uniformity of the grouting, and improve the quality and stability of the reinforcement.

[0038] The second jet hole 4-2 simultaneously jets the slurry and the compressed air to the soft layer 3-2 to form a gas-liquid mixture, the gas-liquid mixture expands the cutting and further crushes the soft layer 3-2, and at the same time, the slurry is injected into the soil layer, the slurry is mixed with the cut and crushed soil to achieve the purpose of reinforcing the soft layer 3-2. The slurry is used to reinforce the soil, and is generally a cement-based slurry. Each second jet hole 4-2 can simultaneously jet the compressed air and the slurry, the slurry is jetted from the center of the second jet hole 4-2, the jetted slurry is columnar, the compressed air is jetted from the periphery of the slurry, and the two are concentrically distributed. Part of the gas-liquid mixture is mixed with the soil of the soft layer 3-2 and forms an expanded pile 5-1, the diameter of the expanded pile 5-1 is generally controlled to be not less than 2.0 m, the remaining gas-liquid mixture naturally returns to the slurry by the gas lifting effect. Part of the returned slurry fills the lead hole and finally forms a lead hole pile 5-2, achieving the primary recycling of the excess slurry; part of the returned slurry enters the loose stratum 3-1 around the lead hole to reinforce the loose stratum 3-1, achieving the secondary recycling of the excess slurry, and the remaining returned slurry is discharged from the hole of the lead hole. The casing 6 is provided with a hole, part of the returned slurry can enter the loose stratum 3-1 around the lead hole from the hole of the casing 6.

[0039] In order to ensure the uniformity and continuity of the rotary jet grouting, during the rotary jet grouting process, the high-pressure rotary jet pipe 4 is lifted upwards, and the high-pressure rotary jet pipe 4 is generally lifted at a uniform speed. When the second jet hole 4-2 of the high-pressure rotary jet pipe 4 is lifted to the top surface of the soft layer 3-2 being processed, the rotary jet grouting is stopped, and the processing of the soft layer 3-2 is completed.

[0040] The lead hole is jetted with high-pressure water, compressed air and slurry, the air pressure in the lead hole is greater than the atmospheric pressure, and the air pressure in the lower part of the lead hole is relatively large, and the relative air pressure in the upper part of the lead hole is small, so that part of the cut soil and the slurry will be sent to the ground by the upward airflow with the lifting of the drill pipe.

[0041] S3-2, increase the pressure of the rotary jet pipe 4 until the first jet hole 4-1 enters the bottom surface of the upper untreated soft layer 3-2, and then repeat step S3-1 until the treatment of all soft layers 3-2 is completed.

[0042] The soft layers 3-2 in the super-deep overburden ground are one or more layers. When the soft layers 3-2 are multiple layers, the water-gas-slurry segmented cutting rotary jet grouting is performed on each layer from bottom to top according to step S3-2 until the treatment of all soft layers 3-2 is completed.

[0043] S3-3, stop the rotary jet grouting, and pull out the high-pressure rotary jet pipe 4 from the guide hole.

[0044] After the rotary jet grouting is completed, the hole is plugged to prevent the leakage of slurry and pollution of the environment, and also to ensure the stability and bearing capacity of the guide hole pile 5-2. In order to effectively plug the hole, special plugging materials and equipment such as cement slurry, inflatable plug, etc. can be used.

[0045] S4, liquefaction-resistant reinforcement of the loose layer 3-1 on the ground surface.

[0046] In order to make full use of the return slurry discharged from the hole of the guide hole, the return slurry can be used for liquefaction-resistant reinforcement of the loose layer 3-1. Specifically, step S1 also constructs a slurry collecting trench 2-11 and a waste slurry pool 2-12 around the drilling construction point 1. For example, the sludge discharge trench is cleaned or modified to serve as the slurry collecting trench 2-11. In step S3, the return slurry discharged from the guide hole through the ground is collected in the waste slurry pool 2-12 to form waste slurry. In step S4, the waste slurry is injected back into the loose layer 3-1 on the ground surface. For example, in step S4, a grid-shaped grouting trench is excavated on the ground, or a grid-shaped grouting pipe 7 is laid on the ground, and the waste slurry is injected back into the loose layer 3-1. The waste slurry is generally injected back into the loose layer 3-1 under normal pressure, and the grouting trench and the grouting pipe 7 are generally arranged in a square grid shape, so that the excess slurry can be uniformly injected into the loose layer 3-1, effectively improving the liquefaction resistance of the shallow ground and reducing the waste of slurry, while reducing the pollution of excess engineering materials to the natural environment.

[0047] S5, maintenance of the high-pressure rotary jet pile after the construction is completed.

[0048] S6, determine the next drilling construction point 1, and repeat steps S1-S5 until the construction of all high-pressure rotary jet piles is completed.

[0049] Steps S5 and S6 can be implemented simultaneously. The drilling construction points 1 are generally multiple and arranged in rows and columns, and the distance between two adjacent drilling construction points 1 is close. In order to avoid the possible slurry mixing problem of water-gas-slug segmented cutting rotary jet grouting in adjacent drilling construction points 1, step S6 determines the next drilling construction point 1, and adopts the hole spacing construction method, that is, the next drilling construction point 1 is not adjacent to the current drilling construction point 1. The next drilling construction point 1 is generally separated by one drilling construction point 1 from the current drilling construction point 1, and can also be separated by two or more drilling construction points 1. In step S6, the construction equipment needs to be moved to the next drilling construction point 1, for example, a crane 2-10 is used to hoist and move the equipment. The hole spacing construction method can avoid the slurry mixing problem between adjacent drilling construction points 1, improve the construction quality and safety, and at the same time reduce the mutual influence and interference in the construction process, and improve the overall construction efficiency.

[0050] The present application is used for reinforcing the soft layer of super-deep and thick covering layer, which can effectively solve the problems of anti-sliding stability and deformation control, and realize the construction of super-high pressure rotary jet grouting pile with a buried depth of 50mm-100m underground. The jetting state is stable during the construction process, the slag and hole cleaning are efficient, and the slurry waste is very little. The present application can also be used for targeted reinforcement of soft soil foundation such as cohesive soil and sandy soil, various soft foundation reinforcement in complex environment, various soft foundation reinforcement in narrow and complex construction site conditions, water stop curtain reinforcement in deep soil layer, water stop curtain reinforcement at the outer joint of super-deep diaphragm wall, repair of various damaged water stop curtains, etc.

Claims

1. A method for targeted reinforcement of ultra-thick overburden foundations using ultra-high pressure jet grouting piles, characterized in that: The steps include: S1, determining a drilling construction point (1) and arranging construction equipment around the drilling construction point (1), including positioning a high-pressure jet grouting pile main unit (2-1) at the drilling construction point (1), and placing a high-pressure slurry pump (2-2), a high-pressure water pump (2-3), and an air compressor (2-4); S2. Drilling to the designed elevation at the drilling construction point (1), the pilot hole obtained by drilling is arranged vertically and penetrates the loose stratum (3-1) of the foundation surface layer, the front end of the pilot hole is located in the weak layer (3-2) or at the bottom surface of the weak layer (3-2), and the diameter of the pilot hole is the minimum value that meets the construction requirements of step S3; S3, lowering the high-pressure rotary jet pipe (4) to the bottom of the guide hole, and then performing water-air-slurry segmented cutting rotary jet grouting on the weak layer (3-2) from bottom to top, wherein: a first spray hole (4-1) and a second spray hole (4-2) are provided around the lower end of the high-pressure rotary jet pipe (4) in a vertical downward direction. When performing water-air-slurry segmented cutting rotary jet grouting, high-pressure water is first sprayed toward the weak layer (3-2) through the first spray hole (4-1), and then slurry and compressed air are simultaneously sprayed toward the weak layer (3-2) through the second spray hole (4-2) to form a gas-liquid mixture. Part of the gas-liquid mixture is mixed with the soil of the weak layer (3-2) and forms an expanded diameter pile (5-1). The rest of the gas-liquid mixture is naturally returned to the slurry by the air lift effect. Part of the returned slurry fills the guide hole and forms a guide hole pile (4-2). Part of the returned slurry enters the loose stratum (3-1) around the guide hole, and the rest of the returned slurry is discharged from the orifice of the guide hole; S3-1, using a high-pressure rotary jet pipe (4) to perform water-air-slurry segmented cutting rotary jet grouting on the weak layer (3-2), raising the high-pressure rotary jet pipe (4) during the rotary jet grouting process, and stopping the rotary jet grouting when the second nozzle hole (4-2) of the high-pressure rotary jet pipe (4) is raised to the top surface of the weak layer (3-2) being processed; S3-2, raising the pressure-spinning nozzle (4) until the first nozzle hole (4-1) enters the bottom surface of the previous untreated weak layer (3-2), and then repeating step S3-1 until all the weak layers (3-2) are treated; S3-3, stop the rotary jet grouting and pull out the high-pressure rotary jet pipe (4) from the lead hole; S4. Carry out liquefaction-resistant reinforcement on the loose stratum (3-1) on the surface of the foundation; S5. Maintain the completed high-pressure jet grouting piles; S6, determine the next drilling construction point (1), and repeat steps S1 to S5 until all high-pressure jet grouting piles are completed.

2. The method for targeted reinforcement of ultra-thick overburden foundation by ultra-high pressure jet grouting piles according to claim 1, characterized in that: In step S1, an electric control cabinet (2-5), a slurry storage tank (2-6), a slurry mixing tank (2-7), a silo (2-8), and a water tank (2-9) are arranged around the high-pressure jet grouting pile main unit (2-1), and a crane (2-10) is also configured around the high-pressure jet grouting pile main unit (2-1).

3. The method for targeted reinforcement of ultra-thick overburden foundation by ultra-high pressure jet grouting piles according to claim 1, characterized in that: In step S1, the distance between the high-pressure slurry pump (2-2) and the high-pressure rotary jet pile main unit (2-1) is 3 to 5 meters, and the distance between the high-pressure water pump (2-3) and the high-pressure rotary jet pile main unit (2-1) is 3 to 5 meters.

4. The method for targeted reinforcement of ultra-thick overburden foundation by ultra-high pressure jet grouting piles according to claim 1, characterized in that: During the drilling in step S2, a casing (6) is used to protect the loose stratum (3-1) on the surface of the foundation, and the casing (6) is provided with holes on its wall.

5. The method for targeted reinforcement of ultra-thick overburden foundation by ultra-high pressure jet grouting piles according to claim 4, characterized in that: In step S2, the holes of the casing (6) are distributed in a plum blossom shape, with a hole diameter of 150 to 200 mm and a hole spacing of 2 to 5 m.

6. The method for targeted reinforcement of ultra-thick overburden foundation by ultra-high pressure jet grouting piles according to claim 1, characterized in that: In step S3, the diameter of the expanded pile (5-1) is not less than 2.0m, the pressure of the high-pressure water is ≥20MPa, the pressure of the slurry is ≥40MPa, and the pressure of the compressed air is 1-2MPa.

7. The method for targeted reinforcement of ultra-thick overburden foundation using ultra-high pressure jet grouting piles according to any one of claims 1 to 6, characterized in that: In step S1, a slurry collecting ditch (2-11) and a waste slurry pool (2-12) are constructed around the drilling construction point (1). In step S3, the slurry returned from the drilling hole is collected in the waste slurry pool (2-12) through the slurry collecting ditch (2-11) to form waste slurry. In step S4, the waste slurry is injected back into the loose stratum (3-1) on the surface of the foundation.

8. The method for targeted reinforcement of ultra-thick overburden foundation using ultra-high pressure jet grouting piles according to claim 7, characterized in that: Step S4 involves excavating a grid-shaped grouting trough on the ground, or laying a grid-shaped grouting pipe (7) on the ground, and injecting the waste slurry back into the loose stratum (3-1).

9. The method for targeted reinforcement of ultra-thick overburden foundation using ultra-high pressure jet grouting piles according to any one of claims 1 to 6, characterized in that: When the next drilling construction point (1) is determined in step S6, the alternate hole construction method is adopted.

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