Construction method for improving bearing capacity and durability of hard stratum mixing pile

Abstract

The application relates to the technical field of pile foundation construction, and discloses a construction method for improving the bearing capacity and durability of hard stratum stirring pile planting, which comprises the following steps: driving a first group of drill rods to drive a soil breaking drill bit to drill into the stratum and synchronously inject a fluid for cooling, lifting upward after reaching the elevation, and injecting cement slurry, solidifying agent slurry or water into the hole in the drilling lifting stage; using a second group of drill rods to drive a multidirectional stirring drill bit to drill downward and synchronously inject grouting, and carrying out multidimensional cross shearing and mixed mixing of the in-situ broken rock-soil body and the slurry in the hole; controlling the multidirectional stirring drill bit to rotate reversely and lift to separate from the hole position, and carrying out grouting or stopping grouting in the lifting stage; and planting a prefabricated pile into the elevation position before the mixed medium in the hole is initially cured. The application adopts the in-situ soil breaking and non-discharge combined internal and external pipe staggered blade reverse stirring process, overcomes the phenomenon that the cutting resistance of the hard stratum is too large and the mud ball at the bottom of the hole is stuck, realizes uniform forced mixing of the medium in the hole, reduces the pile planting penetration resistance, and improves the overall bearing performance of the composite pile.

Description

Technical Field

[0001] This invention relates to the field of pile foundation construction technology, specifically to a construction method for improving the bearing capacity and durability of mixing piles in hard strata. Background Technology

[0002] In foundation reinforcement or deep foundation pit support projects, construction areas often encounter hard strata such as dense sand layers and strongly weathered rock. To meet high bearing capacity requirements, a composite pile-planting process, involving mixing and grouting followed by the implantation of precast piles, is commonly used. When drilling in such hard strata, conventional mixing drills inevitably face significant cutting resistance, making it difficult to penetrate the soil and rock layers smoothly. If the traditional drilling process of first removing soil and removing slag is used, not only are the procedures numerous, but the disturbed strata are also prone to borehole wall collapse. If in-situ forced breaking and cutting is performed, hard debris accumulates directly inside the hole, causing the subsequent mixing drill to experience significant bottom resistance during descent, severely restricting drilling efficiency.

[0003] When the grouting and mixing stage is barely reached, most existing equipment uses drill pipes to drive blades in a unidirectional coaxial rotation. In borehole environments containing a large amount of hard rock fragments and highly viscous soil, the unidirectional rotational force easily causes debris and clay to adhere to the drill bit surface, resulting in mud clumps becoming stuck. Simultaneously, the continuous centrifugal force generated by unidirectional rotation causes physical centrifugal stratification between lighter grout and heavier soil particles. This phenomenon prevents the solidification material from achieving uniform forced mixing with the in-situ aggregate, resulting in uneven strength distribution of the formed cement-soil consolidation body.

[0004] Due to the uneven mixing of the fluid medium within the borehole, often accompanied by localized mud clumps and hardened masses, the precast piles inevitably encounter significant and unevenly distributed vertical penetration frictional resistance during subsequent precast pile implantation. This resistance can easily prevent the pile from being successfully driven to the design depth, or even cause tilting. The resulting composite pile structure, due to the low strength of the solidified medium within the borehole and inadequate individual pile implantation, struggles to achieve the designed vertical bearing capacity and long-term durability standards under hard geological conditions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a construction method for improving the bearing capacity and durability of mixing piles in hard strata. It solves the problems of excessive cutting resistance of existing drilling tools in hard strata such as dense sand or weathered rock, which makes it difficult to penetrate the strata, and the unidirectional mixing mode causing mud clumps to become stuck in the hole and material to separate into layers, resulting in uneven replacement of the medium in the hole, low compressive strength of cement-soil consolidation, and difficulty in pile formation.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a construction method for improving the bearing capacity and durability of mixing piles in hard strata, comprising the following steps:

[0007] The first set of drill rods drives the soil-breaking drill bit at its end to drill into the formation. During the drilling of the first set of drill rods into the borehole, a fluid medium is simultaneously injected to cool the soil-breaking drill bit until the drilling elevation is reached.

[0008] Control the first set of drill rods to lift upwards and exit the hole. During the lifting phase of the first set of drill rods, inject cement grout, curing agent grout, or water into the hole.

[0009] After the first set of drill rods exits the hole, the second set of drill rods is switched to the working hole. The second set of drill rods is driven to propel the multi-directional mixing drill bit at its end into the hole. During the downward drilling phase of the multi-directional mixing drill bit, cement grout or curing agent grout is injected simultaneously. The multi-directional mixing drill bit is used to perform multi-dimensional cross-shearing and mixing of the fractured rock and soil with the grout in the hole until the drilling elevation is reached.

[0010] Control the second set of drill rods to drive the multi-directional stirring drill bit to rotate in the opposite direction and simultaneously lift it upwards until it is out of the hole position;

[0011] Before the mixed medium, after being stirred by a multi-directional stirring drill bit, enters the initial setting state, the precast pile is implanted to the elevation position to form a composite pile structure.

[0012] Preferably, the center of the soil-breaking drill bit is a vertical drill rod, and spiral drill bits are distributed around the outer periphery of the vertical drill rod. The length of the spiral drill bits along the central axis is not less than 0.5m. The bottom of the soil-breaking drill bit is provided with two or more sets of alloy bullet-shaped cutting teeth with different orientations. The soil-breaking drill bit has a pre-machined grouting port inside.

[0013] Preferably, a vertical cylindrical frame is fixedly connected to the periphery of the soil-breaking drill bit. The vertical cylindrical frame contains multiple vertical rods, and each of the vertical rods is equipped with a drill bit at its bottom.

[0014] Preferably, the drive frame mechanism applies a downward driving force to the first set of drill rods, controlling the ground-breaking drill bit to drill deeper into the formation at a feed speed not exceeding 3 m / min, maintaining in-situ ground breaking without soil removal or slag removal. Alloy bullet-shaped cutting teeth are used to cut hard strata, while a spiral drill bit is used to loosen the rock and soil. Simultaneously, while the alloy bullet-shaped cutting teeth are cutting hard strata, clean water or thin slurry is pumped into the ground-breaking drill bit through a fluid grouting device and a pre-reserved grouting port to cool the drill bit. During downward drilling, the operating current and output torque of the drive motor applying the driving force to the first set of drill rods are monitored in real time. When a sudden increase in the operating current and output torque is observed, along with a sudden decrease in the downward feed speed of the ground-breaking drill bit, accompanied by vibration of the first set of drill rods and friction noise emanating from the hole, the feed is maintained until the rock penetration elevation is reached, completing the drilling operation into the formation.

[0015] Preferably, the first set of drill rods is controlled to move upwards from the hole at a speed not exceeding 5 m / min. During the entire upward lifting process of the first set of drill rods, cement grout, curing agent grout, or water is simultaneously injected into the hole using a fluid grouting device. During the overall construction operation of drilling and lifting the drill rods, the total amount of cement or curing agent injected to form the composite pile structure accounts for 15% to 30% of the mass of the soil being reinforced.

[0016] Preferably, the second set of drill rods is a double-tube drill rod structure, consisting of a coaxial inner central drill rod and an outer casing fitted around it. The multi-directional stirring drill bit includes an inner central drill rod and an outer casing fitted around it.

[0017] Preferably, the multi-directional stirring drill bit has a front blade extending laterally from the bottom of the inner central drill rod, a forward blade extending outward from the middle section, and a reverse blade extending outward from the outer casing. The forward and reverse blades are arranged in an alternating pattern in both vertical and horizontal space.

[0018] Preferably, after the first set of drill rods exits the hole, the control frame mechanism moves the second set of drill rods, equipped with a multi-directional mixing drill bit, into the working area. The second set of drill rods is driven downwards, and the multi-directional mixing drill bit is controlled to drill along the hole at a speed not exceeding 2.0 m / min. During downward drilling, the fluid grouting equipment synchronously injects cement slurry or curing agent slurry with a water-cement ratio not exceeding 1.2 into the hole through internal pipelines. Downward drilling and grouting operations continue until the multi-directional mixing drill bit reaches 50% to 100% of the drilling depth of the first set of drill rods. During the dynamic downward movement, the internal central drill rod drives the forward blades and the front blades to rotate continuously in the forward direction, while the external casing synchronously drives the reverse blades to rotate in the reverse direction. The forward and reverse blades cut the mixing medium in opposite directions. Under the multi-dimensional mixing condition of the second set of drill rods drilling downwards, when grouting is only performed inside the hole, the working pressure of the fluid grouting equipment is maintained in the range of 1MPa to 2MPa. Alternatively, when the cement grout or curing agent grout diffuses to the surrounding undisturbed soil outside the hole, high-pressure grouting is used simultaneously when the second set of drill rods is drilling downwards, and the working pressure of the fluid grouting equipment is not less than 15MPa.

[0019] Preferably, the drive state of the second set of drill rods is changed, the internal center drill rod of the multi-directional mixing drill bit is switched to reverse rotation, and the outer casing is switched to forward rotation in a synchronized reverse direction. The drill rod is then lifted upward at a speed not exceeding 3 m / min. During the lifting phase, grouting is continuously injected into the hole or stopped depending on the position of the grouting port of the multi-directional mixing drill bit. During the lifting phase of the drill bit towards the ground, when the grouting port is located at the bottom of the multi-directional mixing drill bit, grouting is stopped during the lifting phase. When the grouting port is located at the upper part of the multi-directional mixing drill bit, grout is pumped continuously during the lifting phase.

[0020] Preferably, the precast pile is inserted into the borehole before the unconsolidated, fluid cement-soil has initially set. The precast pile is pressed into the fluid cement-soil along the central axis of the borehole. Once the final pressure or penetration value is reached, the downward load is stopped, completing the vertical insertion of the precast pile. When multiple precast piles are configured, 2-4 precast piles are inserted into the borehole, and the center-to-center distance between adjacent precast piles is not less than 2.5 times the diameter or side length of the precast pile.

[0021] This invention provides a construction method for improving the bearing capacity and durability of soil mixing piles in hard strata. It has the following beneficial effects:

[0022] 1. This invention utilizes a soil-breaking drill bit with alloy bullet-shaped cutting teeth and a spiral drill bit for in-situ soil breaking without slag removal. During the drill lifting stage, cement grout or curing agent grout is pre-injected into the borehole, effectively cutting and penetrating dense sand layers or weathered rock and other hard strata. This method uses the rock and soil debris generated from in-situ breaking directly as aggregate for borehole formation, reducing soil removal and slag removal processes and the risk of borehole collapse. Furthermore, the pre-grouting creates a preliminary liquid environment within the borehole, reducing cutting and sinking resistance during subsequent re-stirring operations and improving borehole formation efficiency in hard strata.

[0023] 2. This invention employs a second set of drill rods with a double-tube structure to drive a multi-directional mixing drill bit. The alternating vertical and horizontal arrangement of the forward-facing blades of the inner central drill rod and the reverse-facing blades of the outer casing enables synchronous counter-rotation cutting, allowing the drill bit to perform multi-dimensional cross-shearing of the mixed medium within the borehole. This combination of reverse rotation and the staggered blade structure breaks the unidirectional force pattern caused by traditional unidirectional rotation, avoiding the trapping of mud clumps at the bottom of the borehole and the centrifugal stratification of the fluid. This results in a more uniform forced mixing of the solidified material and the fractured rock and soil, thereby improving the overall compressive strength of the solidified fluidized cement-soil within the borehole.

[0024] 3. This invention prepares a uniformly distributed fluid cement-soil mixture within the borehole through multi-directional composite mixing. Before the medium initially sets, the precast pile is pressed in along the central axis of the borehole, ultimately solidifying to form a composite pile structure. The uniformly mixed fluid cement-soil reduces the penetration friction resistance during vertical implantation of the precast pile, ensuring that the precast pile can be smoothly pressed into the design elevation. Simultaneously, the solidified mixture forms a synergistic force-bearing system with the internal precast pile, effectively improving the vertical bearing capacity and durability of a single pile in hard strata. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the soil-breaking drill bit structure of the present invention, wherein, Figure 1 (a) is a schematic diagram of the structure of the soil-breaking drill bit body of the present invention. Figure 1 Image (b) is a bottom view of the main body of the earth-breaking drill bit of the present invention. Figure 1 Image (c) is a schematic diagram of the structure of the earth-breaking drill bit with a cylindrical frame according to the present invention. Figure 1 (d) is a bottom view of the earth-breaking drill bit with a cylindrical frame according to the present invention;

[0026] Figure 2 This is a schematic diagram of the multi-directional stirring drill bit structure of the present invention, wherein, Figure 2 (a) is a schematic diagram of the first blade interlacing structure of the present invention. Figure 2 (b) is a cross-sectional schematic diagram of the first blade interlacing structure of the present invention. Figure 2 Image (c) is a schematic diagram of the second blade interlacing structure of the present invention. Figure 2 (d) is a cross-sectional schematic diagram of the second blade interpenetration structure of the present invention;

[0027] Figure 3 This is a flowchart illustrating a construction method for improving the bearing capacity and durability of mixing piles in hard strata, according to the present invention.

[0028] Figure 4 This is a graph showing the monitoring curves of current, torque, and feed rate as a function of depth during the drilling stage of this invention. Figure 4 In Figure (a), the curve of the operating current of the drive motor of the present invention as a function of depth is shown. Figure 4 Figure (b) shows the curve of the output torque of the present invention as a function of depth. Figure 4 (c) is the curve showing the change in downward feed rate of the soil-breaking drill bit of the present invention with depth;

[0029] Figure 5 This is a comparison diagram of the compressive strength distribution along the depth direction between the multidimensional mixing composite pile of the present invention and the conventional unidirectional mixing pile. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] See attached document Figure 3 This invention provides a construction method for improving the bearing capacity and durability of mixing piles in hard strata, comprising the following steps:

[0032] S10. First drilling operation. Drive the first set of drill rods to drive the soil breaking drill bit at the end into the formation to loosen the soil layer and break the rock. Maintain the soil breaking state in place without soil removal and slag removal. Simultaneously inject fluid medium to cool the soil breaking drill bit while the first set of drill rods is drilling into the hole.

[0033] S20. First drilling operation. Control the first set of drill rods to lift upwards and exit the hole position. During the drilling operation of the first set of drill rods, inject a certain amount of cement grout, curing agent grout or water into the hole position.

[0034] S30. Second drilling operation. After the first set of drill rods exits the hole position, the second set of drill rods is switched to the working hole position. The second set of drill rods drives the multi-directional mixing drill bit at the end to drill along the original hole position. During the downward drilling stage of the multi-directional mixing drill bit, cement slurry or curing agent slurry is injected simultaneously. The multi-directional mixing drill bit is used to perform multi-dimensional cross-shearing and mixing of the mixed medium of broken rock and soil and slurry in the hole position.

[0035] S40. Second drilling operation. Control the second set of drill rods to drive the multi-directional mixing drill bit to rotate in the opposite direction and lift it upwards synchronously until it leaves the hole position. During the lifting stage, control the continuous grouting into the hole position or stop grouting according to the position of the grouting port of the multi-directional mixing drill bit.

[0036] S50. Precast pile implantation operation. Before the cement-soil mixture, after being mixed by a multi-directional mixing drill bit, enters the initial setting state, the precast pile is implanted to the designated design elevation position by static pressure or hammering to form a composite pile structure with the outer layer being the mixed and solidified body and the inner layer being the precast pile.

[0037] To ensure the continuity of the aforementioned multi-step operation and the concentricity of the secondary drilling, the construction work is carried out using specific foundation support equipment. In the specific operation of equipment switching, after the first set of drill rods exits the borehole, the equipment can be directly replaced on the ground with a second set of drill rods equipped with a multi-directional mixing drill bit. As a preferred method to improve construction efficiency and ensure concentricity, the foundation support equipment used on the construction site is preferably a double-drill-rod concentric frame, with the main structure of the frame simultaneously equipped with an independent first set and a second set of drill rods. The end of the first set of drill rods is rigidly connected to a soil-breaking drill bit for breaking hard rock and soil, while the end of the second set of drill rods is rigidly connected to a multi-directional mixing drill bit for fluid mixing. To achieve independent transmission of two different cycloidal forces, the second set of drill rods is a double-tube drill rod structure consisting of a coaxial inner central drill rod and an outer casing.

[0038] A rotating column is located at the central axis of the concentric dual-drill-rod frame. By driving the rotating column to rotate, the first set of drill rods moves out of the working area on the horizontal plane, and the second set of drill rods follows the rotation trajectory to simultaneously cut into the corresponding hole position directly above it. The spatial arrangement angle between the first and second sets of drill rods in the frame structure is limited to 90°. This angle ensures that the geometric coordinates of the centers of the two sets of drills completely overlap when switching between them. In addition, to ensure drilling quality, a drill rod limiting device is fixedly installed on the frame of the drilling equipment at a height of 3m above the ground. When the first or second set of drill rods moves downward, it passes through the drill rod limiting device, thereby limiting the vertical deviation of the drill bit's trajectory on the ground. At the same time, the entire construction operation is associated with an independent fluid grouting equipment, whose output working pressure range is set from 0.5MPa to 20MPa, and the corresponding grouting pressure parameters are adjusted according to the construction stage.

[0039] See attached document Figure 1 To perform the first drilling operation (S10), the end of the first set of drill pipes is equipped with a soil-breaking drill bit specifically designed for breaking through hard strata. In this embodiment, as... Figure 1 (a) and Figure 1As shown in (b), the center of the soil-breaking drill bit is a vertical drill rod, and its main body is surrounded by continuous spiral lines. These spiral lines represent the auger drill bit. The length of the auger drill bit along its central axis is not less than 0.5m. This length provides sufficient in-situ space to accommodate the rock and soil fragments generated during cutting, preventing localized jamming. Figure 1 In (b), the main body of the graphic consists of three concentric circles, with the outermost concentric circle of the spiral drill bit representing the outermost rotating outline of these three concentric circles.

[0040] The bottom of the ground-breaking drill bit features a downward-pointing, inverted triangular serrated structure serving as alloy bullet-shaped cutting teeth. While this structure appears as a single row in the elevation view, the bottom of the drill bit actually contains two or more sets of alloy bullet-shaped cutting teeth facing different directions. The drill bit's interior is machined with pre-drilled grouting ports for fluid conduits, allowing the fluid medium to be directly injected into the bottom of the borehole.

[0041] As a preferred method, for high-hardness strata such as completely weathered rock or strongly weathered rock, such as Figure 1 (c) and Figure 1 As shown in (d), a vertical cylindrical frame is fixedly connected to the periphery of the soil-breaking drill bit. This cylindrical frame contains multiple vertical rods as peripheral drill bits. At the bottom of each of these vertical rods, a small rectangular block is fitted as a drill bit tooth. The peripheral drill bit teeth and the central alloy bullet-shaped cutting teeth form a dual soil-breaking cutting structure that combines inner circular rock breaking with outer ring cutting. Figure 1 In the middle (d), the main body of the graphic consists of four concentric circles. Multiple solid small squares are evenly distributed on the outermost ring to represent the outer cylinder drill. At the same time, the third concentric circle inside the outermost ring represents the outer rotating profile of the inner auger drill bit.

[0042] Based on the above equipment structure, the first drilling operation of S10 is specifically divided into the following execution operations:

[0043] S11. Drill bit downward feed and rock breaking. The drive frame mechanism applies a downward driving force to the first set of drill rods, controlling the soil-breaking drill bit to drill deeper into the strata at a feed speed not exceeding 3 m / min. This control method effectively avoids the reaction force of hard strata causing the frame load to exceed the limit. When encountering dense sand or weathered rock layers during the downward trajectory, the alloy bullet-shaped cutting teeth at the bottom concentrate on pressing and cutting the hard rock and soil surface, while the rotation of the auger bit loosens the broken rock cuttings and soil clods. The rock and soil fragments generated by cutting are loosely accumulated in the in-situ borehole space, and no soil removal or slag removal is performed during the operation.

[0044] S12. Fluid Medium Injection and Physical Cooling. While the alloy bullet-shaped cutting teeth are cutting through hard strata, a low-flow-rate clean water or thin slurry is pumped into the pre-reserved grouting port of the drilling bit via an independent fluid grouting device. The fluid medium directly contacts the alloy components under high-temperature friction, carrying away the heat generated by cutting to maintain the metal structural strength of the drill teeth. As drilling progresses, the fluid medium naturally permeates and flows outwards through the pores of the loosened rock and soil. When water or thin slurry cannot flow outwards, it will surface. Generally, the water inflow is small, relying on natural drainage from the site to ensure that the surface of the construction site does not heave. Only a small amount of soil, equivalent to the volume of the drill rod itself, is squeezed out of the borehole.

[0045] S13. Real-time monitoring of formation conditions and rock penetration determination. During downward drilling, the operating current and output torque of the drive motor applying driving force to the first set of drill rods are monitored in real time. When a sudden increase in the operating current and output torque is observed, along with a sudden slowdown in the downward feed speed of the ground-breaking drill bit, accompanied by vibration of the first set of drill rods and the sound of metal rubbing against rock emanating from within the hole, it is determined, based on the superposition of the above-mentioned multi-dimensional physical feedback characteristics and in conjunction with the corresponding detailed exploration report, that the ground-breaking drill bit has penetrated into hard rock. After rock penetration, if this characteristic is confirmed to be persistent, the equipment continues to maintain this feed state until the drill bit reaches the pre-designed rock penetration depth, thus completing the first drilling operation.

[0046] Based on the above-mentioned ground-breaking and drilling status, the equipment executes the S20 first drilling operation. This operation stage is used to pre-lay the bottom layer of cementitious material within the borehole, and is specifically divided into the following operations:

[0047] S21. The drill rod is lifted upwards at a uniform speed. The first set of drill rods is controlled to move upwards away from the hole at a speed not exceeding 5 m / min. This control method can avoid the negative pressure suction effect inside the hole wall caused by the rapid withdrawal of the drill bit. The hard soil in the hole has been broken and loosened by the alloy bullet-shaped cutting teeth and the auger drill bit, and the withdrawal resistance of the drill bit is at a low level.

[0048] S22. Synchronous quantitative injection of curing grout. During the entire process of raising the first set of drill rods, a quantitative amount of cement grout, curing agent grout, or water is simultaneously injected into the borehole using a fluid grouting device. During the overall construction operation of the two drill rod lowering and raising operations, the total amount of cement or curing agent required to form the composite pile structure is 15% to 30% of the mass of the reinforced soil. For saturated soil, or pile foundations subjected to cyclic loads such as compaction by large construction machinery, traffic of construction vehicles, or vibrations caused by subway operation, as a preferred method, the amount of curing agent or cement can be increased by 20% to 40%, or a curing agent or cement with admixtures (basalt fiber or mineral-based cementitious materials, etc.) added to the curing agent grout and cement grout can be used. The quantitatively injected curing grout penetrates into the pores of the high-water-content crushed rock within the borehole, providing a foundation material for subsequent thorough mixing.

[0049] See attached document Figure 2 After the first set of drill rods completes the soil loosening and exits the borehole, the construction process enters the secondary mixing and shaping stage. To perform the second drilling operation (S30), a multi-directional mixing drill bit is rigidly connected to the end of the second set of drill rods. In this embodiment, the multi-directional mixing drill bit operates using a double-casing reverse transmission technology. Its mechanical body includes an inner central drill rod and an outer casing. The inner central drill rod of the second set of drill rods is rigidly connected to the inner central drill rod of the multi-directional mixing drill bit, and the outer casing of the second set of drill rods is rigidly connected to the outer casing of the multi-directional mixing drill bit.

[0050] The bottom end of the internal center drill pipe extends laterally with a wide horizontal strip structure, which serves as the leading blade. The middle section of the internal center drill pipe extends outwards with forward-facing blades. The outer casing, coaxially arranged with the internal center drill pipe, extends outwards with reverse-facing blades. To achieve sufficient hydrodynamic convection and mechanical shearing within the borehole space, the forward and reverse-facing blades are arranged in a staggered pattern in both vertical and horizontal spaces.

[0051] Specifically, there are two ways to implement the blade interlacing structure of the multi-directional stirring drill bit.

[0052] In the first structural form, such as Figure 2 (a) and Figure 2 As shown in (b), the reverse blade includes a transverse straight structure located in the upper middle part and a wide outer frame structure located on the periphery. Figure 2 In section (b), the reverse-direction blades are characterized by diagonal lines and a reversed arc-shaped arrow indicating the direction of rotation; the forward-direction blades are characterized by an inner frame structure inserted within the outer frame structure, and this inner frame structure has a horizontal line running through its center. Figure 2In the middle (b) the forward blade is filled with a grid, and the bottom front blade (forward) is filled with a coarser grid and a border. Both the forward blade and the front blade (forward) have a forward arc arrow on the outside to indicate the direction of rotation.

[0053] In the second structural form, such as Figure 2 (c) and Figure 2 As shown in (d), the reverse blades appear as multiple layers of horizontally comb-like short columns extending inward from the outer vertical frame. Figure 2 In the middle (d) direction, the reverse-direction blades are characterized by being filled with unidirectional oblique lines, and the outer periphery has a reverse arc-shaped arrow indicating the direction of rotation; the forward-direction blades are characterized by multiple layers of inner horizontal short columns extending outward from the inner center drill pipe, with the inner horizontal short columns interlacing with the outer horizontal comb-shaped short columns and leaving a movement gap. Figure 2 In the middle (d) section, the forward blades are filled with intersecting diagonal lines, and the outer edge of the bottom forward blades (forward) has a thickened grid. Both the forward blades and the front blades (forward) have forward arc-shaped arrows indicating the direction of rotation.

[0054] Based on the dual-casing and staggered blade structure of the multi-directional stirring drill bit, the second drilling operation of S30 is specifically divided into the following execution steps:

[0055] S31. Equipment Concentric Positioning Switching. After the first set of drill rods exits the borehole, the rotating column of the concentric double-drill-rod frame is rotated to move the second set of drill rods, equipped with a multi-directional stirring drill bit, into the working area. As a preferred method, the time interval from the completion of the first set of drill rods being pulled out to the start of the second set of drill rods being lowered into the borehole is generally controlled to be no more than 20 minutes. Controlling the interval to within 20 minutes can prevent the pre-loosened high-moisture-content sand and soil fragments in the borehole from segregating or prematurely settling and consolidating with the previously injected bottom slurry, thereby ensuring the initial flow state of the medium during the second drilling.

[0056] S32. Bidirectional Mixing Feed and Grouting. Drive the second set of drill rods downwards, controlling the multi-directional mixing drill bit to drill along the original hole position at a speed not exceeding 2.0 m / min, to match the rotational shearing cycle of the multi-directional blades, ensuring that the soil and rock mass at each depth level undergoes at least one complete forward and reverse cutting cycle. During downward drilling, the fluid grouting equipment synchronously injects prepared cement grout or curing agent grout with a water-cement ratio not exceeding 1.2 into the hole position through internal pipelines. A lower water-cement ratio parameter is used for the injected grout to balance the overall water-cement ratio within the mixing hole position, avoiding low strength in the final mixture. This drilling action continues until the multi-directional mixing drill bit reaches 50%-100% of the drilling depth of the first set of drill rods.

[0057] S33. Multidimensional Convection Shear Mixing. During the downward dynamic process, the internal central drill pipe drives the forward blades and the bottom front blades (forward) to rotate continuously in the forward direction, while the external casing drives the reverse blades to rotate synchronously in the opposite direction. The forward and reverse blades cut the mixing medium in opposite directions within the closed borehole space, generating multidimensional fluid convection, mechanical shearing, and forced kneading effects. Based on the above-mentioned reverse driving mechanism of the inner and outer double casings, the multi-directional staggered blade structure with different rotation directions breaks the phenomenon of mud clinging to the drill bit that is easily caused by the unidirectional rotation mode, and effectively suppresses the material stratification problem caused by centrifugal force, so that the newly injected slurry, the previously injected bottom slurry, the loosened high water content sand layer, and the crushed weathered rock cuttings achieve physical uniform fusion.

[0058] By using the aforementioned multi-dimensional forced convection mixing in the forward, reverse, and vertical directions, the technical defects of traditional unidirectional mixing methods in forming soil layers with high water content can be effectively solved, thereby increasing the overall strength of the final solidified large-diameter cement-soil pile to 4MPa or higher. Furthermore, by overcoming the resistance bottleneck of unidirectional rotary shearing, the effective reinforcement application range of the outer core mixing pile diameter, which was originally limited to within 1000mm, can be expanded to 3000mm.

[0059] S34. Dynamic control of grouting pressure. In this embodiment of the invention, the pressure range of the fluid grouting equipment is set to 0.5-20MPa. Under the conventional multi-dimensional mixing condition of the second set of drill rods drilling downwards, within the drilling depth range of the first set of drill rods, the working pressure of the grouting equipment is maintained in the range of 1MPa to 2MPa. Under this condition, the cement slurry is mainly transported smoothly from the background to the drill rods.

[0060] As an optional feature under special requirements, if it is necessary to diffuse the cement grout into the undisturbed soil around the borehole, a high-pressure rotary agitation method can be used simultaneously during the downward drilling of the second set of drill rods. This involves the synergistic effect of high-pressure rotary jetting and mechanical mixing. Specifically, the grouting pressure needs to be increased to no less than 15 MPa. The high-pressure fluid jet output from the lateral nozzles on the multi-directional mixing drill bit directly cuts through the surrounding undisturbed borehole wall soil. Combined with the mechanical forced mixing by the staggered blades, this expands the actual reinforcement radius of the drill bit.

[0061] After the grout and soil have been replaced and mixed at a depth within the borehole, the equipment performs the second drilling operation (S40), which is divided into the following steps:

[0062] S41. Reverse Lifting Operation. The drive state of the second set of drill rods is changed, and the internal center drill rod and outer casing of the multi-directional mixing drill bit are reversed (i.e., the internal center drill rod switches to reverse rotation, and the outer casing simultaneously switches to forward rotation), and is controlled to lift upwards at a speed not exceeding 3 m / min. During the lifting stage, grouting is continuously injected into the hole or stopped based on the position of the grouting port of the multi-directional mixing drill bit. This lifting speed parameter setting allows the forward and reverse blades to perform additional full-stroke shearing and mixing of the mixture in the hole as the drill rod is pulled out, further improving the homogeneity of the solidified mixture in vertical space.

[0063] S42. Grouting Compensation Control at the Orifice. During the drilling process of raising the drill string to the surface, the start and stop logic of the grouting operation mainly depends on the position of the grouting port reserved on the multi-directional mixing drill bit. The multi-directional mixing drill bit has a pre-installed pipeline and grouting port that connect to the fluid grouting equipment. Specifically, when the grouting port is located at the bottom of the multi-directional mixing drill bit, grouting can generally be stopped during the raising process; when the grouting port is located at the top of the multi-directional mixing drill bit, grout pumping generally continues during the raising process.

[0064] After the multi-directional mixing drill bit is fully lifted and detached from the borehole, a fluidized cement-soil mixture with uniform mixing in all areas forms within the deep borehole, providing a favorable environment for the subsequent formation of the composite structure. Once the mixture within the borehole exhibits fluid characteristics, construction proceeds to the S50 precast pile implantation stage, which is specifically divided into the following operations:

[0065] S51. In this embodiment, after the second drilling operation is completed, the borehole is filled with unconsolidated fluid cement-soil. To ensure that the precast pile can be successfully driven in and closely adhere to the surrounding medium, the installation of the precast pile needs to be completed before the cement-soil begins to set. Considering the characteristics of conventional cement hydration reaction, the initial setting time of cement-soil is generally 4 hours. The construction operation must be completed within this time window to avoid excessive cement-soil consolidation resistance that could prevent the precast pile from penetrating or cause it to deviate.

[0066] S52. Based on the engineering foundation bearing capacity design requirements, precast piles of matching dimensions are selected. Specific structural types include pipe piles, square piles, or irregularly shaped piles. Vertical dynamic or static loads are applied to the top of the precast piles using pile driving machinery such as static pressure machines or hammers configured on-site. The applied load causes the precast piles to continuously displace the fluid medium along the original borehole center axis and be pressed downwards into the fluidized cement-soil.

[0067] S53. When the precast pile reaches the designed depth, the final static pressure value under static pressure operation or the final pile penetration under hammering operation should be determined comprehensively, taking into account the interval between the completion of mixing and molding and the start of pile implantation, as well as factors such as the hardness parameters of the bearing layer in the geological survey data. After reaching the set final pressure value or penetration, the equipment stops applying downward load, completing the vertical implantation of a single precast pile.

[0068] S54. As a preferred approach, when facing the demand for large-diameter cement-soil mixing pile projects, the number and spatial arrangement of the inner core precast piles need to be adaptively expanded. The specific array rules are as follows: for cement-soil mixing piles with an outer core diameter greater than 1200mm, two precast piles should be installed in the inner core area; for cement-soil mixing piles with an outer core diameter greater than 1600mm, three precast piles should be installed in the inner core area; and for cement-soil mixing piles with an outer core diameter greater than 1800mm, four precast piles should be installed in the inner core area. When the outer core size is too large, adding more precast piles can more evenly distribute the load of the superstructure to the surrounding large-volume cement-soil, avoiding localized stress concentration caused by a single load-bearing body.

[0069] In a system where multiple precast piles share the load, each pile is symmetrically distributed in a circular pattern on its horizontal cross-section within the borehole. No additional mechanical connectors are used between the piles; the overall mechanical transfer is achieved through the solidified high-strength cement-soil. During construction, each precast pile is sequentially driven into the borehole, with the center-to-center distance between adjacent piles being at least 2.5 times the diameter or side length of the pile. By limiting the center-to-center distance to at least 2.5 times, the stress overlap zone caused by the pile group effect is effectively eliminated, ensuring that each core precast pile has an independent load-bearing medium around its periphery.

[0070] By performing precast pile implantation, the fluid medium within the borehole eventually hydrates and solidifies, constructing a composite load-bearing structure in the stratum with an internal precast pile load-bearing skeleton and an external large-section cement-soil core. Due to the multi-dimensional shearing and forced mixing effects generated by the alternating forward and reverse blades during the secondary drilling operation, the problem of incomplete core pile formation during a single drilling operation in existing hard strata mixing pile construction is solved. The solidified cement-soil core exhibits a stable compressive strength of 4 MPa and above.

[0071] Based on the high-strength outer core of the composite pile, the pile perimeter in the calculation of pile side resistance can be directly taken as the diameter of the mixing pile with the outer core. The increased outer working area increases the lateral bearing capacity of the composite pile. Under the same structural bearing requirements, this composite bearing structure can effectively shorten the vertical length of the precast pile in the engineering design, or reduce the dependence on the bearing capacity of the deep, high-hardness pile end bearing layer, thus reducing the overall material cost. At the same time, the multi-dimensional shear mechanism overcomes the torque resistance bottleneck of the conventional unidirectional mixing mode in hard strata, thereby increasing the effective operating cutting radius of the mixing equipment and meeting the construction requirements of ultra-large cross-section mixing piles with an outer reinforcement boundary diameter of up to 3000mm.

[0072] See attached document Figure 4 To be continued Figure 5 To aid in understanding the present invention, a foundation pit support pile foundation project containing a hard, completely weathered rock layer is used as an example. The project design requires an outer core solidified body with a diameter of 1200mm, and the internal implantation of prestressed high-strength concrete pipe piles with a diameter of 600mm, with a designed hole depth of 15m. According to the geological survey report, the underground layer from 0m to 10m consists of sandy clay, and the layer from 10m to 15m consists of completely weathered granite.

[0073] Assemble a concentric frame for the double drill pipes. A rotating column with a geared slewing bearing is mounted at the central axis of the frame, and independent hydraulic power heads are positioned on both sides of the rotating column. The drive spindle of the hydraulic power head integrates strain gauge sensors and electrical parameter acquisition modules for monitoring torque and motor operating current. A 1200mm outer diameter earth-breaking drill bit with alloy bullet-shaped cutting teeth is rigidly connected to the end of the first set of drill pipes.

[0074] Start the hydraulic power head corresponding to the first set of drill rods, and control the soil-breaking drill bit to drill downwards and cut the soil at a feed rate of 2.5 m / min. Do not lift the drill bit or remove soil during drilling. Start the fluid grouting equipment, which uses a high-pressure plunger pump with an output working pressure set to 0.8 MPa. Clean water is output from the high-pressure plunger pump, delivered through the internal channels of the first set of drill rods to the pre-reserved grouting port at the bottom of the soil-breaking drill bit, and sprayed out to provide fluid physical cooling for the alloy bullet-shaped cutting teeth that generate heat from cutting friction.

[0075] The geological conditions of the drill bit's location are assessed based on sensor monitoring data. (Combined with attached...) Figure 4The monitoring data curves show that within the clay layer section from 0 to 10 meters, the operating current of the hydraulic power head remained between 60A and 70A, the output torque remained between 15kN·m and 20kN·m, and the feed rate remained stable at 2.5m / min. When the feed depth exceeded 10 meters and cut into the completely weathered granite layer, the monitoring data suddenly changed: the operating current jumped to over 120A, the output torque rose to 45kN·m, and due to the resistance of the hard rock structure, the feed rate plummeted to between 1.1m / min and 1.3m / min. Based on this simultaneous change in physical parameters, it was determined that the drilling bit had entered the rock, and the current power output was maintained until the drill bit reached a depth of 15 meters.

[0076] The first set of drill rods was pulled upwards from the borehole at a constant speed of 4.5 m / min. During the lifting process, prepared curing agent grout was pumped into the borehole using a fluid grouting device. The curing agent grout was made by mixing ordinary silicate cement with water, and the total amount of cement added throughout the entire construction operation was set at 18% of the mass of the soil to be reinforced. A portion of this total amount was injected in advance during this lifting operation. The grout seeped into the borehole and formed a base grout by the alloy bullet-shaped cutting teeth and the auger bit breaking up the loosened soil and rock pores.

[0077] The slewing bearing mechanism of the rotating column is controlled to rotate 90°, causing the second set of drill rods, connected to the multi-directional stirring drill bit, to move directly above the working hole. The hydraulic power head at the top of the second set of drill rods is equipped with a dual-output shaft gearbox. Through the meshing transmission of the planetary counter-rotating gear set inside the gearbox, the power of the single hydraulic motor is split into two coaxial torques with opposite rotation directions, which are output to the coaxially nested inner center drill rod and outer casing, respectively.

[0078] The second set of drill rods is controlled to drill downwards, with the feed rate of the multi-directional mixing drill bit set to 1.8 m / min. During the downward advance, the fluid grouting equipment simultaneously injects cement slurry with a water-cement ratio of 1.0 at a pressure of 1.5 MPa. The moisture content of the in-situ debris after cutting the completely weathered rock is increased due to the influence of the previously injected clean water; therefore, a low water-cement ratio cement slurry is injected to balance the overall water-cement ratio of the mixture within the borehole. The dual-output shaft gearbox drives the internal central drill rod to rotate the forward blades in the forward direction, while simultaneously driving the external casing to rotate the reverse blades in the reverse direction. The interleaved forward and reverse blades apply multi-directional physical shearing and convection compression to the rock cuttings, soil clods, and slurry accumulated within the borehole until the mixing drill bit reaches the bottom of the 15 m borehole.

[0079] The hydraulic control valve is operated to change the inlet and outlet oil direction of the power head hydraulic motor, thus changing the torque output rotation direction of the second set of drill rods. The inner center drill rod switches to reverse rotation, while the outer casing switches to forward rotation. The second set of drill rods is controlled to be lifted upwards at a speed of 2.5 m / min and then stop injecting grout into the hole. This reverse lifting action performs secondary spatial mechanical kneading of the grout and soil mixture in the hole throughout its entire stroke, ensuring that the solidified medium is evenly distributed across the vertical cross-section.

[0080] The second set of drill rods detaches from the ground hole. Before the fluid cement-soil in the hole reaches its initial setting time, a 600mm outer diameter precast concrete pipe pile is vertically driven into the hole along the original centerline using a static pressure pile driver. When the bottom of the pipe pile reaches an elevation of 15m, the final static pressure value calculated from the pressure of the pile driving cylinder is recorded. Pressurization is stopped after confirming that the engineering bearing capacity design requirements are met.

[0081] The fluid mixture hydrates and solidifies within the pore, forming a composite pile structure with a precast pile at the center and a surrounding cement-soil solidification body. In this structural system, using conventional pile bearing capacity calculation models in this field, the ultimate vertical bearing capacity of a single composite pile is calculated according to the following formula:

[0082] ;

[0083] in, This refers to the ultimate vertical bearing capacity of a single pile in a composite pile system. The perimeter under stress is determined by the fact that, after secondary drilling and multi-dimensional mixing, a solidified body with a complete structure and compressive strength meeting the stress requirements is formed within the borehole. This breaks the limitation of conventional composite pile calculations where the pile side resistance is only determined by the perimeter of the internal precast pipe pile. In this embodiment... The value is taken as the perimeter of the solidified outer core, which is the product of pi and a diameter of 1.2m. For the outer core solidified body in the first Standard value of ultimate lateral resistance between the strata and the surrounding in-situ soil; For the outer core solidified body in the first Thickness of the strata; This is the standard value of the ultimate end resistance of the bearing layer at the pile tip to the bottom end of the internal precast pipe pile; This represents the cross-sectional area of ​​the bottom end of the precast pipe pile. This formula reflects the technical approach of this invention: by increasing the effective diameter of the reinforced outer core, higher lateral resistance is directly obtained to enhance the bearing capacity of a single pile.

[0084] Combined with appendix Figure 5 Based on experimental data, the mechanism of action of multidimensional shearing technology in formation consolidation is analyzed. Figure 5The horizontal axis represents compressive strength (MPa), and the vertical axis represents drilling depth (m). The dashed lines marked with squares represent the control group parameters for conventional unidirectional mixing piles, while the solid lines marked with circles represent the experimental group parameters for the multidimensional mixing composite pile of this invention. Above the boundary of the completely weathered rock strata (i.e., the clay layer interval from 0 to 10m), the compressive strength of the solidified body formed by the conventional unidirectional process remains around 2.0MPa. When the depth exceeds this boundary and enters the completely weathered rock strata below 10m, the blades of the conventional unidirectional mixing process cannot effectively cut and mix the hard rock blocks, resulting in mud clumps sticking to the drill and centrifugal separation. This leads to uneven mixing of the slurry and rock cuttings, causing a sharp drop in compressive strength that fluctuates between 0.5MPa and 1.2MPa, failing to meet the forming structure standards of the outer core pile.

[0085] In contrast, the solid line broken line of the multi-dimensional mixing composite pile of this invention shows that the compressive strength data are consistently within the high-strength range of 4.0MPa to 4.8MPa across the entire borehole depth of 15m. (See attached figure.) Figure 5 Data shows that even in high-hardness, completely weathered rock strata below the boundary of completely weathered rock layers, the independently driven forward and reverse blades of the dual-casing system form a cross-shear network, thoroughly crushing the agglomeration structure of hard rock fragments and eliminating the centrifugal discharge effect caused by single rotation. Multi-dimensional convection and mixing force the high-concentration cement slurry to fully coat each particle of crushed rock aggregate, compensating for the original porosity of the soil and rock mass after excavation. This physical action prevents local dilution of the slurry by the high aquifer, allowing the hydration reaction to occur in a homogeneous environment and reducing the discrete differences in strength. Figure 5 The solid line trajectory in the figure verifies that the scheme can effectively overcome the technical obstacles of mixing and pile forming in hard strata, and ensure the high bearing capacity of the composite pile system under deep and complex geological conditions.

Claims

1. A construction method for improving the bearing capacity and durability of a hard stratum mixing pile, characterized in that, Includes the following steps: The first set of drill rods is driven to drive the soil-breaking drill bit at the end to drill into the stratum. During the drilling of the first set of drill rods into the hole position, a fluid medium is injected simultaneously to cool the soil-breaking drill bit until the drilling elevation position is reached. The soil-breaking state is maintained in place without soil removal and slag removal operations. Control the first set of drill rods to lift upwards and exit the hole position, and inject cement slurry, curing agent slurry or water into the hole position during the drilling phase of the first set of drill rods lifting; After the first set of drill rods exits the hole position, the second set of drill rods is switched to the working hole position. The second set of drill rods drives the multi-directional mixing drill bit at the end to drill along the hole position. During the downward drilling stage of the multi-directional mixing drill bit, the cement slurry or curing agent slurry is injected simultaneously. The multi-directional mixing drill bit is used to perform multi-dimensional cross-shearing and mixing of the mixed medium of broken rock and soil and slurry in the hole position until the drilling elevation position is reached. The second set of drill rods is controlled to drive the multi-directional stirring drill bit to rotate in the opposite direction and simultaneously lift it upwards until it is disengaged from the hole. Before the mixed medium, after being stirred by the multi-directional stirring drill bit, enters the initial setting state in the hole, the precast pile is implanted to the elevation position to form a composite pile structure.

2. The construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 1, characterized in that, The center of the soil-breaking drill bit is a vertical drill rod, and spiral drill bits are distributed around the outer periphery of the vertical drill rod. The length of the spiral drill bits in the central axis is not less than 0.5m. The bottom of the soil-breaking drill bit is provided with two or more sets of alloy bullet-shaped cutting teeth facing different directions, and the interior of the soil-breaking drill bit is machined with a reserved grouting port.

3. The construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 2, characterized in that, The soil-breaking drill bit is fixedly connected to a vertical cylindrical frame, which contains multiple vertical rods, and each of the vertical rods is equipped with a drill bit at its bottom.

4. The construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 2, characterized in that, The drive frame mechanism applies a downward driving force to the first set of drill rods, controlling the soil-breaking drill bit to drill deeper into the strata at a feed speed of no more than 3m / min; The alloy bullet-shaped cutting teeth are used to cut hard strata, and the auger drill bit is used to loosen rock and soil. While the alloy bullet-shaped cutting teeth are cutting through hard strata, clean water or slurry, which serves as the fluid medium, is pumped into the pre-reserved grouting port of the ground-breaking drill bit through a fluid grouting device to cool the ground-breaking drill bit. During the downward drilling process, the operating current and output torque of the drive motor that applies driving force to the first set of drill pipes are monitored in real time. When a sudden increase is observed in the operating current and output torque values, and a sudden slowdown in the downward feed speed of the ground-breaking drill bit, accompanied by vibration of the first set of drill rods and the sound of friction emanating from the hole, the feed state is maintained until the rock entry elevation is reached, thus completing the drilling operation into the strata.

5. The construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 4, characterized in that, Control the first set of drill rods to move upwards away from the hole at a speed not exceeding 5 m / min; During the entire process of lifting the first set of drill rods upward, the cement grout, curing agent grout, or water is simultaneously injected into the borehole through the fluid grouting equipment; During the overall construction operation of drilling and hauling the drill pipe, the total amount of cement or curing agent required to form the composite pile structure is 15% to 30% of the mass of the soil being reinforced.

6. The construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 4, characterized in that, The second set of drill rods is a double-tube drill rod structure. The second set of drill rods includes a coaxial inner central drill rod and an outer sleeve sleeved on the periphery. The multi-directional stirring drill bit includes the inner central drill rod and the outer sleeve sleeved on the periphery.

7. A construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 6, characterized in that, The bottom end of the inner central drill rod of the multi-directional stirring drill bit has a front blade extending laterally, the middle section of the inner central drill rod of the multi-directional stirring drill bit has a positive blade extending outward, and the outer casing of the multi-directional stirring drill bit has a reverse blade extending outward. The positive blade and the reverse blade are arranged in an alternating pattern in vertical and horizontal space.

8. A construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 7, characterized in that, After the first set of drill rods exits the hole, the frame mechanism is controlled to move the second set of drill rods with the multi-directional stirring drill bit into the working area; The second set of drill rods is driven to advance downwards, and the multi-directional mixing drill bit is controlled to drill along the hole at a speed not exceeding 2.0 m / min. During the downward drilling, the fluid grouting equipment synchronously injects the cement slurry or curing agent slurry with a water-cement ratio not exceeding 1.2 into the hole through internal pipelines. The downward drilling and grouting operations are continued until the multi-directional mixing drill bit reaches 50% to 100% of the drilling depth of the first set of drill rods. During the downward dynamic process, the internal central drill pipe drives the forward blade and the front blade to rotate continuously in the forward direction, while the external casing drives the reverse blade to rotate synchronously in the reverse direction. The forward blade and the reverse blade cut the mixed medium in opposite directions. Under the multi-dimensional mixing condition of the second set of drill rods drilling downwards, when grouting is only performed inside the hole, the working pressure of the fluid grouting equipment is maintained in the range of 1MPa to 2MPa. Alternatively, when the cement grout or curing agent grout diffuses to the undisturbed soil around the hole, high-pressure grouting is simultaneously used when the second set of drill rods is drilling downwards, and the working pressure of the fluid grouting equipment is not less than 15MPa.

9. A construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 7, characterized in that, Change the driving state of the second set of drill rods, the internal center drill rod of the multi-directional stirring drill bit switches to reverse rotation, the outer casing switches to forward rotation in a synchronized reverse direction, and controls it to be lifted upward at a speed of no more than 3m / min. During the lifting stage, the grouting is continuously injected into the hole or stopped according to the position of the grouting port of the multi-directional stirring drill bit. During the drilling process of raising the drill string to the ground, grouting is stopped when the grouting port is located at the bottom of the multi-directional mixing drill bit. When the grouting port is located at the upper part of the multi-directional mixing drill bit, grout continues to be pumped during the lifting process.

10. A construction method for improving the bearing capacity and durability of mixing piles in hard strata according to claim 1, characterized in that, Before the unconsolidated fluid cement-soil fills the borehole and begins to set, the precast pile is inserted into the borehole. The precast pile is pressed into the fluid cement-soil along the center axis of the hole. After the final pressure value or penetration degree is reached, the downward load is stopped, and the vertical implantation of the precast pile is completed. When multiple precast piles are configured, 2-4 precast piles are inserted into the hole, and the center-to-center distance between two adjacent precast piles is not less than 2.5 times the diameter or side length of the precast pile.

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