CN113699981A

CN113699981A - Construction method for preventing necking of rotary-digging cast-in-place pile in high-fill field

Info

Publication number: CN113699981A
Application number: CN202111044360.8A
Authority: CN
Inventors: 吴昌莅; 黄虎; 高先略; 赵鑫
Original assignee: China Metallurgical Construction Engineering Group Co Ltd
Current assignee: China Metallurgical Construction Engineering Group Co Ltd
Priority date: 2021-09-07
Filing date: 2021-09-07
Publication date: 2021-11-26

Abstract

The invention discloses a construction method for preventing necking of a rotary-excavated cast-in-place pile in a high-fill field, which comprises the following steps of: measuring lofting and embedding an orifice protection cylinder; drilling downwards for multiple times in the orifice protecting cylinder through the drilling machine until the drilling depth reaches the depth of a necking soil layer, stopping drilling, and removing the drilling machine; preparing a pile hole pile casing and putting the pile hole pile casing into the pile hole; before the pile hole is placed downwards, a layer of drag reducer is fully coated on the outer wall of the pile hole pile casing; a drilling machine is placed in the pile hole casing to continue drilling downwards until the final hole elevation is reached; preparing a steel reinforcement cage, and performing reinforcement treatment on the position of the steel reinforcement cage corresponding to a necking soil layer to form a necking section reinforcement cage, wherein the necking section reinforcement cage comprises an outer reinforcement cage and an inner reinforcement cage which are coaxial; the outer walls of the inner reinforcing cage and the inner reinforcing cage are wrapped with a layer of steel wire mesh; lowering the reinforcement cage to the bottom of the pile, and pouring concrete by adopting a crane lowering conduit; and (4) pulling the guide pipe, and simultaneously adopting a pile machine to vibrate and pull the pile hole protective sleeve while pulling the guide pipe.

Description

Construction method for preventing necking of rotary-digging cast-in-place pile in high-fill field

Technical Field

The invention belongs to the field of pile foundation construction, and particularly relates to a necking prevention construction method for a rotary-digging cast-in-place pile in a high-fill field.

Background

The bored pile foundation is the most widely used foundation form in China, the defects of the pile body related to the safety of the upper structure can be divided into fracture, crack, necking, mud (sundries) inclusion, cavity, looseness and the like, the reasons for different defects are different, and the treatment difficulty is different. The necking treatment is the most difficult, most necking is carried out in cohesive soil, weak soil and high water content, particularly in saturated silt or silt soft soil layers, a free surface is formed by drilling, and water pressure difference and soil pressure difference are main reasons for necking. The probability of necking during construction of a rotary-digging bored pile on a high-fill (filling height is more than 18 m) field is high, if measures such as slowing down drilling speed, improving the performance of retaining wall slurry, recharging and re-drilling by low-grade concrete and the like are adopted to lose effectiveness, the use of long steel retaining cylinder retaining wall pore-forming is the final choice, the steel retaining cylinder is permanently left underground along with pile body concrete, not only is the cost increased, but also the friction force between the pile body concrete and the soil body around the pile is blocked by the steel retaining cylinder so as to reduce the bearing capacity of the pile body.

In order to solve the problems, the Chinese patent with the application number of 2020106389767 discloses construction equipment and a construction method of a necking-resistant sinking pipe bored concrete pile, wherein during construction, a pile is formed in a manner of pulling a pipe and grouting at the same time, and a gap left by pulling the pipe is excessively filled with grout under grouting pressure, so that the necking phenomenon is avoided. The construction mode can effectively reduce the phenomenon of necking down to a certain extent, but in the process of upward pipe drawing, if the pipe drawing is too fast, the lateral pressure resistance of concrete is not enough, and the phenomenon of necking down can be caused, and if the pipe drawing is too slow, the phenomenon that the concrete is solidified in the sleeve and the sleeve can not be drawn out easily occurs. Meanwhile, because the lower end of the steel sleeve is provided with the annular pipe, although the slurry filling amount is effectively increased, and the pile body lateral pressure resistance is correspondingly increased, before the concrete sets for the end, the bottom pile body lateral pressure resistance is limited, and the necking phenomenon cannot be effectively improved.

Disclosure of Invention

Aiming at the defects of the prior art, the technical problems to be solved by the invention are as follows: how to provide a construction method for preventing necking of a rotary-digging cast-in-place pile in a high-fill field, which is convenient for pulling out a steel sleeve, can effectively increase the side pressure of a pile body of a necking section and can prevent locking.

In order to solve the technical problems, the invention adopts the following technical scheme:

a construction method for preventing necking of a rotary-digging cast-in-place pile in a high-fill field is characterized by comprising the following steps: s1, measuring and lofting, determining the position and thickness of a necking soil layer through a rotary drilling rig drill bit, and lofting the position of a pile hole; s2, embedding an orifice casing with the inner diameter larger than the designed pile diameter at the pile hole lofting position; s3, preparing a drilling machine, enabling the drill bit center and the drill rod center of the drilling machine to coincide with the pile center, drilling downwards for multiple times in the orifice casing through the drilling machine until the drilling depth reaches the depth of the necking soil layer determined in the S1, stopping drilling, and removing the drilling machine; s4, preparing a pile hole pile casing with the inner diameter larger than that of the pile hole, putting the pile hole pile casing into the drilled pile hole in the S3, and sinking the pile hole pile casing to the bottom of a necking soil layer by means of self weight and vibration force of a vibration hammer; after the pile hole pile casing is prepared and before the pile hole pile casing is put into the pile hole, a layer of drag reducer is fully coated on the outer wall of the pile hole pile casing; s5, placing a drilling machine in the pile hole casing to continue drilling downwards until the final hole elevation is reached; s6, preparing a steel reinforcement cage, and reinforcing the steel reinforcement cage at a position corresponding to a necking soil layer to form a necking section reinforcing cage, wherein the necking section reinforcing cage comprises an outer reinforcing cage and an inner reinforcing cage which are coaxial, the diameter of the inner reinforcing cage is smaller than that of the outer reinforcing cage, and a plurality of horizontal ribs for connecting the outer reinforcing cage and the inner reinforcing cage are arranged at intervals on the periphery of the inner reinforcing cage; the outer walls of the inner reinforcing cage and the inner reinforcing cage are wrapped with a layer of steel wire mesh; s7, placing the prepared reinforcement cage to the bottom of the pile, placing a guide pipe by a crane, placing the guide pipe in the reinforcement cage, and pouring concrete into the pile hole casing and the reinforcement cage through the guide pipe; after the concrete passes through the guide pipe, the guide pipe is pulled upwards, the pile hole protecting cylinder is pulled upwards while the guide pipe is pulled upwards by adopting a pile machine to vibrate, and grouting is carried out according to the designed grouting pressure while the pile hole protecting cylinder is pulled upwards, so that the grouting pressure and the pipe pulling speed are kept unchanged. Therefore, the soil layer condition is determined by measuring, and after the position of the necking section generated by the pile body is determined according to the soil layer condition, the reinforcement cage is reinforced at the position corresponding to the position, so that the lateral pressure of the pile body can be effectively increased, and the pile body is prevented from necking. When the steel reinforcement cage is consolidated, the side direction compressive capacity of necking down section steel reinforcement cage can effectively be increased to inside and outside reinforcement cage that sets up, and the wire net of wrapping up on inside and outside reinforcement cage can prevent concrete placement back with the effective separation of this section soil layer outside the steel reinforcement cage, and the soil layer gets into or extrudes pile body concrete, appears the necking down phenomenon. The double-layer steel wire mesh can form double-layer separation on the sludge or soil layer at the necking section, and has a good necking prevention effect. Gaps among the steel wire meshes can effectively slow down the entering speed of partial silt or soil layers and slow down the side pressure of the steel wire meshes arranged on the inner side. The layer of drag reducer smeared on the outer wall of the pile hole pile casing can effectively reduce the friction between the pile hole pile casing and the pile wall when the pile hole pile casing is pulled out, and the pile hole pile casing can be pulled up after a period of time is prolonged after the concrete is initially set, so that the side pressure of the pile body is effectively increased, and the necking is prevented.

Furthermore, the drag reducer is prepared by mixing the following components in percentage by weight: 55-62% of water, 3.5-6% of bentonite, 30-36% of clay and 3.5-6% of fly ash; the coating thickness of the drag reducer is 2-5 cm. Therefore, the drag reducer is low in manufacturing cost and has a good drag reduction effect, clay in the drag reducer can effectively ensure that the drag reducer can be well applied to the pile hole casing, and the drag reducer is prevented from falling off in the using process. The workability of the drag reducer is guaranteed by water, the drag reducer can be uniformly coated on the steel casing, the lubricity of the drag reducer is guaranteed by bentonite, the viscosity and the strength of the drag reducer are guaranteed by clay, the dropping of the drag reducer in a sinking process is prevented, and the overall friction force is effectively reduced by small particles of fly ash.

Furthermore, before the reinforcement cage is placed downwards, at least two sound measuring tubes are fixed in the prepared reinforcement cage at equal intervals; the upper end and the lower end of each acoustic pipe are flush with the upper end and the lower end of the steel reinforcement cage, and the top of each acoustic pipe is sealed by a cover; and after the pile body is finally set, removing the cover at the top of each sound measuring pipe, then filling water into the sound measuring pipes, sealing, and detecting the quality of the pile foundation by a sound wave transmission method. Like this, the sounding pipe that sets up can effectively detect pile foundation quality, and sounding pipe circumference sets up, and it is more to measure the position, can effectively measure the quality of pile foundation each position.

Furthermore, before the steel reinforcement cage is placed downwards, a plurality of rotating wheels are circumferentially arranged on the steel reinforcement cage. Like this, the runner that sets up on the steel reinforcement cage can make steel reinforcement cage and pile wall offset, and the downthehole slip of stake is transferred, ensures that the steel reinforcement cage transfers steadily. Simultaneously, the runner that circumference set up can also effectively ensure that the steel reinforcement cage when transferring, and the interval between the cage body and the stake hole is unanimous, ensures that steel reinforcement cage and stake hole are with the axle center.

Furthermore, the orifice casing is formed by rolling a steel plate with the length of 1.5-2.0 meters, the inner diameter of the orifice casing is 0.3-0.4m larger than the designed pile diameter, and a circle of stiffening ring is welded and fixed on the periphery of the upper opening. Like this, the drill way protects a section of thick bamboo buried depth more shallowly, can effectively provide the instruction for digging the pore-forming soon, is convenient for propose and dig soil soon. The stiffening ring welded on the upper opening of the orifice pile casing can effectively ensure that the orifice pile casing is kept at the current position, and the orifice pile casing cannot fall down after the pile hole is excavated to the position below the orifice pile casing.

Further, in S6, the neck reinforcing cage is prepared as follows: a, preparing a plurality of inner reinforcing hoop rings and outer reinforcing hoop rings, and connecting and fixing the inner reinforcing hoop rings and the outer reinforcing hoop rings in pairs by adopting horizontal ribs to form a plurality of concentric frameworks; b, sequentially sleeving all concentric frameworks on a mould frame from bottom to top for positioning; c, welding an equilateral triangle support rib in the inner reinforcing hoop of each concentric framework; d, welding a plurality of longitudinal bars on the outer sides of all the inner reinforcing hoops and the outer sides of all the outer forced hoops in the circumferential direction; e, welding a circle of spiral ribs on the outer side of the longitudinal rib of the inner reinforcing hoop and the outer side of the longitudinal rib of the outer reinforcing hoop; f, wrapping a circle of steel wire mesh outside the two circles of spiral ribs. Therefore, the reinforcing hoop is additionally arranged on the locking section steel reinforcement cage, the arrangement density of the longitudinal ribs and the spiral ribs is increased, and the lateral pressure resistance of the section of the steel reinforcement cage is effectively improved. The equilateral triangle bracing rib fixed in the inner reinforcing hoop can effectively increase the side pressure of the reinforcing hoop.

Further, after the steel bar cage is hung and the guide pipe is put in place, secondary hole cleaning is carried out in a positive circulation mode immediately, whether the performance index of slurry in the hole and the sediment thickness at the bottom of the hole meet the standard and design requirements or not is checked again, the sand content is less than 2%, the relative density is 1.03-1.10 g/cm, and concrete can be poured after all indexes meet the requirements. Therefore, when concrete is poured, no slurry exists in the hole, and the strength of the solidified pile body is not influenced.

Furthermore, after the pile hole pile casing is lifted, the guide pipe is repeatedly lifted and sunk in the hole, feeding is carried out in the upper hopper, and the pressure of concrete and the shaking of the guide pipe are utilized to ensure that the concrete in the hole is self-closed and fill the gap after the pile hole pile casing is lifted. Therefore, the whole pile body of the rotary excavating pile is more compact, the internal structure is better, and partial resistance is provided for preventing the soil body from collapsing and extruding into the pile body.

Furthermore, the height of the top surface of the concrete needs to be measured continuously in the concrete pouring process, and the bottom opening is embedded in the concrete to be not less than 4m when the pile hole pile casing is pulled up; the pile hole protects a section of thick bamboo and puts the time interval after taking one's place to before the pile hole protects a section of thick bamboo and pulls out, the pile hole protects a section of thick bamboo to utilize hoisting machinery to carry out the promotion or shake once every 1 hour, prevent to increase frictional resistance because of the soil body remolding, lead to the difficulty of pulling out, in addition also effectively prevent concrete bonding behind the concrete placement from hindering the effective pulling out of follow-up steel protects a section of thick bamboo on the steel protects a section of thick bamboo, also having played the effect of vibrating the peripheral concrete of steel protects a section of thick bamboo simultaneously, the peripheral concrete of steel protecting a section of thick bamboo that makes is more closely knit, the wholeness of pile body has been improved, also can improve the anti extrusion force of the peripheral soil layer of pile body simultaneously, prevent that the pile body warp or crowd into the soil body.

Drawings

FIG. 1 is a flow chart of construction of a cast-in-place pile in the embodiment;

FIG. 2 is a schematic diagram of an elevational structure of the reinforcement cage of the embodiment;

FIG. 3 is a top view of an embodiment of a cage for rebar of the neck section;

fig. 4 is a schematic diagram of the state of the pile hole casing in the concrete pumping process and the concrete pumping process in the embodiment.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

Example (b):

the construction method for preventing necking of the rotary excavating filling pile in the high-fill field provided by the embodiment comprises the following steps: s1, measuring and lofting, determining the position and thickness of a necking soil layer through a rotary drilling rig drill bit, and lofting the position of a pile hole; determining the center of a pile position by using a total station coordinate lofting, setting guard piles in four directions by using a pile position drawing cross line in order to conveniently recheck the center position of the pile in the drilling process and the positioning of a steel reinforcement cage, performing guard pile protection work, and detecting the horizontal level and the pile top elevation; s2, embedding an orifice casing 1 with the inner diameter larger than the designed pile diameter at the pile hole lofting position; s3, preparing a drilling machine, enabling the drill bit center and the drill rod center of the drilling machine to coincide with the pile center, drilling downwards for multiple times in the orifice casing through the drilling machine until the drilling depth reaches the depth of the necking soil layer determined in the S1, stopping drilling, and removing the drilling machine; s4, preparing a pile hole pile casing 2 with the inner diameter larger than that of the pile hole, lowering the pile hole pile casing 2 into the drilled pile hole in the S3, and sinking the pile hole pile casing 2 to the bottom of a necking soil layer by means of self weight and vibration force of a vibration hammer; after the pile hole pile casing 2 is prepared and before the pile hole pile casing is put into the pile hole, a layer of drag reducer is fully coated on the outer wall of the pile hole pile casing; s5, placing a drilling machine in the pile hole casing to continue drilling downwards until the final hole elevation is reached; when drilling, a drill bit with the same pile diameter is adopted to drill on the bedrock, the rock-socketed depth and the stroke argillite layer are ensured, and a cutting tooth barrel drill and a sand bailing bucket are adopted to drill in a matched mode, so that the drilling speed is improved; constructing bedrock with the aperture of 1.8m or more and the rock hardness of more than 30mpa by adopting a multi-stage hole forming and multi-bell drill bit matched drilling mode, and ensuring that the hole forming diameter meets the design requirement; s6, preparing a steel reinforcement cage 3 (as shown in figure 1), and performing reinforcement treatment on the position of the steel reinforcement cage corresponding to a necking soil layer to form a necking section reinforcement cage, wherein the necking section reinforcement cage comprises an outer reinforcement cage and an inner reinforcement cage which are coaxial, the diameter of the inner reinforcement cage is smaller than that of the outer reinforcement cage, and a plurality of horizontal ribs for connecting the outer reinforcement cage and the inner reinforcement cage are arranged at intervals in the peripheral direction of the inner reinforcement cage; the outer walls of the inner reinforcing cage and the inner reinforcing cage are wrapped with a layer of steel wire mesh; s7, lowering the prepared reinforcement cage 3 to the bottom of the pile, lowering the guide pipe by using a crane to place the guide pipe in the reinforcement cage, and then pouring concrete into the pile hole casing and the reinforcement cage through the guide pipe (as shown in figure 4); after the concrete passes through the guide pipe, the guide pipe is pulled upwards, and a pile machine is adopted to vibrate and pull the pile hole pile casing (as shown in figure 4) while pulling the guide pipe upwards, and grouting is carried out according to the designed grouting pressure while pulling the pile hole pile casing upwards, so that the grouting pressure and the pipe pulling speed are kept unchanged.

The drag reducer is prepared by mixing the following components in percentage by weight: 60% of water, 4% of bentonite, 31% of clay and 5% of fly ash; the coating thickness of the drag reducer is 2-5 cm.

Before the reinforcement cage is lowered, at least two sound measuring tubes are fixed in the prepared reinforcement cage at equal intervals; the upper end and the lower end of each acoustic pipe are flush with the upper end and the lower end of the steel reinforcement cage, and the top of each acoustic pipe is sealed by a cover; and after the pile body is finally set, removing the cover at the top of each sound measuring pipe, then filling water into the sound measuring pipes, sealing, and detecting the quality of the pile foundation by a sound wave transmission method. Simultaneously, before the steel reinforcement cage is transferred, a plurality of runners are circumferentially arranged on the steel reinforcement cage.

The orifice casing 1 in the embodiment is formed by rolling a steel plate with the length of 1.5-2.0 meters, the inner diameter of the orifice casing is 0.3-0.4m larger than the designed pile diameter, and a circle of stiffening rings are welded and fixed on the periphery of the upper opening.

The processing of the reinforcement cage in the embodiment adopts a long-line method for construction, and is divided into 3 or 4 sections, wherein the basic section is 9m long, and the last section is an adjusting section. The reinforcement cage of each pile is segmented and numbered according to the design length, and adjacent segments can be correspondingly matched and bound on the jig frame. As shown in fig. 2 and 3, in S6, the neck reinforcing cage is prepared as follows: a, preparing a plurality of inner reinforcing hoop belts 4 and outer reinforcing hoop belts 5, and connecting and fixing the inner reinforcing hoop belts and the outer reinforcing hoop belts in pairs by adopting horizontal ribs 6 to form a plurality of concentric frameworks; b, sequentially sleeving all concentric frameworks on a mould frame from bottom to top for positioning; c, welding an equilateral triangle supporting rib 7 in the inner reinforcing hoop 4 of each concentric framework; d, welding a plurality of longitudinal ribs 8 on the outer sides of all the inner reinforcing hoop 4 and all the outer forced hoops 5 in the circumferential direction; e, welding a circle of spiral ribs 9 on the outer side of the longitudinal rib 8 of the inner reinforcing hoop 4 and the outer side of the longitudinal rib 8 of the outer reinforcing hoop 5; f, wrapping a circle of steel wire mesh outside the two circles of spiral ribs 9.

After the steel reinforcement cage is hoisted and the guide pipe is put in place, secondary hole cleaning is carried out in a positive circulation mode immediately, whether the performance index of slurry in the hole and the bottom sediment thickness meet the specification and design requirements or not is checked again, the sand content is less than 2%, the relative density is 1.03-1.10 g/cm for carrying out heavy planting, and concrete can be poured after all indexes meet the requirements.

After the pile hole pile casing is lifted, the guide pipe is repeatedly lifted and sunk in the hole, feeding is carried out in the upper hopper, and the pressure of concrete and the shaking of the guide pipe are utilized to ensure that the concrete in the hole is self-dense and fill the gap after the pile hole pile casing is lifted.

The height of the top surface of the concrete is required to be measured continuously in the concrete pouring process, and the bottom opening is ensured to be embedded in the concrete to be not less than 4m when the pile hole protecting cylinder is pulled up; the pile hole pile casing is lifted or vibrated once every 1 hour after the pile hole pile casing is put in place and before the pile hole pile casing is pulled out, so that the problem of difficulty in pulling out caused by increase of frictional resistance due to soil body remodeling is prevented.

The pouring of concrete is an important process for the construction of the bored pile, and the concrete cannot stay in the middle. The concrete transport vehicle is conveyed to a proper position and is fed to a hopper by a top pump, the initial setting time of the concrete cannot be too early, the workability and the fluidity of the concrete are good, and the slump is controlled to be 180-220 mm, so a retarding and water reducing agent is properly added in the process of mixing the concrete, and the concrete pouring of the whole pile is completed before the initial setting of the first-batch poured underwater concrete.

Before the first batch of concrete in the funnel is poured, a disc of mortar is poured in, and the mortar plays a role in lubricating the guide pipe during pouring. In order to embed the first underwater concrete into the conduit by more than 2.0m after the ball shearing, the requirement of the initial filling amount is satisfied according to parameters such as the pile diameter, the pile depth and the conduit diameter (the quantity of the first underwater concrete is calculated by the formula V being more than or equal to pi D/4 (H1 + H2) + pi D/4H 1). If the ball is washed smoothly, a corresponding amount of mud in the hole is turned up and overflows, and no mud water overflows from the guide pipe. After the concrete is poured into the bottom of the hole, the height of the concrete surface in the hole is detected immediately, the embedding depth of the guide pipe is calculated, and if the embedding depth meets the requirement, the pouring construction of the concrete can be continued.

After the pouring is started, the pouring is continuously and compactly carried out, the midway pause is strictly forbidden, the interval time for removing the guide pipe is shortened as far as possible, when the concrete in the guide pipe is not full, the follow-up concrete is slowly poured, and the guide pipe is prevented from being pressed and leaked by a high-pressure air bag in the guide pipe.

In the pouring process, a water head in the hole is always kept to prevent hole collapse, and the depth of the conduit embedded into the concrete is kept within the range of 2-6 m.

When the concrete surface in the hole is close to the steel bar framework, in order to prevent the steel bar framework from being jacked by the concrete to rise, the guide pipe is kept at a slightly larger burial depth, the pouring speed is slowed down, the upward impact force of the concrete after the concrete comes out from the bottom opening of the guide pipe is reduced, after the concrete in the hole enters the steel bar framework to a certain depth, the guide pipe is properly lifted, the embedment depth of the guide pipe is reduced, the burial depth of the steel bar framework under the bottom opening of the guide pipe is increased, and the bond stress of the concrete to the steel bar framework is increased.

When the pouring is nearly finished, the height of the concrete column in the guide pipe is reduced, the overpressure is reduced, the consistency of slurry outside the guide pipe and contained muck is increased, the specific gravity is increased, and when the concrete is difficult to lift, water can be added into the hole to dilute the slurry, and part of settled soil is removed, so that the pouring work is smoothly carried out. When the last section of the guide pipe is pulled out, the pipe pulling speed is slow so as to prevent mud deposited on the pile top from being squeezed into the guide pipe to form a mud core.

In order to ensure the quality of the pile top, 50cm is poured above the designed elevation of the pile top, and after pouring is finished, the sediment and the redundant concrete of the pile top are immediately and manually removed to the position 30cm above the designed elevation of the pile top. The workload of chiseling the pile head after the concrete strength reaches 70 percent can be greatly reduced, and the rest 30cm of concrete is chiseled manually to ensure the good quality of the pile head concrete.

When the underwater concrete is poured, the maintenance personnel are on duty in the whole process and prepare a standby generator. Foreground measurement personnel must carefully record the concrete top surface elevation and the pipe length in the pouring process well to calculate the buried pipe depth, generally require that two buckets of concrete are poured once for measurement (each bucket of concrete is 4m3), three points are uniformly measured along the pile circumference every time, and the buried pipe depth at the lowest point needs to meet the standard requirement.

Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the technical solutions, and although the present invention has been described in detail by referring to the preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention can be made without departing from the spirit and scope of the technical solutions, and all the modifications and equivalent substitutions should be covered by the claims of the present invention.

Claims

1. A construction method for preventing necking of a rotary-digging cast-in-place pile in a high-fill field is characterized by comprising the following steps: s1, measuring and lofting, determining the position and thickness of a necking soil layer through a rotary drilling rig drill bit, and lofting the position of a pile hole; s2, embedding an orifice casing (1) with the inner diameter larger than the designed pile diameter at the pile hole lofting position; s3, preparing a drilling machine, enabling the drill bit center and the drill rod center of the drilling machine to coincide with the pile center, drilling downwards for multiple times in the orifice casing through the drilling machine until the drilling depth reaches the depth of the necking soil layer determined in the S1, stopping drilling, and removing the drilling machine; s4, preparing a pile hole pile casing (2) with the inner diameter larger than that of the pile hole, putting the pile hole pile casing (2) into the pile hole drilled in S3, and sinking the pile hole pile casing (2) to the bottom of a necking soil layer by means of self weight and vibration force of a vibration hammer; after the pile hole pile casing (2) is prepared, before the pile hole pile casing is put into the pile hole, a layer of drag reducer is fully coated on the outer wall of the pile hole pile casing; s5, placing a drilling machine in the pile hole casing to continue drilling downwards until the final hole elevation is reached; s6, preparing a steel reinforcement cage (3), and reinforcing the steel reinforcement cage at a position corresponding to a necking soil layer to form a necking section reinforcing cage, wherein the necking section reinforcing cage comprises an outer reinforcing cage and an inner reinforcing cage which are coaxial, the diameter of the inner reinforcing cage is smaller than that of the outer reinforcing cage, and a plurality of horizontal ribs for connecting the outer reinforcing cage and the inner reinforcing cage are arranged at intervals in the peripheral direction of the inner reinforcing cage; the outer walls of the inner reinforcing cage and the inner reinforcing cage are wrapped with a layer of steel wire mesh; s7, lowering the prepared reinforcement cage (3) to the bottom of the pile, lowering a guide pipe by using a crane, placing the guide pipe in the reinforcement cage, and pouring concrete into the pile hole pile casing and the reinforcement cage through the guide pipe; after the concrete passes through the guide pipe, the guide pipe is pulled upwards, the pile hole protecting cylinder is pulled upwards while the guide pipe is pulled upwards by adopting a pile machine to vibrate, and grouting is carried out according to the designed grouting pressure while the pile hole protecting cylinder is pulled upwards, so that the grouting pressure and the pipe pulling speed are kept unchanged.

2. The construction method for preventing necking of the rotary excavating filling pile in the high fill field according to claim 1, wherein the drag reducer is prepared by mixing the following components in percentage by weight: 55-62% of water, 3.5-6% of bentonite, 30-36% of clay and 3.5-6% of fly ash; the coating thickness of the drag reducer is 2-5 cm.

3. The construction method for preventing necking of the rotary-excavated cast-in-place pile in the high fill field according to claim 1, is characterized in that at least two sound measuring tubes are fixed in the prepared reinforcement cage at equal intervals before the reinforcement cage is placed downwards; the upper end and the lower end of each acoustic pipe are flush with the upper end and the lower end of the steel reinforcement cage, and the top of each acoustic pipe is sealed by a cover; and after the pile body is finally set, removing the cover at the top of each sound measuring pipe, then filling water into the sound measuring pipes, sealing, and detecting the quality of the pile foundation by a sound wave transmission method.

4. The necking prevention construction method for the rotary excavation cast-in-place pile in the high-fill field according to claim 1, 2 or 3, characterized in that a plurality of rotating wheels are circumferentially arranged on the reinforcement cage before the reinforcement cage is lowered.

5. The construction method for preventing necking of the rotary excavating filling pile in the high fill field according to claim 1, wherein the orifice casing (1) is formed by rolling a steel plate with the length of 1.5-2.0 m, the inner diameter of the orifice casing is 0.3-0.4m larger than the designed pile diameter, and a circle of stiffening ring is welded and fixed on the periphery of the upper opening.

6. The necking prevention construction method for the rotary excavation cast-in-place pile in the high-fill field according to claim 1, 2 or 3, wherein in S6, the preparation method for the necking section reinforcement cage is as follows: a, preparing a plurality of inner reinforcing hoop bands (4) and outer reinforcing hoop bands (5), and connecting and fixing the inner reinforcing hoop bands and the outer reinforcing hoop bands in pairs by adopting horizontal ribs (6) to form a plurality of concentric frameworks; b, sequentially sleeving all concentric frameworks on a mould frame from bottom to top for positioning; c, welding an equilateral triangle brace rib (7) in the inner reinforcing hoop (4) of each concentric framework; d, welding a plurality of longitudinal ribs (8) on the outer sides of all the inner reinforcing hoops (4) and all the outer forced hoops (5) in the circumferential direction; e, welding a circle of spiral ribs (9) on the outer side of the longitudinal rib (8) of the inner reinforcing hoop (4) and the outer side of the longitudinal rib (8) of the outer reinforcing hoop (5); f, wrapping a circle of steel wire mesh outside the two circles of spiral ribs (9).

7. The construction method for preventing necking down of the rotary-excavated cast-in-place pile in the high fill field according to claim 6, is characterized in that after a reinforcement cage is hoisted, after a guide pipe is put in place, secondary hole cleaning is carried out in a positive circulation mode immediately, whether the performance index of slurry in a hole and the sediment thickness at the bottom of the hole meet the specification and design requirements or not is checked again, the sand content is less than 2%, the relative density is 1.03-1.10 g/cm, and after all indexes meet the requirements, concrete can be poured.

8. The construction method for preventing necking of the rotary excavating filling pile in the high-fill-field area according to claim 1 or 7, characterized in that after the pile hole casing is lifted, the guide pipe is repeatedly lifted and sunk in the hole, feeding is carried out in the upper hopper, and the concrete in the hole is self-sealed and fills the gap after the pile hole casing is lifted by utilizing the pressure of the concrete and the shaking of the guide pipe.

9. The construction method for preventing necking of the rotary-excavated cast-in-place pile in the high-fill field area according to claim 8, wherein the top surface height of the concrete needs to be measured continuously during the pouring process of the concrete, so that a bottom opening is embedded in the concrete to be not less than 4m when a pile hole casing is pulled up; the pile hole pile casing is lifted or vibrated once every 1 hour after the pile hole pile casing is put in place and before the pile hole pile casing is pulled out, so that the problem of difficulty in pulling out caused by increase of frictional resistance due to soil body remodeling is prevented.