Large-diameter cast-in-place pile construction method under complex condition of super-thick sand layer

Abstract

The invention discloses a large-diameter cast-in-place pile construction method under the complex condition of a super-thick sand layer, which comprises the following steps: three-shaft mixing pile advance support process, hole cleaning process, pile bottom grouting process, pile body detection and grouting treatment after detection, three-shaft mixing pile advance support is used, slurry wall protection and hole forming are carried out, the pile forming quality is improved, and then three times of hole cleaning process are used, so that the pile forming quality is improved. And finally, pile foundation detection is conducted, grouting treatment is adopted according to conditions, and the quality of the large-diameter cast-in-place pile is guaranteed. The method is suitable for the construction quality control of the large-diameter cast-in-place pile, especially the construction quality control of the large-diameter cast-in-place pile under the geological condition of an ultra-thick sand layer, the hole collapse risk can be effectively avoided, and the control effect on the integrity of a pile body and sediment at the bottom of the pile is greatly enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation construction, and particularly relates to a construction method for large-diameter cast-in-place piles under complex conditions of ultra-thick sand layers. Background Art

[0002] In recent years, with the steady improvement of the urbanization level, more and more super high-rise buildings have been constructed in China, and large-diameter cast-in-place piles are mostly used for the foundation. Large-diameter cast-in-place piles have high single-pile bearing capacity, large pile shaft stiffness, and can bear large vertical and horizontal loads. However, the pile hole forming is complex, the drilling time is long, the pouring volume is large, and the difficulty of cleaning slag is high. Especially under geological conditions such as ultra-thick sand layers or sandy soil layers, there are safety risks such as hole collapse and quality risks such as broken piles and too thick bottom sediment of the piles during the construction of large-diameter cast-in-place piles.

[0003] Based on the long construction time, high difficulty, and difficult quality control of large-diameter cast-in-place piles under complex geological conditions of ultra-thick sand layers. The existing patent, "A Large-Diameter Rotary Drilled Cast-In-Place Pile and Its Construction Technology", application number 202211212374.0, records the following steps: (1) Site layout: accurately set out each pile position with a total station, mark the center of the pile position, measure the bottom elevation with a level, and determine the drilling depth; (2) Preliminary preparation; (3) Burying the casing; (4) Preparing the slurry; (5) Manufacturing the steel reinforcement cage; (6) Drilling: While manufacturing the steel reinforcement cage, carry out the drilling operation, (7) Hole cleaning: Introduce clean water and compressed air into the drilled hole to dilute the sediment at the bottom of the drilled hole to form slurry, and then extract the slurry in the drilled hole; (8) Hoisting the steel reinforcement cage; (9) Hoisting the conduit; (10) Pouring: Pour the slurry into the drilled hole through the conduit. During the pouring process, calculate the pouring height according to the pouring volume of the slurry, determine the time and height for lifting the conduit, and do not interrupt and lift the conduit. During this process, the slurry is introduced into the funnel and presses the ball downward, so that the water in the conduit is pressed out from the bottom of the conduit and discharged from the drilled hole. When the pouring volume of the slurry is slightly larger than the designed volume, stop pouring the slurry, and thus obtain the large-diameter rotary drilled cast-in-place pile. However, this technology does not have construction technologies such as special advanced support and the full-sleeve full-rotation construction technology, and it is difficult to ensure the construction quality. Summary of the Invention

[0004] The present invention discloses a construction method for large-diameter cast-in-place piles under complex conditions of ultra-thick sand layers, which can provide guarantee for the construction quality control of large-diameter cast-in-place piles under ultra-thick sand layers, avoid the safety risk of hole collapse, and reduce the quality risks such as poor pile integrity and too thick bottom sediment of the piles.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a large-diameter cast-in-place pile construction method under ultra-thick sand layer and complex conditions, including the process of three-axis mixing pile advance support, hole cleaning process, pile bottom grouting process, pile body detection and grouting treatment after detection, specifically using three-axis mixing pile advance support, mud wall drilling, improving pile quality, and then using three hole cleaning processes, and finally performing pile foundation detection and taking grouting treatment as appropriate;

[0006] Among them, the three-axis mixing pile advance support is an annular support structure formed by a plurality of three-axis mixing piles continuously interlocking along the circumferential direction, and the central axis of the annular support structure coincides with the central axis of the pile hole to be constructed;

[0007] The construction method of the large-diameter cast-in-place pile in the three-axis mixing pile advance support includes the following sub-steps:

[0008] (a) Calculate the diameter of the annular support structure, pile position arrangement and three-axis mixing pile construction parameters based on the engineering pile hole design parameters;

[0009] (b) Use a three-axis pile mixer to continuously construct along a circular path to form a fully closed annular support structure;

[0010] (c) Drilling construction of the engineering pile hole is carried out on the inner side of the annular support structure, and the inner wall of the annular support structure is used as at least a partial support surface of the hole wall during the drilling process; the drilling construction adopts a casing follow-up process, and the gap between the outer wall of the casing and the inner wall of the annular support structure is filled with high-pressure spray cement slurry to form a continuous water stop layer; when the casing is pulled out, compensating slurry is injected simultaneously, and the initial setting time of the compensating slurry is 30%-50% shorter than the initial setting time of the engineering pile concrete;

[0011] (d) A steel cage is placed in the engineering pile hole and concrete is poured to form a composite pile foundation that is partially integrated with the annular support structure.

[0012] Furthermore, the interlocking mode of the three-axis mixing piles is as follows: the center distance L between adjacent three-axis mixing piles and the pile diameter R of a single three-axis mixing pile satisfy L≤0.8R, and the inter-pile interlocking thickness is ≥200mm; the depth H of the annular support structure is greater than the design depth of the engineering pile hole, and the extended part of H exceeds the length of the bottom of the engineering pile hole by ≥2m.

[0013] Furthermore, the engineering pile hole is located on the inner side of the annular support structure, and its outer wall partially overlaps with the inner wall of the annular support structure, and the width W of the overlapping area is 10%-30% of the design diameter of the engineering pile hole; the diameter D of the annular support structure satisfies: D=d+2W+Δ, where d is the design diameter of the engineering pile hole, Δ is the construction error compensation value and Δ∈[50mm,200mm].

[0014] Further, after the construction of sub-step (b) and before the construction of sub-step (c), a polymer reinforcement layer is sprayed on the inner surface of the ring-shaped support structure. The thickness of the polymer reinforcement layer is 20 - 50 mm, and the compressive strength is ≥ 15 Mpa.

[0015] Further, after the construction of sub-step (d), it also includes embedding distributed fiber optic sensors in the ring-shaped support structure to real-time monitor the deformation of the support structure during the construction of the engineering pile hole, and dynamically adjust the drilling parameters according to the monitoring data, and grout to repair the fine voids.

[0016] Further, the hole cleaning process adopts the air-lift reverse circulation hole cleaning method and is carried out three times.

[0017] Further, after the large-diameter cast-in-place pile is poured, high-pressure cement slurry is injected through the previously embedded grouting pipe to fill the gap between the bottom of the cast-in-place pile and the rock stratum, reduce the thickness of the sediment at the pile bottom, and improve the pile quality.

[0018] Further, after the large-diameter cast-in-place pile is poured and reaches the age, ultrasonic testing and core extraction testing are carried out according to the specifications. For large-diameter piles with sediment at the pile bottom meeting the design quality requirements, grouting treatment is carried out. For large-diameter piles with sediment at the pile bottom not meeting the design requirements, high-pressure hole cleaning and then grouting treatment are carried out. For engineering piles with the integrity of the pile body not meeting the design requirements, reinforcement is taken.

[0019] The beneficial effects of the present invention include:

[0020] (1) The ring-shaped support structure formed by the three-axis mixing piles provides stable sidewall support for the construction of the engineering pile hole, effectively preventing the risk of hole wall collapse under complex geological conditions such as ultra-thick sand layers, and ensuring the safety of construction personnel and equipment. The reasonable design of parameters such as the diameter, overlapping area width, and depth of the ring-shaped support structure enables it to better adapt to the construction requirements of the engineering pile hole. At the same time, considering the construction error compensation value, it improves the flexibility and adaptability of construction, and reduces the quality risks such as poor pile integrity and excessive sediment at the pile bottom, providing strong guidance and reference for similar projects.

[0021] (2) Spraying a polymer reinforcement layer on the inner side of the ring-shaped support structure and adopting the casing following process further enhances the stability of the hole wall. At the same time, by high-pressure spraying cement slurry to fill the gap between the casing and the support structure, a continuous water-stop layer is formed, effectively preventing groundwater from seeping in, and further improving the safety of construction.

[0022] (3) The process of three - stage air - lift reverse circulation hole cleaning can more thoroughly remove the sediment and impurities at the bottom of the hole, reducing the impact on the bearing capacity of the pile bottom; the pile - bottom grouting process can fill the gap between the bottom of the cast - in - place pile and the rock formation, reducing the thickness of the sediment at the pile bottom, thereby improving the bearing capacity and overall quality of the pile. Through a variety of innovative processes, the service life of the large - diameter cast - in - place pile in the present invention is longer than that of the existing piles with the same diameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a large - scale drawing of the three - axis mixing pile of the present invention;

[0024] Figure 2 It is a schematic diagram of the principle of the second - stage hole cleaning by air - lift reverse circulation of the present invention;

[0025] Figure 3 It is a schematic diagram of the layout scheme of the grouting pipes of the present invention;

[0026] Figure 4 It is a drawing of the second - stage verification core - drilling holes in the pile foundation inspection of the present invention. SPECIFIC EMBODIMENTS

[0027] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0028] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "middle", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation to the present invention.

[0029] Refer to Figures 1-4 , the present invention discloses a construction method for large - diameter cast - in - place piles under complex conditions of ultra - thick sand layers, including the process of advanced support by three - axis mixing piles, hole - cleaning process, pile - bottom grouting process, pile body detection and post - detection grouting treatment. The advanced support by three - axis mixing piles and mud - wall protection for hole formation are used to improve the pile - forming quality. Then, the three - stage hole - cleaning process is used, and finally, pile foundation detection and grouting treatment are carried out as appropriate to ensure the quality of large - diameter cast - in - place piles;

[0030] Among them, the three-axis mixing pile advance support is an annular support structure 2 formed by continuous engagement of multiple three-axis mixing piles 1 in the circumferential direction, and the central axis of the annular support structure coincides with the central axis of the engineering pile hole to be constructed; the engineering pile 3 hole is located on the inner side of the annular support structure, and its outer wall partially overlaps with the inner wall of the annular support structure, and the width W of the overlapping area is 10%-30% of the design diameter of the engineering pile hole; the diameter D of the annular support structure satisfies: D=d+2W+Δ, where d is the design diameter of the engineering pile hole, Δ is the construction error compensation value and Δ∈[50mm,200mm];

[0031] The construction method of the large diameter cast-in-place pile in the three-axis mixing pile advance support is as follows:

[0032] (a) Calculate the diameter of the annular support structure, pile position arrangement and three-axis mixing pile construction parameters based on the engineering pile hole design parameters;

[0033] (b) Use a three-axis pile mixer to continuously construct along a circular path to form a fully closed annular support structure;

[0034] (c) drilling the engineering pile hole inside the annular support structure, using the inner wall of the annular support structure as at least part of the support surface of the hole wall during the drilling process; the drilling construction adopts a casing follow-up process, and the gap between the outer wall of the casing and the inner wall of the annular support structure is filled with high-pressure sprayed cement slurry to form a continuous water stop layer; when the casing is pulled out, a compensating slurry is injected simultaneously, and the initial setting time of the compensating slurry is 30% to 50% shorter than the initial setting time of the engineering pile concrete; after the construction of step (b) and before the construction of step (c), a polymer reinforcement layer is sprayed onto the inner surface of the annular support structure, and the thickness of the polymer reinforcement layer is 20-50 mm, and the compressive strength is ≥15 MPa;

[0035] (d) Place a steel cage in the engineering pile hole and pour concrete to form a composite pile foundation that is partially integrated with the annular support structure;

[0036] Distributed fiber optic sensors are embedded in the annular support structure to monitor the deformation of the support structure during the construction of the engineering pile hole in real time, and dynamically adjust the drilling parameters according to the monitoring data.

[0037] The interlocking mode of the three-axis mixing piles is as follows: the center distance L between adjacent three-axis mixing piles and the pile diameter R of a single three-axis mixing pile satisfy L≤0.8R, and the inter-pile interlocking thickness is ≥200mm; the depth H of the annular support structure is greater than the design depth of the engineering pile hole, and the length of the extended part of H exceeding the bottom of the engineering pile hole is ≥2m.

[0038] For the drilling process of large-diameter bored piles under ultra-thick sand layers, three-axis mixing pile advance support + mud wall drilling is adopted.

[0039] Working principle:

[0040] ① Design parameters of cement mixing pile materials

[0041] The cement mixing pile adopts the φ800@600 "two spraying and two mixing" process, and is constructed by the spraying method. The water-cement ratio of the cement slurry is 0.6-0.8, and the sinking and lifting speed is 1.0-1.2m / min. The cement used in the construction of the mixing pile is PO 42.5 ordinary silicate cement, and the spraying material is pure cement slurry with a water-cement ratio of 1:1. The cement consumption of the three-axis mixing pile used for engineering pile reinforcement is not less than 500Kg / m per meter, and the cement addition amount is ≥20%.

[0042] ②Cement mixing pile process design parameters

[0043] The mixing piles are drilled at φ800@600 and interlocked with the engineering piles by 200mm to ensure that the outer diameter of the engineering piles is all within the area of ​​the mixing piles. The three-axis mixing piles with a diameter of 10 meters, 9 meters and 8 meters are respectively used for piles with a diameter of 3.2 meters, 3 meters and 2.8 meters. The drilling depth is determined according to the geological columnar diagram. The mixing piles need to pass through the sand layer and the sandy clay layer, and the final hole is 1 meter into the fully weathered rock layer.

[0044] ③Main construction methods of mixing piles

[0045] Measurement and layout: Before construction, according to the design drawings, positioning and layout are carried out, trenches 4 are excavated, and then the first set of piles are placed. The trenches are connected with mud pools 5 and sedimentation pools 6. Partition panels 7 are set between the sedimentation pools and mud pools, and the dimensions are marked according to the design drawings.

[0046] Preparation of cement slurry: The amount of cement slurry should be prepared according to the design standards. During construction, quantitative containers can be used to control the amount of water used.

[0047] Premixed sinking spraying: After the cooling water circulation of the cement mixing pile machine is normal, start the mixing pile machine motor, loosen the mixing pile machine sling, and make the mixing pile machine mix and cut the soil and sink along the guide frame. The sinking speed can be controlled by the motor's current monitoring meter.

[0048] Lifting shotcrete: Start the shotcrete mixing. During the shotcrete mixing process, keep stirring the cement slurry. Observe the operation of the equipment and the changes in the formation at any time. When the drill bit sinks to the designed depth, stop drilling. The sinking and lifting speed is 1.0-1.2m / min, and the pressure at the slurry outlet is maintained at 0.4-0.6Mpa.

[0049] Shift: lift the drill bit to spray. During the spraying process, the cement slurry is continuously stirred to prevent its segregation, and the spraying volume is automatically recorded by the computer. When it is 50cm above the ground, the spraying is stopped.

[0050] ④ Mud wall drilling

[0051] φ2.8m pile hole forming process flow: Step 1, use a 2.8-meter sand bucket to drill to the rock surface; Step 2, change to a 1.4-meter roller bit to drill, and use a 1.4-meter core barrel drill to cooperate with taking rock cores during the drilling process until reaching the final hole depth; Step 3, change to a 1.8-meter roller bit to ream the hole, and use a 1.8-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth; Step 4, change to a 2.2-meter roller bit to ream the hole, and use a 2.2-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process; Step 5, change to a 2.8-meter roller bit to ream the hole, and use a 2.8-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth.

[0052] φ3.2m pile hole forming process flow: Step 1, use a 3.2-meter sand bucket to drill to the rock surface; Step 2, change to a 1.4-meter roller bit to drill, and use a 1.4-meter core barrel drill to cooperate with taking rock cores during the drilling process until reaching the final hole depth; Step 3, change to a 1.8-meter roller bit to ream the hole, and use a 1.8-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth; Step 4, change to a 2.2-meter roller bit to ream the hole, and use a 2.2-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth; Step 5, change to a 2.7-meter roller bit to ream the hole, and use a 2.7-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth; Step 6, change to a 3.2-meter roller bit to ream the hole, and use a 3.2-meter sand bucket to cooperate with removing the fallen rock blocks during the drilling process until reaching the final hole depth.

[0053] During the drilling process, keep the mud in the hole in a full state to enhance the wall protection effect.

[0054] Refer to Figures 2-4 , in a preferred embodiment, the air-lift reverse circulation hole cleaning method is used for three times of hole cleaning. Insert a galvanized pipe about 2 / 3 of the hole depth into the conduit to send high-pressure air to 2 / 3 of the hole depth inside the conduit, mix with the mud inside the conduit, generate a low-pressure area inside the conduit after inflation, continuously increase the pressure difference inside and outside the conduit during continuous inflation, when a certain pressure difference is reached, the balance is broken, then force the mud to spray back from the inside of the conduit under high pressure, and at the same time, the rock debris at the bottom of the hole is carried by the high-speed mud and sprayed out of the hole from the conduit.

[0055] Working principle:

[0056] First hole cleaning: After the hole forming acceptance is passed, lower the conduit first before lowering the cage, and use the air-lift reverse circulation method to conduct the first hole cleaning. During the hole cleaning process, the conduit should rotate around the hole to ensure that every corner at the bottom of the hole is cleaned without leaving dead corners, and use a sand separator to cooperate with sand separation during the process.

[0057] Second hole cleaning: After the first hole cleaning is completed and the sediment at the bottom of the hole is cleared, the steel cage is lowered. After the steel cage is lowered, the second hole cleaning is carried out. Repeat the first hole cleaning steps until all the sediment at the bottom of the hole is cleared, the mud density is controlled below 1.2, the mud sand content is controlled below 4%, and the viscosity is controlled within 18-28s.

[0058] The third hole cleaning: After the installation of the conduit is completed, the hole is cleaned for the third time before concrete pouring. Repeat the second hole cleaning steps. At the same time, remeasure the thickness of the sediment at the bottom of the hole before concrete pouring. Concrete pouring can only be carried out if the sediment thickness is within the specification requirements.

[0059] Reference Figure 3 In a preferred embodiment, a bottom hole grouting scheme as shown in the figure is selected.

[0060] Working principle: In order to ensure the bearing capacity of the pile bottom, the pile bottom grouting is used for reinforcement. The grouting pipe 8 adopts a ф32 grouting pipe, and an opening 9 is set at the bottom of the grouting pipe. It is lowered together with the steel cage 10, spot welded with the main reinforcement of the steel cage and tied tightly. Generally, the normal injection pressure is between 2-5Mpa, and the grouting pump volume is controlled at 32-50L / min. When the conditions for completing grouting are met, stop grouting.

[0061] Reference Figure 4 In a preferred embodiment, ultrasonic testing and core extraction testing are used to inspect the construction quality of large diameter piles.

[0062] In the specific implementation process, the core pulling hole position depends on the actual situation.

[0063] Working principle: For large-diameter engineering piles with insufficient pile body integrity, grouting is used for reinforcement. The thickness of the pile bottom sediment is required to be less than 50mm. The underwater television method is used to check the hole cleaning before pouring. After pouring, the core extraction method is used to detect the thickness of the pile bottom sediment. The core extraction holes 12 are evenly spaced. For engineering piles less than 50mm, grouting is performed after hole cleaning. For engineering piles greater than 50mm, high-pressure hole cleaning and grouting are performed and then the hole cleaning is verified through secondary core extraction holes 11. The test results show that the strength and bending resistance of the pile body of the present invention are much better than those of existing cast-in-place piles of the same diameter.

[0064] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. The replacement may be a replacement of a part of the structure, device, method step, or a complete technical solution. Any equivalent replacement or change according to the technical solution and the inventive concept of the present invention shall be covered within the protection scope of the present invention.

[0065] The annular support structure formed by the three-axis mixing piles in the present invention provides stable sidewall support for the construction of the engineering pile holes, effectively preventing the risk of hole wall collapse under complex geological conditions such as ultra-thick sand layers, and ensuring the safety of construction personnel and equipment; the reasonable design of parameters such as the diameter, overlapping area width, and depth of the annular support structure enables it to better meet the construction requirements of the engineering pile holes. At the same time, the construction error compensation value is considered, improving the flexibility and adaptability of the construction.

[0066] Spraying a polymer reinforcement layer on the inner side of the annular support structure and adopting the casing following process further enhances the stability of the hole wall. At the same time, by injecting high-pressure jet grout to fill the gap between the casing and the support structure, a continuous water-stop layer is formed, effectively preventing groundwater from seeping in and further improving the construction safety.

Claims

1. A method for constructing large-diameter cast-in-place piles under complex conditions of ultra-thick sand layers, characterized by: It includes the process of three-axis mixing pile advance support, hole cleaning, pile bottom grouting, pile body detection and post-detection grouting treatment. Specifically, three-axis mixing pile advance support, mud wall hole forming, improving pile quality, and then three hole cleaning processes are used. Finally, pile foundation detection and grouting treatment are carried out as appropriate. Among them, the three-axis mixing pile advance support is an annular support structure formed by a plurality of three-axis mixing piles continuously interlocking along the circumferential direction, and the central axis of the annular support structure coincides with the central axis of the pile hole to be constructed; The construction method of the large-diameter cast-in-place pile in the three-axis mixing pile advance support includes the following sub-steps: (a) Calculate the diameter of the annular support structure, pile position arrangement and three-axis mixing pile construction parameters based on the engineering pile hole design parameters; (b) Use a three-axis pile mixer to continuously construct along a circular path to form a fully closed annular support structure; (c) Drilling construction of the engineering pile hole is carried out on the inner side of the annular support structure, and the inner wall of the annular support structure is used as at least a partial support surface of the hole wall during the drilling process; the drilling construction adopts a casing follow-up process, and the gap between the outer wall of the casing and the inner wall of the annular support structure is filled with high-pressure spray cement slurry to form a continuous water stop layer; when the casing is pulled out, compensating slurry is injected simultaneously, and the initial setting time of the compensating slurry is 30%-50% shorter than the initial setting time of the engineering pile concrete; (d) A steel cage is placed in the engineering pile hole and concrete is poured to form a composite pile foundation that is partially integrated with the annular support structure.

2. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: The interlocking mode of the three-axis mixing piles is as follows: the center distance L between adjacent three-axis mixing piles and the pile diameter R of a single three-axis mixing pile satisfy L≤0.8R, and the inter-pile interlocking thickness is ≥200mm; the depth H of the annular support structure is greater than the design depth of the engineering pile hole, and the length of the extended part of H exceeding the bottom of the engineering pile hole is ≥2m.

3. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: The engineering pile hole is located on the inner side of the annular support structure, and its outer wall partially overlaps with the inner wall of the annular support structure. The width W of the overlapping area is 10%-30% of the design diameter of the engineering pile hole; the diameter D of the annular support structure satisfies: D=d+2W+Δ, where d is the design diameter of the engineering pile hole, Δ is the construction error compensation value and Δ∈[50mm,200mm].

4. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: After the construction of sub-step (b) and before the construction of sub-step (c), a polymer reinforcement layer is sprayed onto the inner surface of the annular support structure. The thickness of the polymer reinforcement layer is 20-50 mm and the compressive strength is ≥15 MPa.

5. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: After the construction of sub-step (d), it also includes pre-embedding distributed optical fiber sensors in the annular support structure to monitor the deformation of the support structure during the construction of the engineering pile hole in real time, and dynamically adjust the drilling parameters according to the monitoring data, and inject grouting to repair tiny gaps.

6. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: The hole cleaning process adopts an air lift reverse circulation hole cleaning method, and the hole cleaning is performed three times.

7. A method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1 or 2, characterized in that: After the large-diameter bored piles are cast, cement slurry is injected at high pressure through the pre-buried grouting pipe to fill the gap between the bottom of the bored pile and the rock layer, reduce the thickness of the sediment at the bottom of the pile, and improve the quality of the pile.

8. The method for constructing large-diameter bored piles under complex conditions in ultra-thick sand layers according to claim 1, characterized in that: After the large-diameter bored piles are cast and reach the age, ultrasonic testing and core sampling testing are carried out in accordance with the specifications. For large-diameter piles whose bottom sediment meets the design quality requirements, grouting treatment is performed. For large-diameter piles whose bottom sediment does not meet the design requirements, high-pressure hole cleaning and re-grouting treatment is performed. For engineering piles whose pile body integrity does not meet the design requirements, reinforcement is taken.

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