A construction method to improve the bearing capacity of foundations in complex strata

Abstract

This invention discloses a construction method for improving the bearing capacity of foundations in complex strata. It employs a combination of screw piles and lime-soil compaction piles to reinforce foundations with alternating layers of soft and hard soil. Specifically, multiple lime-soil compaction piles are arranged around a central screw pile, connected together by pile caps to form a composite foundation. These piles, the central screw pile, and the pile caps together constitute a foundation unit. Multiple foundation units are arranged in a matrix, and a double-layer cushion layer is constructed above these units to form the foundation and bear the load from the superstructure. This invention solves the technical challenge of pile formation in complex strata with alternating layers of soft and hard soil, and improves pile formation efficiency, thus increasing construction efficiency while meeting requirements for safety, economy, environmental protection, and applicability.

Description

Technical Field

[0001] This invention relates to the field of construction technology for improving the bearing capacity of foundations in complex geological formations. More specifically, this invention relates to a construction method for improving the bearing capacity of foundations in complex geological formations. Background Technology

[0002] China is a vast country with diverse geological structures and conditions, which has led to a series of technical challenges in the construction of high-speed railways. In particular, ensuring controllable foundation settlement during later operation has placed increasingly stringent requirements on foundation treatment technology, as general reinforcement techniques can no longer meet the relevant quality requirements.

[0003] Common reinforcement techniques include steel pipe piles and cement mortar piles. A newer reinforcement technology using cast-in-place screw piles has emerged. Cast-in-place screw piles are constructed by rotating a pile driver to compress the soil and pump concrete into piles. Compared to driven precast piles, this method results in lower construction noise, no vibration, and no impact on existing piles. Compared to grouting piles using full auger drilling and ordinary mud-wall drilling, it eliminates mud pollution and soil disposal issues. Screw piles are a highly practical new type of pile, applying the common-sense principle that "a screw is stronger than a nail" to pile construction, making them more robust. However, screw piles encounter various quality problems when used in complex geological formations, such as foundations with alternating layers of soft and hard soil. These include: 1) slowed drill rod advance and reduced drilling speed; 2) drill bit obstruction leading to overload of the electric drive head and tripping; and 3) prolonged grinding and drilling of the drill bit and rod through the strata causing the core temperature to become unsuitable for concrete pumping, necessitating shutdown for cooling.

[0004] Therefore, in order to address the technical difficulties encountered in the construction of pile foundations in soil layers with alternating soft and hard surfaces, there is an urgent need for a new construction technology to improve the bearing capacity of the foundation in such soil layers. This method should not only overcome the technical difficulties of pile formation in soil layers with alternating soft and hard surfaces, but also improve the efficiency of pile formation and meet the requirements of safety, economy, environmental protection, and applicability. Summary of the Invention

[0005] One objective of this invention is to provide a construction method for improving the bearing capacity of foundations in complex strata, which solves the technical problem of difficult pile formation in complex strata where the surface soil layers are of varying hardness. It can also improve pile formation efficiency, thereby increasing construction efficiency and meeting requirements for safety, economy, environmental protection, and applicability.

[0006] To achieve these objectives and other advantages according to the present invention, a construction method for improving the bearing capacity of foundations in complex strata is provided. This method employs a combination of screw piles and lime-soil compaction piles to reinforce foundations with alternating layers of soft and hard soil. Specifically, multiple lime-soil compaction piles are arranged around a central screw pile, and these piles are connected to the screw pile via pile caps to form a composite foundation. The multiple lime-soil compaction piles, the central screw pile, and the pile caps together form a foundation unit. These foundation units are arranged in a matrix, and a double-layer cushion layer is constructed above each foundation unit to form the foundation and bear the superstructure load.

[0007] Preferably, multiple lime-soil compaction piles are arranged in an equilateral triangle, square, or rectangle with their corresponding screw piles as the center.

[0008] Preferably, the lime-soil compaction piles and screw piles have the same pile diameter. When the compaction depth of the lime-soil compaction piles is less than 12m, the pile diameter is 350-450mm. When the compaction depth exceeds 12m, pre-drilling is used, with a hole diameter of 250-350mm and a hole diameter of 500-600mm.

[0009] Preferably, the spacing between lime-soil compaction piles is 2.0-2.5 times the diameter of the pile hole, or the spacing can be calculated empirically using the following formula:

[0010]

[0011] α - coefficient: for lime-soil compaction piles arranged in an equilateral triangle, α is taken as 0.95; for lime-soil compaction piles arranged in a square, α is taken as 0.89.

[0012] S - Center-to-center distance between pile holes (m);

[0013] D - Diameter of the compacted packing pores (m);

[0014] d - pre-drilled hole diameter (m), d=0 in the compaction method without pre-drilled holes;

[0015] ρ dmax - Light compaction test to determine the maximum dry density of soil between piles (Kg / m³) 3 );

[0016] ρ do - Average dry density of each soil layer within the compressible layer range before foundation compaction (Kg / m³) 3 );

[0017] η c - After the soil between piles is compacted through drilling to reach the diameter D of the compaction filling hole, the average compaction coefficient between multiple lime-soil compaction pile holes is 0.90-0.93.

[0018] Preferably, the screw pile is drilled normally using a screw drilling rig within the set hardness range of the soil layer. When the soil hardness exceeds the set hardness range, the spacing between the screw threads of the screw drilling rig is increased to reduce the ultimate frictional resistance between the drill rod and the soil layer.

[0019] Preferably, after the soil hardness exceeds a set hardness range, multiple high-pressure water jet holes are set in the drill rod core of the screw pile. These holes are arranged in a quincunx pattern at the end of the core. High-pressure water is jetted through the high-pressure water jet holes to soften the corresponding soil layer, thereby reducing the ultimate frictional resistance between the drill rod and the soil layer.

[0020] Preferably, within 7 days of pile completion, the integrity of the pile body is checked using low strain testing, with 10% of the total number of piles being checked; after 28 days, single pile or composite foundation load tests are conducted, with 1% of the total number of piles being tested, and the number of static load tests on the composite foundation for each individual project is not less than 3 points.

[0021] Preferably, the foundation unit is constructed with a double-layer cushion layer consisting of a lower layer of cement-soil-gravel cushion layer with 6% cement content, a bidirectional warp-knitted polyester geogrid, and an upper layer of gravel cushion layer with 6% cement content. The lower cement-soil-gravel cushion layer is 0.5m thick, and the upper cement-soil-gravel cushion layer is 0.3m thick.

[0022] Preferably, the specific steps include the following:

[0023] Step 1: Treat the ground of the construction site and measure and mark each pile position, with the pile position deviation controlled within 50mm;

[0024] Step 2: Construct lime-soil compaction piles and corresponding screw piles according to the marked pile positions. When constructing screw piles where the soil hardness exceeds the set hardness range, replace the drill rod corresponding to the screw pile with a drill rod with increased spacing between the drill rod threads and a high-pressure water jet hole in the drill rod core, and then proceed with drilling. At the same time, water is injected through the high-pressure water jet hole to assist in sinking.

[0025] Step 3: After the concrete of the screw pile reaches the required age, use a pile cutter to cut the pile from three sides, leveling the top of the pile from the perimeter to the center to the design elevation. The elevation deviation of the pile top should be controlled within 0±20mm. The integrity and load of the pile should also be checked.

[0026] Step 4: Connect the corresponding multiple lime-soil compaction piles and screw piles in the same foundation unit into a whole by using pile caps to form a foundation unit. Complete the construction of all foundation units.

[0027] Step 5: Construct the foundation layer. Between the pile caps, construct a lower foundation layer using a mixture of 6% cement, soil, and gravel. The lower foundation layer is 0.5m thick. After the lower foundation layer is completed, lay a bidirectional warp-knitted polyester geogrid on top. Finally, lay a 6% cement, soil, and gravel mixture on top of the geogrid to form an upper foundation layer. The upper foundation layer is 0.3m thick.

[0028] The present invention has at least the following beneficial effects:

[0029] 1) Environmental friendliness: Screw piles are constructed by rotating and squeezing the soil with a pile driver during the construction process to form concrete. This not only results in low construction noise and no vibration, but also has no impact on the already constructed piles, and there is no mud pollution or waste soil problem.

[0030] 2) High efficiency: The hole is drilled from 0 to 22m using a regular screw drill. After entering the hard clay layer for 22m, an improved screw drill is used to assist in water jetting to soften the hole. The single pile is 33m long and the pile can be completed in less than 1 hour.

[0031] 3) Feasibility: Improve the JZU120 type threaded drill rod by injecting high-pressure water, which is then ejected from the drill bit to soften the dry and hard silty clay layer, reduce the drill bit's cutting resistance, and ensure smooth hole formation.

[0032] 4) Cutting-edge nature: The application of screw piles combined with lime-soil compaction piles to reinforce the complex strata section of this application solves the technical problem of difficult pile construction in complex strata where the surface soil layers are of mixed soft and hard textures, and has reference value for the technological advancement of this industry.

[0033] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0034] Figure 1 This is a schematic cross-sectional view of the "lime-soil compaction pile + screw pile" of the present invention;

[0035] Figure 2 This is a schematic diagram of the plan layout of the "lime-soil compaction pile + screw pile" of the present invention;

[0036] Figure 3 This is a detailed schematic diagram of the JZU120 type threaded drill rod of the present invention.

[0037] Explanation of reference numerals in the attached figures:

[0038] 1. Left drainage ditch; 2. Right drainage ditch; 3. Reinforcement layer; 4. Soil-lime compaction pile; 5. Screw pile; 6. Pile cap; 7. Lower 6% cement-soil-gravel cushion layer; 8. Bidirectional warp-knitted polyester geogrid; 9. Upper 6% cement-soil-gravel cushion layer; 10. Left slope toe line; 11. Left geogrid outer edge line; 12. Left reinforcement edge line; 13. Left shoulder line; 14. Right shoulder line; 15. Right geogrid outer edge line; 16. Right slope toe line; 17. Right reinforcement edge line; 18. Drill rod; 19. Thread; 20. Power head. Detailed Implementation

[0039] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0040] It should be noted that, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified. In the description of this invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0041] The construction example corresponding to this application is:

[0042] The surface of the foundation of a certain high-speed railway section DK045+978-DK46+200 is distributed from top to bottom as follows: silt, silty clay (yellow), fine gravel, silty sand, silty clay (brown), and medium sand. Among them, the 22m-27m range is a layer of silty clay (brown), which is dry and the clay is in a hard state. It is reinforced with screw piles. The foundation design scope of this application is 221m long and 8m wide, with a pile diameter of 0.4m and a pile depth of 12-33m, arranged in a square of 2m.

[0043] like Figure 1 and Figure 2 As shown, this invention provides a construction method for improving the bearing capacity of foundations in complex geological formations with alternating layers of hard and soft soil. The method employs a combination of screw piles and lime-soil compaction piles to reinforce this complex geological section. The main procedures include original ground treatment, surveying and setting out, construction of lime-soil compaction piles, installation of screw piles, excavation of the soil between piles, pile head treatment, pile formation testing, pile cap construction, and foundation layer construction. Specifically, the method uses screw piles combined with lime-soil compaction piles to reinforce the foundation in the complex geological section. When the screw piles penetrate the hard silty clay (yellow) layer, a high-pressure water jet is used to soften it.

[0044] 1. Original ground treatment

[0045] Before foundation reinforcement, temporary drainage facilities should be installed around the construction site. Topsoil should be removed, obstacles should be cleared, and underground structures and pipelines within the treatment area should be marked. Drainage ditches 1 and 2 should be installed on the left and right sides outside the toe lines 10 and 16. The subbase of drainage ditch 1 on the left side should be backfilled with 3:7 lime-soil to form reinforcement layer 3. Surface treatment should be carried out in sections by combining manual labor with excavators or bulldozers according to different requirements. The treated base should be flat.

[0046] 2. Measurement and layout

[0047] The construction area was divided according to the design drawings, and each pile was numbered and managed by designated personnel and machinery. The reinforcement edge lines 12 and 17, slope toe lines 10 and 16, geogrid outer edge lines 11 and 15, and shoulder lines 13 and 14 were marked with lime. The lime-soil compaction piles 4 and screw piles 5 were marked with colored flags. Pile locations were laid out according to the pile location plan and design drawings, with holes drilled at the pile locations using steel chisels and filled with lime to mark them. Pile location deviations were controlled within 50mm.

[0048] 3. Construction of lime-soil compaction piles

[0049] A steel pipe is driven into the soil using a hammer to compact it laterally, forming a hole. After the steel pipe is pulled out, the hole is backfilled with 3:7 lime-soil mixture in layers and compacted. In one specific embodiment, the lime-soil piles have a diameter of 0.4m and a spacing of 1m, arranged in a square. The lime-soil compaction piles are used to reinforce the surrounding foundation. The steel pipe is driven to compress the surrounding soil to form piles. The lime-soil compaction piles 4 are connected to the screw piles 5 through screw pile caps 6 to form a composite foundation to bear the upper load. The diameter of the screw piles is the same as that of the lime-soil compaction piles.

[0050] Referring to the Technical Specifications for Foundation Treatment in Railway Engineering, the following conclusions can be drawn:

[0051] (1) Based on the overall stress effect of the foundation, the lime-soil compaction piles should be arranged in equilateral triangles, squares, or rectangles.

[0052] (2) The diameter of the pile hole for lime-soil compaction piles can be determined comprehensively based on the selected drilling equipment, drilling method and foundation treatment depth. When the compaction treatment depth is less than 12m, the pile diameter should be 350-450mm; when the compaction treatment depth exceeds 12m, pre-drilling can be used, the hole diameter should be 250-350mm, and the hole diameter should be 500-600mm.

[0053] (3) The depth of the lime-soil compaction pile treatment for the foundation should be determined comprehensively based on factors such as the soil conditions, engineering requirements, and drilling and compaction equipment. For collapsible loess foundations, it should comply with the relevant provisions of the current national standard "Code for Construction in Collapsible Loess Areas" (GB50025). Lime-soil compaction piles are mainly used to eliminate the collapsibility of the foundation, while screw piles are mainly used to improve the bearing capacity of the foundation soil, enhance its water stability, reduce compressibility, and control foundation settlement and deformation. Therefore, screw piles are shallower than lime-soil compaction piles, and the final depth is sufficient to meet the bearing capacity of the foundation.

[0054] (4) The pile spacing should be 2.0-2.5 times the pile hole diameter, or it can be calculated based on experience using the following formula:

[0055]

[0056] α - coefficient: for lime-soil compaction piles arranged in an equilateral triangle, α is taken as 0.95; for lime-soil compaction piles arranged in a square, α is taken as 0.89.

[0057] S - Center-to-center distance between pile holes (m);

[0058] D - Diameter of the compacted packing pores (m);

[0059] d - pre-drilled hole diameter (m), d=0 in the compaction method without pre-drilled holes;

[0060] ρ dmax - Light compaction test to determine the maximum dry density of soil between piles (Kg / m³) 3 );

[0061] ρ do - Average dry density of each soil layer within the compressible layer range before foundation compaction (Kg / m³) 3 );

[0062] η c - After the soil between piles is compacted through drilling to reach the diameter D of the compaction filling hole, the average compaction coefficient between multiple lime-soil compaction pile holes is 0.90-0.93.

[0063] 4. Construction of screw piles 5

[0064] (1) The layers through which the screw piles pass are silt, silty clay (yellow), fine gravel, silt, silty clay (brown), and medium sand. Among them, the main strata that are 0-22m below the surface are silt, silty clay (yellow), fine gravel, silt, and silty clay (brown), while the main strata that are 22m-33m below the surface are silty clay (brown) and medium sand. The strata are a mixture of soft and hard.

[0065] (2) The section 0-22m below the surface that traverses the strata is mainly drilled using a JZU120 screw drill. During the drilling process, the power head 20 provides a torque of M=300KN.m and a pressure of F=400KN.

[0066] (3) such as Figure 3 As shown, when traversing hard silty clay (brown) 22m below the surface, the JZU120 type threaded drill rod 18 is partially modified to increase the distance between the drill rod threads 19. At the same time, high-pressure water is injected into the drill rod core and ejected from the drill bit. The auxiliary method of "high-pressure water injection softening" is used to soften the dry and hard silty clay layer to a certain extent, reduce the drill bit's advance (cutting) resistance, and ensure smooth hole formation.

[0067] Within 5m of the end of the drill rod, a punching machine is used to make water jet holes. The water jet holes are arranged in a quincunx pattern, with a diameter of φ10mm and a spacing of 15cm. When the drill rod is working in the hard clay layer, a single-stage centrifugal booster pump is connected to the water tank. The booster pump model is ISG20-160 with a head of 30m. Water from the water tank is delivered to the clay layer to soften it and reduce the loss of kinetic energy during the crossing process.

[0068] During the drilling process, the bottom of the drill bit excavates, and the screw threads of the drill body squeeze the soil. The ultimate frictional resistance T between the screw threads and the soil is... f The vertical and horizontal components can be decomposed. The vertical component resists the applied pressure F, and the horizontal component resists the torque M. Increasing the distance between the screw threads can reduce the ultimate frictional resistance. Considering the effect of the screw threads squeezing the soil and controlling the magnitude of the ultimate frictional resistance, the original screw thread spacing S is adjusted to 2S (where S=1m).

[0069] 5. Excavation of soil between piles

[0070] The screw piles should be installed to 0.5m below the ground surface. Soil excavation can only be carried out after the pile strength reaches 70% of the design strength. Soil removal should be carried out using small machinery and manual labor. When using small machinery to excavate, it should be 20cm away from the pile edge. Excess soil should be removed manually to avoid damaging the pile. A 20cm layer of soil should be left at the bottom of the pit for manual removal and leveling.

[0071] 6. Pile head treatment

[0072] After the concrete strength of the screw pile reaches the required age, the pile should be cut off by three-sided circumferential cutting with a pile cutter. It is forbidden to use a heavy hammer or heavy object to strike the pile body laterally, so as to avoid causing the pile body below the top elevation to break laterally. After the pile head is cut off, use a steel chisel and a hand hammer to level the top of the pile from the periphery to the middle to the design elevation of the pile top (the deviation of the pile top elevation is controlled within 0±20mm).

[0073] 7. Pile testing

[0074] Within 7 days of pile completion, the pile depth integrity is checked using low-strain testing, with 10% of the total number of piles being checked. After 28 days, single pile or composite foundation load tests are conducted, with 1% of the total number of piles being tested. The number of static load tests on the composite foundation for each individual project shall not be less than 3.

[0075] 8. Construction of pile cap 6

[0076] After the pile head treatment of the screw pile is qualified, the pile cap is constructed according to the pile foundation location. The steel bars are provided by the steel bar processing plant, transported to the site by flatbed truck, and tied manually. The formwork uses standardized steel formwork, which is transported to the site and assembled manually. C35 concrete mixer trucks are transported to the site, pumped into the formwork by boom, vibrated manually, and covered with geotextile and watered for curing.

[0077] 9. Subbase construction

[0078] The subbase consists of a lower layer of 6% cement-soil-gravel subbase 7, a biaxial warp-knitted polyester geogrid 8, and an upper layer of 6% cement-soil-gravel subbase 9. Subbase construction is carried out after formwork removal. The spaces between pile caps are filled with gravel mixed with 6% cement-soil, with a thickness of 0.5m. After the gravel filling between pile caps is completed, the biaxial warp-knitted polyester geogrid is laid on top. After the geogrid is laid, a 0.3m thick layer of gravel mixed with 6% cement-soil is laid on top.

[0079] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A construction method for improving the load bearing capacity of a complex ground formation, characterised in that, The screw pile and the lime-soil compaction pile are combined to reinforce the foundation with alternating soft and hard soil layers, specifically, a plurality of lime-soil compaction piles are arranged around the screw pile as the center outside the screw pile, the plurality of lime-soil compaction piles are connected together with the screw pile through the pile cap of the screw pile to form a composite foundation, the plurality of lime-soil compaction piles, the central screw pile and the pile cap of the screw pile jointly form a foundation unit, a plurality of foundation units are distributed in a matrix, and a double-layer cushion is arranged above the plurality of foundation units to form a foundation to bear the upper load.

2. The construction method for improving the load bearing capacity of a complex ground formation according to claim 1, wherein, The plurality of lime-soil compaction piles are arranged in a regular triangle or a square with the corresponding screw pile as the center.

3. The construction method for improving the load bearing capacity of a complex ground formation according to claim 2, wherein The diameter of the lime-soil compaction pile is the same as that of the screw pile, when the compaction depth of the lime-soil compaction pile is less than 12 m, the diameter of the pile is 350-450 mm, and when the compaction depth exceeds 12 m, a pre-drilled hole with a diameter of 250-350 mm is used, and the hole diameter is 500-600 mm.

4. The construction method for improving the load bearing capacity of a complex ground formation according to claim 3, wherein The spacing between the lime-soil compaction piles is 2.0-2.5 times the diameter of the hole, or the spacing is calculated according to the following formula: α-coefficient, when the lime-soil compaction piles are arranged in a regular triangle, α is 0.95, and when the lime-soil compaction piles are arranged in a square, α is 0.89; S-center distance between the holes (m); D-diameter of the compaction filler hole (m); d-diameter of the pre-drilled hole (m), d=0 in the compaction method without pre-drilled hole; ρ dmax - Light compaction test to determine the maximum dry density of soil between piles (Kg / m 3 ) ; ρ do - average dry density of each layer of soil in the range of the pre-compacted layer of the foundation (Kg / m 3 ) η c The soil between the piles is compacted by hole-forming and extrusion, and after the hole diameter D of the extruded filler is reached, the average compaction coefficient between the holes of the multiple lime-soil compacted piles is 0.90-0.

93.

5. The construction method for improving the load bearing capacity of a complex ground formation according to claim 1, wherein When the hardness of the soil layer exceeds the set hardness range, the spacing between the threads of the drill rod of the screw pile is increased to reduce the ultimate frictional resistance between the drill rod and the soil layer.

6. The construction method for improving the load bearing capacity of a complex ground formation according to claim 5, wherein When the hardness of the soil layer exceeds the set hardness range, a plurality of high-pressure water jet holes are arranged in the core of the drill rod, which are arranged in a plum blossom shape at the end of the core, and high-pressure water is jetted out through the high-pressure water jet holes to soften the corresponding soil layer to reduce the ultimate frictional resistance between the drill rod and the soil layer.

7. The construction method for improving the load bearing capacity of a complex ground formation according to claim 1, wherein The low-strain test is used to check the integrity of the screw pile within 7 days after the pile is formed, and the number of tests is 10% of the total number of piles; the single pile or composite foundation load test is carried out after 28 days, and the number of tests is 1% of the total number of piles, and the number of tests of the composite foundation static load test of each single project is not less than 3 points.

8. The construction method for improving the load bearing capacity of a complex ground formation according to Claim 1, wherein The double-layer cushion above the foundation unit is composed of a lower layer of cement-soil gravel cushion with 6% cement, a double-layer polyester geogrid, and an upper layer of cement-soil gravel cushion with 6% cement, the thickness of the lower layer of cement-soil gravel cushion is 0.5 m, and the thickness of the upper layer of cement-soil gravel cushion is 0.3 m.

9. The construction method for improving the load bearing capacity of a complex ground formation according to Claim 1, wherein Specifically, the following steps are included: Step one, the ground of the construction site is treated, and each pile position is measured, laid out and marked, and the pile position deviation is controlled within 50 mm; Step two, the lime-soil compaction pile and the corresponding screw pile are constructed according to the marked pile position, and when the screw pile is constructed for the soil layer with hardness exceeding the set hardness range, the drill rod corresponding to the screw pile is replaced with a drill rod with increased spacing between the threads and a high-pressure water jet hole arranged in the core of the drill rod, and then the drill rod is drilled, and the water jet is used to assist the sinking. Step three, after the concrete strength of the screw pile reaches the age, the pile is cut by a pile cutting machine, the top of the pile is flattened to the design elevation from the four sides to the middle, the deviation of the top of the pile is controlled within 0±20mm, and the integrity and load of the pile are inspected; Step four, the corresponding multiple lime-soil compaction piles and screw piles in the same foundation unit are connected by pile caps to form a foundation unit, and all the foundation units are constructed; Step five, the cushion is constructed, the cement-soil gravel mixed with 6% is used to construct the lower cushion between the pile caps to form the lower cushion layer, the thickness of the lower cushion layer is 0.5m, the bidirectional polyester geogrid is laid on the lower cushion layer after the construction of the lower cushion layer, and finally the cement-soil gravel mixed with 6% is laid on the geogrid to form the upper cushion layer, and the thickness of the upper cushion layer is 0.3m.

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