Design and construction method of variable-diameter steel pipe concrete mixing composite pile

Abstract

The present application relates to a kind of variable-diameter steel pipe concrete mixing composite pile design construction method, variable-diameter steel pipe concrete mixing composite pile is designed according to the required pile bearing capacity, variable-diameter steel pipe concrete mixing composite pile includes flexible pile and rigid core pile, rigid core pile is arranged in flexible pile, flexible pile is mixing pile, and rigid core pile is steel pipe concrete;Variable-diameter steel pipe concrete composite pile is designed in section, i.e., variable-diameter steel pipe concrete composite pile is divided into several pile sections along its pile length direction, the diameter of the flexible pile of each pile section, the diameter of rigid core pile and the length of pile section are different, and the bearing capacity of each part of each pile section meets the design requirement of pile bearing capacity.The present application has the advantages that:it can be widely used in various pile foundation engineering, composite foundation engineering, it is reasonable in stress, cost is economic, no waste is produced, it has little influence on surrounding, environmental protection effect is good, pile effect is good, machine is short, construction is simple, work efficiency is high, site adaptability is strong.

Description

Technical Field

[0001] This invention relates to the field of pile foundation design technology, and in particular to a design and construction method for variable diameter steel pipe concrete mixing composite piles. Background Technology

[0002] Currently, traditional rigid piles, flexible piles, and composite piles are generally of equal diameter. However, the vertical load on the pile body (especially friction piles) gradually decreases from the top of the pile downwards. That is, under the action of external vertical load, the vertical load on the pile body gradually decreases with depth. Therefore, the existing traditional pile foundations, which are all designed with equal diameter, have the same vertical load-bearing capacity from top to bottom, resulting in excessive strength in the lower pile body, making the pile design uneconomical.

[0003] Furthermore, current composite piles all employ a post-insertion rigid core pile method, which imposes strict controls on construction accuracy and time. In large-scale construction projects, the core piles are prone to misalignment; additionally, if the insertion time is delayed, problems such as difficulty in inserting the rigid piles may occur. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the prior art by providing a design and construction method for variable-diameter steel-concrete composite piles. This method allows for the design of different inner diameters of the steel-concrete composite pile and outer diameters of the cement-soil mixing pile, varying with depth, based on the load distribution within the pile and the properties of the surrounding soil. This ensures that the bearing capacity provided by each part of the pile matches the vertical load transmitted from the superstructure to each part of the pile, thereby achieving reasonable stress distribution and economical cost of the pile foundation.

[0005] The objective of this invention is achieved through the following technical solutions:

[0006] A design and construction method for variable-diameter steel pipe concrete mixing composite piles, characterized in that the method includes:

[0007] Variable diameter steel-concrete composite piles are designed according to the required pile foundation bearing capacity. The variable diameter steel-concrete composite piles include flexible piles and rigid core piles. The rigid core piles are set inside the flexible piles. The flexible piles are mixing piles. The rigid core piles are steel-concrete composite piles.

[0008] The variable diameter steel pipe concrete mixing composite pile is designed in segments, that is, the variable diameter steel pipe concrete mixing composite pile is divided into several pile segments along its length. The flexible pile diameter, rigid core pile diameter and pile segment length are different for each pile segment, and the bearing capacity of each part of each pile segment meets the design requirements of pile foundation bearing capacity.

[0009] The steel pipe of the rigid core pile is driven down simultaneously with the external mixing pile. The steel pipe also serves as the force transmission member of the mixing pile. After the mixing pile is completed, concrete is poured into the steel pipe to form the variable diameter steel pipe concrete mixing composite pile.

[0010] The statement that the bearing capacity of each part of each pile segment meets the design requirements of the pile foundation bearing capacity means that the bearing capacity provided by the mixing pile as the pile depth changes, the bearing capacity provided by the side friction between the rigid core pile and the mixing pile outside as the pile depth changes, and the bearing capacity provided by the rigid core pile as the pile depth changes, all meet the requirements of the vertical load on the pile foundation as the depth changes.

[0011] The formula for calculating the vertical load N(z) on the pile foundation as a function of depth is:

[0012] ;

[0013] In the formula, z is the depth of the calculation point; n is the nth pile segment from top to bottom at the calculation point; N1 is the vertical load on the pile top; Let u be the perimeter of the composite pile. i =πD i ; is the characteristic value of the side friction resistance between the i-th segment of the composite pile and the soil layer; The length of the i-th pile segment from top to bottom; The perimeter u of the rigid core pile i =πd i ; Let be the characteristic value of the side friction resistance between the i-th rigid core pile and the mixing pile; Let be the length of the i-th segment of the rigid core pile.

[0014] The formula for calculating the bearing capacity R1(z) provided by the mixing pile as a function of depth is:

[0015] ;

[0016] In the formula, L is the total pile length; Let u be the perimeter of the composite pile. i =πD i ; is the characteristic value of the side friction resistance between the i-th segment of the composite pile and the soil layer; This represents the characteristic value of the pile end resistance. n1 is the cross-sectional area of ​​the pile tip; n2 is the number of the first pile segment below the calculation point; n3 is the number of the lowest pile segment.

[0017] The formula for calculating the bearing capacity R2(z) provided by the side friction between the rigid core pile and the mixing pile outside it as a function of depth is as follows:

[0018] ;

[0019] In the formula, L is the total pile length; This represents the characteristic value of the pile end resistance. This is the cross-sectional area at the pile tip; The perimeter u of the rigid core pile i =πd i ;、 Let be the characteristic value of the side friction resistance between the i-th rigid core pile and the mixing pile; is the length of the i-th segment of the rigid core pile; L is the total pile length; n1 is the sequence number of the first segment below the calculation point; n2 is the sequence number of the lowest segment of the pile.

[0020] The formula for calculating the bearing capacity R3(z) provided by the rigid core pile as a function of depth is as follows:

[0021] ;

[0022] In the formula, The cross-sectional area of ​​the solid rigid core pile at the calculation point; This represents the design value of the compressive strength of solid steel-concrete composite at the calculation point.

[0023] The bottom of the steel pipe of the lowest pile segment is sealed and a mixing head is welded. A mixing pile grouting pipe is installed inside the steel pipe and connected to the mixing head. The upper part of the steel pipe is connected to a rotary power device. The mixing pile grouting pipe is connected to a mixing pile grouting machine. The rotary power device is connected to a lifting device.

[0024] After a section of pile is completed, two adjacent sections of steel pipe are connected by a steel pipe variable cross-section joint, and the grouting pipe of the mixing pile inside the steel pipe is extended by a grouting pipe joint.

[0025] The advantages of this invention are:

[0026] 1) Rational stress distribution: Based on the vertical load distribution of the pile foundation with depth and the stratum conditions, the pile foundation can be designed in a targeted segmented manner. Different parameters such as the diameter of the mixing pile, the diameter of the steel-concrete core pile, the wall thickness of the steel pipe, and the grade of the concrete inside the pipe can be designed for each segment to achieve a balance between the vertical load of the pile body, the side friction resistance provided by the flexible pile, and the bearing capacity of the pile body provided by the internal rigid pile. This will maximize the rational stress distribution of each segment of the pile foundation and truly realize the refined segmented design of the pile foundation. Due to the lateral constraint of the steel pipe, the compressive strength of its concrete pile body is significantly improved compared with that of ordinary concrete core piles (unconfined compressive strength).

[0027] 2) Economical cost: Since the side friction resistance is provided by inexpensive mixing piles and the rigid core piles are made of variable diameter steel pipe concrete, the cost is significantly more economical than that of commonly used rigid pile foundations. It can be completed with one set of construction equipment, which is much lower than the two sets of equipment required for general post-insertion composite piles, namely mixing pile machine and pile driving machine.

[0028] 3) Significant environmental protection effect with minimal impact on the surrounding environment: It does not produce mud, slag or other waste, is environmentally friendly, has low construction noise, no soil displacement effect during construction, and has minimal impact on surrounding structures.

[0029] 4) Good pile formation effect: The steel pipe and the mixing pile are constructed and sunk simultaneously, resulting in a good core effect of the composite pile. Due to the lateral confinement effect of the steel pipe, the rigid core pile has a strong vertical bearing capacity. The steel pipes are connected by steel pipe joints, and the joints are filled with concrete. The connection effect between the steel pipes is good and the shear resistance is strong. Compared with precast piles, there will be no weak points at the pipe joints.

[0030] 5) Compact equipment, simple construction, and high efficiency: The equipment used for pile construction can be adjusted in height as needed. Its compact size makes it suitable for pile foundation construction in various sites. Steel pipes are used as the transmission device for the mixing piles. The large diameter of the steel pipes provides strong torsional resistance, allowing for the use of high-power, high-torque motors, resulting in strong pile penetration capabilities. A single piece of equipment is sufficient for pile construction. Compared to bored cast-in-place piles, it only requires two steps: mixing pile construction and in-pipe concrete pouring, significantly improving efficiency compared to traditional bored cast-in-place piles.

[0031] 6) Strong applicability: Steel pipes are common commodities in the market, with a wide variety of pipe diameters and wall thicknesses. The connection between steel pipes is simple, which makes it convenient to design different pile diameters. The equipment is small, requires little construction site conditions, has little environmental impact, and is suitable for various site environments. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of the present invention;

[0033] Figure 2 for Figure 1 Schematic diagrams of the structure of each cross section;

[0034] Figure 3 This is a graph showing the distribution of vertical loads on the pile foundation as a function of depth in this invention.

[0035] Figure 4 The bearing capacity distribution curve of the mixing pile on the outer side of the pile in this invention as a function of depth;

[0036] Figure 5 This is a curve showing the distribution of bearing capacity as a function of depth provided by the side friction between the rigid core pile and the outer mixing pile in this invention.

[0037] Figure 6 This invention provides a curve showing the distribution of the bearing capacity of the steel-concrete core pile as a function of depth.

[0038] Figure 7 This is a cross-sectional view of the rigid core pile in this invention;

[0039] Figure 8This is a graph showing the vertical load and bearing capacity curves of the pile foundation as a function of depth in this invention.

[0040] Figure 9 Construction steps of the present invention Figure 1 ;

[0041] Figure 10 Construction steps of the present invention Figure 2 ;

[0042] Figure 11 Construction steps of the present invention Figure 3 ;

[0043] Figure 12 Construction steps of the present invention Figure 4 ;

[0044] Figure 13 Construction steps of the present invention Figure 5 . Detailed Implementation

[0045] The features and other related features of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments, so as to facilitate understanding by those skilled in the art:

[0046] like Figure 1-13 As shown in the figure, the markings 1-20 and the letter markings represent: 1. Rigid core pile; 2. Flexible pile; 3. Steel pipe variable cross-section joint; 4. First pile segment; 5. Second pile segment; 6. Third pile segment; 7. Mixing head; 8. Steel pipe; 9. Concrete; 10. Mixing pile grouting machine; 11. Pipeline; 12. Mixing grouting pipe; 13. Rotary power device; 14. Frame and lifting device; 15. Grouting pipe joint; 16. First section of steel pipe; 17. Second section of steel pipe; 18. Third section of steel pipe; 19. Concrete pump truck; 20. Concrete pump pipe.

[0047] The length of the first pile segment is L1, the length of the second pile segment is L2, and the length of the third pile segment is L3;

[0048] The diameter of the flexible pile in the first pile segment is D1, the diameter of the flexible pile in the second pile segment is D2, and the diameter of the flexible pile in the third pile segment is D3;

[0049] The diameter of the rigid core pile in the first pile segment is d1, the diameter of the rigid core pile in the second pile segment is d2, and the diameter of the rigid core pile in the third pile segment is d3.

[0050] Example: Figure 1As shown, the main body of the variable-diameter steel-concrete composite pile in this embodiment consists of two parts: a rigid core pile 1 and a flexible pile 2. The rigid core pile 1 is disposed within the flexible pile 2 and is preferably arranged concentrically. The main body of the composite pile is divided into three sections along the pile length direction, specifically including a first pile section 4, a second pile section 5, and a third pile section 6. Each of these three pile sections contains both the rigid core pile 1 and the flexible pile 2. In this embodiment, the steel core pile 1 is a steel-concrete composite pile, and the flexible pile 2 is a mixing pile.

[0051] Combination Figure 1 and Figure 2 As shown, the rigid core pile 1 is composed of a steel pipe 8 and concrete 9. The steel pipe 8 is placed inside the flexible pile 2, while the concrete 9 is filled inside the steel pipe 8 and is laterally constrained by the steel pipe 8, thereby improving the compressive strength of the rigid core pile 1.

[0052] In this embodiment, the first pile segment 4, the second pile segment 5, and the third pile segment 6 all have different diameter dimensions, namely, the flexible pile diameter D1 of the first pile segment, the flexible pile diameter D2 of the second pile segment, and the flexible pile diameter D3 of the third pile segment. At the same time, the rigid core pile diameters d1, d2, and d3 of the first pile segment, as well as the lengths L1, L2, and L3 of the first pile segment, are also different, thereby achieving optimization of the pile foundation structure design.

[0053] Specifically, in this embodiment, as the depth of the composite pile increases, the vertical load on the pile body gradually decreases. The pile foundation is designed in a targeted segmented manner (this embodiment adopts a three-segment segmented design), gradually reducing the diameter of the rigid core pile 1. The mixing piles, which serve as flexible piles 2 on the outside, can also be set with different diameters according to the stratum conditions and the vertical load on the pile body. Ultimately, the vertical load on the pile body, the side friction resistance provided by the external mixing piles, and the bearing capacity of the pile body provided by the internal rigid core pile 1 can be coordinated, thereby achieving reasonable stress distribution and economical cost in each segment of the pile foundation.

[0054] like Figure 1 As shown, in this embodiment, the steel pipes 8 of each rigid core pile 1 are connected by steel pipe variable cross-section joints to form an integral structure, thereby ensuring the integrity and structural strength of the rigid core pile 1 and ensuring that the rigid core pile 1 set in the flexible pile 2 can bear the force as a whole. In order to pour concrete 9 inside the steel pipe 8, the bottom of the steel pipe 8 in the bottommost third pile segment 6 is sealed, for example, by welding a steel plate to the bottom of this segment of steel pipe 8; and in order to realize the synchronous construction and sinking of the steel pipe 8 with the external flexible pile 2, a mixing head 7 is welded to the bottom sealing steel plate of the steel pipe 8.

[0055] In this embodiment, the design of the variable-diameter steel pipe concrete mixing composite pile differs from that of a typical pile foundation, which only calculates whether the bearing capacity at the top of the pile meets the requirements of the superstructure load. The variable-diameter pile requires calculation of whether the bearing capacity of each part of the pile can meet the vertical load requirements transmitted from the superstructure load. Specifically, this includes the following methods:

[0056] 1) such as Figure 3 The figure shows the variation of the vertical load N(z) on the pile foundation with depth:

[0057] ;

[0058] z - Depth of the calculated point;

[0059] At the n-calculation point, the nth segment of variable diameter piles included from top to bottom;

[0060] N1 - Vertical load on the pile top;

[0061] - Composite pile, outer pile perimeter u i =πD i ;

[0062] - Composite pile, characteristic value of side friction resistance between the outer core of the i-th segment and the soil layer (kPa).

[0063] - The length of the i-th pile segment from top to bottom;

[0064] - Composite pile, core pile perimeter u i =πd i ;

[0065] - Composite pile, characteristic value of side friction resistance between the inner core of the i-th segment and the cement-soil (kPa).

[0066] - Length of the i-th segment of the inner core.

[0067] 2) such as Figure 4 As shown, the bearing capacity R1(z) provided by the mixing pile on the outer side of the calculation pile varies with depth:

[0068] ;

[0069] L - Total pile length

[0070] - Composite pile, outer pile perimeter u i =πD i ;

[0071] - Composite pile, characteristic value of side friction resistance between the outer core of the i-th segment and the soil layer (kPa).

[0072] - Characteristic value of the end resistance of the composite pile (kPa);

[0073] -Cross-sectional area of ​​the composite pile tip;

[0074] n1 - The serial number of the first variable-diameter pile in the lower part of the calculation point;

[0075] n2 - The serial number of the lowest diameter-changing pile section.

[0076] 3) such as Figure 5 The figure shows the variation of the bearing capacity R2(z) provided by the rigid core pile through the side friction between the core pile and the outer cement-soil with depth:

[0077] ;

[0078] -Characteristic value of pile end resistance;

[0079] - Cross-sectional area of ​​pile tip;

[0080] - The perimeter u of the rigid core pile i =πd i ;

[0081] - Characteristic value of side friction resistance between the i-th rigid core pile and the mixing pile;

[0082] - The length of the i-th segment of a rigid core pile;

[0083] L is the total pile length;

[0084] n1 is the sequence number of the first pile segment below the calculation point;

[0085] n2 is the sequence number of the lowest pile segment.

[0086] 4) such as Figure 6 and Figure 7 The figure shows the variation of the pile bearing capacity R3(z) provided by the steel-concrete composite core pile with depth:

[0087] ;

[0088] ;

[0089] ;

[0090] ;

[0091] ;

[0092] ;

[0093] i - The sequence number of the variable diameter pile segment at the calculation point;

[0094] - The design value of the core pile bearing capacity at the calculation point;

[0095] - The cross-sectional area of ​​the solid steel-concrete composite member at the calculation point is equal to the sum of the areas of the steel tube and the concrete inside the tube (mm²). 2 );

[0096] -Calculate the design value of the compressive strength (MPa) of solid steel tube concrete at the calculation point;

[0097] , - Calculate the area (mm²) of the steel pipe and the concrete inside the pipe at the calculation point. 2 );

[0098] -Calculate the steel content of the solid steel-concrete composite member (rigid core pile 1) at the calculation point;

[0099] - The confinement coefficient of the solid steel-concrete composite member at the calculation point;

[0100] - The design value of the compressive strength of the steel at the calculation point (MPa);

[0101] - The design value of the compressive strength of the concrete at the calculation point (MPa);

[0102] , -Calculate the influence coefficient of the cross-sectional shape at the calculation point on the hoop effect.

[0103] 5) such as Figure 8 As shown, the bearing capacity is determined:

[0104] In this embodiment, the variable-diameter steel pipe concrete mixing composite pile requires comparative analysis of the load distribution and bearing capacity distribution of various parts of the pile body. The analysis is performed in the form of an internal force distribution diagram. When R1(z), R2(z), and R3(z) at each position of the pile body are all greater than N(z) (that is, when all three bearing capacity R curves are located to the right of the load curve N), the pile bearing capacity meets the requirements of the superstructure load. When the bearing capacity of a certain section or item is much greater than the load or does not meet the load requirements, the corresponding parameters such as the inner and outer pile diameters, steel pipe thickness, and concrete strength inside the pipe can be adjusted to carry out targeted segmented design of the pile foundation, thereby achieving reasonable stress distribution and economical cost in each section of the pile foundation.

[0105] The design value of the pile top bearing capacity is taken as the minimum value of R1(z), R2(z), and R3(z) at the pile top.

[0106] In this embodiment, during construction, the steel pipe and the external mixing pile are sunk simultaneously. The steel pipe also serves as the force transmission member of the mixing pile. After the mixing pile is formed, concrete is poured into the steel pipe to form a steel-concrete composite pile. The specific construction steps are as follows:

[0107] 1) such as Figure 9 As shown, the bottom of the first section of steel pipe 16 is sealed and the mixing head 7 is welded on. The mixing and grouting pipe 12 is set in the first section of steel pipe 16 and connected to the mixing head 7. The upper part of the first section of steel pipe 16 is connected to the rotary power device 13. The mixing and grouting pipe 12 is connected to the mixing pile grouting machine 10 through the pipe 11. The rotary power device 13 is connected to the frame and lifting device 14.

[0108] 2) such as Figure 10 As shown, the rotary power device 13 is started, driving the first section of steel pipe 16 and the mixing head 7 to drill into the stratum. At the same time, the mixing pile grouting machine 10 sprays grout around the mixing head 7 through the pipe 11 and the mixing grouting pipe 12. The mixing head 7 cuts the soil, so that the soil and cement grout are mixed to form cement-soil piles. The frame and lifting device 14 can move the mixing pile up and down to achieve the purpose of re-mixing the mixing pile.

[0109] 3) such as Figure 11 As shown, after a section of the mixing pile is constructed, a second section of steel pipe 17 is connected to the top of the first section of steel pipe 16 through a steel pipe variable cross-section joint 3. The cross-sectional dimensions of the second section of steel pipe 17 can be adjusted according to the stress on the pile body. The mixing and grouting pipe 12 inside the steel pipe is also extended by grouting pipe joint 15. The mixing head 7 can also adjust the blade length according to the stress on the pile body and the surrounding geological conditions to form mixing piles of different diameters.

[0110] 4) such as Figure 12 As shown, repeat step 3) to form multiple sections of steel pipes and mixing piles of different diameters. For example, as shown in this embodiment, continue to extend the third section of steel pipe 18 by using a steel pipe variable cross-section joint.

[0111] 5) such as Figure 13 As shown, after the steel pipe mixing pile construction is completed, the mixing and grouting pipe 12 is pulled out. The mixing head 7 remains inside the pile and is not pulled out. A concrete pump truck 19 is used to pump concrete into the steel pipe through the concrete pump pipe 20, forming the variable diameter steel pipe concrete mixing composite pile in this embodiment; if the pile foundation needs to be connected to the superstructure, a reinforcing cage can also be implanted in the steel pipe.

[0112] In this embodiment, the number of pile body segments of the composite pile can be increased or decreased according to the actual situation. For example, a two-section or four-section segment design can be used. The steel pipes 8 in each pile segment can be connected into a whole and internally connected by a suitable steel pipe variable cross-section joint 3.

[0113] Although the parameters of the first pile segment 4, the second pile segment 5, and the third pile segment 6 in this embodiment are different, including the diameter of the flexible pile, the diameter of the rigid core pile, and the length, in some cases, the parameters of the two pile segments can be the same to meet the stress requirements.

[0114] Although the above embodiments have described the concept and embodiments of the present invention in detail with reference to the accompanying drawings, those skilled in the art will recognize that various improvements and modifications can still be made to the present invention without departing from the scope of the claims, and therefore will not be elaborated here.

Claims

1. A design and construction method for a variable-diameter steel pipe concrete mixing composite pile, characterized in that: The method includes: Variable diameter steel-concrete composite piles are designed according to the required pile foundation bearing capacity. The variable diameter steel-concrete composite piles include flexible piles and rigid core piles. The rigid core piles are set inside the flexible piles. The flexible piles are mixing piles. The rigid core piles are steel-concrete composite piles. The variable diameter steel pipe concrete mixing composite pile is designed in segments, that is, the variable diameter steel pipe concrete mixing composite pile is divided into several pile segments along its length. The flexible pile diameter, rigid core pile diameter and pile segment length are different for each pile segment, and the bearing capacity of each part of each pile segment meets the design requirements of pile foundation bearing capacity. The steel pipe of the rigid core pile is driven down simultaneously with the external mixing pile. The steel pipe also serves as the force transmission member of the mixing pile. After the mixing pile is completed, concrete is poured into the steel pipe to form the variable diameter steel pipe concrete mixing composite pile. The statement that the bearing capacity of each part of each pile segment meets the design requirements of the pile foundation bearing capacity means that the bearing capacity provided by the mixing pile as the pile depth changes, the bearing capacity provided by the side friction between the rigid core pile and the mixing pile outside as the pile depth changes, and the bearing capacity provided by the rigid core pile as the pile depth changes, all meet the requirements of the vertical load on the pile foundation as the depth changes. The formula for calculating the vertical load N(z) on the pile foundation as a function of depth is: ； In the formula, z is the depth of the calculation point; n is the nth pile segment from top to bottom at the calculation point; N1 is the vertical load on the pile top; Let u be the perimeter of the composite pile. i =πD i ; is the characteristic value of the side friction resistance between the i-th segment of the composite pile and the soil layer; The length of the i-th pile segment from top to bottom; The perimeter u of the rigid core pile i =πd i ; Let be the characteristic value of the side friction resistance between the i-th rigid core pile and the mixing pile; Let be the length of the i-th segment of the rigid core pile.

2. The design and construction method of a variable-diameter steel pipe concrete mixing composite pile according to claim 1, characterized in that: The formula for calculating the bearing capacity R1(z) provided by the mixing pile as a function of depth is: ； In the formula, L is the total length of the pile; Let u be the perimeter of the composite pile. i =πD i ; is the characteristic value of the side friction resistance between the i-th segment of the composite pile and the soil layer; This represents the characteristic value of the pile end resistance. n1 is the cross-sectional area of ​​the pile tip; n2 is the number of the first pile segment below the calculation point; n3 is the number of the lowest pile segment.

3. The design and construction method of a variable-diameter steel pipe concrete mixing composite pile according to claim 1, characterized in that: The formula for calculating the bearing capacity R2(z) provided by the side friction between the rigid core pile and the mixing pile outside it as a function of depth is as follows: ； In the formula, L is the total length of the pile; This represents the characteristic value of the pile end resistance. This is the cross-sectional area at the pile tip; The perimeter u of the rigid core pile i =πd i ;、 Let be the characteristic value of the side friction resistance between the i-th rigid core pile and the mixing pile; is the length of the i-th segment of the rigid core pile; L is the total pile length; n1 is the sequence number of the first segment below the calculation point; n2 is the sequence number of the lowest segment of the pile.

4. The design and construction method of a variable-diameter steel pipe concrete mixing composite pile according to claim 1, characterized in that: The formula for calculating the bearing capacity R3(z) provided by the rigid core pile as a function of depth is as follows: ； In the formula, The cross-sectional area of ​​the solid rigid core pile at the calculation point; This represents the design value of the compressive strength of solid steel-concrete composite at the calculation point.

5. The design and construction method of a variable-diameter steel pipe concrete mixing composite pile according to claim 1, characterized in that: The bottom of the steel pipe of the lowest pile segment is sealed and a mixing head is welded. A mixing pile grouting pipe is installed inside the steel pipe and connected to the mixing head. The upper part of the steel pipe is connected to a rotary power device. The mixing pile grouting pipe is connected to a mixing pile grouting machine. The rotary power device is connected to a lifting device.

6. The design and construction method of a variable-diameter steel pipe concrete mixing composite pile according to claim 5, characterized in that: After a section of pile is completed, two adjacent sections of steel pipe are connected by a steel pipe variable cross-section joint, and the grouting pipe of the mixing pile inside the steel pipe is extended by a grouting pipe joint.

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