Combined micro-pile foundation and design method

Abstract

The present application relates to the field of power transmission line foundation design, in particular to a combined micro pile foundation and design method. It comprises a soil layer and a bedrock layer arranged below the soil layer, a pile hole is arranged in the soil layer, a cylindrical pile is arranged in the pile hole, a reinforcement cage is arranged in the cylindrical pile, a plurality of anchor rod holes are arranged in the bedrock layer along the circumference of the axis of the cylindrical pile, the plurality of anchor rod holes intersect with each other to form an anchor hole, an anchor rod pile is arranged in the anchor hole, longitudinal reinforcement is arranged in the anchor rod pile, and the plurality of longitudinal reinforcement is connected by at least two stirrups. The present application sets a plurality of anchor rod holes in the bedrock layer, the anchor rod holes are connected with each other to form an anchor hole, longitudinal reinforcement, stirrups and an anchor rod pile are arranged in the anchor hole, the shear capacity is strong, the horizontal load can be resisted, the size of the cylindrical pile in the soil layer is small, and the cylindrical pile does not need to be embedded in the lower bedrock, which greatly reduces the construction difficulty, improves the construction efficiency, reduces the earthwork excavation amount, and reduces the foundation cost.

Description

Technical Field

[0001] This invention relates to the field of power transmission line foundation design, specifically to a combined micropile foundation and its design method. Background Technology

[0002] Many transmission tower sites have foundations with a certain thickness of overburden on top and bedrock below. For this type of "soil on top and bedrock below" foundation, transmission line projects usually use excavation foundations or pile foundations. These are generally done by manual excavation, which is difficult, slow, and involves a large amount of excavation.

[0003] To reduce excavation and concrete usage in rock foundations and improve construction efficiency, rock anchor foundations are commonly used in power transmission line projects instead of excavated or pile foundations. Rock anchor foundations include various forms such as straight anchor, pile cap, and embedded types. For foundations with a thick overburden layer, such as "soil above rock," pile cap rock anchor foundations are typically used. The anchor holes in rock anchor foundations are drilled mechanically, reducing manual excavation and increasing construction efficiency. Furthermore, the amount of concrete used is less than in excavated foundations. Therefore, rock anchor foundations are a relatively superior foundation type for rock foundations.

[0004] However, rock anchor foundations still face a significant challenge. For instance, Chinese invention patent CN107700478A discloses a one-time forming construction method for single-pile foundations in power transmission line engineering. Each anchor pile in this foundation contains only one anchor rod, which cannot provide substantial shear bearing capacity. For foundations with thick overburden layers, the horizontal loads on the foundation cannot be transferred to the lower anchor rods; they must be borne by the upper pile cap through the friction of the foundation base and the soil resistance on the sides. This necessitates increasing the size of the pile cap. To further enhance resistance to horizontal forces, the pile cap typically needs to be embedded at least 0.5m into the bedrock. This still requires bedrock excavation, which is quite difficult. Therefore, the construction difficulty and efficiency of rock anchor foundations remain unsatisfactory. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a combined micropile foundation and design method, which improves construction efficiency and safety, reduces the amount of foundation pit excavation, and lowers foundation costs.

[0006] The technical solution adopted by this invention to solve its technical problem is a combined micropile foundation, including a soil layer and a bedrock layer below the soil layer. The soil layer has pile holes, and a cylindrical pile is installed in the pile holes. A steel cage is installed inside the cylindrical pile. Multiple anchor holes are arranged circumferentially along the axis of the cylindrical pile in the bedrock layer. The multiple anchor holes intersect each other to form anchor holes. Anchor piles are installed in the anchor holes. Longitudinal bars are installed in the anchor piles. The multiple longitudinal bars are connected by at least two stirrups.

[0007] Furthermore, the axis of the longitudinal reinforcement coincides with the axis of the anchor hole, and the upper end of the longitudinal reinforcement extends into the interior of the cylindrical pile.

[0008] Furthermore, anchor bolts are pre-embedded inside the cylindrical pile.

[0009] A design method for a composite micropile foundation, comprising the following steps:

[0010] S1: Based on the design value of the load generated by the tower on the combined micropile, the lateral area A of the cylindrical pile is obtained through finite element analysis simulation. s Anchor pile bottom area A d The cross-sectional area A of the bedrock layer between the anchor piles T The area A of the lower surface of the cylindrical pile located outside the anchor pile. k and the side area A of the anchor pile q ;

[0011] S2: Design the shape and size of the anchor hole based on the obtained bottom area of ​​the anchor pile and the cross-sectional area of ​​the bedrock layer between the anchor piles. Design the size of the longitudinal reinforcement based on the shape and size of the anchor hole. The longitudinal reinforcement is located at the center of the anchor hole, and the minimum distance from the longitudinal reinforcement to the edge of the anchor hole must be greater than the specified thickness of the longitudinal reinforcement protective layer.

[0012] S3: Design the size of the stirrups in the anchor hole according to the shape and size of the anchor hole; when selecting stirrups, the minimum distance from the stirrup to the anchor hole must be greater than the thickness of the stirrup's protective layer.

[0013] Furthermore, in step S1, the design values ​​of the loads generated by the combined micropiles and the tower include the compressive load R and the tensile load R. T Find the lateral area A of the cylindrical pile. s Anchor pile bottom area A d The cross-sectional area A of the bedrock layer between the anchor piles T The area A of the lower surface of the cylindrical pile located outside the anchor pile. k and the side area A of the anchor pile q The set of solutions is a1, and the formula for finding set a1 is as follows:

[0014] R = 1.1f rk (ξ pk A k +ξ pd A d )+0.5q sik A s

[0015] R t =f rk (ξ pt At +ξ st A q )+0.5q sik A s

[0016]

[0017] In the formula: R is the designed downward pressure load; f rk A represents the saturated uniaxial compressive strength of the bedrock layer. k A represents the area of ​​the lower surface of the cylindrical pile located outside the anchor pile; d A is the area of ​​the bottom of the anchor pile; s ζ represents the lateral surface area of ​​the cylindrical pile; pk ζ represents the end resistance coefficient of the cylindrical pile, taken as 0.6 for moderately weathered and more weathered rocks, and 0.45 for strongly weathered rocks; pd q represents the pile end resistance coefficient, taken as 0.8 for moderately weathered and more weathered rock, and 0.6 for strongly weathered rock; sik R is the ultimate lateral resistance coefficient of the soil layer, which can be found in the "Technical Code for Building Pile Foundations"; T For the designed pull-out load; A T A represents the cross-sectional area of ​​the bedrock layer between the anchor piles. q ζ represents the lateral surface area of ​​the anchor pile; pT The end pull-out coefficient is 0.08 for moderately weathered and more weathered rocks, and 0.06 for strongly weathered rocks; ζ sT The pull-out resistance coefficient is set at 0.03 for moderately weathered and more severely weathered rocks, and at 0.0225 for strongly weathered rocks; i q represents the depth of the soil layer. sik λ represents the ultimate lateral resistance of the soil layer. i The pull-out resistance coefficient is set at 0.5-0.7 for sandy soil and 0.7-0.8 for cohesive soil and silt.

[0018] Furthermore, in step S1, the horizontal displacement X1(Z), rotation angle θ1(Z), and bending moment M of the combined micropile foundation are calculated based on set a1. Z (Z), and compared with the preset horizontal displacement X(Z), rotation angle θ(Z), and bending moment M(Z) of the combined micropile foundation, it is found that X1(Z)≤X(Z), θ1(Z)≤θ(Z), and M Z The set a2 of (Z)≤M(Z); where the horizontal displacement and rotation angle of the combined micropile foundation are divided into two parts: the horizontal displacement X2(Z) and the rotation angle θ2(Z) of the cylindrical pile are calculated using the M method, and the horizontal displacement of the anchor pile is calculated using the following formula:

[0019]

[0020] The rotation angle of the anchor pile is calculated using the following formula:

[0021]

[0022] The bending moment of the anchor pile is calculated using the following formula:

[0023] M z (z)=A m P1T+B m M

[0024] The horizontal displacement of the micropile foundation is X1(Z) = X2(Z) + X Z (Z), rotation angle θ1(Z)=θ2(Z)+θ Z (Z) and bending moment M1(Z);

[0025] P1 is the horizontal load at the top of the micropile foundation, M is the bending moment at the top of the micropile foundation, and A in the formula... x B x A θ B θ A m B m A v B v A p B p Found according to the standard;

[0026] In the formula Where E p I p n represents the bending stiffness of the anchor pile. h This is the reaction coefficient of the bedrock.

[0027] Furthermore, in step S1, the shear force of the anchor pile is calculated based on the values ​​in set a2; the formula is as follows:

[0028]

[0029] In the formula Where E p I p P1 is the bending stiffness of the anchor pile, P1 is the horizontal load at the top of the micropile foundation, and A is the bending stiffness of the anchor pile. v According to the standard, M is the bending moment at the top of the micropile foundation, and n h This represents the reaction coefficient of the bedrock;

[0030] In step S3, based on the pile shear force obtained in step S1, the dimensions of the stirrups are designed. The formula for calculating the shear bearing capacity of the anchor pile is as follows:

[0031]

[0032] PZ (Z)≤V

[0033] In the formula: V is the shear bearing capacity; f c δ represents the design value of the concrete compressive strength; δ represents the average wall thickness of the anchor pile. Equivalent effective height, r is the outer radius of the stirrup, r s f is the radius of the circle containing the centerline of the stirrup; yv A represents the design value of the tensile strength of the stirrups. sv s is the cross-sectional area of ​​the stirrups; s is the stirrup spacing.

[0034] The beneficial effects of this invention are:

[0035] 1. By setting multiple anchor holes in the bedrock layer, the anchor holes are connected to form anchor holes. Longitudinal bars, stirrups and anchor piles are set in the anchor holes. The shear resistance is strong and can resist large horizontal loads. The cylindrical piles in the soil layer are small in size and do not need to be embedded in the lower bedrock, which greatly reduces the construction difficulty, improves the construction efficiency, reduces the amount of earthwork excavation and reduces the foundation cost.

[0036] 2. By precisely designing the anchor piles and cylindrical piles, the bearing capacity of the combined micropile foundation is ensured, avoiding the problem of poor tower stability caused by insufficient bearing capacity of the combined micropile foundation, and also avoiding the problem of material waste caused by excessive bearing capacity of the combined micropile foundation. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure of the present invention;

[0038] Figure 2 This is a schematic diagram of a cylindrical pile;

[0039] Figure 3 This is a schematic diagram of an anchor pile;

[0040] Figure 4 This is a schematic diagram of the anchor bolt hole;

[0041] Figure 5 This is a schematic diagram of one embodiment of an anchor pile.

[0042] Attached diagram labels: 1-soil layer; 2-bedrock layer; 3-pile hole; 4-cylindrical pile; 5-reinforcing cage; 6-anchor hole; 7-anchor hole; 8-anchor pile; 9-longitudinal reinforcement; 10-stirrup; 11-anchor bolt. Detailed Implementation

[0043] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0044] like Figures 1-4 As shown, the present invention discloses a combined micropile foundation comprising a soil layer 1 and a bedrock layer 2 located below the soil layer 1. A pile hole 3 is provided within the soil layer 1, and a cylindrical pile 4 is installed within the pile hole 3. A reinforcing cage 5 is installed inside the cylindrical pile 4. Multiple anchor holes 6 are arranged circumferentially along the axis of the cylindrical pile 4 within the bedrock layer 2. The multiple anchor holes 6 intersect to form anchor holes 7. Anchor piles 8 are installed within the anchor holes 7, and longitudinal reinforcement bars 9 are installed within the anchor piles 8. The multiple longitudinal reinforcement bars 9 are connected by at least two stirrups 10.

[0045] Both the cylindrical pile 4 and the anchor pile 8 are cast from concrete, with the diameter of the cylindrical pile 4 being larger than that of the anchor pile 8. The distance from the reinforcing cage 5 to the outside of the cylindrical pile 4 must be greater than the protective layer thickness of the reinforcing cage 5, which is the thickness of the concrete, and can be found according to the specifications for the thickness of the reinforcing concrete protective layer. The anchor holes 6 are multiple cylindrical holes excavated in the bedrock layer 2 by an anchor drilling machine. The cylindrical holes overlap each other by a certain thickness to form the anchor holes 7. The bedrock portion in the middle of the anchor holes 7 is preserved and not excavated. Then, longitudinal reinforcement 9 is set in the anchor holes 7. Multiple longitudinal reinforcement 9 are connected by stirrups 10, and the connection between the stirrups 10 and the longitudinal reinforcement 9 is achieved by welding.

[0046] By setting multiple anchor holes 6 in the bedrock layer 2, the anchor holes 6 are connected to form anchor holes 7. Longitudinal bars 9, stirrups 10 and anchor piles 8 are set in the anchor holes 7, which have strong shear resistance and can resist large horizontal loads. The cylindrical piles 4 in the soil layer 1 are small in size and do not need to be embedded in the lower bedrock, which greatly reduces the construction difficulty, improves the construction efficiency, reduces the amount of earthwork excavation, and reduces the foundation cost.

[0047] In order to make the protective layer thickness of the longitudinal reinforcement 9 uniform and to improve the connection strength between the cylindrical pile 4 and the anchor pile 8, the axis of the longitudinal reinforcement 9 is further aligned with the axis of the anchor hole 6, and the upper end of the longitudinal reinforcement 9 extends into the interior of the cylindrical pile 4.

[0048] To facilitate subsequent connection with the tower, anchor bolts 11 are pre-embedded inside the cylindrical pile 4.

[0049] A design method for a composite micropile foundation, comprising the following steps:

[0050] S1: Based on the design value of the load generated by the tower on the combined micropile, the lateral area A of the cylindrical pile is obtained through finite element analysis simulation. s Anchor pile 8, bottom area A d The cross-sectional area A of bedrock layer 2 between anchor piles 8 T The area A of the lower surface of cylindrical pile 4 located outside the anchor pile 8. k And the lateral area A of the anchor pile 8 q The design values ​​of the loads generated by the combined micropiles and the tower include the compressive load R and the tensile load R. T The dimensions of the anchor pile (8) and its cylindrical shape are determined based on finite element analysis, including rotation angle, horizontal displacement, and bending moment.

[0051] S2: The shape and size of the anchor hole 6 are designed based on the bottom area of ​​the anchor pile 8 and the cross-sectional area of ​​the bedrock layer 2 between the anchor piles 8. The size of the longitudinal reinforcement 9 is designed based on the shape and size of the anchor hole 6. The longitudinal reinforcement 9 is located at the center of the anchor hole 6, and the minimum distance from the longitudinal reinforcement 9 to the edge of the anchor hole 7 must be greater than the specified thickness of the protective layer of the longitudinal reinforcement 9. The size of the overlapping part of two adjacent anchor holes 6 is determined by the shape and size of the anchor hole 6, and then the shape of the anchor hole 7 is determined.

[0052] S3: Design the dimensions of the stirrups 10 in the anchor hole 7 according to the shape and size of the anchor hole 6; when selecting the stirrups 10, the minimum distance from the stirrups 10 to the anchor hole 7 must be greater than the protective layer thickness of the stirrups 10.

[0053] Furthermore, through finite element analysis simulation, the compressive load of the composite micropile mainly consists of two parts: the end resistance of the lower surface of the cylindrical pile 4 located outside the anchor pile 8 and the end resistance of the bottommost end of the anchor pile 8. Therefore, the calculation formula for the compressive load is designed. Similarly, the tensile load of the composite micropile consists of three parts: the skin friction of the outer surface of the anchor pile 8, the skin friction of the inner surface of the anchor pile 8, and the skin friction of the cylindrical pile 4 within the soil layer 1. Therefore, the calculation formula for the tensile load is designed. In step S1, the design value of the load generated by the tower on the composite micropile includes the compressive load R and the tensile load R. T Find the lateral surface area A of the cylindrical pile. s Anchor pile 8, bottom area A d The cross-sectional area A of bedrock layer 2 between anchor piles 8 T The area A of the lower surface of cylindrical pile 4 located outside the anchor pile 8. k And the lateral area A of the anchor pile 8 q The set of solutions is a1, and the formula for finding set a1 is as follows:

[0054] R = 1.1f rk (ξ pk A k +ξ pd Ad )+0.5q sik A s

[0055] R t =f rk (ξ pt A t +ξ st A q )+0.5q sik A s

[0056]

[0057] In the formula: R is the designed downward pressure load; f rk A represents the saturated uniaxial compressive strength of bedrock layer 2. k A represents the area of ​​the lower surface of cylindrical pile 4 located outside anchor pile 8; d A is the base area of ​​anchor pile 8; s Let ζ be the lateral surface area of ​​cylindrical pile 4; pk The resistance coefficient at four ends of the cylindrical pile is 0.6 for moderately weathered and more weathered rocks, and 0.45 for strongly weathered rocks; ζ pd q represents the pile end resistance coefficient, taken as 0.8 for moderately weathered and more weathered rock, and 0.6 for strongly weathered rock; sik R is the ultimate lateral resistance coefficient of soil layer 1, which can be found according to the "Technical Code for Building Pile Foundations"; T For the designed pull-out load; A T A represents the cross-sectional area of ​​bedrock layer 2 between anchor piles 8; q Let ζ be the lateral surface area of ​​anchor pile 8; pT The end pull-out coefficient is 0.08 for moderately weathered and more weathered rocks, and 0.06 for strongly weathered rocks; ζ sT The pull-out resistance coefficient is set at 0.03 for moderately weathered and more severely weathered rocks, and at 0.0225 for strongly weathered rocks; i q represents the depth of soil layer 1; sik λ represents the ultimate lateral resistance of soil layer 1. i The pull-out resistance coefficient is set at 0.5-0.7 for sandy soil and 0.7-0.8 for cohesive soil and silt.

[0058] Further, in step S1, the cylindrical pile 4 is located within soil layer 1. The horizontal displacement and rotation angle of the pile top of the cylindrical pile 4 are calculated using the m-method, where M refers to the pile foundation M-method. At this stage, it is assumed that the pile end of the cylindrical pile 4 is embedded in bedrock layer 2. For the anchor pile 8, the horizontal displacement and rotation angle of the anchor pile 8 are calculated using the pile foundation k-method, where the foundation deformation modulus remains constant with depth. At this stage, it is assumed that the anchor pile 8 is an independent pile, and the horizontal force and bending moment at its top are calculated using the shear force and bending moment at the pile bottom obtained from the cylindrical pile 4. The horizontal displacement at the top of the combined micropile foundation is the sum of the horizontal displacement of the anchor pile 8 and the horizontal displacement at the top of the cylindrical pile 4; the rotation angle of the combined micropile foundation is the sum of the rotation angle of the anchor pile 8 and the rotation angle of the cylindrical pile 4. The horizontal displacement X1(Z), rotation angle θ1(Z), and bending moment M of the combined micropile foundation are calculated according to set a1. Z (Z), and compared with the preset horizontal displacement X(Z), rotation angle θ(Z), and bending moment M(Z) of the combined micropile foundation, it is found that X1(Z)≤X(Z), θ1(Z)≤θ(Z), and M Z The set a2 of (Z)≤M(Z); where the horizontal displacement and rotation angle of the combined micropile foundation are divided into two parts, the horizontal displacement X2(Z) of the cylindrical pile 4 and the rotation angle θ2(Z) of the cylindrical pile 4 are calculated using the M method, and the horizontal displacement of the anchor pile 8 is calculated using the following formula:

[0059]

[0060] The rotation angle of anchor pile 8 is calculated using the following formula:

[0061]

[0062] The bending moment of anchor pile 8 is calculated using the following formula:

[0063] M z (z)=A m P1T+B m M

[0064] The horizontal displacement of the micropile foundation is X1(Z) = X2(Z) + X Z (Z), rotation angle θ1(Z)=θ2(Z)+θ Z (Z) and bending moment M1(Z);

[0065] P1 is the horizontal load at the top of the micropile foundation, M is the bending moment at the top of the micropile foundation, and A in the formula... x B x A θ B θ A m B m A v B v A p Bp According to the standard, the specific values ​​are shown in Table 1; in the table, Z represents the depth of the anchor pile 8.

[0066] 0.0 2.435 -1.623 0.000 1.000 0.000 1.623 -1.750 1.000 0.000 0.000 0.1 2.273 -1.618 0.100 0.989 -0.227 1.453 -1.650 1.000 -0.007 -0.145 0.2 2.112 -1.603 0.198 0.956 -0.422 1.293 -1.550 0.999 -0.028 -0.259 0.3 1.952 -1.578 0.291 0.906 -0.586 1.143 -1.450 0.994 -0.058 -0.343 0.4 1.796 -1.545 0.379 0.840 -0.718 1.003 -1.351 0.987 -0.095 -0.401 0.5 1.644 -1.503 0.459 0.764 -0.822 0.873 -1.253 0.976 -0.137 -0.436 0.6 1.496 -1.454 0.532 0.677 -0.897 0.752 -1.156 0.960 -0.181 -0.451 0.7 1.353 -1.397 0.595 0.585 -0.947 0.642 -1.061 0.939 -0.226 -0.449 0.8 1.216 -1.335 0.649 0.489 -0.973 0.540 -0.968 0.914 -0.270 -0.432 0.9 1.086 -1.268 0.693 0.392 -0.977 0.448 -0.878 0.885 -0.312 -0.403 1.0 0.962 -1.197 0.727 0.295 -0.962 0.364 -0.792 0.852 -0.350 -0.364 1.2 0.738 -1.047 0.767 0.109 -0.885 0.223 -0.629 0.775 -0.414 -0.268 1.4 0.544 -0.893 0.772 -0.056 -0.761 0.112 -0.482 0.688 -0.456 -0.157 1.6 0.381 -0.741 0.746 -0.193 -0.609 0.029 -0.354 0.594 -0.477 -0.047 1.8 0.247 -0.596 0.696 -0.298 -0.445 -0.030 -0.245 0.498 -0.476 0.054 2.0 0.142 -0.464 0.628 -0.371 -0.283 -0.070 -0.155 0.404 -0.456 0.140 3.0 -0.075 -0.400 0.225 -0.349 0.226 -0.089 0.057 0.059 -0.213 0.268 4.0 -0.050 0.052 0.000 -0.106 0.201 -0.028 0.049 -0.042 0.017 0.112 5.0 -0.009 0.025 -0.033 0.015 0.046 0.000 -0.011 -0.026 0.029 -0.002

[0067] Table 1

[0068] In the formula Where E p I p n is the bending stiffness of anchor pile 8. h This is the reaction coefficient of the bedrock.

[0069] Furthermore, in step S1, the shear force of the anchor pile 8 is calculated based on the values ​​in set a2; the formula is as follows:

[0070]

[0071] In the formula Where E p I p P1 is the bending stiffness of anchor pile 8, P1 is the horizontal load at the top of the micropile foundation, and A is the bending stiffness of anchor pile 8. v According to the standard, the specific values ​​are shown in Table 1; in the table, Z represents the depth of the anchor pile (8). M represents the bending moment at the top of the micropile foundation, and n... h This represents the reaction coefficient of the bedrock;

[0072] In step S3, based on the pile shear force obtained in step S1, the dimensions of the stirrups 10 are designed, and the calculation formula for the shear bearing capacity of the anchor pile 8 is as follows:

[0073]

[0074] P Z (Z)≤V

[0075] In the formula: V is the shear bearing capacity; f c δ represents the design value of concrete compressive strength; δ represents the average wall thickness of anchor pile 8. Equivalent effective height, r is the outer radius of stirrup 10, r s f is the radius of the circle containing the center line of stirrup 10; yv The design value of the tensile strength of stirrup 10; A sv s represents the cross-sectional area of ​​stirrup 10; s represents the spacing of stirrup 10.

[0076] Reference Figure 5 Example of anchor pile 8, which consists of 14 anchor holes 6. The diameter of the anchor holes 6 is 150mm, and the thickness of the thinnest part of the overlap is 112mm, which can meet the requirement of a protective layer thickness of 50mm. The outer diameter of the anchor is 600mm and the inner diameter is 300mm.

[0077] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A design method for a composite micropile foundation, comprising a composite micropile foundation, the composite micropile foundation comprising a soil layer (1) and a bedrock layer (2) disposed below the soil layer (1), wherein a pile hole (3) is disposed within the soil layer (1), a cylindrical pile (4) is disposed within the pile hole (3), and a reinforcing cage (5) is disposed inside the cylindrical pile (4), characterized in that: Multiple anchor holes (6) are provided circumferentially along the axis of the cylindrical pile (4) within the bedrock layer (2). The multiple anchor holes (6) intersect each other to form anchor holes (7). Anchor piles (8) are provided within the anchor holes (7). Longitudinal reinforcement bars (9) are provided within the anchor piles (8). The multiple longitudinal reinforcement bars (9) are connected by at least two stirrups (10). The axis of the longitudinal reinforcement bars (9) coincides with the axis of the anchor holes (6). The upper end of the longitudinal reinforcement bars (9) extends into the interior of the cylindrical pile (4). Anchor bolts (11) are pre-embedded inside the cylindrical pile (4). The feature is that it includes the following steps: S1: Based on the design value of the load generated by the iron tower on the combined micropile, the lateral area of ​​the cylindrical pile (4) is obtained through finite element analysis simulation. 、 Anchor pile (8) bottom area The cross-sectional area of ​​the bedrock layer (2) between the anchor piles (8) The area of ​​the lower surface of the cylindrical pile (4) located outside the anchor pile (8) and the outer area A of the anchor pile (8) q The design values ​​for the loads generated by the combined micropiles on the tower include the downward pressure load. and pull-out load The lateral area of ​​the cylindrical pile (4) is obtained. 、 Anchor pile (8) bottom area The cross-sectional area of ​​the bedrock layer (2) between the anchor piles (8) The area of ​​the lower surface of the cylindrical pile (4) located outside the anchor pile (8) and the outer area A of the anchor pile (8) q The set of solutions is a1, and the formula for finding set a1 is as follows: In the formula: For the designed downward pressure load; The saturated uniaxial compressive strength of the bedrock layer (2); The area of ​​the lower surface of the cylindrical pile (4) located outside the anchor pile (8); The bottom area of ​​the anchor pile (8); Let be the lateral surface area of ​​the cylindrical pile (4); The end resistance coefficient of the lower surface of the cylindrical pile (4) located outside the anchor pile (8) is 0.6 for moderately weathered and above rocks, and 0.45 for strongly weathered rocks. The end resistance coefficient of the bottom surface of the anchor pile (8) is 0.8 for moderately weathered and above rocks, and 0.6 for strongly weathered rocks; The ultimate lateral resistance coefficient of soil layer (1) can be found in the "Technical Specification for Building Pile Foundations"; For the designed pull-out load; A is the cross-sectional area of ​​the bedrock layer (2) between the anchor piles (8); q The outer area of ​​the anchor pile (8); The end pull-out coefficient of the cross section of the central bedrock layer (2) between the anchor piles (8) is 0.08 for moderately weathered and above rocks, and 0.06 for strongly weathered rocks. The pull-out resistance coefficient of the outer side of the anchor pile (8) is 0.03 for moderately weathered and above rocks, and 0.0225 for strongly weathered rocks. The depth of soil layer (1); The ultimate lateral resistance of soil layer (1); Calculate the horizontal displacement X1(Z), rotation angle θ1(Z), and bending moment M of the combined micropile foundation based on set a1. Z (Z), and compared with the preset horizontal displacement X(Z), rotation angle θ(Z), and bending moment M(Z) of the combined micropile foundation, it is found that X1(Z)≤X(Z), θ1(Z)≤θ(Z), and M Z The set a2 of (Z) ≤ M(Z); where the horizontal displacement and rotation angle of the combined micropile foundation are divided into two parts, the horizontal displacement X2(Z) of the cylindrical pile (4) and the rotation angle θ2(Z) of the cylindrical pile (4) are calculated using the M method, and the horizontal displacement of the anchor pile (8) is calculated using the following formula: The rotation angle of the anchor pile (8) is calculated using the following formula: The bending moment of the anchor pile (8) is calculated using the following formula: The horizontal displacement of the micropile foundation is X1(Z) = X2(Z) + X Z (Z), rotation angle θ1 (Z) = θ2 (Z) + θ Z (Z) and bending moment M1(Z); For the horizontal load on the top of the micropile foundation. For the bending moment at the top of the micropile foundation, in the formula... , , , , , , , , , Found according to the standard; In the formula ,in The bending stiffness of the anchor pile (8) is given by... The reaction coefficient of the bedrock S2: Design the shape and size of the anchor hole (6) based on the bottom area of ​​the anchor pile (8) and the cross-sectional area of ​​the bedrock layer (2) between the anchor piles (8), and design the size of the longitudinal reinforcement (9) based on the shape and size of the anchor hole (6); wherein the longitudinal reinforcement (9) is located at the center of the anchor hole (6), and the minimum distance from the longitudinal reinforcement (9) to the edge of the anchor hole (7) must be greater than the specified thickness of the protective layer of the longitudinal reinforcement (9); S3: Design the size of the stirrups (10) in the anchor hole (7) according to the shape and size of the anchor hole (6); when selecting the stirrups (10), the minimum distance from the stirrups (10) to the anchor hole (7) must be greater than the protective layer thickness of the stirrups (10).

2. The design method for a combined micropile foundation as described in claim 1, characterized in that: In step S1, the shear force of the anchor pile (8) is calculated based on the values ​​in set a2; the formula is as follows: In step S3, based on the pile shear force obtained in step S1, the dimensions of the stirrups (10) are designed, and the calculation formula for the shear bearing capacity of the anchor pile (8) is as follows: P Z (Z)≤V In the formula: For shear bearing capacity; This is the design value for the compressive strength of concrete; The average wall thickness of the anchor pile (8); Equivalent effective height, , The outer radius of the stirrup (10) is... The radius of the circle containing the center line of the stirrup (10); The design value of the tensile strength of the stirrup (10); s is the cross-sectional area of ​​the stirrup (10); s is the spacing of the stirrup (10).

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