Design method for pile-anchor bracing combined support structure

Abstract

The present invention discloses a method for determining the arrangement of anchor bolts and the length of anchor bolts in a pile-anchor-strut combined support system, comprising the following steps: Step 1. Determine the physical and mechanical parameters of the soil mass of the foundation pit of the pile-anchor-strut combined support system and the excavation depth at each position; Step 2. Select the cross-sectional retaining types for each area according to the characteristics of the foundation pit; Step 3. Design the cross-sectional retaining; Step 4. Determine the various calculation parameters for the plane design; Step 5. Calculate the plane horizontal deformation curve of the retaining structure; Step 6. Determine the arrangement length of the anchor bolts. The method for determining the arrangement of anchor bolts and the length of anchor bolts in the pile-anchor-strut combined support system provided by the present invention can optimize the design of the pile-anchor-strut combined support structure, not only ensuring that the deformation of each retaining cross-section meets the requirements of foundation pit stability, but also controlling the anchor bolts at a reasonable length and avoiding the decline of foundation pit stability caused by too long arrangement length of the anchor bolts.

Description

Technical Field

[0001] The present invention relates to the field of foundation pit engineering, and mainly relates to a design method for a pile-anchor-brace combined support structure. Background Art

[0002] With the development of urban economy in China, the construction of a large number of high-rise buildings and large-scale underground infrastructure has led to an increasing number of deep and large foundation pit projects, which are developing in the direction of "larger, deeper, and more complex". How to control the deformation of deep and large foundation pits and ensure the stability of foundation pits in a complex urban environment has become the focus of attention of construction designers.

[0003] Traditional foundation pit enclosures mainly adopt pile-brace support structures and pile-anchor support structures. The pile-brace support structure forms an integral body in space by combining retaining piles and braces, and has the advantages of stable bearing capacity and strong deformation control ability. However, there are also some obvious disadvantages: 1. The presence of braces results in a small excavation working surface for earthwork, making construction difficult; 2. The erection and removal processes of braces are complex and time-consuming; 3. The project cost is high. The pile-anchor structure provides a fulcrum for the retaining pile by setting anchor rods, and is more flexible in layout compared with the pile-brace structure, which is beneficial to the subsequent excavation of the foundation pit and the construction of the basement structure, saving the time for support construction and removal, and having good economy at the same time. However, there are also some disadvantages: 1. The bearing capacity of anchor rods is greatly affected by soil conditions, and its application in soft soil layers is limited; 2. The ability to control foundation pit deformation is relatively weak, and it is not suitable for deep and large foundation pits with complex surrounding environments.

[0004] The pile-anchor-brace combined support structure combines retaining piles, anchor rods and braces in the plane and section, arranges anchor rods in the middle of the side length and braces at the corners, minimizing the plane layout area of braces and expanding economic benefits, while also having strong deformation control ability. However, in deep and large foundation pits, due to the weak deformation control ability of the pile-anchor structure, too long arrangement length of anchor rods along the side length direction of the foundation pit will lead to excessive deformation of the foundation pit, and in severe cases, it will lead to the instability and failure of the foundation pit. Therefore, it is necessary to control the arrangement length of anchor rods within a reasonable range.

[0005] For the early-applied pile-brace structure and pile-anchor structure, domestic and foreign experts and scholars have conducted sufficient research on their failure mechanisms, deformation laws and design theories. For the new type of retaining structure, the pile-anchor-brace combined support structure, there is less research at home and abroad at present, and its failure mechanism is not yet clear, and there is also a lack of targeted design methods. Therefore, it is urgent to analyze the failure mechanism of the pile-anchor-brace combined support structure and propose a design method for the pile-anchor-brace combined support structure to provide a basis for engineering design. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a design method for a pile-anchor-support combined retaining structure, which can optimize the design of the pile-anchor-support combined retaining structure, ensure that the deformation of each retaining profile meets the requirements of foundation pit stability, and control the length of the anchor rod within a reasonable range to avoid the decline of foundation pit stability caused by too long layout length of the anchor rod.

[0007] To this end, a design method for a pile-anchor-support combined retaining structure provided by the present invention includes the following steps:

[0008] Step 1. Determine the safety level of the foundation pit, the requirements for surrounding deformation control, and the design parameters of the soil body;

[0009] Step 2. Select the profile retaining types for each area according to the characteristics of the foundation pit;

[0010] Step 3. Determine the parameters of the retaining structure for each profile;

[0011] Step 4. Determine the calculation parameters for the plane design;

[0012] Step 5. Calculate the plane horizontal deformation curve of the retaining structure;

[0013] Step 6. Determine the layout length of the anchor rod.

[0014] Preferably, the calculation method is as follows:

[0015] In step 6, the layout length of the anchor rod in the pile-anchor-support combined retaining system is calculated according to formula (2);

[0016] y max (L m )=v R (2)

[0017] Where: y max is the maximum plane horizontal deformation value of the anchor rod section, related to L m ;

[0018] When the requirements for retaining deformation of the surrounding environment are relatively high, the plane layout range of the anchor rod can be further reduced according to the requirement that the maximum value of the plane horizontal deformation of the retaining structure does not exceed the retaining deformation control value;

[0019] y max (L m )=[R]=v R (3)

[0020] Where: [R] is the retaining deformation control value;

[0021] Establish a simplified model of the retaining structure to calculate the plane horizontal deformation values at each point. The calculation model is based on the following simplifications:

[0022] (1) Simplify the retaining piles into Euler - Bernoulli beams (Euler beams) fixed at both ends;

[0023] (2) Simplify the supports and anchor rods into spring elements, and assume that the displacements of all points on the same section are the same. The spring stiffness at any point on the Euler beam is the combined stiffness of multiple supports or anchor rods on the vertical section at that point. The force and displacement of the spring satisfy the following formula:

[0024] F h =y i ·K i (4)

[0025] Where: y i is the deformation value of the Euler beam at point i; K i is the spring stiffness at point i;

[0026] If the vertical section at the i - th point on the Euler beam is supported by a combination of N anchor rods and M supports, the combined stiffness K i at this point is:

[0027]

[0028] Among them, k mij is the stiffness of the j - th anchor rod at point i on the Euler beam; k zij is the stiffness of the j - th support at point i on the Euler beam;

[0029] k mij 、k zij are calculated according to the following formula:

[0030]

[0031]

[0032]

[0033]

[0034] Where: E s is the elastic modulus of the anchor rod body; E c is the composite elastic modulus of the anchor rod; E m is the elastic modulus of the grouting consolidation body; A p is the cross - sectional area of the anchor rod body; A is the cross - sectional area of the grouting consolidation body; l f is the free - length of the anchor rod; l a is the length of the anchor rod; D is the diameter of the anchor body; k t is the shear stiffness between the anchor body and the soil; α R is the support relaxation coefficient; E Z is the elastic modulus of the support; A Zis the support cross-sectional area; b aZ is the support calculation width; λ Z is the fixed-point adjustment coefficient; l0 is the support length; s Z is the support spacing;

[0035] (3) The soil load outside the pit is uniformly distributed horizontally;

[0036] Take an Euler beam microelement for force equilibrium analysis. From the static equilibrium conditions of the microelement, we can obtain:

[0037]

[0038] According to the theory of mechanics of materials:

[0039]

[0040] Combining the above equations, the differential equation of the deflection curve of the Euler beam deformation is obtained as:

[0041] EIy (4) +hK i y i =q (12)

[0042] In the formula: Q and M are the shear force and bending moment suffered by the microelement respectively; dQ and dM are the increments of the shear force and bending moment respectively; q is the soil pressure suffered by the microelement; θ is the rotation angle of the microelement; h is the vertical calculation width; EI is the stiffness of the retaining pile;

[0043] The soil load q outside the pit is the resultant force of the active earth pressure, which is calculated according to the formula in the Technical Code for Building Foundation Pit Support (JGJ 120 - 2012);

[0044] Before iterative calculation, divide the Euler beam into n segments along the x direction, each segment is of length λ, and introduce virtual nodes 0, 1, n + 1 and first-order, second-order, and fourth-order difference equations (13);

[0045]

[0046] Substitute the second-order and fourth-order difference equations into the deflection curve equation (12) and simplify to obtain:

[0047]

[0048] To solve this equation, let:

[0049] y i =A i y i-1 -B i y i-2 +C i (15)

[0050] It can be obtained:

[0051]

[0052] When \(0\leq i\leq n - 2\), substituting Eqs. (15) and (16) into (14), we get:

[0053]

[0054] When \(i = n\), we have:

[0055] y n = A n y n-1 - B n y n-2 + C n (18)

[0056] According to the boundary conditions at the fixed end, we have \(y n = 0\), \(\theta n = y' n = 0\). Combining with the second - order difference equation, we get:

[0057]

[0058] By combining Eqs. (18) and (19), it is easy to know that \(y n-1 \neq0\), \(y n-2 \neq0\), then:

[0059] A n = B n = C n = 0 (20)

[0060] When \(i = n - 1\), we have:

[0061] y n-1 = A n-1 y n-2 - B n-1 y n-3 + C n-1 (21)

[0062]

[0063] By combining Eqs. (19), (20), and (22), we get:

[0064]

[0065] When \(i = 0\), we have:

[0066] y0 = A0y -1 - B0y -2 + C0 (24)

[0067]

[0068] According to the boundary conditions at the fixed end, y0=0,θ0=y'0=0, combined with the second-order difference equation, we can get:

[0069]

[0070] When i=-1, we have:

[0071] y -1 =A -1 y -2 +C -1 (27)

[0072] Combining equations (24), (26) and (27), we can obtain:

[0073]

[0074] Combining equations (17), (23), and (28), we can obtain A when -1≤i≤n i , B i , C i ;

[0075] When i=1, we have:

[0076] y1=A1y0-B1y -1 +C1 (29)

[0077] Combining equations (26), (27) and (25), we can obtain:

[0078]

[0079] Combined with (15), (29), (30) and A i , B i , C i (-1≤i≤n), Matlab can be used to iterate and calculate the specific L m The Euler beam deformation curve under the load is obtained, and then the layout length of the anchor rod in the pile-anchor combined support system is determined according to formula (2).

[0080] Preferably, the soil parameters within the excavation depth range of the foundation pit include soil parameters of miscellaneous fill, clay silt, clay silt, silty clay intercalated with silt, silty clay intercalated with silt, silty clay and sandy silty clay, and the soil parameters include soil layer thickness, soil weight, cohesion and internal friction angle.

[0081] Preferably, the foundation pit adopts anchor rods arranged in the middle of the side length and concrete supports symmetrically arranged in the corners. The support section is bored cast-in-place piles plus two concrete supports, and the anchor rod section is bored cast-in-place piles plus four anchor rods.

[0082] Technical effects of the present invention:

[0083] 1. For a foundation pit with a pile - anchor - strut combined support structure (such as Figure 1 ), when the strength of the soil outside the pit decreases due to environmental factors, since the strut resistance is relatively stable while the anchor resistance gradually decreases as the soil strength decreases, the anchor in the middle of the pile - anchor section reaches the ultimate bearing capacity first and fails. When the anchor fails, it first develops from the proximal end of the anchorage section to the distal end of the anchorage section until the whole anchor fails, and then causes the continuous failure of the surrounding anchors until the foundation pit is unstable and fails. Moreover, the longer the layout length of the anchors, the more anchors that reach the ultimate bearing capacity first, and the lower the safety redundancy of the foundation pit.

[0084] Therefore, it is very necessary to accurately design the layout length of the anchors. However, there is currently no calculation method for the layout length of the anchors that can be applied in practice. In engineering design, the layout length of the anchors is often determined by empirical values, lacking theoretical basis and unable to ensure the safety and stability of the foundation pit. By using the method of the present invention, accurate calculation and design of the pile - anchor - strut combined support structure can be carried out, ensuring that the deformation of each retaining profile meets the requirements of foundation pit stability and optimizing the pile - anchor - strut combined support structure. By using the method of the present invention for calculation, the accurate value of the layout length of the anchors can be obtained. By controlling the layout length of the anchors within a reasonable range, the decrease in the stability of the foundation pit caused by too long a layout length of the anchors can be avoided.

[0085] 2. When designing the pile - anchor - strut combined support structure of the present invention, the engineering geological and hydrogeological conditions, the requirements for surrounding environmental protection, the excavation depth and shape and size of the foundation pit are comprehensively considered, and combined with local experience, the support types of each retaining profile in each area of the foundation pit are reasonably selected. And the support structure of pile plus strut is adopted at the corner of the foundation pit, and the support structure of pile plus anchor is adopted in the middle of the side length of the foundation pit, so as to minimize the layout area of the struts to the greatest extent and maximize the economic benefits. Description of the Drawings

[0086] Figure 1 It is a schematic diagram of a foundation pit with a pile - anchor - strut combined support structure.

[0087] Figure 2 It is a simplified calculation model diagram of the horizontal deformation of the retaining structure in the plane.

[0088] Figure 3 It is a differential node division diagram.

[0089] Figure 4 It is a design flow chart of the pile - anchor - strut combined support structure

[0090] Figure 5 It is a schematic diagram of the foundation pit in the example.

[0091] Figure 6 It is a comparison diagram between the full - strut and the pile - anchor - strut schemes.

[0092] Figure 7 It is a schematic diagram of the retaining profile design.

[0093] Figure 8 It is a plane horizontal deformation curve diagram of the retaining structure when the arrangement lengths of different anchor rods are different. Specific implementation manners

[0094] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The same components are denoted by the same reference numerals. It should be noted that the terms "front", "rear", "left", "right", "upper" and "lower" used in the following description refer to the directions in the drawings, and the terms "bottom surface" and "top surface", "inner" and "outer" respectively refer to the directions towards or away from the geometric center of a specific component.

[0095] The present invention relates to a method for designing the support of a foundation pit by using a pile-anchor-strut combined support structure, in which the length of the anchor rod used is accurately calculated, breaking through the defect that the pile-anchor-strut combined support system lacks a mature theory and calculation model in the traditional method. For a foundation pit with a traditional pile-anchor-strut combined support structure, when the strength of the soil outside the pit decreases due to environmental factors, since the support resistance is relatively stable while the anchor rod resistance gradually decreases as the soil strength decreases, the anchor rod in the middle of the pile-anchor section first reaches the ultimate bearing capacity and fails. When the anchor rod fails, the failure first develops from the proximal end of the anchorage section to the distal end of the anchorage section until the whole anchor rod fails, and then the continuous failure of the surrounding anchor rods is triggered until the foundation pit is unstable and fails. Moreover, the longer the arrangement length of the anchor rod is, the more the number of anchor rods that first reach the ultimate bearing capacity is, and the lower the safety redundancy of the foundation pit is. Therefore, for the design of the pile-anchor-strut combined support structure, not only should the deformation of each retaining profile be ensured to meet the requirements of foundation pit stability, but also the arrangement length of the anchor rod should be controlled to avoid the decrease of the foundation pit stability caused by the too long arrangement length of the anchor rod.

[0096] When designing the pile-anchor-strut combined support structure, first, the engineering geological and hydrogeological conditions, the requirements for surrounding environmental protection, the excavation depth and shape size of the foundation pit should be comprehensively considered, and combined with the local experience, the support types of the retaining profiles in each area of the foundation pit should be reasonably selected. Generally, the support structure of pile plus support at the corners of the foundation pit and the support structure of pile plus anchor rod in the middle of the side length of the foundation pit can minimize the layout area of the support plane and maximize the economic benefits. After determining the retaining structure parameters of each profile, calculate whether each profile can meet the deformation and stability requirements of the Technical Specification for Building Foundation Pit Support (JGJ 120-2012) or the surrounding environment. If not, the profile structure design parameters should be adjusted and recalculated until the requirements are met. When the properties of some soil layers in the foundation pit are good, the support within the range of this soil layer can be replaced by an anchor rod to further optimize the construction operation and project cost and realize the vertical combination of the pile-anchor-strut structure. However, the new profile retaining structure should also meet the profile deformation requirements.

[0097] After the sectional design of the pile - anchor - strut combined support structure is completed, the plane design is further carried out to determine the layout length of the anchor rods in the middle of the foundation pit.

[0098] Refer to Figure 1-8 As shown, the design method of the pile - anchor - strut combined support structure provided by the present invention is characterized in that it includes the following steps:

[0099] Step 1. Determine the safety level of the foundation pit, the requirements for controlling the surrounding deformation, and the soil design parameters; the soil parameters within the excavation depth range of the foundation pit include: the thickness, soil unit weight, cohesion, and internal friction angle of miscellaneous fill, clayey silt, clayey silt, silt - containing silty clay, silty clay with silt interlayers, silty clay, and sandy silt - containing clay.

[0100] Step 2. Select the sectional retaining types for each area according to the characteristics of the foundation pit; for the foundation pit, anchor rods are arranged in the middle of the side length, and concrete supports are symmetrically arranged at the corners. The support section is a bored pile plus two layers of concrete supports, and the anchor rod section is a bored pile plus four anchor rods.

[0101] Step 3. Determine the parameters of the sectional retaining structures;

[0102] Step 4. Determine the calculation parameters for the plane design;

[0103] Step 5. Calculate the horizontal deformation curve of the retaining structure in the plane;

[0104] Step 6. Determine the layout length of the anchor rods.

[0105] The calculation method is as follows:

[0106] According to the "Technical Code for Building Foundation Pit Support" (JGJ 120 - 2012), the following relationship exists between the bearing capacity of the anchor rod and the horizontal displacement at the anchor rod fulcrum in the section:

[0107]

[0108] In Step 6, the layout length of the anchor rods in the pile - anchor - strut combined support system is calculated according to Equation (2);

[0109] y max (L m )=v R (2)

[0110] Where: y max is the maximum horizontal deformation value of the anchor rod section, which is related to L m ;

[0111] When the surrounding environment has high requirements for the deformation of the retaining structure, the plane layout range of the anchor rods can be further reduced and determined according to the requirement that the maximum value of the horizontal deformation of the retaining structure plane does not exceed the deformation control value of the retaining structure;

[0112] y max (L m ) = [R] = v R (3)

[0113] Where: [R] is the deformation control value of the retaining structure;

[0114] Therefore, such as Figure 2 (a) Establish a simplified model of the retaining structure to calculate the horizontal deformation value of the retaining structure plane at each point. The calculation model is based on the following simplifications:

[0115] (1) Simplify the retaining pile into an Euler-Bernoulli beam (Euler beam) fixed at both ends;

[0116] (2) Simplify the supports and anchor rods into spring elements, and the displacements of each point on the same section are the same. The spring stiffness at any point on the Euler beam is the combined stiffness of multiple supports or anchor rods on the vertical section at that point. The force and displacement of the spring satisfy the following formula:

[0117] F h = y i ·K i (4)

[0118] In the formula: y i is the deformation value of the Euler beam at point i; K i is the spring stiffness at point i;

[0119] If the vertical section at the ith point on the Euler beam uses a combination of N anchor rods and M supports for support, then the combined stiffness K i at this point is:

[0120]

[0121] Among them, k mij is the stiffness of the jth anchor rod at point i on the Euler beam; k zij is the stiffness of the jth support at point i on the Euler beam;

[0122] k mij 、k zij are calculated according to the following formula:

[0123]

[0124]

[0125]

[0126]

[0127] In the formula: E s is the elastic modulus of the anchor rod body; E c is the composite elastic modulus of the anchor rod; E m is the elastic modulus of the grouting consolidation body; A p is the cross-sectional area of the anchor rod body; A is the cross-sectional area of the grouting consolidation body; l f is the free length of the anchor rod; l a is the length of the anchor rod; D is the diameter of the anchor body; k t is the shear stiffness between the anchor body and the soil mass; α R is the support relaxation coefficient; E Z is the support elastic modulus; A Z is the support cross-sectional area; b aZ is the calculated width of the support; λ Z is the fixed-point adjustment coefficient; l0 is the support length; s Z is the support spacing;

[0128] (3) The soil load outside the pit is uniformly distributed horizontally.

[0129] Take an Euler beam microelement for force equilibrium analysis. The extracted microelement is as shown in Figure 2 (b). From the static equilibrium conditions of the microelement, we can obtain:

[0130]

[0131] According to the theory of mechanics of materials:

[0132]

[0133] Combining the above formulas, the differential equation of the deflection curve of the Euler beam deformation is obtained as:

[0134] EIy (4) +hK i y i =q (12)

[0135] In the formula: Q and M are the shear force and bending moment suffered by the microelement respectively; dQ and dM are the increments of the shear force and bending moment respectively; q is the soil pressure suffered by the microelement; θ is the rotation angle of the microelement; h is the vertical calculated width; EI is the stiffness of the retaining pile;

[0136] The soil load q outside the pit is the resultant force of the active earth pressure and is calculated according to the formula in the Technical Code for Building Foundation Pit Support (JGJ 120 - 2012).

[0137] According to equations (12) and (13), the horizontal deformation curve y(x) of the Euler beam in the plane cannot be directly and explicitly expressed. However, it can be strictly proven mathematically that there is a unique correspondence between y(x) and L m ; therefore, the horizontal deformation curve y(x) of the Euler beam in the plane at a specific L m can be calculated by an iterative method, and then the layout length of the anchor rod in the pile-anchor bracing system can be determined according to equation (3).

[0138] Before the iterative calculation, the Euler beam is equally divided into n segments along the x-direction, each segment has a length of λ, and virtual nodes 0, 1, n+1 and first-order, second-order, and fourth-order difference equations (13) are introduced. The difference nodes are shown in Figure 3

[0139]

[0140] Substituting the second-order and fourth-order difference equations into the deflection curve equation (12) and simplifying, we get:

[0141]

[0142] To solve this equation, let:

[0143] y i =A i y i-1 -B i y i-2 +C i (15)

[0144] We get:

[0145]

[0146] When 0≤i≤n-2, substituting equations (15) and (16) into (14), we get:

[0147]

[0148] When i=n, we have:

[0149] y n =A n y n-1 -B n y n-2 +C n (18)

[0150] According to the boundary conditions at the fixed end, we have y n =0, θ n =y' n =0. Combining with the second-order difference equation, we get:

[0151]

[0152] Combining equations (18) and (19), it is easy to know that y n-1 ≠0, y n-2 ≠0, then:

[0153] A n = B n = C n = 0 (20)

[0154] When i = n - 1, we have:

[0155] y n-1 = A n-1 y n-2 - B n-1 y n-3 + C n-1 (21)

[0156]

[0157] Combining equations (19), (20), and (22), we can obtain:

[0158]

[0159] When i = 0, we have:

[0160] y0 = A0y -1 - B0y -2 + C0 (24)

[0161]

[0162] According to the boundary conditions at the fixed end, y0 = 0, θ0 = y’0 = 0. Combining with the second-order difference equation, we can obtain:

[0163]

[0164] When i = -1, we have:

[0165] y -1 = A -1 y -2 + C -1 (27)

[0166] Combining equations (24), (26), and (27), we can obtain:

[0167]

[0168] Combining equations (17), (23), and (28), we can obtain A i , B i , C i .

[0169] When i = 1, there is:

[0170] y1 = A1y0 - B1y -1 + C1 (29)

[0171] By combining equations (26), (27), and (25), we can obtain:

[0172]

[0173] By combining (15), (29), (30) and A i , B i , C i (-1 ≤ i ≤ n), using Matlab iteration, the deformation curve of the Euler beam under a specific L can be calculated, and then the layout length of the anchor rod in the pile-anchor-support combined support system can be determined according to equation (2). m

[0174] Refer to Figure 1-8 shown below, and specific embodiments are provided for detailed description:

[0175] For example: A foundation pit using a pile-anchor-support combined support structure is as Figure 5 shown. The shape of the foundation pit is square, with a side length of 200m. It is planned to have two basements, with a excavation depth of 10m. There are residential buildings with high-rise cast-in-place pile foundations on all sides. Its construction steps are as follows:

[0176] Step 1. Determine the safety level of the foundation pit, the requirements for surrounding deformation control, the design parameters of the soil mass, etc.

[0177] According to the "Technical Code for Building Foundation Pit Support" (JGJ 120 - 2012), the safety level of the foundation pit is determined to be level one, and the requirements for surrounding environmental protection are relatively high. The deformation control value of the foundation pit is determined to be 36mm.

[0178] The soil parameters within the excavation depth of the foundation pit are shown in Table 1.

[0179] Table 1 Soil Parameter Table

[0180]

[0181] Step 2. Select the sectional retaining types for each area according to the characteristics of the foundation pit.

[0182] The shape of the foundation pit is square. If a full-support scheme is adopted, as Figure 6 (a), it will make the construction difficult and the cost high. However, the soil quality within the excavation range of the foundation pit is good, and the anchor rod structure can also have good bearing performance. Therefore, in order to minimize the plane layout area of the foundation pit, optimize the construction and cost, a scheme of arranging supports at the corners of the foundation pit and anchor rods in the middle is adopted, as Figure 6 (b). ​

[0183] Step 3. Determine the parameters of the retaining structure for each section

[0184] Further determine that the support section adopts bored cast-in-place piles plus two layers of concrete supports; the anchor rod section adopts bored cast-in-place piles plus four anchor rods. The structural parameters and schematic diagrams of each section are shown in Figure 7 .

[0185] According to the relevant regulations in the "Technical Code for Building Foundation Pit Support" (JGJ 120-2012), the calculated deformation values of the pile-support and pile-anchor sections are: 20 mm and 36 mm, meeting the requirements of the code and the surrounding environment control

[0186] Step 4. Determine the calculation parameters for the plane design

[0187] According to the design parameters of the retaining structure for each section, the calculated stiffness of the retaining piles is 1.61×10 7 kN·m 2 ; the comprehensive stiffness of the anchor rod fulcrums in Area B is 67.8 MN·m -1 ; when the support length in Area A is 140 m, the comprehensive stiffness of its fulcrums is 274 MN·m -1 ; the external load of the pit is 66 kPa

[0188] Step 5. Calculate the plane horizontal deformation curve of the retaining structure

[0189] Substitute each calculation parameter into Equation (12), and calculate the plane horizontal deformation curve of the retaining structure at a specific L m using the iterative method shown in Equations (13)-(30). Use the loop algorithm in Matlab to automatically calculate the plane horizontal deformation curves of the retaining structure at different L m , and the results are as shown in Figure 8 .

[0190] Step 6. Determine the layout length of the anchor rods

[0191] According to Equation (2), the layout length of the anchor rods in this example should be 60 m

[0192] The above is only the preferred implementation mode of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements should also be regarded as the protection scope of the present invention

[0193] As described above, it is only the preferred embodiment of the present invention and does not impose any formal restrictions on the present invention. Although the present invention has been disclosed above with the preferred embodiment, it is not intended to limit the present utility model. Any person skilled in the art can make some changes or modifications to equivalent embodiments with equivalent changes within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A design method for a combined pile-anchor-support retaining structure, characterized in that: The following steps are involved: Step 1. Determine the foundation pit safety level, surrounding deformation control requirements, and soil design parameters; Step 2. Select the section enclosure type of each area according to the characteristics of the foundation pit; Step 3. Determine the parameters of the enclosure structure of each section; Step 4. Determine the calculation parameters of the plane design; Step 5. Calculate the horizontal deformation curve of the enclosure structure plane; Step 6. Determine the length of the anchor arrangement; The calculation is as follows: The step 6 determines the anchor arrangement length in the pile-anchor combined support system according to formula (2); y max (L m ) = v R (2) Where: y max is the maximum horizontal deformation value of the anchor rod section, related to L m ; When the surrounding environment has high requirements for enclosure deformation, the anchor bolt plane layout range can be further reduced to meet the requirement that the maximum horizontal deformation of the enclosure structure plane does not exceed the enclosure deformation control value; y max (L m ) = [R] = v R (3) Where: [R] is the enclosure deformation control value; A simplified model of the enclosure structure is established to calculate the horizontal deformation value of the enclosure structure plane at each point. The calculation model is based on the following simplifications: (1) The retaining pile is simplified into an Euler-Bernoulli beam fixed at both ends, i.e., an Euler beam; (2) The supports and anchors are simplified into spring units, and the displacements of all points on the same section are the same. The spring stiffness of any point on the vertical section is the combined stiffness of multiple supports or anchors on the vertical section at that point. The force and displacement satisfy the following equation: F h = y i . K i (4) where: y i is the deformation value of the Euler beam at point i; K i is the spring stiffness at point i; If N bolts and M supports are used in combination for vertical support at the i-th point on the Euler beam, the comprehensive stiffness K at this point i is as follows: (5) where k mij is the stiffness of the j th anchor rod at point i on the Euler beam; k zij is the stiffness of the j th support at point i on the Euler beam; k mij , k zij Calculate according to the following formula: (6) (7) (8) (9) Where: E s is the elastic modulus of the anchor rod body; E c is the composite elastic modulus of the anchor rod; E m is the elastic modulus of the grouting consolidation body; A p is the cross-sectional area of the anchor rod body; A is the cross-sectional area of the grouting consolidation body; l f is the free length of the anchor rod; l a is the length of the anchor rod; D is the diameter of the anchor body; k t is the shear stiffness between the anchor body and the soil; α R is the support relaxation coefficient; E Z is the elastic modulus of the support; A Z is the cross-sectional area of the support; b aZ is the calculated width of the support; λ Z is the fixed point adjustment coefficient; l0 is the length of the support; s Z is the support spacing; (3) The soil load outside the pit is evenly distributed in the horizontal direction; Take an Euler beam microelement to analyze the force balance. According to the static balance condition of the microelement, we can get: (10) From the theory of material mechanics, we know that: (11) Combining the above formula, the differential equation of the deflection curve of the Euler beam deformation is obtained as follows: EIy (4) + hK i y i = q (12) Where: Q and M are the shear force and bending moment acting on the infinitesimal element respectively; dQ and dM are the increments of the shear force and bending moment respectively; q is the earth pressure acting on the infinitesimal element; θ is the rotation angle of the infinitesimal element; h is the vertical calculation width; EI is the stiffness of the retaining pile; The soil load q outside the pit is the resultant force of the active earth pressure, which is calculated according to the formula in the Technical Code for Building Foundation Pit Support (JGJ120-2012); Before the iterative calculation, the Euler beam is equally divided into n segments along the x-direction, and the length of each segment is λ , and virtual nodes 0 and 1, n +1 and first, second, and fourth order difference equations (13); (13) Substituting the second-order and fourth-order difference equations into the deflection curve equation (12), we can obtain the following by simplifying: (14) To solve this equation, assume: y i = A i y i-1 - B i y i-2 +C i (15) We can get: (16) When 0 ≤ i ≤ n - 2, substituting equations (15) and (16) into (14), we can obtain: (17) When i=n, ​​we have: y n = A n y n-1 -B n y n-2 +C n (18) According to the boundary conditions at the fixed end, we have y n = 0, θ n = y n ' = 0. Combining with the second-order difference equation, we can obtain: (19) Combining (18) and (19), it is easy to know that n-1 ≠0,y n-2 ≠0, then: A n =B n =C n =0 (20) When i=n-1, we have: y n-1 = A n-1 y n-2 - B n-1 y n-3 + C n-1 (21) (22) Combining equations (19), (20), and (22), we can obtain: (23) When i = 0, we have: y0 = A0y -1 - B0 y -2 + C0 (24) (25) According to the boundary conditions at the fixed end, we have y0 = 0, θ 0 = y0’ = 0. Combining with the second-order difference equation, we can obtain: (26) When i=-1, we have: y -1 = A -1 y -2 +C -1 (27) Combining equations (24), (26) and (27), we can obtain: (28) By combining equations (17), (23), and (28), A, B, and C can be obtained for -1 ≤ i ≤ n. i , B i , C i ; When i=1, we have: y1 = A1y0 -B1y -1 + C1 (29) Combining equations (26), (27) and (25), we can obtain: (30) Simultaneously solve (15), (29), (30) and A i , B i , C i (-1 ≤ i ≤ n), and the Euler beam deformation curve under a specific L m can be calculated by Matlab iteration, and then the layout length of the anchor bolts in the pile-anchor bracing system can be determined according to Equation (2).

2. The determination method of the design method of the pile-anchor bracing combined support structure according to claim 1, characterized in that: The soil parameters within the excavation depth range of the foundation pit include soil parameters of miscellaneous fill, clayey silt, silty clay intercalated with silt, silty clay intercalated with silt, silty clay intercalated with silt, and silty clay containing sand. The soil parameters include soil layer thickness, soil weight, cohesion and internal friction angle.

3. The determination method of the design method of the pile-anchor bracing combined support structure according to claim 2, characterized in that: The foundation pit adopts anchor rods arranged in the middle of the side length and concrete supports arranged symmetrically at the corners. The support section is bored cast-in-place piles plus two concrete supports, and the anchor rod section is bored cast-in-place piles plus four anchor rods.

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