Safety and stability simplified calculation method based on pile stack supporting structure

Abstract

The invention provides a safety and stability simplified calculation method based on a pile stack supporting structure. The safety and stability simplified calculation method comprises the steps that fender posts are equivalent to underground diaphragm walls, and each group of pile stacks are equivalent to sheet piles; selecting a group of pile stacks as a calculation object, respectively selecting half of the stack distance to two sides by taking the center of the group of pile stacks as a symmetric center to form a calculation unit, and calculating the inertia moment of the calculation unit; the calculation unit is wholly equivalent to an underground diaphragm wall; the displacement and internal force of the pile stack are calculated and selected according to the equivalent underground diaphragm wall, and therefore the safety and stability of the pile stack supporting structure are judged. According to the calculation method, segmented calculation can be carried out on each group of pile stacks, the row number of the support piles in each group of pile stacks and the number of the support piles in each row are optimized while safety and stability are considered, the engineering cost can be effectively reduced, the construction time can be effectively saved, and the construction efficiency can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the field of deep foundation pit engineering, and particularly to a simplified calculation method for the safety and stability of a pile stack support structure. Background Art

[0002] In deep foundation pit engineering, reasonable support design is crucial for the safety of surrounding buildings, ground settlement control, and foundation pit deformation control. Currently, for deep foundation pit engineering with strict deformation control requirements, the main support structure types are internal support systems (such as steel supports or concrete supports) or double-row piles. The disadvantage of the double-row pile support structure is that it occupies a large construction space; the disadvantages of the internal support system are long construction period, occupation of the foundation pit operation space, the need to set up column piles (which may pose potential hazards to the quality of the main structure), and there are certain safety risks during the process of removing and replacing the internal support. These problems increase the complexity of underground engineering implementation. In recent years, with the advancement of urbanization and the growth of the demand for underground space development, foundation pit support faces higher technical requirements. Many scholars have begun to explore alternative support forms without internal supports, such as straight and inclined piles, inclined struts, pile stacks, etc. For example, Patent CN109723063A discloses a bundled pile stack support structure, in which bundled pile stacks are added at certain intervals in the middle of the long side of a wide foundation pit, which can achieve supportless and does not need to increase the cross-sectional size of the support piles. However, the bundled pile stacks in this patent are set outside the retaining piles (outside the foundation pit), see the appendix in this patent Figure 3 , and it has weak practicability for foundation pits with limited space. This patent also provides a calculation method based on the bundled pile stack support structure to calculate the stress conditions of the bundled pile stack support structure. This calculation method equivalentizes the front and rear rows of piles in each bundled pile stack structure into a large-rigidity support pile with a pile top rotation angle of 0, and then, by considering the spatial effect of the pile top capping beam, conducts a collaborative stress analysis together with the front row of cantilever support piles outside the bundled pile stack structure area; according to the balance and deformation coordination conditions of the capping beam, establishes the relationship between the shear force and displacement at the pile top of each support pile, and then uses the elastic resistance method to solve the bending moment, shear force, and horizontal displacement of each pile. This calculation method is relatively complex, has a certain calculation difficulty, requires the assistance of software programming and other methods for calculation, and is difficult to promote. Summary of the Invention

[0003] The present invention provides a simplified calculation method for the safety and stability of a pile stack support structure. This calculation method can perform segmented calculations on each group of pile stacks, optimize the number of rows of support piles in each group of pile stacks and the number of support piles in each row while considering safety and stability, and can effectively reduce costs and construction time.

[0004] To solve the above technical problems, the technical solution of the present invention is as follows:

[0005] A simplified calculation method for the safety and stability of a pile-dock support structure, the pile-dock support structure includes a row of retaining piles arranged along the foundation pit edge line and multiple groups of pile-docks located in the foundation pit and arranged at equal intervals and parallel to the retaining piles. The retaining piles are composed of several first support piles arranged at equal intervals, and the pile tops of the first support piles are connected as a whole through a capping beam. Each group of pile-docks is composed of 1 to 3 rows of support piles, and each row of piles is composed of 2 to 3 second support piles arranged at equal intervals. A cover plate is poured on the pile tops of the retaining piles and multiple groups of pile-docks to connect the retaining piles and multiple groups of pile-docks into a whole;

[0006] The simplified calculation method for the safety and stability of the pile-dock support structure includes the following steps:

[0007] (1) According to the basic situation of the foundation pit, determine the pile diameter d 1 and the pile spacing t 1 of the first support piles, 2 the pile diameter d 2 and the pile spacing t

[0008] (2) According to the equivalent flexural rigidity method, take the pile spacing t 1 of the first support piles as the calculation length, and equivalent the retaining piles to a diaphragm wall with a thickness h 1 ; take the actual width of the pile-dock as the calculation length, and equivalent each group of pile-docks to a sheet pile with a thickness h 2 . The center distance between the equivalent diaphragm wall and the sheet pile is taken as the center distance y between the retaining pile and the second support pile closest to the retaining pile in the pile-dock;

[0009] (3) Select a group of pile-docks as the calculation object, and take half of the pile-dock spacing x on both sides with the center of this group of pile-docks as the symmetry center to form a calculation unit, and calculate the moment of inertia of this calculation unit;

[0010] (4) Then, according to the equivalent flexural rigidity method, equivalently the calculation unit in step (3) as a diaphragm wall with a thickness H;

[0011] (5) Calculate the displacement and internal force of the selected pile-dock in step (3) according to the equivalently diaphragm wall in step (4);

[0012] (6) Then repeat steps (3) to (5) to calculate the displacement and internal force of other groups of pile-docks;

[0013] (7) Judge the safety of the pile-dock support structure according to the calculated displacement and internal force of the pile-docks.

[0014] The pile diameter d 2 of the second support piles is not less than the pile diameter d1 。

[0015] When each group of pile stacks is composed of three rows of piles, the row spacing c between two adjacent rows of piles in each group of pile stacks is equal.

[0016] The center distance y between the retaining pile and the second support pile in the pile stack close to the retaining pile is 1 - 2 times the pile diameter d of the second support pile 2 of.

[0017] The stack spacing x between two adjacent groups of pile stacks is 6 - 12 times the pile diameter d of the second support pile 2 of.

[0018] The row spacing c between two adjacent rows of piles in the pile stack is 1 - 2 times the pile diameter d of the second support pile 2 of.

[0019] The thickness of the cover plate is not less than the pile diameter d of the second support pile 2 。

[0020] The number of rows of piles in the pile stack located in the middle of the foundation pit support structure is not less than the number of rows of piles in the pile stacks located on both sides of the foundation pit support structure; the number of support piles in each row of the pile stack located in the middle of the foundation pit support structure is not less than the number of support piles in each row of the pile stacks located on both sides of the foundation pit support structure.

[0021] In step (2) of the simplified calculation method for the safety and stability of the pile stack support structure, the following assumptions should be made during the calculation: a. Assume that the elastic modulus of the first support pile and the equivalent diaphragm wall is equal; b. Assume that the elastic modulus of the second support pile and the equivalent sheet pile is equal.

[0022] In step (2) of the simplified calculation method for the safety and stability of the pile stack support structure: when each row of piles in the pile stack is composed of two second support piles, the actual width l of the pile stack is equivalent to a sheet pile according to l = d 2 +t 2 ; when each row of piles is composed of three second support piles, the actual width l of the pile stack is equivalent to a sheet pile according to l = d 2 +2t 2 equivalent to a sheet pile.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. The calculation method provided by the present invention can perform segmented calculations on each group of pile stacks. While considering safety and stability, it optimizes the number of rows of support piles and the number of support piles in each row in each group of pile stacks, which can effectively reduce costs and construction time; under the premise of the same deformation conditions, it optimizes the stack spacing and pile diameter, thus achieving a balance between safety and economy.

[0025] 2. In the present invention, the layout form of the pile stacks can be flexibly considered. The number of rows of piles in the middle of the foundation pit support structure or the number of retaining piles in each row is set to be relatively large, that is, the stiffness of the pile stacks in the middle of the foundation pit support structure with relatively large deformation is appropriately enhanced to ensure the safety and stability of the middle part of the foundation pit support structure.

[0026] 3. The pile stack support structure provided by the present invention can be used to replace the pile strut support structure composed of one internal support + retaining piles or the double-row pile support structure for foundation pit support. The pile stack is used to replace the internal support node to achieve the purpose of controlling deformation.

[0027] 4. In the present invention, the equivalent diaphragm wall thickness is obtained by an analytical method, and the calculation is relatively simple; moreover, this calculation method can be combined with existing foundation pit calculation software, which is convenient for application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the plan layout diagram of the pile stack support structure provided by the present invention;

[0029] Figure 2 is the plan layout diagram of the pile stack support structure provided by the present invention after being equivalent in step (1);

[0030] Figure 3 is the plan actual layout diagram of the pile stack support structure in the foundation pit in the application example;

[0031] Figure 4 is the schematic diagram of various layout shapes of the pile stacks in the pile stack support structure in the application example;

[0032] Figure 5 is the calculated displacement diagram of the pile stack support structure after being equivalent in the application example;

[0033] Figure 6 is the calculated bending moment envelope diagram of the pile stack support structure after being equivalent in the application example;

[0034] Figure 7 is the calculated soil pressure curve diagram of the pile stack support structure after being equivalent in the application example;

[0035] Figure 8 is the calculated shear force envelope diagram of the pile stack support structure after being equivalent in the application example;

[0036] In the figure: 1 - the first retaining pile, 2 - the second retaining pile, 3 - the cover plate. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The present invention will be further described below with reference to the drawings and embodiments.

[0038] In the present invention, the pile spacing refers to the center distance between two adjacent retaining piles in the same row, the stack spacing refers to the center distance between two adjacent groups of pile stacks, and the row spacing refers to the center distance between two adjacent rows of retaining piles in the pile stack. The pile stack retaining structure in the present invention includes a row of retaining piles arranged along the foundation pit side line and multiple groups of pile stacks parallel to the retaining piles and arranged at equal intervals within the foundation pit. Among them, the retaining piles are composed of several first retaining piles 1 arranged at equal intervals, and the pile tops of the first retaining piles are connected as a whole through a capping beam. Each group of pile stacks is composed of 1 to 3 rows of piles, and each row of piles is composed of 2 to 3 second retaining piles 2 arranged at equal intervals. A cover plate 3 is cast on the pile tops of the retaining piles and the multiple groups of pile stacks, connecting the retaining piles and the multiple groups of pile stacks into a whole, as Figure 1 shown.

[0039] Furthermore, the pile diameter d of the second retaining pile 2 is not less than the pile diameter d of the first retaining pile 1 ; preferably, the pile diameter d of the second retaining pile 2 is the same as the pile diameter d of the first retaining pile 1 .

[0040] Furthermore, when each group of pile stacks is composed of three rows of piles, the row spacing c between two adjacent rows of piles in each group of pile stacks is equal; preferably, each row of piles is composed of three second retaining piles.

[0041] Furthermore, the center distance y between the retaining pile and the second retaining pile closest to the retaining pile in the pile stack is 1 to 2 times the pile diameter d of the second retaining pile 2 ; preferably, the center distance y between the retaining pile and the second retaining pile closest to the retaining pile in the pile stack is 2 times the pile diameter d of the second retaining pile 2 .

[0042] Furthermore, the stack spacing x between two adjacent groups of pile stacks is 6 to 12 times the pile diameter d of the second retaining pile 2 ; preferably, the stack spacing x between two adjacent groups of pile stacks is 6 times the pile diameter d of the second retaining pile 2 .

[0043] Furthermore, the row spacing c between two adjacent rows of piles in the pile stack is 1 to 2 times the pile diameter d of the second retaining pile 2 ; preferably, the row spacing c between two adjacent rows of piles in the pile stack is 2 times the pile diameter d of the second retaining pile 2 .

[0044] Furthermore, the thickness of the cover plate is not less than the pile diameter d of the second retaining pile 2 .

[0045] Furthermore, the number of rows of piles in the pile pier in the middle of the foundation pit support structure is not less than that in the pile piers on both sides of the foundation pit support structure; the number of support piles in each row of the pile pier in the middle of the foundation pit support structure is not less than that in each row of the pile piers on both sides of the foundation pit support structure; that is, the number of rows of piles or the number of piles in each row can be appropriately increased in the middle of the foundation pit support structure with large displacement, so as to enhance the stiffness of the pile pier, as Figure 4 shown.

[0046] The simplified calculation method for the safety and stability of the pile pier support structure includes the following steps:

[0047] (1) Determine the pile diameter d 1 and pile spacing t 1 of the first support pile, the pile diameter d 2 and pile spacing t 2 of the second support pile, the spacing x between adjacent groups of pile piers, the center distance y between the retaining pile and the second support pile closest to the retaining pile in the pile pier, and the number of rows, quantity and row spacing c of the second support piles in each group of pile piers according to the basic conditions (foundation method and layout) of the foundation pit;

[0048] (2) According to the equivalent flexural stiffness method, take the pile spacing t 1 of the first support pile as the calculation length, and equivalent the retaining pile to a diaphragm wall with a thickness h 1 , as Figure 1 and Figure 2 shown, and its calculation formula is:

[0049]

[0050] In the formula, E 1 and E 1 ′ are the elastic moduli of the first support pile and the equivalent diaphragm wall respectively; d 1 is the pile diameter of the first support pile; b 1 is the calculation length, which is taken as the pile spacing t 1 of the first support pile here, that is, b 1 =t 1 ;

[0051] In the calculation process, it is assumed that the elastic moduli of the first support pile and the equivalent diaphragm wall are equal, that is, E 1 =E 1 ′. Since d 1 and t 1 are known, h 1 can be calculated;

[0052] Take the actual width of the pile pier as the calculation length, and equivalent each group of pile piers to a sheet pile with a thickness h 2 . When there is one row of piles in the pile pier, its calculation formula is:

[0053]

[0054] Wherein E 2 and E 2 ′ are the elastic moduli of the second retaining pile and the sheet pile after equivalence, respectively; d 2 is the pile diameter of the second retaining pile; n is the number of the second retaining piles in the pile stack; b 2 is the calculation length, and its value is the actual width l of the pile stack;

[0055] When two rows of piles are arranged in the pile stack, its calculation formula is:

[0056]

[0057] Wherein c is the row spacing of the two rows of retaining piles in the pile stack; n is the number of the second retaining piles in the pile stack;

[0058] When three rows of piles are arranged in the pile stack, its calculation formula is:

[0059]

[0060] Wherein n 1 is the number of the second row of retaining piles in the three rows of piles in the pile stack; n 2 is the number of the other two rows of the second retaining piles;

[0061] Specifically, when one row of piles is arranged in the pile stack and each row of piles is composed of two second retaining piles, according to the actual width l of the pile stack = d 2 +t 2 it is equivalent to a sheet pile; when each row of piles is composed of three second retaining piles, according to the actual width l of the pile stack = d 2 +2t 2 it is equivalent to a sheet pile; there are at most three rows of piles in the pile stack, and at most three second retaining piles in each row.

[0062] In this embodiment, the pile stack is composed of one row of two second retaining piles. Therefore, the value of n is 2; b 2 takes the value of the pile spacing t 2 of the second retaining pile and the pile diameter d 2 of the second retaining pile, and the sum is l, that is, b 2 =l=t 2 +d 2 ;

[0063] In the calculation process, it is assumed that the elastic moduli of the second retaining pile and the sheet pile after equivalence are equal, that is, E 2 =E 2 ′. Since d 2 、t 2 and n are known, then h 2 can be calculated;

[0064] The center distance between the equivalent diaphragm wall and the sheet pile is taken as the center distance y between the retaining pile and the second retaining pile close to the retaining pile among the pile piers.

[0065] (3) Select a group of pile piers as the calculation object, and take half of the pier spacing x on both sides with the center of this group of pile piers as the symmetry center to form a calculation unit, as shown in Figure 2 , and calculate the moment of inertia I of this calculation unit. Its calculation formula is:

[0066] I = I 1 + I 2 ;

[0067]

[0068] A 1 = xh 1 ;

[0069] A 2 = lh 2 ;

[0070]

[0071] In the formula, I 1 and I 2 are the moments of inertia of the equivalent diaphragm wall after the retaining pile and the equivalent sheet pile after the pile pier taking moments about the centroid of the calculation unit respectively; x and l are the lengths of the equivalent diaphragm wall and the sheet pile in the calculation unit respectively; h 1 and h 2 are the thicknesses of the equivalent diaphragm wall and the sheet pile respectively; A 1 and A 2 are the plane cross-sectional areas of the equivalent diaphragm wall and the sheet pile in the calculation unit respectively; y 1 and y 2 are the distances between the centers of the equivalent diaphragm wall and the sheet pile and the centroid of the calculation unit respectively; y is the center distance between the equivalent diaphragm wall and the sheet pile.

[0072] Since x, l, and y are known, and h 1 and h 2 are calculated in step (2), the moment of inertia I of this calculation unit can be calculated.

[0073] (4) Then, according to the equivalent flexural stiffness method, the calculation unit in step (3) is integrally equivalent to a diaphragm wall with a thickness of H; the calculation formula is:

[0074]

[0075] That is

[0076] Since x is known and I is calculated in step (3), the thickness H of the diaphragm wall after the overall equivalence of the calculation unit can be calculated.

[0077] (5) Calculate the displacement and internal force of the selected pile pier in step (3) according to the diaphragm wall after equivalence in step (4); specifically, the displacement and internal force of the pile pier are calculated using Tianhan Foundation Pit Software.

[0078] (6) Repeat steps (3) to (5) to calculate the displacement and internal force of other groups of pile piers.

[0079] (7) Judge the safety and stability of the pile pier support structure according to the calculated displacement and internal force of the pile pier;

[0080] Specifically, according to the provisions in JGJ120 - 2012 "Technical Specification for Building Foundation Pit Support" and DB42 / T 159 - 2024 "Technical Specification for Foundation Pit Engineering", in the design of foundation pit support, the safety grade of the foundation pit support structure and the environmental protection grade of the foundation pit need to be determined according to the specifications, and the horizontal deformation control range values corresponding to each grade are clearly specified in the specifications; verify whether the safety of the support structure is feasible according to the calculated displacement and internal force of the pile pier; if not, adjust the number of rows and quantity of the support piles of the pile pier, the pile diameter of the support piles, the pile spacing, the pier spacing, the center distance between the retaining pile and the support pile closest to the retaining pile in the pile pier, etc., and optimize the support structure while ensuring safety.

[0081] Application Example

[0082] Taking one side of a certain foundation pit support project as an example, the general situation of this side of the project is as follows: The distance between the edge of the bearing platform and the inner edge of the site road is about 2.5m, and the excavation depth is 6.0m; According to the provisions of DB42 / T 159 - 2024 "Technical Specification for Foundation Pit Engineering", the safety grade of the foundation pit support structure of this foundation pit is determined to be grade two, and the environmental protection grade of the foundation pit is determined to be grade one according to the excavation depth of the foundation pit, the surrounding environment, the engineering geology and hydrogeological conditions; A uniform load of 25 kPa is set 2.0m away from the upper opening line of the foundation pit. The physical and mechanical parameters of the soil layer are shown in the following table:

[0083]

[0084] In this application example, the total length of this support section is about 49.31m, the width of the bearing platform is about 5.3m, and the distance between the bearing platforms is between 8 - 8.6m; The foundation pit boundary surrounded by bored cast - in - place piles in the foundation pit is about 0.4m away from the bearing platform and about 3.4m away from the basement exterior wall line, and there is enough space inside the pit.

[0085] According to the calculation method of the present invention, first, it is determined that the pile diameters of the first support pile and the second support pile in the pile pier support structure are both 0.8m, the pile spacings are both 1.2m, and the pile lengths are both 16m; The pier spacing x is considered to be 10.5d 2, a total of 5 groups of pile stacks are set, each group of pile stacks consists of two second retaining piles in a row. The second retaining piles in the pile stacks are coaxial with the first retaining piles in the retaining wall. The center distance y between the retaining wall and the second retaining pile in the pile stack close to the retaining wall is considered 2d 2 , the plan layout of the pile stack support structure is as shown in Figure 3 . According to the equivalent flexural stiffness method, the retaining wall is equivalent to a diaphragm wall with a thickness of h 1 , and each group of pile stacks is equivalent to a sheet pile with a thickness of h 2 . It is calculated that h 1 ≈0.586m, h 2 ≈0.622m; then select a group of pile stacks as the calculation object, and select half of the pile stack spacing x on both sides with the center of this group of pile stacks as the symmetry center to form a calculation unit. Calculate the moment of inertia I of this calculation unit ≈ 2.723m 4 ; then according to the equivalent flexural stiffness method, the aforementioned calculation unit is integrally equivalent to a diaphragm wall with a thickness of H, and it is calculated that H≈1.572m. Since the number of rows and the number of piles of each group of pile stacks are the same in this application example, the calculation unit of each group of pile stacks is equivalent to a diaphragm wall with a thickness of about 1.572m

[0086] Use Tianhan Foundation Pit Software to calculate the pile stack support structure equivalent to a diaphragm wall with a thickness of about 1.572m in this application, and obtain the internal force and displacement of this diaphragm wall. The results are as shown in Figure 5-Figure 8 . From the results, it can be seen that the maximum displacement of this diaphragm wall is 38.8mm, which occurs at the pile top and meets the specification requirement of not exceeding 50mm; the maximum positive moment is 968kN·m, which occurs at 10.5m below the ground surface, and the maximum negative moment is -1kN·m; the maximum positive shear force is 216kN, which occurs at 7.1m below the ground surface, and the maximum negative shear force is -282kN, which occurs at 14m below the ground surface; the minimum safety factor of the passive zone elastic resistance is 1.75. If it is necessary to strengthen the stiffness of the local support structure, it can be adjusted according to the schematic diagram in Figure 4 .

Claims

1. A simplified calculation method for the safety stability of pile support structure, characterized by: The pile support structure includes a row of retaining piles arranged along the edge of the foundation pit and a plurality of piles located in the foundation pit and arranged equidistantly and parallel to the retaining piles, wherein the retaining piles are composed of a plurality of first retaining piles arranged equidistantly, and the pile tops of the first retaining piles are connected as a whole by a cap beam, each pile group is composed of 1 to 3 rows of piles, and each row of piles is composed of 2 to 3 second retaining piles arranged equidistantly, and the retaining piles and the pile tops of the plurality of piles are cast with cover plates, so that the retaining piles and the plurality of piles are connected as a whole; The simplified calculation method for the safety stability of the pile support structure comprises the following steps: (1) According to the basic conditions of the foundation pit, determine the pile diameter d1 and pile spacing t1 of the first supporting pile, the pile diameter d2 and pile spacing t2 of the second supporting pile, the pile spacing x between two adjacent pile piles, the center distance y between the retaining pile and the second supporting pile close to the retaining pile in the pile pile, and the number, quantity and row spacing c of the second supporting piles in each pile pile; (2) According to the equivalent bending stiffness method, the pile spacing t1 of the first supporting pile is taken as the calculation length, and the retaining pile is equivalent to an underground continuous wall with a thickness of h1; the actual width of the pile pile is taken as the calculation length, and each group of pile piles is equivalent to a sheet pile with a thickness of h2. The center distance between the equivalent underground continuous wall and the sheet pile is taken as the center distance y between the retaining pile and the second supporting pile in the pile pile close to the retaining pile; (3) A group of piles is selected as the calculation object, and half of the pile spacing x is selected on both sides of the center of the pile as the symmetry center to form a calculation unit, and the moment of inertia of the calculation unit is calculated; (4) According to the equivalent bending stiffness method, the calculation unit in step (3) is equivalent to an underground continuous wall with a thickness of H; (5) Calculating the displacement and internal force of the pile selected in step (3) according to the equivalent underground continuous wall in step (4); (6) Repeat steps (3) to (5) to calculate the displacement and internal force of other pile groups; (7) Determine the safety of the pile support structure based on the calculated pile displacement and internal forces.

2. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized in that: The pile diameter d2 of the second supporting pile is not less than the pile diameter d1 of the first supporting pile.

3. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized in that: When each pile stack consists of three rows of piles, the row spacing c between two adjacent rows of piles in each pile stack is equal.

4. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: The center distance y between the guard pile and the second guard pile in the pile pile close to the guard pile is 1-2 times the pile diameter d2 of the second guard pile.

5. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized in that: The distance x between two adjacent groups of piles is 6-12 times the diameter d2 of the second supporting pile.

6. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: The row spacing c between two adjacent rows of piles in the pile stack is 1-2 times the diameter d2 of the second supporting piles.

7. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: The thickness of the cover plate is not less than the pile diameter d2 of the second supporting pile.

8. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: The number of pile rows in the pile pile in the middle part of the foundation pit supporting structure shall not be less than the number of pile rows in the pile piles on both sides of the foundation pit supporting structure; the number of supporting piles in each row of the pile pile in the middle part of the foundation pit supporting structure shall not be less than the number of supporting piles in each row of the pile piles on both sides of the foundation pit supporting structure.

9. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: The simplified calculation method for the safety and stability of the pile support structure in step (2) requires the following assumptions to be made during the calculation process: a. Assume that the elastic modulus of the first support pile is equal to that of the equivalent underground continuous wall; b. Assume that the elastic modulus of the second support pile is equal to that of the equivalent sheet pile.

10. The simplified calculation method for safety and stability of pile support structure according to claim 1 is characterized by: In step (2) of the simplified calculation method for the safety and stability of the pile stack support structure: when each row of piles in the pile stack is composed of two second supporting piles, they are equivalent to sheet piles according to the actual width of the pile stack l = d2 + t2; when each row of piles is composed of three second supporting piles, they are equivalent to sheet piles according to the actual width of the pile stack l = d2 + 2t2.

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