A rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation

Abstract

The invention discloses a rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation. It includes a flexible cushion layer, rigid piles and flexible piles. The flexible piles are arranged in a grid shape around the outer contour of the rigid foundation. Rigid piles are arranged in the middle of the flexible piles. A flexible cushion layer is provided at the tops of the flexible piles and the rigid piles. A rigid foundation is arranged above the flexible cushion layer. The invention overcomes the defects that the prior art has too high cost when simply using rigid piles or cannot meet the settlement control requirements when simply using flexible piles; it has the advantages of improving the bearing capacity of the foundation, effectively reducing settlement, being safe and reliable, and being economically reasonable.

Description

Technical Field

[0001] The present invention relates to the technical field of soft foundation treatment, and more specifically, it is a rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation. Background Art

[0002] When the soft clay in the natural foundation is relatively thick, due to the high compression modulus of the soft clay, under the action of the superstructure, the overall settlement of the foundation is large; when the load distribution of the upper building is uneven or the stratum distribution of the foundation is uneven, relatively large differential settlement may also occur. Therefore, effective foundation treatment measures need to be taken to eliminate or reduce the overall settlement and differential settlement to ensure the safety of the building.

[0003] For soft soil foundations, composite foundations are generally used for treatment. Conventional composite foundation treatment measures mainly include: granular material pile (such as sand pile, sand-gravel pile, and gravel pile, etc.) composite foundation; flexible pile composite foundation (such as cement-soil pile, lime-soil pile, and lime pile, etc.); rigid pile composite foundation (such as reinforced concrete pile, plain concrete pile, prestressed pipe pile, large-diameter thin-walled pipe pile, cement fly ash gravel pile, lime-fly ash concrete pile, and steel pipe pile, etc.). Such conventional composite foundation treatment measures generally only involve one kind of vertical reinforcement material, such as prestressed pipe pile composite foundation and cement-soil mixing pile composite foundation.

[0004] For the granular material pile composite foundation, on the one hand, its treatment depth is limited and it cannot transfer the structural load to the deep layer of the foundation; on the other hand, its stiffness is not much different from that of the foundation soil, so it cannot effectively diffuse the additional stress generated by the upper structural load, resulting in little effect on reducing the foundation settlement. For the rigid pile composite foundation, the structural load can be transferred to the deep layer of the foundation by adjusting the pile length to effectively reduce the foundation settlement, but its cost is relatively high; in addition, since the rigid pile determines the settlement of the foundation, when the rigid pile is set too long, especially when the rigid pile is set as an end-bearing pile, it is easy to cause the rigid pile to jack up the foundation, resulting in the separation of the foundation from the natural foundation and forming contact scouring, posing a threat to the engineering safety. The flexible pile composite foundation has a settlement reduction effect between the above two.

[0005] For building projects with a relatively deep soft soil thickness, from the perspective of meeting the foundation bearing capacity, it is advisable to preferably select cement mixing piles, but their settlement amount is difficult to meet the specification requirements; from the perspective of controlling settlement, it is advisable to preferably select precast pipe piles, but the single-pile bearing capacity utilization coefficient is small, resulting in engineering waste.

[0006] Therefore, it is urgently necessary to develop a composite foundation that can improve the foundation bearing capacity and effectively reduce settlement. Summary of the Invention

[0007] The purpose of the present invention is to provide a rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation, thereby improving the bearing capacity of the foundation and effectively reducing the settlement, being safe, reliable, economical and reasonable, and making full use of the settlement reduction effect of the rigid piles to control the vertical settlement of the foundation; utilizing the containment effect of the flexible piles to limit the lateral deformation of the foundation, and further reducing the vertical settlement of the foundation; the present invention effectively combines longer rigid piles with shorter flexible piles, which can effectively reduce the foundation settlement and avoid the defects of excessively high cost of using only rigid piles or failure to meet the settlement control requirements of using only flexible piles.

[0008] In order to achieve the above-mentioned object, the technical solution of the present invention is: a rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation, characterized in that: the settlement reduction effect of the rigid pile is used to control the vertical settlement of the foundation, and the enclosure effect of the flexible pile is used to limit the lateral deformation of the foundation, thereby further reducing the vertical settlement of the foundation;

[0009] The rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation comprises a flexible cushion layer, rigid piles and flexible piles;

[0010] Flexible piles are arranged in a grid shape around the outer contour of the rigid foundation (all around); they are used to reduce the vertical settlement of the foundation and save investment costs; they overcome the defects of the existing rigid-flexible pile foundation that the flexible piles are arranged all over the foundation, which on the one hand results in a waste of investment, and on the other hand, due to the excessive and dense arrangement of flexible piles, the settlement reduction effect of the rigid piles is significantly reduced, which is not conducive to reducing settlement;

[0011] A plurality of rigid piles are arranged at intervals in the middle of the area surrounded by the flexible piles;

[0012] The flexible cushion layer is arranged on top of the flexible piles and the rigid piles;

[0013] A rigid foundation is provided on top of a flexible cushion layer.

[0014] In the above technical solution, the length of the flexible pile is shorter than the length of the rigid pile;

[0015] The length of the flexible pile can be 1 / 3 of the length of the rigid pile or equivalent to twice the equivalent fill height of the additional stress generated under the rigid foundation. For general sluice buildings, it can be 10 to 15 meters. The length of the flexible pile in the above-mentioned rigid-flexible pile composite foundation should be determined according to the lateral deformation characteristics of the foundation. According to on-site monitoring and numerical analysis, under the action of the upper structure load, the lateral deformation of the foundation is "bow" shaped along the depth direction of the foundation. For conventional sluice and pump station loads (100 to 120 kPa), the lateral deformation affects the depth of 10 to 15 meters. Therefore, the length of the flexible pile can be 10 to 15 meters in combination with the load size of the upper building.

[0016] In order to eliminate lateral deformation and reduce overall vertical settlement, flexible piles need to resist lateral deformation caused by additional stress in the foundation soil under the rigid foundation; this lateral deformation will increase the vertical settlement of the foundation; to resist lateral deformation, the flexible piles need to have a certain shear strength; the shear strength of the flexible piles is determined by the static soil pressure k 0 γ′z is determined, where k 0 is the static lateral pressure coefficient, γ′ is the effective bulk density of the soil, z is the depth of the flexible pile, for saturated soft soil, is the internal friction angle of soft soil, which can be 0, k 0 =1; the effective bulk density of saturated soft soil γ′ is about 8~9kN / m 3 Therefore, the length of the flexible pile is 10 to 15 m, and the shear strength of the flexible pile is 80 to 135 kPa.

[0017] In order to achieve the required shear strength of the flexible piles described above, when a single row of piles cannot meet the requirements, double or multiple rows of piles can be used to form a cement soil wall, with the overlap width of the pile rows not less than 15 cm.

[0018] In the above technical solution, the flexible piles are cement-soil mixing piles.

[0019] In the above technical solution, the rigid piles are arranged in a square shape in the area surrounded by the flexible piles; the spacing between the piles should not be too small. For conventional sluice gates and pumping stations, the square spacing between the rigid piles is preferably 4m×4m, so as to give full play to the bearing capacity of the foundation;

[0020] The rigid pile is a friction pile; it prevents the pile-soil deformation from being inharmonious, and the rigid pile directly supporting the rigid foundation, and the rigid foundation being separated from the natural foundation due to the inharmonious pile-soil deformation; it overcomes the problem that the rigid piles in the rigid-flexible pile composite foundation in the prior art are set as end-bearing piles or friction end-bearing piles. Since the foundation settlement is mainly determined by the rigid piles, once the rigid piles are set as end-bearing piles or friction end-bearing piles, it is easy to cause the pile-soil deformation to be inharmonious, and even cause the rigid piles to directly support the rigid foundation, causing the foundation to be separated from the natural foundation and causing the defect of contact scouring;

[0021] Rigid piles are prefabricated pipe piles.

[0022] In the above technical solution, the length of the rigid pile is determined according to the allowable value of foundation settlement, the allowable foundation settlement determines the length of the rigid pile, and the length of the rigid pile depends on the deformation of the rigid pile; since the rigid pile is a friction pile, the force transmitted by the rigid foundation is all borne by the lateral friction resistance of the soil around the pile, and the pile end is not subjected to force; the deformation of the rigid pile is the sum of the depth of the pile penetrating upward into the flexible cushion layer and the compression of the pile body itself; the deformation of the rigid pile s (mm) is calculated by the following formula:

[0023]

[0024] Where: σ is the stress (kPa) transmitted by the rigid foundation to the top of the rigid pile; E and v are the deformation modulus (MPa) and Poisson's ratio of the flexible cushion respectively; r is the radius (m) of the rigid pile body; l is the length (m) of the rigid pile; E p is the compression modulus (MPa) of the rigid pile; [s] is the allowable settlement value (mm) of the foundation.

[0025] According to the above calculation formula (1), the length of the rigid pile that meets the allowable settlement value of the foundation can be obtained.

[0026] In the above technical solution, the flexible cushion is a sand-gravel mixed cushion;

[0027] The particle size of the sand in the flexible cushion is 1 - 2 mm, and the particle size of the crushed stone is 20 - 40 mm, preventing the rigid pile from piercing into the cushion and directly supporting the rigid foundation; to prevent the rigid pile from piercing into the cushion and directly supporting the rigid foundation, the thickness of the sand-gravel mixed flexible cushion should be determined according to the deformation coordination of the pile and soil. The minimum thickness d of the flexible cushion must be greater than the depth h of the upward penetration of the rigid pile into the flexible cushion. The depth h (mm) of the rigid pile penetrating into the flexible cushion is calculated according to the following formula:

[0028]

[0029] Where: σ is the stress (kPa) transmitted by the rigid foundation to the top of the rigid pile; E and v are the deformation modulus (MPa) and Poisson's ratio of the flexible cushion respectively; r is the radius (m) of the rigid pile body.

[0030] According to formula (2), the minimum thickness d of the flexible cushion can be obtained as d ≥ h.

[0031] The volume ratio of sand and gravel in the flexible cushion is 1:1. For conventional sluices and pumping stations, the volume ratio of sand and gravel is 1:1, preventing the rigid pile from piercing into the cushion and directly supporting the rigid foundation, and reasonably determining the stiffness of the cushion.

[0032] In order to give full play to the bearing capacity of the foundation and reduce the number of flexible piles and rigid piles, the thickness of the sand-gravel mixed flexible cushion should not be less than 10 cm; in order to minimize the foundation settlement as much as possible, it is necessary to reduce the load borne by the foundation. Therefore, the thickness of the flexible cushion should not exceed 30 cm to avoid excessive load on the foundation causing foundation failure. For conventional sluices and pumping stations, the thickness of the sand-gravel mixed cushion is 10 - 30 cm.

[0033] The stiffness E and thickness d of the flexible cushion can be designed with variable stiffness according to the deformation coordination of the pile and soil.

[0034] In the above technical solution, the thickness of the flexible cushion is 20 - 30 cm.

[0035] The present invention has the following advantages:

[0036] (1) The rigid-flexible pile composite foundation of the present invention is used to reduce the vertical settlement of the foundation by using longer rigid piles; when the length of the rigid piles that meet the allowable settlement value of the foundation is too long to be constructed or is obviously unreasonable economically, shorter grid-shaped flexible piles (such as cement-soil mixing piles) are arranged around the rigid piles to play a lateral enclosure role for the foundation soil, limit the lateral deformation of the foundation soil within the depth range of the flexible piles, and further reduce the vertical settlement of the foundation by reducing the lateral deformation of the foundation soil. At the same time, due to the lateral enclosure effect of the flexible piles on the foundation, the defect of contact scouring caused by the separation of the rigid foundation and the natural foundation due to the uncoordinated deformation of the piles and soil can be effectively prevented;

[0037] (2) The rigid-flexible pile composite foundation composed of rigid piles and flexible piles of the present invention can fully utilize the characteristic of flexible piles in improving the bearing capacity of the foundation, and fully utilize the advantage of rigid piles in effectively reducing settlement, while having good economic efficiency. The present invention can solve the foundation settlement problem and the separation problem of the foundation from the natural foundation through the reasonable arrangement and reasonable length of flexible piles, the reasonable arrangement and reasonable length of rigid piles, and the reasonable setting of the thickness and stiffness of the cushion layer. The present invention is safe and reliable, economical and reasonable, and is worthy of promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the plan layout of the rigid-flexible pile composite foundation of an example of the present invention.

[0039] Figure 2 It is a schematic diagram of the cross-section of the rigid-flexible pile according to an example of the present invention.

[0040] Figure 3 This is a flow chart of the rigid-flexible pile construction of an example of the present invention, wherein ①, ②, and ③ in the figure represent the construction sequence.

[0041] exist Figure 1 , Figure 3 In the figure, the number below the rigid pile is the number of the corresponding rigid pile.

[0042] exist Figure 1 In the figure, A represents a flexible pile and B represents a rigid pile.

[0043] exist Figure 2 In the figure, A represents flexible piles; B represents rigid piles; C represents cushion layer; D represents foundation; and E represents the direction of load. DETAILED DESCRIPTION

[0044] The following is a detailed description of the implementation of the present invention in conjunction with the accompanying drawings, but they do not constitute a limitation of the present invention and are only used as examples. At the same time, the advantages of the present invention are made clearer and easier to understand through the description.

[0045] Example

[0046] like Figure 1Shown is a rigid-flexible pile composite foundation for reducing the settlement of soft foundation under rigid foundations such as sluices, pumping stations, and ship locks. Among them, the flexible pile is selected as a cement-soil mixing pile, and the rigid pile is selected as a prestressed high-strength concrete pipe pile. The diameter of the cement-soil mixing pile (i.e., the flexible pile) is 500 mm, the length is 12 m, the admixture amount of 42.5-grade cement is 15%, the strength of the cement soil is the average value of the cube compressive strength of the indoor reinforced soil test blocks with the same cement-soil ratio as the pile body cement soil under the standard curing conditions for 90 days, and it is taken as 1.5 MPa according to experience. The compression modulus of the cement soil is taken as 150 - 180 MPa. The diameter of the prestressed high-strength concrete pipe pile (i.e., the rigid pile) is 600 mm, the length is 30 m, the strength grade is C80, and the piles are arranged in a square pattern with a spacing of 4 m × 4 m. The thickness of the sand and gravel mixed cushion (i.e., the flexible cushion) is 30 cm.

[0047] The construction method of the rigid-flexible pile composite foundation for reducing the settlement of soft foundation under rigid foundation in this embodiment is as follows:

[0048] (1) Before construction, first level the site, and conduct technological test piles for rigid piles and flexible piles according to the design requirements, and the number of each shall not be less than 2.

[0049] (2) Since the precast pipe piles will cause soil squeezing effect during the pile forming process, therefore, the precast pipe piles (i.e., the rigid piles) should be constructed first, and finally the cement-soil mixing piles (i.e., the flexible piles) should be implemented. It should be noted that when constructing the precast piles (i.e., the rigid piles), the pile jumping method should be implemented. First, implement the piles numbered 1, 3, 5, 8, 10, 12, 13, 15, 17 shown in Figure 3 (such as the rigid piles indicated by ① in Figure 3 ), then implement the remaining precast pipe piles (such as the rigid piles indicated by ② in Figure 3 ), and finally implement the flexible piles located around the outer contour of the rigid foundation of the foundation (such as the rigid piles indicated by ③ in Figure 3 ).

[0050] (3) When implementing the cement-soil mixing piles (i.e., the flexible piles), ensure that any point in the soil within the reinforcement depth range can be stirred more than 20 times. During construction, the spraying and lifting speed and the stirring times should be strictly controlled. The key point is to ensure that the entire pile length is stirred again.

[0051] (4) Before pouring the foundation, the thickness of the cushion (i.e., the flexible cushion) should be rechecked to ensure that the thickness of the flexible cushion meets the design requirements.

[0052] (5) Construction impact control measures: In order to reduce the impact of construction on the surrounding environment, auxiliary measures such as pre-drilled pile sinking, stress relief holes, sand-filled bags or plastic drainage boards, diaphragm sheet piles or diaphragm walls, excavation of shockproof trenches, and restriction of pile driving rate can be taken.

[0053] Conclusion: In this embodiment, the rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation of the present invention is adopted. The settlement reduction effect of the rigid pile is utilized to control the vertical settlement of the foundation, and the retaining effect of the flexible pile is utilized to limit the lateral deformation of the foundation, further reducing the vertical settlement of the foundation. At the same time, through the reasonable arrangement and reasonable length of the flexible pile, the reasonable arrangement and reasonable length of the rigid pile, and the reasonable setting of the cushion thickness and stiffness, the foundation settlement problem is solved, and the problem of the separation between the foundation and the natural foundation is solved, which is both safe and reliable and economically reasonable.

[0054] In order to more clearly illustrate the advantages of the rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation of the present invention compared with the prior art, the staff compared these two technical solutions, and the comparison results are shown in Table 1 below:

[0055] Table 1 Comparison table of soft soil foundation treatment schemes

[0056]

[0057]

[0058] As can be seen from Table 1 above, compared with the prior art, the rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation of the present invention has a wide range of applications, can not only limit the lateral deformation of the foundation soil but also reduce the vertical settlement of the foundation, and has a good reinforcement effect.

[0059] It should be noted that for those skilled in the art, several changes or deformations can be made to the present invention without changing the essence of the present invention, which also fall within the protection scope of the present invention.

[0060] Other parts not described herein belong to the prior art.

Claims

1. A rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation, Characterized in that: The settlement reduction effect of rigid piles is utilized to control the vertical settlement of the foundation, and the retaining effect of flexible piles is utilized to limit the lateral deformation of the foundation, further reducing the vertical settlement of the foundation; The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation includes a flexible cushion, rigid piles and flexible piles; The flexible piles are arranged in a grid pattern around the outer contour of the rigid foundation; Multiple rigid piles are arranged at intervals in the area enclosed by the flexible piles; The flexible cushion is arranged on the tops of the flexible piles and rigid piles; among them, multiple rigid piles are concentratedly distributed in the middle of the bottom of the flexible cushion, and the flexible piles are arranged around the bottom of the flexible cushion and are located outside the outer circle of the multiple concentrated rigid piles; The rigid foundation is arranged above the flexible cushion; The shear strength of the flexible pile is determined by the static earth pressure k 0 γ'z, where k 0 is the coefficient of static lateral earth pressure, γ' is the effective unit weight of the soil, and z is the depth of the flexible pile; The length of the rigid pile is determined according to the allowable settlement value of the foundation. The allowable settlement of the foundation determines the length of the rigid pile, and the length of the rigid pile depends on the deformation of the rigid pile; since the rigid pile is a friction pile, all the forces transmitted from the rigid foundation are borne by the lateral friction resistance of the soil around the pile, and the pile tip is not stressed; the deformation of the rigid pile is the sum of the depth of upward penetration into the flexible cushion and the compression of the pile body itself; the deformation s of the rigid pile is calculated by the following formula: Where: σ is the stress transferred by the rigid foundation to the top of the rigid pile, kPa; E is the deformation modulus of the flexible cushion, Mpa; ν is the Poisson's ratio of the flexible cushion; r is the radius of the rigid pile body, m; l is the length of the rigid pile, m; E p is the compression modulus of the rigid pile, MPa; [s] is the allowable settlement value of the foundation, mm.

2. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 1, Characterized in that: The shear strength of the flexible pile is 80 - 135 kPa.

3. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 1 or 2, Characterized in that: The length of the flexible pile is less than the length of the rigid pile; The length of the flexible pile is taken as 1 / 3 of the length of the rigid pile or equivalent to 2 times the height of the equivalent fill soil corresponding to the additional stress generated under the rigid foundation.

4. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 3, Characterized in that: The flexible pile is a cement-soil mixing pile; The flexible pile is a single-row pile or a multi-row pile; When the single-row pile cannot meet the shear strength requirement, the method of multi-row pile overlapping is adopted to form a cement soil wall to meet the strength requirement; the overlapping width between pile rows is greater than or equal to 15 cm.

5. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 4, Characterized in that: The rigid piles are arranged in a square pattern in the area enclosed by the flexible piles; The rigid pile is a friction pile; The rigid pile is a precast pipe pile.

6. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 1, Characterized in that: The flexible cushion is a sand-gravel mixed cushion; the volume ratio of sand to gravel in the flexible cushion is 1:1; the particle size of the sand in the flexible cushion is 1 - 2 mm, and the particle size of the gravel is 20 - 40 mm.

7. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 6, Characterized in that: The thickness of the flexible cushion should be determined according to the deformation coordination between the pile and the soil. The minimum thickness d of the flexible cushion is greater than the depth of upward penetration of the rigid pile into the flexible cushion; the depth h of the rigid pile penetrating into the flexible cushion is calculated by the following formula: Where: h is the penetration depth of the rigid pile into the flexible cushion, in mm; σ is the stress transmitted by the rigid foundation to the top of the rigid pile, in kPa; E is the deformation modulus of the flexible cushion, in Mpa; ν is the Poisson's ratio of the flexible cushion; r is the radius of the rigid pile body, in m.

8. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 7, characterized in that: The stiffness E and thickness d of the flexible cushion are determined according to the deformation coordination between the pile and the soil.

9. The rigid-flexible pile composite foundation for reducing the settlement of soft foundation under a rigid foundation according to claim 8, characterized in that: The thickness of the flexible cushion is 20 - 30 cm.

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