A method for adjusting the interaction between piles and soil in end-bearing rigid pile composite foundation

Abstract

The present invention discloses a method for adjusting the interaction of piles and soil in an end-bearing rigid pile composite foundation, belonging to the field of foundation engineering technology. The key points of the technical solution are as follows: the composite foundation comprises rigid piles, soil between piles, a cushion layer, XPS extruded boards, and a foundation raft; the conventional cushion layer is retained, and the XPS extruded boards are vertically embedded into the cushion layer. The XPS extruded boards are protected on both sides with 1-2 mm thick steel plates to prevent the extruded boards from being locally compressed by gravel in the cushion layer during stress. At the same time, the steel plate hoops can also increase the rigidity of the XPS extruded boards in the yield plateau section to a certain extent. The present invention can meet the larger vertical load requirements of high-rise buildings with rigid pile tops using XPS extruded boards with a lower yield strength, while being low in cost and very simple to construct.

Description

Technical Field

[0001] The invention relates to the technical field of foundation engineering, and in particular to a method for adjusting the interaction of piles and soil in an end-bearing rigid pile composite foundation. Background Art

[0002] The overburden on sites in karst areas is generally silty clay or fully (strongly) weathered rock, with a moderately weathered rock layer with certain undulations beneath. Although the bearing capacity of the overburden foundation is relatively high, it still cannot meet the bearing capacity requirements of high-rise buildings. When using pile foundations, since the moderately weathered rock layers in karst areas often have vertical beaded caves, the pile foundations need to pass through multiple layers of caves. Selecting the underlying intact bedrock as the bearing layer for the pile foundation will result in high pile construction costs and increase the risk of collapse caused by passing through caves during construction. The use of rigid pile composite foundations can fully utilize the bearing capacity of the overburden. Since the vertical pressure of the rigid piles in the composite foundation is relatively small, compared with pile foundations, it is easier to meet the stability verification of the roof of small caves. Even if the verification is not satisfied, it is easy to handle.

[0003] When the bearing layer at the pile end is a moderately weathered rock layer, the end-bearing rigid pile cannot penetrate downward to coordinate the deformation of the pile and soil like a friction pile. The difference in the supporting stiffness of the pile and soil is significant. Therefore, in order to achieve the joint action of the pile and soil, it is necessary to eliminate the deformation difference of the pile and soil and ensure the deformation coordination of the pile and soil. Placing a pile top stiffness adjustment device on the top of the rigid pile is an adjustment method for the joint action of the pile and soil in the composite foundation of the end-bearing rigid pile.

[0004] When using a pile-top stiffness adjustment device to coordinate pile-soil deformation, the device's stiffness is pre-designed based on the calculated soil stiffness and cannot be changed once installed. Accurately calculating soil deformation is a prerequisite for achieving pile-soil deformation coordination. When there's a significant discrepancy between the actual and calculated soil settlements, the device struggles to achieve its intended effect. Summary of the Invention

[0005] In view of the shortcomings of existing end-bearing rigid pile composite foundation adjustment devices, such as complex process, high cost and being affected by the calculation accuracy of soil settlement deformation, the present invention proposes a method for adjusting pile-soil deformation coordination of rigid pile composite foundations by adjusting stiffness first and strength later. The present invention retains the traditional pile top cushion layer and adjusts deformation by vertically embedding XPS extruded boards in the pile top cushion layer. When the rigid pile top stress is less than its bearing capacity characteristic value, the XPS extruded boards produce elastic deformation with very small deformation; when the rigid pile top stress exceeds the design bearing capacity characteristic value, the XPS extruded boards of different strengths undergo plastic deformation in succession. The space generated by the plastic deformation is filled by the cushion layer, and subsequent deformation is generated to continue to coordinate the pile-soil stress. The present invention can meet the larger rigid pile top vertical load requirements of high-rise buildings by using XPS extruded boards with smaller yield strength, and the cost is low and the construction is very simple.

[0006] The technical solution of the present invention is achieved as follows:

[0007] End-bearing rigid pile composite foundation includes rigid piles, soil between piles, cushion layer, XPS extruded board and foundation raft;

[0008] The traditional mattress layer is retained, and the XPS extruded board is vertically embedded in the mattress layer. 1-2mm thick steel plates are used on both sides of the XPS extruded board to protect it from being locally compressed by the gravel in the mattress layer during the load process. At the same time, the steel plate hoop effect can also improve the stiffness of the XPS extruded board in the yield platform section to a certain extent.

[0009] At present, the maximum strength of XPS extruded board can reach 900MPa, with three-stage stress characteristics: elastic stage, yield platform stage and densification stage. The strain energy of the yield platform stage reaches above 0.5, and the stress in the densification stage increases sharply with the strain change.

[0010] The elastic modulus of the extruded board is generally 15-30MPa, slightly lower than the elastic modulus of the mattress layer of 25-40MPa. When the XPS extruded board is in the elastic stage, since the thickness of the XPS extruded board is only 10-30mm, the compression deformation is very small, and the deformation in the elastic stage can be basically ignored. When the XPS extruded board reaches the yield stress, the XPS extruded board has a long plastic section. From existing tests, it can be seen that the plastic strain can reach about 0.6. At this stage, the internal pores of the XPS extruded board are gradually squeezed and closed.

[0011] The space that shrinks in volume during the plastic compaction of the XPS extruded board is filled by the cushion layer, thereby promoting the continued deformation of the cushion layer. The subsequent stress and deformation performance of the cushion layer can be controlled by the replacement rate and yield strength gradation of the extruded board.

[0012] When the holes in the XPS extruded board are compacted, the pore walls contact each other, and the extruded board is in the densification stage. The stress of the XPS extruded board increases sharply with the increase of strain, and the compression modulus of the XPS extruded board increases sharply.

[0013] According to the design requirements, 2 to 3 XPS extruded boards with different yield strengths can be vertically embedded in the mattress layer. The XPS extruded boards bear the lateral pressure σ3 of the mattress layer. σ3 can be calculated according to σ3=σ1tan 2 (45°-φ / 2), where σ1 is the vertical compressive stress at the pile top, φ is the internal friction angle of the mattress layer. When the vertical compressive stress σ1 at the pile top takes the characteristic value of the rigid pile bearing capacity, the corresponding σ3 is the minimum yield strength of the XPS extruded board. The yield strengths of the remaining XPS extruded boards can be configured in a step-by-step manner of 1:1.1:1.2, or other configurations can be adopted according to design requirements.

[0014] When the foundation soil settlement is small and the compressive stress on the rigid pile top is less than the characteristic value of the pile foundation bearing capacity, the elastic deformation of the XPS extruded board is very small. When the foundation soil settlement exceeds the design calculation settlement and the stress on the rigid pile top exceeds the characteristic value of the pile foundation bearing capacity, the XPS extruded board with the lowest strength will yield first, creating a plastic deformation space to coordinate the pile-soil stress. If the settlement requirement is still not met, the XPS extruded board with the second lowest yield strength will continue to yield and produce plastic deformation to coordinate the pile-soil deformation, and so on. The overall stiffness of the subsequent deformation is determined by the yield strength step configuration of the XPS extruded boards of different strengths and the amount of plastic deformation of the extruded boards.

[0015] In summary, the present invention has the following beneficial effects:

[0016] 1. Different from the traditional stiffness adjustment method, the present invention adopts a method of cooperative deformation adjustment of rigid pile composite foundation that controls stiffness first and then strength.

[0017] 2. When the method of the present invention is used, the lateral pressure on the extruded board is σ3=σ1tan 2 Calculating the ratio (45° - φ / 2) reveals that when the internal friction angle of the graded gravel cushion is 35 degrees, σ3 = 0.271σ1. When the characteristic bearing capacity of the pile top is 2.0 to 2.5 MPa, the lateral compressive stress on the extruded board is only 542 to 677 kPa. The current strength of the extruded board can meet the application requirements of rigid pile composite foundations. If the yield strength of XPS extruded boards of different strengths is configured in a stepped ratio of 1:1.1:1.2, the maximum stress requirement is 1.2 × 677 kPa = 812 kPa, which can also be met by the current strength of the extruded board.

[0018] 3. XPS extruded board has corresponding national standards and its performance is very stable. The extruded board has a perfect closed-cell honeycomb structure. This structure makes the XPS board have extremely low water absorption (almost no water absorption), low thermal conductivity, high compressive resistance, and aging resistance. There is almost no aging and decomposition phenomenon in normal use, and there will be no gradual increase in compression under long-term load. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic structural diagram of an embodiment of an end-bearing rigid pile composite foundation in which an XPS extruded board is vertically placed on the pile top according to the present invention;

[0020] Markings in the figure: 1, rigid pile; 2, soil between piles; 3, cushion layer; 4, XPS extruded board; 5, raft foundation; 6, bedrock;

[0021] Figure 2 This is a typical stress-strain curve of the XPS extruded board of the present invention;

[0022] Figure 3This is a schematic diagram of an embodiment of the XPS extruded board of the present invention, in which five extruded boards are vertically embedded in a pile top mattress layer;

[0023] Figure 4 This is another embodiment of the XPS extruded board of the present invention, which is a schematic diagram of the arrangement of eight extruded boards vertically embedded in the pile top mattress layer;

[0024] Figure 5 A set of load-settlement test comparison curves of rigid pile composite foundation according to an embodiment of the present invention;

[0025] Figure 6 1 is a set of rigid pile composite foundation load-pile top stress comparison curves according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0027] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0028] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0029] In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connect," "fixed," etc. should be understood broadly. For example, they may refer to fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0030] Example 1:

[0031] A 17-story high-rise building uses a raft foundation. The standard load transmitted from the superstructure to the raft is 300 kPa, but the characteristic bearing capacity of the natural foundation is only 150 kPa. Under a 150 kPa load, the design settlement of the natural foundation is 25 mm. A rigid pile composite foundation is used, and the design requires a pile-soil stress ratio of 10. The characteristic bearing capacity of the rigid pile top is 1.5 MPa. A rigid pile with a diameter of 400 mm is used, and the cushion layer thickness at the pile top is 200 mm according to the standard. Assuming the cushion layer has an elastic modulus of 40 MPa, the cushion layer can only deform by 7.5 mm under a 1.5 MPa pile top load. The cushion layer's deformation capacity cannot meet the soil settlement requirement of 25 mm.

[0032] The method of the present invention is used to adjust the interaction between the pile and the soil. An extruded board is vertically embedded in the cushion layer on the top of the rigid pile. The compression and deformation of the extruded board promotes the continued deformation of the cushion layer to achieve a 17.5mm settlement. The volume required for the cushion layer on the top of a rigid pile with a diameter of 400mm to produce a 17.5mm settlement is 3.14×20cm×20cm×1.75cm=2198cm3. Assuming that the specifications of a single extruded board are 20cm×15cm×3cm and the plastic strain of the cushion layer is 0.5, the volume that can be freed up by compacting each extruded board is 450cm3. Five extruded boards can meet the requirements. The lateral pressure on the extruded board is calculated according to σ3=σ1tan 2 (45°-φ / 2) is used for calculation. When the internal friction angle of the graded crushed stone cushion layer is 35 degrees, then σ3=0.271σ1, and the strength of the extruded board is 406KPa, which can meet the design requirements.

[0033] Example 2:

[0034] A 28-story high-rise building uses a raft foundation. The standard load transmitted from the superstructure to the raft is 420 kPa, but the characteristic value of the natural soil bearing capacity is only 200 kPa. Under a 200 kPa load, the calculated settlement of the natural soil is between 15 and 35 mm, which cannot be precisely defined. A composite foundation using rigid piles with a diameter of 500 mm is used. The design requires a pile-soil stress ratio of 15, and the characteristic value of the pile top bearing capacity of the rigid pile is 3 MPa. The thickness of the pile top cushion layer is 250 mm according to the specification. Assuming the elastic modulus of the cushion layer is 50 MPa, the deformation of the cushion layer under a 3 MPa pile top load can only reach 15 mm. When the actual settlement of the natural soil is 15 mm, the cushion layer alone can meet the deformation coordination requirements. The stress at the pile top will not exceed the design characteristic value of the bearing capacity. The extruded board is in the elastic stage, and the deformation can be basically ignored.

[0035] Because actual settlement is difficult to accurately calculate, when the actual settlement of natural soil under load is 35mm, the extruded board needs to produce plastic deformation to promote the continued deformation of the cushion layer to complete the 20mm settlement. The compressed volume required for the 500mm diameter pile top cushion layer to continue to produce 20mm settlement is 3.14×25cm×25cm×2.0cm=3925cm³. Assuming the specifications of a single extruded board are 20cm×20cm×3cm and the plastic strain of the cushion layer is 0.5, the volume space that can be vacated by compaction of each extruded board is 600cm³. Using 7 extruded boards can meet the cushion layer deformation requirement. As in Example 2, the lateral pressure σ3 of the extruded board is 0.271σ1, and the extruded board strength is 813kPa to meet the requirement. If the extruded board strength is configured according to a 1:1.1 ratio, the extruded board yield strength can be 813 and 894kPa respectively.

[0036] It can be seen from Example 2 that by vertically embedding the extruded board in the cushion layer, the rigid pile composite foundation has a large tolerance to settlement. When the actual settlement is less than 15 mm, the extruded board basically does not play a role. However, when the actual settlement reaches 35 mm, the extruded board is compacted to provide space for the cushion layer at the top of the pile to continue to deform, and the stress at the top of the pile can be set as needed.

[0037] Example 3

[0038] The test of this embodiment was carried out in a geotechnical test box with a size of 1m×1m×0.85m. The diameter of the rigid pile was 300m. The cushion layer was made of coarse sand with a thickness of 170mm. The measured internal friction angle of the cushion layer was 20 degrees. The lateral pressure on the extruded board was calculated as σ3=σ1tan 2 (45°-φ / 2) is used for calculation, σ3=0.49σ1. Two groups of tests are carried out. One group of tests uses only ordinary cushion layer, and the other group of tests inserts 5 extruded boards into the cushion layer on the top of the pile. The measured yield stress of the extruded boards is 210KPa. Figure 5 : is a comparison curve of two groups of rigid pile composite foundation load-settlement tests in an embodiment of the present invention, Figure 6 The load-pile top stress comparison curves of two groups of rigid pile composite foundations of the embodiment of the present invention can be seen from the embodiment test curves. Before the rigid pile composite foundation is loaded to 200kPa, since the extruded board only undergoes elastic deformation, the load-displacement curves of the two groups of tests are slightly different. When the loading exceeds 200kPa, the pile top stress reaches 428kPa, and the lateral pressure of the pile top mattress reaches the extruded board yield stress of 210kpa. The extruded board begins to yield and produce plastic compaction deformation. The deformation capacity of the composite foundation is obviously improved, but the increase in pile top stress is not large. The extruded board has a good effect of controlling the pile top stress.

[0039] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for regulating the interaction between piles and soil in an end-bearing rigid pile composite foundation, wherein the end-bearing rigid pile composite foundation comprises rigid piles, soil between piles, a cushion layer, an XPS extruded board and a foundation raft, characterized in that: Adjustment methods include: A mattress layer is placed on top of the rigid piles; Multiple XPS extruded boards with different yield strengths are vertically embedded in the mattress layer; When the stress at the top of the rigid pile is less than the characteristic value of its design bearing capacity, the deformation adjustment is carried out by the cushion layer; When the stress at the top of the rigid pile is greater than the characteristic value of its design bearing capacity, the XPS extruded boards of different strengths yield one after another under the action of the side pressure of the cushion layer. The space created by the plastic deformation is filled by the cushion layer, and the cushion layer continues to deform to coordinate the stress of the pile and soil.

2. The method for adjusting the interaction between piles and soil in an end-bearing rigid pile composite foundation according to claim 1, characterized in that: XPS extruded board is an elastic-plastic material with three-stage stress characteristics: elastic stage, yield platform stage and densification stage. The strain energy in the yield platform stage reaches above 0.

5.

3. The method for adjusting the interaction between piles and soil in an end-bearing rigid pile composite foundation according to claim 1, characterized in that: The thickness of the XPS extruded board is 10 to 30 mm, and both sides of the XPS extruded board are provided with 1 to 2 mm thick steel plates.

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