Existing engineering pile bearing capacity evaluation method under urban update

Abstract

The invention discloses a method for evaluating the bearing capacity of an existing engineering pile under urban updating, and the method comprises the following steps: drilling, building a model, and testing the bearing capacity, the model establishment is to establish a reduced-scale physical mechanical model according to the actual geometric dimension of the pile foundation and the physical mechanical parameters of the stratum, that is, the physical reduced-scale model of the actual pile foundation and the stratum is manufactured according to the reduced-scale similarity ratio N; according to the bearing capacity test, the physical model of the pile soil is placed in a centrifugal testing machine to carry out the pile foundation bearing capacity test under the action of a centrifugal machine. According to the method for evaluating the bearing capacity of the existing engineering pile under urban update, the stress condition of an actual pile foundation under the load action can be effectively reproduced through detection operation in the model, and responses of the pile foundation in different load stages can be visually observed and analyzed; a reliable basis is provided for accurately evaluating the pile foundation bearing capacity of the existing project in the long-term use process, and the problem that the existing mode is inconvenient to evaluate the pile bearing capacity of the existing project is solved.

Description

Technical Field

[0001] The present invention relates to the technical field related to engineering pile bearing capacity assessment, and specifically to a method for assessing the bearing capacity of existing engineering piles in urban renewal. Background Art

[0002] The engineering pile bearing capacity test is an important means and guarantee for testing the quality of engineering piles, but the existing urban engineering pile bearing capacity test still has certain defects when used; During use, since the design elevation of the top of the engineering pile is generally much lower than the natural ground elevation, the engineering pile test must be carried out after the foundation pit is excavated or by connecting the piles of the same specifications to the ground. The former is very expensive and the test is difficult; the latter will make the test lose its due randomness, which reduces the reliability of the test to a certain extent. In addition, the extra pile body must be cut off after excavation, which also damages the pile body. In order to overcome the above-mentioned defects, prior art one (Chinese patent with application number 202211225359.X and application date 2022-10-09) is a pile test device for the pull-out bearing capacity of a precast pile engineering pile and a method for using the same, comprising a middle rod, an extension tube, a load sensor and a pile connection tube; the middle rod penetrates the extension tube and the pile connection tube; a plurality of independently partitioned extension tube cavities are formed between the extension tube, the middle rod and the extension tube partition; a pile connection tube cavity is formed between the pile connection tube and the middle rod, and a heating tube is installed in the pile connection tube cavity; a second liquid inlet channel and an overflow channel are reserved in the middle rod; a connecting rib is provided at the bottom of the pile connection tube, a soluble nut is provided at the upper end of the connecting rib, or the connecting rib is an integral part with a soluble upsetting head, and after the lower end of the connecting rib is connected to the precast pile, an ordinary pile sinking process is used to sink the pile to the elevation required by the design; after the pull-out test is completed, the soluble nut is dissolved and the device is recovered for reuse. The actual pile top load of the engineering pile obtained by the present invention is accurate, and the installation, use and recycling are convenient, the recycling performance is good, and the economic efficiency is high; There is also prior art 2 (Chinese patent with application number 201711009018.8 and application date 2017-10-25), which is a field test method for the influence of horizontal cyclic load on the vertical bearing capacity of pile foundation. This method eliminates the influence of plastic deformation of foundation soil on the test result to the maximum extent by means of high compressibility cushion layer at the bottom of the test pile and vertical cyclic loading. When the vertical displacement of the test pile foundation meets the requirements, the last vertical loading test result is used as the reference parameter, and then a horizontal load is applied to the upper part of the test pile and circulated as required, and then vertical loading is performed according to the requirements of the specification and the test data of each level is recorded. Finally, the test results of the two vertical loading test piles before and after the horizontal loading are compared, and then the influence of the horizontal cyclic load on the vertical bearing capacity of the pile foundation is calculated; And prior art three (Chinese patent with application number 202010199507.X and application date 2020-03-20) an underground continuous wall structure with high vertical bearing capacity and its construction method, including a continuous wall main body, an inner lining wall and a bottom plate; a plurality of grouting anchor bodies are arranged in the soil body outside the continuous wall main body; each grouting anchor body is fixedly connected to the continuous wall main body by at least two grouting steel pipe piles; the two or more grouting steel pipe piles corresponding to each grouting anchor body are cross-arranged with each other, and each intersection of the cross-arrangement is located inside the corresponding grouting anchor body. During construction, the corresponding grouting anchor bodies and grouting steel pipe piles are built layer by layer from the upper line to the height of the grouting anchor body.

[0003] Although the existing technology can improve the efficiency of engineering pile bearing capacity testing and carry out bearing capacity testing during construction, in the redesign of existing buildings, the bearing capacity of the pile foundation will often increase under the long-term load on the pile top. Therefore, when renovating or reinforcing existing buildings, the above operation cannot accurately detect the data, thereby failing to ensure the safety and stability of the structure.

[0004] In response to the above problems, it is urgently necessary to make innovative designs based on the original bearing capacity assessment method of existing engineering piles under urban renewal. Therefore, we proposed a bearing capacity assessment method of existing engineering piles under urban renewal that can well solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for evaluating the bearing capacity of existing engineering piles under urban renewal, so as to solve the problem that the overall bearing capacity test is carried out during construction in the current market as proposed by the above-mentioned background technology, but in the redesign of existing buildings, the bearing capacity of the pile foundation tends to increase under the long-term load on the pile top. Therefore, when the existing buildings are renovated or reinforced, the above-mentioned operation cannot accurately detect the data, thereby failing to ensure the safety and stability of the structure.

[0006] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: a method for evaluating the bearing capacity of existing engineering piles under urban renewal, comprising the following steps: drilling - model building - bearing capacity test.

[0007] Preferably, the drilling is to determine the stratigraphic division and physical and mechanical parameters of the site through drilling, which provides basic data support for subsequent accurate evaluation, so that the evaluation can be closely aligned with the actual site conditions, thereby improving the accuracy of the evaluation.

[0008] Preferably, the model is established by establishing a scaled physical and mechanical model based on the actual geometric dimensions of the pile foundation and the physical and mechanical parameters of the stratum, that is, a physical scaled model of the actual pile foundation and the stratum is made according to the scale similarity ratio N, which can simulate the complex actual pile foundation conditions at a relatively small cost and space in a laboratory environment, and is convenient for operation and research.

[0009] Preferably, the bearing capacity test is to place the physical model of the pile-soil in a centrifugal testing machine to carry out a pile foundation bearing capacity test under the action of a centrifuge, and the acceleration of the centrifuge is set to N during the test. By centrifuging the scaled physical model, the bearing capacity of the pile foundation under the actual state can be simulated, and a loading device is set at the top of the model test pile to simulate the load applied to the top of the pile in the load test. The above operation can effectively reproduce the stress condition of the actual pile foundation under the action of the load, and can intuitively observe and analyze the response of the pile foundation at different load stages, which provides a reliable basis for accurately evaluating the pile foundation bearing capacity of the existing project during long-term use, helps to discover potential problems in advance and optimize the design scheme, and at the same time greatly reduces the risk and cost of large-scale testing on actual buildings, improves the efficiency and scientificity of the existing building pile foundation bearing capacity assessment, reduces the problem that the existing method is inconvenient to assess the bearing capacity of the existing project piles, and ensures the safety and stability of the reinforcement of the existing building.

[0010] Preferably, in the centrifugal state, the similarity relationship between the physical model and the prototype can be calculated, which not only solves the problem of high cost and difficulty in actual testing, but also enables extreme testing under safe and controllable conditions, thereby improving the rigor of the data.

[0011] Preferably, by carrying out a load test under centrifugal state on the scaled physical model in a centrifuge, the pile foundation load test QS curve is determined, and the bearing capacity characteristic value R of the model pile A is determined. The bearing capacity characteristic value in the model test can be converted into the prototype pile bearing capacity characteristic value N²R, which helps to provide a reliable theoretical basis for pile foundation design in the early stage of the project, makes the design scheme more in line with actual engineering needs, effectively avoids engineering hidden dangers caused by inaccurate bearing capacity estimation, and improves the safety and stability of the project.

[0012] Preferably, another test pile B is selected, and the model pile bearing capacity characteristic value R is used as the pile top holding load to act on the pile top, and the load is loaded to R by loading in a centrifuge, and the load time is stabilized for T, so as to simulate the actual existing engineering piles in actual use. The existing engineering piles are subjected to the continuous action of the pile top load N²R for a period of N²T, which helps to gain a deeper understanding of the durability and long-term stability of the engineering piles, and provide strong data support for the maintenance and renovation of existing projects.

[0013] Preferably, the load on the test pile B is increased until the pile B is destroyed. According to the Qs curve of pile B, the bearing capacity characteristic value of the model pile B is determined to be Ra, and the corresponding bearing capacity characteristic value of the prototype pile is N²Ra. Through calculation, the changes in the mechanical properties of the pile body in the whole process from normal bearing to destruction can be fully understood, and accurate data can be provided for determining the ultimate bearing capacity of the pile foundation, further improving the pile foundation design theory, optimizing the material selection and construction process of the engineering piles, thereby reducing the engineering cost and improving the economic and social benefits of the engineering construction under the premise of ensuring the engineering quality.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: the method for evaluating the bearing capacity of piles in existing projects under urban renewal can effectively reproduce the stress conditions of the actual pile foundation under load through the detection operation in the model, and can intuitively observe and analyze the response of the pile foundation at different load stages, which provides a reliable basis for accurately evaluating the bearing capacity of the pile foundation of the existing project during long-term use, and reduces the problem that the existing method is inconvenient to evaluate the bearing capacity of the piles of the existing project. The specific contents are as follows: (1) Drilling - model building - bearing capacity test. The above operations can effectively reproduce the stress conditions of the actual pile foundation under load, and can intuitively observe and analyze the response of the pile foundation at different load stages, reducing the problem of the existing method being inconvenient to evaluate the bearing capacity of existing engineering piles; (2) The site stratigraphic division and physical and mechanical parameters are determined through drilling, which provides basic data support for subsequent accurate assessment, allowing the assessment to closely match the actual site conditions, thereby improving the accuracy of the assessment; (3) A scaled physical and mechanical model is established based on the actual geometric dimensions of the pile foundation and the physical and mechanical parameters of the stratum. That is, a physical scaled model of the actual pile foundation and stratum is made according to the scale similarity ratio N. This can simulate the complex actual pile foundation conditions in a laboratory environment with a relatively small cost and space. (4) The physical model of the pile-soil is placed in a centrifuge testing machine to conduct a pile foundation bearing capacity test under the action of the centrifuge. During the test, the acceleration of the centrifuge is set to N. By subjecting the scaled physical model to centrifugal action, the bearing capacity of the pile foundation under actual conditions can be simulated. A loading device is set at the top of the model test pile to simulate the load applied to the top of the pile in the load test, which provides a reliable basis for accurately evaluating the bearing capacity of the pile foundation of the existing project during long-term use. (5) Under centrifugal conditions, the similarity relationship between the physical model and the prototype can be calculated. By conducting a load test under centrifugal conditions on the scaled physical model in a centrifuge, the QS curve of the pile foundation load test is determined, and the bearing capacity characteristic value R of the model pile A is determined. The bearing capacity characteristic value in the model test can be converted into the bearing capacity characteristic value N²R of the prototype pile. In the centrifuge, pile B is loaded to R and the load is stabilized for T, thereby simulating the actual existing engineering pile under the load N²R on the pile top during actual use for N²T. Then the load is increased on the test pile B until pile B is destroyed. According to the Qs curve of pile B, the bearing capacity characteristic value of the model pile B is determined to be Ra, and the corresponding bearing capacity characteristic value of the prototype pile is N²Ra. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the evaluation method of the present invention; Figure 2 It is a schematic diagram of the steps of the evaluation method of the present invention; Figure 3 It is a schematic diagram of similarity relationship in the centrifugal model test of the present invention; Figure 4 It is a schematic diagram of the specific operation method of the present invention; Figure 5 It is a schematic diagram of the QS curve of pile A of the present invention; Figure 6 It is a schematic diagram of the QS curve of pile B after loading according to the present invention; Figure 7 Schematic diagram of the evaluation steps of the present invention. DETAILED DESCRIPTION

[0016] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Embodiment 1:

[0017] In this embodiment, the stress condition of the actual pile foundation under load can be effectively reproduced through the detection operation in the model, and the response of the pile foundation at different load stages can be intuitively observed and analyzed, which provides a reliable basis for accurately evaluating the bearing capacity of the pile foundation of the existing project during long-term use, and reduces the problem that the existing method is inconvenient to evaluate the bearing capacity of the pile foundation of the existing project, such as Figure 1-Figure 3The technical solution shown in the figure determines the stratum division and physical and mechanical parameters of the site through drilling, which provides basic data support for subsequent accurate evaluation, so that the evaluation can be closely matched with the actual site conditions, thereby improving the accuracy of the evaluation. Then, a scaled physical and mechanical model is established according to the actual geometric dimensions of the pile foundation and the physical and mechanical parameters of the stratum, that is, a physical scaled model of the actual pile foundation and the stratum is made according to the scaled similarity ratio N. It can simulate the complex actual pile foundation conditions at a relatively small cost and space in a laboratory environment, which is convenient for operation and research. Then, the physical model of the pile soil is placed in a centrifuge testing machine for a pile foundation bearing capacity test under the action of the centrifuge. In the test, the acceleration of the centrifuge is set to N, and the scaled physical model is centrifuged. The effect can simulate the bearing capacity of the pile foundation under the actual state. A loading device is set on the top of the model test pile to simulate the load applied to the top of the pile in the load test. The above operation can effectively reproduce the stress condition of the actual pile foundation under the load, and can intuitively observe and analyze the response of the pile foundation at different load stages, which provides a reliable basis for accurately evaluating the bearing capacity of the pile foundation of the existing project during long-term use, helps to discover potential problems in advance and optimize the design scheme, and greatly reduces the risk and cost of large-scale tests on actual buildings, improves the efficiency and scientificity of the assessment of the bearing capacity of the pile foundation of existing buildings, reduces the problem that the existing method is inconvenient to assess the bearing capacity of the piles of existing projects, and ensures the safety and stability of the reinforcement of existing buildings; By conducting a load test on the scaled physical model in a centrifuge, the QS curve of the pile foundation load test is determined, and the bearing capacity characteristic value R of the model pile A is determined. The bearing capacity characteristic value in the model test is based on Figure 3 , which can be converted into the characteristic value of the prototype pile bearing capacity N²R, helps to provide a reliable theoretical basis for pile foundation design in the early stage of the project, makes the design scheme more in line with the actual project needs, effectively avoids engineering risks caused by inaccurate bearing capacity estimation, and improves the safety and stability of the project; Another test pile B is selected, and the model pile bearing capacity characteristic value R is used as the pile top holding load to act on the pile top. The load is loaded to R in the centrifuge, and the load is stabilized for T. This simulates the actual existing engineering pile under the continuous action of the load N²R on the pile top for N²T during actual use. This helps to gain a deeper understanding of the durability and long-term stability of engineering piles, and provides strong data support for the maintenance and renovation of existing projects. Then, the load on the test pile B is increased until pile B is destroyed. According to the Qs curve of pile B, the bearing capacity characteristic value of model pile B is determined to be Ra, and the corresponding bearing capacity characteristic value of the prototype pile is N²Ra. Through calculation, the changes in the mechanical properties of the pile body from normal bearing to destruction can be fully understood, providing accurate data for determining the ultimate bearing capacity of the pile foundation, further improving the pile foundation design theory, optimizing the material selection and construction technology of engineering piles, thereby reducing engineering costs and improving the economic and social benefits of engineering construction while ensuring engineering quality. Embodiment 2:

[0018] In this example, an existing building has been in use for 10 years. Now, it is necessary to evaluate the bearing capacity of its pile foundation because the building needs to be renovated with additional floors. Through drilling, it is determined that the upper part of the stratum is an 8m thick fine sand layer, and the lower part is a 22m thick medium sand layer. The prototype pile is a pipe pile with a buried length of 20m and a diameter of 800mm. Now it is necessary to determine the characteristic value of the bearing capacity of the existing engineering pile under 10 years of continuous load. First, the similarity ratio is determined to be 50 according to the centrifuge acceleration of 50g. Aluminum alloy hollow pipes are used to simulate pipe piles. The model pile has an inner diameter of 12mm, an outer diameter of 16mm, an aspect ratio of 25, and a length of 450mm. The buried length is 400mm and the exposed length is 50mm. The exposed section is set to facilitate the application of load on the pile top. The corresponding prototype pile is a pipe pile with a buried length of 20m and a diameter of 800mm. The upper layer of 0-16cm is fine sand and the lower layer of 16-60cm is medium sand to simulate the stratum. The corresponding stratum conditions are the geological conditions of an 8m thick fine sand layer on the upper part and a 22m thick medium sand layer below. Two test model piles are set, labeled A and B respectively; After the centrifuge is started and the load motor is started to run stably until 50g is reached, a load test is carried out on pile A to determine the bearing capacity limit value, and then the load acting on the pile top before the subsequent load test is determined based on this value. Figure 5 The figure shows the test results converted to the prototype. The QS curve of pile A shows a slow-changing characteristic without obvious inflection points, which is consistent with the characteristics of friction piles. According to the Technical Specifications for Testing of Building Foundation Piles (JCJ106-2014)

[18] , the load corresponding to S = 40 mm (settlement 0.05D) is taken as the ultimate value of its bearing capacity, and its characteristic value of bearing capacity is 3300 kN. In order to simulate the load-bearing time of the existing building piles for 10 years, for the condition that the similarity ratio N in the centrifuge test is 50, according to Figure 3 In the model test, the model pile B needs to be run in the indoor centrifugal model for 25 hours, so as to simulate the continuous load time of the pile top of the existing building pile during use for 10 years; After holding the load in the centrifugal model for 25 h, the loading was started again to obtain the QS curve of model pile B until pile B failed. Figure 6As shown, the centrifuge model test was carried out for 25 hours to simulate the pile top load of 10 years. The prototype pile top load was 3300kN, and the corresponding model test load was 1.32kN. The corresponding prototype was the upper structure acting on the pile top load of 3300kN for about 10 years, and then loaded to the single pile failure. The load test curve of model pile B is slowly varying without obvious inflection points. The load of 8560 kN corresponding to S = 40 mm (settlement 0.05D) is taken as its bearing capacity limit value, and the characteristic value of pile bearing capacity is 4280 kN. That is, considering the existing load on the pile top for 10 years, the bearing capacity characteristic value of 3300 kN when the load on the pile top is not considered is increased by about 29.7%.

[0019] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for evaluating the bearing capacity of existing engineering piles under urban renewal, comprising the following steps: drilling - model building - bearing capacity test.

2. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: The drilling is to determine the site stratigraphic division and physical and mechanical parameters through drilling.

3. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: The model building is to build a scaled physical and mechanical model according to the actual geometric dimensions of the pile foundation and the physical and mechanical parameters of the stratum, that is, to make a physical scaled model of the actual pile foundation and the stratum according to the scale similarity ratio N.

4. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: The bearing capacity test is to place the physical model of the pile-soil in a centrifuge testing machine to perform a pile foundation bearing capacity test under the action of the centrifuge. During the test, the acceleration of the centrifuge is set to N. By subjecting the scaled physical model to centrifugal action, the bearing capacity of the pile foundation under actual conditions can be simulated.

5. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: In the centrifugal state, the similarity relationship between the physical model and the prototype can be calculated.

6. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: By conducting a load test on the scaled physical model in a centrifuge under centrifugal state, the QS curve of the pile foundation load test is determined, and the bearing capacity characteristic value R of the model pile A is determined. The bearing capacity characteristic value in the model test can be converted into the prototype pile bearing capacity characteristic value N²R.

7. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1 is characterized by: Another test pile B is selected, and the characteristic value R of the model pile bearing capacity is used as the pile top holding load. The load is loaded to R in a centrifuge and the load is stabilized for T to simulate the actual existing engineering pile under the continuous action of the existing engineering pile top load N²R for N²T during actual use.

8. The method for evaluating the bearing capacity of existing engineering piles in urban renewal according to claim 1, characterized in that: The load on the test pile B is increased until pile B is destroyed. According to the Qs curve of pile B, the characteristic value of the bearing capacity of the model pile B is determined to be Ra, and the corresponding characteristic value of the bearing capacity of the prototype pile is N²Ra.

Images