A mechanical property simulation method for large-diameter ring-shaped foundation piles

Abstract

The present invention discloses a method for simulating the mechanical properties of large-diameter annular foundation piles, including determining the relevant relationships and schemes of model experiments, establishing model piles, preparing the soil materials around the model piles, preparing model boxes, installing experimental equipment and loading, analyzing the results of model experiments, and comparing and analyzing the results. The beneficial effects of the present invention are as follows: By describing the pile settlement, bearing capacity, and compression in three aspects, and referring to the result laws of numerical simulation experiments for guidance, demonstrating from multiple angles and considering comprehensively, it is finally determined that the annular piles, bearing performance, and mechanical properties can all reach the same level as traditional solid piles and can be applied in actual projects. To verify the change of the mechanical properties of annular piles with the thickness-to-diameter ratio K, in the range of the thickness-to-diameter ratio set in this project, the saving range of concrete is 0% - 68%, which fully proves that annular piles can save a large amount of building materials and meet the requirement of reducing construction costs while ensuring that the bearing capacity meets the requirements.

Description

Technical Field

[0001] The present invention relates to a method for simulating the mechanical properties of foundation piles, specifically a method for simulating the mechanical properties of large-diameter ring foundation piles, and belongs to the field of construction technology. Background Technique

[0002] As a very ancient form of building foundation, the application of pile foundations by people has been at least tens of thousands of years. With the development of corresponding materials, the construction technology of pile foundations has also been greatly affected.

[0003] At present, according to different construction methods, they are mainly divided into mechanically constructed piles and manually dug piles. Among them, manually dug piles are mainly developed by learning from ancient well-digging techniques. They can achieve large pile diameters, fast construction speed, convenience and quickness, do not require large construction machinery, have strong seismic performance, and low cost, saving costs. The large-diameter manually dug solid pile is based on the construction of large-diameter solid piles. By setting up molds, a hollow part is formed inside the pile body. Compared with solid piles, ring foundation piles theoretically have multiple advantages: 1) When designing manually dug piles, according to the specifications, generally a certain safety reserve needs to be retained, that is, there is a large gap between the characteristic value of the pile body bearing capacity and the ultimate value of the pile body bearing capacity. This also makes the pile body strength have a relatively rich strength, so the materials cannot be fully utilized. Especially the concrete near the center of the pile body. This part of the concrete does not contact the soil layer and only plays the role of force transmission. If the concrete strength of the pile body permits, removing this part of the concrete, then for the same pile body diameter, the ring pile can save a huge amount of concrete usage on the basis of meeting the bearing capacity requirements and save project funds; 2) Although the pile foundation is a supporting component of the upper building, as the upper load increases, it naturally leads to the need to increase the length and diameter of the lower pile foundation to meet the requirements. This makes the self-weight of the pile foundation increase accordingly, increasing the burden on the bearing capacity of the foundation, especially in areas with soft soil layers. The ring pile can just solve this problem, saving material usage and reducing the structural self-weight.

[0004] Generally speaking, the pile-forming process of the ring pile is to first excavate the pile hole and construct the retaining wall, then form a mold in the pile hole, and finally pour concrete. During the pile-forming process, workers need to enter the hollow part. By observing the inner wall, the pile-forming quality can be grasped more intuitively and accurately, which greatly improves the overall safety of the project. In addition, if necessary, the ring pile can also be used as a water storage well.

[0005] At present, in the research on large-diameter ring piles and traditional large-diameter solid piles at home and abroad, there are still some problems, specifically as follows:

[0006] (1) As a widely used type of foundation, the pile foundation is rarely applied in high-rise buildings within the city. This is mainly because there is still a lack of overall understanding of the ring-shaped pile. At present, more research focuses are on traditional solid piles. There have been a large number of studies on various aspects of traditional solid piles, such as settlement calculation algorithms, deformation laws, bearing capacity prediction, etc. However, research on ring-shaped piles is extremely rare. Most of the existing studies rely on a certain actual project to verify whether the bearing capacity of the ring-shaped pile can meet the requirements or study the bearing capacity of more novel variable cross-section ring-shaped piles;

[0007] (2) As a special type of pile, the ring-shaped pile is characterized by a hollow center in the middle, and only the ring-shaped side wall of the pile body is relied on to bear the upper load. In the actual operation of the ring-shaped pile, the stress conditions, strain magnitudes of various parts of the pile body, or the law of pile body load transfer are still unknown, and there is little complete information in the existing literature to confirm;

[0008] (3) Although there are already engineering examples of successfully using ring-shaped piles as building foundations, there is still no reference standard for the actual design of ring-shaped piles, and it is unknown which factors are related to the size of the hollow range in the middle of the ring-shaped pile.

[0009] In addition, the construction of ring-shaped piles is also an issue that needs to be emphasized. At present, the mainly used is a one-time thin steel plate formwork system, which has a long working time and high costs. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for simulating the mechanical properties of large-diameter ring-shaped foundation piles to solve the problems.

[0011] The present invention realizes the above purpose through the following technical solutions: A method for simulating the mechanical properties of large-diameter ring-shaped foundation piles,

[0012] The large-diameter ring-shaped foundation pile includes:

[0013] A pile body, which is formed by pouring concrete and is cylindrical, and there is a hollow pile core at the center of the pile body;

[0014] A pre-embedded steel reinforcement cage, which is embedded in the concrete of the pile body.

[0015] Its method for simulating mechanical properties includes:

[0016] Step 1: Determine the relevant relationships and schemes of the model experiment.

[0017] Determine the similarity coefficient of the model. The size of the experimental model is proportionally reduced according to the similarity coefficient for the engineering prototype structure. According to the experimental prototype and the similarity coefficient, the model experiment takes the thickness-diameter ratio as the first standard and the concrete saving amount as the second standard. The outer diameter of the model pile is used as a fixed quantity, and the simulation experiment is carried out by changing the inner diameter of the hollow of the ring-shaped pile;

[0018] Step 2: Establish model piles. According to the determined similarity coefficient and experimental scheme, use C30 concrete and steel reinforcement cages to pour model piles with corresponding dimensions.

[0019] Step 3: Prepare the soil materials around the model piles. According to the site geological exploration report of the prototype project, prepare soil materials around the model piles similar to those of the prototype project site.

[0020] Step 4: Prepare the model box. The prepared model box adopts a cylindrical shape with uniform stress on the barrel wall and is not prone to deformation, and add stiffening ribs to prevent stress relaxation.

[0021] Step 5: Install experimental equipment and apply load. Install the equipment for the experiment and the detection device on the prepared simulated piles, apply a load that is always maintained at the corresponding magnitude, complete all static load tests of the model piles, and collect detection data.

[0022] Step 6: Analyze the results of the model experiment. According to the obtained model experiment data, respectively analyze the settlement law of the model piles, the compression law of the pile body of the model piles, the strain law of the pile body of the model piles, and the axial stress law of the pile body of the model piles.

[0023] Step 7: Compare and analyze the results. Expand the force data of the model piles in the model experiment according to the similarity coefficient to obtain the relevant data of the restored actual project, and then conduct a comparative analysis of the pile body settlement and bearing capacity and a comparative analysis of the compressibility of the pile body.

[0024] As a further scheme of the present invention: In the above Step 2, for the steel reinforcement cage part of the model pile, according to the provisions in the "Technical Code for Building Pile Foundations JGJ94 - 2008", HPB300 is used as the steel reinforcement, the reinforcement ratio is 0.65%, and the cross-sectional area of the steel reinforcement of a single model pile is 1276 mm 2 , and accordingly, ribbed round steel with a diameter of 8 mm is selected as the longitudinal reinforcement of the steel reinforcement cage, and ribbed round steel with a diameter of 5 mm is selected for the stirrup part, and it is bundled in a spiral manner, and the spacing of each circle of stirrups is 150 mm.

[0025] As a further scheme of the present invention: In the above Step 3, the soil layer of the soil materials around the model piles is divided into seven different particle size gravel soil layers from top to bottom, and a mixture of coarse sand and small gravel is selected to simulate the gravel layer, and a small amount of water is added to make it slightly moist, which helps to increase the density of the soil during the ramming process.

[0026] As a further scheme of the present invention: In the above Step 4, the model box includes two parts, a box body and a reference beam, where:

[0027] The barrel of the box body is formed by rolling and welding a steel plate with a thickness of 5 mm. The diameter of the cylinder is 1000 mm and the height is 2000 mm. The bottom of the cylinder is a steel plate with a diameter of 990 mm and a thickness of 20 mm, which is welded to the barrel to seal the bottom of the barrel.

[0028] The reference beam is formed by bending and welding a square steel pipe with a thickness of 2 mm. Four connecting rods are welded around the ring and fixed to the barrel by bolts to ensure that the reference beam is fixed.

[0029] As a further solution of the present invention: in the fifth step, the experimental equipment includes a loading device composed of a reaction frame and a hydraulic jack, and a measurement sensor composed of a laser displacement meter and a wire-pulling displacement meter; the acquisition of measurement data is carried out through the "Geotechnical Engineering Structure Displacement Remote Real-Time Monitoring System" independently developed by Beijing University of Technology and the supporting acquisition box for data acquisition of the two displacement meters and the earth pressure gauge; the DH3816N static strain test acquisition system is used for data acquisition of the strain gauge and the top pressure sensor.

[0030] The loading includes:

[0031] ① Place the model box directly below the reaction frame, and first pour 50 cm high of soil. This part of the soil is used as the bearing soil at the pile end, and more small stones and a small amount of water need to be mixed in, and it is tamped with a wooden mallet to improve the degree of consolidation. Place 4 earth pressure gauges horizontally in the middle of the soil surface.

[0032] ② Lift the first solid model pile and evenly press it on the earth pressure gauge, fill the soil around the pile to a position 230 mm away from the pile top, and tamp it.

[0033] ③ Install the reference beam, connect the steel wire and the wire-pulling displacement sensor, and fix it with a magnetic support to ensure that the movable part of the steel wire is parallel to the pile body.

[0034] ④ Place the rubber pad, bearing plate, force transfer column and hydraulic jack. Place 4 hard iron sheets on the bearing plate and extend them directly above the laser displacement sensor.

[0035] ⑤ Connect the wires to the acquisition instrument and the balance sensor.

[0036] ⑥ Apply the load according to the load application table to ensure that the load always maintains the corresponding magnitude, and complete all the static load tests of the model piles according to the above process.

[0037] As a further solution of the present invention: in the sixth step, it specifically includes:

[0038] Analysis of the settlement law of the model pile. Record all the change data within one hour of each stage of loading. On average, each stage of load lasts for one hour and is recorded once per minute. Finally, extract the data within the first 5 minutes before the load change as the experimental results, and correspond the settlement amount with the load one by one to obtain the corresponding Q-s (load-settlement) curve. According to the determination method of the ultimate bearing capacity of a single pile and the settlement curve graph of the model experiment, the starting point of the obvious steep drop section is regarded as the ultimate bearing capacity of the pile body;

[0039] Analysis of the compression law of the model pile body. Remove the singular points and take the average of the data within 30 minutes before the load change to obtain the pile body compression-load curve. Draw a table with the corresponding pile body compression amount and the load at the current stage, and sort according to the size of the compression amount. The thinner the wall thickness, the greater the compression amount;

[0040] Analysis of the strain law of the model pile body. According to the measured concrete strain through the conversion relationship, convert the strain into stress. If it is lower than the strength of the concrete itself, it can be determined that in this case, the pile body strength is satisfied and will not be damaged.

[0041] Analysis of the axial stress law of the model pile body. Measure the pressure transmitted from the upper part according to the 4 earth pressure gauges evenly arranged at the center of the pile bottom to judge the transmission situation of the axial force of the annular pile and the solid pile for the upper load.

[0042] The beneficial effects of the present invention are:

[0043] 1) By describing the three aspects of pile body settlement, bearing capacity and compression amount, and using the result law of the numerical simulation experiment as a reference for guidance, comparing with the actual measurement results of the model experiment, demonstrating from multiple angles and comprehensively considering, it is finally determined that the annular pile, as a special pile type, can achieve the same level of bearing performance and mechanical performance as the traditional solid pile and can be applied in actual projects;

[0044] 2) To verify the change of the mechanical properties of the annular pile with the thickness-diameter ratio K, in the thickness-diameter ratio range set in this technical solution, the concrete saving range is 0% - 68%, which fully proves that the annular pile can save a large amount of building materials and meet the requirement of reducing construction costs while ensuring that the bearing capacity meets the requirements;

[0045] 3) At the same time, due to the special nature of the pile body of the annular pile, the pile forming process requires the construction of a formwork to form a hollow part in the middle, and then pour concrete. After solidification, the formwork can be retained or removed. In this construction step, the engineering personnel can enter the pile hole. During the formwork support and pouring process, they can pay attention to the pile body structure at any time, which can well ensure the project quality and avoid special situations such as waste piles and defective piles. Description of the Drawings

[0046] Figure 1 Schematic diagram of the annular foundation pile structure of the present invention;

[0047] Figure 2 Schematic diagram of the system flow of the present invention;

[0048] Figure 3 Schematic diagram of the pile top load - settlement curve of the second embodiment of the present invention;

[0049] Figure 4 Schematic diagram of the pile bottom load - settlement curve of the second embodiment of the present invention;

[0050] Figure 5 Schematic diagram of the pile body compression curve of the second embodiment of the present invention;

[0051] Figure 6 Schematic diagram of the strain of the No. 1 solid pile body in the second embodiment of the present invention;

[0052] Figure 7 Schematic diagram of the strain of the No. 2 annular pile body in the second embodiment of the present invention;

[0053] Figure 8 Schematic diagram of the strain of the No. 3 annular pile body in the second embodiment of the present invention;

[0054] Figure 9 Schematic diagram of the strain of the No. 4 annular pile body in the second embodiment of the present invention;

[0055] Figure 10 Schematic diagram of the strain of the No. 5 annular pile body in the second embodiment of the present invention;

[0056] Figure 11 Schematic diagram of the pile bottom pressure curve of the second embodiment of the present invention;

[0057] Figure 12 Comparison diagram of the pile top settlement between the numerical simulation results and the model experiment results of the second embodiment of the present invention;

[0058] Figure 13 Comparison diagram of the pile body compression amount between the numerical simulation results and the model experiment results of the pile in the second embodiment of the present invention;

[0059] In the figure: 1. Pile body, 2. Pile core, 3. Embedded steel reinforcement cage. Specific implementation manners

[0060] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0061] Example 1

[0062] As Figures 1 to 2 shown: A method for simulating the mechanical properties of a large-diameter annular foundation pile, wherein the large-diameter annular foundation pile includes:

[0063] A pile body 1, which is formed by pouring concrete and is cylindrical, and a hollow pile core 2 is disposed at the center of the pile body 1;

[0064] A pre-embedded steel reinforcement cage 3, which is embedded in the concrete of the pile body 1.

[0065] Its mechanical property simulation method includes:

[0066] Step 1: Determine the relevant relationships and schemes of the model experiment.

[0067] Determine the similarity coefficient of the model experiment, and scale down the size of the experimental model according to the similarity coefficient for the engineering prototype structure. Based on the experimental prototype (engineering prototype structure) and the similarity coefficient, the model experiment takes the thickness-diameter ratio as the first criterion, the concrete saving amount as the second criterion, and the outer diameter of the model pile as a fixed quantity, and conducts simulation experiments by changing the inner diameter of the hollow of the annular pile.

[0068] Step 2: Build the model pile. According to the determined similarity coefficient and experimental scheme, pour a model pile of corresponding size with C30 concrete and a steel reinforcement cage.

[0069] For the steel reinforcement cage part of the built model pile, according to the provisions in the "Technical Code for Building Pile Foundations JGJ94-2008", HPB300 is used as the reinforcement, the reinforcement ratio is 0.65%, and the cross-sectional area of the steel reinforcement of a single model pile is 1276 mm 2 , and accordingly, ribbed round steel with a diameter of 8 mm is selected as the longitudinal reinforcement of the steel reinforcement cage, and ribbed round steel with a diameter of 5 mm is selected for the stirrup part, and it is bundled in a spiral manner, and the spacing of each circle of stirrups is 150 mm.

[0070] Step 3: Prepare the soil material around the model pile. According to the site geological exploration report of the prototype project, prepare the soil material around the pile similar to that of the prototype project site.

[0071] The prepared soil material around the model pile is divided into seven different particle-size gravel soil layers from top to bottom, and a mixture of coarse sand + small gravel is selected to simulate the gravel layer, and a small amount of water is added to make it slightly moist, which helps to increase the density of the soil during the ramming process.

[0072] Step 4: Prepare the model box. The prepared model box adopts a cylindrical shape with uniform stress on the barrel wall and is not prone to deformation, and stiffening ribs for preventing stress relaxation are added.

[0073] The model box mainly includes two parts: a box body and a reference beam, wherein:

[0074] The barrel body of the box is formed by rolling and welding a steel plate with a thickness of 5 mm. The diameter of the cylinder is 1000 mm and the height is 2000 mm. The bottom of the barrel is a steel plate with a diameter of 990 mm and a thickness of 20 mm, which is welded to the barrel body to seal the bottom of the barrel;

[0075] The reference beam is formed by bending and welding a square steel pipe with a thickness of 2 mm. Four connecting rods are welded around the ring and fixed to the barrel body by bolts to ensure that the reference beam is fixed.

[0076] Step 5: Install and load the experimental equipment. Install the equipment for the experiment and the detection device on the prepared simulated pile, apply a load that is always maintained at the corresponding magnitude, complete all static load tests on the model piles, and collect the detection data;

[0077] The experimental equipment includes a loading device composed of a reaction frame and a hydraulic jack, and a measurement sensor composed of a laser displacement meter and a wire-pulling displacement meter; the acquisition of measurement data is carried out through the "Geotechnical Engineering Structure Displacement Remote Real-Time Monitoring System" independently developed by Beijing University of Technology and the supporting acquisition box for the data acquisition of the two displacement meters and the earth pressure gauge; the DH3816N static strain test acquisition system is used for the data acquisition of the strain gauge and the top pressure sensor.

[0078] The said loading includes:

[0079] ① Place the model box directly below the reaction frame, first pour 50 cm high of soil. This part of the soil serves as the bearing soil at the pile tip, and more small stones and a small amount of water need to be mixed in, and it is tamped with a wooden mallet to improve the degree of consolidation. Place 4 earth pressure gauges horizontally in the middle of the soil surface;

[0080] ② Lift and place the first solid model pile, evenly press it on the earth pressure gauge, fill the soil around the pile to a position 230 mm away from the pile top, and tamp it;

[0081] ③ Install the reference beam, connect the wire and the wire-pulling displacement sensor, and fix it with a magnetic support to ensure that the movable part of the wire is parallel to the pile body;

[0082] ④ Place the rubber pad, bearing plate, force transfer column and hydraulic jack. Place 4 hard iron sheets on the bearing plate and extend them directly above the laser displacement sensor;

[0083] ⑤ Connect the wires to the acquisition instrument and the balance sensor;

[0084] ⑥ Apply the load according to the load application table to ensure that the load is always maintained at the corresponding magnitude, and complete all static load tests on the model piles according to the above process.

[0085] Step 6. Analysis of model experiment results: According to the obtained model experiment data, analyze the settlement law of the model pile, the compression law of the model pile body, the strain law of the model pile body, and the axial stress law of the model pile body respectively.

[0086] In the embodiment of the present invention, in the said Step 6, it specifically includes:

[0087] Analysis of the settlement law of the model pile: Record all the change data within one hour of each stage of loading. On average, each stage of load lasts for one hour and is recorded once per minute. Finally, extract the data within the first 5 minutes before the load change as the experimental results, and correspond the settlement amount with the load one by one, then the corresponding Q-s (load-settlement) curve can be obtained. According to the determination method of the ultimate bearing capacity of a single pile and the settlement curve graph of the model experiment, the starting point of the obvious steep drop section is regarded as the ultimate bearing capacity of the pile body.

[0088] Analysis of the compression law of the model pile body: Remove the singularities from the data within 30 minutes before the load change and take the average value, then the pile body compression-load curve can be obtained. Make a table by corresponding the obtained pile body compression amount with the load at the corresponding stage, and sort according to the size of the compression amount. The thinner the wall thickness, the greater the compression amount.

[0089] Analysis of the strain law of the model pile body: According to the conversion relationship of the measured concrete strain, convert the strain into stress. If it is lower than the strength of the concrete material itself, it can be determined that in this case, the pile body strength is satisfied and will not be damaged.

[0090] Analysis of the axial stress law of the model pile body: Measure the pressure transmitted from the upper part according to the 4 earth pressure gauges evenly arranged at the center of the pile bottom to judge the axial load transfer situation of the annular pile and the solid pile for the upper load.

[0091] Step 7. Comparative analysis of results: Expand the force data of the model pile in the model experiment according to the similarity coefficient, then the relevant data of the restored actual project can be obtained, and further conduct comparative analysis of pile body settlement and bearing capacity and comparative analysis of pile body compressibility.

[0092] Embodiment 2

[0093] As Figures 3 to 13 shown: A method for simulating the mechanical properties of a large-diameter annular foundation pile specifically includes the following steps:

[0094] A) Similarity coefficient of model experiment

[0095] In model experiments, the size of the experimental model needs to be reduced proportionally according to the similarity coefficient for the engineering prototype structure. The similarity coefficient can be used to represent the relevant relationship, that is, the ratio of the same physical quantities between the experimental prototype (p) and the model (m), which is represented by the letter C. The determination of the similarity coefficient needs to consider various factors, such as: the limitations of the experimental site, the accuracy of the experimental instruments, the feasibility of the experiment, and the size effect, etc.

[0096] The engineering project on which this experiment relies is a large-diameter pile with a diameter of 10m and a length of 33m. It belongs to a special pile type with a relatively low usage frequency in general engineering. There is a size effect caused by the large diameter. If the model size is designed relatively small, it may increase the influence of the size effect. The closer the size is to the prototype, the more the actual mechanical laws of the model can be restored. And in existing research, there are few indoor model experiments on large-size annular piles.

[0097] Therefore, in order to restore the working conditions of the actual project as much as possible, the size similarity coefficient of this model experiment is set at 20:1, and the specific weight similarity coefficient is set at 1:1. According to the equilibrium equation, geometric equation, and physical equation, the relationships between other similarity indices can be deduced, as shown in Table 1 specifically.

[0098] Table 1: Similarity coefficients between various physical quantities of the model

[0099]

[0100]

[0101] B) Experimental scheme

[0102] Based on the experimental prototype and the similarity coefficient, in this model experiment, the thickness-diameter ratio is used as the first criterion, the concrete saving amount is used as the second criterion, and the outer diameter of the model pile is used as a fixed quantity. By changing the inner hollow diameter of the annular pile, according to the size similarity coefficient of 20:1, 5 groups of experiments as shown in the table are designed. Among them, the 1st solid pile experimental group is the control group, as shown in Table 2.

[0103] Table 2: Model experiment scheme

[0104]

[0105] According to the similarity coefficient, the pile length of the model pile corresponding to the prototype pile can be obtained as 1670mm. For the convenience of placing the loading device and measuring instruments, an additional 230mm is added to the upper part of the pile. The total length of the model pile is 1900mm, and the part actually buried in the soil and participating in the static load experiment is 1670mm.

[0106] C) Design of model pile

[0107] According to the above similarity coefficients, in order to obtain accurate experimental results and be able to restore to the actual real engineering after the experiment, the pile body materials are selected as the same materials commonly used in existing major projects, that is, the steel cage made of C30 concrete and HPB300 steel bars.

[0108] According to the aforementioned experimental scheme, model piles of corresponding sizes are cast with C30 concrete and steel cages. Among them, for the steel cage part, in accordance with the provisions of the "Technical Code for Building Pile Foundations JGJ94-2008", HPB300 is used as the reinforcement, the reinforcement ratio is 0.65%, and the cross-sectional area of the steel reinforcement of a single model pile is 1276 mm 2 , based on this, ribbed round steel bars with a diameter of 8 mm are selected as the longitudinal bars of the steel cage. The length of a single steel cage is 1900 mm, and 26 steel bars are used. For the stirrup part, ribbed round steel bars with a diameter of 5 mm are selected and bundled in a spiral manner. The spacing of each circle of stirrups is 150 mm, with a total of 13 circles.

[0109] The same steel cage is configured for 5 different types of model piles to ensure controlling variables and excluding other factors that may affect the experimental results.

[0110] D) Soil materials around the model pile

[0111] According to the site geological exploration report of the prototype project, the soil layer can be divided into gravel soil layers with seven different particle sizes from top to bottom. Considering the size effect between the contact surface of the large-diameter ring pile and the soil layer, and at the same time, it is impossible to completely achieve the consolidation degree of the original soil when simulating the soil layer in the model experiment.

[0112] In this experiment, the soil around the pile is selected as a mixture of coarse sand and small gravel to simulate the gravel layer. And a small amount of water is added to make it slightly moist, which helps to increase the density of the soil during the ramming process.

[0113] E) Model box design

[0114] The design of the model box is another important part of this embodiment. Whether the design of the model box is reasonable will affect the accuracy of the experimental results, so special attention needs to be paid.

[0115] First is the shape of the model box. According to the existing research results, most of the model boxes for pile foundation static load model experiments are designed as large cuboids. The similarity coefficient of the experimental model pile is relatively large, the physical model is relatively small, the load applied on the upper part is relatively small, and the soil squeezing phenomenon of the model pile during the experiment can be ignored.

[0116] To reduce the influence of size effect, the model pile has a relatively large size. During the experiment, the extrusion of the pile body on the soil may cause the rectangular model box to be extruded and deformed, resulting in stress relaxation. Moreover, multiple sets of experiments need to be carried out for comparison in this experiment. Therefore, the model box will adopt a cylindrical shape and be equipped with stiffeners. During the experiment, the circular cylinder wall is evenly stressed and not easily deformed. With the protection of the stiffeners, it can ensure that the stress will not easily relax and affect the experimental results.

[0117] Taking into account all the influencing factors of the experiment, the overall model box adopts a cylindrical steel cylinder to simulate the real situation of traditional in-situ static load tests. The model box consists of two parts: the box body and the reference beam.

[0118] The barrel body of the box body part is formed by rolling and welding a 5-mm-thick steel plate. The diameter of the cylinder is 1000 mm and the height is 2000 mm. The bottom of the barrel is a steel plate with a diameter of 990 mm and a thickness of 20 mm, which is welded to the barrel body to seal the bottom of the barrel.

[0119] During the experiment, it is necessary to tamp the soil around the pile and the settlement of the model pile will also cause extrusion on the soil around the pile. To prevent the box body from deforming and damaging the stress field.

[0120] The reference beam is the most important part of the entire model box. All the measuring instruments during the experiment need to be fixed through the reference beam. Secondly, it can also locate the placement position of the model pile to ensure that the pile body is in the exact middle of the box body. The reference beam is mainly formed by bending and welding square steel pipes with a thickness of 2 m. The four "claws" around the ring can be fixed to the barrel body through bolts to ensure that the reference beam remains stationary.

[0121] F) Loading device

[0122] The loading device consists of a reaction frame and a hydraulic jack with a maximum stroke of 100 mm. The maximum pressure that the jack can provide is 100 tons (1000 kN), which is suitable for this experiment.

[0123] G) Measuring sensors

[0124] Laser displacement gauges and wire-pull displacement gauges are used to measure the settlement of the pile top and the pile bottom of the model pile respectively. The accuracy of the laser displacement gauge is 0.1 mm, and the accuracy of the wire-pull displacement gauge is 0.01 mm.

[0125] The pressure sensors used are top pressure sensors and bottom earth pressure gauges, which are used to measure the application of the top load and the conduction of the bottom pressure respectively. The top pressure sensor is of the semiconductor piezoresistive type, which changes the passing electrical signal by changing the resistance of the internal semiconductor pressure sheet to achieve data output. The maximum range is 100 tons (1000 kN) and the accuracy is 0.1 kN. The bottom earth pressure sensor is a vibrating wire sensor, which changes the electrical signal by changing the vibrating wire frequency to achieve data output.

[0126] The strain of the pile body is measured using concrete strain gauges. The length of the strain gauge is 80 mm, the width is 3 mm, the resistance is 119.9 ± 0.1 Ω, and the sensitivity coefficient is 2.08 ± 1%. The 1 / 4-bridge wiring method is adopted.

[0127] H) Data acquisition system

[0128] A total of two sets of data acquisition systems are required for this experiment.

[0129] (1) The "Geotechnical Engineering Structure Displacement Remote Real-time Monitoring System" independently developed by Beijing University of Technology and its supporting acquisition box. This set of systems is mainly responsible for the data acquisition of two types of displacement gauges and earth pressure gauges.

[0130] The acquisition box is equipped with a battery, a signal antenna, and a mobile phone card, and can be connected to the network. By logging in to the corresponding website on the computer side, commands can be remotely sent to the acquisition box to achieve the acquisition of sensor data. Each acquisition box has a total of 8 channel interfaces and can connect 8 sensors to work together simultaneously.

[0131] The data acquisition frequency is at least 1 Hz, and the experimental data collected is stored in the cloud server, which can be classified and stored and retrieved according to time, data points, and sensors.

[0132] (2) DH3816N static strain test acquisition system. This set of acquisition systems is mainly responsible for the acquisition of strain gauges and top pressure sensors. It has a total of 60 channels, and the acquisition frequency can reach 1 Hz.

[0133] I) Load prediction

[0134] In the above-mentioned numerical simulation experiment part, the ultimate bearing capacity of a single large-diameter solid pile with a prototype diameter of 10 m has been calculated to be 389169 kN. According to the similarity coefficient, the predicted ultimate load of the model pile can be obtained as 48 kN.

[0135] When calculating the bearing capacity of the prototype pile, the values of each coefficient are on the conservative side, and only considering the similarity coefficient cannot fully and accurately take into account the actual situation. Therefore, during the load application process of this model experiment, a maximum vertical axial load of 100 kN will be applied to the top of the model pile, as shown in Table 3.

[0136] Table 3: Load application situation

[0137]

[0138] Preparation work

[0139] (1) Stick strain gauges on the pile body according to the standard process. The direction is parallel to the pile body. Select one route in each of the four directions of the southeast, northwest, east, and west of the pile body, and stick 7 strain gauges on each route from top to bottom. A total of 28 strain gauges are stuck on a single pile.

[0140] (2) To ensure the accuracy of the data, four corresponding sensors are arranged for measuring the displacements at the top and bottom of the pile and the soil pressure at the bottom of the pile, and are fixed on the reference beam and the bottom of the pile. Finally, the measured data are averaged. Four protruding bolts are fixed at the bottom of the pile body, and thin steel wire ropes are tied and extended to the top of the pile.

[0141] J) Installation of experimental equipment and loading

[0142] Place the model box directly under the reaction frame. First, pour 50 cm high of soil. This part of the soil serves as the bearing soil at the pile end and needs to be mixed with more small stones and a small amount of water, and then tamped with a wooden mallet to improve the degree of consolidation. Four earth pressure cells are placed horizontally in the middle of the soil layer surface, and small stones are avoided under the lower part of the earth pressure cell, otherwise stress concentration may occur and affect the experimental results.

[0143] Lift the first solid model pile and evenly press it on the earth pressure cells. Fill the soil around the pile to a position 230 mm away from the top of the pile and tamp it.

[0144] Install the reference beam, connect the steel wire and the pull-wire displacement sensor, and fix them with magnetic supports to ensure that the movable part of the steel wire is parallel to the pile body.

[0145] Place rubber pads, bearing plates, load transfer columns and hydraulic jacks. Place 4 hard iron sheets on the bearing plate and extend them directly above the laser displacement sensor.

[0146] Connect the wires to the data acquisition instrument and balance the sensor.

[0147] Apply the load according to the load application table to ensure that the load always maintains the corresponding magnitude.

[0148] Complete the static load test of all model piles according to the above process.

[0149] Analysis of the settlement laws of the top and bottom of the model test pile

[0150] The process of recording the experimental data is to record all the changes within one hour of each stage of loading. On average, each stage of load lasts for one hour and is recorded once per minute. Finally, the data within the first 5 minutes before the load change are extracted as the experimental results, and the settlement amounts are corresponded to the loads one by one, and the corresponding Q-s (load-settlement) curve can be obtained.

[0151] The degree of consolidation of the soil in the indoor model experiment will directly affect the accuracy of the experimental results. In this experiment, the soil of each set of experimental schemes is made to reach the same degree of consolidation as much as possible.

[0152] According to the determination method of the ultimate bearing capacity of a single pile and the settlement curve graph of the model experiment, an obvious steep drop section appears. Therefore, the starting point of the steep drop stage of the curve is regarded as the ultimate bearing capacity of the pile body.

[0153] In this experiment, obvious inflection points appeared in the Q-s curves of the pile tops of the 5 model piles. The positions where the inflection points occurred were basically concentrated in the load range of 40 kN to 55 kN. Within this range, the settlement of the pile body changed from a gentle increase before the inflection point to a steep increase after the inflection point. It can be judged that this inflection point is the ultimate bearing capacity of the model pile.

[0154] Through the pull-wire displacement sensor connected to the bottom of the model pile, the load-settlement curve (Q 底 -s curve) of the pile bottom of the model pile can be obtained. Corresponding to the Q 顶 -s curve, the inflection points of the curve are also concentrated in the load range of 40 kN to 55 kN, and the settlement trend of the pile body changes significantly before and after the inflection point. It can be determined that the inflection point is the ultimate bearing capacity of the pile body, as shown in Table 4.

[0155] Table 4: Comparison of settlement amounts of 5 pile types

[0156]

[0157]

[0158] The data shows that from the start of loading to 20 kN, the settlement of the pile body is very small and there is no obvious change. The settlement amounts of the 5 pile types are all about 0.5 mm. When the load is increased from 20 kN to 40 kN, the settlement amount begins to increase gradually. An inflection point appears between 40 kN and 55 kN, and it is considered that the normal working state of the pile foundation has been damaged at this time. When the upper load application amount is continued to be increased, the settlement amount after the inflection point begins to increase significantly. When the maximum load of 100 kN is reached, the maximum settlement amount of the pile body can reach 10 mm.

[0159] Whether it is the pile top or the pile bottom, for the measured Q-s curves, the inflection points all appear in the same interval, that is, within the range of 40 kN to 55 kN. Considering the non-controllable non-human factors during the experiment, it can be determined that when the bottom is sealed, the pile body diameter is the same, and it does not exceed the self-strength of the pile body material, and only judged from the perspective of the ultimate bearing capacity, changing the wall thickness of the pile body (changing the size of the middle hollow) has no obvious impact on it, that is, the change of the thickness-diameter ratio K has no obvious impact on the ultimate bearing capacity of the pile body.

[0160] Excluding the influence of the pile top settlement of the solid pile as a singularity, the settlements of the model piles are all within the range of 1 mm ± 0.2 mm corresponding to the ultimate bearing capacity. Comparing the load-settlement amounts of the 5 pile types at the inflection point, the comprehensive performance of No. 3 and No. 5 is better, and the settlement amount is 0.1 mm less than that of other pile types.

[0161] According to the similarity principle of similar model experiments, based on the size similarity ratio C L = 20. Return the experimental results to the prototype engineering pile. The ultimate bearing capacity of the pile shaft of the engineering prototype is approximately in the range of 320,000 kN to 440,000 kN, and the settlement is in the range of 20 mm ± 2 mm. Return the comparative advantages of the best-performing No. 3 and No. 5 piles to the prototype pile as well, and the difference will be about 2 mm. That is, there is an error of about 8%, which can be ignored compared with actual construction projects.

[0162] Through model experiments and the analysis of settlement, for different types of annular piles and solid piles, from the perspective of pile shaft settlement, there is not much difference between solid piles and piles with different thickness-to-diameter ratios (different wall thicknesses). When returning from model piles to prototype piles, the differences generated can be ignored. That is, it is considered that the settlement amounts during normal operation of annular piles and solid piles are the same within a reasonable range.

[0163] K) Analysis of the law of pile shaft compression of model experiments

[0164] Generally, the total settlement of a single pile is jointly composed of the compression of the soil at the pile tip and the compression of the pile shaft. Due to its own structure and material strength, traditional solid piles generally hardly undergo large self-compression of the pile shaft. However, the inner wall of the annular pile is in a free surface. If applied to actual projects, the outside of the pile shaft will also receive lateral extrusion from the soil around the pile. Therefore, when evaluating the mechanical properties of annular piles, the compression of the pile shaft is also one of the factors that need to be considered.

[0165] Remove the singularities and take the average of the data within 30 minutes before the load change, and the pile shaft compression-load curve can be obtained. It can be clearly seen from the figure that the influence of the pile shaft wall thickness on the pile shaft compression is relatively obvious. Generally, the thinner the wall thickness, the greater the compression.

[0166] When the load is initially applied and reaches 10 kN, only the compression of the No. 4 annular pile is slightly larger, and the compressions of the other 4 piles are relatively small, only within the range of 0.1 mm. As the load increases to 30 kN, the compressions of different pile types all have slight changes, but the change of the No. 2 pile is obvious, reaching 0.4 mm.

[0167] When the load increases to 40 kN, that is, within the range determined as the inflection point, the compressions of all 5 pile types increase significantly.

[0168] According to the aforementioned data collection method, draw a table with the pile shaft compression corresponding to the load at that stage, and sort it according to the size of the compression, and draw a table as shown in Table 5.

[0169] Table 5: Comparison of compressions of 5 pile types

[0170]

[0171] If the possibility of singularities in the No. 2 annular pile is excluded, it can be determined that within the inflection point range, as the thickness-diameter ratio decreases, the pile body compression gradually increases, but all are less than 0.8 mm. As the load continues to increase, at the next level of 60 kN of the inflection point load, except for the No. 4 annular pile, other pile types reach the peak value of the pile body compression. Continuing to increase, due to the combined support of the bearing soil at the pile bottom and the soil around the pile body reaching the limit, after exceeding the ultimate bearing capacity of the pile, punching failure will occur at the pile tip, the pile tip soil will be rapidly compressed and squeezed outwards, and then a certain amount of stress will be released. Therefore, the axial force of the pile body is released to a certain extent at this stage.

[0172] During the experiment, the loading process of this level of load can also obtain direct feedback. When the load before the inflection point increases, it can quickly increase to the specified load. However, when the load after the inflection point increases, the pressure sensor shows that the pressure has been decreasing, and within a certain period of time, the top load can never reach the specified value, and pressure needs to be continuously supplied. Therefore, the pile body compression will decrease here, and then the pile body compression will rise slightly.

[0173] L) Analysis of the strain law of the model test pile body

[0174] The strain of the pile body is also the most concerned experimental factor in this experiment. Under the same load, different-sized stress-bearing structures will inevitably produce different degrees of strain. Paying attention to the magnitude and law of this strain has an important impact on using the thickness-diameter ratio K to judge the quality of the pile body.

[0175] In this experiment, all measured strains are microstrains - με.

[0176] Compared with the annular pile, the overall strain level of the solid pile is less than that of the annular pile. After receiving the pressure from the upper load, the strain at the pile head part increases slightly, and then in the section from the pile head to the middle position of the pile body, as the depth increases, the overall change law of the strain is decreasing. Although there will be a small increase during the process, when it reaches the middle position of the pile body, the strain will reach the minimum value, and then the strain begins to increase. The closer to the bottom of the pile body, the greater the strain, but at the pile bottom, the strain has a small retraction.

[0177] In this experiment, the strain gauges are pasted on the surface of the pile body, and the strain gauge at the pile bottom is close to the edge. The bearing soil at the pile bottom is relatively low-strength sand, which can be regarded as having a soft cushion layer. In addition, the pile bottom is a sealed plane, and the pressure is mostly borne in the middle of the pile bottom, which results in a relatively weak stress concentration phenomenon at the edge, so the strain will decrease.

[0178] The strain development law of annular piles with 4 different wall thicknesses has certain similarities with that of solid piles. The strain at the pile top is slightly larger and there is an obvious pattern, that is, the smaller the thickness-diameter ratio K, the larger the strain at the pile tip. Although the pile top strains U of piles No. 1, No. 2, No. 3 and No. 4 are different, the numerical differences are small. However, the pile top strain of the No. 5 annular pile is significantly different from the others, being 5 times larger than the strains of the other 4 piles under the same load.

[0179] At a position some distance below the pile top, 0.75 m from the pile top, which is the middle position of the section from the pile top to the middle part of the pile body, there are significant numerical differences in the strain growth with different thickness-diameter ratios K: The change of the No. 2 annular pile (K = 0.6) is basically not much different from that of the solid pile, with a small decrease or increase; The strains of the No. 3 annular pile (K = 0.4) and the No. 5 annular pile (K = 0.2) will have an obvious surge at this point, but the numerical value of the strain surge of the No. 5 pile is about 10 times that of other pile types.

[0180] When reaching the middle part of the pile body, the strains of the 4 annular piles all decrease to varying degrees.

[0181] When crossing the middle position, like the solid pile, the strains of the 4 annular piles increase with the increase of depth. When reaching the position 0.4 m from the pile bottom, the strain begins to decrease and will not increase anymore until the pile bottom position.

[0182] When manufacturing the annular pile, there is a 30-cm-thick bottom seal at the bottom of the pile body, making the bottom of the pile body can be regarded as a small part of the solid pile. The inner wall of the pile body becomes thicker, and the strain is naturally smaller than that of the upper hollow part.

[0183] In addition, by observing the position points of the strain peak values on the pile body strain curve, it can be found that with the decrease of the thickness-diameter ratio K, the position of the strain peak value, that is, the position where the pile body strain is the largest, is gradually moving up. The pile body depths corresponding to the peak strain values are tabulated as shown in Table VI.

[0184] Table VI: Position and magnitude of the peak strain of the pile body

[0185]

[0186] When the pile body settlement and bearing capacity meet the requirements, it is still necessary to evaluate the material strength of the pile itself, which is also an important purpose of measuring the strain in this experiment. The relevant parameters of C30 concrete are shown in Table VII.

[0187] Table VII: Relevant parameters of C30 concrete

[0188]

[0189] According to the measured concrete strain, through the conversion relationship, the strain is converted into stress. If it is lower than the strength of the concrete material itself, it can be determined that in this case, the pile body strength is satisfied and will not be damaged. Calculate the stress at the peak strain of the annular pile pile body according to the formula and list it, as shown in Table VIII.

[0190] Table VIII: Stress corresponding to the peak strain of the pile body

[0191]

[0192] Through calculation, the peak stresses at the most unfavorable positions of the 5 pile types are all much smaller than the strength of the pile material itself, and the pile body will not be damaged by compression. According to the similarity theory and the stress-strain similarity coefficient, restoring to the actual project, the pile body will not be damaged by extreme compression due to insufficient pile material strength. Therefore, it can be determined that for annular piles and solid piles with the same pile body diameter, the same pile body length, different hollow volumes, and different thickness-to-diameter ratios, within the same formation environment and the range of the ultimate bearing capacity of the pile body, they are equally safe.

[0193] If the error influence during the experiment of the 2nd annular pile experimental group is excluded, comparing with the other 4 piles, as the thickness-to-diameter ratio K decreases, the peak stress of the pile body increases significantly. And when the thickness-to-diameter ratio K decreases from 0.4 to 0.28, the stress increase is huge, and the increase amount can reach 300%. When the thickness-to-diameter ratio is within 0.4, the stress fluctuation of the pile body is small.

[0194] Analysis of the law of pile bottom pressure of model test piles

[0195] Four earth pressure gauges evenly arranged at the center of the pile bottom measure the pressure transmitted from the upper part, which can be used to judge the axial load transfer situation of the annular pile and the solid pile to the upper load.

[0196] The earth pressure gauges at the pile bottom were all balanced and zeroed before the loading started, so the measured result is the actual force conducted by the pile body.

[0197] The bottom pressures of the 5 pile types increase linearly with the application of the top load, and the values are basically the same as the actual applied values of the upper load, indicating that during the experiment, the upward supporting force provided by the side friction of the pile is not obvious. The differences in the pile bottom earth pressures measured for the 5 piles under the same load are not large. At the same time, considering that the only difference in each experimental group is the different central hollows, the outer diameters and the actual contact areas with the surrounding soil are the same. Therefore, it can be considered that in actual work, the force conduction of the pile body of the annular pile and the solid pile is the same, and for large-diameter annular piles and large-diameter solid piles, the main bearing capacity is the pile tip bearing capacity.

[0198] Comparative analysis of pile body settlement and bearing capacity

[0199] The data of the pile top settlement in the model experiment can be enlarged by a similarity coefficient of 20 to obtain the relevant data of the restored actual project. Plot the results obtained from the numerical simulation against these results.

[0200] As mentioned above, in the actual working state of the pile foundation, the pile top is in direct contact with the upper load. Therefore, when describing the settlement law of the pile foundation, the focus is usually on the pile top settlement. In the results of the numerical simulation experiment, the load-settlement curve shows a slow descent trend, and the ultimate loads of the solid pile and the ring pile are concentrated in the range of 280 MN to 367 MN. According to the curves obtained from the indoor model experiment, there are obvious "inflection points", and the ultimate loads of the pile bodies of the five pile types are roughly concentrated in the range of 320 MN to 440 MN.

[0201] The numerical simulation experiment is completed by computer software, which can fully ensure the stability of controlling variables, and the output data results are stable. However, from the perspective of the uncertainty factors in the model experiment, after each group of experiments is completed, it needs to be refilled. Only within the allowable range of capabilities can the experimental environment of each group be ensured to be the same as much as possible, which may lead to deviations between the experimental results and the expected results.

[0202] Comparing the data result curves of the two experiments comprehensively, the result curve of the numerical simulation is on the left, and the result curve of the model experiment is on the right. Although the overall data results of the model experiment are 40 MN to 80 MN larger than those of the numerical simulation, the order of magnitude of the data is the same, and the error is between 14% and 20%, which is within the acceptable range. As mentioned above, in engineering design, to ensure that the pile body has sufficient safety reserves, the ultimate bearing capacity of the pile body needs to be divided by the safety factor of 2 to obtain the bearing capacity characteristic value of the pile body, and this is used as the design basis. Under the regulations, the difference between the ring pile and the traditional solid pile will be further reduced. Therefore, it can be determined that the ring pile has sufficient safety.

[0203] According to the curve graph drawn based on the numerical simulation results, it is not difficult to see that the ultimate bearing capacity of the large-diameter traditional solid pile is the largest, and the ultimate bearing capacity of the ring pile will decrease as the wall thickness becomes thinner, that is, as the wall thickness T decreases and the thickness-diameter ratio K decreases, the ultimate bearing capacity of the pile body will decrease. In the pile top settlement-load curve graph of the indoor model experiment, excluding the curve of the solid pile with errors caused by experimental factors, the curves of the remaining ring piles also show a similar pattern - as K decreases, the bearing capacity decreases. However, within the range of the thickness-diameter ratio K = 1 to 0.2 set in this project, regardless of which experiment, the obtained pile body bearing capacity results are concentrated in a relatively small range. Therefore, it can be shown that within the range of the thickness-diameter ratio K = 1 to 0.2, the bearing capacity of the ring pile can fully reach the same level as that of the solid pile.

[0204] In addition, before conducting all the experiments, the prototype test pile relied on in this project obtained through the current "Pile Foundation Code" had an estimated ultimate pile shaft load of 389 MN. The calculation method specified in the code was for traditional solid piles, and there were multiple range parameters determined based on engineering experience in the calculation formula. To obtain more accurate and conservative experimental conclusions, the values of the parameters were all taken as moderately large values within the given ranges. Comparing the model test results, due to the influence of uncertain factors in the model test, it was inevitable that the degree of consolidation of the real stratum could not be restored, resulting in a certain reduction in the restored bearing capacity. In this case, the experimental results were still larger than the values obtained from the code, which further indicated that within the range of the thickness-diameter ratio K = 1 - 0.2 specified in this project, the bearing capacity of the annular pile could fully reach the same level as that of the solid pile.

[0205] Comparative analysis of pile shaft compressibility

[0206] The pile shaft settlement generally consists of the compression of the bearing soil at the pile tip and the compression of the pile body itself. However, for traditional solid piles widely used in engineering, the pile shaft compression is very small and generally not considered in the settlement calculation. For annular piles, due to their special pile body structure, the pile shaft compression may increase, so it needs to be added to the safety evaluation.

[0207] Draw a comparative chart of pile shaft compression based on the numerical simulation experiment and model experiment results.

[0208] The data results corresponding to the curve graph of the numerical simulation results were directly extracted and processed. The results of the model experiment were obtained by magnifying the real experiment results according to a similarity ratio of 20.

[0209] The most intuitive result presented by the two experiments is that under the same load, the pile shaft compression amount will increase with the decrease of the wall thickness T and the thickness-diameter ratio K. The pile shaft compression of the solid pile is the smallest, and that of the annular pile is larger. Moreover, with the increase of the load, the difference in the compression amount becomes gradually obvious. In the model results, although there are individual data as singularities, the overall development trend still conforms to the results of the numerical simulation experiment.

[0210] Corresponding to the numerical simulation results, the pile shaft compression amount corresponding to the ultimate bearing capacity of the pile shaft is between 3 mm and 5 mm. Corresponding to the model experiment results, the pile shaft compression amount corresponding to the ultimate bearing capacity of the pile shaft is between 2 mm and 14 mm. The pile shaft compression amounts of each pile type under the ultimate load are plotted in a table, as shown in Table 9.

[0211] Table 9: Comparison of pile shaft compression amounts of two test piles

[0212]

[0213]

[0214] As shown in the table, under the same working conditions, the pile body compression of the annular pile is significantly increased compared with the traditional solid pile, which also fully explains why the bearing capacity of the annular pile is slightly reduced.

[0215] The increase in the compression of the annular pile will bring about the growth of stress and strain. However, through comparison, under the action of the ultimate bearing capacity of the pile body, the pile body stress at the most unfavorable position of the No. 5 annular pile (K = 0.2) with the most unfavorable thickness-diameter ratio is still less than its own material strength (concrete with C30 grade).

[0216] Therefore, it can be determined that within the range of thickness-diameter ratio = 1 - 0.2 set in this study, the safety and bearing performance of all pile types meet the requirements. And in actual engineering, to ensure safety reserves, when the characteristic value of the pile body bearing capacity is used as the actual design standard, the load intensity on the upper part of the pile generally will not reach the ultimate bearing capacity. Therefore, it can be further determined that the safety and strength of the annular pile and the traditional solid pile are almost the same, and it can achieve the purpose of replacing the traditional solid pile and being applied in actual engineering.

[0217] Working principle: Conduct experiments on a model that is scaled down proportionally. Apply loads in proportion on the model made of appropriate proportions and similar materials, so that the model can reproduce the actual working state of the engineering structure to the greatest extent. In order to obtain relevant data and phenomena. And restore the model results obtained through the similarity theory to the actual engineering structure.

[0218] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed claims.

[0219] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for simulating the mechanical properties of a large-diameter ring-shaped foundation pile, characterized in that: The large-diameter ring-shaped foundation pile includes: A pile body (1), which is cast from concrete and is cylindrical. There is a hollow pile core (2) at the center of the pile body (1); A pre-embedded steel reinforcement cage (3), which is embedded in the concrete of the pile body (1); Its method for simulating mechanical properties includes: Step 1: Determine the relevant relationships and schemes of the model experiment Determine the similarity coefficient of the model experiment, and scale down the size of the experimental model according to the similarity coefficient for the engineering prototype structure. Among them, the model experiment takes the thickness-diameter ratio as the first standard, the concrete saving amount as the second standard, the outer diameter of the model pile as a fixed quantity, and the hollow inner diameter of the model pile is changed to conduct the simulation experiment; Step 2: Build a model pile. According to the determined similarity coefficient and experimental scheme, use C30 concrete and a steel reinforcement cage to pour a model pile of corresponding size; Step 3: Prepare the soil material around the model pile. According to the site geological exploration report of the prototype project, prepare the soil material around the model pile similar to the prototype project site; Step 4: Prepare a model box. The prepared model box adopts a cylindrical shape with uniform stress on the barrel wall and not easily deformed, and add stiffening ribs to prevent stress relaxation; Step 5: Install the experimental equipment and apply load. Install the equipment for the experiment and the detection device on the prepared simulation pile, apply a load that is always maintained at the corresponding magnitude, complete all static load tests of the model pile, and collect the detection data; Step 6: Analyze the results of the model experiment. According to the obtained model experiment data, analyze the settlement law of the model pile, the compression law of the model pile body, the strain law of the model pile body, and the axial stress law of the model pile body for the model pile respectively; Step 7: Compare and analyze the results. Expand the stress data of the model pile in the model experiment according to the similarity coefficient to obtain the relevant data of the restored actual project, and then conduct a comparison and analysis of the pile body settlement and bearing capacity and a comparison and analysis of the pile body compressibility.

2. The mechanical property simulation method of a large-diameter annular foundation pile according to claim 1, wherein: In the second step, for the steel cage part of the model pile, in accordance with the provisions of the "Technical Code for Building Pile Foundations JGJ94-2008", HPB300 is used as the reinforcement, the reinforcement ratio is 0.65%, and the cross-sectional area of the steel reinforcement of a single model pile is 1276mm 2 , accordingly, ribbed round steel with a diameter of 8mm is selected as the longitudinal reinforcement of the steel cage, and ribbed round steel with a diameter of 5mm is selected for the stirrup part. The spiral binding method is adopted, and the spacing of each stirrup is 150mm.

3. A method for simulating the mechanical properties of a large-diameter ring-shaped foundation pile according to claim 1, characterized in that: In the said Step 3, the soil layer of the soil material around the model pile is divided into seven gravel soil layers with different particle sizes from top to bottom, and a mixture of coarse sand + small gravel is selected to simulate the gravel layer, and water is added to make it moist.

4. A method for simulating the mechanical properties of a large-diameter annular foundation pile according to claim 1, characterized in that: In the said Step 4, the model box includes a box body and a reference beam, where: The barrel body of the box body is rolled and welded by a 5-mm-thick steel plate. The diameter of the cylinder is 1000 mm and the height is 2000 mm. The bottom of the barrel is a 990-mm-diameter and 20-mm-thick steel plate, which is welded to the barrel body to seal the bottom of the barrel; The reference beam is bent and welded by a square steel pipe with a thickness of 2 mm. Four connecting rods are welded around the ring and fixed to the barrel body by bolts to ensure that the reference beam is fixed.

5. A mechanical property simulation method for a large-diameter annular foundation pile according to claim 1, characterized in that: In the said Step 5, The experimental equipment includes a loading device composed of a reaction frame and an oil jack and a measurement sensor composed of a laser displacement meter and a wire-pulling displacement meter; The acquisition of measurement data is carried out through the "Geotechnical Engineering Structure Position Remote Real-Time Monitoring System" and the supporting acquisition box for the data acquisition of the two displacement meters and the earth pressure meter; The DH3816N static strain test acquisition system is used for the data acquisition of the strain gauge and the top pressure sensor; The said loading includes: ①Place the model box directly below the reaction frame. First, pour 50 cm high of soil. This part of the soil serves as the bearing soil at the pile tip and needs to be mixed with small stones and water, and then tamped with a wooden mallet to improve the degree of consolidation. Place 4 earth pressure gauges horizontally in the middle of the soil layer surface; ②Lift and place the first solid model pile, evenly press it on the earth pressure gauges, fill the soil around the pile to a position 230 mm away from the pile top, and tamp it; ③Install the reference beam, connect the steel wire and the pull-wire displacement sensor, and fix it with a magnetic support to ensure that the movable part of the steel wire is parallel to the pile body; ④Place the rubber pad, bearing plate, load transfer column and hydraulic jack. Place 4 hard iron sheets on the bearing plate and extend it directly above the laser displacement sensor; ⑤Connect the wires to the acquisition instrument and the balance sensor; ⑥Apply the load according to the load application table, ensure that the load always maintains the corresponding magnitude, and complete all static load tests of the model piles according to the above process.

6. A method for simulating the mechanical properties of a large-diameter annular foundation pile according to claim 1, characterized in that: In the sixth step above, it specifically includes: Analysis of the settlement law of the model pile. Record all the change data within one hour of each stage of loading. On average, each stage of load lasts for one hour and is recorded once per minute. Finally, extract the data within 5 minutes before the load change as the experimental results, and correspond the settlement amount with the load one by one to obtain the corresponding load-settlement curve. According to the determination method of the ultimate bearing capacity of a single pile and the settlement curve graph of the model experiment, the starting point of the obvious steep drop section is regarded as the ultimate bearing capacity of the pile body; Analysis of the compression law of the model pile body. Remove the singular points from the data before the load change and take the average value to obtain the pile body compression-load curve. Draw a table with the pile body compression amount corresponding to the load at that stage and sort it according to the magnitude of the compression amount. The thinner the wall thickness, the greater the compression amount; Analysis of the strain law of the model pile body. According to the measured concrete strain through the conversion relationship, convert the strain into stress. If it is lower than the strength of the concrete material itself, it can be determined that in this case, the pile body strength is satisfied and will not be damaged; Analysis of the axial stress law of the model pile body. Measure the pressure transmitted from the upper part according to the 4 earth pressure gauges evenly arranged at the center of the pile bottom to judge the transmission situation of the annular pile and the solid pile axially for the upper load.

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