A pile foundation negative skin friction testing system and method

Abstract

This invention provides a pile foundation negative skin friction testing system and method, belonging to the fields of geotechnical engineering and roadbed engineering. The testing system obtains negative skin friction test data by vertically loading a soil sample and using an electronic scale positioned below a simulated pile, which is simple and convenient. The pile foundation negative skin friction testing system includes: a model box, a vertical loading unit, and a data acquisition unit; the model box is used to hold the simulated pile and the soil sample surrounding it; the vertical loading unit vertically loads the soil sample using a top hydraulic bladder; the data acquisition unit monitors the vertical load changes of the simulated pile using an electronic scale positioned below it.

Description

Technical Field

[0001] This invention relates to a testing system and method, specifically a testing system and method for negative skin friction of pile foundations, belonging to the fields of geotechnical engineering and roadbed engineering. Background Technology

[0002] With the deepening of the Western Development Strategy and the high-quality development of engineering construction in the new era, a new wave of infrastructure construction is emerging in the mountainous areas of western my country. However, high excavation and high embankment projects are common in the construction of airports in mountainous areas, especially in high embankment areas where large-diameter boulders are often used for filling. These filling materials have large particle sizes, high porosity, and low compressibility, but they can still experience significant settlement under long-term loads or environmental factors (such as rainfall and groundwater changes). This settlement generates negative skin friction (i.e., downward frictional force exerted by the surrounding soil on the pile) in the pile foundation, leading to increased additional stress in the pile and even causing a decrease in the bearing capacity of the pile foundation, structural deformation, or failure, seriously affecting the safety of the project. Therefore, it is necessary to design a system capable of testing the negative skin friction of pile foundations. Summary of the Invention

[0003] In view of this, the present invention provides a pile foundation negative skin friction testing system, which can directly obtain negative skin friction test data by vertically loading a soil sample and using an electronic scale set under the simulated pile, which is simple and convenient.

[0004] The technical solution of the present invention is: a pile foundation negative skin friction testing system, comprising: a model box, a vertical loading unit, and a data acquisition unit;

[0005] The model box is used to hold the simulated pile and the soil sample around the simulated pile;

[0006] The vertical loading unit applies vertical loading to the soil sample via a top hydraulic bladder;

[0007] The data acquisition unit monitors the vertical load changes of the simulated pile using an electronic scale positioned below it.

[0008] As a preferred embodiment of the present invention, a bottom hydraulic bladder is also provided at the bottom of the soil sample.

[0009] As a preferred embodiment of the present invention: the model box includes: a top plate, a box body, and a bottom plate;

[0010] The enclosure is a rectangular structure made of steel plates, which is closed on all sides and open at the top and bottom. A top plate is detachably installed at the top opening and a bottom plate is fixed at the bottom opening, thus forming a closed test space.

[0011] As a preferred embodiment of the present invention, the inner wall of the box is provided with stainless steel sheets.

[0012] As a preferred embodiment of the present invention, the lower part of the base plate is provided with rollers having a braking function.

[0013] As a preferred embodiment of the present invention: an electronic weighing scale is provided at the center of the bottom surface of the model box; a simulated pile is vertically arranged above the electronic weighing scale; and soil samples are filled around the simulated pile.

[0014] As a preferred embodiment of the present invention: the simulated pile is a cast-in-place concrete simulated pile; when pouring the simulated pile, a protective casing with open ends is first placed above the electronic scale inside the model box, and then concrete is poured in the protective casing. The protective casing is pulled out simultaneously when pouring the concrete.

[0015] Furthermore, based on the aforementioned testing system, this invention provides a method for testing the negative skin friction of pile foundations:

[0016] First, the test system is set up: a protective casing is placed inside the model box, and soil samples are filled outside the protective casing; before filling the soil samples, the bottom hydraulic bladder is pressurized, and then the soil samples are filled in layers outside the protective casing; after the soil samples are filled, concrete is poured in the protective casing, and the protective casing is pulled out simultaneously when pouring the concrete; finally, a top hydraulic bladder is placed on top of the soil samples.

[0017] During testing, the top hydraulic bladder is activated to load the soil sample according to the set loading rate, loading amount, and loading method, causing the soil sample to settle and displace, generating negative skin friction on the simulated pile; at the same time, the bottom hydraulic bladder is controlled to slowly depressurize; during the loading process, the electronic scale monitors the vertical load of the simulated pile in real time and sends it to the external display and data processing unit.

[0018] Beneficial effects:

[0019] (1) The test system of the present invention can directly obtain test data related to negative skin friction by vertically loading the soil sample and using an electronic scale set under the simulated pile; the test system can not only test the negative skin friction of coarse soil pile foundation, but also test the negative skin friction of pile foundation in fine soil or sand filling area.

[0020] (2) The pressure application method of the test system of the present invention is a hydraulic bladder, which can achieve uniform loading of soil sample, thereby reducing pressure error and obtaining more reliable test data; and the loading rate and loading amount can be precisely adjusted by controlling the top hydraulic bladder.

[0021] (3) In the testing system of the present invention, a bottom hydraulic bladder is set below the soil sample to support the soil sample. The bottom hydraulic bladder slowly releases pressure during the test to ensure that the soil sample around the pile has sufficient settlement and that the negative skin friction is fully utilized, thereby further improving the accuracy of the test.

[0022] (4) The test system of the present invention is equipped with a data transmission interface, which can transmit the test data in the display and data processing unit to the external control unit via wired or wireless means, so as to facilitate remote monitoring and data analysis by the user. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the overall structure of the pile foundation negative skin friction testing system of the present invention;

[0024] Figure 2 for Figure 1 Schematic diagram of section AA;

[0025] Figure 3 for Figure 1 Schematic diagram of the BB section;

[0026] Figure 4 for Figure 1 Schematic diagram of the CC section (each grid represents the vesicle of the bottom hydraulic bladder).

[0027] Among them: 1-top plate, 2-box body, 3-fastener, 4-electronic scale, 5-bottom plate, 6-simulated pile, 7-soil sample, 8-top hydraulic bladder, 9-bottom hydraulic bladder, 10-casing, 11-casing limit ring, 12-stainless steel sheet. Detailed Implementation

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0029] Example 1:

[0030] This embodiment provides a pile foundation negative skin friction testing system. This testing system can test the negative skin friction of pile foundations with coarse soil particles, and it can also be used to test the negative skin friction of pile foundations in areas filled with fine soil or sand.

[0031] like Figures 1-4 As shown, the pile foundation negative skin friction testing system includes: a model box, a vertical loading unit, and a data acquisition unit; wherein the model box includes: a top plate 1, a box body 2, and a bottom plate 5; the vertical loading unit is a top hydraulic bladder 8; and the data acquisition unit is an electronic scale 4.

[0032] The model box is a soil sample container used to hold simulated pile 6 and soil sample 7.

[0033] As an example, the housing 2 is a rectangular structure made of steel plates, sealed on all sides and open at the top and bottom. A top plate 1 is detachably installed at the top opening via fasteners 3, facilitating installation and disassembly. Fasteners 3 can be screws made of high-strength stainless steel with rust-proof treatment to ensure that rust will not affect the sealing and stability of the device during long-term use. A bottom plate 5 is fixed (e.g., welded) to the bottom opening. The top plate 1 is made of steel plate of the same material and thickness as the bottom plate 5, thus forming a closed test space together with the housing 2 and the bottom plate 5. This testing system can test the negative skin friction of coarse-grained pile foundations using a large-sized model box, thereby realistically simulating actual field conditions.

[0034] As an example, one side panel of the box 2 is removable (e.g., by bolting) to facilitate the removal of soil sample 7 from the model box after the test is completed.

[0035] As an example, the inner wall of the housing 2 is provided with an anti-corrosion coating.

[0036] As an example, the inner wall of the enclosure 2 is provided with stainless steel sheets 12 (e.g., bonded with universal steel adhesive), which not only prevents rust but also increases the smoothness of the sides of the enclosure 2 and reduces the influence of the friction of the enclosure 2 on the measurement results.

[0037] As an example, the base plate 5 is made of thickened steel plate with a surface flatness error of no more than ±0.05mm to ensure the verticality and stability of the simulated pile 6 during installation.

[0038] As an example, the base plate 5 is equipped with multiple sets of rollers to facilitate the movement of the model box. Furthermore, the rollers are equipped with braking devices, which can be used to lock the rollers and prevent the model box from moving when it needs to be fixed.

[0039] An electronic weighing scale 4 is installed at the center of the inner bottom surface of the base plate 5 to monitor the vertical load changes of the simulated pile 6 in real time. As an example, the electronic weighing scale 4 has multiple parallel piezoelectric load cells inside its bearing platform. The sensors have an accuracy of 10g. The electronic weighing scale 4 is electrically connected to an external display and data processing unit, which displays its measured values ​​in real time. The electronic weighing scale 4 has high-precision load cells, and its measuring range can meet the measurement requirements of the negative skin friction of the simulated pile 6 under different test conditions, with a measurement accuracy of ±0.1%. The electronic weighing scale 4 needs to be sealed. After further correction and calculation, the negative skin friction of the pile foundation (i.e., the simulated pile 6) can be obtained from the data displayed by the electronic weighing scale 4. Here, the correction refers to the correction factor considering the influence of surrounding friction; the negative skin friction of the pile foundation is the reading of the electronic weighing scale 4 divided by the lateral area of ​​the simulated pile 6 (i.e., the length of the simulated pile 6 multiplied by its perimeter).

[0040] Inside the box 2, a simulated pile 6 is vertically installed above the electronic scale 4. The surface of the simulated pile 6 is roughened to simulate the roughness of the pile foundation surface in actual engineering, thereby improving the consistency between the test results and the actual situation.

[0041] Inside the box 2, the outer perimeter of the simulated pile 6 (i.e., between the outer surface of the simulated pile 6 and the inner surface of the box 2) is filled with soil sample 7 (particle size not exceeding 60mm); the soil sample 7 is standard sand or clay that has been screened and proportioned, and its physical and mechanical properties meet the requirements of relevant test standards. It has also been pre-compacted before being loaded into the box 2 to eliminate the initial porosity inside the soil sample.

[0042] The simulated pile 6 is a cast-in-place concrete simulated pile. When preparing this test system, a stainless steel protective cylinder 10 is first placed above the electronic scale 4 inside the box 2. The protective cylinder 10 is a cylindrical structure with open ends and the same diameter as the electronic scale 4. Then, soil sample 7 is filled between the protective cylinder 10 and the box 2. After the soil sample 7 is filled, concrete is poured in the protective cylinder 10. The protective cylinder 10 is pulled out at the same time when the concrete is poured.

[0043] As an example, a protective sleeve limiting ring 11 is provided above the electronic scale 4 (specifically the outer sealing layer of the electronic scale) to limit the protective sleeve 10. The bottom of the protective sleeve 10 is embedded and positioned in the protective sleeve limiting ring 11 to ensure that the protective sleeve 10 is quickly and accurately positioned and installed.

[0044] Inside chamber 2, hydraulic bladders (specifically oil bladders in this example, connected to the top and bottom of chamber 2 via an oil gauge to transmit pressure data in real time) are placed on top of soil sample 7. The hydraulic bladder at the top of soil sample 7 is designated as the top hydraulic bladder 8, used to apply vertical force to soil sample 7. The top hydraulic bladder 8 is arranged around the simulated pile 6, with the same cross-sectional shape and dimensions as soil sample 7. The top hydraulic bladder 8 is connected to an external hydraulic control unit, which, under its control, applies vertical pressure to soil sample 7. The top hydraulic bladder 8 allows for uniform vertical pressure application to the soil sample; and the hydraulic control unit allows for precise adjustment of the loading rate and amount.

[0045] The hydraulic bladder located at the bottom of soil sample 7 is called bottom hydraulic bladder 9, which is used to support soil sample 7.

[0046] Both the top hydraulic bladder 8 and the bottom hydraulic bladder 9 are made of high-strength rubber material, which has good elasticity and sealing properties, and can withstand a certain pressure without leakage or rupture.

[0047] As an example, a pore water pressure gauge was embedded in soil sample 7 to monitor changes in pore water pressure during the loading process.

[0048] As an example, displacement sensors are installed at the top and bottom of the simulated pile 6 to measure the settlement and displacement of the simulated pile 6.

[0049] As an example, a settlement gauge is vertically embedded in soil sample 7 to measure the settlement of soil sample 7 in real time. The settlement gauge is electrically connected to an external display and data processing unit, and sends the monitoring data to the display and data processing unit.

[0050] All data collected by the aforementioned sensors is transmitted to the display and data processing unit for analysis and processing. The testing system is also equipped with a data transmission interface, enabling the transmission of test data from the display and data processing unit to an external control unit (such as a computer or mobile terminal) via wired or wireless means, facilitating remote monitoring and data analysis by the user.

[0051] This testing system can accurately simulate different engineering geological conditions and produce samples (i.e., soil sample 7) that are similar to actual working conditions. The top hydraulic bladder applies uniform vertical pressure to the soil sample 7. During the sinking process of the soil sample 7, friction is generated on the simulated pile 6. The force on the simulated pile 6 can be measured by the automatic reading electronic scale 4. The display and data processing unit can calculate the negative skin friction of the simulated pile 6 based on the test data of the electronic scale 4. This realizes the real-time recording and display of the negative skin friction data of the pile foundation.

[0052] Example 2:

[0053] Based on the testing system in Embodiment 1 above, this embodiment provides a method for testing the negative skin friction of pile foundations.

[0054] When setting up the testing system, first, a casing 10 is placed inside the housing 2, and soil sample 7 is filled outside the casing 10. Before filling the soil sample 7, the bottom hydraulic bladder 9 is pressurized. Then, the soil sample 7 is filled in layers outside the casing 10, with each layer controlled to a thickness of 0.5m to 1.0m. After each layer is filled, it is compacted using a vibrator until the designed compaction degree is achieved. During each layer filling process, the interlayer bonding needs to be checked to avoid segregation. After the soil sample 7 is filled, concrete is poured inside the casing 10, and the casing 10 is simultaneously pulled out during concrete pouring. Finally, a top hydraulic bladder 8 is placed on top of the soil sample 7.

[0055] Before the experiment, the test system was initialized, including sensor calibration and loading parameter settings; the loading rate, loading amount and loading method (such as graded loading) were determined.

[0056] During the test, the top hydraulic bladder 8 was activated (i.e., pressurized), and a quantitative load was applied to the soil sample 7. The soil sample 7 underwent settlement displacement, generating negative skin friction on the simulated pile 6. During this process, the bottom hydraulic bladder 9 was slowly depressurized to prevent the soil sample 7 from collapsing rapidly under stress, which would affect the measurement results. During the loading process, the display and data acquisition unit displayed the test data of the electronic scale 4 in real time.

[0057] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for testing negative skin friction of pile foundations, characterized in that, First, the test system is set up: the test system includes a model box, a vertical loading unit, and a data acquisition unit; The model box is used to place the simulated pile (6) and the soil sample (7) around the simulated pile (6). The vertical loading unit applies vertical loading to the soil sample (7) via a top hydraulic bladder (8); The data acquisition unit monitors the vertical load change of the simulated pile (6) by using an electronic scale (4) located below the simulated pile (6); an electronic scale (4) is located at the center of the bottom surface of the model box; the simulated pile (6) is vertically installed above the electronic scale (4); and soil samples (7) are filled around the simulated pile (6). The bottom of the soil sample (7) is also provided with a bottom hydraulic bladder (9) for supporting the soil sample (7). When setting up the test system: place a protective casing (10) above the electronic scale (4) inside the model box, and fill the outside of the protective casing (10) with soil sample (7); before filling the soil sample (7), pressurize the bottom hydraulic bladder (9), and then fill the soil sample (7) in layers outside the protective casing (10); after the soil sample (7) is filled, pour concrete in the protective casing (10), and pull out the protective casing (10) at the same time when pouring concrete to form a cast-in-place concrete simulated pile; finally, place a top hydraulic bladder (8) on top of the soil sample (7). During the test, the top hydraulic bladder (8) is activated to load the soil sample (7) according to the set loading rate, loading amount and loading method, so that the soil sample (7) will settle and displace, generating negative skin friction on the simulated pile (6); at the same time, the bottom hydraulic bladder (9) is controlled to slowly depressurize; during the loading process, the electronic scale (4) monitors the vertical load of the simulated pile (6) in real time and sends it to the external display and data processing unit.

2. The method for testing negative skin friction of pile foundations as described in claim 1, characterized in that, The model box includes: a top plate (1), a box body (2), and a bottom plate (5); The box (2) is a rectangular structure with closed sides and open top and bottom, made of steel plates; a top plate (1) is detachably installed at the top opening and a bottom plate (5) is fixed at the bottom opening, thus forming a closed test space.

3. The method for testing negative skin friction of pile foundations as described in claim 2, characterized in that, The inner wall of the box (2) is provided with stainless steel sheet (12).

4. The method for testing negative skin friction of pile foundations as described in claim 2, characterized in that, The bottom plate (5) is provided with a roller with braking function.

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