A pile freezing force testing device

Abstract

The present invention discloses a pile body frozen pullout force testing device, including a model box, a water tank, a cold bath box, an insulation box, a sensor reserved hole, a water permeable baffle, a water replenishment bottle, a dynamometer, a test pile and a data collector, wherein the inner side of the model box is marked with scales and the sensor hole is reserved on the side, the water permeable baffle is fixedly installed inside the model box, the water tank is arranged at the bottom of the model box, the dynamometer is installed on the dynamometer connection base, the test pile is fixedly installed on the dynamometer, a water thermal sensor is installed in the sensor reserved hole, the cold bath box is fixedly installed at the bottom of the model box, the water replenishment bottle is connected to the water tank through a pipeline, and the dynamometer and the water thermal sensor are respectively connected to the data collector through wires. The test device is simple to make and has a clear principle. Compared with the traditional strain gauge, the principle of using a dynamometer to measure the frozen pullout force of the pile body is more reliable and the result is more accurate.

Description

Technical field:

[0001] The invention belongs to the field of indoor model testing of frozen soil mechanics, and in particular relates to a pile body freezing pullout force testing device. Background technology:

[0002] Frost pullout mainly occurs in shallow, light-weight columnar foundations or structures in cold regions, such as utility poles and shallow, thin pile foundations of solar panels in photovoltaic power generation. However, for pile foundations with great burial depth and heavy weight, frost pullout force will also act as an additional force to destroy the overall structural integrity of the pile foundation, bringing potential risks to the project.

[0003] In cold-region structures, the pile foundation is in direct contact with the soil. When the air temperature reaches negative temperatures, the upper soil around the pile reaches the freezing temperature first, begins to freeze, and expands in volume. Under the action of the negative temperature gradient, the unfrozen water in the lower soil will migrate and accumulate through the pores in the soil toward the freezing front. During the continuous migration of the unfrozen water, the frost pullout force on the pile continues to increase, and the pile body appears to be pulled up, tilted, or even broken, causing the pile foundation to lose stability. For example, the uprooting of power poles in the Qinghai-Tibet region and permafrost areas in Northeast China, the uprooting of solar panels for photovoltaic power generation in high-latitude and high-altitude areas, the uprooting of pile foundations of small bridges, and the uprooting of the foundation of lakeshore structures are all caused by frost pullout. Frost pullout of piles or poles is the main frost damage problem faced by cold-region power, cold-region road and cold-region energy projects. When designing pile foundations in cold regions, the mechanism and magnitude of frost pullout force should be fully understood. The premise of these works is to accurately measure the frost pullout force of piles in projects and tests. Therefore, accurate measurement of frost pullout force in projects and tests is an important indicator for achieving frost pullout prevention and control, and an important basis for guiding the design and calculation of frost pullout measures.

[0004] In the indoor model test of the freeze-pullout force of piles, the commonly used methods are strain gauge method, reaction beam method and pull-out method.

[0005] The strain gauge method is to stick strain gauges on the pile body, and obtain the tangential frost heave force on the pile body through the stress values ​​corresponding to two adjacent strain gauges on the pile body. However, this test method has the following disadvantages: the strain gauge is greatly affected by temperature, and temperature compensation needs to be considered; the strain gauge can only measure the strain of a single point, but cannot perform global measurement, resulting in a large error when converted into the frost pullout force on the pile body as a whole; in addition, the test method of sticking strain gauges on the pile body is limited by the shape of the pile body. For example, special-shaped piles such as spiral piles and bamboo piles cannot be stuck with strain gauges.

[0006] The reaction beam method is to place a dynamometer on the top of the test pile body, and place a crossbeam on the top as a reaction frame, and use the dynamometer on the reaction frame to test the frozen pullout force at the top of the pile body during frost heave. This test method has the following disadvantages: if the reaction beam bracket is installed in the model box, the contact with the soil during the test will also produce frozen pullout, and it is impossible to accurately indicate that the measured frozen pullout force is the frozen pullout force of the pile body; if the reaction beam is installed outside the model box, the vertical height of the test model will be increased, occupying more space; in addition, if the pile body tilts under the action of the frozen pullout force, the frozen pullout force tested on the top of the pile body is not the vertical frozen pullout force that the pile body is actually subjected to.

[0007] The pull-out method is to pull out or press the pile in the frozen soil at negative temperature to test the force required for the pile to separate from the frozen soil. This method has the following disadvantages: pulling out or pressing the pile in the frozen soil destroys the bonding force of the ice-soil interface, which is not the freeze-pull force in the strict sense; the freeze-pull force is a quantity that changes continuously with time and is also affected by factors such as water content and temperature. The pull-out method destroys the integrity of the pile-soil, and it is impossible to test the change of the freeze-pull force during the water replenishment and temperature change process. Moreover, only a single shear force limit value of the pile-soil contact surface can be obtained in one test.

[0008] Therefore, it is particularly necessary to develop a frost pullout force testing device and testing method with a reliable testing principle, adaptable to various pile types, meeting water replenishment and temperature control conditions during the test, able to simulate actual working conditions, and able to detect the change law of frost pullout force in real time. Summary of the invention:

[0009] The technical problem to be solved by the present invention is: in view of the shortcomings of the above-mentioned original technical testing principle being unreliable and the existing technology testing frost pull-out force being inaccurate, a pile frost pull-out force testing device and testing method are proposed, which are to achieve reliable testing principles, be adaptable to a variety of pile types, meet water replenishment and temperature control conditions during the test, simulate actual working conditions, and monitor the changes in frost pull-out force in real time.

[0010] In order to solve the above technical problems, the present invention is achieved through the following technical solutions: a pile body frozen pullout force testing device, including a model box, a water tank, a cold bath box, an insulating box, a sensor reserved hole, a permeable baffle, a water replenishment bottle, a dynamometer, a test pile and a data collector, the inner side of the model box is marked with scales and the sensor hole is reserved on the side, the permeable baffle is fixedly installed inside the model box, the water tank is arranged at the bottom of the model box, and the permeable baffle is located on the upper part of the water tank, the insulating box is covered and arranged on the outside of the model box, the permeable baffle is provided with mesh holes, the permeable baffle is welded with a dynamometer connecting base, the dynamometer is installed on the dynamometer connecting base, the test pile is fixedly installed on the dynamometer, a hydrothermal sensor is installed in the sensor reserved hole, the cold bath box is fixedly installed at the bottom of the model box, the water replenishment bottle is connected to the water tank through a pipeline, and the dynamometer and the hydrothermal sensor are respectively connected to the data collector through wires.

[0011] Preferably, the water replenishing bottle is a Marriott bottle, and a scale is marked on the water replenishing bottle.

[0012] Preferably, the dynamometer and the dynamometer connecting base are installed and fixed by means of threaded connection.

[0013] Preferably, a casing is sleeved and installed on the dynamometer, and the lower part of the casing is welded and fixedly installed on the water-permeable baffle.

[0014] Preferably, filter paper is laid on the upper portion of the permeable partition.

[0015] The water-permeable partition includes an upper partition, a lower partition and bolts. The upper partition and the lower partition are provided with corresponding water-permeable holes. The upper partition is provided with a waist-shaped hole, and the lower partition is provided with a threaded hole. The bolts are installed in the waist-shaped hole and the threaded hole, and the bolts connect and fix the upper partition and the lower partition.

[0016] Preferably, a scale is provided on the inner wall of the model box 1 .

[0017] Compared with the prior art, the present invention is beneficial in that:

[0018] 1. The test device is simple to make and the principle is clear. Compared with the traditional strain gauge, the principle of using a dynamometer to measure the frozen pullout force of the pile is more reliable and the result is more accurate. Compared with the reaction frame test method of frozen pullout force, since the dynamometer is installed inside the model box, at the bottom of the pile, the horizontal and vertical space volume occupied by the model box is reduced;

[0019] 2. One end of the dynamometer is fixed to the dynamometer connection seat on the waterproof bottom plate, and the other end is directly connected to the bottom of the pile. The frozen pullout force of the pile is the tensile force exerted on the dynamometer. The dynamometer is connected to an external data collector to test the variation law of the frozen pullout force of the pile and the maximum total frozen pullout force during freezing under various conditions, and also to test the dissipation law of the frozen pullout force during any melting process.

[0020] 3. The outside of the model box is wrapped by an insulation box to prevent heat transfer, and the bottom cold bath box is detachable. After the cold bath box is installed, the water in the water tank at the bottom of the model box can be temperature controlled. When it is removed, the five-sided insulation effect can keep the water in the water tank at a constant temperature;

[0021] 4. The water tank at the bottom of the model box is separated from the soil by a permeable partition. Filter paper is laid on the permeable partition to prevent the soil from falling. The mesh size of the permeable partition can be adjusted. Changing the mesh size can adjust the water replenishment rate. During the test, any water replenishment conditions can be achieved for the entire bottom area of ​​the soil. The process of underground water migration at a certain plane in different projects can be widely simulated. Compared with the single-point water replenishment test conditions, it better meets the actual water replenishment conditions, and the obtained water distribution law is more valuable for guiding the actual project;

[0022] 5. After being wrapped in an insulating box, the bottom surface and five surrounding surfaces of the entire device are insulated. The device can be directly placed in a simple refrigerator to meet the one-way freezing or melting conditions. No other temperature control device or cold bath is required. During the test, the ambient temperature in the refrigerator is pre-adjusted, and then the refrigerator is cooled to cool the soil on the surface of the model box, which can simulate the surface cooling process. The temperature change process in the test is close to the actual temperature change conditions, and the obtained temperature change law has guiding significance for engineering practice;

[0023] 6. Connect a water supply bottle to the side of the sink. The water supply bottle is a Malliot bottle with scales. During the freezing process, the water supply amount can be monitored in real time through the scale changes of the liquid level in the water supply bottle.

[0024] 7. A sensor hole is reserved on the side of the model box. During the pile freezing and pulling force test, other sensors such as moisture and temperature are buried through the sensor hole to monitor the changes of other soil parameters at the same time;

[0025] 8. In this device, since the pile body and the dynamometer are fixedly connected to the dynamometer connection seat at the permeable baffle, the pile body does not move, while the soil around the pile can frost heave freely, so that the frost heave displacement change of the top surface of the soil can be directly observed through the scale marked on the inside of the model box;

[0026] 9. This device can perform freeze-pull force test and freeze-pull displacement test (without connecting dynamometer) on two piles of the same size, so as to establish the relationship between freeze-pull force and freeze-pull amount, and guide the research and development of freeze-pull measures;

[0027] 10. The device can connect multiple dynamometer bases at the permeable baffle, and multiple dynamometers can be installed through multiple dynamometer bases, so that piles of various sizes can be tested for freeze-draw force comparison under the same soil and water-heat conditions at the same time;

[0028] 11. The test device can adapt to various shapes of model piles. The test piles can be set into various shapes such as cylindrical piles, conical piles, spiral piles or bamboo piles according to the test conditions. If it is a concrete pile, the idea provided is to connect the internal bolts with the dynamometer. If it is a pile of other materials, it can be flexibly applied according to the characteristics of the material by referring to the connection mode of concrete piles;

[0029] 12. The method of using a dynamometer to test the frost pullout force can also be directly applied to the in-situ frost pullout force monitoring of piles in actual projects (in actual projects, the lowest part of the pile is buried below the frost depth line).

[0030] 13. The outer side of the dynamometer is protected by a casing to prevent the dynamometer from direct contact with the soil and avoid contact between the bottom of the test pile and the soil, ensuring that the data tested by the dynamometer is the freeze-pullout force of the soil on the side of the pile. Description of the drawings:

[0031] The present invention is further described below in conjunction with the accompanying drawings.

[0032] Figure 1 It is a three-dimensional structural schematic diagram of the present invention.

[0033] Figure 2 It is a schematic diagram of the cross-sectional structure of the present invention.

[0034] Figure 3 This is a diagram showing the variation of freezing pull force during the freezing process.

[0035] Figure 4 This is a graph showing the variation of freeze pull force during the melting process at -30℃.

[0036] Figure 5 It is a structural schematic diagram of a permeable baffle. Specific implementation method:

[0037] The present invention is described in detail below in conjunction with specific embodiments:

[0038] like Figure 1 and Figure 2The pile body frost pullout force test device shown in the figure comprises a model box 1, a water tank 2, a cold bath box 3, an insulation box 4, a sensor reserved hole 5, a permeable baffle 6, a water replenishment bottle 7, a dynamometer 8, a test pile 9 and a data collector 11. The model box 1 is made of welded steel plates. The inner side of the model box 1 is marked with scales and the sensor hole 5 is reserved on the side. The permeable baffle 6 is fixedly installed inside the model box 1. The inner wall of the model box 1 is provided with scales. During the test, the scales marked on the inner side of the model box 1 can be used to measure the strength of the pile body. The frost heave displacement change of the top surface of the soil can be directly observed. The water tank 2 is arranged at the bottom of the model box 1, and the permeable baffle 6 is located on the upper part of the water tank 2. A water replenishment bottle 7 is installed on the side of the water tank 2. The insulating box 4 is arranged to cover the outside of the model box 1. The insulating box 4 can realize the insulation of the surroundings and the bottom plate of the steel model box 1. Due to the insulation effect of the insulating box 4, the device can be directly placed in an ordinary refrigerator to freeze or melt, that is, it can realize a one-way freezing and one-way melting process without other temperature control devices; at the same time, due to the insulation The hot box is detachable, and any initial temperature field can be pre-realized for the steel model box, the test soil and the pile body of the entire experimental device, and then loaded into the insulation box for freezing and melting tests. The permeable partition 6 is provided with mesh holes, and the permeable partition 6 includes an upper partition 61, a lower partition 62 and bolts 63. The upper partition 61 and the lower partition 62 are provided with permeable holes, and the upper partition 61 is provided with waist-shaped holes, and the lower partition 62 is provided with threaded holes. The bolts 63 are installed in the waist-shaped holes. and threaded holes, and bolts 63 connect and fix the upper baffle 61 and the lower baffle 62. The mesh size of the water-permeable baffle 6 is adjustable, and different water replenishment rates can be controlled by adjusting the mesh size. The water-permeable baffle is a double-layer baffle, and bolts 63 are installed on both sides of the lower baffle 62. Waist-shaped holes are opened on both sides of the upper baffle 61 as sliding grooves. The area of ​​the lower baffle 62 is smaller than that of the upper baffle 61. The bolts of the lower baffle 63 can slide in the sliding groove of the upper baffle 61, and the upper and lower baffles are fixed by bolts 63. Loosen the bolts 63, and the relative displacement of the upper and lower baffles changes the size of the water-permeable holes. When the mesh size of the upper and lower baffles overlap, the contact area between the sample and water is the largest, and the water replenishment rate is the largest.During the test, the densely distributed mesh holes can evenly replenish water at multiple points on the entire bottom area of ​​the soil, which is closer to the actual working condition than single-point water replenishment. A plurality of dynamometer connection bases 12 are welded at different positions on the permeable baffle 6, and a plurality of dynamometers 8 are connected through the plurality of bases 12. At the same time, a comparison test of the freezing pullout force of piles of various sizes is carried out under the same soil and the same hydrothermal conditions. The dynamometer 8 is installed on the dynamometer connection base 12, and the test pile 9 is fixedly installed on the dynamometer 8. The dynamometer 8 is fixed on the base 12 and is directly connected to the bottom of the test pile 9. A casing 13 is sleeved and installed on the dynamometer 8. The lower part of the casing 13 is welded and fixedly installed on the permeable baffle 6. The casing 13 protects the dynamometer 8, keeps the measuring meter 8 isolated from the soil filling, and ensures that the data tested by the dynamometer 8 is the freezing pullout force of the soil on the side of the pile. The freezing pullout force of the pile is the tension exerted on the dynamometer, and the freezing pullout force change of the pile during the freezing process under various conditions can be directly measured. The freezing law and the maximum total freezing pull-out force and the dissipation law of the freezing pull-out force during the melting process are measured. A hydrothermal sensor 10 is installed in the sensor reserved hole 5. The hydrothermal sensor 10 monitors the changes in temperature and moisture in the soil during the freezing process. The cold bath box 3 is installed at the bottom of the model box 1. The cold bath box 3 below the model box 1 can be disassembled. When the cold bath box 3 is removed, the insulation box 4 wrapped outside the model box 1 can maintain the water temperature in the water tank 2 constant. The cold bath box 3 can be temperature controlled by a program. The water replenishment bottle 7 is connected to the water tank 2 through a pipeline. During the freezing process, the water replenishment amount can be observed in real time through the change of the liquid level of the water replenishment bottle 7. The dynamometer 8 and the hydrothermal sensor 10 are respectively connected to the data acquisition device 11 through wires. The water replenishment bottle 7 is a Malliot bottle. The water replenishment bottle 7 is marked with scales. The dynamometer 8 is fixed to the dynamometer connection base 12 by threaded connection. During the test, filter paper is laid on the upper part of the permeable partition 6 to prevent the soil from falling.

[0039] The test pile 9 can be a cylindrical pile, a tapered pile, a spiral pile, a bamboo pile, etc. of different materials. If the pile is a concrete cylindrical pile or a tapered pile, the present invention provides an implementation method of connecting the embedded bolts with the dynamometer. If the pile is an iron pile or a steel pile, the present invention provides an implementation method of connecting the welded bolts at the bottom of the pile with the dynamometer. If it is a pile of other materials, a reasonable connection method should be selected according to the characteristics of the material. As a preferred embodiment, the model box of this device selects a steel box with a size of 60cm long, 40cm wide, and 50cm high. The thickness of the insulation box is 5cm, and the model box is wrapped. The test pile body is a concrete tapered pile with a top diameter of 4cm, a bottom diameter of 6cm, a height of 25cm, and embedded bolts.

[0040] During the test, the outside of the model box 1 is wrapped by the insulating box 4 to prevent heat transfer around and on the bottom of the model box 1. The cold bath box 3 at the bottom of the water tank 2 is installed or removed according to whether the water temperature needs to be regulated. When the entire device is placed in a low-temperature temperature control room or an ordinary refrigeration refrigerator, the test can meet the one-way freezing and one-way melting conditions. At the same time, the insulating box 4 is detachable, and any initial temperature field can be pre-realized for the entire experimental device of the steel model box 1, the test soil and the pile body, and then placed in the insulating box for one-way freezing and melting tests.

[0041] A sensor hole is reserved on the outside of the model box 1. A hydrothermal sensor 10 can be buried in the reserved sensor hole 5 according to the test requirements. The hydrothermal sensor 10 simultaneously measures the temperature change of the water content of the soil around the pile during the freezing process. During the test, a comparison pile can also be buried in the model box 1. The comparison pile and the test pile are at different positions at the same horizontal height in the model box. The comparison pile is not connected to a dynamometer, and a displacement sensor is placed above the comparison pile. The freezing and pulling displacement data of the comparison pile can also be collected, thereby establishing the relationship between freezing and pulling force and freezing and pulling displacement. In addition, multiple piles of the same size can be connected to the bottom plate at the same time, and the freezing and pulling force change law of piles of different sizes can be tested under the same soil and the same hydrothermal conditions.

[0042] Before the test begins, water is poured into the water tank 2 until the water just covers the permeable baffle 6. Filter paper is laid on the bottom plate of the permeable baffle 6, and the bottom of the test pile is connected to the dynamometer. The configured soil is evenly filled in layers around the pile and compacted. During this period, sensors such as moisture and temperature are buried in appropriate positions in the soil around the pile. After the soil is filled, fill the water supply bottle with water and record the initial scale. Then turn on the refrigerator switch and adjust it to the set negative temperature. During the cooling process of the refrigerator, the surface of the soil serves as the cold end. If the water temperature of the bottom water tank 2 in the model box 1 needs to be regulated, a cold bath box 3 needs to be installed. The water tank 2 supplies water to the sample and freezes the sample in one direction. During the test, the entire process of the freeze-pull force change during freezing and thawing is recorded by an external data acquisition device, and the water and heat changes in the soil are observed through the arranged moisture temperature sensors. In addition, the frost heave amount on the top surface of the soil can also be observed through the scale lines on the inside of the model box. Attachment Figure 3 This is a diagram showing the variation of the freezing force during freezing. Figure 4 The data are the freeze-pullout test data of concrete test piles at -30℃.

[0043] Embodiment 1:

[0044] 1. Make test piles and prepare soil samples with initial moisture content.

[0045] 2. Connect the dynamometer 8 to the base 12 on the water-permeable partition 6 in the model box 1 , connect and fix the test pile 9 to the dynamometer 8 , and connect the wire to the data collector 11 .

[0046] 3. Insert the water thermal sensor 10 into the model box 1 through the sensor reserved hole 5 on the side of the model box 1. The water thermal sensor 10 is a temperature and moisture sensor that can measure temperature and moisture data.

[0047] 4. Pour water into the water tank 2 in the model 1. Stop pouring water when the water just covers the water-permeable baffle 6. Lay a layer of filter paper on the waterproof bottom plate 6. Then load the soil samples into the model box 1 in layers. During the soil loading process, bury the water and heat sensors in the soil around the pile at a certain interval, and compact the soil in layers.

[0048] 5. After the soil is filled, the entire experimental device including the model box 1, the test soil and the test pile 9 is placed in a predetermined temperature field to achieve the initial temperature condition, and then the entire device is wrapped with an insulating box 4 (if the water temperature of the water tank 2 needs to be controlled, a cold bath box 3 is installed at the bottom of the steel model box 1; if the water temperature does not need to be controlled, the cold bath box 3 does not need to be installed). The entire device wrapped in the insulating box is placed in a refrigeration refrigerator. The refrigerator temperature is adjusted to a set negative temperature before the test, and the temperature inside the refrigerator is used as the cold end. The insulating box 4 prevents heat transfer around and at the bottom of the model box 1, achieving a one-way freezing condition, and starting to freeze the sample.

[0049] 6. Record the initial height of the soil filling according to the scale of the scale inside the model box 1. At the same time, energize the dynamometer 8 and the water thermal sensor 10 to record the change law of the freezing force during the freezing process, the change of temperature and moisture at different positions, and the amount of frost heave on the soil surface. In addition, the amount of water replenishment can also be monitored by the change of the liquid level scale of the Marriott water replenishment bottle 7.

[0050] It should be emphasized that it is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-restrictive from all points of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims be included in the present invention.

Claims

1. A pile frost pullout force testing device, characterized in that: The invention comprises a model box (1), a water tank (2), a cold bath box (3), an insulating box (4), a sensor reserved hole (5), a water permeable baffle (6), a water replenishment bottle (7), a dynamometer (8), a test pile (9) and a data acquisition device (11). The inner side of the model box (1) is marked with scales and the sensor reserved hole (5) is reserved on the side. The water permeable baffle (6) is fixedly installed inside the model box (1). The water tank (2) is arranged at the bottom of the model box (1), and the water permeable baffle (6) is located on the upper part of the water tank (2). The insulating box (4) is covered on the outside of the model box (1). The water permeable baffle (6) is provided with mesh holes. A dynamometer connecting base (12) is welded on the water permeable baffle (6). The dynamometer (8) is installed on the dynamometer connecting base (12). The test pile (9) is fixedly installed on the dynamometer (8). A water thermal sensor (10) is installed in the sensor reserved hole (5), the cold bath box (3) is installed at the bottom of the model box (1), the water replenishment bottle (7) is connected to the water tank (2) through a pipeline, the dynamometer (8) and the water thermal sensor (10) are respectively connected to the data acquisition device (11) through wires, the water permeable partition (6) includes an upper partition (61), a lower partition (62) and a bolt (63), the upper partition (61) and the lower partition (62) are respectively provided with water permeable holes, the upper partition (61) is provided with a waist-shaped hole, the lower partition (62) is provided with a threaded hole, the bolt (63) is installed in the waist-shaped hole and the threaded hole, and the bolt (63) connects and fixes the upper partition (61) and the lower partition (62), the water replenishment bottle (7) is a Malliot bottle, and the water replenishment bottle (7) is marked with scales.

2. The pile frost pullout force testing device according to claim 1, characterized in that: The dynamometer (8) and the dynamometer connecting base (12) are installed and fixed by means of threaded connection.

3. The pile frost pullout force testing device according to claim 1, characterized in that: Filter paper is laid on the upper part of the water-permeable partition (6).

4. The pile frost pullout force testing device according to claim 1, characterized in that: A casing (13) is sleeved and installed on the dynamometer (8), and the lower part of the casing (13) is welded and fixedly installed on the water-permeable partition (6).

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