Apparatus and method for testing deformation characteristics of expansive soil for reducing side wall friction effect

Abstract

The application discloses a swelling-shrinking soil deformation characteristic test device capable of reducing side wall friction effect, which comprises a water bath type temperature control device and a data acquisition and image processing system connected with a core measuring device capable of reducing side wall friction effect respectively, the core measuring device capable of reducing side wall friction effect is assembled into a cylindrical structure by a top disc, an outer cylinder and a base, and the inside of the core measuring device comprises a sliding resistance reduction device and an expansion force measuring device arranged in an up-down mode. The application also discloses a measuring method using the test device. The application can not only reduce the adverse effect of the friction resistance of the sample cylinder side wall on the swelling-shrinking deformation and expansion force measurement in the existing equipment, but also can measure the swelling-shrinking deformation and expansion force change rule of the swelling soil in the wet-dry cycle process, and can also combine the image processing and analysis method to observe the change rule of the crack shape and size of the soil sample bottom under the wet-dry cycle condition.

Description

Technical Field

[0001] This invention relates to a special soil engineering testing device, and particularly to a testing technology that improves the accuracy of measuring the swelling and shrinkage deformation and swelling force of expansive soil, especially a testing device and method for expansive soil deformation characteristics that reduces the sidewall friction effect. Background Technology

[0002] Swelling and shrinkage (i.e., expansibility) is one of the important properties of expansive soils. If wet expansion deformation is restricted, expansive force will inevitably be generated. Expansibility is the main source of engineering defects in expansive soils; therefore, accurately measuring the force and deformation generated during the swelling and shrinkage process of expansive soils is of great engineering significance. Most existing experimental studies on the deformation characteristics (swelling and shrinkage deformation and expansive force) of expansive soils are carried out on consolidation apparatus.

[0003] For example, the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019) discloses a device for measuring expansive force based on the loading balance method, which mainly consists of a consolidation apparatus and a ring sampler. After the sample begins to swell upon immersion in water, the vertical stress is gradually increased until the sample stops expanding within 2 hours under a certain level of vertical stress, at which point the test ends. While this technique is simple, it does not consider the significant error in the measurement results caused by the frictional effect of the sample's sidewalls.

[0004] Chinese patent CN203772856U discloses a model test chamber with low sidewall friction. It significantly reduces the frictional constraint of rigid sidewalls on soil sample settlement deformation through multiple rigid sliding plates, more realistically transmitting vertical loads and making test results more accurate. However, this patent can only measure soil sample settlement (shrinkage) rather than expansion, and cannot guarantee the integrity of the soil sample when downward loads are applied. When the soil sample undergoes expansion deformation, the flexible cushion layer does not expand, leading to damage to the soil sample sidewalls. Furthermore, during the soil sample shrinkage stage, this patent cannot measure the elastic modulus of the flexible cushion layer and the soil sample, and cannot accurately assess the drag reduction effect.

[0005] Chinese patent 202210519818 discloses a device and method for measuring the swelling rate and swelling force of soil samples under segmented wetting conditions. It can obtain the swelling force or swelling rate under different moisture content gradients and simulate the dynamic swelling characteristics of soil as the moisture content gradually increases, enabling the determination of swelling parameters under unsaturated wetting paths. However, this patent only measures the swelling force and swelling rate separately by replacing the pressure sensor and displacement sensor, lacking synchronous measurement capabilities and limiting its ability to obtain the real-time relationship between swelling force and deformation. The infiltration device can only achieve bottom-up wetting, affecting the in-situ drying process in wet-dry cycle tests. Furthermore, this patent does not develop a measuring device to reduce sidewall friction, converting the static friction between the sample and the instrument into sliding friction, which cannot accurately measure the swelling deformation.

[0006] Chinese patent 201911070825 discloses a device for testing the unsaturated expansion force of high-pressure compacted bentonite assemblies. This device can measure expansion force under single-stage or multi-stage suction control, and can also study the self-healing conditions of the assembly interface under suction, thus evaluating the expansion characteristics of engineering barrier materials in unsaturated states. However, this patent only controls suction through gas-phase circulation, lacking temperature regulation and simultaneous measurement of expansion deformation, limiting its ability to control the coupling effect of temperature and deformation. The sample pressing auxiliary device can only perform sample separation pressing and splicing, affecting the simultaneous monitoring of expansion deformation and the implementation of wet-dry cycle tests. This patent does not reduce the adverse effects of frictional resistance on the sidewall of the sample cylinder on expansion deformation and expansion force testing in existing equipment, and cannot accurately measure expansion deformation.

[0007] For example, the paper "Studies of Swell and Swell Pressure Characteristics of Compacted Clays" published in the *Highway Research Board Bulletin* by HB Seed, JK Mitchell, and CK Chan discusses the loading expansion test method disclosed by Seed et al. in their 1962 monograph. The researchers applied a predetermined pressure to the treated sample before injecting water; then, they applied progressively increasing constant pressures to different samples. During hydration, the soil sample would overcome this pressure and expand upwards until the dial gauge reading stopped changing, thus obtaining the final expansion rate under that specific pressure. In other words, for several samples with the same initial conditions, different vertical stresses were progressively increased. This technique cannot perform continuous testing on the same sample. To implement the loading expansion method, multiple parallel samples must be tested independently under different constant loads. Differences between samples will introduce errors in the test results, and the significant error caused by sidewall friction during the test is not considered. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, this invention provides a test device and method for testing the deformation characteristics of expansive soil by reducing the sidewall friction effect. This method can reduce the adverse effects of the sidewall friction resistance of the sample tube on the measurement of expansive deformation and expansion force in existing equipment, and can also determine the variation law of expansive soil in expansive deformation and expansion force during wet-dry cycle. At the same time, it can also combine image processing and analysis methods to observe the variation law of the morphology and size of the cracks at the bottom of the soil sample under wet-dry cycle conditions.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: The test apparatus for reducing the sidewall friction effect of expansive soil consists of three parts: a core measuring device for reducing sidewall friction effect, a water bath temperature control device, and a data acquisition and image processing system.

[0010] (1) Core measuring device for reducing sidewall friction effect The core measuring device for reducing sidewall friction is externally assembled from a top plate, outer cylinder, and base, and internally consists of a sliding drag-reducing device and an expansion force measuring device. The components of this device will be described in detail below, from the inside out.

[0011] The sliding drag-reducing device includes a track base plate, a top ring, sliders, a moving ring, a guide rail, and a bottom ring. The top ring has the same structure as the moving ring, and its position is at the top, contacting the pressure plate, and then sequentially contacting the moving rings; the last moving ring is connected to the bottom ring. The bottom ring is not fixed to the guide rail by sliders, nor is it fixed to any other structure; it only serves a fixing function during sample loading. Each independent moving ring is in close contact to form a whole, rather than being fixed together. During the expansion and deformation of the sample, each moving ring is driven individually to reduce friction. Each moving ring, as well as the top and bottom rings, has a height of 1 cm and a diameter of 10 cm. The 18 moving rings and the top and bottom rings in close contact together form a sample placement area with a height of 20 cm and a diameter of 10 cm. Each top ring and each moving ring is equipped with 3 sliders, and the sliders are connected to the moving rings by screws to ensure that there is no relative displacement between the sliders and the moving rings. Three sliders are located on separate track plates, and the connection between the sliders and the track plates will not cause relative displacement. The track plates are connected to guide rails (a total of 9) installed on the inner wall of the outer cylinder. At the connection point between the guide plate and the guide rail, there are two rows of ball bearings to ensure that each moving ring can slide freely on the guide rail under the action of the three sliders.

[0012] The moving ring is connected to the track plate via a slider, and the height of both the slider and the track plate is higher than the height of the moving ring. When the sliders of adjacent moving rings are on the same guide rail, the height difference between the slider and the moving ring will cause gaps between the adjacent moving rings, preventing them from making close contact and weakening the sidewall drag reduction effect of the core measuring device. To enhance the sidewall drag reduction effect, as many moving rings and sliders as possible need to be arranged. Therefore, when assembling the core measuring device of this invention, an assembly scheme with optimal sidewall drag reduction effect is proposed by arranging and combining the slider arrangement and the number of guide rails. Specifically, a total of 9 guide rails are arranged on the inner wall of the outer cylinder, and the sliders are arranged in a staggered manner on the 9 guide rails according to a specific arrangement. This arrangement method can ensure that gaps will not be generated between adjacent moving rings due to slider contact, thereby obtaining the optimal sidewall drag reduction effect. After eliminating the adverse effects of the slider arrangement, to ensure that the sample sidewalls are not damaged and that adjacent moving rings maintain tight contact during the test, the inner wall of the moving rings is designed in a vertical "elbow" shape, with a net distance of 1 cm between adjacent moving rings. This ensures that the top of the next moving ring can tightly engage with the bottom of the previous one while also increasing the number of moving rings. For the top and bottom rings, a groove is provided only at the bottom of the top ring, and a protrusion is provided at the top of the bottom ring. Because the assembly process of the core measuring device is complex, it is necessary not only to use the above-mentioned slider arrangement but also to ensure that the number of small-diameter balls in the two rows of ball bearings on the track base plate is sufficient and not easily dislodged. After the core measuring device is assembled, all moving rings can slide freely on the guide rails of the outer cylinder under the action of the sliders and the track base plate, thus achieving optimal drag reduction on the sidewalls.

[0013] The expansion force measuring device includes a permeable plate, a pressure plate, a force sensor, and a mandrel. The permeable stone is embedded in a groove at the bottom of the pressure plate, allowing water to evenly permeate into the sample during water saturation. A permeable hole at the top of the pressure plate connects to the bottom groove, and the inner wall of the permeable hole is threaded for installing an air pipe connector. A plastic tube can be directly installed on the air pipe connector for adding water and venting air, respectively. The force sensor (range 500N, accuracy 0.01%) is located on top of the pressure plate; its size is smaller than the pressure plate for easy installation of the air pipe connector. The force sensor has a screw hole for fixing to the pressure plate with screws, and a data output line is connected to its side to output real-time data on the expansion force changes over time. The bottom of the mandrel is threaded and can be positioned at the center of the force sensor.

[0014] The outer cylinder is made of stainless steel. Since the test materials include expansive soil and bentonite (whose expansion force is much higher than that of expansive soil), the rigidity of the outer cylinder must be ensured to be suitable for these two types of soil materials. The outer cylinder has eight guide rails arranged evenly along its interior; its upper and lower parts are connected to the base and top plate, respectively, forming the core of the test device. In addition, an observation hole is provided in the middle of the outer cylinder's side wall to observe the movement of the moving ring during the test, ensuring that the sliding drag-reducing device can fully exert its drag-reducing effect during the deformation of the expansive soil. Two circular holes (connected to the water inlet pipe and the exhaust pipe, respectively) and a force sensor data output connector are provided at the upper end of the outer cylinder's side wall.

[0015] The base includes an observation window, a flange, and O-rings. The observation window is located inside the flange, and an O-ring is installed on the inner wall of the flange to ensure waterproofing at the connection between the observation window and the flange. Additionally, an O-ring is also installed in a groove on the outside of the flange; during sample loading, any excess latex film on the sample can be placed here, also serving a waterproofing function. The bottom ring rests on the observation window and does not require fixing. During sample loading, the bottom ring is first placed on the bottom of the sample, and then placed on the observation window to relatively fix the sample position for subsequent loading. After the test is completed and the sample is removed, the bottom ring can also be placed here to prevent the moving ring from falling off.

[0016] The top plate includes a linear bearing, a plate surface, an observation port, and a knob nut. Before the test, all components are assembled and the plate surface is secured with hexagonal screws. The top of the mandrel passes precisely through the linear bearing at the center of the plate surface. Ball bearings are installed inside the linear bearing to ensure axial accuracy during expansion deformation and expansion force measurement, and to reduce sliding friction of the mandrel within the linear bearing during deformation. A knob nut is located on the upper part of the plate surface; tightening the knob nut constrains the sample, and the expansion force value can be measured using data acquisition software connected to a force sensor.

[0017] (2) Water bath temperature control device The water bath temperature control device includes a stainless steel water tank, a miniature water pump, a semiconductor water-cooled head heat sink, an electric thermostatic environment chamber, an intelligent time-temperature controller, and a test enclosure. The test enclosure is made of acrylic glass, with insulation material at the bottom to reduce temperature control errors. Its internal space houses the core measuring device, which reduces sidewall friction. An external water inlet connected to the core measuring device is also provided. A semiconductor water-cooled head heat sink is installed on its inner wall, connected to the electric thermostatic environment chamber via inlet and outlet water pipes. The semiconductor water-cooled head heat sink is connected by flexible hoses, achieving water bath heating through water circulation. The intelligent time-temperature controller, equipped with a temperature probe, monitors the ambient temperature and adjusts the heating temperature of the electric thermostatic environment chamber accordingly. Before the experiment began, the circulating water was heated to the preset temperature using an electric heating constant temperature chamber, and then the water circulation was started using an intelligent time and temperature controller. During the experiment, the temperature in the electric heating constant temperature chamber was adjusted to control the temperature of the circulating water and the semiconductor water cooling head heat sink, so that the temperature difference was controlled within ±0.5 ℃.

[0018] (3) Data acquisition and image processing system The data acquisition and image processing system includes a data acquisition device for expansion and contraction deformation, a data acquisition device for expansion force, a high-resolution mirrorless camera, and a computer.

[0019] The expansion and contraction deformation data acquisition device includes a laser displacement sensor, an RS485 communication module for the laser displacement sensor, a 220V to 24V power supply, a USB to RS485 serial port adapter, and a lifting bracket. Before the test, the laser displacement sensor (range 120 mm to 280 mm, accuracy 0.1 mm) is fixed to the lifting bracket with screws. Then, the data output line of the laser displacement sensor is connected to the RS485 communication module. By adjusting the height and position of the lifting bracket, the laser point is accurately positioned at the top of the mandrel, and the settings ensure that its variation range is within the range. Finally, the RS485 communication module is connected to the computer using the USB to RS485 serial port adapter, and then the power is turned on. The displacement changes on the expansive soil surface can be recorded in real time using data acquisition software. The deformation of the sample can be obtained by the difference between the real-time data collected during the test and the initial value.

[0020] The expansion force data acquisition device includes an RS485 communication module for the force sensor, a single-channel digital transmitter, a 220V to 24V power supply, and a USB to RS485 serial port adapter. Before starting the experiment, connect the data output line of the force sensor to the single-channel digital transmitter; then, after conversion by the single-channel digital transmitter, connect it to the RS485 communication module. Use the USB to RS485 serial port adapter to connect the RS485 communication module to the computer; finally, turn on the power, and the changes in expansion force can be recorded in real time through the data acquisition software.

[0021] A high-resolution mirrorless camera (3648×2736 pixels) is located on one side of the observation window on the base of the core measuring device, designed to capture high-definition images of cracks on the sample surface.

[0022] (4) Sample preparation device In addition to the components of the aforementioned testing apparatus, sample preparation is also achieved through a sample preparation device. This device mainly includes a molding die and a sample-specific protective device. The molding die includes a base, an outer mold, a gasket, and a pressure head. The outer mold has a diameter of 99 mm and a height of 200 mm. During sample pressing, it is divided into 5 layers, each 40 mm thick. For demolding, the three support legs are installed on the base, and the pressure head can be used for demolding. The sample-specific protective device includes a sample sleeve, an air pipe connector, an air valve, screws, and a vacuum pump. Before preparing the sample, first place the latex film on the inner wall of the sample protector, and fold the excess part out and place it on the outside of the sample protector. Then, install the air valve in the air hole reserved on the outside of the sample protector, and connect the air pipe connector to the air valve. Then, use a vacuum pump to remove the gas generated between the latex film and the inner wall of the sample protector, ensuring that there are no gaps between the latex film and the sample protector. At this time, the sample can be placed into the sample protector. Finally, fold the excess latex film back, unscrew the 6 screws on the side wall of the sample protector, and the sample protector with the latex film on can be opened and the sample taken out.

[0023] The method for determining the deformation characteristics of expansive soil using the above-mentioned device includes the following steps: Step (1), sample preparation: First, the expansive soil is dried and the sand and gravel are removed. Then, the expansive soil is crushed, passed through a 2mm sieve, and dried to obtain loose soil. Then, according to the test requirements, distilled water is sprayed evenly into the loose soil and stirred thoroughly. Then, the prepared loose soil is placed in a plastic bag, sealed, and left to stand for 24 hours to ensure that the moisture in the loose soil is evenly distributed. Then, the prepared loose soil is divided into 5 portions for later use. Due to the large size of the sample, a press is used to compact the sample. The specific operation steps are as follows: First, talcum powder is applied to the inner wall of the molding mold to facilitate the removal of the press head and subsequent demolding after each layer of sample is pressed. Then, the outer mold of the molding mold is placed on the base plate, and a portion of loose soil is poured into the outer mold. Then, a gasket is placed on the press head to compact the loose soil. Then, the press head is removed and roughened with a scraper to ensure good contact between sample layers. After each portion of loose soil is placed, a pad ring is added to the pressure head for compaction. Repeat the above compaction operation to compact 5 portions of loose soil. After that, remove all the pad rings from the pressure head, then install the support legs on the outer mold. The sample can then be pressed out completely with the pressure head.

[0024] During sample preparation, the following two aspects should be noted: First, after the first layer of sample is compacted, the indenter is difficult to remove. In this case, the base plate can be removed by inverting the indenter and the forming mold. Then, a pad of the same size as the forming mold can be placed on the sample, and the indenter can be pressed out and removed by pressing lightly with a press. Second, during the demolding process of the sample, the indenter and the forming mold should be kept coaxial to avoid the sample breaking during demolding due to uneven force. After the sample is demolded, the sample surface should be checked for smoothness, cracks or defects.

[0025] Step (2), applying the latex film: First, place the latex film on the inner wall of the sample protector and then fold the excess part out and place it on the outside of the sample protector; then, install the air valve in the air hole reserved on the outside of the sample protector, and connect the air pipe connector to the air valve; then use a vacuum pump to remove the gas generated between the latex film and the inner wall of the sample protector to ensure that there is no gap between the latex film and the sample protector; then put the sample into the sample protector and fold the excess latex film back in; finally, unscrew the 6 screws on the side wall of the sample protector to open the sample protector and take out the sample with the latex film on.

[0026] Step (3), placing the sample: First, fold up the excess latex film at the bottom of the sample (already covered with latex film), and then place the bottom ring on the bottom of the sample; then, fold down the excess latex film at the bottom of the sample and place it into the groove of the flange to secure it. The remaining latex film can be repeatedly folded and placed on the flange. Note: During the folding process of the latex film, it should not be placed too deep, otherwise it will affect the installation of the outer cylinder.

[0027] Step (4), Install the outer cylinder: After fixing the bottom ring on the bottom of the sample (with the latex film already fitted), apply lubricating oil to the surface of the latex film to reduce the friction effect between the moving ring and the latex film, so that the outer cylinder can be installed smoothly. When installing the outer cylinder, first lift it and all the moving rings at the same time from the observation hole of the outer cylinder to prevent the moving rings from falling off and avoid damaging the ball bearings in the guide rail seat plate; then, lower the outer cylinder from the top of the sample, while preventing excess latex film from the top of the sample from being trapped between the moving ring and the sample; after placing the outer cylinder on the chassis, fine-tune the position of the outer cylinder so that it is aligned with the screw holes on the chassis, and then fix the outer cylinder to the chassis with screws.

[0028] Step (5), install the expansion force measuring device: First, fix the force sensor on the pressure plate with screws, and then install two air pipe connectors on the pressure plate; then, connect the hose to the air pipe connectors, and then install the permeable stone in the groove at the bottom of the pressure plate; then, open the latex film on the top of the sample and place the assembled expansion force measuring device in it. During the placement process, tighten the latex film until the pressure plate and the sample are in good contact; then, pass the hose through the water inlet and air outlet holes reserved on the side wall of the outer cylinder to facilitate the subsequent addition of water to the sample, and then pass the data output line connector of the force sensor through the hole reserved on the top plate; finally, place the top plate on the upper part of the outer cylinder, so that the mandrel passes through the linear bearing in the center of the top plate, and then finely adjust the position of the top plate so that the screw hole on it is aligned with the screw hole on the outer cylinder. Then, the outer cylinder can be fixed to the base plate with screws.

[0029] Step (6), control the test environment temperature: first, place the core measuring device and the corresponding sensor inside the test shell of the water bath temperature control device, then heat the circulating water to a suitable temperature using an electric heating constant temperature environment chamber, and then start the water circulation; then determine whether the test set temperature has been reached by the temperature control probe of the intelligent time temperature controller, and thus regulate the water circulation according to the change of the test environment temperature, so as to ensure that the test can be carried out in a stable temperature environment.

[0030] Step (7), debug the data acquisition and image processing system: Debugging the expansion and contraction deformation data acquisition device: First, fix the laser displacement sensor on the bracket, turn on the power to display data, and then adjust the universal table on the bracket so that the laser is perpendicular to the top surface of the core measuring device. Next, place the core measuring device to one side of the laser displacement sensor so that the light source illuminates the center of the mandrel. Since the laser displacement sensor's range is 120 mm to 280 mm, and the mandrel moves outward during sample expansion and deformation, the bracket height should be adjusted so that the laser displacement sensor reading is within the range of 270 mm to 280 mm to ensure that the range is not exceeded during the test. Then, connect the sensor's data output lines to the TR+ and TR- serial ports of the RS485 communication module, and then fix it to the computer using screws via a USB to RS485 serial adapter. Finally, open the laser displacement sensor data acquisition software on the computer, select the corresponding serial port of the sensor, connect it, check if the sensor matching is normal, and then set the data acquisition time interval and data display mode to begin data acquisition.

[0031] Debugging the expansion force data acquisition device: First, pass the data output connector of the force sensor through the pre-drilled hole on the top plate. Since the sensor is located inside the flange and has a different signal output format, it needs to be connected to a single-channel digital transmitter. Then, connect the adapter through the single-channel digital transmitter to the RS485 communication module, and then fix it to the computer using screws via a USB to RS485 serial adapter. Finally, open the force sensor data acquisition software CurveDisplay on the computer, select the port corresponding to the sensor, connect it, and check if the sensor matching is normal. After zero-point calibration, set the data acquisition time interval, and data acquisition can begin.

[0032] A high-resolution mirrorless camera (3648×2736 pixels) is fixed to one side of the observation window on the base of the core measuring device via a tripod. It can then capture high-definition images of the cracks on the sample surface in real time, and then process and analyze the crack images using the computer image processing software ImageJ.

[0033] Step (8), saturate the sample by vacuuming: First, determine the position of the observation hole and the position of the chassis outlet on the side wall of the outer cylinder. The position of the outlet is parallel to the bottom ring. Connect a gas pipe connector to the hole at the outlet position. One end of the plastic tube is connected to the gas pipe connector, and the other end passes through the observation hole and is connected to a vacuum pump with a vacuum gauge and regulating valve. In the process of adding water for saturation, the initial stage is the conventional water addition method, that is, adding water to the sample at constant pressure through the Marvin bottle. After the water volume in the Marvin bottle has not changed for 24 hours, vacuuming saturation begins. In the vacuuming saturation stage, the entire system is in an open state. The vacuum pump is used to evacuate at the bottom. The negative pressure of the entire system is kept within a small safe range by controlling the valve to prevent damage to the sample. Water seeps downward under the action of positive pressure and gravity. Combined with the negative pressure applied by the vacuum pump, most of the free air in the sample can be discharged, thereby increasing the saturation of the sample.

[0034] Step (9) involves conducting the following three types of experiments: A. Swelling and Contraction Deformation Test under Alternating Salt Solution Conditions: This test simulates the swelling and contraction deformation of the sample caused by chemical permeation and hydration resulting from the circulation of salt solution (NaCl solution) and distilled water. Another salt solution (KCl solution) is then introduced. The circulation path for NaCl solution and distilled water is: NaCl solution - distilled water - NaCl solution - distilled water; the circulation path for NaCl solution, distilled water, and KCl solution is: NaCl solution - distilled water - KCl solution - distilled water. Salt solution and distilled water are sequentially introduced into the sample through the inlet according to the above paths. Simultaneously, the laser displacement sensor and data acquisition system are activated to record the displacement changes of the mandrel during the circulation process. During the test, if the displacement change of the mandrel within 24 hours after the infiltration of a certain concentration of salt solution or distilled water is less than 0.01 mm, the test is considered complete for this stage. At this point, the next solution can be introduced according to the circulation path, and so on, following the preset circulation path until the test is completed.

[0035] B. Expansion and contraction deformation test under wet-dry cycle conditions: The test temperature is controlled at 25℃ using a water bath temperature control system. Water is added to the sample through the inlet until saturation is achieved. During this process, the sample will undergo axial deformation, causing displacement of the mandrel. The amount of expansion deformation of the sample during saturation is recorded using a data acquisition system. If the displacement change of the mandrel is less than 0.01 mm within 24 hours, the test is considered complete for this stage. After that, water addition is stopped, and the dehumidification stage begins. As the moisture content of the sample decreases, axial shrinkage will occur. The amount of shrinkage deformation of the sample during dehumidification is recorded using a data acquisition system. If the displacement change of the mandrel is less than 0.01 mm within 24 hours, the test is considered complete for this stage.

[0036] C. Expansion Force Test: This device measures expansion force using the constant volume method. After the sample is placed, tighten the knob nut on the top plate, and then start adding water through the inlet to saturate the sample. During sample saturation, the knob nut contacts the mandrel. During deformation, the knob nut hinders the displacement of the mandrel, thus measuring the expansion force during the expansion process. The expansion force generated by the sample during the expansion process is recorded by the data acquisition system. Combined with the 0.1 N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors in Geotechnical Engineering Instruments", the test end time when the pressure test data tends to stabilize is determined.

[0037] Compared with existing testing equipment for the deformation characteristics of expansive soil, the advantages of this invention are as follows: 1. To enhance the sidewall drag reduction effect of the core measuring device of the present invention, a set of assembly schemes with optimal sidewall drag reduction effect is proposed by arranging the slider and the number of guide rails in a specific combination during the assembly process.

[0038] 2. The core measuring device that reduces the sidewall friction effect transforms the static friction between the sample and the instrument into sliding friction, which significantly reduces the significant error caused by the sidewall friction effect of existing expansive soil deformation testing equipment on the expansive deformation and expansive force test data, making the test results closer to the true deformation characteristics of expansive soil compared to existing testing equipment.

[0039] This invention provides an instrument that can accurately and simultaneously detect changes in the volume and force of a sample's expansion. In the initial expansion stage of the sample's expansion deformation process, the expansive soil initially undergoes a small amount of lateral expansion deformation until it adheres tightly to the moving ring, thus limiting the lateral expansion. Furthermore, with the sample encased in a latex film, the contact between the sample and the moving ring is tight, and the soil sample sidewalls remain undamaged. Subsequently, the sample only undergoes axial expansion deformation, and each moving ring is individually driven to reduce friction, transforming the static friction with the sidewall into sliding friction, significantly reducing the impact of sidewall friction on the amount of expansion deformation. This invention develops a device for measuring the deformation characteristics of expansive soil with reduced sidewall friction, converting the static friction between the sample and the instrument into sliding friction. Based on the characteristics of this device, a corresponding experimental method is proposed, enabling relatively accurate measurement of expansion deformation even with minimal sidewall friction.

[0040] 3. Traditional apparatuses have a small height-to-diameter ratio for the specimens, and the sidewall constraints can lead to an overestimation of the deformation. In contrast, the height-to-diameter ratio of the specimens in this apparatus is much larger than that of traditional specimens, which not only ensures that the deformation of the specimens is axial but also improves the accuracy of the deformation measurement.

[0041] 4. This device can provide expansion deformation-moisture content relationship curves and expansion force-moisture content relationship curves, which can optimize engineering design parameters and guide actual engineering such as foundation depth and replacement thickness. Attached Figure Description

[0042] Figure 1 This is an overview diagram of the device of the present invention; Figure 2 This is the front view of the core measuring device for reducing sidewall friction effect; Figure 3 This is a top view of the core measuring device for reducing sidewall friction. Figure 4 This is a top view of the pressure plate of the expansion force measuring device; Figure 5 This is a top view of the force sensor mounted on the pressure plate; Figure 6 It is a side sectional view of the arrangement of the guide rail, moving ring, and slider; Figure 7 It is a top view of the arrangement of the guide rails, moving ring, and slider; Figure 8 Top view of the chassis of the core measuring device for reducing sidewall friction effect; Figure 9 This is the front view of the sample preparation apparatus; Figure 10 This is the front view of the protective gear for the test specimen; Figure 11 This is the front view of a water bath temperature control device; Figure 12 This is an overview diagram of a traditional device for measuring the deformation characteristics of expansive soil. Figure 13 The graph shows the variation of swelling strain of expansive soil in Ankang over time, measured using different testing devices. Figure 14 The graph shows the change in expansion strain of sodium-based bentonite over time, measured using different testing devices. Figure 15 The graphs show the changes in expansion strain over time of saturated sodium-based bentonite using 1 mol / L NaCl solution, measured with different experimental setups. Figure 16 The graph shows the change in expansion strain over time of saturated sodium-based bentonite using a 1 mol / L NaCl solution, measured using the same experimental setup. The components include: A) a core measuring device for reducing sidewall friction; B) a water bath temperature control system; C) a displacement acquisition system; and D) a data acquisition and image processing system. The components are: 1. Laser displacement sensor; 2. Laser displacement sensor 485 module; 3. Laser displacement sensor USB connector; 4. Force sensor 485 module; 5. Force sensor USB connector; 6. Computer; 7. Lifting bracket; 8. Metal bracket; 9. Camera; 10. Linear bearing; 11. Top plate; 12. Knob nut; 13. Mandrel; 14. Force sensor; 15. Pressure plate; 16. Permeable plate; and 17. Force sensor water inlet. 18. Force sensor vent, 19. Air pipe connector, 20. Force sensor water inlet pipe, 21. Force sensor vent pipe, 22. Groove, 23. Pressure plate screw hole, 24. Hexagonal screw, 25. Outer cylinder, 26. Outer cylinder outlet, 27. Outer cylinder outlet, 28. Top plate hexagonal screw, 29. Bottom plate hexagonal screw, 30. Observation hole, 31. Top ring, 32. Slider, 33. Moving ring, 34. Guide rail base plate, 35. Guide rail, 36. Bottom ring, 3 7. Observation window; 38. Flange; 39. O-ring; 40. Chassis vent outlet; 41. Sample preparation mold chassis; 42. Sample preparation mold outer membrane; 43. Gasket; 44. Indenter; 45. Support leg; 46. Sample sleeve; 47. Sample protection air pipe connector; 48. Sample protection screw; 49. Air valve; 50. Visualization test shell; 51. Semiconductor water-cooled heat sink; 52. Silicone tubing bend; 53. Heat sink outlet; 54. Heat sink. 55. Water inlet; 56. Miniature water pump; 57. Water pump outlet pipe; 58. Water pump inlet pipe; 59. Temperature probe; 60. Stainless steel container; 61. Electric heating constant temperature environment chamber; 62. Safety hole socket; 63. Intelligent time and temperature controller; 64. Temperature setting button; 65. Glass outer cylinder of traditional device; 66. Upper plate of traditional device; 67. Lower plate of traditional device; 68. Double-ended screw; 69. Hexagonal screw of traditional device; 60. Knob nut of traditional device. Detailed Implementation

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0044] The structures, proportions, and sizes illustrated in the accompanying drawings are merely for illustrative purposes and to aid those skilled in the art in understanding and reading the invention. They are not intended to limit the scope of the invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and purpose of the invention, should still fall within the scope of the technical content disclosed herein. Furthermore, the terms "upper," "lower," "left," "right," "middle," and "one" used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0045] like Figure 1 As shown, the test device for reducing the sidewall friction effect of expansive soil includes a core measuring device for reducing sidewall friction effect (A), a water bath temperature control system (B), a displacement acquisition system (C), and a data acquisition system (D).

[0046] The core measuring device for reducing sidewall friction includes a laser displacement sensor 1, a laser displacement sensor-compatible 485 communication module 2, a laser displacement sensor-compatible USB connector 3, and a lifting bracket 7. The laser displacement sensor 1 is fixed to the lifting bracket 7 with screws. The data output line of the laser displacement sensor 1 is connected to the laser displacement sensor-compatible 485 communication module 2. Before the test, the height of the lifting bracket 7 is adjusted so that the laser point accurately hits the center point of the top of the mandrel, and the initial value of the laser displacement sensor is set. The laser displacement sensor-compatible 485 communication module 2 is connected to the computer 6 via the laser displacement sensor-compatible USB adapter 3, and the monitoring software is opened. The corresponding port is searched and connected to perform real-time displacement recording.

[0047] like Figure 2 , Figure 3 , Figure 4 , Figure 5As shown, the force measuring device of the core measuring apparatus for reducing sidewall friction effect includes a permeable plate 16, a pressure plate 15, a force sensor 14, a mandrel 13, and a linear bearing 10. The permeable plate 16 is embedded in the groove 22 at the bottom of the pressure plate 15 and fixed with glue to prevent it from falling off; the permeable plate 16 is a test consumable that can be replaced at any time. The upper part of the pressure plate 15 is provided with a water inlet hole 17 and an air outlet hole 18, which are connected to the bottom groove 22; the water inlet hole 17 and the air outlet hole 18 are threaded; the air pipe connector 19 is fixed on the pressure plate 15 and connected to the sample water inlet pipe 20 and the air outlet pipe 21, so that water can be evenly added through the groove 22 and the permeable plate 16. The pressure plate 15 is provided with screw holes 23; the force sensor 14 is fixed on the pressure plate 15 with hexagonal screws 24; the mandrel 13 is tightened at the center position of the force sensor 14. The data acquisition connector of the force sensor 14 is connected to the 485 module 4 through the outlet 26 located at the upper end of the side wall of the outer cylinder 25. The water inlet pipe 20 and the air outlet pipe 21 are used for water addition and air release through the outlet 27 located at the upper end of the side wall of the outer cylinder. After tightening the knob nut 12 on the top of the top plate 11, the soil sample generates expansion and contraction forces, and the force changes can be recorded by the monitoring software. After the instrument is assembled, the top of the mandrel 13 passes through the linear bearing 10. The linear bearing 10 is equipped with ball bearings, which ensures that the axial expansion force and axial deformation are measured, and also reduces the friction force when the mandrel slides in the linear bearing during deformation. The force sensor and its matching 485 communication module 4 are connected to the computer 6 through the force sensor and its matching USB adapter 5. The data acquisition software is opened, the corresponding port is searched for and connected, and the force changes can be recorded in real time.

[0048] The outer cylinder 25 of the core measuring device for reducing sidewall friction is made of stainless steel. It has eight guide rails 36 evenly distributed inside. The upper and lower parts of the outer cylinder 25 are fixedly connected to the top plate 11 and the base plate by hexagonal screws 28 on the top plate and 29 on the base plate, respectively, forming the main body of the core measuring device for reducing sidewall friction. The outer cylinder 25 has observation holes 30 on its sidewalls; these are designed to allow the camera 9 to record and observe the condition of the moving ring 33 during the test, verifying whether the sliding device reduces side friction during the deformation of expansive soil; and can also serve as the outlet for the pipe when the air is saturated. The sample can be loaded by tightening the knob nut 12 on the top plate 11; the specific pressure value is determined using the data acquisition software of the force sensor 14.

[0049] The sliding device includes a top ring 31, sliders 32, moving rings 33, guide rail base plate 34, guide rail 35, and bottom ring 36. The top ring 31 is located at the top and contacts the pressure plate 15 during the test. Immediately below the top ring 31 are 18 moving rings 33. The bottom ring 36 is connected to the last moving ring 33. Each moving ring 33 is independent, but they are in close contact with each other. All moving rings 33, as well as the top ring 31 and bottom ring 36, are 1 cm high and 10 cm in diameter, forming a 20 cm high, 10 cm diameter area for placing the sample. The top ring 31 and each moving ring 33 are equipped with three sliders 32. The bottom ring 36 is not equipped with any sliders and is not fixed to the observation window 37 at the bottom. During sample loading, the sample is first placed on the bottom ring 36 to fix its position, facilitating subsequent sample loading stages. Each slider 32 is located on a guide rail plate 34; the guide rail plate 34 is connected to the guide rail 35 provided on the inner wall of the outer cylinder; the guide rail plate 34 is provided with two rows of ball bearings; each moving ring 33 can slide freely on the guide rail 35 under the drive of the three sliders 32.

[0050] like Figure 6 , Figure 7 As shown, the sliders 32 on the moving ring 33 are arranged in a staggered manner in each layer; all the rings are in close contact with each other. After assembly, all the guide rail base plates 34 are on the guide rails 35, and all the moving rings 33 slide freely inside the outer cylinder 25.

[0051] like Figure 8 As shown, the chassis includes an observation window 37, a flange 38, an O-ring 39, and an extraction outlet 40. The observation window 37 is installed in a fixing groove inside the flange 38; the flange 38 has a groove on its exterior and is fitted with an O-ring 39; excess latex film from the bottom of the sample is placed over the groove of the flange and sealed with the O-ring 39 for waterproofing. The flange sidewall has an extraction outlet 40, which allows the sample to be saturated with air from the outside.

[0052] like Figure 9 As shown, the sample forming device includes a forming mold and a special protective device for the sample. The forming mold includes a base plate 41, an outer mold 42, a gasket ring 43, a pressure head 44, and support legs 45. The sample is pressed in five layers, each layer being 4 cm. For demolding, the three support legs 45 are installed on the base plate, and the sample can be demolded using the pressure head.

[0053] like Figure 10As shown, the sample protection device includes a sample sleeve 46, an air pipe connector 47, screws 48, an air valve 49, and a vacuum pump. The latex sleeve is placed over the inner wall of the sample protection device, with the excess portion folded out and placed over the outer wall. The air valve 49 is then installed in the pre-drilled air hole on the outer wall of the sample protection device, and the air pipe connector 47 is connected to the air valve 49. The vacuum pump is then used to remove the gas generated between the latex sleeve and the inner wall of the sample protection device, ensuring there are no gaps between them. The sample can then be placed inside the sample protection device. The excess latex sleeve is then folded back over. The six screws 48 on the side wall of the sample protection device can be unscrewed to open the device and remove the sample with the latex sleeve on.

[0054] like Figure 11 As shown, in the water bath temperature control system, a semiconductor water-cooling head heat sink 51 is installed on the visual test shell 50 and connected in series with silicone tubing 52. The connected semiconductor water-cooling head heat sink 51 needs to be connected to the outlet pipe 56 and inlet pipe 57 of the micro water pump 55 through the outlet 53 and inlet 54 respectively, forming a closed water circulation loop. The temperature probe 58 is placed near the half-box, and the stainless steel container 59 and the water pump 55 are both placed in the electric heating constant temperature environment chamber 60. To control the temperature inside the visual test shell 50, the electric heating constant temperature environment chamber 60 needs to be started first, and the test temperature is preset to heat the water in the stainless steel container; then the intelligent time temperature controller 62 is adjusted, and the water circulation temperature is dynamically controlled by the temperature setting button 63. That is, if the actual test temperature is not detected by the temperature probe 58, the power is turned on to heat the water in the circulation loop until the test temperature reaches the preset value, and then the power is turned off.

[0055] like Figure 12 As shown, the traditional testing apparatus includes a force measuring device, a displacement measuring device, and an outer cylinder. The force measuring device and displacement measuring device are almost identical to the force measuring device in the core measuring device for reducing sidewall friction; only the range of the tension / compression sensors and the dimensions of some components differ. The outer cylinder 64 of the traditional testing apparatus is made of acrylic material; its diameter is 10 cm and its height is 30 cm. To meet its rigidity requirements, an upper plate 65 and a lower plate 66 are connected to the upper and lower parts of the outer cylinder, respectively. Grooves are provided in both the upper plate 65 and the lower plate 66 for fixing the outer cylinder. The upper plate 65 and the lower plate 66 are connected by four double-ended screws 67. The upper plate 65 is fixed to the outer cylinder with hexagonal screws 68, forming the main body of the traditional testing apparatus. A knob nut 69 is provided on the upper part of the upper plate 65, which can be tightened to load the sample; the specific pressure value can be determined by combining the data acquisition software of the force sensor. The upper plate 65 linear bearing has balls inside, which can fix the position of the sensor core rod, reduce friction, and detect axial expansion force and axial deformation data.

[0056] A test apparatus for reducing sidewall friction effect on expansive soil deformation characteristics, and a method for determining swelling and shrinkage deformation and swelling force, including the following steps: Step (1), sample preparation: First, the expansive soil is dried and the sand and gravel are removed. Then, the expansive soil is crushed, passed through a 2mm sieve, and dried to obtain loose soil. Then, according to the test requirements, distilled water is sprayed evenly into the loose soil and stirred thoroughly. Then, the prepared loose soil is placed in a plastic bag, sealed, and left to stand for 24 hours to ensure that the moisture in the loose soil is evenly distributed. Then, the prepared loose soil is divided into 5 portions for later use. Due to the large size of the sample, a press is used to compact the sample. The specific operation steps are as follows: First, talcum powder is applied to the inner wall of the molding mold to facilitate the removal of the press head and subsequent demolding after each layer of sample is pressed. Then, the outer membrane of the molding mold is placed on the base plate, and a portion of loose soil is poured into the outer membrane. Then, a gasket is placed on the press head to compact the loose soil. Then, the press head is removed and roughened with a scraper to ensure good contact between sample layers. After each portion of loose soil is placed, a pad ring is added to the pressure head for compaction. Repeat the above compaction operation to compact 5 portions of loose soil. After that, remove all the pad rings from the pressure head, then install the support legs on the outer mold. The sample can then be pressed out completely with the pressure head.

[0057] During sample preparation, the following two aspects should be noted: First, after the first layer of sample is compacted, the indenter is difficult to remove. In this case, the base plate can be removed by inverting the indenter and the forming mold. Then, a pad of the same size as the forming mold can be placed on the sample, and the indenter can be pressed out and removed by pressing lightly with a press. Second, during the demolding process of the sample, the indenter and the forming mold should be kept coaxial to avoid the sample breaking during demolding due to uneven force. After the sample is demolded, the sample surface should be checked for smoothness, cracks or defects.

[0058] Step (2), applying the latex film: First, apply the latex film to the inner wall of the sample protector and then fold the excess part out and apply it to the outside of the sample protector; then, install the air valve in the air hole reserved on the outside of the sample protector, and connect the air pipe connector to the air valve; then use a vacuum pump to remove the gas between the latex film and the inner wall of the sample protector to ensure that there is no gap between the latex film and the sample protector; then put the sample into the sample protector and fold the excess latex film back in; finally, unscrew the 6 screws on the side wall of the sample protector to open the sample protector and take out the sample with the latex film applied.

[0059] Step (3), placing the sample: First, fold up the excess latex film at the bottom of the sample (already covered with latex film), and then place the bottom ring on the bottom of the sample; then, fold down the excess latex film at the bottom of the sample and place it into the groove of the flange to secure it. The remaining latex film can be repeatedly folded and placed on the flange. Note: During the folding process of the latex film, it should not be placed too deep, otherwise it will affect the installation of the outer cylinder.

[0060] Step (4), Install the outer cylinder: After fixing the bottom ring on the bottom of the sample (with the latex film already fitted), apply lubricating oil to the surface of the latex film to reduce the friction effect between the moving ring and the latex film, so that the outer cylinder can be installed smoothly. When installing the outer cylinder, first lift it and all the moving rings at the same time from the observation hole of the outer cylinder to prevent the moving rings from falling off and avoid damaging the ball bearings in the guide rail seat plate; then, lower the outer cylinder from the top of the sample, while preventing excess latex film from the top of the sample from being trapped between the moving ring and the sample; after placing the outer cylinder on the chassis, fine-tune the position of the outer cylinder so that the position of the chassis air extraction outlet is aligned with the position of the outer cylinder observation hole and the position of the screw hole of the outer cylinder is aligned with the position of the screw hole on the chassis, and then fix the outer cylinder to the chassis with screws.

[0061] Step (5), install the expansion force measuring device: First, fix the force sensor on the pressure plate with screws, and then install two air pipe connectors on the pressure plate; then, connect the hose to the air pipe connectors, and then install the permeable stone in the groove at the bottom of the pressure plate; then, open the latex film on the top of the sample and place the assembled expansion force measuring device in it. During the placement process, tighten the latex film until the pressure plate and the sample are in good contact; then, pass the hose through the water inlet and air outlet holes reserved on the side wall of the outer cylinder to facilitate the subsequent addition of water to the sample, and then pass the data output line connector of the force sensor through the hole reserved on the top plate; finally, place the top plate on the upper part of the outer cylinder, so that the mandrel passes through the linear bearing in the center of the top plate, and then finely adjust the position of the top plate so that the screw hole on it is aligned with the screw hole on the outer cylinder. Then, the outer cylinder can be fixed to the base plate with screws.

[0062] Step (6), control the test environment temperature: first, place the core measuring device and the corresponding sensor inside the test shell of the water bath temperature control device, then heat the circulating water to a suitable temperature using an electric heating constant temperature environment chamber, and then start the water circulation; then determine whether the test set temperature has been reached by the temperature control probe of the intelligent time temperature controller, and thus regulate the water circulation according to the change of the test environment temperature, so as to ensure that the test can be carried out in a stable temperature environment.

[0063] Step (7), debug the data acquisition and image processing system: Debugging the expansion and contraction deformation data acquisition device: First, fix the laser displacement sensor on the bracket, turn on the power to display data, and then adjust the universal table on the bracket so that the laser is perpendicular to the top surface of the core measuring device. Next, place the core measuring device to one side of the laser displacement sensor so that the light source illuminates the center of the mandrel. Since the laser displacement sensor's range is 120 mm to 280 mm, and the mandrel moves outward during sample expansion and deformation, the bracket height should be adjusted so that the laser displacement sensor reading is within the range of 270 mm to 280 mm to ensure that the range is not exceeded during the test. Then, connect the sensor's data output lines to the TR+ and TR- serial ports of the RS485 communication module, and then fix it to the computer using screws via a USB to RS485 serial adapter. Finally, open the laser displacement sensor data acquisition software on the computer, select the corresponding serial port of the sensor, connect it, check if the sensor matching is normal, and then set the data acquisition time interval and data display mode to begin data acquisition.

[0064] Debugging the expansion force data acquisition device: First, pass the data output connector of the force sensor through the pre-drilled hole on the top plate. Since the sensor is located inside the flange and has a different signal output format, it needs to be connected to a single-channel digital transmitter. Then, connect the adapter through the single-channel digital transmitter to the RS485 communication module, and then fix it to the computer using screws via a USB to RS485 serial adapter. Finally, open the force sensor data acquisition software CurveDisplay on the computer, select the port corresponding to the sensor, connect it, and check if the sensor matching is normal. After zero-point calibration, set the data acquisition time interval, and data acquisition can begin.

[0065] A high-resolution mirrorless camera (3648×2736 pixels) is fixed to one side of the observation window on the base of the core measuring device via a tripod. It can then capture high-definition images of the cracks on the sample surface in real time, and then process and analyze the crack images using the computer image processing software ImageJ.

[0066] Step (8), saturate the sample by vacuuming: First, determine the position of the observation hole and the position of the chassis outlet on the side wall of the outer cylinder. The position of the outlet is parallel to the bottom ring. Connect a gas pipe connector to the hole at the outlet position. One end of the plastic tube is connected to the gas pipe connector, and the other end passes through the observation hole and is connected to a vacuum pump with a vacuum gauge and regulating valve. In the process of adding water for saturation, the initial stage is the conventional water addition method, that is, adding water to the sample at constant pressure through the Marvin bottle. After the water volume in the Marvin bottle has not changed for 24 hours, vacuuming saturation begins. In the vacuuming saturation stage, the entire system is in an open state. The vacuum pump is used to evacuate at the bottom. The negative pressure of the entire system is kept within a small safe range by controlling the valve to prevent damage to the sample. Water seeps downward under the action of positive pressure and gravity. Combined with the negative pressure applied by the vacuum pump, most of the free air in the sample can be discharged, thereby increasing the saturation of the sample.

[0067] Step (9) involves conducting the following three types of experiments: A. Swelling and Contraction Deformation Test under Alternating Salt Solution Conditions: This test simulates the swelling and contraction deformation of the sample caused by chemical permeation and hydration resulting from the circulation of salt solution (NaCl solution) and distilled water. Another salt solution (KCl solution) is then introduced. The circulation path for NaCl solution and distilled water is: NaCl solution - distilled water - NaCl solution - distilled water; the circulation path for NaCl solution, distilled water, and KCl solution is: NaCl solution - distilled water - KCl solution - distilled water. Salt solution and distilled water are sequentially introduced into the sample through the inlet according to the above paths. Simultaneously, the laser displacement sensor and data acquisition system are activated to record the displacement changes of the mandrel during the circulation process. During the test, if the displacement change of the mandrel within 24 hours after the infiltration of a certain concentration of salt solution or distilled water is less than 0.01 mm, the test is considered complete for this stage. At this point, the next solution can be introduced according to the circulation path, and so on, following the preset circulation path until the test is completed.

[0068] B. Expansion and contraction deformation test under wet-dry cycle conditions: The test temperature is controlled at 25℃ using a water bath temperature control system. Water is added to the sample through the inlet until saturation is achieved. During this process, the sample will undergo axial deformation, causing displacement of the mandrel. The amount of expansion deformation of the sample during saturation is recorded using a data acquisition system. If the displacement change of the mandrel is less than 0.01 mm within 24 hours, the test is considered complete for this stage. After that, water addition is stopped, and the dehumidification stage begins. As the moisture content of the sample decreases, axial shrinkage will occur. The amount of shrinkage deformation of the sample during dehumidification is recorded using a data acquisition system. If the displacement change of the mandrel is less than 0.01 mm within 24 hours, the test is considered complete for this stage.

[0069] C. Expansion Force Test: This device measures expansion force using the constant volume method. After the sample is placed, tighten the knob nut on the top plate, and then start adding water through the inlet to saturate the sample. During sample saturation, the knob nut contacts the mandrel. During deformation, the knob nut hinders the displacement of the mandrel, thus measuring the expansion force during the expansion process. The expansion force generated by the sample during the expansion process is recorded by the data acquisition system. Combined with the 0.1 N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors in Geotechnical Engineering Instruments", the test end time when the pressure test data tends to stabilize is determined.

[0070] To prevent damage to the ball bearing caused by the dynamic ring falling off, the disassembly steps after the test are as follows: Step (1): Turn off the power to the force sensor and displacement sensor; remove the screws on the top plate 11; remove the top plate 11 and take out the force measuring device; tie the excess latex film on the upper side of the sample with rubber bands; fix the moving ring 33 through the outlet 26 and outlet 27 at the upper end of the outer cylinder 25 to prevent the moving ring from falling off when the instrument is inverted.

[0071] Step (2): Invert the test device for expansive soil deformation characteristics to reduce side wall friction; remove the base screws 29; gently lift the base; remove the latex film on the base; slowly pull the latex film to completely pull out the soil sample, while gently pressing the moving ring 33 to ensure that the moving ring does not come off the guide rail 35 with the latex film.

[0072] Step (3): After sampling, reinstall the chassis onto the outer cylinder 25 and place the entire device upright; after the test, apply lubricating oil to the guide rail 35 and gently slide the ring for maintenance.

[0073] Traditional test setups use the same test procedures and materials for comparative tests, aiming to verify the reduction of sidewall friction effect by the sliding device.

[0074] To verify the advantages of this invention over traditional testing devices in reducing lateral friction and improving the accuracy of expansion deformation testing, the inventors, referring to the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019), also constructed a traditional device for measuring the deformation characteristics of expansive soil (hereinafter referred to as the traditional testing device), such as... Figure 12 As shown, the sample size in this conventional device is the same as that of the present invention, and it ensures that comparative tests can be conducted under the same test conditions as the present invention. Figure 12 It is also known that the outer cylinder of the traditional device is made of acrylic material, without considering the reduction of side wall friction. To meet its rigidity requirements, the upper and lower parts of the outer cylinder are connected to the base plate and the top plate, respectively. The base plate is provided with grooves for fixing the outer cylinder. The upper and lower plates are connected by four double-ended screws, and then the top plate is fixed to the outer cylinder with hexagonal nuts, forming the main body of the traditional device.

[0075] This paper evaluates the advantages of the present invention over traditional devices in terms of reducing lateral friction and improving the accuracy of expansion deformation testing by conducting a rationality verification test on the deformation characteristics of expansive soil.

[0076] (1) Soil used for testing The verification experiments used expansive soil and sodium-based bentonite from Ankang as the research objects. The expansive soil was collected from the Zhenping Enclave Industrial Park, Gaoxin 13th Road, Ankang High-tech Industrial Development Zone, Shaanxi Province, located in the northwest of Ankang City, at a sampling depth of approximately 6 m. After air-drying and crushing, the collected soil samples underwent basic physical and mechanical property tests and swelling rate tests. The corresponding main physical and mechanical property parameters are shown in Table 1. The sodium-based bentonite was collected from Tongqiao Village, Gaodian Township, Luoshan County, Xinyang City, Henan Province. The corresponding main physical and mechanical property parameters are shown in Table 2.

[0077] Table 1. Main physical and mechanical properties of expansive soil in Ankang 2.70 18.8 1.73 42.1 23.2 56 7.52 82.4 10.08 Table 2. Main physical and mechanical properties of sodium-based bentonite 2.70 1.73 256 35 294 (2) Comparative test plan The experiment aimed to verify the instrument's effect in reducing side friction. Soil was dried and mixed with an appropriate amount of distilled water to the target moisture content. After standing for 24 hours, it was compacted using a layered compaction method to form soil column samples with a diameter of 95 mm and a height of 200 mm. These samples were then fitted with latex diaphragms and placed into both the experimental apparatus of this invention and the conventional apparatus. After sample loading, the position and height of the laser displacement sensor were adjusted to ensure the light source accurately illuminated the center of the mandrel and its height was within the sensor's range. The required moisture content for sample saturation was calculated using the sample preparation parameters. Water was then gradually added to both apparatuses using a vacuum saturation method until the samples reached saturation. During saturation, the samples underwent axial expansion deformation due to the continuously increasing moisture content. The relationship between the deformation and time was measured using the laser displacement sensor. The experiment ended when the change in the laser displacement sensor reading was less than 0.01 mm over 24 consecutive hours. The results of the two sets of experiments were compared to verify the invention's reduction of sidewall friction during the expansion deformation process. Note: Three parallel tests were performed for each type of soil during the expansion deformation test.

[0078] (3) Analysis of comparative test results like Figure 13As shown, the expansion deformation of Ankang expansive soil in the traditional test apparatus is very small, mainly because Ankang expansive soil itself is a moderately weak expansive soil with inherently small expansion deformation. Additionally, the resistance generated by the latex film on the sample also contributes to the small amount of expansion deformation. However, the results from the traditional test apparatus show that the sample hardly deforms, due to the large height-to-diameter ratio and the significant frictional effect between the barrel sidewall and the sample.

[0079] like Figure 14 As shown, the expansion deformation curves measured by both the traditional testing apparatus and the present invention include three stages: (1) rapid expansion stage; (2) deceleration expansion stage; and (3) stabilization stage. However, in the early stage of the test, the sample, which should have been in the rapid expansion stage, hardly deformed. This is because the prepared test dimensions are 95 mm in diameter and 200 mm in height, while the diameter of the moving ring is 100 mm. There is actually a 5 mm distance between the sample and the moving ring. Therefore, in the early stage of the expansion deformation process, the sample first undergoes radial expansion deformation, and thus no axial strain is generated.

[0080] like Figure 15 As shown, only in the tests of sodium-based bentonite in the new and traditional test setups, when all other initial test conditions are the same, the only difference is that the distilled water is replaced with a 1 mol / L NaCl solution.

[0081] like Figure 13 , Figure 14 , Figure 15 As shown, under the same experimental conditions, the expansion strain of Ankang expansive soil and sodium-based bentonite measured by this invention in distilled water and 1 mol / L NaCl solution increased by 2.27 times, 1.56 times, and 2.08 times, respectively, compared with the traditional experimental device. This demonstrates that the sliding drag-reducing device of this invention significantly reduces sidewall friction during the expansion deformation process.

[0082] like Figure 16 As shown, in the sodium-based bentonite experiment in the new experimental setup, ensuring all other initial experimental conditions are the same, replacing distilled water with a 1 mol / L NaCl solution will suppress the expansion strain of sodium-based bentonite. The salt solution reduces the expansion of the clay mineral crystal layer by lowering the water activity, preventing water molecules from entering the voids between the clay mineral crystal layers. Secondly, the cation concentration also affects the expansion of the electric double layer; a high concentration of salt solution will suppress the expansion of the electric double layer. Therefore, the analysis of the influence of the expansion between clay mineral crystal layers shows that the salt solution has a significant inhibitory effect on the expansion deformation of bentonite.

[0083] Regarding the microscale study of bentonite clay mineral crystal layers, the paper "Influence of layer charge on swelling of smectites" published in *Applied Clay Science* by DA Laird, C. Shang, ML Thompson, et al., used X-ray diffraction experiments to determine the interlayer spacing of sodium-based bentonite by controlling NaCl concentrations (0.35 mol / L–3.00 mol / L). The results showed that for sodium-based bentonite, when the clay minerals were in distilled water, the interlayer spacing was greater than 40 Å; when the clay minerals were in NaCl solution, the interlayer spacing decreased to 15.5 Å–19 Å with increasing NaCl concentration, a reduction of at least 1.11–1.58 times compared to the interlayer spacing in distilled water. Figure 16 It can also be seen that the expansion strain of the sodium-based bentonite used in the experiment of this invention under the condition of 1 mol / L NaCl solution was reduced by 1.38 times compared with that under the condition of distilled water, which is exactly within the range of the reduction factor of interlayer spacing described in the paper "Influence of layer charge on swelling of smectites". This indicates that in the comparative experiment of this invention, if the sodium-based bentonite is replaced with 1 mol / L NaCl solution instead of distilled water, the reduction in its expansion strain is mainly due to the inhibitory effect of the salt solution on the interlayer voids of clay minerals. It can be seen that the electrochemical phenomena on the surface of clay mineral crystals have a significant impact on the macroscopic expansion deformation of expansive soil. When predicting the deformation characteristics of expansive soil, it is necessary to consider the micromechanical behavior between clay mineral crystal layers. The experimental results based on this invention provide important experimental basis for the theoretical study of the deformation characteristics of expansive soil.

Claims

1. A test device for the deformation characteristics of swelling and shrinking soil to reduce sidewall friction effect, characterized in that, It includes a core measuring device for reducing sidewall friction, a water bath temperature control device, and a data acquisition and image processing system; The core measuring device for reducing sidewall friction effect has an external cylindrical structure assembled from a top plate, an outer cylinder, and a base, and its internal components include a sliding friction reduction device and an expansion force measuring device arranged vertically. The sliding drag reduction device includes a track base plate, a top ring, sliders, a moving ring, and a bottom ring. Multiple moving rings are stacked vertically and located between the top ring and the bottom ring. Each top ring and each moving ring is equipped with three sliders, which are located on the same track base plate. The track base plate is clamped onto the guide rail on the inner side wall of the outer cylinder. The moving rings on adjacent layers are staggered. The top ring is fitted around the pressure plate of the expansion force measuring device, and the bottom ring is located on the base. The expansion force measuring device includes a permeable plate and a pressure plate. The permeable stone is embedded in a groove at the bottom of the pressure plate. The upper part of the pressure plate is provided with a water inlet and an air outlet that communicate with the bottom groove. The water inlet and the air outlet are connected to the water inlet pipe and the air outlet pipe, respectively. A force sensor is fixedly installed on the pressure plate. A mandrel is installed at the center of the force sensor. The force sensor is connected to a data acquisition and image processing system. The upper end of the mandrel moves through the top plate.

2. The experimental device for reducing sidewall friction effect in expansive and contractile soil deformation characteristics as described in claim 1, characterized in that, Each moving ring, as well as the top and bottom rings, is 1 cm high and 10 cm in diameter. The 18 moving rings and the top and bottom rings are in close contact to form a sample placement area that is 20 cm high and 10 cm in diameter.

3. The experimental device for reducing sidewall friction effect in expansive and contractile soil deformation characteristics as described in claim 1, characterized in that, There are a total of 8 guide rails, which are arranged in parallel and equally spaced along the inner wall of the outer cylinder. At the position where the guide rails connect to the guide rails inside the guide rail base plate, there are 2 rows of ball bearings to ensure that each moving ring can slide freely on the guide rails under the drive of 3 sliders.

4. The experimental device for reducing sidewall friction effect in expansive and contractile soil deformation characteristics as described in claim 1, characterized in that, The outer cylinder is made of stainless steel, and its upper and lower parts are connected to the chassis and the top plate, respectively. An observation hole is provided in the middle of the side wall of the outer cylinder, and two round holes for the water inlet pipe and the exhaust pipe to pass through, as well as a force sensor data output connector, are provided at the upper end of the side wall of the outer cylinder.

5. The test device for reducing sidewall friction effect of swelling and shrinking soil deformation characteristics as described in claim 1, characterized in that, The base includes an observation window, a flange, and an O-ring. The observation window is placed inside the flange, and O-rings are provided on both the inner wall and the outer groove of the flange. The bottom ring is placed on the observation window and does not need to be fixed.

6. The test device for reducing sidewall friction effect of swelling and shrinking soil deformation characteristics as described in claim 1, characterized in that, The top plate includes a linear bearing, a plate surface, an observation port, and a knob nut. The plate surface is fixed to the top of the outer cylinder. The top of the mandrel passes through the linear bearing at the center of the plate surface. Ball bearings are installed inside the linear bearing. A knob nut is installed on the upper part of the plate surface. The sample is constrained by tightening the knob nut, and the expansion force value is measured by data acquisition software connected to the force sensor.

7. The test device for reducing sidewall friction effect of swelling and shrinking soil deformation characteristics as described in claim 1, characterized in that, The water bath temperature control device is connected to the core measuring device for reducing sidewall friction. The water bath temperature control device includes a stainless steel water tank, a micro water pump, a semiconductor water-cooled head heat sink, an electric heating constant temperature environment chamber, an intelligent time and temperature controller, and a test shell. The test shell is made of acrylic glass and has insulation material at the bottom. Its internal space is used to house the core measuring device for reducing sidewall friction, and its exterior is equipped with a water inlet connected to the core measuring device. A semiconductor water-cooled head heat sink is installed on its inner wall and is connected to the electric heating constant temperature environment chamber through inlet and outlet water pipes. The semiconductor water-cooled head heat sink is connected by a flexible hose to achieve the water bath heating effect in water circulation. The intelligent time and temperature controller has a temperature probe to monitor the test environment temperature and adjust the heating temperature of the electric heating constant temperature environment chamber according to the monitored temperature.

8. The test device for reducing sidewall friction effect of swelling and shrinking soil deformation characteristics as described in claim 5, characterized in that, The data acquisition and image processing system is used to acquire and process data from the core measuring device for reducing sidewall friction effect. The data acquisition and image processing system includes a expansion and contraction deformation data acquisition device, an expansion force data acquisition device, a high-resolution micro-camera, and a computer. The expansion and contraction deformation data acquisition device includes a laser displacement sensor, an RS485 communication module for the laser displacement sensor, a 220V to 24V power supply, a USB to RS485 serial port adapter, and a lifting bracket. The laser displacement sensor is fixed on the lifting bracket, which enables the laser point to be accurately positioned at the top of the mandrel. The data output line of the laser displacement sensor is connected to the RS485 communication module, and the USB to RS485 serial port adapter connects the RS485 communication module to the computer. The expansion force data acquisition device includes an RS485 communication module for a force sensor, a single-channel digital transmitter, a 220V to 24V power supply, and a USB to RS485 serial port adapter. The data output line of the force sensor is connected to the single-channel digital transmitter; the single-channel digital transmitter is connected to the RS485 communication module, and the USB to RS485 serial port adapter connects the RS485 communication module to the computer. The high-resolution mirrorless camera is located on one side of the observation window on the base of the core measurement device.

9. The test device for reducing sidewall friction effect of swelling and shrinking soil deformation characteristics as described in claim 1, characterized in that, It also includes a sample preparation device, which includes a molding die and a sample protector. The molding die includes a base, an outer mold, a gasket, and a pressure head. The sample protector includes a sample sleeve, a gas pipe connector, a gas valve, screws, and a vacuum pump.

10. A method for determining the deformation characteristics of swelling and shrinking soil using the test apparatus for reducing sidewall friction effect according to any one of claims 1-9, comprising the following steps: Step (1), prepare the sample; Step (2), apply latex film; Step (3), place the sample; Step (4), Install the outer cylinder: After fixing the bottom ring on the bottom of the sample, apply lubricating oil to the surface of the latex film. First, lift the outer cylinder and all the moving rings at the same time from the observation hole. Then, lower the outer cylinder from the top of the sample, while preventing excess latex film from the top of the sample from getting stuck between the moving ring and the sample. After placing the outer cylinder on the base plate, fine-tune the position of the outer cylinder so that it is aligned with the screw hole on the base plate, and then fix the outer cylinder on the base plate with screws. Step (5), install the expansion force measuring device: first, fix the force sensor on the pressure plate with screws, and then install two air pipe connectors on the pressure plate; then, connect the hose to the air pipe connectors, and then install the permeable stone in the groove at the bottom of the pressure plate; then, open the latex film on the top of the sample, place the assembled expansion force measuring device in it, and then pass the hose through the water inlet and vent holes reserved on the side wall of the outer cylinder, and then pass the data output line connector of the force sensor through the hole reserved on the top plate; finally, place the top plate on the upper part of the outer cylinder, so that the mandrel passes through the linear bearing in the center of the top plate, and then finely adjust the position of the top plate so that the screw hole on it is aligned with the screw hole on the outer cylinder, and then fix the outer cylinder on the base plate with screws; Step (6), control the test environment temperature: first, place the core measuring device and corresponding sensors inside the test shell of the water bath temperature control device, then heat the circulating water to the set temperature using an electric heating constant temperature environment chamber, and then start the water circulation; then determine whether the test set temperature has been reached by the temperature control probe of the intelligent time temperature controller, and adjust the water circulation according to the change of the test environment temperature. Step (7), debug the data acquisition and image processing system: Debugging the expansion and contraction deformation data acquisition device: First, fix the laser displacement sensor on the bracket, turn on the power to make it display data, and then adjust the universal table on the bracket so that the laser is perpendicular to the top surface of the core measuring device; then place the core measuring device on one side of the laser displacement sensor so that the light source is exactly illuminating the center of the mandrel; then, connect the sensor's data output line to the computer; open the laser displacement sensor data acquisition software on the computer and use the laser user software to collect data. Debugging the expansion force data acquisition device: First, pass the data output connector of the force sensor through the hole reserved on the top plate and connect the sensor to the single-channel digital transmitter; then, connect the single-channel digital transmitter to the computer; finally, open the force sensor data acquisition software CurveDisplay on the computer to acquire data. A high-resolution mirrorless camera is fixed to one side of the observation window of the base of the core measuring device via a tripod, and then high-definition images of the cracks on the sample surface are captured in real time. The crack images are then processed and analyzed by the computer image processing software ImageJ. Step (8): Saturate the sample by evacuation. Step (9) involves conducting expansion and contraction deformation tests under alternating salt solution conditions, expansion and contraction deformation tests under wet-dry cycle conditions, and expansion force tests.

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