Large-scale visualized direct shear test device and test method for geosynthetics

Abstract

The invention discloses a large-scale visualized direct shear test device and method for geosynthetics. The device comprises a main frame, an upper shear box, a lower shear box, a normal cyclic loading system, a horizontal cyclic shear loading system, a data acquisition system and a visualized monitoring system. The direct shear test device is characterized in that the direct shear test device can perform normal and horizontal cyclic loading at the same time, and normal stress with different frequencies and amplitudes can be applied according to needs to simulate a test of the characteristics of the reinforcement-soil interface under the cyclic loading effect; one side wall of the upper shear box and one side wall of the lower shear box are transparent observation windows, and a high-speed camera and a digital image analysis method are combined, so that the shear band evolution process can be visually observed and quantitatively analyzed; the shear box is large in size, and the size effect is effectively overcome. According to the invention, the interface characteristics under the cyclic load effect in the road engineering can be reflected more truly and comprehensively, the size and the function of the shear box are comprehensive, and the interface mechanical characteristics and the interaction mechanism under various working conditions can be simulated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geotechnical engineering test, in particular to a large-scale visual direct shear test device and test method for studying the interaction between geosynthetic material and soil. BACKGROUND

[0002] The macroscopic mechanical properties of soil are closely related to its microstructure characteristics, such as particle distribution and displacement behavior. Direct shear test is a basic method for determining the shear strength of geotechnical materials and their interface with reinforced materials. However, the shear box of the soil direct shear apparatus currently used in the laboratory is made of brass, which makes it impossible to directly monitor the changes of the soil sample during the shearing process and only the physical and mechanical parameters can be obtained, while the particle distribution and movement state are difficult to observe. Currently, many researchers use numerical simulation techniques to study the displacement, stress distribution and porosity of cohesive soil. However, these simulation results often lack universal applicability and differ significantly from actual engineering conditions. Although some researchers have made simple modifications to the indoor soil direct shear apparatus and achieved preliminary visual monitoring, there are still obvious shortcomings: single observation angle, poor coordination between image processing system and instrument, lack of innovation in soil mechanical behavior research methods, and increased measurement error of the modified instrument. The traditional direct shear apparatus has the following limitations:

[0003] (1) Single function: It is difficult for one device to simulate the shear characteristics of different normal stress amplitudes and vibration frequency conditions, and it is impossible to fully evaluate the performance of the reinforcement.

[0004] (2) Size effect: The size of the conventional shear box is small, and for materials such as geogrids with large aperture and undulating structure, the test results cannot truly reflect their macroscopic mechanical behavior.

[0005] (3) Mechanism is not clear: The traditional device cannot monitor the mesoscopic movement of soil particles and reinforcement in the shear band, and the understanding of strength development and deformation failure mechanism is limited to the macroscopic level.

[0006] (4) Data is not comprehensive: Only the macroscopic force-displacement relationship can be obtained, and there is a lack of quantitative description of key information such as the formation process, thickness of the shear band, and deformation of the reinforcement. The stress-strain curve method cannot accurately match the complete evolution process of the shear band failure of geotechnical materials exhibited by the visualization means.

[0007] It can be seen that the existing visualized geotechnical direct shear apparatus cannot effectively solve these technical problems. However, the current international academic circles generally agree that the shear characteristics and internal mechanisms of rock-soil bodies can be explored by analyzing the formation process of shear bands. With the aid of visualization technology, the growth, evolution and deformation characteristics of shear bands can be observed, which can provide strong support for the risk prediction and evaluation of geological disasters. These research results not only lay a theoretical foundation for the durability research of geosynthetic reinforced soil structure, but also can effectively improve the safety performance of engineering projects, and promote the virtuous cycle development of social and economic benefits. Therefore, there is an urgent need for a direct shear test device and method which can realize full-process visualization and be used for large sample testing. SUMMARY

[0008] The purpose of the present application is to overcome the shortcomings of the prior art, provide a visualized and large-scale geosynthetic direct shear test device and method, which can complete various modes of shear test on one equipment and accurately reveal the mechanical mechanism. The equipment is a large-scale visualized research equipment for the friction characteristics of the interface between soil and reinforcement, which can not only conduct shear characteristics test of soil itself, but also conduct test research on the friction characteristics between soil and contact surface (including concrete contact surface, contact surface of other different types of soil, geosynthetic materials, etc.). The large-scale direct shear equipment can apply dynamic load (including static load) with different loading frequency and amplitude in vertical and horizontal directions, and can also conduct equal shear plane and variable shear plane direct shear test, and conduct related comparative test analysis. The test device is beneficial to obtain test data more consistent with actual working conditions, and has high adaptability.

[0009] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0010] A large-scale visualized geosynthetic direct shear test device, comprising a main frame;

[0011] A direct shear apparatus, comprising an upper shear box and a lower shear box, which are arranged in the main frame in a corresponding manner, and a replaceable clamping module is arranged at the contact surface of the two, which is used for adapting to different test modes;

[0012] A normal loading system arranged above the upper shear box, which is used for applying vertical load, and the device is composed of a servo loading device and an automatic lifting mechanism, which can apply cyclic load with different frequency and amplitude;

[0013] A horizontal shear loading system connected with the lower shear box, which is used for driving the lower shear box to generate horizontal relative displacement;

[0014] A measuring component, comprising a vertical load sensor and a vertical LVDT displacement sensor;

[0015] On the right side of the main frame is a horizontal load application device, which consists of a servo motor and an automatic lift. At the bottom of the load application device are a shear load sensor, a horizontal LVDT displacement sensor, and a computer-controlled data acquisition device.

[0016] The data acquisition system is used to collect normal force, shear force, and displacement data in real time.

[0017] The visualization monitoring system includes a transparent observation window located on the side of the lower shear box and an image acquisition device facing the transparent observation window.

[0018] Furthermore, the normal load application device can apply cyclic loads, enabling the study of the interface friction characteristics between soil and geosynthetic materials under cyclic loads.

[0019] Furthermore, the clamping module includes a rigid clamp that is detachably mounted on the front end of the lower shear box. The rigid clamp is used to clamp the front half of the geosynthetic material in the direct shear test at the reinforcement-soil interface to ensure that the geosynthetic material does not slip during the shearing process.

[0020] Furthermore, at least one side of the lower shear box is a transparent observation window made of a high-strength transparent material; the image acquisition device is a high-speed camera and is connected to a computer equipped with digital image correlation analysis software.

[0021] Furthermore, the lower shear box of the direct shearing device is longer than the upper shear box.

[0022] Furthermore, the data acquisition system includes a normal force sensor, a shear force sensor, a displacement sensor, and a miniature earth pressure cell embedded at the interface between the soil and the reinforcement.

[0023] Furthermore, the horizontal shearing loading system is a ball screw actuator driven by a servo motor, which can control the shearing rate.

[0024] Furthermore, the normal load application device can apply cyclic loads of different frequencies and amplitudes, and can conduct tests on the characteristics of the reinforced soil interface under different vertical load conditions, including static and dynamic loads.

[0025] According to another aspect of the present invention, a method for large-scale visual direct shear testing of geosynthetics using the above-described apparatus is provided, characterized by comprising the following steps:

[0026] S1. Test mode selection and installation: Select and install the corresponding clamping module according to the test objective;

[0027] S2. Sample preparation: Soil is filled into the upper and lower shear boxes to a predetermined density. If it is a reinforcement-soil interface test, the geosynthetic material is installed according to the corresponding mode.

[0028] S3. Apply normal stress: Apply a predetermined normal stress through the normal loading system;

[0029] S4. Synchronous Start-up and Cutting: Start the data acquisition system and visualization monitoring system, and then control the horizontal cutting loading system to cut at a constant rate;

[0030] S5. Data Processing and Analysis: Based on the collected data, draw shear stress-shear displacement curves and calculate shear strength parameters; at the same time, based on the images recorded by the visualization monitoring system, analyze the entire process of shear band development and evolution.

[0031] Furthermore, in step S5, a set of tests are conducted under different normal stresses to obtain the peak shear strength, and the Coulomb strength envelope is fitted to determine the cohesion c and the internal friction angle φ. At the same time, the thickness of the shear band, the entire evolution process, and the deformation and contribution of the reinforcing material in the composite are obtained through digital image correlation analysis.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) Multi-purpose: Through modular design, one device can complete three core direct shear tests, which greatly improves the utilization rate of the equipment and the comparability of the tests. It can simulate the shear characteristics under different normal stress amplitudes and vibration frequencies.

[0034] (2) Mechanism visualization: The combination of transparent observation window and DIC technology transforms the "black box" operation into a visible process, enabling direct observation and analysis of the generation, development, thickness and deformation of shear bands and reinforcing materials, greatly deepening the understanding of the mechanism.

[0035] (3) Reliable results: The large-scale shear box ensures the representativeness of the test results and the reliability of the engineering.

[0036] (4) Rich data dimensions: Combining macroscopic mechanical parameters with microscopic deformation field information provides extremely valuable verification data for theoretical model development and numerical simulation.

[0037] The unique advantage of this invention lies in its experimental device's ability to flexibly adjust the frequency and amplitude of vertical forces, thereby accurately simulating the impact of cyclic loading on the soil-reinforcement interface and more realistically reproducing the actual loading conditions in road engineering. Furthermore, the system possesses efficient data acquisition and processing capabilities, enabling the rapid generation of accurate test charts. This experimental equipment can be used to observe phenomena such as morphological changes, geometric characteristics, internal structural evolution, and crack development patterns during the crushing and extrusion of soil and rock materials. By integrating non-destructive testing technology with a direct shear apparatus, this device pays particular attention to the expansion and contraction characteristics of soil and rock materials during shearing. This innovative design not only significantly expands the applicability of the equipment but also solves technical problems such as insufficient compatibility between traditional image processing systems and instruments, and the difficulty in intuitively presenting the microscopic mechanisms of soil and rock shearing under multi-factor coupling, thus effectively enhancing the matching degree between geotechnical testing and practical engineering applications. Therefore, this invention has strong applicability and broad development prospects. Attached Figure Description

[0038] Figure 1 This is the front view of an example of the present invention.

[0039] Figure 2 This is a top view of an example of the present invention.

[0040] Figure 3 This is a schematic diagram of the shear box structure in an embodiment of the present invention.

[0041] Figure 4 This is a schematic diagram of the top loading plate in an example of the invention.

[0042] The reference numerals in the attached figures are explained as follows:

[0043] 1-Main frame, 2-Upper shear box, 3-Lower shear box, 3-1-Transparent observation window, 4-Normal loading system, 4-1-Loading plate, 5-Shear loading system, 6-Data acquisition system, 6-1-Vertical displacement sensor, 6-2-Horizontal displacement sensor, 7-Visual monitoring system, 7-1-High-speed camera, 8-Geosynthetic material, 9-Soil, 10-Clamp, 11-Control panel, 12-Slide rail, 13-Horizontal fixed support frame. Detailed Implementation

[0044] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0045] It should be noted that the following detailed description is for illustrative purposes only and is intended to supplement the explanation of this technical solution. Unless otherwise specified, all technical and scientific terms used in this document are consistent with the general understanding of those skilled in the art.

[0046] It should be noted that the specific expressions used herein are for illustrative purposes only and are not intended to limit the scope of the exemplary solutions of this application. In the context of this document, unless otherwise expressly stated, the singular form also encompasses the plural meaning. Furthermore, it should be noted that when the terms "comprising" and / or "including" are used in this specification, it indicates that the technical solution possesses corresponding characteristic elements, implementation steps, operation processes, hardware modules, functional components, and / or any combination thereof.

[0047] Figures 1-4 A large-scale visual direct shear test device for geosynthetics includes a main frame, a direct shear apparatus, a normal loading system, a horizontal shear loading system, a data acquisition system, and a visual monitoring system (image acquisition equipment); the direct shear apparatus includes fully transparent upper and lower shear boxes, with the upper shear box placed on the lower shear box.

[0048] The upper and lower shear boxes are positioned correspondingly within the main frame. The core innovation of this design lies in its unique normal loading design. Specifically, a servo-driven loading system is innovatively configured in the top frame structure, capable of precisely applying normal cyclic loads with variable frequency and amplitude. This system integrates a high-precision vertical load measurement unit and a vertical displacement sensor (LVDT). Furthermore, a horizontal shear loading mechanism is specially designed on the right side of the frame. This mechanism achieves precise control of the horizontal load through a servo motor and is equipped with a horizontal load detection module and a horizontal displacement sensor (LVDT).

[0049] At least one side wall of the lower shear box is a transparent observation window made of a high-strength transparent material (such as tempered glass or polycarbonate sheet). The visualization monitoring system includes a high-speed camera facing the observation window and a computer connected to the camera. The computer is equipped with digital image correlation (DIC) analysis software for non-contact full-field measurement of deformation during the shearing process.

[0050] The shearing boxes are designed for large-scale operation. The preferred internal dimensions are: upper shearing box (length = 500mm, width = 500mm, height = 300mm), and lower shearing box (length = 700mm, width = 500mm, height = 300mm). This represents a 200mm increase in longitudinal dimension compared to the upper shearing box to mitigate the impact of reduced shearing area. The tempered glass dimensions are: length = 400mm, height = 200mm.

[0051] The lower shear box has five screw holes on the top of the side plate on the other side along the shearing direction, and also includes a stainless steel pressure strip to fix the geosynthetic material to the surface of the lower shear box. Then, the shear box is installed on the fixed geosynthetic material and filled with soil.

[0052] The normal loading system applies a constant normal stress to the upper shear box via a loading plate. The horizontal shear loading system preferably uses a servo motor-driven actuator to push the lower shear box (which is fixed to the upper shear box) to produce a horizontal displacement. The data acquisition system includes a force sensor, a displacement sensor, and an optional miniature earth pressure cell.

[0053] Based on the same inventive concept, this invention also provides a testing method using the device, including: sample preparation, application of normal stress, simultaneous initiation of high-definition visualization imaging and shearing, data processing and analysis, and other steps. By analyzing macroscopic force-displacement curves and microscopic DIC images, the strength parameters of the material can be comprehensively obtained and the microscopic mechanism of shear failure can be revealed.

[0054] like Figure 1 As shown, the device of the present invention includes a rigid main frame. An upper shear box and a lower shear box are disposed within the frame, with one side wall of each shear box serving as a transparent observation window. A normal loading system applies a load to the upper shear box via a loading plate. The shear loading system is connected to the lower shear box, propelling it horizontally. Sensors in the data acquisition system collect data in real time, and a high-speed camera in the visualization monitoring system faces the observation window.

[0055] Example: Taking the direct shear test at the reinforcement-soil interface as an example. When preparing for the direct shear test, consistency in the operating procedures should be maintained as much as possible to reduce experimental errors. The sample loading process mainly consists of five steps:

[0056] (1) Apply Vaseline to the inner walls of the upper and lower shear boxes, and then push the lower shear box out along the linear guide rail.

[0057] (2) Place the filler in five layers into the lower shear box and compact each layer to ensure the consistency of density of each shear sample.

[0058] (3) After filling the lower shear box, fix the geosynthetic material sample (530 mm (width) × 780 mm (length)) on the surface of the lower shear box, and then clamp it with steel clamps at the front edge of the lower shear box to ensure that it will not be relatively displaced from the lower shear box during the shearing process.

[0059] (4) Push the lower shear box into the equipment along the guide rail, place the upper shear box at a specific position above the lower shear box, and then fill the upper shear box with packing material in layers and compact it.

[0060] (5) Place the loading plate on top of the upper shear box, tighten and secure all bolts of the instrument. Prepare DIC speckle patterns on the soil surface near the transparent observation window, and record the entire process using a high-definition digital camera. Subsequently, apply σ through the normal loading system. n =100 kPa stress.

[0061] (6) Start the direct shearing machine and conduct a test. After starting all systems, control the shearing loading system to shear at a rate of 1.0 mm / min.

[0062] After the experiment, shear stress-displacement curves were obtained, and the interfacial friction angle was calculated. Simultaneously, DIC analysis clearly showed the concentration of shear strain bands near the contact surfaces between the soil and reinforcement in the upper and lower boxes, as well as the movement of soil particles around the grid ribs, directly verifying the mechanism of interfacial friction.

[0063] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A large-scale visual direct shear test device for geosynthetics, characterized in that, include: Main framework; The direct shearing apparatus includes an upper shearing box and a lower shearing box, which are arranged correspondingly in the main frame. The contact surfaces of the two are provided with replaceable clamping modules to adapt to different test modes. A normal loading system, located above the upper shear box, is used to apply vertical loads. This device consists of a servo loading device and an automatic lifting mechanism, and applies cyclic loads of different frequencies and amplitudes. A horizontal shear loading system, connected to the lower shear box, is used to drive it to generate a horizontal relative displacement; Measurement components include vertical load cells and vertical LVDT displacement cells; On the right side of the main frame is a horizontal load application device, which consists of a servo motor and an automatic lift. At the bottom of the load application device are a shear load sensor, a horizontal LVDT displacement sensor, and a computer-controlled data acquisition device. The data acquisition system is used to collect normal force, shear force, and displacement data in real time. The visualization monitoring system includes a transparent observation window located on the side of the lower shear box and an image acquisition device facing the transparent observation window.

2. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, The normal load application device can apply cyclic loads, enabling the study of the interface friction characteristics between soil and geosynthetic materials under cyclic loads.

3. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, The clamping module includes a rigid clamp that is detachably mounted on the front end of the lower shear box. The rigid clamp is used to clamp the front half of the geosynthetic material in the direct shear test at the reinforcement-soil interface to ensure that the geosynthetic material does not slip during the shearing process.

4. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, At least one side of the lower shear box is a transparent observation window made of high-strength transparent material; the image acquisition device is a high-speed camera and is connected to a computer equipped with digital image correlation analysis software.

5. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, The lower shear box of the direct shearing device is longer than the upper shear box.

6. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, The data acquisition system includes a normal force sensor, a shear force sensor, a displacement sensor, and a miniature earth pressure cell embedded at the interface between the soil and the reinforcement.

7. The large-scale visual direct shear test device for geosynthetics according to claim 1, characterized in that, The horizontal shearing loading system is a ball screw actuator driven by a servo motor, which can control the shearing rate.

8. The large-scale visual direct shear test device for geosynthetics according to claim 2, characterized in that, The normal load application device can apply cyclic loads of different frequencies and amplitudes, and can conduct tests on the characteristics of the reinforced soil interface under different vertical load conditions, including static and dynamic loads.

9. A method for large-scale visual direct shear testing of geosynthetics based on the apparatus described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Test mode selection and installation: Select and install the corresponding clamping module according to the test objective; S2. Sample preparation: Soil is filled into the upper and lower shear boxes to a predetermined density. If it is a reinforcement-soil interface test, the geosynthetic material is installed according to the corresponding mode. S3. Apply normal stress: Apply a predetermined normal stress through the normal loading system; S4. Synchronous Start-up and Cutting: Start the data acquisition system and visualization monitoring system, and then control the horizontal cutting loading system to cut at a constant rate; S5. Data Processing and Analysis: Based on the collected data, draw shear stress-shear displacement curves and calculate shear strength parameters; at the same time, based on the images recorded by the visualization monitoring system, analyze the entire process of shear band development and evolution.

10. The test method according to claim 9, characterized in that, In step S5, a set of tests are conducted under different normal stresses to obtain the peak shear strength, and the Coulomb strength envelope is fitted to determine the cohesion c and the internal friction angle φ. At the same time, the thickness of the shear band, the entire evolution process, and the deformation and contribution of the reinforcing material in the composite are obtained through digital image correlation analysis.

Images