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The protocol describes procedures to acquire high-spatial resolution computed tomography (CT) images of a granular soil during triaxial compression, and to apply image processing techniques to these CT images to explore the grain-scale mechanical behavior of the soil under loading.
The rapid development of X-ray imaging techniques with image processing and analysis skills has enabled the acquisition of CT images of granular soils with high-spatial resolutions. Based on such CT images, grain-scale mechanical behavior such as particle kinematics (i.e., particle translations and particle rotations), strain localization and inter-particle contact evolution of granular soils can be quantitatively investigated. However, this is inaccessible using conventional experimental methods. This study demonstrates the exploration of the grain-scale mechanical behavior of a granular soil sample under triaxial compression using synchrotron X-ray micro-tomography (μCT). With this method, a specially fabricated miniature loading apparatus is used to apply confining and axial stresses to the sample during the triaxial test. The apparatus is fitted into a synchrotron X-ray tomography setup so that high-spatial resolution CT images of the sample can be collected at different loading stages of the test without any disturbance to the sample. With the capability of extracting information at the macro scale (e.g., sample boundary stresses and strains from the triaxial apparatus setup) and the grain scale (e.g., grain movements and contact interactions from the CT images), this procedure provides an effective methodology to investigate the multi-scale mechanics of granular soils.
It is widely recognized that the macro-scale mechanical properties of granular soil, such as stiffness, shear strength and permeability, are critical to many geotechnical structures, for example, foundations, slopes and rock-fill dams. For many years, on-site tests and conventional laboratory tests (e.g., one-dimensional compression tests, triaxial compression tests and permeability tests) have been used to evaluate these properties in different soils. Codes and standards for testing soil mechanical properties have also been developed for engineering purposes. While these macro-scale mechanical properties have been intensively studied, the grain-scale mechanical behavior (e.g., particle kinematics, contact interaction and strain localization) that governs these properties has attracted much less attention from engineers and researchers. One reason is the lack of effective experimental methods available to explore the grain-scale mechanical behavior of soils.
Until now, most of the understanding of the grain-scale mechanical behavior of granular soils has come from discrete element modeling1 (DEM), because of its ability to extract particle-scale information (e.g., particle kinematics and particle contact forces). In earlier studies of using DEM techniques to model granular soil mechanical behaviors, each individual particle was simply represented by a single circle or sphere in the model. The use of such over-simplified particle shapes has led to the over-rotation of particles and thereby a lower peak strength behavior2. To achieve a better modeling performance, many investigators have used a rolling resistance model3,4,5,6 or irregular particle shapes7,8,9,10,11,12 in their DEM simulations. As a result, a more realistic understanding of particle kinematic behavior has been acquired. Aside from particle kinematics, DEM has been increasingly used to investigate grain contact interaction and to develop theoretical models. However, because of the requirement to reproduce real particle shapes and the use of sophisticated contact models, DEM requires extremely high computational capability in the modeling of granular soils with irregular shapes.
Recently, the development of optical equipment and imaging techniques (e.g., the microscope, laser-aided tomography, X-ray computed tomography (CT) and X-ray micro-tomography (μCT)) has provided many opportunities for experimental examination of the grain-scale mechanical behavior of granular soils. Via acquisition and analysis of soil sample images before and after triaxial testing, such equipment and techniques have been utilized in the investigation of soil microstructures13,14,15,16,17,18,19. More recently, in situ tests with X-ray CT or μCT have been increasingly used to investigate the evolution of void ratio20, strain distribution21,22,23,24, particle movement25,26,27,28, inter-particle contact29,30,31 and particle crushing32 of granular soils. Here, “in situ” implies X-ray scanning conducted at the same time as loading. In contrast to general X-ray scanning, in situ X-ray scanning tests require a specially fabricated loading apparatus to deliver stresses to soil samples. With the combined use of the loading apparatus and X-ray CT or μCT device, CT images of the samples at different loading stages of the tests can be acquired non-destructively. Based on these CT images, particle-scale observations of granular soil behavior can be acquired. These CT image-based particle-level observations are extremely helpful to verify numerical findings and to gain novel insights into the grain-scale mechanical behavior of granular soils.
This article aims to share the details of how an X-ray in situ scanning test of a soil sample can be carried out, using an exemplary experiment that observes particle kinematics, strain localization and inter-particle contact evolution within a soil sample. The results show that X-ray in situ scanning tests have a great potential to explore the grain-level behavior of granular soils. The protocol covers the choice of X-ray μCT device and the preparation of a miniature triaxial loading apparatus, and detailed procedures to carry out the test are provided. In addition, the technical steps for using image processing and analysis to quantify the particle kinematics (i.e., particle translation and particle rotation), strain localization, and inter-particle contact evolution (i.e., contact gain, contact loss and contact movement) of the soil are described.
1. Designing the experiment well in advance
2. Carrying out in situ triaxial compression testing
3. Image processing and analysis
4. CT image-based exploration of grain-scale mechanical behavior of soils
NOTE: The following image-based analysis is not applicable to idealistically spherical particles or samples with very narrow grading ranges (i.e., monodisperse samples). However, for particles with high roundness and poor grading (e.g., 0.3~0.6 mm glass beads), the methodology yields good results (see Cheng and Wang31).
Figure 5 depicts the particle kinematics results of a Leighton Buzzard sand (LBS) sample at a 2D slice during two typical shear increments, I and II. Most of the particles are successfully tracked and their translations and rotations are quantified following the above protocol. During the first shear increment, neither particle displacements nor particle rotations show clear localization. However, a localized band is developed in both the particle displacement map and particle rotation map d...
High-spatial resolution X-ray micro-CT and advanced image processing and analysis techniques have enabled the experimental investigation of the mechanical behavior of granular soils under shear at multi-scale levels (i.e., at macro-scale, meso-scale and grain-scale levels). However, CT image-based meso- and grain-scale investigations require the acquisition of high-spatial resolution CT images of soil samples during loading. The most challenging aspect of this process is perhaps the fabrication of a miniature triaxial lo...
The authors have nothing to disclose.
This study was supported by the General Research Fund No. CityU 11213517 from the Research Grant Council of the Hong Kong SAR, Research Grant No. 51779213 from the National Science Foundation of China, and the BL13W beamline of the Shanghai Synchrotron Radiation Facility (SSRF).
Name | Company | Catalog Number | Comments |
Confining pressure offering device | GDS | STDDPC | |
De-aired water | N/A | N/A | Water de-aired in the lab |
Leighton Buzzard sand | Artificial Grass Cambridge | Drained Industrial Sand 25 kg | Can be replaced with different soils |
Miniature triaxial loading device | N/A | N/A | The miniature loading device is specially fabricated by the authors |
Silicon grease | RS company | RS 494-124 | |
Synchrotron radiation X-ray micro CT setup | Shanghai Synchrotron Radiation Facility Center (SSRF) | 13W1 | The triaxial testing is carried out at the BL13W beam-line of the SSRF |
Vacuum pump | Hong Kong Labware Co., ltd. | Rocker 300 |
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