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Frontiers of Structural and Civil Engineering

Front. Struct. Civ. Eng.    2020, Vol. 14 Issue (2) : 357-373
A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning
Zhuang CHENG1, Jianfeng WANG2(), Matthew Richard COOP3, Guanlin YE4
1. Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong 999077, China
2. Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, China
3. Department of Civil, Environmental and Geomatic Engineering, University College London, London WC1E 6BT, UK
4. Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, China
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The development of a miniature triaxial apparatus is presented. In conjunction with an X-ray micro-tomography (termed as X-ray μCT hereafter) facility and advanced image processing techniques, this apparatus can be used for in situ investigation of the micro-scale mechanical behavior of granular soils under shear. The apparatus allows for triaxial testing of a miniature dry sample with a size of 8mm×16mm (diameter × height). In situ triaxial testing of a 0.4–0.8 mm Leighton Buzzard sand (LBS) under a constant confining pressure of 500 kPa is presented. The evolutions of local porosities (i.e., the porosities of regions associated with individual particles), particle kinematics (i.e., particle translation and particle rotation) of the sample during the shear are quantitatively studied using image processing and analysis techniques. Meanwhile, a novel method is presented to quantify the volumetric strain distribution of the sample based on the results of local porosities and particle tracking. It is found that the sample, with nearly homogenous initial local porosities, starts to exhibit obvious inhomogeneity of local porosities and localization of particle kinematics and volumetric strain around the peak of deviatoric stress. In the post-peak shear stage, large local porosities and volumetric dilation mainly occur in a localized band. The developed triaxial apparatus, in its combined use of X-ray μCT imaging techniques, is a powerful tool to investigate the micro-scale mechanical behavior of granular soils.

Keywords triaxial apparatus      X-ray μCT      in situ test      micro-scale mechanical behavior      granular soils     
Corresponding Author(s): Jianfeng WANG   
Just Accepted Date: 08 January 2020   Online First Date: 11 March 2020    Issue Date: 08 May 2020
 Cite this article:   
Zhuang CHENG,Jianfeng WANG,Matthew Richard COOP, et al. A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning[J]. Front. Struct. Civ. Eng., 2020, 14(2): 357-373.
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Zhuang CHENG
Jianfeng WANG
Matthew Richard COOP
Guanlin YE
Fig.1  Schematic of a typical parallel beam X-ray μCT facility.
Fig.2  The triaxial system: (a) schematic of the triaxial system; (b) photograph of the triaxial apparatus.
Fig.3  A closer view of the axial loading device.
Fig.4  A schematic of the seal design of the apparatus.
Fig.5  Data acquisition and controlling system.
Fig.6  Photograph of the sample maker.
Fig.7  The process of making a sample. (a) A porous stone and a membrane; (b) sample maker; (c) fixing of the membrane; (d) filling of sand grains; (e) installation of a cushion plate; (f) removal of the sample maker.
Fig.8  The triaxial apparatus being used in conjunction with the synchrotron radiation facility: (a) a photograph; (b) a schematic of the connection between the apparatus and the synchrotron radiation facility.
Fig.9  Stress-strain curves of the LBS sample: (a) deviatoric stress vs. axial strain, (b) volumetric strain vs. axial strain.
Fig.10  Vertical slices of the sample at different scans. (a) 0%; (b) 0.98%; (c) 4.94%; (d) 10.40%; (e) 15.34%.
Fig.11  Illustration of the image processing of a 2D horizontal slice to determine sample porosity: (a) raw CT image; (b) filtered CT image; (c) binary image; (d) after 12 times of dilation of image (c); (e) after filling holes of image (d); (f) after 12 times of erosion of image (e).
Fig.12  Intensity histograms of the CT image before and after image filtering.
Fig.13  Illustration of the image processing of a 2D horizontal slice to determine local porosities: (a) a binary image of separated particles; (b) a binary image of an extracted particle; (c) distance transformation of image (a); (d) distance transformation of image (b); (e) extracted local void region; (f) the local void region superimposed on image (a).
Fig.14  Comparison between the volumetric strains of the sample as calculated by two methods.
Fig.15  A vertical slice of local porosity distributions of the sample at different axial strains.
Fig.16  Normalized frequency distributions of local porosity of the sample at different axial strains.
Fig.17  Particle displacement and rotation of the sample during the axial strain increments of: (a) 0~0.98%; (b) 0.98~4.94%; (c) 4.94~10.40%; (d) 10.40~15.34%.
Fig.18  Volumetric strain distributions of the sample during the axial strain increments of: (a) 0~0.98%; (b) 0.98~4.94%; (c) 4.94~10.40%; (d) 10.40~15.34%.
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