A micro-investigation of unsaturated sand in mini-triaxial compression based on micro-CT image analysis

Abstract

The mechanical behaviours of unsaturated soils are highly related to the water content and pore water and air distributions. Under the context of climate change, geo-disasters related to soil moisture change attract more and more research attentions. Due to the heterogeneity of soil textures and the complicated morphology of liquid phases, it is crucial to understand the microstructural features of unsaturated soils. This work presents a study based on a miniaturized suction controlled triaxial device which is suitable for micro-CT image analysis. A fine sand is sheared in this device under different suction levels while CT scans are taken at different strain stages. After image 3D reconstruction, image trinarization, label analysis, contact detection and other customized image analysis and calculations, the micro-mechanisms of unsaturated granular soils upon triaxial shearing are investigated. It is observed that the inter-particle contact coordination number is reduced after shearing due to the dilation behaviour and the sample with the highest capillary strength has the highest coordination number. Although there is an initial fabric anisotropy due to gravity and sample compaction, triaxial loading will further enlarge the fabric anisotropy of the solid phase and the solid fabric anisotropy is also associated with shear strength. With the development of shear band, water drains out and the quantity of small-volume liquid clusters in the liquid bridge increases. This shifts the distributions of inter-facial areas. The effective stress tensor is interpreted microscopically based on small RVEs. Based on the CT image analysis, the suction-induced stress component is not an isotropic term and the anisotropy of the water phase is increased with triaxial deformation as well as decrease in degree of saturation when there are more isolated water bridges formed around solid contacts.

Microscale Mechanical Behaviour Evaluation of Unsaturated Granular Soils via 4D in situ Triaxial Test on μ-CT

“…A marker-based watershed algorithm was utilised to accomplish image segmentation [18] and subsequent analyses, such as volume calculation, interface extraction, and label analysis. Figure 4 presents the segmented images of a sample at the initial and end states of the laboratory in situ triaxial compression CT test; these include both 3D reconstructions and cross-sections of the central horizontal and vertical planes.…”

Pore scale investigation of unsaturated granular soil behaviour by means of in situ CT experiments

“…Especially the spatially and temporarily highly resolved CT images at the synchrotron allow to study interfacial areas [4] as well as pore filling and emptying processes [5] and relate the pore scale findings to macroscopic capillary pressure [6]. Other authors have focused on the hydromechanical coupling in unsaturated soils and have studied their shear behaviour by means of triaxial tests in a CT scanner [7][8][9][10]. By analysing the evolution of the pore water distribution as well as fluid clusters, the influence of shearing on capillary state variables such as liquid clusters could be studied.…”