Understanding the microstructural characteristic of clay samples is crucial in determining the corresponding macroscopic properties. However, several issues that are often encountered during sample preparations may induce measurement bias, and at the same time, the microstructure of clay has rarely been characterized statistically and quantitatively. Hence, the main objective of this research is to propose a pragmatic guide for preparing high-quality load-preserved fabric clay samples for microstructural characterizations and to characterize the microstructural response of clay samples in a more quantitative and statistical manner in order to gain deeper insight into the deformation of one-dimensional (1-D) consolidation.

To prepare high-quality load-preserved fabric clay samples subj...[ Read more ]

Understanding the microstructural characteristic of clay samples is crucial in determining the corresponding macroscopic properties. However, several issues that are often encountered during sample preparations may induce measurement bias, and at the same time, the microstructure of clay has rarely been characterized statistically and quantitatively. Hence, the main objective of this research is to propose a pragmatic guide for preparing high-quality load-preserved fabric clay samples for microstructural characterizations and to characterize the microstructural response of clay samples in a more quantitative and statistical manner in order to gain deeper insight into the deformation of one-dimensional (1-D) consolidation.

To prepare high-quality load-preserved fabric clay samples subjected to 1-D consolidation for the pore size and fabric characterizations, a practical guide, together with a real sample tested as an example, is therefore proposed in this study. In this guide, several issues often encountered in sample preparations that may induce measurement bias are tackled with feasible solutions proposed. A tailor-made oedometer is also invented and produced using a 3D printing technique to help achieve the goal. First, a homogenous and uniform kaolinite sample is prepared from a slurry state, and then positioned in the 3D-printed oedometer for 1-D consolidation tests. This device allows the applied loading to be maintained during rapid freezing of the sample at different loading stages, using liquid nitrogen to preserve the fabric associations not affected by the unloading effects. Afterwards, for the purpose of using the same sample for different tests to facilitate comparisons, the sample is cut in half while remaining frozen. This on one hand, creates an observation plane along the center of the sample with the morphological information preserved for the subsequent Scanning Electron Microscopy (SEM) analyses. On the other hand, each half of the sample undergoes the Mercury Intrusion Porosimetry (MIP) and Nitrogen Adsorption (NA) analyses to obtain complementary information on the pore-size distribution. The samples have to be dewatered through freeze drying before conducting these tests. During the freeze drying process the cut sample is protected by a 3D-printed container, which has a reference mark to indicate the sample orientation. In each of the SEM images taken, the associated position and orientation are controlled, and the number of the images taken for analyses is maximized to enhance the statistical representation of the analyzed results.

Next, through micromechanical analyses, the microstructural responses of kaolinite samples subjected to 1-D consolidation were quantitatively and statistically characterized. For each sample at different loading stages, the applied loading was maintained during rapid freezing of the sample to preserve the fabric associations not affected by the unloading effects for the subsequent characterizations. The MIP and SEM were used in parallel to measure the pore-size distribution and to take images of soil structures. There were about 20 ~ 30 SEM images taken and at least ~3000 particles identified in each samples to provide representative data for micromechanical analyses. With the boundary established between intra- and inter-aggregate pores based on the MIP results, the quantitative SEM analyses further reveal that the inter-aggregate pores exhibit a significantly large area fraction and therefore dominate the deformation responses. Fabric tensors were used to further quantify the directional behavior of the voids and particles. As expected, the particles tend to align horizontally during 1-D consolidation, and the void fabric follows the same trend. To further comprehend the deformation mechanism, the shape evolution of inter-aggregate pores was examined based on the SEM images. It was found that increasing loading gradually flattens those pores aligned horizontally and makes those pores oriented vertically become rounder. These scenarios describe the compression process of the inter-aggregate pores during the collapse of the card-house structures forming by the aggregates.