Pore structure evolution due to loess collapse: A comparative study using MIP and X-ray micro-CT

Abstract

Loess is a structured soil that tends to undergo collapse when loaded and wetted. X-ray micro-computed tomography (μ-XCT) is increasingly employed to study loess collapse, but some questions still need to be addressed including the method’s capabilities to effectively characterize loess micro-fabrics and how this can be best achieved. This study addresses these questions by investigating the evolution of pore structure during loess collapse through a combined approach involving the oedometer test, mercury intrusion porosimetry (MIP) and X-ray micro-computed tomography (μ-XCT). The pore size distribution (PSD) in terms of entrance diameter was obtained from μ-XCT images. This PSD corresponds well with that obtained from MIP. For the characterisation of pore evolution during loess collapse, a voxel size of 1 μm is shown to provide good results relevant to the evolution of PSD as a result of loess collapse. The effectiveness of PSD characterization for loess materials progressively deteriorates and rapidly loses significant value when the voxel size is increased. Different geometry concepts and pore size descriptors were evaluated and the discrete PSD approach was found to result in larger pore sizes compared to the continuous PSD method. A so-called ‘ink-bottle’ effect (where the volume of large pores that are only accessible through smaller pore entrances is incorrectly associated with the smaller pore fraction) was observed in both intact and collapsed samples. This highlights that the hierarchal morphology of loess pore structure is preserved during collapse. The PSD evolution due to loess collapse can be captured by either continuous or discrete descriptor demonstrating that μ-XCT forms a powerful complementary tool in characterizing loess pore structure and its evolution caused by mechanical loading or wetting.