Characterizing and modeling the pore-size distribution evolution of a compacted loess during consolidation and shearing

Abstract

The microstructure of a given soil is neither unique nor fixed as it evolves during deformation of the soil. Quantitative characterization of the microstructural evolution is crucial for interpretation of the soil mechanical behavior and development of the mechanical model taking account of the structural effect. This study aims to investigate the pore-size distribution (PSD) evolution of a compacted loess during deformation associated with consolidation and shearing. The studied loess was a low-plasticity clay and collected from Xi’an, China. Compacted loess specimens having the same initial condition were loaded in triaxial cells to different final stress–strain states. Investigations of the initial and final microstructures were carried out using the mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) techniques. The PSD evolution during consolidation was examined by comparing the PSDs of the specimens consolidated to different confining stresses, and the PSD evolution during shearing was examined by comparing the PSDs of the specimens sheared at the same confining stress to a series of increasing axial strains. Inter-aggregate pores (those greater than 1 μm in particular) were compressed from the larger to the smaller as the soil specimen contracts, intra-aggregate pores were almost unaffected, irrespective of the stress path. According to these findings, the inter-aggregate pore-size density function (PSDF) evolution is suggested to be depicted using two scaling factors (α and β). The inter-aggregate PSDF is assumed as a normal distribution function for simplicity and the empirical relationship between the cumulative intrusion void ratio, ein, and the dominant macro pore diameter, dmacro, is used, α and β can be related to ein. Thus, the PSD of compacted loess in any deformed state can be predicted from the reference PSD provided that ein in the deformed state is estimated by a mechanical model. Aggregates would not be destroyed due to loading, and the mechanical responses of compacted loess are the interactions among aggregates. The proposed model assumes the inter-aggregate PSDF scales along the horizontal axis, which is consistent with the experimental findings; and it provides a simple way to predict the PSD variation using two scaling factors.