Formation mechanism of metastable internal support microstructure in Malan Loess and its implications for collapsibility

Abstract

Loess is formed from dust through wind transportation, sedimentation, and pedogenesis. Studying its initial sedimentary structure can elucidate the origin of loess microstructures and various engineering geological properties closely related to them. This study conducted wind tunnel experiments, scanning electron microscopy (SEM), and numerical simulations to analyze the microstructures of deposited dust and intact loess. The results revealed the following: (1) A novel internal support structure was widely observed in both deposited dust and intact loess, characterized by coarse silt particles as the skeleton and fine silt and adhesive components as the supporting elements. The former exhibited minimal adhesive components and was classified as “unstable,” whereas the latter possessed some adhesive components and was classified as “metastable.” (2) During pedogenesis, adhesive particles form bonding components under the influence of water, causing deposited dust to gradually evolve into intact loess without significant disturbance to the internal support structure. (3) The collapsibility of loess was primarily due to the breakdown of the bonding components in the metastable internal support structure of intact loess upon exposure to water. The metastable internal support structure identified in this study can be used to describe the collapse of loess from both macro- and micro-perspectives, providing new insights into the loess microstructure and the relationship between loess structure and engineering geological properties.