Three-stage collapsibility evolution of Malan loess in the Loess Plateau

Abstract

Soil collapse forms a major hazard in large parts of Canada, the United States, Europe, China, and Africa, especially in the Loess Plateau of China. The Malan loess overlying the Loess Plateau with porosity and water sensitivity due to its typical wind-deposited characteristics, which also determines its collapsibility. Human activities continue to increase in the Loess Plateau underlain by collapsible soils, so that the hazards posed are increasing in both relative and absolute terms. The purpose of this paper was to reveal the collapsibility mechanism of Malan loess in the Loess Plateau by combining macro and micro research methods. The aim was to identify the internal and external factors affecting loess collapse from the origin of loess and to reconstruct the collapse process as a key to theory so as to enhance utilization and diminish hazard. Oedometer-collapse tests were used to explore the relationship between pressure, water content and loess collapsibility coefficient. Scanning electron microscopy (SEM) and computed tomography (CT) scanning tests were used to reveal the qualitative and quantitative characteristics of loess microstructure before and after the test, to comprehensively reproduce the loess collapsibility process. The results revealed the three-stage collapsibility evolution process of Malan loess. The weak connection mode and vertically distributed particles constitute the natural loess with metastable structure. Under the action of wetting and loading, the contact mode of soil particles changes, the large and medium pores are compressed, and the loess is transformed into unsaturated compacted structure before yielding pressure. The soil exhibited a saturated collapsed structure with the decrease of pore volume and the increase of soil particle cementation after structural yield stress. At the same time, it is determined that the structural yield stress of soil determines the peak value of collapsibility pressure.