Experimental study of the effect of bound water on the shear strength and structural units of Malan loess

Abstract

The structural specificity and hydrological sensitivity of loess have a big impact on its strength, deformation and long-term stability and safety. This topic is being actively researched, and focuses on the macromechanical behaviour of the shear strength of loess and its micromechanisms from the perspective of bound water. In this study, based on the combined determination of isothermal adsorption and the liquid–plastic limit, bound water is analysed qualitatively and quantitatively, and the entire humidity sequence is divided in a new method. The shear strength of Malan loess measured by the direct shear test is also analysed to investigate the relationship between bound water, shear strength and basic structural units. The results show that the adsorption patterns for the Malan loess in the three regions studied are approximately the same but, at the same humidity, their maximum moisture content ( W мг ) was, from highest to lowest, 7.13% in Chan he, 6.54% in Yan'an and 5.54% in Hei fang tai, which is related to clay minerals and their contents in the soil. In addition, the change in strength of the loess is divided into three stages using the maximum moisture content ( W мг ) and plastic limit ( W P ) as the characteristic moisture content. The patterns of strength change for the loess in the three regions is similar throughout the moisture content sequence: all stages show a negative correlation between moisture content and shear strength but the internal mechanism of each stage is not the same due to the different connection forces between the basic structural units of the loess. The change in bound water content caused by the cementation of glue and clay particles also yields certain changes in cohesion, which is linearly fitted to the shear strength and its parameters. Finally, the bound water in the soil changes the microstructure of the soil, and determines the combination characteristics and contact mode of the ‘core clothes’ in the microagglomerate structure of the loess. This study describes the essence of soil–water structural interactions and provides theoretical references for applications in geotechnical engineering fields such as slope protection and road construction.