| In strict scientific significance,Malan loess formed by wind accumulation is an soil-like deposit formed by wind transport and accumulation in the terrestrial environment.It is loose and porous and easy to permeate water.It has experienced many dry and wet cycles during tens of thousands of years of accumulation,resulting in unique engineering properties.The unique properties of loess are closely related to its texture and microstructure formed during dry and wet cycles.This paper starts with the dry-wet cycle test of Malan loess,learns the basic physical properties of the sample loess through the soil test,and makes observation and analysis of the recycled loess sample through the preparation of microscopic samples and the scanning electron microscopy(SEM)and X-ray fluorescence spectroscopy(XRF),so as to explore the influence of dry-wet cycle on the microstructure of loess and its internal mechanism.Direct shear tests were carried out on the cyclic samples to analyze the changes of shear strength of the samples before and after the cyclic tests,in order to reveal the causes of loess structure and its influence on the shear characteristics of loess.The results show that:(1)In an open environment,the ion content of the loess samples decreased after the dry-wet cycles,indicating that the cementation of the loess particles had weakened due to the loss of cementation materials as the cycle number increased.The shear strength of the loess also showed a decreasing trend with an increase in the number of cycles,indicating a decrease in the structural strength of the loess.(2)The ion content of the soil samples did not change significantly after dry-wet cycles in a closed environment.However,after several cycles,the clay and colloid particles migrated and were adsorbed around the skeleton particles,and soluble salt ions were enriched in the form of cements.The shear strength of the loess slightly increased with the cycle number,suggesting an improvement in the loess structure.(3)Scanning electron microscopy(SEM)showed that the microstructure of the loess body consisted of single mineral particles and aggregates.The fine sand and silt particles in the aggregate units are connected by cementation materials,and the connection modes of single particles are mainly basement-type cemented connection and contact-type cemented connection.The connection modes between aggregates are point,line,and surface contact connection.The structural effects of the migration of clay particles and soluble salts depended on the strength of the aggregates and the aggregate connection strength..The proposed dry-wet cycle model showed that the loess structure was weaker in an open environment and stronger in a closed environment,explaining the genetic mechanism of the Malan loess structure at the microscale.The results of this study provide very important theoretical significance for deep understanding of the mechanical and hydraulic properties of loess. |
