EVOLUTION OF SOIL MICROSTRUCTURE DURING DRYING AND WETTING

Variation of moisture is an inevitable process in natural soil, which influences engineering properties significantly. Mastering the evolution characteristics of soil structure in wetting/drying process systematically is meaningful for understanding the macro mechanical properties. Based on a large number of literatures, the evolution rules during drying, wetting and wetting/drying cycling processes are summarized and some insights are as follows. (1)Water content is the key factor to microstructure. The soils on the wet side of optimum water content have a matrix type of structure with a single pore size distribution, whereas the soils on the dry side of optimum water content display an aggregate structure with a bimodal pore size distribution. (2)On the drying path before shrinkage limit, the decrease of total volume is mainly caused by the shrink of macro-voids. Different sizes of voids are influenced in different suction domain. (3)On the wetting path, inter-aggregate voids and intra-aggregate voids both increase and the latter are dominant. The evolution characteristic is relative to confining condition closely. (4)The change of soil structure in wetting/drying cycling isn't reversible completely, and the accumulated expansion/shrink derives from the deformation of macro-voids. With the development of wetting/drying cycling, the volumetric strain will reach an equilibrium, which can be described by the elastic void ratio. Additionally, the wide spread applied techniques to soil structure research are summarized and include SEM, ESEM, MIP, and CT. Finally, based on the current situation, writers proposed the researching emphasis and direction in the future, ranging from new sample preparation methods, further research on the wetting process, combining between micro-structural parameters and macro mechanical models etc.