A model for the formation and evolution of structure of initial loess deposits

Abstract

The wetting and drying process is one of the commonest natural conditions the loess deposit encounter in its life span, and drives the formation and evolution of loess structure. Understanding the genesis of loess structure is essential for reconstructing paleoclimate and predicting the future structure. The common belief is that drying-induced shrinkage is responsible for formation and propagation of cracks in soil, while wetting-induced swelling minimizes pre-existing cracks. Contrary to this, it was lately found that the wetting rather than drying dominates the initiation of vertical cracks in loose and uncemented initial loess deposits. In present study, we investigated the structural evolution mechanism of initial loess deposits subjected to wetting and drying using physical monitoring test, numerical simulation and theoretical analysis. Results show that the structural evolution was controlled by capillarity. This can be interpreted by a mechanical model which indicates that due to the loose particle packing, the interparticle force caused by the capillarity, especially in the wetting process, is enough to overcome the resistance (mainly friction) and pull adjacent particles close to each other, modifying the overhead pores into vertical cracks. Our findings confirm the main contribution of wetting-induced shrinkage to the formation and evolution of early loess structure, which challenges the current understanding of loessification process.