Microstructure and permeability evolution of remolded loess with different dry densities under saturated seepage

Understanding water seepage in loess is important for engineering constructions and geo-hazard prevention in loess regions. However, the saturated seepage behavior of remolded loess with different dry densities has been found to be inconsistent. In order to gain a deeper insight into the changes of saturated permeability for remolded loess with different dry densities and the underlying microscopic mechanisms, the saturated seepage tests were designed for loess samples, and scanning electron microscope (SEM) imaging was carried out to obverse the changes of loess microstructure before and after the seepage test. The results of this study highlight that the water seepage in loess samples with dry densities smaller than or equal to 1.35 g·cm−3 would lead to the movement of particles within the sample, thus blocking the pores and even causing the collapse of the loess structure. In this process, the pores in the sample transform from macropores to mesopores and small pores, and from mesopores to small pores, such that the saturated permeability of remolded loess decreases with the seepage time. While, for loess samples with dry densities greater than or equal to 1.45 g·cm−3, the saturated permeability of loess increases with the seepage time. The microstructure obtained by SEM revealed that the transition from small pores and micropores to mesopores after seepage. The enlargement of the pores is mainly caused by chemical reactions during the seepage, namely, the dissolution of soluble salt and the leaching of calcium cement. For the loess samples with dry density of 1.40 g·cm−3, the changes of saturated permeability over seepage time can be defined as a transition stage from the decreasing to increasing. Leaching enlarged the pores of soil samples and enhanced their connectivity. At the same time, it also formed the space for particle migration which could lead to the blocking of pores, and caused the reduction of soil strength which may result in the structural collapse of the loess. Therefore, the saturated permeability of the sample first increases and then decreases with seepage time. These findings have practical significance for the construction and protection of loess engineering in loess area.