Creep mechanical and microstructural insights into the failure mechanism of loess landslides induced by dry-wet cycles in the Heifangtai platform, China

Affected by groundwater fluctuations, the loess around the hydro-fluctuation zone near the groundwater table is frequently subjected to periodic dry-wet cycles. The investigation on the creep characteristics and microstructural evolution associated with the mechanical response of loess after the application of dry-wet cyclings is of great significance to better understand the loess landslide mechanism. Triaxial creep tests and scanning electron microscopy (SEM) were conducted on intact loess specimens obtained from Heifangtai platform, Gansu, China. The micrographs were processed using IPP software and the microstructural information of soil pores were obtained. Results of triaxial creep test revealed that the application of dry-wet cyclings causes the creep deformation of loess soils to increase significantly, and they showed that the long-term strength and dry-wet cycle follow an exponentially decreasing function, indicating that the reduction in the long-term strength of loess soils is greater in the first five dry-wet cycling compared with that in the subsequently dry-wet cyclings. The microstructural analysis showed that aggregates were found to cluster upon the application of drying-wetting cycling, followed by the rearrangement of soil skeletons, transforming the initial relatively dense structure into a loosen one with a greater percentage of macro-pores which are more sensitive to creep deformation of loess soils. Additionally, it is observed that the microstructural evolution is dependent on the number of dry-wet cyclings. As the dry-wet cycles increase, the flatter and irregular macro-pores increased gradually at the expense of meso-pores. In addition, the mechanism of irrigation-induced loess landslides is revealed from the perspective of variation in creep characteristics of loess after the dry-wet cycles, which can be summarized into three stages, namely, the dry-wet cycle stage of the loess around the hydro-fluctuation zone; the long-term creep-liquefaction stage of the deep loess, and the slip surface transfixion stage.