Failure mechanism and movement process of three loess landslides due to freeze-thaw cycle in the Fangtai village, Yongjing County, Chinese Loess Plateau

In the seasonal freeze-thaw region of China, the long-term freeze-thaw cycle (FTC) weakens the strength of the soil's structure leading to a series of geohazard events such as landslides and collapses. In order to understand the formation mechanism of the landslide during periods of freeze-thaw in the Chinese Loess Plateau (CLP). In this study, we analyzed the evolution process of the microstructure and the strength combined with triaxial test, resistivity experiments and CT technology. Furthermore, we collected meteorological, irrigation, groundwater level and landslide data to discuss landslide inducing factors. The results show that (1) the FTC has a significant damage effect on the strength of the original loess. After 10 FTCs, the strength of the original loess with various moisture content (ωmc) is reduced by at least 50%. The higher the ωmc of the original loess sample, the greater the strength reduction and the more apparent the deterioration characteristics. (2) FTC causes the expansion of the pore structure, that gradually presents non-uniformity and complexity, resulting in the continuous increase of resistivity. The pore area of macropores and mesopores are the most significant feature after 10 FTCs, and the increase of pore structure leads to the gradual loosening of the soil structure, which reflects the essence of strength deterioration. (3) Fangtai 1# landslide has the typical characteristics of high-speed and long-distance. The primary movement period of the landslide was from 4 s to 30s, the maximum speed of the landslide was 20 m/s, and the apparent friction angle was 0.34. (4): The most direct inducing factor of the three consecutive landslides in the Fangtai village was a temperature change lasting >10 days. Through mechanical and microstructure analysis, we found that the increase of FTCs leading to soil strength damage is the trigger factor, especially in the slope softening zone, which promotes the frequent occurrence of landslides. The rise of groundwater level caused by irrigation is the fundamental condition for the formation of landslide, and the irrigation plan of the Heifangtai tableland needs to be paid reasonable attention.