Loess covers approximately 6.6% of China and forms thick extensive deposits in the northern and northwestern parts of the country. Natural erosional processes and human modification of thick loess deposits have produced abundant, potentially unstable steep slopes in this region. Slope deformation monitoring aimed at evaluating the mechanical behavior of a loess slope has shown a cyclic pattern of contraction and expansion. Such cyclic strain change on the slope materials can damage the loess and contribute to slope instability. The site showing this behavior is a 70-m high loess slope near Yan’an city in Shanxi Province, northwest China. A Ground-Based Synthetic Aperture Radar (GB-SAR) sensor and a displacement meter were used to monitor this cyclic deformation of the slope over a one-year period from September 2018 to August 2019. It is postulated that this cyclic behavior corresponds to thermal and moisture fluctuations, following energy conservation laws. To investigate the validity of this mechanism, physical models of soil temperature and moisture measured by hygrothermographs were used to simulate the observed cyclic deformations. We found good correlations between the models based on the proposed mechanism and the exhibited daily and annual cyclic contraction and expansion. The slope absorbed energy from the time of maximum contraction to the time of maximum expansion, and released energy from the time of maximum expansion to the time of maximum contraction. Recoverable cyclic deformations suggest stresses in the loess are within the elastic range, and non-recoverable cyclic deformations suggest damage of the loess material (breakage of bonds between soil grains), which could lead to instability. Based on these observations and the models, we developed a quantitative relationship between weather cycles and thermal deformation of the slope. Given the current climate change projections of temperature increases of up to 3.5 °C by 2100, the model estimates the loess slope to expand about 0.35 mm in average, which would be in addition to the current cyclic “breathing” behavior experienced by the slope.

Plant roots are important in ecological slope protection and reinforcement, significantly affecting soil’s water-holding characteristics and shear strength. The typical herb Festuca Arundinacea root-loess composite in the Loess Plateau was taken as the research object in this paper. The matrix suction test, unsaturated shear strength test, nuclear magnetic resonance (NMR) test, and scanning electron microscope (SEM) test were used to systematically study the root–soil composite’s suction state and strength characteristics under different water content conditions and reveal its internal physical mechanism. The main research results are as follows: (i) The incorporation of roots can increase the air entry value (AEV), reduce the residual water content, and significantly enhance the matrix suction within a specific water content range, thereby increasing the unsaturated shear strength and the enhancement effect of shear strength gradually decreases with the increase of water content. (ii) The microscopic test results show that the root system accelerates the water loss of the soil under low suction conditions by changing the pore structure, increasing the total porosity and the proportion of medium and large pores. At the same time, the increase of tiny pores enhances the capillary action, increasing matrix suction. (iii) Roots can also improve the soil stress state, enhance the adsorption strength of matrix suction, and effectively improve soil shear strength. The results of this study are helpful to understanding the evolution mechanism of unsaturated strength characteristics of root–soil composite and provide the experimental basis and theoretical reference for vegetation slope protection engineering in the loess area.