In order to investigate the impact of plant root systems on the stability of loess shallow slope, this study conducted plant morphology investigations and direct soil shear tests to analyse the morphological characteristics of alfalfa and the shear characteristics of alfalfa root-loess composites under different soil bulk densities and soil moisture saturation levels. Additionally, the reinforcing effect of the alfalfa root system on the reliability of loess slopes was assessed using the Monte Carlo method. Slope reliability analysis refers to the estimation of the probability of slope failure under specific conditions. The results showed that plant weight and root weight both decreased following an exponential function with increasing soil bulk density. Root weight had a positively linear correlation with plant weight. The cohesion and internal friction angle of both loess samples without roots and with roots increased with increasing soil bulk density. The cohesion and internal friction angle of the two kinds of samples could decreased at less and more than 30% soil moisture saturation. The cohesion and internal friction angle of the root-soil composites were significantly higher than those of the rootless soil. The decrease of soil bulk density and the increase of soil moisture could increase the difference of the two mechanical parameters between the two kinds of samples. Assuming the thickness of the landslide body was 0.3 m, the failure probability of loess slopes covered with alfalfa significantly decreased from 34.97 to 14.51% compared to slopes without vegetation cover. Alfalfa roots significantly increased the reliability of the loess slopes in stability.

The spatial distributions of hydraulic conductivity and shear strength parameters are influenced by the soil structure, property and mineral composition. However, hydraulic conductivity is not only determined by the intrinsic soil property but also influenced by external factors such as fractures and interlayers. This study investigates the impact of the asynchronism between the spatial distribution of hydraulic conductivity and shear strength parameters on the reliability assessment and failure mechanism of unsaturated soil slopes with different titled stratifications under rainfall conditions. The results indicate that the asynchronism in the rotational angles (alpha) of hydraulic conductivity and shear strength parameters shows the greatest impact on the probability of failure (Pf) of slopes. By contrast, the asynchronism in the scales of fluctuation of hydraulic conductivity and shear strength parameters and employing different autocorrelation functions (ACFs) show minor impact on the Pf. The impact of using different ACFs, alpha, and scales of fluctuation to characterise the spatial variability of hydraulic conductivity on sliding mass and failure modes is minimal.

Natural slopes often exhibit tilted stratification with rotated transverse anisotropy in multiple soil properties, including the mechanical (e.g., friction angle phi and cohesion c) and hydraulic properties (e.g., saturated hydraulic conductivity ks). This phenomenon indicates that to achieve an accurate assessment of slope reliability under rainfall infiltration, the anisotropic spatial variation of the shear strength parameters and ks should be incorporated in a combined manner. Thus, this paper discusses the combined influence of rotated transverse anisotropy in the shear strength parameters and ks on the reliability and failure mechanism of an unsaturated slope under rainfalls. It is found that for a slope with tilted stratification, the failure mechanism and reliability estimator are dominantly influenced by the rotated transverse anisotropy in the shear strength parameters, while the influence induced by that in ks is slight. In particular, only incorporating the rotated transverse anisotropy in ks may lead to an incorrect reliability estimator of a slope with tilted stratification. Herein, the reliability index beta would be estimated to be dramatically higher, and beta of a slope with horizontal bedding would be higher than that of an anti-dip slope, which is inconsistent with the engineering experiences. Nonetheless, the rotated transverse anisotropy in ks should not be ignored in slope reliability assessment, because ignoring the rotated transverse anisotropy in ks would lead to overestimation of the reliability index beta, which is adverse to the safety design of a slope.