The South China region is characterized by diverse landforms and significant stratification of geological materials. The rock and soil layers in this area have obvious layering characteristics. The stability of layered slopes is a critical issue in the safe mining of southern ion-type rare earth ores. This study investigates the morphological changes, pore water pressure, and moisture content variation of layered ion-type rare earth ore slopes under the combined effects of rainfall and liquid infiltration through indoor model tests. A numerical simulation was conducted to analyze the variations in pore water pressure, moisture content, slope displacement, and safety factor under different working conditions. As rainfall intensity increases, the interface between soil layers in sandy-silty clay slopes is more likely to form a saturated water retention zone, causing rapid pore water pressure buildup and a significant reduction in shear strength. For the silty-sand clay slopes, the low permeability of the upper silty clay layer limits the infiltration rate of water, resulting in significant interlayer water retention effects, which induce softening and an increased instability risk. The higher the initial moisture content, the longer the infiltration time, which reduces the matrix suction of the soil and significantly weakens the shear strength of the slope. When the initial moisture content and rainfall intensity are the same, the safety factor of the silty-sand clay slope is higher than that of the sandy-silty clay slope. When rainfall intensity increases from 10 mm/h to 30 mm/h, the safety factor of the sandy-silty clay slope decreases from 1.30 to 1.15, indicating that the slope is approaching a critical instability state.

This study aims to investigate the effects of antislide piles and cohesion anisotropy on seismic displacements of three-dimensional (3D) layered slopes. A discrete mechanism generated by the point-to-point technique is employed as the deterministic model, and the particle swarm optimization algorithm is used to determine the least upper-bound solutions. By combining the pseudostatic approach and Newmark's method, the yield acceleration coefficients ky and earthquake-induced displacements of two-layer slopes are further analyzed in varying positions of strong/weak layers, ratios of layer strength, reinforcement locations of piles, and anisotropy coefficients of cohesion. The results indicate that for the seismic slopes (strength ratio Sr = 1.5), displacement can be reduced by an order of magnitude after pile reinforcement; considering the anisotropy results in higher safety evaluations, typically, there is generally about a 65% reduction in the seismic displacement of Sr = 1.5 slopes when coefficient kc decreases from 1 to 0.7; the optimal pile locations in anisotropic slopes may be further away from the slope toe; the presence of a strong layer at the bottom of the slope is more conducive to slope stability than in the top, but it also makes the slope stability more sensitive to changes in layer strength ratio; the destabilizing/stabilizing effect of the weak/strong layer at the slope bottom is most pronounced at low values of its proportion; switching the strong layer from the bottom to the top, the maximum values of ky experience a 25%-40% reduction, while this percentage would be magnified when calculating its impact on displacement. Moreover, different from single-layer slopes, layer heterogeneity may also result in nonuniqueness in the optimal pile locations.

Slopes in nature usually present layered characteristics, and its stability is susceptible to rainfall events. Considering that current analytical solutions are only suited to simulate the rainfall infiltration of double-layered infinite unsaturated slopes, an analytical procedure is hence proposed in this study to tackle the consideration of multiple layers. The variable separation method and transfer matrix method are combined to derive the analytical solution of pore water pressure (PWP) for simulating rainfall infiltration in layered infinite unsaturated slopes. After having validated the proposed model and analytical solutions by comparing with existing literature and numerical simulation, the closed-form solution of PWP is incorporated into the limit equilibrium for assessing slope stability. A three-layer slope is selected as an example for further discussion. PWP distribution and factor of safety are calculated, considering the effects of saturated hydraulic conductivity and thickness of the upper layer, intensity of antecedent and subsequent rainfall, and varied soil unit weight along the depth. The slope stability subjected to rainfall effects is consistent with the variation of PWP. The proposed analytical solutions provide a simple and practical avenue for computing PWP distribution and evaluating the stability of multi-layered slopes under rainfall conditions, which can also serve as a benchmark for numerical solutions.

The process of rainwater infiltration into unsaturated multi-layered slopes is complex, making it extremely difficult to accurately predict slope behaviors. The hydrological mechanisms in multi-layered slopes could be significantly influenced by the varying hydraulic characteristics of different soils, thus influencing slope stability. A numerical model based on Hydrus 2D was constructed to investigate the hydrological mechanisms of multi-layered slopes under different slope inclinations and rainfall intensities. The results revealed hydraulic processes in response to rainfall in unsaturated multi-layered slopes, in which layered soils retard the advance of wetting fronts and affect seepage paths in the slope. The results also showed the characteristics of hydraulic parameters, including pore water pressure and moisture content, under different conditions, and explained the crucial factors at play in maintaining slope stability.