The weak mechanical properties of weak interlayers are crucial for controlling landslide deformation and failure under water level fluctuation. The instability and failure of landslides in reservoirs can lead to unpredictable consequences. In this study, the reservoir bank landslide with a weak interlayer was selected as the research subject. The material composition, structural characteristics, mechanical properties, and permeability of the landslide were determined through field investigations and tests. Additionally, a physical model test was conducted to explore the groundwater variation rules and deformation failure modes of landslides with weak interlayers under different water level fluctuation rates. The results indicate that due to the low permeability of the interlayer, there was a significant lag in monitoring data such as pore water pressure within the interlayer under the same water level fluctuation rate. At the same point, the faster the water level fluctuation rate, the greater the degree of lag. The deformation and failure mode of landslide with weak interlayer under reservoir water level fluctuation can be summarized as the following five stages: slope toe erosion stage, cracks on slope surface and interlayer stage, micro-collapse of slope toes and crack expansion of slope surface and interlayer stage, local micro-collapse of slope toe and crack penetration of slope body stage, crack development leads to landslide of slope body stage. This study provides theoretical support for prevention and control of landslides with weak interlayers in the gravel soils of reservoirs.

In order to study the force characteristics and reinforcement mechanisms of the bank protection capacity of micropile groups under rain seepage, two different scale models were employed using model tests and the finite element method. Focusing on the stress within the micro-piles, the lateral soil pressure against the piles, the displacement at the pile tops, and the overall stability of the embankment reinforced by the micro-piles, engineers can assess the performance and durability of the structure during rainwater scouring. The study shows that rainfall leads to increased soil saturation, which in turn reduces the soil's shear strength. When subjected to loading after rain, micropiles within the same row exhibit similar strains. The thrust from potential landslides at the top of the slope causes the rear row of piles to experience greater flexural deformation. The difference between the soil pressure values of the same row after rainwater infiltration is small, and the overall soil pressure value increases in a stepwise manner with the increase of loading volume. The micropile support helps reduce soil displacement. The displacement of the middle and front row of piles is significantly lower compared with the back row by 32.3 % and 35.7 %, respectively. The pile group can limit the soil displacement within a certain range, which is beneficial for improving the stability of bank slopes under rainfall scouring. The micropile group enhances the overall slip resistance of the bank slope and can inhibit the development of the slip and crack surface of the bank slope to a certain extent. In engineering design, it is crucial to determine an appropriate pile spacing. A too small spacing can prevent the piles from achieving their optimal bending strength, whereas too large a spacing may lead to the risk of the bank slope as a whole experiencing overturning damage.

Water level rise and fall can lead to changes in seepage flow in the bank slopes of reservoirs. Cyclic seepage flow can result in the redistribution and even loss of fine particles in the slope, which eventually influences the slope stability. In this study, numerical simulations were conducted based on a multispecies transport finite element method (FEM) to investigate the influence of periodic water level fluctuations on the reservoir bank slope. The unsaturated soil was treated as a five-constituent mixture, by which erosion and deposition behaviours were captured by phase transition between deposited and fluidized fines. A constitutive model for suffusion considering the hydraulic fluctuation effect was employed to quantify the erosion process. The change in the seepage field and the corresponding evolution of the erosion rate and fines eroded ratio in the slope during the fluctuation period were investigated. The influence of the water level fluctuation frequency on the fines redistribution in the slope was analysed. Furthermore, the evolution patterns of the slope stability under different fluctuation frequencies were compared. The numerical results indicated that a higher fluctuation frequency could promote fine particle loss in the zones near the slope surface, consequently inducing greater deterioration in the slope stability.