The effect of rainfall on the failure of slope can be analysed through experimental testing, field investigation, and numerical simulations. In numerical simulation for rainfall-triggered slope failure, how accurate boundary conditions of infiltration are initialized and initial pore water pressure is obtained in a dynamic flowing boundary condition are the most important aspects. To solve these problems, firstly, disadvantages of traditional numerical boundary in infiltration are found through numerical simulation and experimental results. Thus, new boundary conditions of infiltration are presented, in which the rainfall infiltration is processed through air units, as a dynamic boundary condition, and then into unsaturated soils. Considering these new infiltration conditions, a variety of numerical simulation is carried out in a soil column test for infiltration. It is shown that the results from simulation are consistent with those from experimental tests and field monitoring by others. The proposed method is proved to be effective and accurate for the rainfall-triggered slope failure. Meanwhile, the tensile strength for water at nodes is set to express suction in unsaturated soils. Because the relative permeability coefficient and suction are the function of saturation, the changes of saturation in each node can update the relative permeability coefficient and suction in each step of simulation. Then the seepage is developed using FISH language in the FLAC software. Moreover, a dynamic flowing boundary condition is proposed to consider the presence of suction in unsaturated soils. In this condition, the natural rainfall infiltration is simulated through the change of groundwater, and the initial pore water is precisely obtained after the development of seepage modules in the FLAC software.
