High leachate levels seriously affect slope stability in landfills. Heavy rainfall and degradation of waste are the main causes of high leachate levels. Here, we analyze rainfall patterns, simulate leachate levels, and suggest methods for reducing the impact of heavy rain on slope stability. Heterogeneous characteristics of waste piles are simulated by combining a user subroutine with the finite element method. Finally, under maximum daily rainfall conditions at the landfill, seepage is simulated in three dimensions. We find that heterogeneities in hydraulic conductivity are often responsible for perched leachate levels. For a given rainfall, the leachates are perched in multiple places with the least rainfall duration; for a given rainfall duration, the leachates are perched in multiple places with the greatest rainfall intensity. High leachate levels exist downstream of the landfill slope and they dissipate slowly. From the initial rainfall to 2.8 hours after the rain, more saturated zones exist in the simulated landfill, especially near landfill slopes. In order to stabilize landfills for safety, we suggest the use of drainage measures, including silo systems, at the top of the waste pile and near slopes in order to reduce leachate levels.