In Japan, the government reinforces the environmental regulation and people worry about the environmental stability of waste landfill, so the site of landfill is diversified from inland to coast near urban areas. However, as the standard of the environmental stability and the monitoring procedure are not established, the coastal landfills have been built and used without a long-term evaluation of the performance. Therefore this study shows the methods of evaluations for the impervious walls that prevent leakage of the waste exudation water. This study uses Weibull distribution such as statistical method. Furthermore, this study assumes that the environmental damages occur by the leakage through the defects.