A numerical model is used to estimate the service life and clogging of gravel leachate drainage layers with a filter separator layer, such as those used in municipal solid waste landfills. A filter separator layer between the waste and the drainage layer is shown to reduce the leachate strength entering the gravel drainage layer and to extend the time that it takes to clog the drainage layer to the point when the leachate level exceeds the maximum design value (the service life). The filter layer is shown to have a far more significant effect in extending the service life when pea gravel () is used in the drainage layer, and less effect when coarse gravel () is used in the cases examined, although most of the benefit of a filter separator layer was achieved by using a 3-mm-thick needle-punched nonwoven geotextile. This geotextile may be sufficient for many practical purposes that are similar to those examined. The results from modeling leachate collection systems with filter separator layers subjected to both constant and variable leachate strength show that the characteristics of the leachate entering the drainage layer can substantially affect the service life of the drainage layer, and that high-strength leachate entering the systems for a limited time early in the life (acid phase) of a landfill can greatly reduce the service life of the leachate drainage layer.