A semianalytic (finite layer) technique that will readily allow modelling of the interaction between different landfill cells is presented. The cells may be either adjacent or separated and may have different dimensions (length, thickness) and differentsource leachate characteristics. The technique also allows modelling of the finite service life of engineered components of thebarrier system such as the service life of the primary leachate collection system and, if present, geomembrane liner. Theapplication of the technique is illustrated by first considering the longitudinal expansion along the alignment of leachate migrationof a landfill and examining the interaction between the original and subsequent landfill in terms of impact on the underlyingaquifer. Consideration is then given to the vertical expansion of existing landfills and finally to the modelling of the finite servicelife of a primary leachate collection system and geomembrane liner. It is demonstrated that a vertical expansion hasconsiderable potential for increasing long-term impact and that the impact is also controlled by the service life of the engineeredsystem.Key words: landfill, expansion, analysis, service life.