The aim of this paper is to present the installation procedure and design method used in the case of a real vertical extension project on an old landfill, an operation also known as “piggyback expansion”. First, the installation steps are presented for a project in Champigny- sur-Yonne (France). Then, the assumptions and design method which allowed to estimate the tensile strength of the geosynthetic reinforcement required for the project are exposed. The method used in this project is the standard French method (RAFAEL) for the design of geosynthetics in the case of soil subsidence and sinkholes. Latest improvements suggested in the literature for this method are also exposed in this paper.