In landfills, geosynthetics (GSYs) are implemented on the bottom and side slopes of the waste cells to prevent the leachate infiltration into groundwater, as a lining system. In piggy-back landfill expansions (PBLE) where a new landfill is built over an older one, this lining system can be subject to high tensile forces (tear) and interface shear stresses (interface failure) due to the overlying waste mass. Certainly, the GSY tensile behaviour and their interface shear strength have been widely studied, but very little attention has been paid to the deformation and slippage processes. Understanding these mechanisms in order to improve the design of GSY lining system remains an outstanding challenge and this is the focus of our work. Using the finite difference code FLAC 2D, numerical modelling was conducted on a typical PBLE based on realistic conditions. The model includes a multi-layered GSY system and takes into consideration the interface strain softening at interfaces, the nonlinear stiffness of GSYs and the differentiation between their compressive and tensile behaviour. A new parameter named stability ratio is proposed to better understand the failure mechanisms at the various interfaces and their evolution as backfilling progresses. This parameter calculated for each individual portion of the interfaces allows for the location of the local instability. The numerical results showed that the interface failure mainly begins at the rightmost part of the lower flat area of PBLE and near the corner of the inner slope. The numerical results also indicated that when the GTX/GMB interface exhibits a low shear strength, the significant relative shear displacements calculated for this interface leads to a great tensile deformation of the GTX.