This study examines the extent to which the results of numerical calculations can be influenced both by the differing compressive and tensile behavior of multiple geosynthetics GSYs and by the assumption of strain softening at interfaces between GSYs. Several numerical models are implemented using the finite-difference code FLAC 2D on a typical piggyback landfill expansion (PBLE) that involves four GSYs and six interfaces. The present work applies comprehensive, state-of-the-art numerical modelling to study the interactions between multiple layers of GSYs. It also investigates the nonlinear axial stiffness of GSYs through a series of uniaxial tensile tests. The numerical results show that, if the GSY axial compressive and tensile characteristics are the same, then tensile force is minimized, which induces significant compressive force in the GSYs. The results also indicate that neglecting strain softening at the interface between GSYs affects interface shear stresses, displacements of GSYs at the interface, and the GSY force distribution, potentially rendering the model unrealistic. Including strain softening, however, allows the assessment (location) of unstable areas along the interface where large displacements occur.