Numerical modelling to identify key factors controlling interface behaviour of geosynthetic lining systems

Geosynthetics have been extensively used in landfills as a lining system to prevent leachate infiltration into groundwater. In piggy-back landfill expansion (PBLE), consisting of building a new landfill over an existing one, a lining system is implemented between the old and new waste. In this context, interface failure (stability) and deformation (integrity) of the lining system should be considered for the design. Such stability and integrity mainly depend on the PBLE geometry and the mechanical properties of the geosynthetics. Comprehensive numerical modelling simulations were performed to show how these factors influence the shear stresses, shear displacements, translational stability and the axial strains/forces within the various geosynthetics. The numerical modelling was conducted using the finite difference code FLAC 2D, focusing on a typical PBLE and considering geosynthetic interface strain softening, the nonlinear stiffness of geosynthetics, and the differentiation between the compressive and tensile behaviours of geosynthetics. The simulations showed that the lateral length of the PBLE, the type of geomembrane (textured or smooth) and the level of the leachate table in new waste are the factors that most influenced the mechanical behaviour of the lining system and its stability. Finally, a parameter called the stability ratio was proposed as a complement to the traditional factor of safety, to analyse the progressive slippage along the geosynthetic interfaces. The numerical results indicated that interface failure concurrently begins at the rightmost part of the lower flat area of the PBLE and near the corner of the inner slope before spreading out to the left as backfilling progresses.