This paper presents the numerical method GPILE to calculate the scattered EM field by any stratified 1D medium composed of three random rough interfaces separating homogeneous media. GPILE is an extension of the PILE (Propagation Inside-Layer-Expansion) method, which considers only twointerfaces. Both methods rely on the rigorous implementation of the Maxwell equations, with a simple matrix formulation and has a straightforward physical interpretation. In particular, this method allows us to distinguish the primary echo of the upper surface, and also, the multiple echoes arising from the intermediate and lower interfaces. This method is applied in this paper to simulate the ground penetrating radar (GPR) signal atnadir. The simulated signals are analyzed to study the sensitivity of the GPR signal to any material debonding within the pavementlayered structure.