Analytical solution of soil deformation and fluid pressure change for a two-layer system with an upper unsaturated soil and a lower saturated soil under external loading

•A theoretical model is developed for consolidation of a double-layer (unsaturated–saturated) soil.•Normal coordinates are derived to decouple model equations into analytically-solvable formulation.•A boundary-value problem is physically consistent with the continuity of pore water flux and pressure at the interface.•Closed-form, analytical solution is determined to describe excess pore fluid pressure in each layer.