A vibration response calculation model of elastic layered pavement under the falling weight deflectometer (FWD) load considering the unsaturated characteristics of the subgrade is established to provide the theoretical basis for accurately identifying subgrade performance deterioration by using FWD. The dynamic governing equations of the model are derived and solved by Laplace-Hankel transformation

and the dynamic stiffness matrices of the pavement and unsaturated subgrade are established. The global dynamic stiffness matrix of the highway structure is synthesized from the interlayer continuity condition

and the vibration response solution of the entire highway structure in the transform domain under the FWD load is derived by combining the boundary conditions. The Laplace-Hankel inverse transform is utilized to obtain the numerical solution of the vibration response in the space-time domain. The effects of FWD load frequency and subgrade saturation on the vibration response and the correlation between subgrade performance deterioration and pavement surface vibration signals under FWD load are analyzed. It is found that the vertical dynamic displacement of the pavement surface is mainly influenced by the low frequency component of FWD load and increases with the increase of saturation. The vertical dynamic displacement of the pavement surface is more sensitive to subgrade performance deterioration than the vibration velocity of the pavement surface and the vertical dynamic stress of the subgrade surface. Moreover

the change rate of vertical dynamic displacement of the pavement surface at 1.2 m from the load center reaches the maximum. The results of the study can provide a scientific basis for selecting feedback indicators and locations of subgrade performance deterioration.