Based on the multiphase pore elasticity theory

the propagation equations for the various elastic waves in unsaturated soils are derived. According to the boundary conditions at the interface

the relationships between different wave amplitudes of various potential functions are established; and then the reflection and transmission problems of incident shear wave at the interface between unsaturated soil layers with different saturation degrees are discussed. Three compressional waves and one shear wave exist in the infinite unsaturated soils

therefore four reflected waves and four transmitted waves are generated when the shear wave propagates at the interface between different unsaturated soil layers. The theoretical expressions for the amplitude ratios and energy ratios for the various waves are derived; and the numerical analysis is then performed. In the numerical examples

the influences of incident frequency

incident angle and saturation degrees of the upper and lower soil layers on the proportionality coefficients of the reflected wave energy and transmitted wave energy (namely energy reflection ratios and energy transmission ratios) are discussed respectively. It is shown that the energy reflection ratios and energy transmission ratios are affected by both the incident angle and the incident frequency. Additionally

the influence of the saturation degrees of the upper and lower soil layers on the reflected and transmitted waves should not be ignored.