Horizontal dynamic responses of pile groups in unsaturated soil considering the second-order effects

This paper proposes an analytical solution applicable to the second-order effects of pile group foundations embedded in unsaturated soil. This solution incorporates the three-phase nature of soil, thereby accounting for the impact of groundwater level variations on the dynamic pile responses. Unsaturated soils are described using porous continuum mechanics and seepage theory. Through Helmholtz decomposition of the governing equations for unsaturated soils, the resistance of unsaturated soils is derived. Then, the impedance and interaction factors of the pile group in the frequency domain were determined by employing the superposition method as well as the transfer matrix method. Subsequently, the significance of second-order effects on the dynamic responses of pile groups is investigated under varying soil moduli and groundwater conditions. Parametric analysis reveals that the interaction factors, impedance, and the shear force at the pile top all exhibit pronounced frequency dependency. This phenomenon becomes more prominent with increasing pile spacing; however, the inclusion of vertical static load can mitigate it. Furthermore, variations in groundwater level will lead to a redistribution of the shear force at the pile top.