COUPLED DYNAMIC RESPONSES BETWEEN SATURATED SOIL AND PILE SYSTEMS UNDER VERTICAL EARTHQUAKES

To reveal the coupling dynamic characteristics of pile-soil systems under vertical seismic loads, the pile foundation was firstly regarded as a three-dimensional axisymmetric bar with radial and vertical deformation, and its motion equation was established by using a Hamiltonan variational principle. The soil around the pile was treated as a three-dimensional fluid-filled porous continuous medium, and its dynamic behavior was described by using Boer’s poroelastic media model. Without the introduction of potential functions, the volumetric strain of soil skeleton and pore fluid pressure were taken as intermediate variables to deal with the soil motion equation, and then the motion equations of soil and pile were solved by the method of variable separation. Combined with the boundary and continuity conditions of the pile-soil system, the analytical solutions of the kinematic amplification factor and kinematic response factor of the pile top were derived. The correctness of the proposed solution was verified by comparing the numerical results of the corresponding finite element model with the existing solutions. Finally, the influence of the main pile-soil parameters on the dynamic characteristics of the pile-soil coupling system was analyzed, and some meaningful conclusions were obtained, which can provide a reference for related engineering practice. The results show that: when the pile length-radius ratio is small, the radial deformation of pile foundation has a significant effect on the dynamic response of saturated soil-pile system. If the radial deformation of the pile foundation could be ignored, the resonance behavior of the pile-soil system might be overestimated. For single-phase soil, in the low frequency range, the response of a pile top is smaller than that of a free field surface and, in the high frequency range, it is larger with the increase of pile length-radius ratio. The resonance behavior of a pile-soil system occurs when the excitation frequency is close to the natural frequency of a soil free field. As the length-radius ratio of piles increases, the magnifying effect of a pile-soil system on bedrock movement tends to increase. For saturated soil, the response of saturated soil surface is basically the same as the movement of bedrock. With the increase of pile length-radius ratio, the magnifying effect of a pile-soil system on bedrock movement tends to decrease.