FACTORS EFFECTING THE KINEMATIC BENDING MOMENT IN THE PILES GROUP

Kinematic soil -pile interaction during earthquake motions has received large attention in recent years. In fact, post-earthquake underground eld measurement by many researchers proof that bending moments due to kinematic interaction an addition to superstructure inertial eects damaged the pile foundation in recent earthquake. Kinematic forces arise from the passage of seismic waves through the surrounding soil and become signicant in the presence of strong discontinuities in stiness of the soil prole. In this case, the large curvatures imposed to the piles by the vibrating soil in turn generate bending moments; these moments will develop even in the absence of a substructure and are referred to as \kinematic" moments, to be distinguished from moments generated by structural loading at the pile head (\inertial" moments). Kinematic soil - pile interaction is a complex problem involving a number of factors, such as soil prole, soil properties, nonlinear soil behavior, induced pore -pressure and pile properties. Evaluation of the above factors and impact of each on the nal response of piles can help engineers to better design. In this paper the results of parametric study of seismic behavior of pile groups embedded in layered soil is evaluated by focusing the attention on the kinematic bending moments induced by the transient motion using the three dimensional nite dierence program FLAC3D. Elcentro earthquake excitation that has a low predominant frequency was applied as an accelerationtime history at the base bedrock of the nite dierence mesh. Lateral side grids were horizontally xed for static analysis. In dynamic analyses, lateral boundaries were considered as \Quite Boundaries", a built-in boundary condition in FLAC for dynamic analysis. Analysis is carried out by varying the main parameters governing the dynamic response of piles like the soil properties, the diameter, the spacing between pile and the depth of soft soil layer.The results showed that 1) Kinematic bending moment is increased by increasing the diameter of the pile and increasing the depth of soft soil layer or increase the space between the piles enhance the eect of pile diameter on the bending moment; 2) Increase the space between the piles reduce kinematic bending moments; and 3) Increasing the depth of soft soil layer is increase the kinematic bending moment and by increasing the space between the piles eects of soft soil layer depth on bending moment increases slightly.