effect of pile foundation on natural frequency of soil layer

Free vibration of soil often occurs during earthquakes. since the vibration caused by earthquake does not have (steady state harmonic vibration) continuity, the alluvium vibrates with its natural frequency between two natural seismic waves. this study evaluates the effect of piles on the period of free vibration of a soil layer using numerical method. in the first stage, using analytical equations for calculation of vibration period of a soil layer and a column with continuous mass, the results were analyzed by the software. in the second step, piles with the same dimensions and distance were added step by step, and the vibration period for the soil layer with piles was calculated. the friction or floating effects of the piles on alluvial soil vibration period was also examined. the results show that as the number of piles increases, the differences between the results of one dimensional analysis of alluvium soil and the results of the software become different, and this creates the need for specific arrangements for seismic analysis of this kind of alluvium (with inserted piles). the results also suggest that endbearing piles have a greater effect on alluvial soil vibration period, and with increased amount of the floating of these piles, these effects decline.