Finite element analysis of the axial dynamic response and efficiency of pile groups

Dynamic pile group effect can either increase or decrease the response of pile-supported structures. This paper presents the results of a three-dimensional finite element model of the pile-to-pile interaction that considers the effect of the surrounding soil to determine the dynamic stiffness and damping for vertical end bearing pile groups subjected to a vertical harmonic loading. The results were generated for a wide range of the dimensionless frequency parameter (a(o) ) for a 9 x 9-pile group with three different spacings: two-, four- and six-pile diameter. Both the stiffness and the damping showed an oscillatory behaviour with the dimensionless frequency parameter a(o), as well as with the soil shear modulus. In addition, the group efficiency was determined as a function of the pile spacing and the soil shear modulus. The efficiency factor for the stiffness can be as high as 1.15 and as low as 0.7 and for the damping as high as 3.75 and as low as 0.4 as a function of the dimensionless frequency parameter a(o).