Horizontal dynamic stiffness of pile groups: Approximate expressions

A number of solutions and computer programs are already available to determine the dynamic stiffness of complete pile foundations, assuming linear elastic soil behavior and perfect bonding between the piles and the surrounding soil. These are assumptions that would be generally valid for properly designed machine foundations where very small strains should be expected. A number of approximate formulations have also been developed. Among these the most commonly used one is that proposed by Poulos (1971) [12] for the static case, computing interaction coefficients between the heads of two piles considered by themselves, then forming a matrix of these coefficients to obtain the interaction between the heads of all the piles in the group. Additional approximations have been suggested, particularly for the computation of the interaction coefficients, using closed form expressions. In this paper, approximate expressions that can be used for preliminary estimates, at the very early stages of the design, without the need of computers, are presented. They are intended for pile groups with pile spacing of the order of 3 diameters, typical relations between the modulus of elasticity of the piles and that of the soil between 100 and 1000, and very small amplitude vibrations.