Analysis of the applicability of wave model solution to evaluate the variation of the dynamic stiffness in the pile foundation versus the distance between piles at the vertical vibrations; both experimental data from references and our own measurements in field are involved. To justify the reliability of the solutions of the wave models used to determine the dynamic stiffness values of the pile foundations at the vertical vibrations, we use the data obtained experimentally for the determination of natural frequencies in the cap-bound groups. The data obtained for the natural frequencies of 3×3 floating piles with different distances between them are considered. In addition, the data found at the forced vertical vibrations of the cap-bound groups of 2×2 piles under different loads and at different distances between the piles are involved. Processing of available amplitude-frequency curves involves the solution of the inverse problem with the theory of nonlinear vibrations to determine the parameters of the “pile group – soil” system, namely the effective mass, stiffness, and damping. The correlation between the measured and predicted data is evaluated by using the data obtained at the description of the pile groups in soil behavior. It has been found that the relations obtained at the solution of wave models and used to calculate the dynamic stiffness at the vertical vibrations of pile foundations consider the mutual effect of the piles in the group and permit satisfactory accuracy of the results. The maximum discrepancy between the results and experimental data is 15 %.