Experimental and Theoretical Studies on the Nonlinear Characteristics of Soil-Pile Systems under Coupled Vibrations

This study includes the determination of soil-pile separation lengths and frequency-amplitude responses of single and group piles under rotating machine-induced coupled (horizontal and rocking) vibrations by experimental investigation. Estimation of boundary zone soil parameters responsible for the measured nonlinear responses of the soil-pile systems with an analytical approach are another aspect of the present study. To fulfill these objectives, forced vibration tests were performed in the field on a single pile and a $2\times 2$ pile group made of hollow steel piles of length 5.8 m with an outer diameter of 0.166 m to determine the frequency-amplitude responses for different eccentric moments under a static load of 15 kN. In addition to that, the soil-pile separation lengths of single and group piles are determined by measuring the contact pressure between the pile and soil. The numerical analysis of the pile foundations is performed with a continuum approach using the soil-pile separation lengths obtained from the dynamic pile tests. Comparing the analytical frequency versus amplitude responses with the experimental response curves, the best possible variation of boundary zone soil parameters is determined for different eccentric moments. From the results, it can be concluded that the continuum approach analysis is very capable of predicting the accurate nonlinear responses of the single and group piles. The variations of stiffness and damping of the soil-pile systems with frequency are also presented for both horizontal and rocking modes of vibration using the numerical analysis. The pile-soil-pile interaction phenomenon of the $2\times 2$ pile group is also investigated by calculating the group efficiency ratios (GER) of the pile group for different eccentric moments and vibration modes.