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Most of the man-made structures in cold regions could expose in seasonally frozen conditions which could have significant impact on the soil-pile systems of structures. Little research has been conducted to investigate the thawed frozen soil effects on the dynamic performance of pile structures. An experimental investigation of the thawed frozen soil effects on the dynamic behavior of a pile structure is presented. Free-decay response approach is used in summer to estimate the dynamic properties of a pile partially embedded in Fairbanks silt one year ago. The frequency spectrum analyses are used to evaluate system's vibration properties. The empirical modal decomposition is used to decompose response signal for system parameter identification. Results show that two dominant modes can be used to characterize pile dynamic response, one is a component at 14 Hz and the other one is at 920 Hz. The former is a pile rocking mode in which pile behaves like a rigid body; the latter is the first order structural bending mode of the embedded pile. The rocking mode exhibits nonlinear characteristics in the time-frequency domain which is dominated by strong soil nonlinearity. The structural modes of embedded pile exhibit weak nonlinear characteristics. The damping ratios are identified, which can be used to characterize pile-thawed frozen soil interactions. A theoretical nonlinear model including equivalent nonlinear soil spring and nonlinear damping coefficients is developed to correlate with the experimental results.

A free-decay response signal approach is proposed for reliable estimation of frozen soil–pile dynamic properties of a partially embedded pile. Theoretical consideration and approximations are given for the free vibration of pile structures, with 20% of the pile cantilevered aboveground and the remaining embedded in Fairbanks silt. Winter measurements were taken for free-decay response of the pile. A comprehensive frequency spectrum analysis that includes fast Fourier transform, power spectrum density, and spectrograms is used to evaluate the system's vibration properties. Empirical mode decomposition is then used to decompose the signal to extract specific components for parameter identification. Results show that the response exhibits time-variant and nonlinear characteristics in the time–frequency domain. Experimental data show that the tested system exhibits weak nonlinearity. Dominant system parameters, used to characterize frozen soil–pile interactions, are identified. Two dominant frequencies for a stiff pile embedded 20 ft (6.096 mm) deep in frozen Fairbanks silt are 97 Hz and 1080 Hz. Damping was found to be approximately 0.016.