Characterization of the viscoelastic effects of thawed frozen soil on pile by measurement of free response

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.