Abstract
A time-frequency approach based on wavelet transform is employed to examine the transient vibration characteristics of two 2×2 pile-group models tested in a shake table and subjected to three different records consisting of: white noise input and two differently scaled records from the 2011 Christchurch earthquake. In contrast to the conventional Fourier transform, the proposed method has the advantage of being capable of monitoring the temporal variation in structural frequencies and frequency content of ground motion due to liquefaction. Results are presented in time-frequency planes that enable displaying these time-varying processes in an effective way. It is found that the onset of liquefaction can have a substantial effect on the vibration characteristics, resulting in a reduction of natural frequencies in the range of 34- 51%, and shift of frequency content of the ground motion towards lower frequencies, along with narrowing of its overall frequency bandwidth. A final discussion on the practical implications of the main findings highlights that such non-stationary phenomena have important effects on the seismic response of pile-supported structures founded in liquefiable soils.