Identification of transient vibration characteristics of pile-group models during liquefaction using wavelet transform

A time-frequency approach based on the wavelet transform is employed to examine the transient vibration characteristics of two 2×2 pile-group models tested in a shake table. The models are subjected to three different records consisting of white noise input and two differently scaled records from the 2011 Christchurch Earthquake. In contrast to conventional Fourier analysis, the proposed method has the advantage of enabling the visualisation of the temporal variation in structural frequencies and frequency content of ground motion due to liquefaction in an effective way. It is found that liquefaction causes a decrease in structural frequency, whose reduction depends on the rate of excess pore pressure build-up, whereby high rates (“fast liquefaction”) lead to greater reduction, ie, up to 51%. Liquefaction is also responsible for the elongation of the predominant period of the ground motion and narrowing of its overall frequency bandwidth. The combined effect of reduction in structural frequency and filtering of high frequency components of the ground motion may lead to moving resonance condition, resulting in amplification of structural response. After the onset of liquefaction, there is a redistribution of maximum bending moment toward deeper elevations, indicating that kinematic soil-structure interaction dominates the overall seismic response.