Abstract
A time–frequency approach based on the wavelet transform is used 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.
Highlights
Extensive damage to pile-supported structures, bridges, high-rise buildings, coastal structures, have been observed in liquefiable soils after most major earthquakes, including recent events in India [11], China [24], Japan [5,16], Italy [31], Nepal [9] and New Zealand [37]
This paper shows the potential of the wavelet transform for the analysis of a highly non-stationary and non-linear process involving soil-structure interaction during seismically-induced liquefaction
Specific conclusions that can be drawn from the present work may be summarised as follows: Wavelet analysis is a useful tool for identifying the temporal variation in vibration characteristics of structures and ground motion due to soil liquefaction
Summary
Extensive damage to pile-supported structures, bridges, high-rise buildings, coastal structures, have been observed in liquefiable soils after most major earthquakes, including recent events in India [11], China [24], Japan [5,16], Italy [31], Nepal [9] and New Zealand [37]. In piles passing through saturated cohesionless soil, the analysis is further complicated since the shaking leads to development of excess pore pressure, resulting in a temporary reduction of stiffness and strength of the foundation soil This phenomenon, commonly referred to as soil liquefaction, leads to further non-stationarity and non-linearity, including filtering effect of the liquefied deposit to seismic waves, which results in a temporal shift of the frequency content of the shaking towards lower frequencies, and temporal variation of the vibration characteristics of the foundation-structure system. To quantify the variation in vibration characteristics of pile-supported structures caused by soil liquefaction, Lombardi and Bhattacharya [31] applied a Fourier spectral analysis to acceleration responses of models subjected to a white noise input applied by means of a shake table. The paper concludes with a discussion on the practical implications of the research findings on the seismic design of pilesupported structures in liquefiable soils
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