Experimental investigation of transient bending moment of piles during seismic liquefaction

The purpose of this paper is to study the behavior of pile-supported structures in liquefiable soils, specifically when the soil surrounding the pile transits from no-liquefaction to full-liquefaction. A series of shaking table tests were performed on four pile-supported structures subjected to different input motions and, as a result, with different times to reach full-liquefaction. The bending moment of the piles during the transient phase is compared with those predicted in pre-and post-liquefaction stages. The experimental results showed that the maximum bending moment may occur during the transient phase (i.e. during the development of excess pore pressure before the soil is fully liquefied). Arguably, the observed amplification in bending moment is caused by the the tuning effect between the predominant frequency of the input motion and the frequency of the pile supported structure, which is progressively decreasing during the liquefaction process. Results are presented using a non-dimensional framework whose parameters are derived from the governing mechanics. A new parameter TAF (Transient Amplification Factor) is defined to predict the design bending moment during the transient phase. It is shown that the transient bending moment can be obtained from the newly introduced parameter TAF, and the values of maximum bending moments in the pre-and post-liquefaction stages. It is found that TAF is a function of two easily obtainable parameters: (a) time taken to reach full liquefaction, this can be obtained through site response analysis; (b) elongation of natural period of vibration, expressed as ratio of the time period of the structure at full liquefaction to the time period at zero-liquefaction. Finally, practical implications of the main findings are discussed.

Automated summary

This paper investigates the behavior of pile-supported structures in liquefiable soils, specifically focusing on the transition from no-liquefaction to full-liquefaction. Shaking table tests were conducted on four pile-supported structures, which were subjected to different input motions and reached full-liquefaction at different times. The experimental results showed that the maximum bending moment can occur during the transient phase, and this amplification in bending moment can be attributed to the resonance effect between the input motion frequency and the decreasing frequency of the pile supported structure during the liquefaction process. A non-dimensional framework and a new parameter TAF (Transient Amplification Factor) were introduced to predict the design bending moment during the transient phase. It was found that TAF can be determined based on easily obtainable parameters such as the time taken to reach full liquefaction and the elongation of natural period of vibration.