Liquefaction evaluation using frequency characteristics of acceleration records in KAIST centrifuge tests for LEAP

Abstract

As part of the LEAP-UCD-2017 and the LEAP-ASIA-2019 exercises, dynamic centrifuge model tests were conducted at KAIST on a submerged 5 degrees sloped medium-dense and dense model grounds of Ottawa F-65 sand. In this paper, the experimental procedure along with the testing results are presented. It was found that liquefaction occurred in the medium-dense model indicated by large lateral surface displacements and pore pressure buildup, while soil layers only partially liquefied in the dense models. The frequency characteristics of liquefied and partially liquefied models were observed using Fast Fourier Transform (FFT), ratio of response spectra (RRS) and time-frequency analysis of recorded accelerations. A shift in the fundamental frequency of the model grounds to lower frequencies due to softening associated with excess pore pressure buildup was seen in the FFT analysis. From the RRS, it was found that high-frequency amplification in accelerations due to soil dilatancy was similar within the liquefied zone in the medium-dense model regardless of confining pressure, while in the partially liquefied dense models, the high-frequency amplification was different according to the soil depth. Moreover, using time-frequency analysis, a shift in mean frequency of base motion (1 Hz) to lower frequencies was evident after liquefaction but not after partial liquefaction. Hence, by observing the frequency characteristics of liquefied and partially liquefied soil layers, the occurrence and depth of liquefaction can be ascertained. This provides an independent check for liquefaction in dynamic centrifuge testing using acceleration data.