The monopile response to lateral load under different liquefaction phases via centrifuge modeling tests is reported in this paper. The test was performed in viscous scaling mode to match the loading time and flight speed at a centrifuge acceleration of 80 g. A mixture of methylcellulose powder in water increased the viscosity by 80 times compared with water at a concentration of 0.47%. The seismic input is applied with a frequency of 1 Hz to achieve soil liquefaction. During the test, the monopile received the lateral load. The liquefaction regime was divided into four distinct phases based on the excess pore water pressure ratio during loading: total liquefaction, two tests during the period of degradation of pore pressure when the excess pore water pressure ratio was 0.67 and 0.32, and the last test at the end of liquefaction. The results reveal that pore water pressure distribution slightly differs in the free field and surrounding pile. The shallow layer started to liquefy early in response to the ground acceleration. The entire monopile body rotated owing to 0.48 m of pile head displacement during full liquefaction when the lateral load applied reached 290 kN. During the liquefaction regime, the profile of the pile behaviors migrating from a slender pile to a rigid pile is a significant discovery in this study. A large shear force appeared one-third of the pressure to the bottom of the monopile, and the maximum bending moment location became deeper, with a value of 65% greater than at the end of liquefaction.

The magnitude (Mw) 8.3 Tokachi-oki earthquake occurred in September 2003, causing extensive damage in Hokkaido, Japan, and triggering extensive soil liquefaction in the region. The Port of Kushiro was one of the locations where surficial evidence of liquefaction was observed but was also a well-instrumented location with four pore-water pressure transducers installed in the backfill of the quay wall. However, all of the sensors malfunctioned during the earthquake. As a result, the pore-water pressure response recorded by those sensors were inaccurate and unusable with regard to evaluating liquefaction triggering and extent. This study introduced the energy-based soil liquefaction evaluation to estimate the excess pore water pressure responses at the Port of Kushiro based on the cumulative strain energy of the soil during the 2003 Tokachi-oki earthquake. In order to apply the energy-based method to this case history, this study explored the empirical equation describing a relationship between normalized cumulative energy and excess pore water pressure ratio while incorporating the bidirectional shaking effect on strain energy development. Although the energy-based method allowed for the estimation of the time needed to trigger liquefaction at a target site, it was derived using the empirical coefficients that were developed for a different soil from those at the site of interest. This indicated that an adjustment to the estimated timing of liquefaction was needed, which was accomplished by additional evaluation through a Stockwell transform and Arias intensity-based liquefaction assessment. Both procedures indicated a similar timing of liquefaction at the site. Based on the updated time of liquefaction triggering, the empirical coefficient was recalibrated to estimate the excess pore water pressure ratio, and the result provided reasonable excess pore water pressure responses at the backfill of the Port of Kushiro during the 2003 Tokachi-oki earthquake.

The evaluation of the excess pore water pressure ratio (ru), the ratio of the excess pore water pressure of the soil, is a defining approach to assessing liquefaction occurrence. Rarely is ru measured, so surficial observations of sand boils, fissures, and soil settlements have provided indirect evidence of liquefaction occurrence in case histories. Acceleration responses during undrained cyclic loadings incorporate shear strain and stress responses of the liquefied soil. Therefore, the use of acceleration responses can provide another indirect indication of liquefaction as the sudden drop in the frequency in the time-frequency domain in acceleration records. This study aimed to develop strain-based and energy-based methods for estimating the pore water pressure buildup based on the acceleration responses of liquefiable sand layers. The strain-based method linked the liquefaction-induced shear strain of the soil with ru through the shear modulus that is a function of the effective stress. An alternative approach used an energy-based method that linked pore-pressure generation with the energy dissipated in the soil. Centrifuge model tests for the liquefaction of soil were used to develop and validate the two methods, and these were applied to a case history, the 1987 Superstition Hill earthquake at the Wildlife site, for validation. To capture the variation of ru from its contractive to dilative responses, the amount of ru drop was estimated based on the peak shear stress when dilation spikes occurred. For the energy-based method, the centrifuge test results were used to derive empirical relations between ru and cumulative dissipated energy done by liquefiable soil. The estimated ru time-histories from the established methods were consistent with the measured responses in the centrifuge tests and the case history.