2024 Volume 10 Issue 12 Pages 334-339
The 2011 Great East Japan Earthquake caused long-duration earthquake ground motions in the eastern region of Japan, leading to ground liquefaction in many areas owing to its massive magnitude of MW = 9.0. Subsequently, the authors (2014) conducted quantitative studies on the effect of the waveform shape of seismic waves on liquefaction strength and proposed a reasonable method for predicting liquefaction. This study discusses the liquefaction process of sandy soil under long-duration earthquake waveforms, based on the results of cyclic hollow torsional shear tests wherein shear stress is the driving force. The tests replicated the long-duration earthquake motions observed at K-NET Haramachi and K-NET Urayasu during the Great East Japan Earthquake while varying the density and fine-grain content. From the effective stress paths obtained for each seismic waveform, it was confirmed that the effective shear stress ratio, which is the historical maximum in the shear stress loading direction, was updated at the point where the excess pore pressure increased owing to the plastic deformation of the specimen. This point of change coincided with the yield surface indicated by Ishihara et al. (1975). Furthermore, the effective confining pressure decreased significantly when the effective shear stress ratio was updated when the shear stress history was subjected to a large number of irregular shear stress histories before updating the shear stress history. Therefore, owing to the higher number of cyclic waves, the seismic waveform of K-NET Haramachi was more prone to liquefaction than that of K-NET Urayasu. This characteristic is independent of the density or fine-grain content of the specimens.
