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The aim of this paper is to provide new insight into the catastrophic mobility of the earthquake-induced flow-type Takanodai landslide that occurred on 16 April 2016, which had fatal consequences. A geological and geotechnical interpretation of the site conditions and experimental investigations of the mechanical behavior of weathered Kusasenrigahama (Kpfa) pumice are used to characterize the landslide failure mechanism. The results of large-strain undrained torsional shear tests indicate that Kpfa pumice has the potential to rapidly develop very large shear strains upon mobilization of its cyclic resistance. To evaluate the actual field performance, a series of new liquefaction triggering analyses are carried out. The liquefaction triggering analyses indicate that Kpfa pumice did not liquefy during the Mw6.2 foreshock event, and the hillslope remained stable. Instead, it liquefied during the Mw7.0 mainshock event, when the exceedance of the cyclic resistance of the Kpfa pumice deposit and subsequent flow-failure type of response can be considered the main cause of the landslide. Moreover, the combination of large cyclic stress ratios (CSR = 0.21–0.35)—significantly exceeding the cyclic resistance ratio CRR = 0.09–0.13)—and static shear stress ratios (α = 0.15–0.25) were critical factors responsible for the observed flow-type landslide that traveled more than 0.6 km over a gentle sloping surface (6°–10°).

This paper reports on the influence of initial static shear on large deformation behavior of very loose (Dr = 24-30%) Toyoura sand subjected to undrained cyclic torsional loading. A series of isotropically consolidated torsional simple shear tests were carried out on hollow cylindrical specimens up to single amplitude shear strain exceeding 50%. Two types of cyclic loading patterns, namely reversal stress and non-reversal stress, were employed by varying the magnitude of combined initial static shear and cyclic shear stresses. The observed types of failure were distinguished as liquefaction and residual deformation based on the difference in the effective stress paths and the modes of development of cyclic residual shear strain. Test results revealed that, similar to the case of medium-dense Toyoura sand (Dr = 44-50%) previously investigated by the Authors, under reversal stress loading, failure could be associated with liquefaction followed by extremely large deformation during cyclic mobility. Contrarily, under non-reversal stress loading, a progressive accumulation of residual deformation brought specimens to failure although liquefaction did not occur. Moreover, the presence of initial static shear does not always lead to a decrease in the liquefaction resistance or strain accumulation of very loose sand. In fact, its resistance can increase or decrease with an increase in initial static shear stress, but it strongly depends on the combination of static and cyclic shear stresses and, thus, on the type of loading. However, under the same magnitude of combined shear stresses applied, very loose sand is much weaker against large cyclic shear strain accumulation than the medium-dense sand.

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