Influence of Intra-Particle Saturation Ratio on Strength Degradation of Pumice Soil

The 2018 Hokkaido Eastern Iburi Earthquake occurred on 6 September 2018 and caused more than 7000 landslides in and around Atsuma and Abira towns. The geology of this area is mainly formed by pyroclastic fall deposits. The rain gauges in the epicentral area showed little rain from typhoon Jebi (5 September 2018) and heavy rain in the middle of August 2018, about 3 weeks before the earthquake. A series of direct shear tests and single particle fragmentation tests were conducted to investigate the effect of soaking time on the strength reduction of Tarumae d (Ta-d) pumice. The median Single particle fragmentation strength of Ta-d particles decreases with the soaking time. The Intra-particle saturation ratio of Ta-d has increased with the logarithmic value of the soaking time. Samples were soaked in water for 1, 6, 24, 168 and 508 h before shearing. Peak shear strength was decreased with the soaking time. The residual strength is about 2 kPa for each soaking condition, which shows that there is no significant variation in residual strength with soaking time. The friction angle shows a sharp reduction from the peak value during the first 800 mm of shear strain and thereafter gradually stabilizes to the residual value. The peak shear stress of Ta-d pumice is reduced with the intra-particle saturation ratio. The mobilized frictional coefficient of Ta-d is not sensitive to the intra-particle saturation ratio.