Mechanism of earthquake-induced rapid landslides occurring along the bedding plane of weathered soft rock

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  • 地震時に風化軟岩層理面をすべり面として発生した高速地すべりの発生機構
  • ジシンジ ニ フウカナンガンソウ リメン オ スベリ メン ト シテ ハッセイ シタ コウソク ジスベリ ノ ハッセイ キコウ

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Abstract

  In recent earthquake events in Japan, many largely-displaced landslides occurred on dip slopes of Neogene sedimentary rocks. In this study, in order to examine the mechanism of earthquake-induced landslides, we took Nigorisawa landslides which occurred in the 2008 Iwate-Miyagi Nairiku earthquake for the research site, and investigated physical and mechanical properties of slip surface soils existing along the bedding plane. Results of ring shear tests performed under shear speeds ranging from 0.02 to 500mm/min revealed that shear strength decreases during high shear rates. To clarify the mechanism of the negative rate effect, additional experiments were performed on medium-plasticity cohesive soils. Test results showed that weakening of shear strength is likely to occur when the shear box bath is filled with water. On the other hand, a long-time fast shearing test under drained condition suggests that shear weakening did not continue and strength recovery gradually occurred due to dissipation of excess pore water pressure. Temporal increase in water content within shear zone is an essential cause for strength weakening during fast shearing. In addition, according to field survey on groundwater conditions, water flow is significant just above the bedding plane, possibly influencing on shear weakening of slip surface soils during the earthquake.

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