ABSTRACT
A landslide-triggered-debris flow killed 14 persons working inside a torrent to construct debris flow prevention dams in Nagano, Japan. Debris flows usually occur at the time of heavy rainfall. However, this debris flow took place in a very dry season of early December in this area. Therefore, warning was not enough to evacuate from this debris flow. This paper presented the mechanism of debris flow initiation due to “sliding surface liquefaction” of partly saturated torrent deposits, which was caused by undrained loading due to a displaced mass from the scarp of previous landslide in a valley slope. This phenomenon was reproduced by the undrained dynamic loading ring shear apparatus using volcanic soils taken from the debris flow area.
