Abstract
Huge debris flows and slope failures occurred at Ryu-ga-mizu district, Kagoshima City on August 6, 1993. They were directly induced by a long rainfall of over 2, 000 mm, continuing at a rate of 259.5 mm per day. The topographical factor of the slope angles from 30 degree to 40 degree in the wall of Aira caldera also contributed to the disaster. We estimated three mechanisms of the occurrence of those debris flows and slope failures as a result of field survey. These mechanisms are as follows;1) Rain water penetrated into well-permeable pumice flow deposit layer and permeable welded tuff layer superposed on non-permeable silt layer. Then, because of an increasing ground water flow rate, collapsing occurred at surface of pumice flow deposit layer and welded tuff layer.2) Collapsing of pumica flow deposit layer occurred on account of an increased gush out of ground water through the boundary between silt layer and pumice flow deposit layer.3) Rain water penetrated into pumice flow deposit layer. Increasing pore-water pressure in the layer caused the collapsing.
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