The mechanism inducing the infiltration rate lowering of the Unzen volcanic ash was studied through permeability tests applying varying electrolyte concentration, ESP (Exchangeable Sodium Percentage) measurement of the fallout deposits, and SEM (Scanning Electron Microscope) observation. The results of the permeability tests showed that the permeability decreased with the number of experiments using destilled water, whereas the permeability kept constant using electrolyte. The ESP in the field condition is sufficiently higher than the forest soil. These results indicate that the mechanism of infiltration lowering of the Unzen volcanic ash is due to formation of chemical crust by physico-chemical processes of the ash. The observation of the SEM also support the results. The experimental results suggest that infiltration rate of Unzen volcanic ash could be increased by applying electrolyte for reducing surface runoff and debris flow initiation.
Japan Society of Erosion Control Engineering