Estimation of the occurrence of a volcanic mud flow based on the theory of liquefaction of a volcanic ash layer caused by vibration of an earth quake or impact force of a slope failure

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In this study, first it was tried to make clear the mechanism of the fluidization of volcanic ash layer by the vibration of an earth quake or impact force of a slope failure. This volcanic ash layer lain between volcanic accumulations is made by falling ash in an area of active volcano, next it was to predict the occurrence of a volcanic mud flow based on the theory of liquefaction of the mass of volcanic ash caused by the action of vibration or impact force stated above. As a result, the followings could be clarified.Judging from the result of the rheological test of a slurry consisted of volcanic ash and water, it can be defined as an entirely volcanic mud flow when the volumetric concentration of it is over 15%. When the vibration with low frequency acts on a slurry with the concentration of over 48.5% which consists of the only volcanic ash, the effect of dilatancy presents conspicuously itself, and the shearing resistance force of it increases in accordance with the number of vibr … More ation. When the concentration of it gets to about 66%, it presents entirely plasticity and the nobility of it decreases greatly. But the effect of dilatancy is prevented and the mobility of it increases when small gravels are mixed into that slurry, because the particle sizes in the slurry are not so homogenious as those in the slurry consisted of the only volcanic ash. The mobility of it gets the highest at the mixing ratio of about 40%. The mass of volcanic ash is easily fluidized in only few seconds by the vibration with low frequency of 5 cycle/sec and the amplitude of lcm- 2cm in the concentration below 66%.When a volcanic ash layer suffers from the vibration with high frequency like an earth quake, the critical angle for the occurrence of the movement of it decreases by 20ﾟ - 30ﾟ in comparison with that in the usual state when any vibration dose not act on it. The apparent internal friction angle of a slurry with high concentration falls much lower than that with low concentration. This means that liquefaction of volcanic ash layer would be easily caused by the vibration like an earth quake or impact force of a slope failure in an area of active volcano even if in a little rain fall. Less