An eruption of Mt. Ontake on September 27, 2014, which was small scale stream eruption with about 500,000 ton volcanic products, caused 47 deaths and 6 missing. Though volcanoes take on serious aspect in actively in Japan, the concern with the effect of falling of volcanic ash in huge eruption to buildings has been growing. A large volume of ash fall can destroy a building or lead to a catastrophe causing tragic loss of human life. It is important to understand of a characteristic of sliding of volcanic ash depositing on the roof to estimate an amount of accumulation of volcanic ash on the roof.
The aim of the present work is to observe behavior of volcanic ash on pitched roof and to understand the basic characteristics of the sliding of volcanic ash on the roof using a model testing.
The model testing was carried out using a roof model consisted of two components: one was a roof and another was a supporting structure that held up the roof. These two components were jointed each other with a pin-connected which allowed the roof rotate and replace the roof to another type of roofs; an opposite side of the joint was connected to a crane attached on a ceiling in a laboratory. Four types of roof were prepared to investigate an effect of surface roughness of roof materials and shape of a surface of the roofs on the sliding. The types of roof covers were cement tile, plane Galvalume steel plate, plane Galvalume steel plate with straight line roofing and Galvalume steel plate with seven steel plates substituted for snow steps.
The sliding of volcanic ash on the roof could be occurred when the crane pulled a free end of the roof upward. The pitch of the roof was increased until the falling of volcanic ash depositing on the roof was observed and a drop amount of the volcanic ash from a roof and a pitch of a roof were measured when the sliding of the volcanic ash stopped. Two kinds of volcanic ash which were spouted from Mt. Sakurajima in 2014 and from Mt. Shinmoedake in 2011 were accumulated on the roofs.
Each roof had a unique characteristic of sliding of volcanic ash when a drop amount of the volcanic ash from the roof reached to same value. This result suggested that the sliding of volcanic ash on the roof depended on surface roughness of roof material and shape of a surface of the roof.
A trapezoid model which approximated to a cross-section shape of volcanic ash on the roof was developed to calculate an angle of a slope φ of the volcanic ash which was a value of the angle of slope when the sliding was over.
The angle of a slope φ was closed to an angle of internal friction when the pitch of roof was small; it gradually decreased close to 70% of an initial value. A pitch of the roof θMAX which was maximal angle that the volcanic ash could accumulate was calculated by extrapolation of the angle of a slope φ. The values were in the range of 21° to 26°. This result suggested that sliding would occur and could cause serious damage to buildings in huge eruption.
These results indicated that the sliding of the volcanic ash would be one of issues in volcanic disaster prevention in Japan and the angle of a slope φ could be main factor to understand the sliding. Further research on angle of a slope φ was needed to clarify the characteristics of the sliding.
