MECHANICAL PROPERTIES OF "SHIRASU" AND THE CAUSES : Pesearch on "SHIRASU" Disaster Prevention Part 2 No.1

Having used part 1 for the general remarks, especially for the history of "Shirasu Study", now the author is to relate his own works in part 2. In this paper he examines the method, which, the author conciders, will be the mother of the product of the researches, making it deep or shallow, giving meaning to, characterizing, sometimes colouring the child. The chapter is divided into two paragraphs, a general opinion and the instance concerned. He first thinks about the significance and points out four important matters, explains respectively. (1) Characteristics or rather particularity of "Shirasu", which seems to be half-rock in nature and yet resembles mere soil when disturbed. Orthodox soil mechanics do not serve. (2) Mathematical analysis is not always possible, available and it should not be required to excess. (3) The investigation and selection of the data, be it experimental or of literature, of others or of his own, will be indispensable. We are sometimes apt to lean upon the papers of others without full caution and to quote from them or to form our theory on them, but the letter is not the truth, and they might contain errors and defects so we must get rid of them in case of application. (4) Logical coordination should be retained through the whole. A part, making up the whole, is to be regulated by the whole and the contradiction must be avoided within the logic system at all. Formally to join partial arguments, mere arrangement of the data and conventional theory of them, and the elongation and subdividing of formal logic of not so much sense will work little. Above all, basic study of the subject of investigation is required at the beginning. Obscure understanding of it sometimes gives us a translucent view. Next he settles three fundamental concepts, he makes clear the purpose of the research, gives a definition to Shirasu, and utters the most distinguished nature of the subject materlal. Paragraph 2 is the space for the good experimental case which reveals the entire difference betWeen undisturbed and disturbed Shirasu behaviour under the varying loads. Man can easily recognize the fact and the cause that natural Shirasu is solidified with some substance and it turns into a mass of granular material and loses the solidification when disturbed. The full size concrete block retaining wall of 3m high by dry masonry both of the disturbed Shirasu banking and of the natural cut slope were subjected to the same load tests. The data were analysed and, in the former case, the wall was proved to be a pressure free construction. The earth pressure distribution is of the Japanese handdrum type, having the fixed zero apex in the middle part with both the lower bell type distribution of active and upper flare of passive earth pressure. Every maximum active pressure at the bottom part is always kept constant while the minimum passive, the total active and passive pressures vary according to the steps of loading. The disturbed Shirasu showed plastic behaviour. In the case of the cut slope it was found to be nearly elastic. The difference is caused by the solidification