シラスの土質力学的特性とその原因 : シラス防災の研究第一部その1

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Studies and researches of the mechanical properties of Shirasu made poor progress for these twenty years, naturally the final solution has apparently gone to the wall. It comes from the fact that they, lacking the understanding for the essential qualities of Shirasu, indiscreetly tried to just apply the slide theory of ordinary soils. The point of the method is to observe shearing resistance values after Coulomb's equation and to use them as the basic materials for stability theory. But they can, mainly for the two reasons, offer no scientifically reliable data. For the first reason undisturbed test pieces of natural Shirasu is difficult or almost impossible to gain. As for Shirasu difference of mechanical properties between disturbed and undisturbed test pieces is large and so the above circumstances are fatal to the laboratory tests. For the second they disregard the fact that the theoretical hypothesis does not agree with the actual phenomenon. Except a special case the failure of Shirasu always occurs by tension cracks while the theory of ordinary soils is for failure by a slide. Using the hypothesis unfit for the phenomenon why can we solve the problem? The author, from the view of the fact that how natural Shirasu is stable and how it fails, studied and investigated manyways, presents a new idea quite contradictory to the usual theory. This idea starts in reconsidering the essential qualities of Shirasu. This report is only an introduction but it makes clear brief and plain the mechanical properties of Shirasu and its special structural feature from which the idea comes out. Details will be reported later, to be frank, epitomizing the essential qualities of Shirasu chiefly in the sight of mechanical properties, we can relate as follows.After geologists we understand Shirasu as sediments of nonwelded pumice flows from past volcanos. The main factor how the flows stop and deposit will, leaving the outer conditions out of consideration, be their fluidity. And it having to do with grain-size, shape of particles, and specific weight, depends on their viscosity. Assuming the three factors are roughly alike fluidity is at all decided by viscosity and, in the concrete, it shows itself as the thickness of the deposition and as its slope angle. The force of the flow has, of course, something to do with, but generally speaking steep heights are thought to be viscider than those which are lower and easier. The setting power when cooled down will probably be closely related with the viscosity of the hot flow. These circumstances, as the results, will support the author's idea that a vertical cliff is stable to the height of the original deposition.Shirasu is brittle concrete-like rock with aggregates of volcanic glass and pumice, having very week setting power of some chemical components and its coeficients of fragility are l5~20 sometimes it reaches 40. Compressive and tensile strengths are about 1kg/cm^2 and 20~60g/cm^2 and so the breaking always occurs by tention, never by shear. Grains and particles stick together on the surface and form collection of minute irregular solid arches. Shirasu is of large coid ratio and is not anisotropic both in composition and in strength. Setting power will come from chemical substance, perhaps, colloid of silicic acid.The setting power is easily lost when the normal state of water existence in the composition is disturbed by any cause and the Shirasu fails. The causes can be either too much or little water content or sprinkling absolute mineral oil or alcohol. The reason why Shirasu is extremely weak against water is as mentioned above. Water content of natural Shirasu covered with a surface layer is in usual case some 20% and that is the proper water quantity to be stable.If it reaches to 30% the setting power lowers to the degree that Shirasu begins to just slide. This is the only natural case of slide of Shirasu slope.Shirasu is brittle, somewhat elastic, very weak rock so the strength test should be simple compression test.These are the summary of our opinion.
1969-09-25

著者

上田通夫
鹿児島大学

シラスの土質力学的特性とその原因 : シラス防災の研究第一部その1

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シラスの土質力学的特性とその原因 : シラス防災の研究第一部その1