砂防学会誌

In order to examine shear strength of unsaturated sandy soil, unsaturated triaxial tests on 3 kinds of sandy soil were carried out, and some analysis were also carried out based on the results. The shear strength characteristics of unsaturated sandy soil are mentioned below.
(1) Internal friction angle is constant even if suction changes.
(2) Cohesion added by suction is proportional to logarithm of suction (pF).
(3) Because linear relation exists between pF and moisture ratio, Cohesion added by suction is proportional to moisture ratio.
Based on these results, stability of plain sliding slope composed by unsaturated sandy soil is examined. The results are that critical slope gradient becomes smaller in proportion with moisture ratio and drop of critical slope gradient becomes higher as the decrease of cohesion added by suction becomes larger.

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Slit sabo dams are increasingly attracting attention because the use of this type of sabo dam is more effective not only for environmental preservation, but also for disaster prevention than conventional sabo dams. The role of slit sabo dams as a function of sediment control was examined through numerical simulation. A sabo dam with narrow and deep slits acts to lower its peak sediment discharge rate but are not able to decrease its total sediment discharge because most of the sediment deposited at the slit sabo dam is re-eroded by the water flow every time a recession occurs. A series of slit sabo dams is, however, not necessarily effective for sediment control, as is true of ordinary sabo dams. Rather, sabo dams that are capable of trapping a large quantity of sediment, when they are empty of sediment, contribute most to checking the quantity of sediment discharge.

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To contribute for delineation of the hazardous area by the Merapi type pyroclastic flows, a theory for the mechanics of the flow and deposition is presented. In this model, the flow mechanism transits from the granular flow upstream to the fluidized one downstream associated with the crush of the particles caused by inter-particle collisions. In the granular flow the particles are mainly sustained by the inelastic collisions, whereas in the fluidized flow, the particles are sustained by the large scale turbulence and up-lifting force of the upward gas flow emitted from the material itself. The flow continues to run down leaving the base layer in which the driving force due to gravity does not exceed the resistance force. Using this model, the 1991 pyroclastic flow at Mt. Unzen-lake were reproduced by the 2-D numerical simulation. The results of calculation well explain the actual phenomena.

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