Stability of a levee modelled in a geotechnical drum centrifuge.

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The first drum centrifuge in Japan was designed and constructed to study the stability problems of long soil structures, such as embankments, levees. The drum has the diameter of 0.8 m , the width of 0.3 m and the depth of 0.1 m , and can be spinned up to 150 g, producing the prototype behaviour of a soil model of 377 m long, 45 m wide and 15 m deep. The achievement of this study can be summarized in the following points.1. Safety operation of the drum centrifuge has continued for over one thousand hours by the end of Feburary,1987, which gives us a confidence of utilizing a drum centrifuge as an useful experimental tool for modeling geotechnical problems.2. Experimental technique of production of a normally consolidated clay layer overlain by a levee of compacted soils in the drum has been established.3. Modeling technique of levee failures during flood has been sucessfully developed. A series of model tests with different soil type was carried out. Close observation suggests that the process of failure of a levee may be understood by using an analogy of stability of shallow tunnels. Observed process of failure was as follows:As the level of water raises, the first erosion is occurred at the surface of a levee slope, and that a pipe flow is created by hydraulic fracture and seeping water reaches at the surface of the levee slope on the inland side. As the result of flushing out the soil particles, the diameter of the pipe flow gradually widens. Our knowlegde of the stability of of a shallow tunnel tells us that the stability of the pipe is gorverned by the ratio of the cover of the pipe to the diameter of the pipe, and by the cohesion and the unit weight of the soil. For soils having higher cohesion, the pipe remains stable up to relatively larger diamter, whereas soils having less cohesion, the pipe collapes when the diamter becomes a certain value, resulting the total failure of the levee.