ABSTRACT
If a granular material, such as soil, contains macroscopic cavities and is exposed to viscous flow, these cavities tend to collapse and grow in large scale, which may result in landslides, destruction of river banks and cliffs. We have already shown both theoretically and experimentally a significant effect of flow concentration into the cavity regions in a granular material, i.e. the volume flux and the velocity at the center of a twodimensional circular cavity amount to 2 times and 3 times, respectively, (and 3 times and 6 times, respectively, in the case of a spherical cavity) compared to those without the cavity (Sano, 1983; RajaSekhar & Sano, 2000; 2001 & 2003; Kaneko & Sano, 2003). Consequent increase of stress on the boundary of the cavity may destroy the local equilibrium configuration of particles around the cavity region above a certain critical magnitude of velocity (Sano, 2003 & 2004; Kaneko & Sano, 2005). Particles, which detach from the bulk granular medium, will be carried in the downstream direction, until another local equilibrium configuration is attained. In certain situations the collapse of the cavity induces changes of the local flow field, which may enhance further destruction of the upstream cavity boundary. If these nonlinear processes are repeated, the cavity region or fluidized region containing floating particles will grow toward upstream direction in the granular material (see Figure 1). In this paper we shall confine our attention to the fundamental mesoscopic processes connecting microscopic configuration changes due to viscous flow with
Flow
Collapse Cavity
Upward growth of fluidized region Stress
enhancement
macroscopic fluidization, and give evidence to support the conjectures mentioned above.
