The Stability Analyses of Cut-off Slopes on the Basis of the Classification of Original Ground Shirasu

The undisturbed Shirasu belongs to a none or weakly welded part of pyroclastic pumice-flow soil widely distributed in southern Kyushu and is counted to be an unusual soil for its frequent damages owing to rainfalls and earthquakes. As its original ground is of property like a weak rock, no rotational shear slide ever occurs in fact in the cut-off and natural Shirasu slopes. On the other hand, this soil is extremely erosive against water flow and the nearly vertical cut-off slope has traditionally been adopted. However a few kinds of local damages were unavoidable in the slope besides the tensile failures inevitably cause even at the ordinary time. This paper reports, after made clear the elastic characteristics of undisturbed Shirasu material, the elastic analysis for various cut-off slopes on the basis of the classification of original ground Shirasu (JSF standard : M 2-81), which has been proposed by JSSMFE in 1979. These analyses were carried out by the finite element method, assuming that the Shirasu material is an isotropic and homogeneous elastic body. The discussions were made from the view point of tensile failure of slope, and the following findings were obtained