土質材料における塑性理論の修正

抄録

The theory of plasticity in soil has been established in such a framework that the hardening modulus h vanishes when volumetric compressive strain rate ε_p is zero. In general, soil shows dilatancy under relatively low pressure and h becomes negative. This is called strain softening. We have discussed the criterion of plastic instability in soil. In plastic instability the state which permits multiplicity of the solution arises. Consequently, the criterion is shown to be characterized by h=0. However, at ε_p=0 the plastic instability is not generally observed in practice. Furthermore, for h<0, stable deformation has been observed in many cases. Such inconsistencies exist between the theory and the observation. Next, we point out another inconsistency in the loading criterion of strain softening. The loading criterion does not clearly distinguish strain softening from unloading. For resolving these inconsistencies, it is necessary to make the expansion characteristics of the yield surface consistent with the hardening properties of soil. Then, we can exclude the phenomenon of strain softening. Such conformity of hardening characteristics in yield surface with hardening properties of soil makes it possible to develop a unified theory of plasticity in soil.