SUPERLOADING YIELD SURFACE CONCEPT FOR HIGHLY STRUCTURED SOIL BEHAVIOR

AKIRA ASAOKA, MASAKI NAKANO and TOSHIHIRO NODA

ABSTRACT The superloading yield surface concept is newly introduced to the original Cam-clay model in order to describe some aspects of the mechanical behavior of highly structured soils, in which destructured soils are assumed to follow the original Cam-clay model. Following are typically those aspects: (a) structured soils are always "bulky" compared with destructured soils, and if they are in the normally consolidatcd state they always take their state variables outside the "Roscoe surface" of the Cam-clay model (b) when void ratios are the same, structured soils exhibit strengths higher than those of destructured soils; (c) for the same stresses, the void ratios of structured soils are greater than remolded soils. The structured state of a soil is simply defined as the size ratio of the original Cam-clay yield surface and the superloading yield surface that should lie above the Cam-clay yield surface. On the basis of "unconventional plasticity" theory the superloading yield surface concept, together with Hashiguchi's subloading yield surface concept, describes the degradation processes from both an overconsolidated state to a normally consolidated state and a structured state to a destructured state. These degradation processes continue gradually with ongoing plastic deformation. Since plastic deformation is irreversible, the decay of soil structure is also irreversible: The degraded state can not come back to the original state again through elasto-plastic mechanical operation alone. Chemical and/or thermal effects with "aging", that are said to newly generate both overconsolidated state and structured state without any change of stresses, are beyond the scope of this study.

Key words: constitutive equation, elasto-plasticity, normally consolidated soil, overconsolidated soil, remolded soil, structured soil (IGC: D6)