Mechanisms of quasi-preconsolidation stress development in clays: A rheological model

This paper deals with the development of a mechanistic model for the ageing consolidation behavior of clays with the focus on aspects related to the development of quasi-preconsolidation pressure. The initial use of such pressure in design met with criticism, but field and laboratory evidence, which highlights its significance, continues to accumulate. A nonlinear rheological model is used to numerically simulate the consolidation process of clay in laboratory tests and to identify the basic mechanical parameters that contribute to the development of the quasi-preconsolidation phenomenon. Methods to identify the parameters of the model from oedometer tests are described. It is shown that while the variation in soil modulus can be characterized by a linear form in the virgin compression region, it is nonlinear in the recompression region and is best characterized by a hyperbolic function. Changes to the modulus in the recompression region, due to ageing, is shown to be the dominant cause of the development of the quasi-pc phenomenon. Observed results as well as numerical simulations demonstrate that specimens that had aged longer show increased quasi-pc values. While the variation in soil modulus controls the EOP curve of clays, the observed time effects, such as the “vanishing pc” phenomenon, are controlled primarily by changes in soil viscosity. However, this has no bearing on the development of the quasi-pc phenomenon.