Elastoplastic constitutive modelling of clay behaviour under principal stress rotation

Abstract

Recent experimental investigations have highlighted the impact of principal stress rotation on soil behaviour. This impact is fundamental to a wide range of soil types covering from cohesive to non-cohesive. However, most existing constitutive models that account for the effect of principal stress rotation are essentially dedicated to sand modelling. Clay models are very different from sand models in terms of the yield surface, flow rule, etc. The consideration of principal stress rotation in clay models can also be different. To fill this gap, this thesis develops a number of constitutive models dedicated to the simulation of clay behaviour that moreover consider principal stress rotation. All these models are incrementally linear, which facilitates easy numerical implementations. Firstly, considering that the yield-vertex based tangential loading (TL) theory has been frequently used in geotechnical problems that involve significant principal stress rotation, its difference from the true pure principal stress rotation should be investigated. To this end, a principal stress rotational loading (RL) theory is constructed to represent the true pure principal stress rotation. A comparison between the TL and RL theories shows that the tangential stress rate includes the rotational stress rate and an additional coaxial term associated with a variation of the Lode angle. Numerical applications of both theories are also conducted to reveal their respective performance under different circumstances. Moreover, the conditions for maintaining the incremental linearity of the TL and RL theories are discussed. Inspired by the approach of multiple loading mechanisms used in the TL and RL theories, a simple non-associative model for clay that considers principal stress rotation is developed and applied to the simulation of simple shear test. The effect of principal stress rotation is considered by the incorporation of an additional loading mechanism. This mechanism caters for the plastic volumetric strain and non-coaxiality of soil under principal stress rotation. A comparison of model simulations with test data justifies the importance of considering principal stress rotation in the clay simple shear simulation. The model can satisfactorily capture the undrained shear strength of clay. The soil non-coaxial behaviour is also well reproduced. Finally, this thesis develops an anisotropic bounding surface model for clay. The distinctive feature of this model is the inclusion of a second mapping rule in the stress ratio deviatoric plane with a relocatable mapping centre, in addition to the classical radial mapping rule in the meridional plane with a fixed mapping centre. This feature allows for the plastic loading under principal stress rotation to occur without introducing additional loading mechanism or stress-strain incremental nonlinearity. The model response to proportional loading such as the triaxial loading is not affected by the second mapping rule. The performance of the model under principal stress rotation is firstly presented under various conditions, followed by a comparison between model predictions and experimental data.

Date of Award	7 Jul 2019
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Yunming Yang (Supervisor) & Andrew Dawson (Supervisor)