A non-coaxial theory of plasticity for soils with an anisotropic yield criterion

Yuan, Ran (2015) A non-coaxial theory of plasticity for soils with an anisotropic yield criterion. PhD thesis, University of Nottingham.

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A novel, non-coaxial soil model is developed in the context of perfect plasticity for the plane strain condition whilst incorporating initial soil strength anisotropy. The anisotropic yield criterion is developed by generalising the conventional isotropic Mohr-Coulomb yield criterion to account for the effects of initial soil strength anisotropy described by the variation of internal friction angles at different principal stress directions. The model is implemented into the commercial finite element(FE)software ABAQUS via the user defined material subroutine(UMAT).

The proposed model is used to predict material non-coaxiality in simple shear tests. The non-coincidence of the directions of principal stresses and plastic strain rates can be reproduced. A faster rate of approaching coaxiality is observed when soil yield anisotropy is presented when compared to the model with an isotropic yield criterion.

A semi-analytical solution of the bearing capacity for a smooth strip footing resting on an anisotropic, weightless, cohesive-frictional soil is developed based on the slip line method. A good match of the bearing capacity can be obtained between numerical and semi-analytical results. The results show that the vertical load at plastic collapse of a strip footing resting on an anisotropic soil is lower than that on an isotropic soil. The settlement prior to collapse is larger when the non-coaxial assumption is involved; however, no significant impacts can be observed on the ultimate failure load.

In addition, the non-coaxial soil model is applied to investigate tunnelling induced displacement. The results are compared with the results from the centrifuge tests performed by Zhou (2015). For equal volume loss, the normalised settlement trough can be improved by adopting the soil anisotropic parameter β as compared to the experimental results. The maximum settlement is larger in light of larger non-coaxial coefficient for the same degree of the stress reduction.

The cross-section of the anisotropic yield criterion developed is a rotational ellipse. Other types of the ellipse are possible. In addition, for simplicity we only consider the effect of initial anisotropy without considering induced anisotropy, and only the simple case of perfect plasticity is investigated. It is suggested that in order to capture the soil behaviours under more complex stress paths, the non-linear and anisotropic elasticity should be associated with the current model, and the development of hardening/softening rules is worth investigating.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Yu, H-S.
Keywords: Non-coaxial behaviour, soil strength anisotropy, numerical simulations, strip footings, tunnel excavations
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA 703 Engineering geology. Rock and soil mechanics
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Civil Engineering
Item ID: 29007
Depositing User: Yuan, Ran
Date Deposited: 08 Sep 2015 10:00
Last Modified: 13 Oct 2017 19:42
URI: https://eprints.nottingham.ac.uk/id/eprint/29007

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