The recent activity in the design and construction of offshore, nuclear and large earth structures has given considerable impetus to research on accurate description of stress-strain relationship of soils subjected to static and cyclic loading. A number of constitutive models for soils has been proposed and the main line of approach has been to treat soil as an elasto/plastic or elasto/viscoplastic material. However, none of the models accounts for the rotation of the principal stress axes which invariably takes place in most problems. The main objective of this thesis has been to develop constitutive relationships based on the microstructural behaviour of soils, which would respond to the rotation of principal stress axes and plastic flow-induced anisotropy. The constitutive laws have been incorporated in a simple computer program to analyse triaxial problems and, also in a finite element program, to solve boundary value problems. The behaviour of soil has been assumed basically as represented by an elasto/viscoplastic rheological model. Both triaxial and boundary value problems have been analysed for two classes of soil behaviour which represent either the short-term fully undrained conditions or the long-term fully drained situations. It has been assumed that there are an infinite number of contact planes or asperities for each stress point - and the yield function and potential function are written for each contact plane in terms of effective normal and shear stresses. The following yield functions have been developed: the Mohr-Coulomb yield function on contact plane (MCYCF), and Critical State Model Accounting for Rotation (CSMAR). In addition, no-tension yield criterion on the contact planes has been used to analyse boundary value problems. Associated flow rule has been used except on the dry side of CSMAR where non-associated flow rule has been used. A test is devised to demonstrate the effect of rotation of principal axes of stress on plastic strains for a homogeneous stress situation, and it is shown that plastic strains can increase by as much as 100% due to this rotation when the CSMAR model is used. It has also been shown that CSMAR can be conveniently used to analyse triaxial tests and boundary value problems. Further, the CSMAR model has been extended to describe the cyclic soil behaviour under undrained conditions using the concept of cyclic strain softening. Solutions of many triaxial problems have been compared with experimental results, and the predictions of the model are very close to experimental results. One boundary value problem of footing subject to cyclic loading has been analysed and the model predictions are consistent with experimental results qualitatively. It is concluded that the model CSMAR is potentially useful. The model is conceptually very simple and very easy to implement in the finite element codes.

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