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A Study of Plasticity Theories and their Applicability to Soils

A review of the types of rate independent theories for soils is given, together with a more deatailed survey of plasticity theories. The contributions of particulate mechanics are briefly summarised. The remainder of the dissertation is divided into two parts, dealing respectively with a theoretical and experimental study of the applicability of plasticity theory to soils. Some additional information is given in Appendices.

Part I begins with some theoretical preliminaries. Kinematic variables and their conjugate forces, internal variables and the theoretical restrictions on plasticity theory are introduced. The need for a less restrictive approach is explained, and this is met by a thermomechanical formulation of plasticity theory. Some implications of the theory and some specific examples are given, including models involving non-associated flow and the principle of effective stress.

A detailed discussion of the derivation of the Modified Cam-Clay model from the thermomechanical method is given, including comparisons with earlier energy theories for clays. The model is extended to general stress states and to large strain theory. Some modifications of the model are then considered, including changes to the yield locus and a study of a shear modulus dependent on pressure or preconsolidation pressure.

Experimental data on the variation of the shear modulus are discussed. Part I concludes with two aspects of soil behaviour not yet included in the thermomechanical approach: the generalisation of yield loci in the octahedral plane and the development of anisotropy.

The material for experimental study is described, and the types of test for the applicability of plasticity theory discussed. An investigation of the effect of stress and of stress history on a dense sand in triaxial compression, using a programme of stress cycle tests, is outlined. A computer controlled triaxial testing machine necessary for the tests is described, with details of the datalogging and control system and the sample preparation procedure.

The method of analysis for the tests is given, including a method of fitting elastic and plastic properties to the data. The results of the tests are presented. The elastic properties are anisotropic and depend on stress and stress history. The plastic properties are strongly history dependent, and significant secondary plastic strains on unloading were observed.

Finally some conclusions from both the theoretical and experimental work are drawn together and some topics for future work suggested. The application of the thermomechanical approach in describing soils is emphasised.

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