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Abstract

This dissertation describes a series of methods and approaches to the modelling of features of the stress-strain behaviour of natural stiff clays. A review of experimental work on these clays has shown that natural structure, anisotropy and creep are major causes of these features. Nevertheless, work by several authors (Burland, 1990, Cotecchia, 1996) has demonstrated that the behaviour of natural stiff clays fits reasonably well into the Critical State framework developed for reconstituted clays, in particular, those clays with a stable natural structure, which are the focus of much of the work in this dissertation.

The methods that have been developed to model the effects of natural structure, anisotropy and creep are extensions to a constitutive model developed from tests on reconstituted clay. They have been evaluated by modelling both laboratory tests, where conditions are well defined, and field problems, where conditions are more complex.

A series of laboratory tests carried out on samples of reconstituted and natural Boom clay were modelled to investigate the influence of assuming a natural state boundary surface on predictions for the natural clay. Tests on natural samples of Oxford clay were modelled to investigate the application of the model to a clay which has a response characterised by depositional anisotropy. Analyses were carried out using a version of the constitutive model adapted to allow periods of volumetric creep to test whether the consequences of modelling creep by increasing the size of the state boundary surface at constant stress were consistent with observations.

The methods were also used in the analysis of field problems. A series of analyses were undertaken to investigate the importance of allowing for the effects of natural structure and anisotropy on predicted ground movements around a tunnel, a foundation and an open excavation. These analyses also investigate the effect of assuming different scenarios for the formation of natural structure during the geological history of the deposit. Analyses were also carried out to investigate the consequences of allowing periods of creep on the predicted displacements around a tunnel.

The work showed that the methods used to model the features of natural stiff clays affected predictions in a manner consistent with observations, for the analyses of laboratory element tests. Analyses of field problems showed that using these methods significantly affects predictions of ground movements, but that the application of the methods depends on a detailed understanding of the processes undergone during the geological history of the clay, which are in practice often unclear.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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