Modelling of viscous effects in natural clays
A new approach to allow the modelling of the viscous behaviour of natural clay soils, including creep, stress relaxation and the effects of applied strain rate on soil stiffness, has been developed based on the BRICK constitutive model (published by Simpson in 1992). The new model, "strain rate-dependent" (SRD) BRICK, was used in a series of simulations to demonstrate its capabilities in predicting realistic behaviour during one-dimensional compression and undrained triaxial tests in which applied strain rates were varied. Triaxial stress path tests conducted by Gasparre et al. in 2007 to assess the influence of creep, resulting from recent stress history, on soil stiffness were also simulated. The observed trends concerning the stiffness at small strains were successfully modelled.
Modelling the effect of recent stress history on the deformation of overconsolidated soils
The aim of the research was to study the behaviour of overconsolidated soils subjected to small changes of strain or stress appropriate to the investigation of ground movements around excavations, retaining walls or foundations, and to develop a constitutive soil model that can predict such behaviour. The principal feature of soil behaviour investigated was the effect of recent stress history, defined by 9 the angle of rotation between the previous and current stress path directions. Stress path triaxial tests were carried out on both reconstituted and undisturbed samples of speswhite kaolin and London clay. The tests, which followed on from previous work by Richardson (1988), examined details of the influence of recent stress history, which was found to have a significant influence on the stress-strain response of the soil for the current loading path. The data from the tests together with a re-evaluation of the existing experimental data and a limited investigation of the effect of recent stress history in 3D stress space, enabled the main features of the soil behaviour to be identified. The stress-strain response of the soil was found to be highly non-linear, inelastic and dependent on recent stress history; if the stress path rotation was 18O, i.e. a complete reversal, the soil stiffness was at a maximum and was at a minimum for no rotation. As the loading path continued the influence of the recent stress history gradually diminished until it was no longer evident. Recent stress history also affects strain paths and effective stress paths measured during drained and undrained loading respectively. The significance of mean effective pressure and overconsolidation ratio was also investigated. Soil models which can predict this behaviour need to include at least one kinematic yield surface which allows plastic straining inside the state boundary surface. A two-surface yield model of this type, formulated by Al Tabbaa (1987) was evaluated. The predictions of this model did not compare well with several important aspects of the experimental observations and it was shown that an additional kinematic surface is necessary to model non-linearity, inelasticity and the recent stress history effect. A new three-surface model based on the two-surface model was developed, within the framework of Critical State soil mechanics, which successfully predicts all the main features of the soil behaviour. It is described by eight parameters which are all soil properties and, with one exception, all have a clear physical meaning.
Geological, mineralogical and chemical characterization of Devonian kaolinite-bearing sediments for further applications in the ceramic (tiles) industry in La Paz, Bolivia
Clay minerals are widely distributed in Bolivia; among them, illitic clays are most common in the Altiplano where they are mined for use in the ceramic industry that has been growing in the last few years. In addition to illitic sediments, kaolinitic sediments have been recently discovered in sedimentary units in the Bolivian Altiplano. Residual ball clay occurrences in Devonian sedimentary units were studied as part of this work. Geological mapping and geophysical studies (ERT and GPR) were done for better understanding the origin of the deposits and were part of a preliminary study of the mineral potential to define the relationship with the host rock. Chemical and mineralogical techniques such as X-Ray Diffraction, Scanning Electron Microscopy and Inductively Coupled Plasma analyses were performed in samples from the studied area to verify the presence of kaolinite. Atterberg limits and behaviour of the raw material in ceramic specimens supported by chemical analyses show that this material is suitable for manufacturing tile ceramics. This study provides fundamental knowledge for deposit exploitation and future generation of an alternative source of employment for the inhabitants of Micaya.