Dalili, A; (1991) Disparate particulate materials subjected to principal stress rotation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Many geotechnical engineering designs must accommodate the action of wind, waves and earthquakes. These loadings cause principal stress rotations on the foundation soil. The experimental simulation of these in situ stress paths can only be achieved by using sophisticated laboratory test devices capable of controlling both magnitude and direction of principal stresses. This research uses a new Directional Shear Cell with a modified shear sheet design that avoids hysteresis particularly under cyclic loading and allows tests to be carried out at constant mean stress level. Sample preparation techniques have been improved to provide fully saturated clay samples with minimum sample disturbance and a technique has been developed to measure pore water pressure accurately in the centre of the sample. The experimental work investigates the stress-strain behaviour of dry Leighton Buzzard sand, damp coal and saturated kaolinite at low stress levels when subjected to continuous and cyclic rotation of principal stress directions under plane strain conditions. The results of cyclic principal stress rotation tests on damp coal and saturated kaollnite show parallels with dry sand In that large strains occur at low applied stress ratios. Dilatency is suppressed by continuous rotation of principal stress and no failure planes were observed. However, the underlying mechanism of deformation was different for partly saturated kaolin. The onset of flow in this material leads to the occurrence of discontinuities. These may be tension cracks as well as the more readily anticipated shear rupture layers. Findings show that the principal stress rotation is the governing factor in initiating strain, and that cycling the stress level magnitude is secondary. It has been found that for the samples that have been previously sheared under cyclic rotation of principal stress directions and subsequently tested under monotonic loading, large increases In stiffness, dilation rate and strength-brittleness were observed, i.e. loose sand can become as strong and stiff as dense sand. It has been shown that soil strain can relate to two limiting stress-dllatancy relationships In slow shear flow in which volume change occurs, one attributed to energy dissipated through friction and the other through the dissipation of stored elastic energy.

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