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UBC Theses and Dissertations

Behaviour of sand under general stress paths in the hollow cylinder torsional device Sayao, Alberto S.F.J.

Abstract

A fundamental investigation of the stress-strain behaviour of inherently anisotropic sands is presented. The study was carried out in a newly developed hollow cylinder torsional apparatus (HCT). The HCT is the only apparatus that permits a soil specimen to be subjected to multiaxial stress states with controlled variations in four stress parameters: the magnitudes of the three principal stresses (σ₁, σ₂ and σ₃) and the direction of one of these stresses. The experimental program was aimed at assessing the isolated effects of continuous changes in only one derived stress parameter, σ[sub m] (mean effective stress), R (principal stress ratio), b (intermediate stress parameter), or α (direction of σ₁ relative to the vertical) on deformation response. Minimization of stress and strain nonuniformities, inherenly present in HCT specimens, was achieved through careful selection of specimen geometry and regions of the stress space to be investigated. A new approach to define these nonuniformities is proposed in terms of the distribution of stress ratio R across the specimen wall. All tests were carried out on saturated specimens of pluviated sands under fully drained conditions. Inherent anisotropic behaviour of sand is clearly illustrated by the deformation response that is strongly dependent on the loading direction α. Contractive volume changes and shear deformations are shown to increase with increase in α. When compared to the principal stress directions, principal strain increment directions always deviate towards the horizontal, with the exception of loading at α =0 (vertical compression). Effects of induced strain anisotropy were observed to be negligible at R-levels less than 2.0. Continuous principal stress rotation at constant principal stress magnitudes induces progressive accumulation of both volumetric contractions and shear distortions. Additional cycles of rotation in the same direction result in progressively smaller incremental strains, implying hardening effects of previous rotations. Deformations are shown to increase significantly with decrease in relative density and increase in R and σ[sub m]. The effect of continuous variations in b, under constant R, σ[sub m] and α, has been evaluated for the first time. Deformations increase progressively as b increases from 0 to 1, regardless of the direction a. During proportional loading in general stress space, which corresponds to changes in σ[sub m] alone, the strain directions remain constant, and hence the nature of inherent anisotropy is essentially preserved at R-levels up to about 2.0 and at relative densities greater than about 30%.

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