Invariant stress space

Abstract The paper presents results of an experimental study of thermal effects on the mechanical behaviour of a saturated clay, with emphasis on the determination of the onset of yielding. The study was performed on CM clay (Kaolin) using a temperature-controlled u-iaxial apparatus. Applied temperatures were between 22 °C and 90 °C. Various methods are used to identify the yield points (pseudo-elastic limit) and to define the shape of the yield surface in the invariant stress space p (effective mean pressure)- q (deviatoric stress). Yield surface obtained at 90 °C is compared with results at ambient temperature. Based on this comparison, thermo-mechanical yielding is discussed and yield limit evolution with temperature is presented. 

Other coordinate systems may be used for failure surface representations in addition to stress space. Blatz and Ko (11) indicate that either stress (Stress space is most commonly used because the failure surface concept was originally applied to metals, for which stress and strain are more simply related. Viscoelastic materials, on the other hand, may show a multitude of strain values at a given stress level, depending on test conditions. 

This model is based on the mean features of the Mohr-Coulomb model and is expressed with stress invariants [Maleki (1999)] instead of principal stresses. Until plasticity is reached, a linear elastic behaviour is assumed. It is fully described by the drained elastic bulk and shear moduli. The yield surface of the perfectly plastic model is given by equation 7. Function 7i(0) is chosen so that the shape of the criterion in the principal stress space is close to the Lade criterion. 

The VCS-MD used here[ll] is based on a Eangragian formulation very similar to the original dynamics[16j however it uses strains as dynamical variables, instead of the Cartesian coordinates of the simulation cell vectors. This new choice of dynamical variables makes the Eagrangian (and the trajectories) invariant with respect to the choice of simulation cell vectors. It also gives a more physical strain/stress relationship which preserves throughout the trajectories the space group symmetry of the initial condition. This latter property is physical and does not imply in any constraints. This point will be further discussed when presenting examples in the next section. 

Settlement can present a problem in clayey soils, so that the amount that is likely to take place when they are loaded needs to be determined. Settlement invariably continues after the construction period, often for several years. Immediate or elastic settlement is that which occurs under constant-volume (undrained) conditions when clay deforms to accommodate the imposed shear stresses. Primary consolidation in clay takes place due to the void space being gradually reduced as the pore water and/or air are expelled therefrom on loading. The rate at which this occurs depends on the rate at which the excess pore water pressure, induced by a structural load, is dissipated, thereby allowing the structure to be supported entirely by the soil skeleton. Consequently, the permeability of the clay is all important. After sufficient time has elapsed, excess pore water pressures approach zero, but a deposit of clay may continue to decrease in volume. This is referred to as secondary consolidation and involves compression of the soil fabric. 

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