A mathematical model is formulated to represent the stress-strain behavior of overconsolidated soil. The full range of behavior from lightly to heavily overconsolidated states is covered. The basis of the model is the utilization of a constitutive equation for a work-hardening plastic material. The paper is confined to modeling behavior under the stress conditions imposed in the conventional triaxial apparatus and oedometer, but extension to more general stress systems is possible.
"They can capture the behavior of normally consolidated and lightly overconsolidated clays fairly well; however, they fail to reproduce the experimental behavior of highly overconsolidated clays with satisfaction. This is mainly because the Cam-clay models are based on the classical plasticity theory (Hill 1950), and they do not allow plastic deformation to occur inside the yield surface (e.g., Pender 1978; Yu 1998). However, experimental observations show that plastic deformation may occur before the stress state reaches the yield surface and when the soil behavior does not show an obvious transition from being perfectly elastic to elastoplastic (e.g., Pestana et al. 2002; Prashant and Penumadu 2004). "
[Show abstract][Hide abstract] ABSTRACT: Most clays, either naturally deposited or man-made, possess a certain degree of overconsolidation owing to tamping, cyclic loading, erosion, excavation, and/or changes in groundwater tables. An easy-to-use constitutive model for overconsolidated clays is useful for relevant engineering applications. In this paper, a simple model is proposed for overconsolidated clays based on the unified-hardening (UH) model. To evaluate the potential peak stress ratio of overconsolidated clays, a parabolic Hvorslev envelope rather than a straight envelope (used in the original UH model) is adopted. The proposed parabolic Hvorslev envelope passes through the origin of the mean stress-deviatoric stress plane. It has a slope of 3 as the overconsolidation ratio (OCR) approaches infinity and intersects with the critical state line as the OCR reaches unity. This modification leads to more realistic predictions for highly overconsolidated clays than does the original UH model with a straight Hvorslev envelope and is consistent with the critical state soil mechanics in which the higher peak stress ratio in overconsolidated clays is a result of interlocking (or dilatancy) rather than cohesion. The modified UH model retains the same parameters as those in the modified Cam-clay model. Reasonable agreement between the model predictions and experimental data demonstrates that the modified model is capable of addressing the fundamental behavior of overconsolidated clays. The present model is developed for reconstituted clays with an isotropic fabric. The potential improvement of the model, taking into account anisotropy and structural effects, is discussed.
Journal of Geotechnical and Geoenvironmental Engineering 07/2012; 138(7):860-868. DOI:10.1061/(ASCE)GT.1943-5606.0000649 · 1.60 Impact Factor
"Nevertheless, much research on OC clays has been carried out since the early years in the development of soil mechanics. The work of Wroth (1971), Nadarajah (1973), Amerasinghe & Parry (1975), Atkinson (1975) and Pender (1978) in the 1970s has set a good foundation for the study of OC behaviour. Since that time, the concepts of the bounding surface (Dafalias & Herrmann, 1982, 1986; Dafa- Dafalias, 1986a, 1986b) and the subloading surface (Hashiguchi, 1978, 1989; Hashiguchi & Chen, 1998) have been proposed. "
[Show abstract][Hide abstract] ABSTRACT: Based on the relationship between the current yield surface and the reference yield surface, a new model, called the three-dimensional unified hardening model for over-consolidated clays (the UH model), is proposed in this paper. A current yield surface is used to describe over-consolidated behaviour, and a reference yield surface to describe the yield characteristics corresponding to normally consolidated clays. The UH model can model many characteristics of overconsolidated clays well, including stress-strain relationships, shear dilatancy, strain-hardening and softening, and stress path dependence behaviour. The key feature of the model is the adoption of a unified hardening parameter that is independent of stress paths. Based on the SMP criterion and the corresponding transformed stress method, the proposed model can be applied conveniently to three-dimensional stress states. Compared with the Cam-clay model, the UH model requires only one additional clay parameter, the slope of the Hvorslev envelope. The validity of this new model is confirmed by data from triaxial drained and undrained compression and extension tests for clays with different overconsolidation ratios, true triaxial tests with different Lode's angles, and cyclic loading tests.
"P R O O F S to be elastic in the original model. It was recognised that real soils do not behave in this manner, and, for example, Pender (1978) made an assumption that undrained stress paths below yield are of a parabolic shape, therefore being non-linear instead of linear as in the original model. Van Eekelen & Potts (1978) introduced several improvements to the model: (a) a cut-off surface on the dry side to reduce the mobilised peak strength of overconsolidated soils (b) a Mohr–Coulomb yield surface in the deviatoric plane to better represent soil failure (c) non-associated plasticity in the deviatoric plane, with circular plastic potential surface (d ) a single parameter cyclic model below the yield surface to enable modelling of cyclic processes. "
[Show abstract][Hide abstract] ABSTRACT: A review of the first 60 years of Géotechnique publications shows clearly how the subject of soil mechanics has evolved. In terms of constitutive and numerical modelling of soil, early forms of numerical analysis involved hand calculations of ultimate states applying classical methods of analysis: limit equilibrium, limit analysis or stress field solutions. Consequently, the soil was considered to behave as a rigid plastic material, and to follow one of the two basic failure laws of classical soil mechanics, namely the Tresca or Mohr-Coulomb failure criteria. For assessing the deformation of structures, soil was normally considered to be linear elastic. The foundations of modern numerical analysis and constitutive modelling were laid in the early to mid 1960s, with the development of the finite element method and the postulation of the critical state framework of soil behaviour respectively. Clearly, the continuous advancement of computer power has been essential in applying new developments to modern geotechnical analysis. This paper reviews some of the main milestones in the evolution of geotechnical analysis in the past 60 years, commenting, where appropriate, on what problems still lie ahead.
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