Constitutive modelling of unsaturated soils

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113

SUMMARYThis paper addresses various issues concerning the modelling of solid-liquid-air coupling in multiphase porous media with an application to unsaturated soils. General considerations based on thermodynamics permit the derivation and discussion of the general form of field equations; two cases are considered: a three phase porous material with solid, liquid and gas, and a two phase porous material with solid, liquid and empty space. Emphasis is placed on the presentation of differences in the formulation and on the role of the gas phase. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The two formulations are then analysed with respect to a documented drainage experiment carried out by the authors. The merits and shortcomings of the two approaches are shown.

“…This further affects the degree of saturation (through Equation (55)), as can be seen in Figure (10). Note that the maximum degree of saturation is limited to 99% for the sake of …”

Section: Two and Three Phase Calculationsmentioning

confidence: 97%

“…A short review on the constitutive modelling of unsaturated soils can be found in Laloui et al [10].…”

Section: Introductionmentioning

confidence: 99%

Solid–liquid–air coupling in multiphase porous media

Laloui¹,

Klubertanz²,

Vulliet³

2003

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113

“…Secondly, the assumption of a continuous gaseous phase with a uniform pressure (equal to the atmospheric pressure datum) is usually accepted in engineering applications [3], thus leading to a further simplification. In Reference [4], the authors explained on the contrary the interest to use the 'saturated effective stress' ðr À p l 1Þ rather than the net stress ðr À p g 1Þ in order to, among others, simplify the transition from partial to total saturation. Some significant works among these attempts can be cited like for example Alonso et al model [5] or Wheeler et al contributions [6,7].…”

Section: Introductionmentioning

confidence: 98%

Adaptation of existing behaviour models to unsaturated states: application to CJS model

Pereira

Wong

Dubujet

et al. 2005

SUMMARYThis paper proposes a general formulation of an elastoplastic model adapted to unsaturated soils. This formulation enters within the framework of two independent state variables descriptions. The choice of a particular effective stress combined with suction is made. The definition of this effective stress is based on the formulation of an equivalent pore pressure which is an essential point of this type of models. It will be discussed in this paper.This general formulation can be seen as a methodology allowing to adapt in a straightforward way most of elastoplastic behaviour models classically used in saturated soils mechanics to unsaturated states. It is shown that this synthesis can include most of recent models developed within the same framework.The last part of this paper is devoted to the adaptation of an existing complex elastoplastic model (CJS model) to unsaturated states using the methodology previously exposed. The model thus obtained is validated on various loading paths including oedometric, isotropic or triaxial compressions and also wetting tests simulating collapse phenomenon.This model extension shows the easiness introduced by the proposed methodology to adapt a given elastoplastic model to unsaturated states. Its validation illustrates by the way the abilities of the extended model to reproduce complex volumetric responses of an unsaturated soil.

“…A relevant consequence of the unified stress approach lies thus in the sufficiency of a single mechanical yield surface to comprehend any elasto-plastic behaviour resulting in skeleton deformations. So, for the mechanical stress-strain behaviour, no second yield surface in suction is required, setting the model formulation free from any kind of SI or SD yield curves, which are proper to constitutive models written in terms of independent stress variables such as Barcelona Basic Model family [27] or others [52]. In the end, the LC curve should include a hydro-mechanical coupling, which leads to the following formulation:…”

Section: Unicity Of Yield Limitmentioning

confidence: 99%

Effective stress concept in unsaturated soils: Clarification and validation of a unified framework

Nuth¹,

Laloui²

2007

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431

243

SUMMARYThe effective stress principle, conventionally applied in saturated soils, is reviewed for constitutive modelling purposes. The assumptions for the applicability of Terzaghi's single effective stress are recalled and its advantages are inventoried. The possible stress frameworks applicable to unsaturated soil modelling are reassessed in a comparative manner, specifically the Bishop's single effective stress, the independent stress variables approach and the generalized stress framework. The latter considerations lead to the definition of a unified stress context, suitable for modelling soils under different saturation states. In order to qualify the implications brought by the proposed stress framework, several experimental data sets are re-examined in the light of the generalized effective stress. The critical state lines (CSLs) at different saturation states tend to converge remarkably towards a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with conventional net stress conception. The accent is finally laid on a second key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface besides the unique CSL.