Modélisation du processus de sédimentation-consolidation des vases estuariennes: approche continue unidimensionnelle

Résumé

L'étude de la sédimentation et de la consolidation des sédiments cohésifs présente un grand intérêt scientifique par son aspect interdisciplinaire, à la frontière hydraulique-géotechnique et en raison de l'importance de ses applications à la gestion des zones cotières. Nous proposons d'associer les points de vue hydraulique et géotechnique en vue d'établir un modèle continu du processus de sédimentation-consolidation basé sur les équations de conservation de deux phases compressibles. Ce modèle, général et évolutif, permet d'envisager la prise en compte de phénomènes négligés jusqu'alors: fluage, floculation, écoulements non-darciens, ... Il constitue donc un élément fondamental pour une modélisation globale et précise de la dynamique sédimentaire en estuaire.

Abstract

 Nowadays numerical models tend to replace physical models for the study of sediment transport processes in fluvial or marine environments. The variability of both the materials and the environments make the processes complex in areas of major economic importance such as estuaries. The representativeness of current models depends much on the evaluation of the material which is deposited and/or re-suspended, hence the modelling of the deposit and consolidation phenomena. The paper proposes a unidimensional analysis of the sedimentation-consolidation process based on a continuous action and taking account of the complexity of the sedimentary materials. The distinction between sedimentation (the fall of isolated grains in a fluid medium) and consolidation (compression of a solid deposit by evacuation of pore water) is difficult to determine in the case of muds. In muds, particles frequently interact (for example in flocculation and deflocculation) although the grains remain separated by a film of water at a microscopic scale. In the 1980s and 1990s, several authors attempted to unify the two phenomena. This paper reviews the most significant works on sedimentation and consolidation and discusses their limitations, noting that there is always a boundary between the two processes. The model proposed in this paper assumes a continuity of the sedimentation-consolidation process which remains compatible with previous theories. It is based on wide assumptions, postulating the effective stress can be time dependent and the infiltration velocity is not necessarily governed by Darcy's law. A system of equations is derived which are valid for a polyphasic medium (solid, liquid, gas) and where the solid phase can be composed of isolated grains and aggregates. State equations, equations of mass conservation and equations of mechanical equilibrium are completed by laws derived from the experimental approaches described in this paper or from those of other authors. The model allows the introduction of various shapes of constitutive laws, incorporating time in the effective stress function (taking account of creep phenomena) and a filtration velocity which does not follow Darcy's law (notably for the very low hydraulic gradients). This new model is applicable to a great number of sedimentation and/or consolidation cases, e.g.

– Under water sedimentation-consolidation cyclic   load (swell, tide, etc.).

– Consolidation of mud in ground deposits.

– Consolidation of soft clay layers.

The model has been validated by means of the sedimentation-consolidation simulation tests as undertaken by Been (1981). Despite the use of simple constitutive laws, the model gives a theoretical evolution of density profiles which is very similar to the experimental one for the same initial conditions. This model of the transition between sedimentation and consolidation has a wide field of application, including civil engineering, medicine, the food industry, chemical engineering and ecology.