Rheological properties of cohesive sediments and rheological adjustment under wave action Ricardo Silva Jacinto Ifremer, France Thierry Aubry Université de Bretagne Occidentale, France

Mechanical characterization of cohesive sediments Soil mechanics – used for the characterization of soil stability, i.e. statical applications and civil engineering applications Rheology – used for the characterization of the flow of complex fluids, i.e. dynamical applications and industrial (processing) applications.

Types of parameters Composition parameters: particle size distribution, density, concentration, content of minerals and organic matter… Structural state parameters: consolidation (effectif stresses), void ratio, water content, porosity, viscoelasticity, sismic and acoustic parameters… Yield parameters: penetration, scissometry, undrained cohesion, yield stress…

Types of correlations Direct correlations: between descriptif parameters (e.g. effectif stresses vs. Density, effectif stresses vs. Void ratio). Undirected correlations: each time one uses yield parameters (unreversible) against descriptif parameters: density vs. yield stress; effectif stresses vs. Yield stresses or triaxial tests. Merckelbach (2000) shows that soil mechanical and rheological parameters are often correlated.

Rheology Rheology gives relations (constitutive relations) between stress tensors and strain and strain rate tensors. Cohesive sediments rheology depends on: density, mineral and organic content, pH, ionic strength…. (Migniot, 1986). Experimental problems: edge effects, wall slip, cracks, fracture… (Coussot, 1997).

Applications on sediment transport

Application on sediment transport

Bulk erosion in the Seine estuary

Sediment structures Calm: soft consolidating mud over stiffer layers. Waves: only the stiffer layers remain. Storms: even part of the consolidated bed is eroded and dispersed.

Conceptual model of the fine sediments dynamics in the Seine estuary

Rheological models Viscous (Newton) Elastic (Hook) Viscoelastic (Voigt) F F F

Yield stress

Rheological tests 1. Creep tests : before flow and yield conditions. 2. Dynamic or Oscillatory tests. Mud

Creep tests

Oscillatory tests

Density effects

Oscillatory tests

Conclusions on rheology Microscopic behaviour do not depend on sample density. One starts to find some parameters that could characterize the rheological (macroscopic) behaviour of cohesive sediments : before it flows (deformation), yield conditions, flow behaviour Correlation with sediment composition could give tools to predict the behaviour of cohesive sediments and not only characterize a given sediment under a given condition (test).

Rheological model

SUAVE - Analytical modelling of wave-mud viscoelastic interaction

Results

Deep homogeneous layer (1460 kg/m3). Waves of 5 s period and 28 cm high.

Conclusions The model ables us to conceptualize the observed erosion in the Seine estuary tidal flat: liquefaction of the soft mud ; failure of the consolidated mud near the rigid bottom. Rheological changes must be accounted for to predict liquefaction and/or mud failure.

What do we call liquefaction? Liquefaction corresponds to a rheological evolution of the mud (structural failure) that allows the material to approach a Newtonian behaviour. A one phase approach (rheological approch) remains possible.