rheological properties of cohesive sediments and rheological adjustment under wave action ricardo...
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Rheological properties of cohesive sediments and rheological adjustment
under wave actionRicardo 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
.G
..G
Yield stress
Rheological tests
1. Creep tests : before flow and yield conditions.
2. Dynamic or Oscillatory tests.
Mud
Creep tests
Creep tests
Creep tests
Creep tests
Creep tests
Creep testsEvolution de apparent / plateau en fonction de '
1,E-08
1,E-07
1,E-06
1,E-05
1,E-04
1,E-03
1,E-02
1,E-01
1,E+00
1,E+01
1,E-06 1,E-04 1,E-02 1,E+00 1,E+02 1,E+04
taux de cisaillement en s-1
cour
bes
rédu
ites
à 1420kg/m3 à 1470kg/m3 à 1500kg/m3 à 1540kg/m3
' = 0,0004s-1
instabilités
écoulement newtonien
Creep tests
Creep tests
Oscillatorytests
Density effects
Oscillatory tests
Oscillatory tests
Oscillatory tests
Oscillatory testsà une fréquence de 1Hz
1,E-08
1,E-07
1,E-06
1,E-05
1,E-04
1,E-03
1,E-02
1,E-01
1,E+00
1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04
déformations en %
G'/
G'0
à 1440kg/m3 à 1450kg/m3 à 1510kg/m3 à 1540kg/m3 à 1580kg/m3
Faible non linéarité à 0,04% <
thixotropie à 3% < 4%
liquéfaction à 100< %
régi
me
linéa
ire
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
Rheological model
Rheological model
SUAVE - Analytical modelling of wave-mud viscoelastic interaction
Results
Results
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.