Nautical bottom2010b

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Mud: Mud: more complex than you think! more complex than you think!

Revising the concept of nautical depth in the light of Revising the concept of nautical depth in the light of

micromicro--structure dynamics and implications for instructure dynamics and implications for in--situ surveyingsitu surveying

byby

Erik A. ToormanErik A. Toorman

Hydraulics LaboratoryHydraulics Laboratory

Civil Engineering DepartmentCivil Engineering Department

HSB Workshop “HSB Workshop “HarboursHarbours and Specific Survey Problems”and Specific Survey Problems”8 December 2010, Flanders Hydraulics, 8 December 2010, Flanders Hydraulics, BorgerhoutBorgerhout (B)(B)

SEDIMENT SEDIMENT MECHANICS RESEARCH MECHANICS RESEARCH

@ @

K.U.LEUVENK.U.LEUVEN

Research Unit Coordinator:

E-mail: [email protected]

http://www.kuleuven.be/hydr/SedMech.html

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Understanding “mud”Understanding “mud”

and its behaviour under shear

Defining Mud

Mud is a mixture of water and fine sediments

(clay, silt and sand) and organic matter,

which behaviour is characterized as cohesive,

when it is dominated

by the clay fraction (>15%).

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Mud particles

Formed by flocculation

= aggregation (+ break-up) in a suspension

© van Leussen (1993)

after Krone

Mud layer micro-structure

3D network structure for ρ > 1100 kg/m3 (gel point)

“card house”

after Partheniades

© van © van OlphenOlphen ���� primary kaolin particles (SEM image)

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Micro-structure in mud-sand mixture

Clay bridges glue the sand skeletonScanning-electron microscopy images (left: air-dried – right: oven dried)

© KULeuven (Torfs , PhD, 1995)

“Fluid” Mud

= “dense”, flocculated suspension with

concentration (slightly) above the gel point

(i.e. the structure formation threshold)

(soil at rest � liquefied when sheared)

Mud rheology = macroscopic description of

its deformation and flow behaviour

Time-dependence due to (micro-)cracks and

restructuring

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Experimental equilibrium flow curve

bentonite suspension (9.91% by weight)

Weissenberg Rheogoniometer + parallel plates (Wright & Krone, 1989)

retained

viscous

total

SHEAR RATE (s-1)

SH

EA

R S

TR

ES

S (

Pa)

Experimental mud viscosity

Apparent viscosity for River Parrett (UK) mud at different densities

Carri-Med CS rheometer data by Jones & Golden (1990)

“Shear thinning”

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Thixotropy: conceptual

Barnes (J. Non-Newtonian Fluids, 1997)

Thixotropy

Time-dependent behaviour under shear of dense

flocculated suspensions:

– structural break-down when sheared (liquefaction),

– structural recovery at rest (aggregation).

(+ self-weight consolidation)

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Thixotropy @ microscale

Complex floc motion and interaction during simple shear flow in a 2D dispersion of PS particles at decane/water interface

Masschaele & Vermant (2008)

ThixotropyThixotropy @ @ macroscalemacroscalePlacement of ADV instruments on mud bank along the Surinam coastPlacement of ADV instruments on mud bank along the Surinam coast

© Hydraulics Laboratory, © Hydraulics Laboratory, KULeuvenKULeuven

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0

10

20

30

40

50

60

70

80

90

100

17:16:48 17:24:00 17:31:12 17:38:24 17:45:36 17:52:48

Time(hh:mm:ss)

Vis

co

sit

y(%

)

60

30

12

6

3

1.5

0.6

0.3

Measurement: Rheometry

“Shear-rate-step-change” experiment

Surinam Coast mud (diluted to ρ = 1130 kg/m3 )

Brookfield rotoviscometer + vane spindle

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140

SHEAR RATE (s-1

)

SH

EA

R S

TR

ES

S (

Pa)

TOTAL STRESS

RESIDUAL STRESS

Equilibrium flow curve: Bingham

ρ = 1130 kg/m3 – after shear rate correction

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Rheological models

γτµ &/=

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70 80 90 100

SHEAR RATE (s-1

)

SH

EA

R S

TR

ES

S /

BIN

GH

AM

YIE

LD

ST

RE

SS

Newtonian

Herschel-Bulkley

Worrall-Tuliani

Bingham

“true” yield stress

effective viscosity ���� shear thinning

Thixotropy Model

• Constitutive equation:

• Structural kinetics (1st order):

• with: and

( ) γλγλγλ

&&& )(1)( badt

d−−=

00 // τηβ == abγβ

γλ&

&

+=

1

1)(e

( )

0

00),(

τλ

λγµ

λτγληµγλτ

e

+=

++=

∞

∞

&

&&

(Toorman, 2005)

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Fluid mud rheology

equilibrium flow curveequilibrium flow curve

constant structure curvesconstant structure curves

�������� �������� ddata Cheng & Evans (1965)ata Cheng & Evans (1965)

bentonite

Fluid mud rheology

ddata ata JoyeJoye & & PoehleinPoehlein (1971)(1971)

hysteresis

hectorite

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Implications Implications for for

the definition of the definition of

Nautical Nautical BottomBottom

Navigability• Thickness & viscosity of disturbed mud

= function of speed & ac-/deceleration

• Drag force unevenly distributed

• Interface between disturbed and undisturbed = artificial (and temporal!) “rheological transition”

disturbed muddisturbed mud

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CFD of drag on a ball in a Bingham fluid

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Implications for Implications for

surveyingsurveying

Problems

• Intrusive instruments (profiling or towed)

disturb the original microstructure.

• The degree of disturbance is determined by

the speed of intrusion and the size and

shape of the device

• No equilibrium data can be obtained

• Usually the device moves too fast such that

the structure is cut and not truly sheared

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ConclusionsConclusions

Conclusions (1)

• Mud is a thixotropic gel (a “structured” fluid).

• Its instantaneous strength and apparent viscosity

depend on its micro-structure.

• A rheological transition indicates a discontinuity in

micro-structure (i.e. in history).

• The state of the micro-structure requires knowledge

of the shear history.

• The actual nautical depth depends thus also on the

state of the mud layer (which varies along the ship!)

and the shear history caused by the movement of

the ship and that of previous passages.

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Conclusions (2)

• Nautical depth cannot be defined in terms of

rheological parameters (and composition) alone!

• It also depends on the manoeuvre and ship

characteristics.

• Mud rheology can be characterized by dedicated

laboratory experiments.

• A rheological closure for CFD applications is

available.

• 3D CFD studies are needed to understand the

problem of nautical depth for each ship.

Thank you

Questions?

E-mail: [email protected]

SELECTED PUBLICATIONS

Berlamont, J., Ockenden, M., Toorman, E. & Winterwerp, J. (1993). The characterisation

of cohesive sediment properties. Coastal Engineering, Vol.21:105-128.

Toorman, E.A. (1996). Sedimentation and self-weight consolidation: general unifying

theory. Géotechnique, Vol.46(1):103-113.

Toorman, E.A. (1998). Sedimentation and self-weight consolidation: general unifying

theory. Discussion. Géotechnique, Vol.48 (2):295-298.

Toorman, E.A. (1997). Modelling the thixotropic behaviour of dense cohesive sediment

suspensions. Rheologica Acta Vol.36 (1):56-65.