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Experiments on subaqueous mass transport with variable sand-clay rati o

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Fabio De Blasio

Trygve Ilstad

Anders Elverhøi

Dieter Issler

Carl B. Harbitz

International Centre for Geohazards

Norwegian Geotechnical Institute, Norway

Dep. of Geosciences, University of Oslo, Norway.

.

In cooperation withthe SAFL group, University of Minnesota

Basic problem!

- How can we explain that 10 - 1000 km3 of sediments can
- move100 - > 200 km
- on < 1 degree slopes
- at high velocities
- ( -20 - > 60 km/h)

- Debris
- flow

- Field observations (long runout, outrunner blocks, geometry of sandy bodies, velocity...)
- Experiments:
- (Experiments +Numerical modeling) × Extrapolation Field
- composition change

- Physical understanding and numerical simulation
- Important application:
- Emplacement of massive sand in deep water
- Offshore geohazards

Experimental Flume: “Fish Tank”

turbidity current

debris flow

6° slope

10 m

Video (regular and high speed) and

pore- and total pressure measurements

How to explain the various styles of run out!

Subaerial

Short and thick

Subaqueous

Thin and long

Turbidity current

Hydroplaning debris flow

Flow behavior - High clay content( 30 % kaolinite)

Turbidity current

Dense flow

Deposition of sand

Turbulent front Deposition of sand

- High clay content-
- - Plug flow- “Bingham”
- High sand content
- Macro-viscous flow?
- Divergent flow in the
- shear layer

Thickness of sandy deposits – versus clay content

Pore pressure

Total pressure

Flow

Pore pressure

Flow

Pressure

Flow

Time

Grains in constant contact with bed

Total pressure

Pressure

Fluidized flow

Time

Pressure

Rigid block over a fluid layer

Time

Low clay contentHigh clay content

Total pressure

Hydrostatic pressure

L2

Ls

L1

H2

Hs

H1

Downslope gravitational forces

Bottom shear stresses

Detachment/stretching dynamics

Neglected physics:

- Changing tension due to slope and velocity changes
- Friction, drag and inertial forces on neck
- Changes in material parameters of neck due to
- shear thinning, accumulated strain and wetting, crack formation

- More sophisticated treatment is possible
- Coupled nonlinear equations, use a numerical model
- Main difficulty is quantitative treatment of crack formation and wetting and lubricating effects

- Visco-plastic materials
- Model approach:
- ”Classical Bingham fluid” (“BING”)
- R-BING: Remolding of the sediment during the flow
- H-BING: Hydroplaning/Lubricating

Shear layer

- Classical Bingham fluid:
- Yield strength: constant during flow

- Bingham fluid – with remolding (R-BING):
- The yield strength is allowed to vary during flow

Plug layer

Pluglayer

Water film/lubricating layer shear stress reduction in a Bingham fluid

u=1

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, m, um

1+

1

u

1

1

1-

1-

(R-)/

1

u

Plug layer

Shear layer

Velocity

Shear stress

1+

R(1+)/

Simulation: final deposit of the large-scale Storegga

What happens during flow at low clay content?

- 1) disintegration of the mass: the yield stress drops dramatically
- 2) settling and sand stratification within few seconds

dependent on the clay content

Reference solid fraction

solid fraction in the slurry

- Mud with plug and shear layers
- plasticity, viscosity, and visco-elasticity
- dry friction (no cohesion in code)
- dynamic shear (thinning)
- dispersive pressure

- plasticity, viscosity, and visco-elasticity

- Depth integrated, three-dimensional model
- Accounts for the exchange of fluid between different parts of the slurry due to diffusion and advection.
- Limitations for our purpose: water content of the slurry must not change, no cohesion, no turbulence

- Viscoplastic behaviour
- Vertically quasi-homogeneous
- Hydroplaning/lubrication
- Dynamical forces important
- The material remains compact
- Front detachment/outrunner block
- Modeling: rheological flow,
- Modified “BING”

- THEY ARE VERY MOBILE BECAUSE OF LUBRICATION

Granular + turbulent behaviour

Settling and vertical layering (“Brazil Nut Effect” )

Lubrication only at the very beginning

The material breaks up catastrophically

Blocks do not form

Modeling: Fluid dynamics + granular

THEY ARE VERY MOBILE BECAUSE OF DRAMATIC DROP IN YIELD STRESS AND FLUIDISATION IN THE SAND LAYER

- Slurries with a high clay content:
- transported over long distances preserving the initial composition

- Slurries with low clay content:
- sandy materials may drop out during flow, alternatively being transformed into turbidity currents

- Flow behavior:
- Strongly influenced by the amount of clay versus sand in the initial slurry

- the Coulomb frictional force (diminished of the water pressure at the base of the debris flow),
- the fluid viscous shear stress,
- the earth-pressure force (namely, the lateral forces generated in the debris flow due to differences in the lateral pressure),
- the earth-pressure contribution of the bed pressure,
- a diffusive term of water escaping from the bottom,
- an earth-pressure term along the lateral (z) direction,
- the diffusive term of water along the lateral direction, and finally
- the pressure at the base of the debris flow.

- At high clay content:
- a thin water layer intrudes underneath the front part = lubrication!
- progressive detachment of the head
- the thin water underneath the head is a supply for water at the base of the flow
- a shear wetted basal layer with decreased yield strength is formed

- At low clay content:
- water entrainment at the head of the mass flow
- low slurry yield stress = particles settlement and continuous deposition
- a wedge thickening depositional layer is developed some distance behind the head
- viscous effects in the diluted flow, Coulomb frictional behavior within the dense flow. High pore pressures → near liquefaction.

- When solid particles are present
- Particles forced apart
- Ability to move large particles
- proportional to square of the particle size for given shear rate (Bagnold, 1954)
- larger particles forced towards area of least shear (up and front)

- Further research required

shear stress

dynamic viscosity

yield strength

shear rate

Plug layer

Shear layer

Yield strength: constant during flow

u=1

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, m, um

1+

1

u

1

1

1-

1-

(R-)/

1

u

Water film shear stress reduction in a Bingham fluid

Plug layer

Shear layer

Velocity

Shear stress

1+

R(1+)/

A: 32.5 wt% clay, hydroplaning front

Dilute turbidity current

B: 25 wt% clay hydroplaning front

D: Behind the head, increasing

concentration in overlying turbidity current

Turbulent front Deposition of sand