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

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

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Experiments on subaqueous mass transport with variable sand clay rati o

Experiments on subaqueous mass transport with variable sand-clay ratio

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


Experiments on subaqueous mass transport with variable sand clay rati o

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


Inferring the dynamics of subaqueous debris flow

Inferring the dynamics of subaqueous 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 settings st anthony falls laboratory

Experimental settingsSt. Anthony Falls Laboratory

Experimental Flume: “Fish Tank”

turbidity current

debris flow

6° slope

10 m

Video (regular and high speed) and

pore- and total pressure measurements


Experiments on subaqueous mass transport with variable sand clay rati o

How to explain the various styles of run out!

Subaerial

Short and thick

Subaqueous

Thin and long


High clay content video record

High clay content – video record

Turbidity current

Hydroplaning debris flow


High speed video record 250 frames sec

High speed video record (250 frames/sec)

Flow behavior - High clay content( 30 % kaolinite)


Low clay content video record

Low clay content – video record

Turbidity current

Dense flow

Deposition of sand


Debris flows low clay content 5

Debris flows- low clay content (5%)

Turbulent front Deposition of sand


Experiments on subaqueous mass transport with variable sand clay rati o

  • High clay content-

  • - Plug flow- “Bingham”

  • High sand content

  • Macro-viscous flow?

  • Divergent flow in the

  • shear layer


Experiments on subaqueous mass transport with variable sand clay rati o

Thickness of sandy deposits – versus clay content


Pressure interpretation

Pore pressure

Total pressure

Flow

Pore pressure

Flow

Pressure

Flow

Time

Pressure interpretation

Grains in constant contact with bed

Total pressure

Pressure

Fluidized flow

Time

Pressure

Rigid block over a fluid layer

Time


Pressure measurements at the base of a debris flow as pressure develops during the flow

Pressure measurements at the base of a debris flow as pressure develops during the flow

Low clay contentHigh clay content

Total pressure

Hydrostatic pressure


High clay content viscoplastic hydroplaning lubrication

High clay content viscoplastic/hydroplaning/lubrication


Material from the base of the debris flow is eroded and incorporated into the lubricating layer

Material from the base of the debris flow is eroded and incorporated into the lubricating layer.

L2

Ls

L1

H2

Hs

H1

Downslope gravitational forces

Bottom shear stresses


Experiments on subaqueous mass transport with variable sand clay rati o

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


Clay rich sediments

Clay rich sediments

  • Visco-plastic materials

  • Model approach:

    • ”Classical Bingham fluid” (“BING”)

    • R-BING: Remolding of the sediment during the flow

    • H-BING: Hydroplaning/Lubricating


Velocity profile of debris flows bingham fluid

Shear layer

Velocity profile of debris flows Bingham fluid

  • 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


Experiments on subaqueous mass transport with variable sand clay rati o

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

u=1

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, m, um

1+

1

u

1

1

1-

1-

(R-)/

1

u

Plug layer

Shear layer

Velocity

Shear stress

1+

R(1+)/


Experiments on subaqueous mass transport with variable sand clay rati o

Simulation: final deposit of the large-scale Storegga


Experiments on subaqueous mass transport with variable sand clay rati o

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


Low clay content turbulence disintegration layering

Low clay content Turbulence, disintegration, layering


Existing models adapted to low clay debris flows e g nis model

Existing models adapted to low clay debris flows: e.g.: NIS model

  • Mud with plug and shear layers

    • plasticity, viscosity, and visco-elasticity

      • dry friction (no cohesion in code)

      • dynamic shear (thinning)

      • dispersive pressure


Iverson dellinger model

Iverson- Dellinger model

  • 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


In short high clay debris flows

In short: high clay debris flows

  • 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


In short low clay debris flows

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

In short: low clay debris flows


Conclusions

Conclusions

  • 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


Iverson dellinger model1

Iverson-Dellinger model

  • 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.


Conclusions1

Conclusions

  • 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.


Dispersive pressure

Dispersive pressure

  • 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


Velocity profile of debris flows bingham fluid1

shear stress

dynamic viscosity

yield strength

shear rate

Velocity profile of debris flows Bingham fluid

Plug layer

Shear layer

Yield strength: constant during flow


Experiments on subaqueous mass transport with variable sand clay rati o

u=1

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, 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+)/


Debris flows high clay content

Debris flows- high clay content

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


Debris flows low clay content 51

Debris flows- low clay content (5%)

Turbulent front Deposition of sand


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