Erosion in fractures Rheological properties of bentonite in different phasesLennart Börgesson, Clay Technology AB
Erosion in fractures Rheological properties of bentonite in different phases
Lennart Börgesson, Clay Technology AB
Bentonite penetration and transport in fractures Overview of processes
Four different phases that need to be modelled
Bentonite penetration and transport in fractures Overview of processes
Two different directions controlled by different processes (simplification)
Radially (rel. dep. hole) by repulsion in still water (swelling)
Tangentially (rel. dep. hole) by fluid/water flow driven by water pressure gradient (erosion)
Bentonite penetration and transport in fractures Overview of processes
Zone A (solid phase)
Limited in time by hydraulic conductivity K
Limited in space by friction
Tangential transport (from hydraulic gradient)
Not possible Radial transport (from swelling)
The penetration depth z and distribution of the swelling pressure along
the fracture can be estimated:
Zone A (solid state)
The solid state of zone A is defined as follows (proposal):
A water ratio below the liquid limit
The shear strength higher than 1 kPa
MX-80 in non saline water: w<500% (e≈11, rm≈1 150 kg/m3)
Ca-bentonite in non saline water: w<100% (e≈2.8, rm≈1 500 kg/m3)
The main properties controlling the rheological behaviour are
Swelling pressure – density relation
Shear strength and friction
Hydraulic conductivity
Mechanical material model for FEM calculations
Zone B (gel)
Radial transport (from swelling)
Same as for zone A
Tangential transport (from pressure gradient)
Goverend by rheological flow properties of the gel
Zone B (gel state)
The gel state of zone B is defined as follows (proposal):
A water ratio between the liquid limit and when the gel becomes almost Newtonian
The shear strength lower than 1 kPa and higher than 1 Pa
MX-80 in non saline water: 400-500%
Zone B (gel state)
There may be no tangential transport if the hydraulic gradient is low due to the Bingham strength
Important issue: where is the transport limit
Thixotropy helps (and complicates)
Zone C (fluid)
Radial transport (from swelling/separation)
Diffusion?
Tangential transport (from pressure gradient)
Goverend by the viscosity of the fluid
Zone C (fluid state)
The fluid state of zone C is defined by (proposal)
~Newtonian viscous behaviour
Viscosity higher than pure water
Shear strength lower than 1 Pa
MX-80 in non saline water: 2 000-5 000%
Zone D (water)
Low clay concentration that does not affect the viscosity of the water
Radial transport (from swelling/separation)
Diffusion?
Tangential transport (from pressure gradient)
Water flux
Colloidal conditions
Transition from C to D is gradual in distil water but distinct in saline water
Overview of processes
The coloured mechanisms and properties may be investigated with soil mechanics and rheological tools
Rheometer and Viscometer
Non-Newtonian behaviour
Power law
Peak strength
Tixotropic behaviour
Clay solutions
Laboratory program for investigating and modelling the geotechnical and rheological behaviour
Zones A-C
Influence of
water ratio,
salt content
resting time
Instruments
viscometers
rheometers
geotechnical equipment
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