Particle Dynamics Investigations of Geologic Materials Lecture 2: Granular Shear Noncohesive Fault R

1
Particle Dynamics Investigations of Geologic
MaterialsLecture 2 Granular Shear -
Noncohesive Fault Rocks
Julia K. Morgan Rice University Collaborator
Chris Marone Pennsylvania State University
Deformation and Failure of Geomaterials,
Brindisi, Italy (June 14-19, 2009)
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Outline of Talks

• Background, Motivation, Geologic Examples
  Methodology
• Applications Fault Zones, Fault Gouge, Particle
  Size Evolution Effects
• Applications Gravitationally Driven Deformation
• Landsliding
• Gravity Spreading
• Salt Tectonics
• Applications Tectonically Driven Deformation
• Contractional Tectonics
• Extensional Tectonics

3
Fault Gouge

• Softens the fault zone. But how and how much?
• Depends on variations in gouge characteristics
• Mean particle size (ltdgt) -- Roundness /
  angularity
• Particle size distribution (D) -- Thickness (T)
• Modulated by variations in imposed or intrinsic
  properties
• Normal stress (sn) -- Interparticle friction
  (µp)
• Cohesive strength, e.g., composition (sucs )
• Manifest as variations in mechanical response
• Shear strength, i.e., friction (µ) -- Dilatancy
• Grain fracture -- Grain rolling
• How do these vary with shear zone evolution, with
  what effect on slip behavior??

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Death Valley Detachment Faults

• Mean grain size ltdgt decreases towards principal
  slip plane
• Thought to be shear strain indicator

(Cowan et al., 2003)
gouge zone
Slip weakening?
grain size reduction
breccia zone
damage zone
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Theoretical Particle Size Distribution for
Cataclastic Rocks (D2D1.6 D3D2.6)
(Morgan et al., 1996)
Increasing shear strain
N/N0(L/L0)-D
(Sammis et al., 1986, 1987)
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Experimental Fault Gouge
10 mm
(Beeler et al., 1996)
Simulated Fault gouge
65 mm
375 mm
Bare rock surfaces
407 mm
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Laboratory Experiments
µ 0.6 (Byerlees Law)
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Numerical Simulations

• Use DEM simulations to determine the effects of
  particle size and PSD on granular friction.
• Variable parameters
• Grain size distribution and abundance
• Normal stress 40, 70, 140 MPa - but no grain
  breakage.
• Interparticle friction 0.1, 0.5, 0.75.
• Analysis
• Particle displacements, rotations, and slip
  gradients.
• Asemblage friction.
• Compare to laboratory granular experiments.

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Numerical Simulations ofGranular Shear Zones
(Morgan and Boettcher, 1999)

• Look inside actively deforming systems
• Quantify displacements, interparticle forces,
  stress distributions
• Document grain scale micromechanics, and their
  intrinsic controls (e.g., friction, grain size,
  grain strength, etc.)

Course-grained fault gouge
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Numerical Simulations ofGranular Shear Zones
(Morgan and Boettcher, 1999)

• Look inside actively deforming systems
• Quantify displacements, interparticle forces,
  stress distributions
• Document grain scale micromechanics, and their
  intrinsic controls (e.g., friction, grain size,
  grain strength, etc.)

Fine-grained fault gouge
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Effect of D on Granular Friction
Increasing abundance of small particles ---gt
(Morgan, 1999)

• Low sliding friction, m 0.3.
• Stick slip and strain localization (gray bars).
• Strength and stress drop depend on particle size
  and size distribution.

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Effect of D on Granular Friction
Increasing abundance of small particles ---gt
(Morgan, 1999)
Strength drops as D-gt1.6 Potential slip
weakening mechanism.
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Rolling Self-Organization
(Morgan and Boettcher, 1999)

• Similar particle sizes rotate in same direction
• High stress resisting contacts

• Counter-rotating small particles lubricate
  large particles
• Low stress contacts

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Comparison of Laboratory and Numerical Results

• First-order discrepancies
• - Sliding friction significantly lower for
  numerical simulations.
• - Numerical simulations show greater tendency
  for stick slip.

Lab data
Angular quartz sand (granular fault gouge) (Frye
and Marone, 2002)
Numerical results
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Comparison of Laboratory and Numerical Results

• First-order discrepancies
• - Sliding friction significantly lower for
  numerical simulations.
• - Numerical simulations show greater tendency
  for stick slip.

Possible Reasons

• Particle shape
• Particle-size distribution
• Fracture
• 2D vs. 3D particles

Angular quartz sand (Granular Fault Gouge) (Frye
and Marone, GRL, 2002)
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Laboratory Measurements of Granular Friction
2D vs. 3D
(Fry and Marone, 2002)
2D Particles

• Glass rods
• Brass rods
• Pasta

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Comparison of Laboratory and Numerical Results
2D

• Character and mean value of laboratory data are
  very similar.

Quartz Rods (Frye and Marone, 2002)

• Validates numerical simulations.

Distinct element model (Morgan, 1999)

• But numerical methods still missing critical
  elements.

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Laboratory Measurements of Granular Friction
3D Particle shape and size distribution without
fracture
Ang
Angular Quartz, 105-149 µm
Glass Beads, 1-800 µm
Sph 1
Glass Beads, 105-149 µm
Sph 2
(Mair, Frye and Marone, 2002)
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3DLaboratory Results

• Glass beads or angular quartz in non-fracturing
  regime.
• Rough and smooth shear zone boundaries.
• Range of sliding velocities.

(Anthony Marone, 2005)
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3DLaboratory Results

• Angular gouge is stronger than spherical gouge.
• Rough walls are stronger than smooth walls.
• Spherical gouge exhibits distinct stick-slip
  sliding.

(Anthony Marone, 2005)
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Interpreted Micromechanics

• Rough boundaries favor distributed shear. Grain
  bridges support load across layer.
• Smooth boundaries lead to grain-boundary sliding.
  Grain bridges absent.
• Angular grains interlock and resist motion.

(Anthony Marone, 2005)
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Stick-Slip Sliding
(Anthony Marone, 2005)
Hypothesis The nature, geometry, and durability
of force chains determines shear zone behavior
strength
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Numerical Simulations

• Use DEM simulations to examine the nature and
  evolution of granular force chains.
• Design numerical experiments to match Anthonys
  and Marone (2005) experiments
• Grain shape, size and size distribution.
• Shear zone thickness 2, 3, 5, and 8 mm.
• Normal stress 5 and 10 MPa.
• Wall roughness.
• Differences
• 2D instead of 3D.
• No angular particles.
• High sliding velocity.

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Granular Force Chains - Low Velocity

• Low sliding velocities -gt asymmetric stick-slip
  events..

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Granular Force Chains - Low Velocity

• Force chain networks span the shear zone.
• Force chains dissipate during stress drops -gt
  localized slip high angle chains persist.

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Granular Force Chains - High Velocity

• High sliding velocities -gt symmetric, oscillating
  stress fluctuations.

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Granular Force Chains - High Velocity

• Force chain networks span the shear zone.
• Force chains dissipate during stress drops -gt
  distributed deformation high angle chains
  persist.

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Rough Walls - Ultrafine Grained (JEN1)

• Force chain network spans shear zone.
• Low contact forces evolve rapidly.
• Distributed deformation
• Uniform strength, low stress drops.

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Force Chains - Fine Gouge
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Rough Walls - Fine Grained (JEN2)

• Force chain network spans shear zone.
• Moderate contact forces evolve rapidly.
• Distributed deformation.
• Uniform strength, moderate stress drops.

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Rough Walls - Medium Grained (JEN4)

• Force chain network spans shear zone.
• Moderate contact forces.
• Distributed deformation.
• Paired force chains.
• Moderate stress drops.

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Rough Walls - Course Grained (JEN7)

• Force chain network spans shear zone.
• High contact forces.
• Paired force chains.
• Irregular strength, high stress drops.

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Force Chains - Coarse Gouge
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Rough Walls - Poorly Sorted (JEN30)

• Force chain network spans shear zone.
• Wide range of contact forces.
• Distributed deformation
• Irregular strength, high stress drops.
• Periodic??

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Force Chains - Results

• Complicated, evolving networks of contact forces,
  dependent on grain size and distribution.
• Generally, contact force magnitudes scale up with
  particle size.
• Force chain distributions and evolution control
  shear zone friction and stress fluctuations.

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Smooth Walls - Medium Grains (JEN9)

• Force chain network spans shear zone.
• Localized boundary slip.
• Internal force network is stable.
• Very irregular strength, high stress drops.

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Smooth Walls - 2 mm (JEN22)

• Force chain network spans shear zone.
• Horizontal force chains.
• Internal force network is stable.
• Localized boundary slip.
• Very irregular strength, high stress drops.

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Shear Zone Friction
rough walls
rough walls
smooth walls
smooth walls

• Shear zone strength is a function of particle
  size, shear zone thickness, normal stress, and
  wall roughness.
• Results are consistent with the laboratory
  experiments.

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Mean Stress Drop
smooth walls
smooth walls
rough walls
rough walls

• Stress drop decreases with increasing particle
  abundance.
• Stress drop higher for smooth walls!!
• - Reflects onset and cessation of boundary slip.

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Summary

• Force chains exist in ALL granular assemblages.
• Shear zone friction and stress fluctuations
  depend on the rate of change in force chain
  configuration.
• Rough walled assemblages exhibit periodic
  stress fluctuations associated with breakage of
  high force grain bridges.
• Smooth walled assemblages show surprisingly high
  aperiodic stress drops, associated with cessation
  and reinitiation of boundary slip.
• Subject of ongoing research.