Transcript Slide 1
Induced Slip on a
Large-Scale Frictional Discontinuity:
Coupled Flow and Geomechanics
Antonio Bobet
Purdue University, West Lafayette, IN
Matthew Mauldon
Virginia Tech, Blacksburg, VA
Research Objectives
OBJECTIVES:
Determine mechanisms that produce onset of slip on a
large-scale frictional discontinuity
Determine conditions necessary for slip rupture
Quantify pore pressure response during slip
Assess coupled flow-deformation effects of large scale
discontinuities under large stresses
Estimate scale effects: comparison between laboratory
and DUSEL experiments
Develop theoretical fracture mechanics framework for
quantification and modeling of progressive slip
Apply and develop imaging technologies for monitoring
flow and deformation
Research Applications
Stability of tunnels and
underground space
Stability of rock slopes
Earthquake geomechanics
Coupled processes
Resource recovery
Vaiont Dam. In 1963 a
block of 270 million m3
slid from Mt Toc.
A wave overtopped the
dam by 250 m and
swept onto the valley
below, resulting in the
loss of about 2500 lives.
Slip surface has non-uniform
strength. Failure occurs before entire
frictional strength is mobilized
Modes of fracture
Displacements across fracture
A. Mode I: Perpendicular to fracture; perpendicular to fracture front
B. Mode II: Parallel to fracture; perpendicular to fracture front
After S. Martel
C. Mode III: Parallel to fracture; parallel to fracture front
Mode I
Opening
Mode II
Sliding
Mode III
Tearing
Shearing modes
Proposed research will investigate Mode II on field scale
Preliminary work needed
Determine stress field at DUSEL site, including pore
pressures
Determine rock mass properties at the test site
Identify and characterize suitable frictional
discontinuities: fault(s) or bedding planes
Estimate frictional strength and permeability of suitable
discontinuities
Laboratory-scale experiments
Shear
Load
Frictional discontinuity
Normal Load
Slip induced by
increasing shear stress
Energy release occurs
with drop from peak to
residual friction
Measure:
GIIC Critical energy release rate
P Critical displacement
Laboratory: small scale tests
Shear tests on frictional discontinuities at laboratory-scale
indicate that:
GIIC (critical energy release rate) and C (critical
displacement) appear to be fundamentally related to the
initiation of slip on a frictional discontinuity
GIIC strongly depends on:
normal stress
frictional properties of slip surface
critical slip, C (slip from peak to residual strength)
GIIC is ~ a quadratic function of normal stress
C is ~ a linear function of normal stress
slip initiation predicted by fracture mechanics theory.
Shear stress (MPa)
Load-displacement results of shear test
Displacement (mm)
Proposed Research
Continuously test coupled flow and deformations related to slip
initiation along selected large-scale discontinuities and faults.
Induce slip by:
Altering stress field through excavation of drifts
Injection of fluid inside discontinuity
Induce flow by:
Injection of fluid in the discontinuity
Generation of excess pore pressures by slip
Continuous behavior monitoring
Use results to scale-up fracture mechanics theories for
Mode II crack growth (fault slip )
Fluid pressure can produce slip on fault
Deformation, fault slip, normal stress & pore-pressure monitored
Rock Mechanics
Laboratory (DUSEL)
Observation
holes
Plan view
Seals
Packers
Induced Slip
Pressurized
holes
Measure deformation
Fluid pressure from multiple boreholes
Increase slip zone; monitor slip, normal stress & pore-pressure
Rock Mechanics
Laboratory (DUSEL)
Observation
holes
Plan view
Seals
Packers
Induced Slip
Pressurized
holes
Measurement of pore pressures
Pressure
transducers
Large-scale frictional discontinuity
Measurement of acoustic emissions
Acoustic
emission
sensors
Large-scale
frictional
discontinuity
Reconstruct displacement pattern using seismic tomography
Dependency of GIIC on sn (lab scale)
ratemm)
release
Energy
Energy
Release
Rate (MPa
3.0
higher f riction - no cohesion
low er f riction - cohesion
low er f riction - no cohesion
2.5
2.0
1.5
1.0
0.5
0.0
0
0.1
0.2
0.3
Normal
sc c
Normalstress
Stress s
s n/ / s
n
0.4
0.5
Dependency of C on sn (lab scale)
Critical
Displacement (mm)
(mm)
displacement
Critical
1.0
0.8
0.6
0.4
0.2
higher f riction - no cohesion
low er f riction - cohesion
low er f riction - no cohesion
0.0
0
0.1
0.2
0.3
Normal
Stressssn // s
sc
Normal
stress
n
c
0.4
0.5
Rock mass attributes
Conductive
fractures
Nonconductive
fractures
Large-scale
features
Pre-existing
stresses
Coupled stress
and flow
Multiscale
fracture
networks
Strength
heterogeneity
Conclusions
Mode II fracture initiation and propagation important in
rock mechanics (slope stability, tunnels, underground
caverns, earthquake geomechanics).
Lab-scale experiments show that critical energy release
rate and critical displacement are not material properties
(as previously thought) but are stress-dependent
DUSEL will enable research into slip rupture on largescale frictional discontinuities (faults and bedding
planes)
Experiments can be carried out at many scales
Long-term experiments are possible
Ideal experimental environment is a layered rock mass
with large-scale (persistent) frictional faults