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