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Induced Slip on a Large-Scale Frictional Discontinuity:
Coupled Flow and Geomechanics
Antonio BobetPurdue University, West Lafayette, IN
Virginia Tech, Blacksburg, VAMatthew Mauldon
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
Mode IOpening
Mode IISliding
Mode IIITearing
Shearing modes
A. Mode I: Perpendicular to fracture; perpendicular to fracture front
B. Mode II: Parallel to fracture; perpendicular to fracture front
C. Mode III: Parallel to fracture; parallel to fracture front After S. Martel
Modes of fracture
BA C
Displacements across fracture
Proposed research will investigate Mode II on field scale
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
Preliminary work needed
Laboratory-scale experiments
Shear Load
Frictional discontinuity
No
rma
l Lo
ad
GIIC
P
Critical energy release rate
Critical displacement
Slip induced by increasing shear stress
Energy release occurs with drop from peak to residual friction
Measure:
Laboratory: small scale tests
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 tests on frictional discontinuities at laboratory-scale indicate that:
Load-displacement results of shear test
Displacement (mm)
She
ar s
tres
s (M
Pa)
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 driftsInjection of fluid inside discontinuity
Induce flow by:Injection of fluid in the discontinuityGeneration 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
Pla
n v
iew
Seals
Pressurizedholes
Observationholes
Frictio
nal d
iscon
tinui
ty
Rock MechanicsLaboratory (DUSEL)
Packers
Induced Slip
Deformation, fault slip, normal stress & pore-pressure monitored
Measure deformation
Pla
n v
iew
Seals
Pressurizedholes
Observationholes
Frictio
nal d
iscon
tinui
ty
Rock MechanicsLaboratory
Packers
Induced Slip
Fluid pressure from multiple boreholesIncrease slip zone; monitor slip, normal stress & pore-pressure
Rock MechanicsLaboratory (DUSEL)
Measurement of pore pressures
Pressure transducers
Large-scale frictional discontinuity
Measurement of acoustic emissions
Large-scale frictionaldiscontinuity
Acoustic emission sensors
Reconstruct displacement pattern using seismic tomography
Dependency of GIIC on n (lab scale)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0.1 0.2 0.3 0.4 0.5
lower friction - cohesion
lower friction - no cohesion
higher friction - no cohesionE
ne
rgy
Re
lea
se
Ra
te (
MP
a m
m)
Normal Stress n /
c
Ene
rgy
rele
ase
rate
Normal stress n / c
Dependency of C on n (lab scale)
0.0
0.2
0.4
0.6
0.8
1.0
0 0.1 0.2 0.3 0.4 0.5
higher friction - no cohesion
lower friction - no cohesionlower friction - cohesion
Cri
tic
al
Dis
pla
ce
me
nt
(mm
)
Normal Stress n /
cNormal stress n / c
Crit
ical
dis
plac
emen
t (m
m)
Rock mass attributes
Coupled stressand flow
Conductivefractures
Nonconductivefractures
Multi-scalefracturenetworks
Large-scalefeatures
Pre-existingstresses
Strength heterogeneity
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 large-scale frictional discontinuities (faults and bedding planes)
Experiments can be carried out at many scalesLong-term experiments are possible Ideal experimental environment is a layered rock mass
with large-scale (persistent) frictional faults
Conclusions