induced slip on a large-scale frictional discontinuity: coupled flow and geomechanics antonio bobet...
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![Page 1: Induced Slip on a Large-Scale Frictional Discontinuity: Coupled Flow and Geomechanics Antonio Bobet Purdue University, West Lafayette, IN Virginia Tech,](https://reader030.vdocuments.net/reader030/viewer/2022033106/56649e5c5503460f94b53f96/html5/thumbnails/1.jpg)
Induced Slip on a Large-Scale Frictional Discontinuity:
Coupled Flow and Geomechanics
Antonio BobetPurdue University, West Lafayette, IN
Virginia Tech, Blacksburg, VAMatthew Mauldon
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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
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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
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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
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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
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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:
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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:
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Load-displacement results of shear test
Displacement (mm)
She
ar s
tres
s (M
Pa)
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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 )
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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
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Measure deformation
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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)
![Page 13: Induced Slip on a Large-Scale Frictional Discontinuity: Coupled Flow and Geomechanics Antonio Bobet Purdue University, West Lafayette, IN Virginia Tech,](https://reader030.vdocuments.net/reader030/viewer/2022033106/56649e5c5503460f94b53f96/html5/thumbnails/13.jpg)
Measurement of pore pressures
Pressure transducers
Large-scale frictional discontinuity
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Measurement of acoustic emissions
Large-scale frictionaldiscontinuity
Acoustic emission sensors
Reconstruct displacement pattern using seismic tomography
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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
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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)
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Rock mass attributes
Coupled stressand flow
Conductivefractures
Nonconductivefractures
Multi-scalefracturenetworks
Large-scalefeatures
Pre-existingstresses
Strength heterogeneity
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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