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