1. Induced Slip on a Large-Scale Frictional Discontinuity: Coupled Flow and Geomechanics Antonio Bobet Purdue University, West Lafayette, IN Virginia Tech, Blacksburg, VA Matthew Mauldon

2. 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

3. 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 m 3 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

4. Mode I Opening Mode II Sliding Mode III Tearing 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 B A C  Displacements across fracture  Proposed research will investigate Mode II on field scale

5.  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

6. Laboratory-scale experiments Shear Load Frictional discontinuity Normal Load  G IIC   P Critical energy release rate Critical displacement  Slip induced by increasing shear stress  Energy release occurs with drop from peak to residual friction  Measure:

7. Laboratory: small scale tests  G IIC (critical energy release rate) and  C (critical displacement) appear to be fundamentally related to the initiation of slip on a frictional discontinuity  G IIC strongly depends on:  normal stress  frictional properties of slip surface  critical slip,  C (slip from peak to residual strength)  G IIC 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:

8. Load-displacement results of shear test Displacement (mm) Shear stress (MPa)

9. 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 )

10. Fluid pressure can produce slip on fault Plan view Seals Pressurized holes Observation holes Frictional discontinuity Rock Mechanics Laboratory (DUSEL) Packers Induced Slip Deformation, fault slip, normal stress & pore-pressure monitored

11. Measure deformation

12. Plan view Seals Pressurized holes Observation holes Frictional discontinuity Rock Mechanics Laboratory Packers Induced Slip Fluid pressure from multiple boreholes Increase slip zone; monitor slip, normal stress & pore-pressure Rock Mechanics Laboratory (DUSEL)

13. Measurement of pore pressures Pressure transducers Large-scale frictional discontinuity

14. Measurement of acoustic emissions Large-scale frictional discontinuity Acoustic emission sensors Reconstruct displacement pattern using seismic tomography

15. Dependency of G IIC on  n (lab scale) Energy release rate Normal stress  n /  c

16. Dependency of  C on  n (lab scale) Normal stress  n /  c Critical displacement (mm)

17. Rock mass attributes Fluid pressure will be used to produce slip on a fault patch. Rock mass deformation, fault slip, normal stress and pore-pressure will be monitored Coupled stress and flow Conductive fractures Nonconductive fractures Multi- scale fracture networks Large-scale features Pre-existing stresses Strength heterogeneity

18.  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 scales  Long-term experiments are possible  Ideal experimental environment is a layered rock mass with large-scale (persistent) frictional faults Conclusions