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Induced Slip on a Large-Scale Frictional Discontinuity: Coupled Flow and Geomechanics

This research investigates the mechanisms and conditions necessary for slip rupture on large-scale frictional discontinuities. It aims to quantify the pore pressure response during slip and assess the coupled flow-deformation effects. The study also aims to develop a theoretical framework for quantification and modeling of progressive slip and apply imaging technologies for monitoring flow and deformation. The research applications include stability of tunnels and underground space, stability of rock slopes, earthquake geomechanics, coupled processes, and resource recovery.

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Induced Slip on a Large-Scale Frictional Discontinuity: Coupled Flow and Geomechanics

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  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

  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 Slip surface has non-uniform strength. Failure occurs before entire frictional strength is mobilized 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.

  4. Modes of fracture • Displacements across fracture • Mode I: Perpendicular to fracture; perpendicular to fracture front • Mode II: Parallel to fracture; perpendicular to fracture front • Mode III: Parallel to fracture; parallel to fracture front After S. Martel B C A Mode I Opening Mode II Sliding Mode III Tearing Shearing modes • Proposed research will investigate Mode II on field scale

  5. 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

  6. 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 • P Critical energy release rate Critical displacement

  7. 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 • Cis ~ a linear function of normal stress • slip initiation predicted by fracture mechanics theory.

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

  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 Deformation, fault slip, normal stress & pore-pressure monitored Rock MechanicsLaboratory (DUSEL) Frictional discontinuity Observationholes Seals Plan view Packers Pressurizedholes Induced Slip

  11. Measure deformation

  12. Fluid pressure from multiple boreholes Increase slip zone; monitor slip, normal stress & pore-pressure Rock MechanicsLaboratory Rock MechanicsLaboratory (DUSEL) Frictional discontinuity Observationholes Seals Plan view Packers Pressurizedholes Induced Slip

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

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

  15. Dependency of GIIC on sn (lab scale) Energy release rate Normal stress sn / sc

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

  17. 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 Rock mass attributes Conductivefractures Nonconductivefractures Large-scalefeatures Strength heterogeneity Multi-scalefracturenetworks Pre-existingstresses Coupled stressand flow

  18. 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 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

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