Laboratory Determination of Rock Fracture Shear Stiffness Using Seismic Wave Propagation and Digital Image Correlation

Seismic wave propagation and digital image correlation were used during direct shear experiments on Indiana limestone specimens to investigate the stiffness of rock discontinuities (fractures) approaching shear failure. An instrumented direct shear apparatus was used to apply shear stress to the discontinuity. Compressional and shear wave pulses were transmitted through and reflected from the discontinuity, whereas digital images of the specimen surface were acquired during the test. To measure the dynamic shear stiffness of the rock discontinuities, the displacement discontinuity theory was used and the stiffness was calculated based on the ratio of transmitted to reflected wave amplitudes. The static shear stiffness was calculated based on the ratio of an increment in the applied shear stress to the corresponding increment of relative shear displacement (slip) along the discontinuity. The dynamic shear stiffness measured by seismic wave propagation showed roughly five to ten times greater magnitude than the static values measured by digital image correlation technique. This observation is found to be in agreement with available studies indicating that the frequency-dependent fracture stiffness arises from probabilistic and spatial distributions of stiffness and that dynamic moduli are typically greater than the static values.