Evolution of contact area and aperture during the shearing process of natural rock fractures

The role of surface geometry and its evolution during shearing process on the hydromechanical behaviour of rock discontinuities has long remained an open question for geologist. During the shearing process, only a fraction of discontinuity is in contact and the actual stresses on these contacts are considerably greater than nominal stresses. However, measuring the contact area during shearing process has been a significant experimental challenge due to difficulties in detection of the surface evolution while the shearing process takes place. In this study, the evolution of contact area and fracture aperture during the shearing process were measured using a novel experimental set-up coupled with X-ray micro computed tomography technique. In these experiments, the contact area and aperture of natural fractures were measured during the shearing process in slightly smooth (shale) and rough surfaces (sandstone) specimens. It was shown (within resolution limit) that for the shale specimen, the contact area decreases from 24% to 11%. Contrarily, the contact area increased from 5% to 13% for the sandstone specimen. The volumetric aperture of both specimens, on the other hand, increased during shearing process indicating that the evolution of contact area and aperture is a complex process. This process is likely to be controlled by initial mismatching of the surfaces, dilation, damage of the asperities, and gouge production. The obtained results of this study have significant implication on prediction of shearing behaviour of faults and fractures through measuring the actual stresses acting on the fracture surfaces.