Three-dimensional observations of faulting process in Westerly granite under uniaxial and triaxial conditions by X-ray CT scan

Abstract

Observations of spatial fault development in granite undergoing compression provide new insights into the process of faulting. Dry intact Westerly granite samples were loaded under a confining pressure of 100 MPa (triaxial conditions) and 5 MPa (∼ uniaxial conditions), and the progress of faulting was controlled by maintaining the increment of circumferential displacement at a constant rate, which apparently stiffened the machine. The samples were unloaded after they experienced some degree of stress drop and were successfully recovered before faulting progressed further. A conventional medical X-ray CT scanning system was used to image the sample interiors. Three-dimensional fault systems were detected with sequential X-ray CT images. It was found that three-dimensional reconstruction by X-ray CT images yields not only three-dimensional images of the fault system, but also provides fault cross-section images with much less artificial noise (artifacts) than does direct X-ray CT imaging. Three-dimensional images show that a fault system that developed under uniaxial conditions is much more complicated than a fault system produced by triaxial conditions. In addition, the fault plane produced under uniaxial conditions is inclined at a lower angle to the maximum compressive axis than under triaxial conditions. Comparing X-ray CT images, we show that a fault nucleates locally on the sample surface just after peak stress, then develops into the final fault plane in the residual stress stage of the complete stress–strain relationship under triaxial conditions.